Background

A full set of tyres is a major constituent of any car or truck because it has the exclusivity of contact with the road. Progress in automotive technology is impossible without synchronous improvement in tyre developments.
Types of Tyres

Although the tyre appears to be a simple thing, it is, in fact, a complex composite with as demanding a combination of requirements as any part of the vehicle structure. Since the introduction of the first steel-reinforced tyres, a continuing series of innovations has enabled cold drawn 0.8% C steel cord to remain the most economical and efficient way to manufacture high performance tyres in mass production. Consequently the market share for textile and glass fibres has been reduced or reserved for specific niches.
Factors Leading to Increased Life of Tyres

The aims have been to reduce the unsprung mass of the vehicle, with consequential improvements in fuel economy and reduced emissions, and to increase the lifetime of tyres. These have been achieved by increasing the strength of steel cord filaments by 15%, with a corresponding increase in fatigue performance as well as maintaining the level of ductility of individual filaments and optimising manufacturing methods to control costs. A change in the design of the cord assembly was made in order to improve the reinforcement functionalism of the tyre.
Why use Steel?

Steel is the only material available to reinforce tyres which has a stabilised endurance limit, i.e. below a characteristic stress level no fatigue crack propagation will occur with infinite fatigue life. Above this threshold stress level, the higher strength steel cord gives between 10 and 30% longer life and enables truck tyres to be safely retreaded twice, giving lives of 500,000 km.
Steel Developments

A new high strength steel has been developed through the adoption of continuously casting with low segregation levels and high surface quality. Modifications and improvements to the drawing and heat treatment equipment have enabled steels with higher carbon contents to be drawn. The cold work deformation ψ increased from 3.2 to 3.5. The availability of the higher strength steel has facilitated the use of more productive stranding equipment. New strand and cable assemblies have been developed to make it possible to transmit an increased shear stress from the reduced cord surface to the surrounding rubber.

The strength of steel cord filaments is related to the logarithmic value of the filament size, table 1.

Table 1. Strength of steel cord filaments in relation to filament size.

Diameter (mm)	Regular tensile (N/mm2)	High tensile (N/mm2)
0.15	2950	3400
0.20	2815	3240
0.25	2720	3130
0.30	2650	3000
0.35	2580	2960

A full set of tyres is a major constituent of any car or truck because it has the exclusivity of contact with the road. Progress in automotive technology is impossible without synchronous improvement in tyre developments.
Types of Tyres

Although the tyre appears to be a simple thing, it is, in fact, a complex composite with as demanding a combination of requirements as any part of the vehicle structure. Since the introduction of the first steel-reinforced tyres, a continuing series of innovations has enabled cold drawn 0.8% C steel cord to remain the most economical and efficient way to manufacture high performance tyres in mass production. Consequently the market share for textile and glass fibres has been reduced or reserved for specific niches.
Factors Leading to Increased Life of Tyres

The aims have been to reduce the unsprung mass of the vehicle, with consequential improvements in fuel economy and reduced emissions, and to increase the lifetime of tyres. These have been achieved by increasing the strength of steel cord filaments by 15%, with a corresponding increase in fatigue performance as well as maintaining the level of ductility of individual filaments and optimising manufacturing methods to control costs. A change in the design of the cord assembly was made in order to improve the reinforcement functionalism of the tyre.
Why use Steel?

Steel is the only material available to reinforce tyres which has a stabilised endurance limit, i.e. below a characteristic stress level no fatigue crack propagation will occur with infinite fatigue life. Above this threshold stress level, the higher strength steel cord gives between 10 and 30% longer life and enables truck tyres to be safely retreaded twice, giving lives of 500,000 km.
Steel Developments

A new high strength steel has been developed through the adoption of continuously casting with low segregation levels and high surface quality. Modifications and improvements to the drawing and heat treatment equipment have enabled steels with higher carbon contents to be drawn. The cold work deformation ψ increased from 3.2 to 3.5. The availability of the higher strength steel has facilitated the use of more productive stranding equipment. New strand and cable assemblies have been developed to make it possible to transmit an increased shear stress from the reduced cord surface to the surrounding rubber.

The strength of steel cord filaments is related to the logarithmic value of the filament size, table 1.

Table 1. Strength of steel cord filaments in relation to filament size.

Diameter (mm)


Regular tensile (N/mm2)


High tensile (N/mm2)

0.15


2950


3400

0.20


2815


3240

0.25


2720


3130

0.30


2650


3000

0.35


2580


2960

The amount of cold work is limited to levels that assure an acceptable ductility, and this is size dependent (figure 1).

Figure 1. Tensile test data for 0.175mm diameter filaments for (1) normal product, (2) high tensile grade and (3) an experimental super high tensile grade. An increase of 18 and 28% in breaking load was produced over the standard grade for the latter 2 materials.

Substitution of a high tensile steel cords for the normal cords has resulted in a projected weight reduction of up to 16% for both truck and passenger tyre varieties (figure 2). This weight reduction has come with no loss of ply strength.

Figure 2. Reinforcement of a typical truck tyre. The carcass is reinforced with cords in a radial arrangement from rim to rim. The belt zone in this example has three layers with steel cord reinforcement.
Load Transfer

As far as tyre design is concerned, the increase in the ultimate strength of the filaments must be effectively transferred to the functional surroundings of the reinforcement. To increase its efficiency the coated cord ply must have a lower weight for the same strength or a higher strength for the same weight. Higher strength cord also requires less special rubber providing adherence to the brass-coated cord.
Implications of High Tensile Cords

Because the wall thickness of the tyre body is decreased with the higher strength steel cord the hysteresis loss of deformation energy is also reduced, leading to lower fuel consumption and lower running temperature. The overall benefits include the following:

· Reduced weight tyres

· Reduced unsprung mass

· Longer life tyres

· Reduced fuel consumption

· Reduced vehicle emissions

· Reduced running costs

· Lower consumption of natural resources.

Background

When steel sections or fabrications are immersed in molten zinc, their temperature is raised to that of the molten zinc which is typically 455°C. The rate at which the steel will reach this temperature across its entire surface will depend on:

· The thickness of the individual sections making

· The total mass of the item

· The dimension of the item

At galvanizing temperatures, there is no change to steel’s metallurgical microstructure and the galvanizing process is not hot enough to have any heat treating effects on the mechanical properties of the steel after galvanizing.

However, at galvanizing temperatures, the yield strength of steel is lowered by approximately 50%. If the adjacent steel is not at the same temperature and any stresses exist, the weaker area will be subject to movement by the stronger area. There is a responsibility on the designer, the fabricator and the galvaniser to co-operate in ensuring that distortion risks are minimised or eliminated.
Basic Design Rules

1. Design to use uniform thickness sections throughout the fabrication.

2. Ensure welding and assembly techniques minimize stresses in components making up the item.

3. Ensure that venting and draining are adequate. This will allow the item to be immersed in and withdrawn from the molten zinc as quickly as possible.

4. Ensure that the structural design of the item is sufficient to support its own weight at 50% of the steel’s specified yield strength. Consider temporary bracing if potential to yield exists.

5. Avoid using large areas of thin (under 8 mm) flat plate.

6. Guillotine cut plate is preferred to oxy cut plate.
Case Study

The pattern of distortion in this 3 mm floorplate is clearly the result of the welding technique used. Attaching the channels with fasteners after galvanizing would significantly reduce the risk of distorting.

It is an old cliche, but a consortium of companies really has reinvented the wheel. A ground-breaking new design of off-road wheel disc has been developed using advanced high strength steels (HSS) that are up to four times stronger than ordinary mild steel. Successful research based on the premise that doubling the strength of a product allows the material thickness to be reduced by one third has allowed the creation of a prototype that is 30% thinner and 30% lighter than a conventional wheel disc.

The collaborating companies, GKN Wheels, Corus and SSAB Swedish Steel, believe that this is the first off-highway wheel disc to use HSS. The wheel is more cost-effective and environmentally friendly than those made from mild steel. One of the main drivers behind the development of this wheel disc was the increasing specifications of agricultural, construction and other off-road vehicles. This has meant rising vehicle weight, at a time when the supply chain needs to cut weight and costs to make vehicles more fuel efficient.

The disc uses advanced HSS with minimum tensile strengths typically ranging between 400 N/mm² and 1,400 N/mm², and yield strengths in excess of 550 N/mm². Mathematical modelling proved that the disc gauge could be significantly reduced from 12.5mm to 8mm by substituting grade 550 hot rolled steel for grade 355 material, and adding stiffening ribs for rigidity. Cyclic fatigue tests gave positive results, and sample discs were successfully produced from 8mm hot rolled material in grades 500, 550, 600 and 650.

Paul Taylor from Corus said, ‘This new disc will help to maximise the load-bearing capacity and fuel efficiency of the vehicle. We have also succeeded in reducing total supply chain costs, which provides a significant competitive advantage.’

The HSS wheel disc prototype was unveiled at the Agritechnica 2003 exhibition in Germany last month, and the group are now looking to bring the new disc into full production.

Background

High nitrogen steels (HNS) are a new class of high alloy martensitic, austenitic or duplex grades with up to 0.9 mass% of N in solid solution. They are applied e.g. to stainless tools and bearings, in chemical engineering and for high strength non-magnetic components.

Dissolving Nitrogen into Steels

In contrast to carbon, nitrogen is a volatile element and therefore requires special measures to be dissolved in the melt. The first one consists of alloying. The Nitrogen solubility increases in the following order of elements Xi = N, C, Si, Al, Ni, Co, Cu/W, Mo, Mn, Cr, Nb, V, Ti, the oblique indicating the change from repulsing to attracting elements. As the solubility product of Nb, V and Ti in austenite is rather low, the Nitrogen solubility of HNS is mainly based on Cr, shifting the majority of alloys into the stainless range. The second measure is to further raise the Nintrogen content of the melt by applying nitrogen pressure. Pressurised electroslag remelting (PESR) is commercially available producing ingots of up to 20 tons in weight. The third measure makes use of the high uptake of Nitrogen in austenite. Respective steel powder is subjected to solid state nitriding and than compacted by hot isostatic pressing (HIP). This allows for the highest Nitrogen content, e.g. 3 mass% in stainless powder metallurgical (PM) tool steels containing e.g. wear resistant NbN nitrides embedded in a martensitic matrix. Measures two and three may lead to Nitrogen contents in the steel, which are above the solubility at temperatures of hardening or solution annealing, if carried out at the partial pressure of nitrogen in air.

Interactions of Alloy Nitrogen

Thus the interaction of alloy nitrogen with the furnace atmosphere or vacuum may differ considerably from carbon and require specific precautions. At first one has to look at the thermodynamic equilibrium between the steel surface and the surrounding atmosphere. Next the kinetics of phase transformations in the whole cross section are of interest to avoid e.g. embrittling precipitates. This is especially important for stainless austenitic steels, which are usually low in carbon but may contain up to 0.9 mass% nitrogen. Finally the expected changes in volume and the resistance to oxidation of HNS during heat treatment have to be considered in comparison to respective carbon grades.

Thermodynamic considerations

Nitrogen Concentration

During manufacturing of high Nitrogen steels the [N] content dissolved in the steel is in equilibrium with the partial pressure pN₂ in the atmosphere along ½N₂ = [N].

The reaction constant of this equation is related to the free energy

which becomes zero in case of equilibrium leading to

Activity of Nitrogen

The activity of nitrogen aN is expressed by the concentration [N] times the activity coefficient of nitrogen in a steel alloyed with repulsing and attracting elements Xi as mentioned above. The latter are described by negative interaction parameters changing the sign of the first term to positive. Thus elements like Cr and Mn raise [N] as does p_N2. For these elements the standard heat of solution ΔH₀, is negative causing a decrease of [N] if the temperature T in the third term is raised. The final term contains the change of standard entropy ΔS, and the gas constant R.

The main difference of equation (1) in respect to [C] of carbon grades is the pronounced pressure dependence of [N]. While stainless carbon steels are satisfactorily heat treated in vacuum furnaces, an effusion of Nitrogen is to be expected for high Nitrogen steels. If the latter are treated in pure nitrogen of 1 bar pressure the dissociation of N₂ is low up to 700°C which amounts to a shielding atmosphere. Above 900°C N₂ becomes increasingly thermally dissociated and as terms one to three of (1) are set by a given steel and temperature, effusion or infusion of Nitrogen is liable to occur except for some narrow range of parameters. Therefore the safest way to proceed is to calculate the required PN2 in equilibrium with [N] of a given steel at a required temperature. However, this implies that for p_N2 > 1 bar a pressure chamber is used, while at p_N2 <>

The Effect of Oxygen on Carbon and Nitrogen Containing Steels

Even at a small activity of oxygen within the furnace atmosphere, steels of high Cr content tend to oxidise during heat treatment. In stainless carbon grades, carbon is also oxidised leading to decarburisation and fissures in the scale caused by carbon monoxide molecules on their way out. Not so in high Nitrogen steels, in which the scale goes unharmed and adheres well to the steel surface. In a near surface zone, though, the Nitrogen content was found above that of the available p_N2 pointing to a higher aN at the scale/steel interface, which may be supported by a displacement of Nitrogen from the oxidised layer of the steel inwardly. An example is given in Table 1 for solution annealing in air, respectively in nitrogen of equivalent p_N2 = 0.8 bar. While the latter treatment meets the calculated N content except for a disturbed surface film, the former exceeds the calculation considerably.

Solution Nitriding

The equilibrium between pure N₂ and a stainless steel surface is also used to intentionally dissolve [N] in the outer case of near net shape parts. This new heat treatment is called solution nitriding, which is done at a temperature of 1100±50°C and a pressure of 0.1 <>N2 <>

Kinetic considerations

As shown above, high nitrogen steels require a high Cr content besides other elements to influence the constitution and properties. In high alloy steels diffusion is retarded. Fig. 2 summarises the effect of temperature and alloy content of different steels on the coefficient of nitrogen diffusion. Interstitial nitrogen is quicker than substitutional elements by several orders of magnitude.

Figure 1. Effect of temperature on the diffusion co-efficient of Nitrogen in pure iron and stainless steels.

The Importance of Heating

During heating the dissolution of precipitates and other phase transformations are slow in high alloy steels and require a sufficient soaking time. One is well aware of the phenomenon as far as martensitic steels of high Cr and C content are concerned. Little difference is expected for respective high nitrogen steels in which Cr nitrides instead of Cr carbides have to be dissolved prior to hardening. Stainless austenitic and duplex steels are commonly of low interstitial content, though, and in this respect high nitrogen steels are quite different. They take longer to reach a homogeneous distribution of e.g. Cr atoms after the dissolution of nitrides, which is a prerequisite of a high resistance to corrosion.

The Importance of Cooling

During cooling the high interstitial content of high nitrogen steels tends to provoke a precipitation of nitrides if a critical cooling time t_8/5 is exceeded referring to the duration of cooling from 800 to 500°C. If all nitrides are dissolved during austenitisation no nuclei for precipitation during cooling are left and a decoration of grain boundaries will occur first, followed by a discontinuous growth of M2N lamellae into austenitic grains. The resulting microstructure is termed “nitrogen pearlite”. For the sake of toughness and corrosion resistance nitrides have to be subdued by quenching.

Scale formation

The interaction of scale and interstitial elements was discussed earlier. In stainless steel dense chromium oxide forms a barrier against the migration of oxygen and metal ions and retards the oxidation of carbon and nitrogen grades. Comparing both types of stainless steel, nitrogen seems to improve the resistance to scaling. Of martensitic, creep resistant steels with (mass%) Cr9W2Mo0.5V0.2Nb and 0.045 respectively 0.168 N the latter showed a considerably reduced weight gain in air between 500 and 900°C and up to 104 hours. An exchange of (mass%) 0.5C + 0.5 N by 0.9 N in stainless austenitic steels for exhaust valves enhanced the resistance to oxidation at 850°C and ~500 hours. Therefore material loss by scaling during heat treatment of heavy components in air is impeded by Cr and apparently further retarded by N.

Distortion

The term distortion comprises a change of shape or of size and size stability. The first part depends mainly on external parameters like shape, taking, loading, supporting and quenching of the work pieces and little difference is expected between stainless carbon or nitrogen grades. The size change, however, relies on internal, microstructural and thermal changes of volume during heat treatment. The thermal ones are size dependent and again little difference is anticipated between stainless carbon and nitrogen grades considering the relatively low heating and cooling rates of high alloy steels. Volume changes by phase transformation are bound to be different, though.

Martensitic Stainless Steels

Looking at martensitic stainless steels an exchange of carbon by nitrogen enhances short range atomic ordering of Cr atoms and stabilises the austenitic phase, which results in a higher content of retained austenite (RA) and a smaller size of high Nitrogen steels after hardening. Therefore deep freezing and tempering in the range of secondary hardening at about 450°C is required to reduce RA. The highest degree of ordering and RA content is met in steels with C+N like in PESR steel Cr15Mo1C0.3N0.4 used e.g. for stainless bearings. After tempering at 450°C martensitic high nitrogen steels are prone to reveal a good size stability during service at room or slightly elevated temperature. As the nitride precipitates responsible for secondary hardening are not enriched in Cr the corrosion resistance is retained. In contrast respective carbon grades are restricted to lower tempering temperatures and small volume changes by retarded precipitation or RA transformation during service may impair the size stability. Austenitic stainless steels are commonly of low interstitial content. Respective high Nitrogen Steels contain up to 0.9 mass% N, though, the major part of which is precipitated as nitrides during slow cooling after hot working. The PESR steel Cr16Mn14Mo3N0.9 is an example and used for high strength, non-magnetic retaining rings holding the wiring of electric power generators. Solution annealing entails a slight increase of volume and therefore should be carried out before final machining, which is a common procedure, though.

The Softening Processes
Annealing

Used variously to soften, relieve internal stresses, improve machinability and to develop particular mechanical and physical properties.

In special silicon steels used for transformer laminations annealing develops the particular microstructure that confers the unique electrical properties.

Annealing requires heating to above the As temperature, holding for sufficient time for temperature equalisation followed by slow cooling.

Normalising

Also used to soften and relieve internal stresses after cold work and to refine the grain size and metallurgical structure. It may be used to break up the dendritic (as cast) structure of castings to improve their machinability and future heat treatment response or to mitigate banding in rolled steel.

This requires heating to above the As temperature, holding for sufficient time to allow temperature equalisation followed by air cooling. It is therefore similar to annealing but with a faster cooling rate. Curve 3 in Figure I would give a normalised structure.
The Hardening Processes
Hardening

In this process steels which contain sufficient carbon, and perhaps other alloying elements, are cooled (quenched) sufficiently rapidly from above the transformation temperature to produce Martensite, the hard phase already described, see Curve 1 in Figure 1.

There is a range of quenching media of varying severity, water or brine being the most severe, through oil and synthetic products to air which is the least severe.
Tempering

After quenching the steel is hard, brittle and internally stressed. Before use, it is usually necessary to reduce these stresses and increase toughness by 'tempering'. There will also be a reduction in hardness and the selection of tempering temperature dictates the final properties. Tempering curves, which are plots of hardness against tempering temperature. exist for all commercial steels and are used to select the correct tempering temperature. As a rule of thumb, within the tempering range for a particular steel, the higher the tempering temperature the lower the final hardness but the greater the toughness.

It should be noted that not all steels will respond to all heat treatment processes, Table 1 summaries the response, or otherwise, to the different processes.

	Anneal	Normalise	Harden	Temper
Low Carbon <0.3%	yes	yes	no	no
Medium Carbon 0.3-0.5%	yes	yes	yes	yes
High Carbon >0.5%	yes	yes	yes	yes
Low Alloy	yes	yes	yes	yes
Medium Alloy	yes	yes	yes	yes
High Alloy	yes	maybe	yes	yes
Tool Steels	yes	no	yes	yes
Stainless Steel (Austenitic eg 304, 306)	yes	no	no	no
Stainless Steels (Ferritic eg 405, 430 442)	yes	no	no	no
Stainless Steels (Martensitic eg 410, 440)	yes	no	yes	yes

Thermochemical Processes

These involve the diffusion, to pre-determined depths into the steel surface, of carbon, nitrogen and, less commonly, boron. These elements may be added individually or in combination and the result is a surface with desirable properties and of radically different composition to the bulk.

Carburising

Carbon diffusion (carburising) produces a higher carbon steel composition on the part surface. It is usually necessary to harden both this layer and the substrate after carburising.

Nitriding

Nitrogen diffusion (nitriding) and boron diffusion (boronising or boriding) both produce hard intermetallic compounds at the surface. These layers are intrinsically hard and do not need heat treatment themselves.

Nitrogen diffusion (nitriding) is often carried out at or below the tempering temperature of the steels used. Hence they can be hardened prior to nitriding and the nitriding can also be used as a temper.

Boronising

Boronised substrates will often require heat treatment to restore mechanical properties. As borides degrade in atmospheres which contain oxygen, even when combined as CO or C02, they must be heat treated in vacuum, nitrogen or nitrogen/hydrogen atmospheres.

Processing Methods

In the past the thermochemical processes were carried out by pack cementation or salt bath processes. These are now largely replaced, on product quality and environmental grounds, by gas and plasma techniques. The exception is boronising, for which a safe production scale gaseous route has yet to be developed and pack cementation is likely to remain the only viable route for the for some time to come.

The gas processes are usually carried out in the now almost universal seal quench furnace, and any subsequent heat treatment is readily carried out immediately without taking the work out of the furnace. This reduced handling is a cost and quality benefit.

Table 2 (Part A). Characteristics of the thermochemical heat treatment processes.

Process	Temp (°C)	Diffusing Elements	Methods	Processing Characteristics
Carburising	900-1000	Carbon	Gas. Pack. Salt Bath. Fluidised Bed.	Care needed as high temperature may cause distortion
Carbo-nitriding	800-880	Carbon Nitrogen mainly C	Gas. Fluidised Bed. Salt Bath.	Lower temperature means less distortion than carburising.
Nitriding	500-800	Nitrogen	Gas. Plasma. Fluidised Bed.	Very low distortion. Long process times, but reduced by plasma and other new techniques.
Nitro-carburising	560-570	Nitrogen Carbon mainly N	Gas. Fluidised Bed. Salt Bath.	Very low distortion. Impossible to machine after processing.
Boronising	800-1050	Boron	Pack.	Coat under argon shield. All post coating heat treatment must be in an oxygen free atmosphere even CO and CO2 are harmful. No post coating machining.

Table 2 (Part B). Characteristics of the thermochemical heat treatment processes.

Process	Case Characteristics	Suitable Steels	Applications
Carburising	Medium to deep case. Oil quench to harden case. Surface hardness 675-820 HV (57-62 HRC) after tempering.	Mild, low carbon and low alloy steels.	High surface stress conditions. Mild steels small sections <12mm. Alloy steels large sections.
Carbo-nitriding	Shallow to medium to deep case. Oil quench to harden case. Surface hardness 675-820 HV (57-62 HRC) after tempering.	Low carbon steels.	High surface stress conditions. Mild steels large sections >12mm.
Nitriding	Shallow to medium to deep case. No quench. Surface hardness 675-1150 HV (57-70 HRC).	Alloy and tool steels which contain sufficient nitride forming elements eg chromium, aluminium and vanadium. Molybdenum is usually present to aid core properties.	Severe surface stress conditions. May cinfer corrosion resistance. Maximum hard ness and temperature stability up to 200°C.
Nitro-carburising	10-20 micron compound layer at the surface. Further nitrogen diffusion zone. Hardness depends on steel type carbon & low alloy 350-540 HV (36-50 HRC) high alloy & toll up to 1000 HV (66 HRC).	Many steels from low carbon to tool steels.	Low to medium surface stress conditions. Good wear resistance. Post coating oxidation and impregnation gives good corrosion resistance.
Boronising	Thickness inversely proportional to alloy content >300 microns on mild steel 20 microns on high alloy. Do not exceed 30 microns if part is to be heat treated. Hardness >1500 HV typical.	Most steels from mild to tool steels except austenitic stainless grades.	Low to high surface stress conditions depending on substrate steel. Excellent wear resistance.

Techniques and Practice

As we have already seen this requires heating to above the As temperature, holding to equalise the temperature and then slow cooling. If this is done in air there is a real risk of damage to the part by decarburisation and of course oxidation. It is increasingly common to avoid this by ‘bright’ or ‘close’ annealing using protective atmospheres. The particular atmosphere chosen will depend upon the type of steel.

Normalising

In common with annealing there is a risk of surface degradation but as air cooling is common practice this process is most often used as an intermediate stage to be followed by machining, acid pickling or cold working to restore surface integrity.

Hardening

With many components, hardening is virtually the final process and great care must taken to protect the surface from degradation and decarburisation. The ‘seal quench’ furnace is now an industry standard tool for carbon, low and medium alloy steels. The work is protected at each stage by a specially generated atmosphere.

Some tool steels benefit from vacuum hardening and tempering, salt baths were widely used but are now losing favour on environmental grounds.

Tempering

Tempering is essential after most hardening operations to restore some toughness to the structure. It is frequently performed as an integral part of the cycle in a seal quench furnace, with the parts fully protected against oxidation and decarburisation throughout the process. Generally tempering is conducted in the temperature range 150 to 700°C, depending on the type of steel and is time dependent as the microstructural changes occur relatively slowly.

Caution : Tempering can, in some circumstances, make the steel brittle which is the opposite of what it is intended to achieve.

There are two forms of this brittleness

Temper Brittleness which affects both carbon and low alloy steels when either, they are cooled too slowly from above 575°C, or are held for excessive times in the range 375 to 575°C. The embrittlement can be reversed by heating to above 575°C and rapidly cooling.

Blue Brittleness affects carbon and some alloy steels after tempering in the range 230 to 370°C The effect is not reversible and susceptible steels should not be employed in applications in which they sustain shock loads.

If there is any doubt consult with the heat treater or in house metallurgical department about the suitability of the steel type and the necessary heat treatment for any application.

Introduction

Heat treatment processes fall into two distinct groups, those which harden and those which soften. They all use time and temperature to alter the microstructure, and hence the mechanical properties of the steel.

It is important to recognise that these changes are accompanied by changes in volume and hence part size. With good design, material selection, manufacturing and heat treatment practice it is possible to accommodate and allow for, but never eliminate, these changes.

The temperatures are also sufficient to relieve any internal stresses in the component from cold work or prior heat treatment. This too may cause distortion of the part.
The Designer's Contribution

As far as possible avoid sudden changes of part section. Where this is not possible minimise any stress concentration by the most generous fillet radii possible and the smoothest undercuts.

As far as possible avoid mixing thick and thin sections in the same component.

If this is not possible then remove excess metal from the thick section, to equalise the cooling rates in the thin and thicker sections.

The layout of any cutouts and holes across the section should be as even as possible, again to equalise cooling rates.

Avoid sharp edged slots, stamp marks or rough surface finishes which will act as stress concentrators and crack initiation sites.

In collaboration with your in-house metallurgical department, or sub contract heat treater, select a steel with sufficient hardenability to achieve your desired properties in the component section without the need for an over severe quench.

Do not economise on inter stage annealing or normalising to relieve machining or cold forming stresses. Pennies saved here may cost pounds in scrap or rectification later.

If you are thinking about thermochemical treatments remember that nitriding and nitrocarburising are carried out at lower temperatures than carburising and may cause lower distortion. Again discuss with your in-house or sub contract heat treatment department.
The Heat Treater's Contribution

Always try to achieve the most uniform temperature, heating and cooling rates across the furnace load.

Always try to properly jig and adequately support the part in the furnace to prevent sagging between supports etc., long shafts for example are best heat treated suspended vertically.

To ensure uniform heating of the part by allowing sufficient soaking time to minimise warpage.

To recommend the steel with the optimum heat treatment characteristics. Where appropriate suggest special processes and steels with lower intrinsic distortion, (such as austempering and martempering).

Always use the most economic and appropriate equipment and process for the application.

To protect the work from oxidation, decarburisation or other surface degradation as far as possible.

Introduction

Steels owe their dominance of the field of engineering materials to their ability to respond to heat treatment and provide appropriate properties at economic cost on a production scale.

The consistent and reproducible nature of the properties developed by heat treatment.
Why Heat Treat?

Steels can be softened, hardened and have their surface properties altered by heat treatment.
The Softening Processes

Annealing and normalising, reduce hardness, refine grain size and improve machinability. Their principal uses are therefore to make further processing operations easier or possible.
The Hardening Processes

Hardening (quenching) and tempering, develop the appropriate bulk and surface properties. Their principal use is to render the part fit for final use or purpose.
The Thermochemical Processes

Carburising, nitriding and boronising, are used to develop specific surface properties, again to make the part fit for final use or purpose.
Fundamentals

All steels are alloys of iron and carbon, while other alloying elements are added to confer particular properties. The manipulation of heat treatment response is a prime reason for adding alloying elements to steels.

An appreciation of the thermal behaviour, with the accompanying microstructural changes, is fundamental to the understanding of heat treatment and the mechanical properties so generated.

These curves describe the decomposition of austenite into ferrite and cementite or martensite with time and temperature. They are the scientific basis for modern heat treatment and exist for all commercially available steels.

Figure 1 shows an idealised TTT Curve. In this figure, A represents austenite, F represents ferrite, C represents cementite. M represents martensite. As is the austenite/ferrite transformation temperature and Ms is the martensite start transformation temperature.

Cooling at different rates from point X, i.e. above the As temperature will develop very different microstructures, and therefore properties, in the steel.

When the steel is cooled rapidly, following Curve 1, to get below the 'nose temperature' of 520°C in less than approximately one second, it will begin to transform at the Ms temperature to martensite. The steel is said to be hardened by this process.

Martensite is a strong, hard, but brittle structure. After tempering, which increases toughness and reduces brittleness, it has widespread use throughout engineering.

Conversely if the steel cools very slowly (Curve 2) then the austenite transforms to ferrite and cementite and a much softer structure will result.

In summary, the rate of cooling from the austenite phase is the main determinant of final structure and properties.
Hardenability

All steels have TTT Curves of essentially the same shape.

Alloying elements influence the As and Ms temperatures significantly and move the position of the 'nose' to the right. This will allow slower cooling rates to 'miss the nose' and still permit transformation to martensite. In metallurgical terms this is described as increased hardenability.

Increased hardenability has two important practical effects. Less severe quenches can be used to achieve martensite, and therefore hardening. The risks of quench cracking and distortion are consequently reduced.

As the centre of a section will always cool more slowly than the edge it allows thicker sections to through harden. With sufficient alloying element content the 'nose' can move so far to the right that an air cool will permit transformation to martensite. Such steels, and many are tool steels, are described as air hardening.

Certain alloying elements, eg Nickel, Manganese and Nitrogen individually and collectively and in sufficient quantity, depress the As temperature below room temperature making the steel austenitic (hence not hardenable or indeed magnetic) at ambient temperatures.

Background

Hardenability is the term used to describe the effect of various elements on the transformation characteristics of steels and the resultant microstructure after cooling from the austenitising (or hardening) temperature.
Depth of Hardening

In plain carbon steels the depth of hardening will be shallow, even for small sections given a relatively fast quench rate. As the alloying element content increases so does the depth of hardening. This means that the alloy steels can be hardened in many cases right through sections to the desired microstructures and give optimum service life.

It is at the design stage that these basic considerations should be made and the right steel grade chosen for the application.
Specifying Heat Treatments

In the final analysis, it is the heat treater who is asked to transform an expensive shape into a serviceable product. Remember, it is not the heat treaters function to guess the steel grade or required heat treatment. Full details of the steel grade must be given together with a property specification sheet. By far the most commonly specified property is hardness and it is normally given as a range in either Brinell, Rockwell or Vickers scale. It is normal for the heat treater to hardness test sample components and the area of the component which can be treated for a hardness test should be indicated.
Tensile Strength and Hardness

For heat treated steels a relationship exists between tensile strength and hardness. By referring to published figures the hardness values can be converted to tensile strength usually measured in N/mm2 or MPa.
Properties That Can be Affected by Heat Treatment

When service conditions demand higher hardness areas on components to resist wear, abrasion, erosion, indentation or increase fatigue resistance, various treatments can be specified.
Surface Hardening

Case carburisation followed by quench hardening is a process which is well documented and has occupied a leadership role in surface hardening technology for decades. It is still the major surface hardening treatment with wide application in the gear industry and automotive components manufacturing.
Nitriding

Nitriding and various related processes are lower temperature solid state reactions which as a consequence are very valuable for surface hardening items requiring minimal distortion during processing.

By using special steel grades the surface hardness of nitrided components can be very high, say 1100 Vickers compared to 750-800 Vickers for a carburised and quenched structure.

It is important to note that for nitriding to be successful, the steel used should be hardened and tempered or the components should be rough machined then hardened and tempered, usually to a hardness range of HRc 30 - HRc 35. Nitriding must be carded out on a correct microstructure of tempered martensite to minimise the occurrence of spalling.
Flame Hardening

This process consists of hardening the surface of an object by heating it above the transformation temperature using a high intensity flame from a specially designed burner and then progressively quenching it in water, oil or suitable synthetic quenchant. The depth of hardened layer can range from 1 to 10 mm.

A considerable level of skill and understanding is required for the process and depending upon the complexity of the part to be hardened, a large number of variables need to be examined before the job should proceed.
Induction Hardening

This process is capable of a high degree of automation and as such can be used for high volume production. It involves heating the component by an induced eddy current to temperatures at which the rate of formation of austenite is rapid and then quenching to transform austenite to martensite. Induced currents are generated in the metal surface using specially designed inductors which suit the surface shape and using an alternating current in the inductors at frequencies from 50Hz to 1000kHz, depending upon the depth of hardening required. The advantage of induction hardening is that the process is quick, efficient and surface degradation is minimal. The disadvantage is that capital and set up costs are high.