The fatigue life of a component is made up of initiation and propagation stages. This is illustrated schematically in Fig. 1 Figure 1. Initiation and propagation portions of fatigue life

The size of the crack at the transition from initiation to propagation is usually unknown and often depends on the point of view of the analyst and the size of the component being analyzed. For example, for a researcher equipped with microscopic equipment it may be on the order of a crystal imperfection, dislocation, or a 0,1 mm-crack, while to the inspector in the field it may be the smallest crack that is readily detectable with nondestructive inspection equipment.

Nevertheless, the distinction between the initiation life and propagation life is important. At low strain amplitudes up to 90% of the life may be taken up with initiation, while at high amplitudes the majority of the fatigue life may be spent propagating a crack. Fracture mechanics approaches are used to estimate the propagation life.

Fracture mechanics approaches require that an initial crack size be known or assumed. For components with imperfections or defects (such as welding porosities, inclusions and casting defects, etc.) an initial crack size may be known. Alternatively, for an estimate of the total fatigue life of a defect-free material, fracture mechanics approaches can be used to determine propagation. Strain-life approaches may then be used to determine initiation life, with the total life being the sum of these two estimates.

Linear Elastic Fracture Mechanics Background

Linear elastic fracture mechanics (LEFM) principles are used to relate the stress magnitude and distribution near the crack tip to:

• Remote stresses applied to the cracked component
• The crack size and shape
• The material properties of the cracked component

Historical Overview

In the 1920s, Griffith formulated the concept that a crack in a component will propagate if the total energy of the system is lowered with crack propagation. That is, if the change in elastic strain energy due to crack extension is larger than the energy required to create new crack surfaces, crack propagation will occur.

Griffith`s theory was developed for brittle materials. In the 1940s, Irwin extended the theory for ductile materials.
He postulated that the energy due to plastic deformation must be added to the surface energy associated with the creation of new crack surfaces. He recognized that for ductile materials, the surface energy term is often negligible compared to the energy associated with plastic deformation. Further, he defined a quantity, G, the strain energy release rate or "crack driving force," which is the total energy absorbed during cracking per unit increase in crack length and per unit thickness.

In the mid-1950s, Irwin made another significant contribution. He showed that the local stresses near the crack tip are of the general form

(1)

where r and q are cylindrical coordinates of a point with respect to the crack tip (see Fig. 2) and K is the stress intensity factor. He further showed that the energy approach (the "G" approach above) is equivalent to the stress intensity approach and that crack propagation occurs when a critical strain energy release rate, G, (or in terms of a critical stress intensity, K_c) is achieved.

Figure 2. Location of local stresses near a crack tip in cylindrical
coordinates

LEFM Assumptions

Linear elastic fracture mechanics (LEFM) is based on the application of the theory of elasticity to bodies containing cracks or defects. The assumptions used in elasticity are also inherent in the theory of LEFM: small displacements and general linearity between stresses and strains.

The general form of the LEFM equations is given in Eq. 1. As seen, a singularity exists such that as r, the distance from the crack tip, tends toward zero, the stresses go to infinity. Since materials plastically deform as the yield stress is exceeded, a plastic zone will form near the crack tip. The basis of LEFM remains valid, though, if this region of plasticity remains small in relation to the overall dimensions of the crack and cracked body.

Loading Modes

There are generally three modes of loading, which involve different crack surface displacements (see Fig. 3). The three modes are:

Mode 1:	opening or tensile mode (the crack faces are pulled apart)
Mode 2:	sliding or in-plane shear (the crack surfaces slide over each other)
Mode 3:	tearing or anti-plane shear (the crack surfaces move parallel to the leading edge of the crack and relative to each other)

The following discussion deals with Mode 1 since this is the predominant loading mode in most engineering applications. Similar treatments can readily be extended to Modes 2 and 3.

Figure 3. Three loading modes

Stress Intensity Factor

The stress intensity factor, K, which was introduced in Eq. 1, defines the magnitude of the local stresses around the crack tip. This factor depends on loading, crack size, crack shape, and geometric boundaries, with the general form given by

(2)

where:

s = remote stress applied to component (not to be confused with the local stresses, s_ij, in Eq. 1)
a = crack length
f (a/w) = correction factor that depends on specimen and crack geometry

Figure 4 gives the stress intensity relationships for a few of the more common loading conditions. Stress intensity factors for a single loading mode can be added algebraically. Consequently, stress intensity factors for complex loading conditions of the same mode can be determined from the superposition of simpler results, such as those readily obtainable from handbooks.

Figure 4.	Stress intensity factor for
	(a) Center-cracked plate loaded in tension,
	(b) Edge-cracked plate loaded in tension,
	(c) Double-edge-cracked plate loaded in tension
	(d) Cracked beam in pure bending

Ductile fracture

A pure and inclusion free metal can elongate under tension to give approx. 100% RA and a point fracture, Fig. 1. Most alloys contain second phases which lose cohesion with the matrix or fracture and the voids so formed grow as dislocations flow into them. Coalescence of the voids forms a continuous fracture surface followed by failure of the remaining annulus of material usually on plane at 45° to the tension axis. The central fracture surface consists of numerous cup-like depressions generally called dimples. The shape of the dimples is strongly influenced by the direction of major stresses-circular in pure tension and parabolic under shear. Dimple size depends largely on the number of inclusion sites. Fig. 2a shows typical dimples.

Figure 1.	(a) Stages in ductile fracture from inclusions
	(b) Fracture toughness n thickness

Some important features of ductile fracture can be summarised as follows:

• Pure metals and solid solutions that are relatively free from second phase particles (including impurity particles) are usually more ductile than strong two-phase alloys.
• The local stress required for whole nucleation at particles depends on their resistance to cracking and the strength of their bond with the matrix.
• The local stress generated at the particles depends on the flow strength of the alloy, the applied strain and the shape and size of the particles.
• Growth of the holes, so that they coalesce to form a macroscopic fracture, depends on the applied stresses being tensile. Much higher ductilities are achieved in compressive straining.

In cleavage fracture the material fails along well defined crystallographic planes within the grain but the crack path is affected by grain boundaries and inclusions. Basically a cleavage fracture surface contains large smooth areas separated by cleavage steps and feathers, river markings and cleavage tongues which are the direct result of crack path disturbances-Fig. 2b.

Intercrystalline fracture is characterized by separation of the grains to reveal a surface composed of grain boundary facets, Fig. 2c. This type of fracture is found in stress-corrosion, creep hot tearing and hydrogen embrittlement.

Fatigue fractures are characterized by striations (Fig. 2d) representing the extent of crack propagation under each cycle of loading.

a) Dimples in a ductile fracture of mild steel (x5000)	b) Cleavage patterns in HS steel fracture (x12000)
c) Intergranular fracture in low alloy steel (x1500)	d) Fatigue striations in Nimonic 80A (x7000)(A.Strang)

Figure 2. Fracture in Scanning electron micrographs

Compound stresses and brittle fracture

The failure of some American all-welded ships during the Second World War has stimulated much work on the causes of brittle fracture in steel. In the tensile test plastic deformation involves shearing slip along crystal planes within the crystals, but in the presence of tension of equal magnitude in each principal direction, shearing stresses are absent, plastic deformation is prevented and a brittle fracture occurs as soon as the cohesive strength of the material is exceeded. Equal triaxial tension stresses do not arise frequently in practice, but it is common to find a triaxial tension superimposed on a unidirectional tension, and if the margin between cohesive strength and plastic yield strength is small, a brittle fracture may occur in a material ordinarily considered highly ductile. Compound stresses arise in a weld in very thick plate and in a tube under internal pressure and an axial tension.

This is shown in Fig. 3 with cohesive stress-strain curves, B, N, and F. If the two curves intersect at Y, brittle fracture occurs preceded by plastic deformation, which decreases as the cohesive strength curve becomes lower with respect to the yield stress-strain curve. Orowan has shown that if the yield stress is denoted by Y, the strength for brittle fracture by B (both Y and B depend on the plastic strain), and the initial value of Y (for strain = 0) by Y₀ we have the following relation:

• The material is brittle if B <>0;
• The material is ductile but notch-brittle if Y₀ <>0
• The material is not notch-brittle if 3Y <>

The factor 3 takes into account the stress increase at a notch. Whether the material is notch-brittle or not depends on the very small margin between B and 3Y.

Figure 3.	(a) Yield and cohesive stress curves
	(b) Slow notch bend test
	(c) Effect of temperature on the Izod value of mild steel

Carbon steel is an exceptional material, because 3Y and B are so close, and this is the reason why the results of Izod tests seem to be so erratic, and why notch brittleness is so sensitive to slight variations of composition, previous treatment and temperature.

Brittle fracture is characterised by the very small amount of work absorbed and by a crystalline appearance of the surfaces of fracture, often with a chevron pattern pointing to the origin of fracture, due to the formation of discontinuous cleavage cracks which join up (Fig. 4). It can occur at a low stress of 75-120 MPa with great suddenness; the velocity of crack propagation is probably not far from that of sound in the material in this type of fracture plastic deformation is very small, and the crack need not open up considerably in order to propagate, as is necessary with a ductile failure.

Figure 4. Steel brittle fracture surface with chevron markings. Micrograph shows discontinuous cracks ahead of main crack

The work required to propagate a crack is given by Griffith`s formula:

(1)

where:
s = tensile stress required to propagate a crack of length c
g = surface energy of fracture faces
E = Young`s modulus

Orowan modified the Griffith theory to include a plastic strain energy factor, p, since some plastic flow is always found near the fracture surface:

(2)

When the temperature is above the brittle-ductile transition temperature, p is large and the stress, s, required to make the crack grow will also be large. Below the transition temperature the metal is brittle and p will be smaller. The stress necessary to cause crack growth, therefore, will be reduced. The reason for the increasing speed of crack propagation, once a crack has started, is clear from both Griffith`s and Orowan`s equations: as the crack grows in length, the stress required for propagation continually decreases.

Fire, water and wind are the essential elements of the ironworks. The wind fans the fire and gives it the power to melt the iron; the cooling effects of the water holds the destructive power of fire under control.

In the Voelklingen Ironworks, which was founded in 1883 and now is awarded the status of a World Cultural Heritage by UNESCO, history of creating the wind element can be seen directly. The heart of the entire plant is over 6000 m² (65000 sq feet) large blowerhouse, in which gigantic machines produced the blast necessary for iron making.

The blasting engines
The striking idea for the new technology was late in coming, but it was to revolutionize iron production: in 1878 German engineers Otto and Langen constructed the first gas motor. The mechanical engineering works Deutz builds the first the first blast furnace gas engine in 1894.

It was an ingenious innovation: at last it was now possible to use the gas, which was produced by the blast furnace in gigantic quantities in the process of smelting iron, to drive engines. The iron and steel industry was now able to recycle waste material.

Voelklingen Ironworks puts the new technology to use at once. The first large scale gas engine was ordered from M.A.N. in Nuremberg in 1899, as a generator unit for electricity, and went into operation in 1901. A total of 30 gas engines were in operation in Voelklingen Ironworks, and they were not only used not only as blast engines and power generators, but also to drive pumps and rolling mills.

In July 1903 Ironworks ordered the oldest surviving machine in the hall from the Augsburg-Nuernberg mechanical engineering works "at a price of 300 000 marks, transported by rail to Voelklingen station, fully installed and assembled and including one week of test operation". The gas engine was a twin blaster: two units were linked by a flywheel. This was latter put to a good use: when an irreparable damage occurred on a part of the twins in 1968, this part served as a "spare parts warehouse".

Another three gas engines were ordered from Thyssen AG in 1906, and the blast capacity of the Ironworks was dramatically increased with the commissioning of the engines in 1908. At that time, there was still no crane available for installing the colossus, and Hermann Roehling, the owner of the Ironworks, included the instruction that "the heaviest parts are to be transported using our available tools" in the contract. Those machines were producing blast until 80`s, when the Ironworks were shut down.

The rest of the engines were acquired until early 40`s. They could produce either electricity or air blast as required. When the blast furnaces needed less air blast, the machines were used to generate electricity for the Ironworks own power grid. To do this, the gas engines had to be driven at a higher speed, and the operators then received a wage bonus.

These ten blast engines produced up to 110000 cubic meters of blast air per hour for each blast furnace in the Steelworks. The cold airflow was forced into the blast air heaters along six pipelines. Hot stones in the air heaters heated the air up to 1100°C.

The hot air entered the smelting zone of the furnace through 16 blast openings. This raised the temperature of the glowing mass up to 2000°C. The oxygen blown in combined with the carbon from the coke and left the furnace flue as blast furnace gas. The blast furnace gas was then cleaned of dust and ash and taken back to the blasting hall, where it was used to drive the blasting engines. The circulation loop was closed.

Working in the blasting hall
The blasting hall had a 12-men shift. Each blast engine has its own operator. The flywheels rotated and dispersed an uninterrupted fine oil spray into the hall which was inhaled by the operators while they worked. In addition to this came the noise of the blast engines, monotonous rhythm of the engines and hum of the flywheels as they rotated.

The men worked extra shifts when malfunctions occurred: heavy machine parts and outsized tools had to be moved. There were no fixed break times and operators generally ate the food they brought with them towards the middle of the shift. They also kept their eyes on the machine while they ate: there was a simple table and chair next to the each blast engine. Only from the mid 1970`s, a break room offered protection from noise and oil.

The operators in the blasting hall worked for a long time worked a three-shift system. This was a constant round of early, day and night shifts, 56 hours a week without a single day off. When the pattern changed a "long shift" had to be done, working through from midday on Sunday to Monday morning. In 1960 a four-shift system was introduced. After working seven days a worker had a couple days free-- time for himself and his family.

Starting up and shutting down, oiling and monitoring the blast was the daily routine of the machine operators. But each shift has its own special tasks. The early shift was responsible for cleaning the machines. All parts were cleaned with a mixture of oil and petroleum. The cleaning of the cellar and the maintenance of spare parts was the task of day shift. The night shift ended by scrubbing the hall floor with potassium soap.

Fire, water and wind are the essential elements of the ironworks. The wind fans the fire and gives it the power to melt the iron; the cooling effects of the water holds the destructive power of fire under control.

In the Voelklingen Ironworks, which was founded in 1883 and now is awarded the status of a World Cultural Heritage by UNESCO, history of creating the wind element can be seen directly. The heart of the entire plant is over 6000 m² (65000 sq feet) large blowerhouse, in which gigantic machines produced the blast necessary for iron making.
The blasting engines
The striking idea for the new technology was late in coming, but it was to revolutionize iron production: in 1878 German engineers Otto and Langen constructed the first gas motor. The mechanical engineering works Deutz builds the first the first blast furnace gas engine in 1894.

It was an ingenious innovation: at last it was now possible to use the gas, which was produced by the blast furnace in gigantic quantities in the process of smelting iron, to drive engines. The iron and steel industry was now able to recycle waste material.

Voelklingen Ironworks puts the new technology to use at once. The first large scale gas engine was ordered from M.A.N. in Nuremberg in 1899, as a generator unit for electricity, and went into operation in 1901. A total of 30 gas engines were in operation in Voelklingen Ironworks, and they were not only used not only as blast engines and power generators, but also to drive pumps and rolling mills.

In July 1903 Ironworks ordered the oldest surviving machine in the hall from the Augsburg-Nuernberg mechanical engineering works "at a price of 300 000 marks, transported by rail to Voelklingen station, fully installed and assembled and including one week of test operation". The gas engine was a twin blaster: two units were linked by a flywheel. This was latter put to a good use: when an irreparable damage occurred on a part of the twins in 1968, this part served as a "spare parts warehouse".

Another three gas engines were ordered from Thyssen AG in 1906, and the blast capacity of the Ironworks was dramatically increased with the commissioning of the engines in 1908. At that time, there was still no crane available for installing the colossus, and Hermann Roehling, the owner of the Ironworks, included the instruction that "the heaviest parts are to be transported using our available tools" in the contract. Those machines were producing blast until 80`s, when the Ironworks were shut down.

The rest of the engines were acquired until early 40`s. They could produce either electricity or air blast as required. When the blast furnaces needed less air blast, the machines were used to generate electricity for the Ironworks own power grid. To do this, the gas engines had to be driven at a higher speed, and the operators then received a wage bonus.

These ten blast engines produced up to 110000 cubic meters of blast air per hour for each blast furnace in the Steelworks. The cold airflow was forced into the blast air heaters along six pipelines. Hot stones in the air heaters heated the air up to 1100°C.

The hot air entered the smelting zone of the furnace through 16 blast openings. This raised the temperature of the glowing mass up to 2000°C. The oxygen blown in combined with the carbon from the coke and left the furnace flue as blast furnace gas. The blast furnace gas was then cleaned of dust and ash and taken back to the blasting hall, where it was used to drive the blasting engines. The circulation loop was closed.

Working in the blasting hall
The blasting hall had a 12-men shift. Each blast engine has its own operator. The flywheels rotated and dispersed an uninterrupted fine oil spray into the hall which was inhaled by the operators while they worked. In addition to this came the noise of the blast engines, monotonous rhythm of the engines and hum of the flywheels as they rotated.

The men worked extra shifts when malfunctions occurred: heavy machine parts and outsized tools had to be moved. There were no fixed break times and operators generally ate the food they brought with them towards the middle of the shift. They also kept their eyes on the machine while they ate: there was a simple table and chair next to the each blast engine. Only from the mid 1970`s, a break room offered protection from noise and oil.

The operators in the blasting hall worked for a long time worked a three-shift system. This was a constant round of early, day and night shifts, 56 hours a week without a single day off. When the pattern changed a "long shift" had to be done, working through from midday on Sunday to Monday morning. In 1960 a four-shift system was introduced. After working seven days a worker had a couple days free-- time for himself and his family.

Starting up and shutting down, oiling and monitoring the blast was the daily routine of the machine operators. But each shift has its own special tasks. The early shift was responsible for cleaning the machines. All parts were cleaned with a mixture of oil and petroleum. The cleaning of the cellar and the maintenance of spare parts was the task of day shift. The night shift ended by scrubbing the hall floor with potassium soap.

Fire, water and wind are the essential elements of the ironworks. The wind fans the fire and gives it the power to melt the iron; the cooling effects of the water holds the destructive power of fire under control.

In the Voelklingen Ironworks, which was founded in 1883 and now is awarded the status of a World Cultural Heritage by UNESCO, history of creating the wind element can be seen directly. The heart of the entire plant is over 6000 m² (65000 sq feet) large blowerhouse, in which gigantic machines produced the blast necessary for iron making.
The blasting engines
The striking idea for the new technology was late in coming, but it was to revolutionize iron production: in 1878 German engineers Otto and Langen constructed the first gas motor. The mechanical engineering works Deutz builds the first the first blast furnace gas engine in 1894.

It was an ingenious innovation: at last it was now possible to use the gas, which was produced by the blast furnace in gigantic quantities in the process of smelting iron, to drive engines. The iron and steel industry was now able to recycle waste material.

Voelklingen Ironworks puts the new technology to use at once. The first large scale gas engine was ordered from M.A.N. in Nuremberg in 1899, as a generator unit for electricity, and went into operation in 1901. A total of 30 gas engines were in operation in Voelklingen Ironworks, and they were not only used not only as blast engines and power generators, but also to drive pumps and rolling mills.

In July 1903 Ironworks ordered the oldest surviving machine in the hall from the Augsburg-Nuernberg mechanical engineering works "at a price of 300 000 marks, transported by rail to Voelklingen station, fully installed and assembled and including one week of test operation". The gas engine was a twin blaster: two units were linked by a flywheel. This was latter put to a good use: when an irreparable damage occurred on a part of the twins in 1968, this part served as a "spare parts warehouse".

Another three gas engines were ordered from Thyssen AG in 1906, and the blast capacity of the Ironworks was dramatically increased with the commissioning of the engines in 1908. At that time, there was still no crane available for installing the colossus, and Hermann Roehling, the owner of the Ironworks, included the instruction that "the heaviest parts are to be transported using our available tools" in the contract. Those machines were producing blast until 80`s, when the Ironworks were shut down.

The rest of the engines were acquired until early 40`s. They could produce either electricity or air blast as required. When the blast furnaces needed less air blast, the machines were used to generate electricity for the Ironworks own power grid. To do this, the gas engines had to be driven at a higher speed, and the operators then received a wage bonus.

These ten blast engines produced up to 110000 cubic meters of blast air per hour for each blast furnace in the Steelworks. The cold airflow was forced into the blast air heaters along six pipelines. Hot stones in the air heaters heated the air up to 1100°C.

The hot air entered the smelting zone of the furnace through 16 blast openings. This raised the temperature of the glowing mass up to 2000°C. The oxygen blown in combined with the carbon from the coke and left the furnace flue as blast furnace gas. The blast furnace gas was then cleaned of dust and ash and taken back to the blasting hall, where it was used to drive the blasting engines. The circulation loop was closed.

Working in the blasting hall
The blasting hall had a 12-men shift. Each blast engine has its own operator. The flywheels rotated and dispersed an uninterrupted fine oil spray into the hall which was inhaled by the operators while they worked. In addition to this came the noise of the blast engines, monotonous rhythm of the engines and hum of the flywheels as they rotated.

The men worked extra shifts when malfunctions occurred: heavy machine parts and outsized tools had to be moved. There were no fixed break times and operators generally ate the food they brought with them towards the middle of the shift. They also kept their eyes on the machine while they ate: there was a simple table and chair next to the each blast engine. Only from the mid 1970`s, a break room offered protection from noise and oil.

The operators in the blasting hall worked for a long time worked a three-shift system. This was a constant round of early, day and night shifts, 56 hours a week without a single day off. When the pattern changed a "long shift" had to be done, working through from midday on Sunday to Monday morning. In 1960 a four-shift system was introduced. After working seven days a worker had a couple days free-- time for himself and his family.

Starting up and shutting down, oiling and monitoring the blast was the daily routine of the machine operators. But each shift has its own special tasks. The early shift was responsible for cleaning the machines. All parts were cleaned with a mixture of oil and petroleum. The cleaning of the cellar and the maintenance of spare parts was the task of day shift. The night shift ended by scrubbing the hall floor with potassium soap.

Rolling is an indirect compression process. Normally the only force or stress applied is the radial pressure from the rolls. This deforms the metal and pulls it through the roll gap. The process can be compared to compression or forging but differs in two respects in that compression takes place between a pair of platens at various inclinations to each other, and that the process is continuous,
Rolling is the most widely used deformation process and for the reason that there are so many versions the process has its own classification. This can be according to the arrangement of the rolls in the mill stand or according to the arrangement of the stands in sequence.
The two-high mill was the first and simplest but production rates tended to be low because of the time lost in returning the metal to the front of the mill. This obviously led to the reversing two-high mill where the metal could be rolled in both directions. Such a mill is limited in the length that it can handle, and if the rolling speed is increased, the output is almost unchanged because of the increased time spent in reversing the rotation at each pass. This sets an economic maximum of about 10 meters.

The next obvious development was the three-high mill, which has the advantages of both the two high reversing and non-reversing mills. Such a mill must, of course, have elevating tables on both sides of the rolls. The roll gap on a three-high mill cannot be adjusted between passes, therefore grooves or passes must be cut into the roll face to achieve different pass reductions.

All three kinds of mill suffer from the disadvantage that all stages of rolling are carried out on the same rolled surface and the surface quality of the product tends to be low. Roll changes on such mills are relatively frequent and time consuming. This type of mill is therefore used for primary rolling where rapid change of shape is required, even at the expense of surface quality.

Four-high mills are a special case of two-high, and in an attempt to lower the rolling load, the work roll diameter is decreased.

There is, however, a risk of roll bending which is avoided by supporting the small work rolls by larger backing rolls. The backing roll diameter cannot be greater than about 2-3 times that of the work rolls, and as the work roll diameter is decreased more and more (to accommodate processes with exceedingly high rolling loads) the size of the backing rolls must also decrease. A point is reached when the backing rolls themselves begin to bend and must be supported hence the ultimate design - the cluster mill.

The principal criticism of the traditional mill is this tendency for roll bending due to its inherent design - the beam principle. Sendzimir proposed a design which eliminated this limitation based on the castor principle where the work roll is supported over ali its face by an array of backing rolls. This principle can be applied to much mills and an installation for rolling stainless steel 1600 mm wide is fitted with work rolls 85 mm diameter.

Continuous rolling mills can be classified according to the arrangement of stands or passes. These are in line in a continuous mill and line abreast in a looping or cross-country mill. Looping and cross-country mills require the workpiece to be bent or turned between stands and are used therefore for rolling rods, rails or sections. Continuous mills are used for plates, strip or sheets. They all require a large capital outlay and are only justified when a large demand for the product is guaranteed.

It is possible to derive an expression for this friction force. Pressure acts radially on the ends of this element, and if the element is located between the point of entry and the neutral point a frictional force acts toward the neutral point. The radial pressure has a horizontal component which tends to reject the metal and prevent it from entering the rolls, whilst the friction force has a horizontal component dragging the metal inward. Whether the metal passes through the rolls depends upon the values of the two horizontal force components.

Primary rolling is a process where large maximum reductions are required in order that the metal can be deformed quickly and cheaply. Such mills have large diameter rolls with surfaces that are roughened or ragged to increase the coefficient of friction.

The rolling load can be minimized by making the radius as small as possible and the roll surface as smooth as possible. This principle is used in the design of cluster mills which are used extensively for foil rolling and consist of small work rolls supported by larger back-up rolls to prevent bending. Even with such mills the rolling loads can still be excessive and recourse is made to devices which apply front and back tension to the metal being rolled.

Foil rolling and finishing mills are generally very different from primary mills which as already seen tend to use large diameter rolls with roughened surfaces.

It is an essential of metal-deformation processes that the tool is only loaded elastically, while the workpiece is flowing plastically. This elastic deformation is generally so small that it can be ignored, but this is not the case in rolling. There are two reasons. One is that rolling loads and stresses can be very large, especially when the workpiece is thin and work-hardened. The other is that the tool in rolling comprises the whole mill-rolls and housing with overall dimensions measurable in meters. This combination can result in very large strains due to elastic deformation divided between mill stand extension "mill spring", roll flattening and roll bending.

Roll flattening. The workpiece passing between a pair of rolls is compressed by the radial stress in them, but the reaction is transferred to the mill bearings and housing, which are capable of only limited yield because of their large dimensions. If an attempt is made to compress thin hard material further, the reaction becomes so large that the rolls deform elastically and the radius of curvature of the arc of contact is increased. The extent of this flattening depends on the magnitude of the reaction stress and the elastic constants of the rolls.

Roll flattening has another effect in that for a given mill there is a minimum gauge below which it is not possible to roll. Any attempt to do so results in greater deformation of the rolls, without any plastic deformation of the strip. With thin gauges as already seen the friction hill becomes very large producing reaction stresses in the arc of contact which exceed the yield stress of the rolls, therefore it is easier to deform the rolls than the metal. As long as the mill is running the rolls will remain circular, but if the load is not removed when it is stopped, deformation will take place to flatten the surface over the area of contact between the rolls.

Attempts to avoid or limit roll bending have involved ways of decreasing the rolling load. This has resulted in small work rolls and four-high mills. But even with these mills a certain amount of roll bending still occurs and is accommodated by cambering the rolls, i.e. making them barrel shaped. With multistand continuous rolling, interstand tension is adjusted to maintain the rolling load to a constant value and so achieve a flat surface. This is an important aspect of shape control in the rolling of strip.

A recent development has been the introduction of hydraulic jacks onto the roll necks thereby altering the roll camber by actually bending the rolls. Results to date indicate that this method will be very successful in controlling strip shape.

All the methods described so far have involved continuous rolling where front and back tension or interstand tension can be used. With single sheet rolling this technique for controlling rolling load cannot be used and therefore the problem of shape control is tackled in another way.

Mill spring or plastic distortion. The reaction to rolling load is called the roll separating force and if the rolls were not held in the mill housing they would indeed separate and reduction of metal would not be possible. The upper roll pushes the top of the housing upwards whilst the bottom roll pushes the base of the housing downwards. The housing is therefore subjected to a tensile stress, which is obviously below the yield stress of the cast steel normally used, but there is a measurable elastic deformation.

The extent depends upon (a) the rolling load, (b) the cross-sectional area of the housing, and (c) the height of the housing. If the extent of this deformation is small the mill is said to be hard or rigid, whilst if it is large, the mill is said to be soft or springy.

It is a characteristic of the mill and can be determined in the following way. The mill is set to a constant roll gap and a series of different pieces of metal are rolled. These produce different rolling loads which are measured. The rolling loads can be varied either by using different gauges of the same metal or by using different metals. A graph is drawn relating rolling load to gauge, the gauge being found by measuring the thickness of the rolled pieces.

Specification according to the Standard F 2282, establishes quality assurance requirements for the physical, mechanical, and metallurgical requirements for carbon and alloy steel wire, rods, and bars in coils intended for the manufacture of mechanical fasteners which includes: bolts, nuts, rivets, screws, washers, and special parts manufactured cold.

The term "quality" is being used to designate characteristics of a material which make it particularly well suited to a specific fabrication and/or application and does not imply "quality" in the usual sense.

Material is furnished in many application variations. The purchaser should advise the supplier regarding the manufacturing process and finished product application as appropriate. Five application variations are:

* Cold heading
* Recessed head
* Socket head
* Scrapples nut
* Tubular rivet

Forming is the primary manufacturing operation in the fastener industry and the term includes heading, upsetting, extruding, and forging. These formed parts are produced at very high speeds by metal flow due to machine-applied pressure.

The primary forming operation self-inspects the quality of the raw material and imperfections such as seams, laps, and internal pipe which may not be visible are revealed when the material is upset. The absence of bursts, forging cracks, and open seams is strong evidence that the quality of material selected was that intended for the severe upsets of today’s fastener manufacturing.

Manufacture

Melting Practice:	The steel shall be melted in a basic oxygen or electric furnace process.
Casting Practice:	Steel shall be ingot cast, or continuous cast with controlled procedures to meet the requirements of this specification.
Deoxidation Practice and Grain Size:	The material shall be furnished in one of the deoxidation and grain size. When not specified, the practice shall be at the option of the manufacturer. Silicon killed fine grain shall be produced with aluminum for grain refinement. The material purchaser’s approval shall be obtained for the use of vanadium or columbium for grain refinement. Silicon killed coarse grain practice. Silicon killed fine grain practice. Aluminum killed fine grain practice.
Hardenability:	Hardenability for steels with a specified minimum carbon content of 0.20% or greater shall be determined for each heat and the results furnished to the purchaser when requested on the purchase order.
Thermal Treatments:	The purchaser shall specify one of the following options for thermal treatment on the purchase order: No thermal treatment. Annealed. Spheroidized. Drawn from annealed rod or bar. Drawn from spheroidize annealed rod or bar. Spheroidized at finished size wire. Annealed-in-process wire. Spheroidized annealed-in-process wire.

Rimmed or capped steels are characterized by a lack of uniformity in their chemical composition, especially for the elements carbon, phosphorus, and sulfur, and for this reason product analysis is not technologically appropriate unless misapplication is clearly indicated.

Coarse Austenitic Grain Size: When a coarse grain size is specified, the steel shall have a grain size number of 1 to 5 inclusive. Conformance to this grain size of 70 % of the grains in the area examined shall constitute the basis of acceptance.

Fine Austenitic Grain Size: When a fine grain size is specified, the steel shall have a grain size number greater than five. Conformance to this grain size of 70 % of the grains in the area examined shall constitute the basis of acceptance. When aluminum is used as the grain refining element, the fine austenitic grain size requirement shall be deemed to be fulfilled if, on heat analysis, the total aluminum content is not less than 0.020 % total aluminum or, alternately, 0.015 % acid soluble aluminum. The aluminum content shall be reported.

Materials and Processing
While standard steel grades for rods and bars have been in existence for many years, and have, with modifications or restrictions of one or more elements, long been used for cold forming, ASTM standard presents a distinct selected series of twenty steel grades for cold forming. These have been jointly developed by steel producers and cold heading and forging users under the aegis of the Industrial Fasteners Institute. These twenty grades are designated steel grades and the ranges and limits for the thirteen carbon steel grades for carbon, manganese, phosphorus, and sulfur and alloy steels with copper, nickel, chromium, molybdenum, tin, and silicon.

A significant area of improvement is in the decarburization control and measurement for cold heading rods and bars.

To prepare a material for cold forming it is often spheroidized, which is an annealing treatment that transforms the microstructure of steel to its softest condition with maximum formability. In the hot rolled or normalized condition, steels containing less than 0.80 % carbon consist of the microconstituents pearlite and ferrite. Pearlite, the harder of the two constituents, causes the steels to resist deformation. The harder pearlite is comprised of alternating thin layers or shells of ferrite and cementite, a very hard substance.

In spheroidize annealing, the cementite layers are caused by time and temperature to collapse into spheroids or globules of cementite. This globular form of cementite tends to facilitate cold deformation in such processes as cold heading, cold rolling, forming, and bending.

Boron is extremely effective as a hardening agent in carbon steels, contributing hardenability which generally exceeds the result of many commercial alloying elements. It does not adversely affect the formability or machinability of plain carbon steels. Actually, the reverse is true since boron permits the use of lower carbon content which contributes to improved formability and machinability.

In its early development, some unsatisfactory results produced product which did not have uniform hardness or toughness along with reduced ability to resist delayed fracture. However, many of these problems were overcome by exhaustive research which demonstrated that for boron to be effective as an alloying agent, it must be in solid solution in a composition range of 0.0005% to 0.003%. During deoxidation, failure to tie up the free nitrogen results in the formation of boron nitrides which will prevent the boron from being available for hardening. Research also revealed boron content in excess of 0.003% has a detrimental effect on impact strength because of the precipitation of excess boron as iron borocarbide in the grain boundaries. Many European steels contain higher boron levels than in North America.

When producing a boron steel, titanium and/or aluminum is added and the resulting product is subjected to thermal processing. These two additions are designed to tie up nitrogen to stop it from reacting with boron. The resulting free boron is available to provide excellent hardenability in steel. Both titanium and aluminum nitrides reduce the machinability of the steel, however, when the nitrogen becomes tied up, the formability of the steel is improved.

Silicon and aluminum act as somewhat similar elements with respect to their behavior when added during the steel making process. They both have a high affinity for oxygen and are, therefore, used to deoxidize or "kill" the steel. Deoxidation or "killing" is a process by which a strong deoxidizing element is added to the steel to react with the remaining oxygen in the bath to prevent any further reaction between carbon and oxygen.

When carbon and oxygen react in the bath a violent boiling action occurs which removes carbon from the steel. When the bath or heat reaches the desired carbon content for the grade being produced, the carbon-oxygen reaction must be stopped quickly to prevent further elimination of carbon. This addition is accomplished by the addition of deoxidizers such as silicon and aluminum which have a greater affinity for oxygen than does carbon. This effectively removes oxygen, eliminating the "carbon boil" and killing the heat. Elements other than silicon and aluminum can be used, but these are the most common.

Silicon and aluminum can be added together or individually. This is determined by the type of steel desired. If silicon only is added, that particular batch of steel is referred to as a silicon killed coarse grain practice grade because silicon acts as a deoxidizer without the formation of fine precipitates allowing the formation of large or coarse austenitic grains.

Austenitic grain size is not usually a factor for consideration in cold forming, but has a significant effect in subsequent fastener heat treatment. Aluminum, on the other hand, not only deoxidizes the steel, but also refines the grain size. Like silicon, aluminum removes oxygen from the bath, effectively killing the heat. Aluminum also reacts with nitrogen in the steel to form aluminum nitride particles which precipitate both at the grain boundaries and within the austenitic grains thus restricting the size of the grains; even when the steel is reheated for carburizing or neutral hardening, hence the term fine grain.

When aluminum only is added, the steel is referred to as aluminum killed, fine grain. A third group of steels are referred to as silicon killed, fine grain. In steels of this type, silicon is added as the deoxidizer followed by the addition of aluminum for grain size control.

In the two types where silicon is added, the silicon content can have several ranges with the most common being 0.15 % to 0.30 %. When aluminum is added to these steels for grain size control, the aluminum content is generally in the 0.015 % to 0.030 % range. The aluminum content in fully aluminum killed steels is generally 0.015 % to 0.055 %, somewhat higher on average since the aluminum must both deoxidize and control grain size at the same time.

In selecting the type of deoxidation practice for a particular carbon grade of steel to be used in fastener manufacturing, a number of factors should be considered, such as, heat treated property requirements, heat treat conditions, fastener size, and steel availability, to name a few. Silicon acts as a ferrite strengthener and, therefore, in the absence of aluminum, has somewhat greater hardenability. For the same carbon grade and heat treat conditions with and without aluminum, complete transformation of the fastener core during heat treatment can take place in a larger section using a coarse grain steel.

The disadvantage of silicon killed steels can be reflected in reduced ductility and tool life during cold heading because of its ferrite strengthening characteristic. Aluminum killed steels are usually more formable and hence provide somewhat improved tool life but reduced heat treatment response during heading, particularly in larger size fasteners. For this reason, the recommended maximum diameter for oil quenched aluminum killed carbon grades is typically 0.190 in.

The future developments in mechanical engineering, vehicle and energy system engineering must concentrate on solutions for processes, machines and materials which carefully treat resources and energy and at the same time keep the technical lead with new and innovative products. Lightweight construction concepts are able to be maintained and operated costs efficiently, reduce production costs, increase the product life for economic reasons reliability of use or optimize the freight of payloads.

Steel has become less favorable in previously dominated areas, e.g. in the automobile industry since lightweight materials such as aluminum and magnesium based alloys as well as synthetic materials and composite materials have gained a broad range of acceptance.

Steels with a higher strength and a higher young modulus than conventional steel cannot quite compensate the advantage of these materials for lightweight construction, despite the advantage of a lower price, a better forming behavior, a higher strength and the possibility of recycling without problems. A trend-setting solution for a higher demand of steel use seems possible by the development of high-strength, austenitic steels with a large manganese content. These steels show comparable mechanical qualities, and at the same time are more economical and in addition permit lightweight construction.

Sandwich systems represent an interdisciplinary concept by combining the areas of material choice, production engineering, design and function integration for the fulfillment of the high demands on modern materials. The sandwich material connects the advantages of miscellaneous materials (e.g. low density, high bend resistance, sound and vibration insulation, energy absorption, high load-capacity at a low weight, need adapted qualities) with each other.

Applications
Nowadays these new materials and designs are appreciated as key technologies for innovative research and development. The further development of the materials, the optimization of material applications and the necessary manufacturing method with reduced costs and time are permanent research objectives.

These new compound systems open new, future-oriented applications. The weight reduction is considerable for this task. A combination of steel/synthetic material/steel has the advantage of a higher strength opposite to corresponding steel and, depending on the choice of the steel grade, a high corrosion resistance. These sandwich materials find their way in more and more industrial applications such as automotive-, building-, transport-, chemical-, aerospace- and airplane industry.

The first essays and theoretical based works from to the "sandwich" topic are from 1935-1945. Applications are found not only in aircraft construction but also in the automobile manufacturing industry, in architecture, in shipbuilding engineering as well as in the sports and leisure industry. Some examples are described in the following.

Sandwich sheet metals increasingly find their way into the automobile industry. They are used for car bodies both for of lightweight reasons and for sound reduction. Sandwich materials are used with a homogeneous or inhomogeneous core of foams and other hard materials.

Examples of components of sandwich constructions are cowl application, gear box covers, hoods, car boot covers, oil pumps and chassis frame components. A well known example for the use of sandwich sheet metals in the automobile industry is the lightweight construction bodywork (Ultra Light Steel Auto Body).

Some of the components, such as spare wheel hollow and cowl application were manufactured of steel sandwich sheets. These components can be executed up to 50% lighter with the same properties concerning geometry and function than with normal deep drawing steel. The material consists of two thin steel sheets which are bonded with a thin polypropylene material layer as core material.

Material manufacturing
In one investigation sandwich materials with high-grade steel sheets were researched. They combine a good corrosion behavior and acid resistance with good damping behavior and noise reduction. For the production of the sandwich materials a 0.5 mm thick polyolefin foil was used.

The first manufacturing method to be tested was a press-joining process. This was performed discontinuously by an 8" and 10" rolling stand. The high-grade steel sheet metals [X2CrNiMo17 12-2 (1.4404) and X6CrNiMoTi17 12-2 (1.4571)] with a thickness of 0.5 mm were first cleaned and degreased. The steel was than coated with a defined layer of adhesive. The used adhesive agent is a conventional product based on epoxy with resin. After activating the adhesive the upper sheet metal was joined with a 0.5 mm thick PP/PE-foil in a rolling process. During the next step the produced upper sandwich was bonded with the lower sheet metal, also by rolling. For durable and reproducible adhesive bonding an activation temperature of the adhesive of 254°C +/-2°C was needed. The necessary dwell time of the coated sheet metals was 30 seconds in a stationary convection oven.

The other way to produce sandwich material is the discontinuous method. This manufacturing method was carried out with a cooling and heating system deduced in a laboratory press system. For the sandwich production a sufficient set of the granular material was mixed with the adhesive agent. This mixture was inserted as a packed bed between the cleaned and degreased sheet metals. At temperatures between 260 to 300°C the sandwich materials were then pressed for about 60s. To reach an even core layer thickness, the sandwich material was pressurized at 445 MPa. After the press process the sandwich material annealed to room temperature with a cooling rate of 10°C/min. For adjusting the core layer thickness and the thickness of the complete sandwich material a metal frame was used as a spacer.

These sandwich materials were examined in different tests for the bond strength of the individual layers and for their forming behavior. Deep drawing behavior is for example examined in the Erichsen Test. The height of the cup is a reference value to compare different sheet materials.

The difficulties in the deep drawing process of sandwich systems dwells from the different behavior of the used materials, e.g. polymer and high-grade steel.

The wrinkling of the metal can be counteracted with blank holders for mono materials. The material is forced into the desired flow direction. With sandwich systems, e.g. metal/PP/PE/metal the metal layer can flow despite a blank holder in the polymers, so that it can come to wrinkling in the metal layer. The higher the resistance of the polymer is brought into line with that of the metal, the bigger the resistance is against wrinkling.

For the deep drawing process of sandwich materials the knowledge about the blank holder pressure and force was necessary. Too little blank holder pressure increases the risk of wrinkling.

Conclusion
If some years ago sandwich systems were used only in individual sections, then they will find the way to more and more industry areas today. By the combination of construction and material they offer the substitution of classic mono materials, because next to lightweight construction they offer quantities like anti-corrosion protection and damping.

The development of new materials and technologies still stands at the beginning. New adapted material systems like natural fiber composites, hybrid structures of metals, polymers and ceramics increasingly gain meaning in future. The development for adapted composites, the processing of a material construction matrix for composite materials as well as the improvement on the adhesion and cohesion qualities by shift transitions graduated are future design objectives. Furthermore at the beginning of the material design process the aspects of the environmental protection and recycling have to consider.

Tool concepts and procedures should also be reconsidered for the component production from sandwich materials or be developed newly or adapted to the materials.

Aspects of the environmental protection and recycling are getting more important in these considerations from the beginning of the development. The use of natural fibers can serve as reinforcements in a matrix material between two metal sheets.

The interest in research and development in the area of these new materials has increased strongly during the last few years. In combination with other fields of research, like the nanotechnology, the biotronics, the mechatronics and the material development, the sandwich materials offer a large and important spectrum for the future.

Several types of hot-rolled dual-phase sheet steels prepared by simple temperature control in hot strip mill or by heat treatment on a continuous annealing line have been compared in this article with conventional micro alloyed steels through various forming tests.

Thickness of these steels ranges from 1.8 to 2.5 mm and yield strength from 300 to 520 MPa. Forming tests employed include stretching, drawing, flange stretching or hole expansion, and simulative model forming of automotive parts such as rear axle housing and spring support, and the behavior of these sheets is discussed.

Interest in high-strength steel has a lengthy history in the steel industry. Recent development of high-strength low-alloy (HSLA) sheet steel depends upon the large amount of technical information available in this field. In response to the automobile industry’s demand to reduce overall vehicle weight and thereby improve fuel economy, and to satisfy safety and crash-worthiness requirements, the steel industry has developed a large variety of steels and processes for producing high-strength hot-rolled and cold-rolled steel sheets.

The overall suitability of various steels for automobile body panel applications is assessed by evaluating their characteristics with regard to the performance requirements (formability, weldability, paintability, etc.). The formability of the steel sheets is perhaps the most important requirement for automotive component applications.

The aim of this article is to shed some light on the properties of steels which are controlled by the manufacturing conditions, and to recover the loss of formability that occurs as strength increases. The possible applications to automotive parts can be divided into two general categories, namely body panels and structural and safety-related parts. This article describes the formability of hot-rolled high-strength sheet steels for the latter category and the principal material properties which become the indication when producing such materials.

Dual phase steels, which have much better ductility for a given strength than conventional high-strength steels, have been developed. They have microstructures consisting of two major phases: martensite and ferrite. The suitable method of making these steels is to roll to the required thickness and then make use of heat treatment on a continuous annealing line. Another method is to find out the cooling condition and steel compositions which achieve typical dual phase properties directly from a continuous hot strip mill. These lead to the availability of hot-rolled dual phase steels made by two different methods and substantially different compositions.

Despite the differences between the steels, it is necessary for the automotive industry that they should have similar forming behavior and performance. This study therefore compares some of the properties for nine as- hot-rolled dual phase steels, two continuously heat treated dual phase steels, two conventional high-strength steels, and a commercial low carbon steel with yielding strength of 300 to 520 MPa.

The press forming of these steels is studied to gain an understanding of the influence of increasing strength on formability parameters. The formability investigation is performed through an evaluation of the response of the sheet steel in three deformation modes in the forming limit diagram: stretching, plain strain, and drawing.

Stampings are judged acceptable if there are no obvious tears, cracks, buckles, wrinkles, or necks in the finished stamping. In the forming of hot-rolled steels applied to the frame members of automobiles, which generally require thicker sheet than that of exposed panels, it is important that the steels exhibit good stretch flanging and punch stretching ability.

Tension testing is performed on parallel-sided specimens with a nominal width of 25 mm. Testing is carried out using a constant cross head speed, and elongation to fracture measured with a 50-mm gage length extensometer. Average mechanical properties are obtained from a minimum of five specimens in three test directions.

Hole expansion testing is carried out as follows: a 20-mm hole is punched into the sheet before deformation and is expanded with a conical punch. The expansion of this hole prior to the point of failure is referred to as the ratio of hole expansion.

The stretch forming test is performed with a hemispherical and flat bottom punch in which 400 and 450 mm square blanks are held in the die.

Simulative model forming is carried out with two types of dies. One is a spring support of which a character is stretching, and the other is a rear axle housing of which a character is drawing.

Springback is measured after a simple bending over three dies of different radius of curvature. Thickness of specimen is reduced to 1.7 mm by surface grinding in order to establish a constant strain of bending.

Formability parameters affect the ability of a material to be transformed from its original shape into a defined final shape by a specific forming process. Material, process, and shape interact in forming parts; therefore, they must be considered simultaneously in a formability study.

Mechanical properties such as yield strength, tensile strength, total elongation, work hardening exponent, plastic strain ratio, and strain rate sensitivity exponent, which are determined in the tension test, generally indicate the forming behavior of the material. The importance of these material parameters, which all interact in forming processes, depends upon the shape of the part and the manufacturing processes. Better understanding and accurate determination of these forming parameters help to predict the behavior of these steels in stamping operations.

The work hardening behavior of sheet steels is often characterized by the n-value, defined as the exponent in the Ludwig’s equation. For most dual phase steels, and also for highly formable interstitial free steels, the stress-strain curves do not conform to the Ludwig’s equation. To compare the work hardening behavior of the steels, it is suggested that the most useful parameter is the instantaneous work hardening rate normalized with respect to the flow stress. The distinct expression of the work hardening behavior is obtained by this parameter. However, it is tedious to establish the curves of the normalized work hardening rate in the function of the tensile strain.

Hole expansion ratio is influenced by the plastic strain ratio, by total elongation (which affects the critical hole expansion), and by quantity and shape of inclusions (which cause cracks). Results indicates that the hole expansion ratio decreases with the increase of total quantity of inclusions.

As reported previously, sulfide shape control becomes important in achieving a higher ductility along the sheared edge. Without sulfide shape control in these hot-rolled steels, lower expansion can occur due to the tearing which initiates on the punched edge at elongated sulfide inclusions. However, even in a material with sulfide shape control, there is a rather important degradation of sheared edge ductility as strength increases.

It is noted that a high-strength material which has a hole expansion ratio of more than 1.5 may be considered satisfactory, compared with the low-carbon steels. An investigation is made of the influence of the clearance between punch and die when a hole is punched into the sheet. It is indicated that the clearance has relatively little effect on the hole expansion ratio.

For automotive components the formability of sheet steel is determined principally by biaxial stretchability and deep draw ability. The total elongation and work hardening exponent are measures of the biaxial stretchability of sheet, and these parameters decrease as the yield strength of the sheet steel increases. As a general rule, the average plastic strain ratio, which is a measure of deep draw ability, also decreases as strength increases. For all the steels examined, the values are in a very narrow range and similar to those for low-carbon steel.

There is a good correlation between the forming index and work hardening exponent. This test is performed both parallel and transverse to the rolling direction, so the fracture properties of the sheet in both directions can be evaluated. There is a difference in formability due to the rolling direction.

The shape of automotive sheet components is apt to deviate from the design configurations because of various elastic recovery effects including springback. Defects in shape precision of finished parts are responsible for difficulties in assembly processes. Materials must be as uniform as possible with regard to thickness and properties in order to minimize springback after stamping.

Various types of hot rolled dual phase steels are examined by forming tests. Dual phase steels containing manganese and silicium are characterized by improved formability. Good correlation is obtained between the hole expansion ratio and inclusion shape control.

The work hardening exponent is the principal factor determining the press performance of hot-rolled dual-phase steels. In particular, n-value from 5 to 10 percent strain in tension testing is shown to have a good correlation with formability. This will allow the setting of guidelines for optimizing manufacturing conditions for these steels.

It is expected that the superior properties of dual phase steels will result in significant increases in their use for automotive applications in the immediate future.

Forging was the first of the indirect compression-type process and it is probably the oldest method of metal forming. It involves the application of a compressive stress, which exceeds the flow stress of the metal. The stress can either be applied quickly or slowly. The process can be carried out hot or cold, choice of temperature being decided by such factors as whether ease and cheapness of deformation, production of certain mechanical properties or surface finish is the overriding factor.

There are two kinds of forging process, impact forging and press forging. In the former, the load is applied by impact, and deformation takes place over a very short time. Press forging, on the other hand, involves the gradual build up of pressure to cause the metal to yield. The time of application is relatively long. Over 90% of forging processes are hot.

Impact forging can be further subdivided into three types:

* Smith forging,
* Drop forging,
* Upset forging.

Smith Forging
This is undoubtedly the oldest type of forging, but it is now relatively uncommon. The impact force for deformation is applied manually by the blacksmith by means of a hammer. The piece of metal is heated in a forge and when at the proper temperature is placed on an anvil. This is a heavy mass of steel with a flat top, a horn which is curved for producing different curvatures, and a square hole in the top to accommodate various anvil fittings. While being hammered the metal is held with suitable tongs.

Formers are sometimes used; these have handles and are held onto the work piece by the smith while the other end is struck with a sledgehammer by a helper. The surfaces of the formers have different shapes and are used to impart these shapes to the forgings. One type of former, called fuller, has a well-rounded chisel-shaped edge and is used to draw out or extend the work piece. A fuller concentrates the blow and causes the metal to lengthen much more rapidly than can be done by using a flat hammer surface. Fullers are also made as anvil fittings so that the metal is drawn out using both a top and bottom fuller. Fittings of various shapes can be placed in the square hole in the anvil.

The working chisels are used for cutting the metal, punches and a block having proper-sized holes are used for punching out holes. Welding can be done by shaping the surfaces to be joined, heating the two pieces then adding a flux to the surfaces to remove scale and impurities. The two pieces are then hammered together producing welding.

The easiest metals to forge are the low and medium carbon steels and most smith forgings are made of these metals. The high carbon and alloy steels are more difficult to forge and require great care.

Drop Forging
This is the modern equivalent of smith forging where the limited force of the blacksmith has been replaced by the mechanical or steam hammer. The process can be carried out by open forging where the hammer is replaced by a tup and the metal is manipulated manually on an anvil.

The quality of the products depends very much on the skill of the forger. Open forging is used extensively for the cogging process where the work piece is reduced in size by repeated blows as the metal gradually passes under the forge.

The cogging of a prismatic bar can be used to assess the parameters involved and how they are controlled. The objective is to reduce the thickness of the work piece in a stepwise sequence from end to end. Several passes may be required to complete the work and edging is usually carried out to control the width. The reduction in thickness is accompanied by elongation and spreading. The relative amounts of elongation and spread cannot be calculated theoretically but they have been determined experimentally for mild steel.

Die drop forging. Closed-die drop forging is widely used and the tup and anvil are replaced by dies. Matching dies fit into the anvil and the tup. The dies have a series of grooves and depressions cut into them and the work piece is passed in sequence through a shaping series.

These stations have names such as fullering, blocking, edging, bending and cut off. Where several stages are involved, care must be taken to ensure that the metal does not become excessively chilled before the last station is reached. To ensure that the die cavity is completely filled the volume of the starting billet is greater than that of the final forging. The excess metal appears as a "flash" at each stage, this is a thin fin around the perimeter of the forging at the parting line. This flash is cut away in a further press operation generally at a high temperature. The weight of flash may be a small percentage of the total weight for forgings of simple shapes but may exceed the weight of the actual forging for those of complex shape.

Each size and shape of forging will thus require a separate set of forging and trimming dies. The production tolerance for the initial metal must involve excess, e.g. ~10 mm. The over-tolerance metal is accommodated by a gutter around the die cavity which allows the formation of the fin referred to earlier.

Upset Forging
This process was developed originally to gather, or upset metal to form heads on bolts. Today the purpose of this machine has been broadened to include a wide variety of forgings.

It is essentially a double-acting press with horizontal motions rather than vertical. The forging machine has two actions. In the first, a movable die travels horizontally towards a similar stationary die. These two dies have semi-circular horizontal grooves, which grip the bars. A bar heated at the end is inserted between the movable and stationary die. While thus held, the end of the bar is upset or pressed into the die cavity by a heading tool mounted on a ram, which moves towards the front of the machine.

If hexagon heads are desired, a heading tool will upset some of the metal into a hexagon-shaped die cavity. For more complex forgings, as many as six different dies and heading tools may be used in turn in a similar manner to the different stations in die drop forging.

Press Forging
Whereas impact forging usually involves a mechanical press, press forging, on the other hand, requires hydraulic power. The largest forgings are invariably produced on large hydraulic presses. These have vertically moving rams, which move down slowly under considerable pressure.

A typical press forge would be capable of loads of the order of 6000 to 10000 tones. Forgings up to 100 tones weight can be handled easily in this forge and the highest-quality products are manufactured by this technique.

Structure and Properties of Forgings
Forgings are invariably produced by the hot-working process and this controls the resultant structure and properties. There are, however, important differences in forgings produced by different techniques.

The fact that the impact forge applies a stress for a very short period compared to the long period for the press forge results in totally different structures in the product. In the case of impact, the mechanical working is concentrated in the surface layers, since rapid removal of the stress after the blow results in metal relaxation before the effect of the blow has penetrated into the center. Impact forging of a large "as cast" piece of metal at high temperature will result in a very inhomogeneous structure, the outside layers showing a typical hot-worked structure whilst the center is still as cast. Any attempt to achieve greater penetration by increasing the impact load usually leads to internal cracking. Impact forging is therefore limited to relatively small work pieces.

Press forging invariably results in total penetration of the effect of the applied stress into the center of the work piece. The process is generally less severe on the metal than impact. The end result is a more homogeneous product having very high quality. Since the process is much slower and the equipment used is much larger, press forged articles are more expensive than impact forged components.

Wear costs money, sometimes lots of money. Numerous structures, such as dump bodies, materials handling equipment and crushing machines, for instance, are exposed to continuous, abrasive and impact wear, which is costly. As a solution, has developed special structural steels that are highly resistant to wear and abrasion.

Wear-resistant special structural steels are, as a rule, quenched or quenched and tempered, and have a fine martensitic or martensitic-bainitic microstructure. They are produced in thicknesses up to 120 mm. Further new developments complement the range of wear-resistant steels, alongside such established, wear-resistant special structural grades as CrMnMo steels 1.8703, 1.8710, 1.8713, 1.8714, 1.8734 etc., under the trade name XAR, BRINAR, DILLIDUR, HARDOX which depends on the manufacturer.

Normalized special structural steel with hardness of 300HB is now available for structures exposed to low or moderate levels of wear, such as scrap grabs, while the HB 600 grade meets extreme wear resistance requirements. Covering a hardness spectrum from 300 to 600 Brinell, a suitable material is thus available every type of wear-exposed application.

The grade most in use at present is the steel with hardness of 400 HB which, is around five times as durable as conventional structural steel. The steels with 450HB, a further modified grade, displays even higher hardness and, at the same time, good toughness. It enables the realization of more stable and lighter structures that are also highly resistant to impact wear.

The main fields of use for the 450 HB steel include the manufacture of dump bodies and cutting edges. All the wear resisting steels contain chromium as an alloying element, which has proven very effective especially in low-acid media. The high strength ensures good shape stability and thus little deformation. Thin-plate structures allowing a greater net load are also possible. The steels have a level of toughness that guarantees a high impact resistance even under the most difficult conditions, such as subzero temperatures, for example. Wear resisting steels present no problems when subjected to flame, plasma and laser cutting. They display good weldability and low susceptibility to cold cracking.

Steels with hardness of 400 and 450 HB, with improved bending properties for commercial vehicle manufacture, are highly suitable for brake press forming and bending (dump bodies) because of their balanced alloying concept, which includes sulfide shape control. Wear resisting steels can also be supplied as plates cut from strip with close thickness tolerances of less than ±0.20 mm and the resultant processing advantages.

Wear resistant special structural steels
To complement established wear-resistant steels 400 and 500 HB, the following grades are added which offer notable benefits:

* Steel with 300 HB for structures exposed to low or moderate level of wear and abrasion - high operational efficiency and surfaces with low scale formation
* Steel with 450 HB with high hardness and, at the same time, good toughness
* Steel with 600 HB with super high hardness for extreme wear applications.

Quenched and tempered special ballistic steels
Steel is an important alternative when it comes to protecting vehicles and buildings against firearm threat. The most important criterion in these applications is achieving a high degree of security for occupants through ballistic protection. Many mills have many years of experience in the manufacture of armor plate for ballistic protection.

As a solution, ballistic steels with hardness levels up to 600 HB are available and have been used in the civilian sector for years. Thanks to our advanced steel making and rolling technologies, ballistic steels have good cold forming properties despite their high hardness and can be readily welded using ferritic and austenitic consumables.

Ballistic steels owe their high hardness and good ballistic properties to their characteristic alloying elements carbon, chromium, molybdenum, vanadium and nickel and to appropriate heat treatment by water or oil quenching and tempering. For fabricators, compliance with minimum sheet thickness and close thickness tolerances is particularly important for manufacturing and weight reasons. Material produced meeting thickness tolerances down to 0.4 mm through the use of a state-of-the-art hot strip mill.

The thickness tolerance on plate cut from hot-rolled strip is roughly half that on material produced via the four-high mill. Close thickness tolerances allow customers to provide ballistic protection and optimized weight levels.

The advantages of ballistic steels can also be favorably combined with those of fiber composite materials.

Advantages of ballistic steels can be summarized in the following way:

* Defined ballistic properties
* Very close thickness tolerances
* Maximum flatness tolerance 6 mm/m for all thicknesses
* Good cold forming properties
* Available thicknesses 3 to 150 mm
* Good weldability.

The ferritic stainless steels are somewhat stronger than austenitic stainless steels, the yield stresses being in the range 300-400 MPa, but they work harden less so the tensile strengths are similar, being between 500 and 600 MPa. However, ferritic stainless steels, in general, are not as readily deep drawn as austenitic alloys because of the overall lower ductility. However, they are suitable for other deformation processes such as spinning and cold forging.

Welding causes problems due to excessive grain growth in the heat affected zone but, recently, new low-interstitial alloys containing titanium or niobium have been shown to be readily weldable. The higher chromium ferritic alloys have excellent corrosion resistance, particularly if 1-2% molybdenum is present.

Finally, there are two phenomena which may adversely affect the behavior of ferritic stainless steels. Firstly, chromium-rich ferrites when heated between 400 and 500°C develop a type of embrittlement, the origins of which are still in doubt.

The most likely cause is the precipitation of a very fine coherent chromium-rich phase arising from the miscibility gap in the Fe-Cr system, probably by a spinodal type of decomposition. This phenomenon becomes more pronounced with increasing chromium content, as does a second phenomenon, the formation of sigma phase. The latter phase occurs more readily in chromium-rich ferrite than in austenite, and can be detected below 600°C. As in austenite, the presence of sigma phase can lead to marked embrittlement.

Some austenitic steels are often close to transformation, in that the Ms temperature may be just below room temperature. This is certainly true for low-carbon 18Cr8Ni austenitic steel, which can undergo a martensitic transformation by cooling in liquid nitrogen or by less severe refrigeration. The application of plastic deformation at room temperature can also lead to formation of martensite in metastable austenitic steels, a transformation of particular significance when working operations are contemplated.

In general, the higher the alloying element content the lower the Ms and Md temperatures, and it is possible to obtain an approximate Ms temperature using empirical equations. Useful data concerning the Md temperature are also available in which an arbitrary amount of deformation has to be specified. The martensite formed in Cr-Ni austenitic steels either by refrigeration or by plastic deformation is similar to that obtained in related steels possessing an Ms above room temperature.

Manganese can be substituted for nickel in austenitic steels, but the Cr-Mn solid solution then has much lower stacking fault energy. This means that the fee solid solution is energetically closer to an alternative close-packed hexagonal structure, and that the dislocations will tend to dissociate to form broader stacking faults than is the case with Cr-Ni austenites. Manganese on its own can stabilize austenite at room temperature provided sufficient carbon is in solid solution. The best example of this type of alloy is the Hadfields manganese steel with 12 % Mn, 1.2 % carbon which exists in the austenitic condition at room temperature and even after extensive deformation does not form martensite.

However, if the carbon content is lowered to 0.8%, then Md is above room temperature and transformation is possible in the absence of deformation at 77°K. Both ε and α’ martensites have been detected in manganese steels. Alloys of the Hadfields type have long been used in wear resistance applications, e.g. grinding balls, railway points, excavating shovels, and it has often been assumed that partial transformation to martensite was responsible for the excellent wear resistance and toughness required. However, it is likely that the very substantial work hardening characteristics of the austenitic matrix are more significant in this case.

In general, fee metals exhibit higher work hardening rates than bee metals because of the more stable dislocation interactions possible in the fee structure. This results in the broad distinction between the higher work hardening of austenitic steels and the lower rate of ferritic steels, particularly well exemplified by a comparison of ferritic stainless steels with austenitic stainless steels.

The advantages obtainable from the easily fabricated austenitic steels led naturally to the development of controlled transformation stainless steels, where the required high strength level was obtained after fabrication, either by use of refrigeration to take the steel below its Ms temperature, or by low temperature heat treatment to destabilize the austenite. Clearly the Ms - Mf range has to be adjusted by alloying so that the Ms is just below room temperature. The Mr is normally about 120°C lower, so that refrigeration in the range -75 to -120°C should result in almost complete transformation to martensite.

Alternatively, heat treatment of the austenite can be carried out at 700°C to allow precipitation of M23C6 mainly at the grain boundaries. This reduces the carbon content of the matrix and raises the Ms so that, on subsequent cooling to room temperature, the austenite will transform to martensite. Further heat treatment is then necessary to give improved ductility and a high proof stress; this is achieved by tempering in the range 400-450°C.

Another group of steels has been developed to exploit the properties obtained when the martensite reaction occurs during low temperature plastic deformation. These steels, which are called transformation induced plasticity (TRIP) steels, exhibit the expected increases in work hardening rate and a marked increase in uniform ductility prior to necking. Essentially the principle is the same as that employed in controlled transformation steels, but plastic deformation is used to form martensite and the approach is broader as far as the thermomechanical treatment is concerned.

In one process, the composition of the steel is balanced to produce an Md temperature above room temperature. The steel is then heavily deformed (80%) above the Md temperature, usually in the range 250-550°C, which results in austenite which remains stable at room temperature. Subsequent tensile testing at room temperature gives high strength levels combined with extensive ductility as a direct result of the martensitic transformation which takes place during the test.

For example, a steel containing 0.3% C, 2% Mn, 2% Si, 9% Cr, 8.5% Ni, 4% Mo after 80% reduction at 475°C gives the following properties at room temperature:
• 0.2% Proof stress 1430 MPa
• Tensile strength 1500 MPa
• Elongation 50 %

Higher strength levels (proof stress ~2000 MNm2) with ductilities between 20-25% can be obtained by adding strong carbide forming elements such as vanadium and titanium, and by causing the Md temperature to be below room temperature. As in the earlier treatment, severe thermomechanical treatments in the range 250-550°C are then used to deform the austenite and dispersion strengthen it with fine alloy carbides. The Md temperature is, as a result, raised to above room temperature so that, on mechanical testing, transformation to martensite takes place, giving excellent combinations of strength and ductility as well as substantial improvements in fracture toughness.

The commonest austenitic steel is so-called 18/8 containing around 18% Cr and 8% Ni. It has the lowest nickel content concomitant with a fully austenitic structure. However in some circumstances, e.g. after deformation, or if the carbon content is very low, it may partially transform to martensite at room temperature. Greater stability towards the formation of martensite is achieved by increasing the nickel content, as illustrated in the 301 to 310 types of steel. 18/8 stainless steel owes its wide application to its excellent general resistance to corrosive environments. However, this is substantially improved by increasing the nickel content, and increasing the chromium gives greater resistance to intergranular corrosion.

Austenitic steels are prone to stress corrosion cracking, particularly in the presence of chloride ions where a few ppm can sometimes prove disastrous. This is a type of failure which occurs in some corrosive environments under small stresses, either deliberately applied or as a result of residual stresses in fabricated material. In austenitic steels it occurs as transgranular cracks which are most easily developed in hot chloride solutions. Stress corrosion cracking is very substantially reduced in high nickel austenitic alloys.

Type 316 steel contains 2-4% molybdenum, which gives a substantial improvement in general corrosion resistance, particularly in resistance to pitting corrosion, which can be defined as local penetrations of the corrosion resistant films and which occurs typically in chloride solutions. Recently, some resistant grades with as much as 6.5% Mo have been developed, but the chromium must be increased to 20% and the nickel to 24% to maintain an austenitic structure.

Corrosion along the grain boundaries can be a serious problem, particularly when a high temperature treatment such as welding allows precipitation of Cr23C6 in these regions. This type of intergranular corrosion is sometimes referred to as weld-decay. To combat this effect some grades of austenitic steel, e.g. 304 and 316, are made with carbon contents of less than 0.03% and designated 304L and 316L. Alternatively, niobium or titanium is added in excess of the stoichiometric amount to combine with carbon, as in types 321 and 347.

The austenitic steels so far referred to are not very strong materials. Typically their 0.2% proof stress is about 250 MPa and the tensile strength between 500 and 600 MPa, showing that these steels have substantial capacity for work hardening, which makes working more difficult than in the case of mild steel. However, austenitic steels possess very good ductility with elongations of about 50% in tensile tests.

The Cr/Ni austenitic steels are also very resistant to high temperature oxidation because of the protective surface film, but the usual grades have low strengths at elevated temperatures. Those steels stabilized with Ti and Nb, types 321 and 347, can be heat treated to produce a fine dispersion of TiC or NbC which interacts with dislocations generated during creep. One of the most commonly used alloys is 25Cr20Ni with additions of titanium or niobium which possesses good creep strength at temperatures as high as 700°C.

To achieve the best high temperature creep properties, it is necessary first to raise the room temperature strength to higher levels. This can be done by precipitation hardening heat treatments on steels of suitable composition to allow the precipitation of intermetallic phases, in particular Ni3(Al Ti).

The importance of controlling the γ-loop in achieving stable austenitic steels was emphasized. Between the austenite and δ-ferrite phase fields there is a restricted (α+γ) region which can be used to obtain two-phase or duplex structures in stainless steels. The structures are produced by having the correct balance between α-forming elements (Mo, Ti, Nb, Si, Al) and the γ-forming elements (Ni, Mn, C and N). To achieve a duplex structure, it is normally necessary to increase the chromium content to above 20%. However the exact proportions of α+γ are determined by the heat treatment. It is clear from consideration of the γ-loop section of the equilibrium diagram, that holding in the range 1000-1300°C will cause the ferrite content to vary over wide limits.

The usual treatment is carried out between 1050 and 1150°C, when the ferrite content is not very sensitive to the subsequent cooling rate The duplex steels are stronger than the simple austenitic steels, partly as a result of the two-phase structure and also because this also leads normally to a refinement of the grain size. Indeed, by suitable thermomechanical treatment between 900°C and 1000°C, it is possible to obtain very fine microduplex structures which can exhibit superplasticity, i.e. very high ductilities at high temperatures, for strain rates less than a critical value.

A further advantage is that duplex stainless steels are resistant to solidification cracking, particularly that associated with welding. While the presence of δ-ferrite may have an adverse effect on corrosion resistance in some circumstances, it does improve the resistance of the steel to transgranular stress corrosion cracking as the ferrite phase is immune to this type of failure.

There is another important group of stainless steels which are essentially ferritic in structure. They contain between 17 and 30% chromium and, by dispensing with the austenite stabilizing element nickel, possess considerable economic advantage. These steels, particularly at the higher chromium levels, have excellent corrosion resistance in many environments and are completely free from stress corrosion.

These steels do have some limitations, particularly those with higher chromium contents, where there can be a marked tendency to embrittlement. This arises partly from the interstitial elements carbon and nitrogen, e.g. a 25% Cr steel will normally be brittle at room temperature if the carbon content exceeds 0.03%. An additional factor is that the absence of a phase change makes it more difficult to refine the ferrite grain size, which can become very coarse after high temperature treatment such as welding. This raise still furthers the ductile/brittle transition temperature, already high as a result of the presence of interstitial elements. Fortunately, modern steel making methods such as argon-oxygen refining can bring the interstitial contents below 0.03%, while electron beam vacuum melting can do better still.