Non-oriented silicon steels do not use a secondary recrystallization process to develop their properties, and high temperature annealing is not essential. Therefore, a lower limit on silicon, such as is required for the oriented grades, is not essential.

Non-oriented grades contain between 0.5 and 3.25% Si plus up to 0.5% Al, added to increase resistivity and lower the temperature of primary recrystallization. Grain growth is very desirable in the nonoriented grades, but is generally much smaller than for the oriented grades.

Processing to hot rolled band is similar to that described for the oriented grade. After surface conditioning, the bands are usually cold rolled directly to final gage, and sold to the transformer manufacturer in one of two conditions - fully-processed, or semi processed. After final cold rolling, the strip is annealed, decarburizing it to 0.005% C or lower and developing the grain structure needed for the magnetic properties. Samples are then taken from each coil end, and tested.

Fully processed nonoriented silicon steels are generally used in applications in which:

* Quantities are too small to warrant stress relieving by the consumer, or
* Laminations are so large that good physical shape would be difficult to maintain after an 843°C stress relief anneal.

Non-oriented steels are not as sensitive to strain as the oriented product. Consequently, shearing strains constitute the only strain effects, which should degrade the magnetic quality. Because laminations are generally large, these shearing strains can be tolerated. Most of the fully processed grades are used as stamped laminations in such applications as rotors and stators.

The non-oriented steels have a random orientation. They are commonly used in large rotating equipment, including motors, power generators, and AC alternators. Fully processed steels are given a "full" strand anneal (to develop the optimum magnetic quality), making them softer and more difficult to punch than semi-processed products. Grades with higher alloy content are harder and thus easier to punch.

Improved punchability can be provided in fully processed steels by adding an organic coating, which acts as a lubricant during stamping and gives some additional insulation to the base scale. If good inter-lamination resistance is required, fully processed material can be purchased with core plate.

Semi processed products are generally given a lower-temperature decarburizing anneal after the final cold rolling. Carbon is not necessarily removed to the same low level as in fully processed material. The transformer manufacturer will subsequently stress relief anneal the material in a wet decarburizing atmosphere to obtain additional decarburization and develop the magnetic properties. Samples are taken after the mill decarburization anneal, cut into specimens, decarburized at 843°C for at least one hour and tested to grade the coil.

Semi processed nonoriented silicon steels are used for applications in which the customer does the stress relief anneal. In general, such products have good punching characteristics, and are used in a variety of applications including small rotors, stators, and small power transformers. Semi processed steels can be purchased with a tightly adherent scale, or with an insulating coating over the oxide. The organic coating acts as a lubricant during punching, but it does not withstand stress relief annealing temperatures; therefore, it is not applied to semi-processed material.
Table 1. The most important silicon steel designations specified by different standards

IEC 404-8-4 (1986)	EN 10106 (1995)	AISI	ASTM A677 (1989)	JIS 2552 (1986)	GOST 21427 0-75
-	M235-50A	-	-	-	-
250-35-A5	M250-35A	M 15	36F145	35A250	2413
270-35-A5	M270-35A	M 19	36F158	35A270	2412
300-35-A5	M300-35A	M 22	36F168	35A300	2411
330-35-A5	M330-35A	M 36	36F190	-	-
-	M250-50A	-	-	-	-
270-50-A5	M270-50A	-	-	50A270	-
290-50-A5	M290-50A	M 15	47F168	50A290	2413
310-50-A5	M310-50A	M 19	47F174	50A310	2412
330-50-A5	M330-50A	M 27	47F190	-	-
350-50-A5	M350-50A	M 36	47F205	50A350	2411
400-50-A5	M400-50A	M 43	47F230	50A400	2312
470-50-A5	M470-50A	-	47F280	50A470	2311
530-50-A5	M530-50A	M 45	47F305	-	2212
600-50-A5	M600-50A	-	-	50A600	2112
700-50-A5	M700-50A	M 47	47F400	50A700	-
800-50-A5	M800-50A	-	47F450	50A800	2111
-	M940-50A	-	-	-	-
-	M310-65A	-	-	-	-
-	M330-65A	-	-	-	-
350-65-A5	M350-65A	M 19	64F208	-	-
400-65-A5	M400-65A	M 27	64F225	-	-
470-65-A5	M470-65A	M 43	64F270	-	-
530-65-A5	M530-65A	-	-	-	2312
600-65-A5	M600-65A	M 45	64F360	-	2212
700-65-A5	M700-65A	-	64F400	-	2211
800-65-A5	M800-65A	-	-	65A800	2112
-	-	M 47	64F500	-	-
1000-65-A5	M1000-65A	-	64F550	65A1000	-

Oriented silicon steel is more restricted in composition than non-oriented varieties. The texture is developed by a series of careful working and annealing operations, and the material must remain essentially single-phase throughout processing, particularly during the final anneal because phase transformation destroys the texture. To avoid the y loop of the Fe-Si phase system, today`s commercial steel has about 3.25% Si. Higher silicon varieties, which might be favored on the basis of increased resistivity and lower magnetostriction, are precluded by difficulties in cold rolling.

Temperature, atmosphere composition, and dew point are closely controlled to decarburize the strip without oxidizing the surface. During this treatment, primary recrystallization occurs, forming small, uniform, equiaxed grains. The coating of magnesium silicate glass which forms will provide electrical insulation between successive laminations when assembled in a transformer core. At this stage, the steel is graded by cutting Epstein samples from the coil; the samples are stress relief annealed and flattened at 790°C, and tested for core loss.

Applications for oriented silicon steel include transformers (power, distribution, ballast, instrument, audio, and specialty), and generators for steam turbine and water wheels.

Lay-up cores, in general, utilize the whole spectrum of grain oriented quality and gages. The gage and grade of material for a given application are determined by economics, transformer rating, noise level requirement, loss requirements, density of operation, and even core size. Because the strip must be flat to produce a good core, coils are flattened after the high temperature anneal. Then, the strip is coated with an inorganic phosphate for insulation. Samples from each coil end are graded after a laboratory stress relief anneal, as previously described. From such strip, the transformer manufacturer cuts his required length improves the insulation of the strip. Consequently, it decreases the eddy current losses and heat buildup, which is of particular importance in transformers which must withstand an impulse test.

As noted earlier, an important requirement in the manufacture of lay-up cores is minimizing transformer noise. Noise is a function of manufacturing and core design factors, the core material characteristic being one of the most important. The dependence of magnetostriction on silicon content has already been noted. In addition, magnetostriction is reduced by improving the texture and by introducing tensile stresses through application of glass-type insulation coatings. Because compressive stresses affect magnetostriction adversely, it is important that the lamination remains flat for assembly. Operating induction is also a factor that affects noise, and indeed affects the transformer`s general operating characteristics. Operating inductions of lay-up transformers are usually in the 10,000 to 17,000 G range; power ratings extend over the 500 to 1,000,000 kVA range.

Wound cores are wound toroidally with the [100] crystallographic direction around the strip. Processing steps are somewhat different from those used for lay-up transformers though the starting material is the same-large toroidally annealed coil coated with magnesium silicate, which usually provides sufficient insulation.

For wound core application, unreacted MgO powder is removed from the strip surface, and a sample from each coil end is cut into Epstein strips to be tested as before. After being graded, the coil is shipped to the transformer manufacturer either as slit multiples or as a full-width coil for subsequent slitting. The slit multiple, wound to the given core dimension, must be stress relief annealed at 790°C in a dry nonoxidizing atmosphere. Annealing trays and plates must be of low carbon steel to eliminate any carbon contamination, which can be very detrimental to quality.

After being stress relief annealed, the cores are cut, and the transformer core is assembled by lacing the steel around the copper (or aluminum) current-carrying coils. In the stress relief annealed condition, grain-oriented steel is sensitive to mechanical strain; therefore, cores must be assembled carefully. Regardless of how carefully assembly is accomplished, the final core quality is always poorer than it was in the stress-relief annealed, uncut condition.

The difference in quality, commonly referred to as the "destruction factor", is due to the relative strain sensitivity of the grain-oriented steel, the handling procedure in fabrication, and the uniformity and amount of air gap in the core. Being a function of the transformer design and fabrication, the latter two factors are controlled best by the manufacturer. Most wound cores are utilized in distribution transformer applications of 25 to 500 kVA.

Silicon steel is undoubtedly the most important soft magnetic material in use today. Applications vary in quantities from the few ounces used in small relays or pulse transformers to tons used in generators, motors, and transformers. Continued growth in electrical power generation has required development of better steels to decrease wasteful dissipation of energy (as heat) in electrical apparatus and to minimize the physical dimensions of the increasingly powerful equipment now demanded.

The earliest soft magnetic material was iron, which contained many impurities. Researchers found that the addition of silicon increased resistivity, decreased hysteresis loss, increased permeability, and virtually eliminated aging.

Substantial quantities of oriented steel are used, mainly in power and distribution transformers. However, it has not supplanted nonoriented silicon steel, which is used extensively where a low-cost, low-loss material is needed, particularly in rotating equipment. Mention should also be made of the relay steels, used widely in relays, armatures, and solenoids. Relay steels contain 1.25 to 2.5% Si, and are used in direct current applications because of better permeability, lower coercive force, and freedom from aging.

Important physical properties of silicon steels include resistivity, saturation induction, magneto-crystalline anisotropy, magnetostriction, and Curie temperature. Resistivity, which is quite low in iron, increases markedly with the addition of silicon. Higher resistivity lessens the core loss by reducing the eddy current component. Raising the silicon content will lower magnetostriction, but processing becomes more difficult. The high Curie temperature of iron will be lowered by alloying elements, but the decrease is of little importance to the user of silicon steels.

The magnetization process is influenced by impurities, grain orientation, grain size, strain, strip thickness, and surface smoothness. One of the most important ways to improve soft magnetic materials is to remove impurities, which interfere with domain-wall movement; they are least harmful if present in solid solution. Compared with other commercial steels, silicon steel is exceptionally pure. Because carbon, an interstitial impurity, can harm low induction permeability, it must be removed before the steel is annealed to develop the final texture.

The mechanism for the growth of grains with cube-on-edge orientation during the final anneal is not completely understood. The process involves secondary recrystallization, which, by definition, is characterized by accelerated growth of one set of grains in an already recrystallized matrix.

For secondary recrystallization, normal grain growth must be inhibited in some manner. As the temperature is raised, certain grains break loose from the inhibiting forces, and grow extensively at the expense of their neighbors. Producers know that, on a practical basis, appropriate cold rolling and recrystallization sequences must be carefully followed to obtain the desired secondary recrystallization nuclei and the correct texture. Today`s silicon steels use MnS as the grain growth inhibitor, but other compounds, such as carbides, oxides, or nitrides, are also effective.

By far the largest tonnage of alloy steels is of the types containing generally 0.25 to 0.55% C, or less, and usually quenched and tempered for high strength and toughness. Manganese, silicon, nickel, chromium, molybdenum, vanadium, aluminum and boron are commonly present in these steels to enhance the properties obtainable after quenching and tempering.

These alloy steels are ordinarily quench-hardened and tempered to the level of strength desired for the application. Even though the strength level at which the steel is used may be as low as, or lower than, that which could be achieved by the microstructure (fine pearlite or upper bainite) developed by a simple cooling from forging or normalizing temperature, the steels are quench-hardened and tempered, indirectly reflecting the engineering and economic basis of the demand for this type of steel.

The microstructure (tempered martensite or bainite) produced by quenching and tempering these alloy steels is characterized by a greater toughness or capacity to deform without rupture at any strength level. Similarly, under the adverse state of stress below a notch in bending, the tempered martensite may flow considerably at a testing temperature far below that at which a pearlitic steel of equal strength would break in a brittle manner; the Charpy or Izod values are thus improved. The basic phenomenon of developing this favorable microstructure by heat treatment is manifested in plain carbon steels, but only in small sections; thus the most important effect of the alloying elements in these steels is to permit the attainment of this microstructure, and the accompanying superior toughness in larger sections.

Alloying Elements Dissolved in Austenite. The general effect of elements dissolved in austenite is to decrease the transformation rates of the austenite at subcritical temperatures. The only one among the common alloying elements to behave exceptionally in this regard is cobalt. Since the desirable products of transformation in these steels are martensite and lower bainite, formed at low temperatures, this decreased transformation rate is essential; it means that pieces can be cooled more slowly, or larger pieces can be quenched in a given medium, without transformation of austenite to the undesirable high-temperature products, pearlite or upper bainite.

This function of decreasing the rates of transformation, and thereby facilitating hardening to martensite or lower bainite, is known as hardenability and is the most important effect of the alloying elements in these steels. Thus, by increasing hardenability the alloying elements greatly extend the scope of enhanced properties in hardened and tempered steel to the larger sections involved in many applications.

The several elements commonly dissolved in austenite prior to quenching, increase hardenability in approximately the following ascending order: nickel, silicon, manganese, chromium, molybdenum, vanadium and boron. The effect of aluminum on hardenability has not been accurately evaluated. But at 1% Al, as used in "nitralloy" steels, the effect on hardenability seems to be relatively small. Further, it has been found that the addition of several alloying elements in small amounts is more effective in increasing hardenability than the addition of much larger amounts of one or two.

In order to increase hardenability effectively, it is essential that the alloying elements are dissolved in austenite. The steels containing the carbide-forming elements - chromium, molybdenum and vanadium - require special consideration in this respect. These elements are present predominantly in the carbide phase of annealed steels, and such carbides dissolve only at higher temperatures and more slowly than iron carbide. Since the basic function of the alloying elements in these steels is to increase hardenability, the selection of steel and the choice of suitable austenitizing conditions should be based primarily on the assurance of adequate hardenability. More than adequate hardenability is rarely a disadvantage, except in cost.

Alloying Elements in Quenching. Since the sections treated are often relatively large, and since the alloying elements have the general effect of lowering the temperature range at which martensite is formed, the thermal and transformational stresses set up during quenching tend to be greater in these alloy steel parts than those involved in quenching the necessarily smaller sections of plain carbon steels. In general, this means greater distortion and risk of cracking.

The alloying elements, however, have two functions that tend to offset these disadvantages. The first and probably the most important of these functions is that of permitting the use of lower carbon content for a given application. The decrease in hardenability accompanying the decrease in carbon content may be offset very readily by the hardenability effect of the added alloying elements, and the lower-carbon steel will exhibit a much lower susceptibility to quench cracking. This lower susceptibility results from the greater plasticity of the low-carbon martensite and from the generally higher temperature range at which martensite is formed in the lower-carbon materials. Quench cracking is seldom encountered in steels containing 0.25% C or less, and the susceptibility to cracking increases progressively with increasing carbon content.

The second function of the alloying elements in quenching is to permit slower rates of cooling for a given section, because of increased hardenability, and thereby generally to decrease the thermal gradient and, in turn, the cooling stress. It should be noted, however, that this is not altogether advantageous, since the direction, as well as the magnitude, of the stress existing after the quench, is important in relation to cracking.

In order to prevent cracking, the surface stresses after quenching should be either compressive or at a relatively low tensile level and, under certain circumstances, lowering the cooling rate may lead to increased tensile stresses at the surface, thus increasing the tendency to crack. In general, though, unless a study of the particular piece being quenched indicates that it falls within this category of increased susceptibility with decreased quenching rates, the use of a less drastic quench suited to the hardenability of the steel will result in lower distortion and greater freedom from cracking.

Furthermore, the increased hardenability of these alloy steels may permit heat treatment by "austempering" or "martempering", and thereby the level of adverse residual stress before tempering may be held to a minimum. In "austempering", the work piece is cooled rapidly to a temperature in the lower bainite region and is allowed thereafter to transform, completely at some chosen temperature. Since this transformation occurs at a relatively high temperature and proceeds rather slowly, the stress level after transformation is quite low and distortion is held to a minimum.

In "martempering", the piece:

1. is cooled rapidly at the surface to a temperature that permits very little martensite to form, if any;
2. is equalized at this temperature; and
3. is then cooled slowly so that transformation throughout the whole section occurs more or less simultaneously, thereby holding transformational stresses at a very low level and minimizing distortion and danger of cracking.

Alloying Elements in Tempering. Hardened steels are softened by reheating, but this effect is not the one actually sought in tempering. The real need for increasing the capacity of the piece is to flow moderately without fracture, and this is inevitably accompanied by a loss of strength. Since the tensile strength is very closely related to hardness in this class of steels, as heat-treated, it is satisfactory to follow the effects of tempering by measuring the Brinell or Rockwell hardness. The statistical mean of the relationships among Brinell hardness, tensile strength and yield strength, is shown in Fig. 1, drawn from many data.

Alloy steels are defined as those steels that:

1. contain manganese, silicon, or copper in quantities greater than the maximum limits (1.65% Mn, 0.60% Si, and 0.60% Cu) of carbon steel; or
2. that have specified ranges or minimums for one or more other alloying additions.

The low-alloy steels are those steels containing alloy elements, including carbon, up to a total alloy content of about 8.0%.

Except for plain carbon steels that are micro alloyed with just vanadium, niobium, and/or titanium, most low-alloy steels are suitable as engineering quenched and tempered steels and are generally heat treated for engineering use. Low-alloy steels with suitable alloy compositions have greater hardenability than structural carbon steel and, thus, can provide high strength and good toughness in thicker sections by heat treatment. Their alloy contents may also provide improved heat and corrosion resistance. However, as the alloy contents increase, alloy steels become more expensive and more difficult to weld. Quenched and tempered structural steels are primarily available in the form of plate or bar products.

Alloying Elements and Their Effect on Hardenability and Tempering. Quenched and tempered steels have carbon contents in the range of 0.10 to 0.45%, with alloy contents, either singly or in combination, of up to 1.5% Mn, 5% Ni, 3% Cr, 1% Mo, 0.5% V, 0.10% Nb; in some cases they contain small additions of titanium, zirconium and/or boron. Generally, the higher the alloy content, the greater the hardenability and the higher the carbon content, the greater the available strength. The response to heat treatment is the most important function of the alloying elements in these steels.

Microalloyed Quenched and Tempered Grades. Although fittings with 0.69% Mn and induction bends use quenching and tempering as a standard practice, mild steels (plain, low-carbon steels with less than 0.7% Mn) with microalloying additions of vanadium, niobium, or titanium are seldom used as quenched and tempered steels.
However, elements such as boron and vanadium are considered as substitutes for other elements that enhance hardenability. The titanium was added in order to form titanium nitride, thereby retaining an increased amount of vanadium in solution. This provided for a more efficient use of vanadium as a hardenability agent.

Some scientist investigated completely V-substituted variants of 4140 base series (0.4C-1Cr) with titanium additions, as well as partially V-substituted variants with and without titanium additions. These studies concluded that:

* Complete substitution of molybdenum by vanadium does not increase the hardenability over standard 4140 (0.20% Mo) even when all the vanadium is dissolved during austenitization
* Steels containing 0.1 to 0.2% V and 0.04% Ti are characterized by significantly increased hardenability (10 to 25% in D1) over standard 4140
* Microalloy combinations of V + Mo + Ti (~0.06-0.06-0.04%) provide very high hardenability with D1 being up to 60% greater than the D1, in standard 4140 with 0.20% Mo. This effect is completely absent in partially substituted steel without titanium.

The pronounced effect on hardenability of molybdenum-vanadium combinations without titanium as observed by Manganon in 4330 steels, can probably be reconciled with the third result of Sandberg in that the latter studied steels containing 0.06% Al, which would be expected to remove nitrogen to about the same extent as 0.04% Ti.

Quenched and tempered alloy steels can offer a combination of high strength and good toughness. In addition, quenched and tempered alloy steel plate is available with ultrahigh strengths and enhanced toughness. Enhanced toughness and high strength are achieved in the nickel-chromium-molybdenum alloys, which include steels such as ASTM A 543, HY-80, HY-100 and HY-130. These steels use nickel to improve toughness.

High-Nickel Steels for Low-Temperature Service. For applications involving exposure to temperatures from 0 to -195oC, the ferritic steels with high nickel contents are typically used. Such applications include storage tanks for liquefied hydrocarbon gases and structures and machinery designed for use in cold regions. These steels utilize the effect of nickel content in reducing the impact transition temperature, thereby improving toughness at low temperatures. Carbon and alloy steel castings for subzero-temperature service are covered by ASTM standard specification A 757.

The 5% Ni alloys for low-temperature service include HY-130 and ASTM A 645. For steel purchased according to ASTM A 645 minimum Charpy V-notch impact requirements for 25 mm plate are designated at -170oC for hardened, tempered, and reversion-annealed plate.

Double normalized and tempered 9% nickel steel is covered by ASTM A 353, and quenched and tempered 8% and 9% nickel steels are covered by ASTM A 553 (types I and II). For quenched and tempered material, the minimum lateral expansion in Charpy V-notch impact tests is 0.38 mm. Testing of typical tensile properties of 5% and 9% Ni steels at room temperature and at subzero temperatures shows that yield and tensile strengths increase as testing temperature is decreased. These steels remain ductile at the lowest resting temperatures.

Ferritic nickel steels are too tough at room temperature for valid fracture toughness (KIc) data to be obtained on specimens of reasonable size, but limited fracture toughness data have been obtained on these steels at subzero temperatures by the J-integral method. The 5% Ni steel retains relatively high fracture toughness at -162oC and the 9% Ni steel retains relatively high fracture toughness at -196oC. These temperatures approximate the minimum temperatures at which these steels may be used.

High-strength carbon and low-alloy steels have yield strengths greater than 275 MPa and can be more or less divided into four classes:

* As-rolled carbon-manganese steels
* As-rolled high-strength low-alloy (HSLA) steels (which are also known as micro alloyed steels)
* Heat-treated (normalized or quenched and tempered) carbon steels
* Heat-treated low-alloy steels.

These four types of steels have higher yield strengths than mild carbon steel in the as-hot-rolled condition. The heat-treated low-alloy steels and the as-rolled HSLA steels also provide lower ductile-to-brittle transition temperatures than do carbon steels.

These four types of high-strength steels have some basic differences in mechanical properties and available product forms. In terms of mechanical properties, the heat-treated (quenched and tempered) low-alloy steels offer the best combination of strength and toughness.

Structural Carbon Steels
Structural carbon steels include mild steels, hot-rolled carbon-manganese steels, and heat-treated carbon steels. Mild steels and carbon-manganese steels are available in all the standard wrought forms: sheet, strip, plate, structural shapes, bar, bar-size shapes, and special sections. The heat-treated grades are available as plate, bar and. occasionally, sheet and structural shapes.

Mild (low-carbon) steels are normally considered to have carbon contents up to 0.25% C with about 0.4 to 0.7% Mn, 0.1 to 0.5% Si and some residuals of sulfur, phosphorus, and other elements. These steels are not deliberately strengthened by alloying elements other than carbon; they contain some manganese for sulfur stabilization and silicon for deoxidation. Mild steels are mostly used in the as-rolled, forged, or annealed condition and are seldom quenched and tempered.

The largest category of mild steels is the low-carbon (<0.08%>C, with <0.4%>Mn) mild steels used for forming and packaging. Mild steels with higher carbon and manganese contents have also been used for structural products such as plate, sheet, bar, and structural sections.

High-strength structural carbon steels have yield strengths greater than 275 MPa and are available in various product forms:

* Cold-rolled structural sheet
* Hot-rolled carbon-manganese steels in the form of sheet, plate, bar, and structural shapes
* Heat-treated (normalized or quenched and tempered) carbon steels in the form of plate, bar, and occasionally, sheet and structural shapes.

The heat treatment of carbon steels, which typically attain yield strengths of 290 to 690 MPa, consists of either normalizing or quenching and tempering. These heat treatments can be used to improve the mechanical properties of structural plate, bar, and occasionally, structural shapes.

Normalizing involves air-cooling from austenitizing temperatures and produces essentially the same ferrite-pearlite microstructure as that of hot-rolled carbon steel, except that the heat treatment produces a finer grain size. This grain refinement makes the steel stronger, tougher, and more uniform throughout.

Quenching and tempering, that is heating to about 900oC, water quenching, and tempering at temperatures of 480 to 600oC or higher, can provide a tempered martensitic or bainitic microstructure that results in better combinations of strength and toughness. An increase in the carbon content to about 0.5%, usually accompanied by an increase in manganese, allows the steels to be used in the quenched and tempered condition.

The iron-cobalt SOFCOMAG family of alloys are characterized by moderately high permeability and very high saturation induction. While iron-nickel Softmag alloys attain the maximum saturation induction of about 1.5 teslas, the SOFCOMAG alloys can achieve saturation induction values as high as 2.3 teslas. They are also marked by their low electrical resistivity and high hysteresis loss.

SOFCOMAG can be broadly classified into two series:

* SOFCOMAG 25 (25% Co, Fe rest)
* SOFCOMAG 49 (49% Co, Fe rest)

SOFCOMAG 25 is an alloy which exhibits the highest saturation flux density of all the magnetic alloys. Its high curie point enables its use in areas where the magnetic properties have to remain unimpaired even at high temperatures of about 500°C. This alloy is designed for application in electrical equipment requiring high saturation; induction in high magnetic field as in. electric motor parts of aircraft for which weight is an important consideration. It is also used for magnetic poles of electromagnets.

SOFCOMAG 49A is similar to SOFCOMAG 25 in respect of its high saturation flux density. However, it offers a higher resistivity than SOFCOMAG 25. This yields low eddy current losses at high induction levels. In addition to electric motors for aircraft, SOFCOMAG 49A is also used for its high positive magnetostriction in sonar applications and ultrasonic equipment.

SOFCOMAG 49B is similar to SOFCOMAG 49A but is given special properties due to different processing methods. Hence the hysteresis cycle of this grade is rectangular.

The iron-nickel SOFTMAG alloys exhibit a wide range of magnetic properties in relation to their nickel content. The high nickel alloys have high initial permeability but low saturation, whereas the low nickel alloys are lower in initial permeability but higher in saturation induction. Small amounts of other alloying elements, particularly molybdenum and copper, are added to these alloys and special processing techniques such as annealing in hydrogen are employed in order to develop or accentuate specific characteristics.

Softmag can be broadly classified into six categories:

* SOFTMAG 30 Series (30%Ni, Fe rest) - low curie point, temperature compensator alloys. In this type of alloys, by slight compositional variation, the curie point can be brought down to between +40 and +100°C which is mostly used for temperature compensation in magnetic circuit, temperature-sensitive switches and relays.
* SOFTMAG 36 Series (36%Ni, Fe rest) - low-permeability, high resistivity alloys. These are two alloys having the same composition but different magnetic properties due to different processing methods.
36A Series is distinguished by the linearity of its magnetic property and finds application in weak fields in the form of laminations.
36B Series is characterized by very high electrical resistivity, good permeability and low electrical loss. This alloy is mostly used in relays and pulse transformers.
* SOFTMAG 48 Series (48%Ni, Fe rest) - medium-permeability, high saturation alloys. These are alloys-with similar composition but different magnetic properties.
48A Series is specially heat treated to attain special properties in low fields, for example, to lower the Rayleigh Region coefficient (g/m). This alloy is supplied in finishing heat-treated condition only, in the form of cores and laminations. They are used in telephone equipment and in some measuring devices.
48B Series shows high initial permeability in low fields. It is used for making relays, transformers, solenoids, current transformers, safety plugs for gas applications.
48C Series is a superior version of 48B Series and exhibits very high permeability and low hysteresis loss. It is available in the form of thin strips, cores and laminations. This alloy is used for making electrical components, small sensitive relays, current transformers, differential detectors, transducers, etc.
48D Series is a square loop version of 48B Series produced by adjusting the composition and subsequent rolling and annealing process achieving a high remanence of flux density due to the cube texture. 48D Series alloy is supplied in the form of strips, cores used in magnetic amplifiers, DC-DC transformers, memories, switching devices, etc.

* SOFTMAG 53 Series (53%Ni, Fe rest) - high-permeability, medium saturation alloys. 53 Series is a vacuum melted nickel-iron alloy offering a high induction at saturation in conjunction with high permeabilities. It is used only in the form of cores in current transformers, differential detectors, etc.
* SOFTMAG 76 Series (76%Ni, Fe rest) - high permeability, low saturation alloys. 76 is an alloy with saturation induction which is higher than that of 78 Series (-8500 G). This alloy has been specially developed for split armature coils of telephones.
* SOFTMAG 78 Series (78%Ni, Fe rest) - very high permeability, l ow saturation alloys. This family of alloys shows very high initial and maximum permeability- at low magnetizing forces, low core losses and very good magnetic shielding characteristics. There are six grades of alloys under this series, composed basically of 78% Ni-Fe-Mo and classified according to their permeability characteristics.
78A Series, 78B Series & 78C Series contain small quantities of copper in addition to molybdenum as an alloying element. They are supplied in the form of sheets, strips, cores and laminations.
78D Series & Series 78E are supplied only as tape wound cores in the heat-treated condition to gain optimum magnetic properties higher permeability and reduced losses.
Series 78F is an alloy which exhibits a rectangular hysteresis loop due to special heat treatment. It is obtained from 78D Series heats and is used in the form of cores for magnetic amplifiers, DC transformers, memories, etc.

Soft magnetic material includes a wide variety of nickel-iron and nickel-cobalt soft magnetic alloys and pure iron for high performance components requiring high initial and maximum permeability coupled with ease of fabrication.

Sophisticated equipment, advanced technology and the expertise developed for producing aeronautical grade alloys are employed for manufacturing, high performance soft magnetic material.

Starting with ultra clean raw materials, special processes and techniques are used for melting and refining this material under controlled atmospheric conditions in the Air Induction Melting, Vacuum Induction Refining and Vacuum Induction Melting furnaces. The final product is manufactured through a combination of forging, hot and cold rolling, wire drawing and heat treatment depending on the customer`s specifications.

Soft magnetic alloys includes two alloy systems:

1. Softmag Alloys
2. Sofcomag Alloys

As the section size of manganese steel increases, tensile strength and ductility decrease substantially in specimens cut from heat-treated castings. This occurs because, except under specially controlled conditions, heavy sections do not solidify in the mold fast enough to prevent coarse grain size, a condition that is not altered by heat treatment.

Fine grain specimens may exhibit tensile strength and elongation as much as 30% greater than those of coarse-grain specimens. Grain size is also the main reason for the differences between cast and wrought manganese steels -- the latter are usually on fine grain size.

Mechanical properties vary with section size. Tensile strength, tensile elongation, reduction in area and impact strength are substantially lower in 102 mm (4 inches) thick sections than in 25 mm (1 inch) thick sections. Because section thicknesses of production castings are often from 102 to 152 mm (4 to 6 inches), this factor is an important consideration for proper grade specification.

Austenitic manganese steel remains tough at subzero temperatures above the Ms temperature. The steel is apparently immune to hydrogen embrittlement. There is gradual decrease in impact strength with decreasing temperature. The transition temperature is not well defined because there is no sharp inflection in the impact strength-temperature curve down to temperatures as low as -85oC. At a given temperature and section size, nickel and manganese additions are usually beneficial for enhancing impact strength, while higher carbon and chromium levels are not.

Resistance to crack propagation is high and is associated with very sluggish progressive failures. Because of this, any fatigue cracks that develop might be detected, and the affected part or parts removed from service before complete failure occurs.

Yield strength and hardness vary only slightly with section size. The hardness of most grades is about 200 HB after solution annealing and quenching, but this value has little significance for estimating machinability or wear resistance.

Heat treatment strengthens austenitic manganese steel so that it can be used safely and reliably in a wide variety of engineering applications. Solution annealing and quenching, the standard treatment that produces normal tensile properties and the desired toughness, involves austenitizing followed quickly by water quenching. Variations of this treatment can be used to enhance specific desired properties such as yield strength and abrasion resistance.

Usually, a fully austenitic structure, essentially free of carbides and reasonably homogeneous with respect to carbon and manganese, is desired in the as-quenched condition, although this is not always attainable in heavy sections or in steels containing carbide-forming elements such as chromium, molybdenum, vanadium and titanium. If carbides exist in the as-quenched structure, it is desirable for them to be present as relatively innocuous particles or nodules within the austenite grains rather than as continuous envelopes at grain boundaries.

Austenitic steels with higher manganese contents (>15%) have recently been developed for applications requiring low magnetic permeability, low temperature (cryogenic) strength and low temperature toughness. This applications stem from the development of superconducting technologies used in transportation systems and nuclear fusion research and to meet the need for structural materials to store and transport liquefied gases.

For low magnetic permeability, these alloys have lower carbon content than the regular Hadfield steels. The corresponding loss in yield strength is compensated by alloying with vanadium, nitrogen, chromium, molybdenum, and titanium. Chromium also imparts corrosion resistance, as required in some cryogenic applications.

The alloys are used in the heat-treated (solution-annealed and quenched) condition except for those that are age-hardenable. Wrought alloys are available in the hot-rolled condition. The microstructure is usually a mixture of g (face-centered cubic or fcc) austenite and e (hexagonal close-packed, or hcp) martensite.

These alloys are characterized by good ductility and toughness, both especially desirable attributes in cryogenic applications. Further, the ductile-brittle transition is gradual, not abrupt. Because the stability of the austenite is composition dependent, a deformation-induced transformation can occur in service under certain conditions. This is usually undesirable because it is accompanied by a corresponding increase in magnetic permeability.

Additions of sulfur, calcium, and aluminum are made to enhance the machinability of these alloys where required. Because of their lower carbon content, most of these alloys are readily weldable by the shielded metal arc welding (SMAW), gas metal arc welding (GMAW), and electron beam welding (EBW) processes. The composition of the weld metal is similar to that of the base metal and tailored for low magnetic permeability. The phosphorus content is generally maintained below 0.02% to minimize the tendency for hot cracking.

Another class of austenitic steels with high manganese additions has been developed for cryogenic and for marine applications with resistance to cavitation corrosion. These alloys have been viewed as economical substitutes for conventional austenitic stainless steels because they contain aluminum and manganese instead of chromium and nickel. Consequently, these alloys are generally of higher strength but lower ductility than conventional stainless steels such as type 304. The microstructure of these alloys is a mixture of g (fcc) austenite and e (hcp) martensite, and in some cases (especially when the aluminum content exceeds about 5%) a (bcc) ferrite. There is a tendency for an embrittling b-Mn phase to form in the high manganese compositions during aging at elevated temperatures. The result is a significant decrease in ductility. The addition of aluminum to some extent suppresses the precipitation of this compound.

The original austenitic manganese steel, containing about 1.2% C and 12% Mn, was invented by Sir Robert Hadfield in 1882. Hadfield`s steel was unique in that it combined high toughness and ductility with high work-hardening capacity and, usually, good resistance to wear.

Consequently, it rapidly gained acceptance as a very useful engineering material. Hadfield`s austenitic manganese steel is still used extensively, with minor modifications in composition and heat treatment, primarily in the fields of earthmoving, mining, quarrying, oil well drilling, steelmaking, railroading, dredging, lumbering, and in the manufacture of cement and clay products. Austenitic manganese steel is used in equipment for handling and processing earthen materials (such as rock crushers, grinding mills, dredge buckets, power shovel buckets and teeth, and pumps for handling gravel and rocks). Other applications include fragmentizer hammers and grates for automobile recycling and military applications such as tank track pads.

Many variations of the original austenitic manganese steel have been proposed, often in unexploited patents, but only a few have been adopted as significant improvements. These usually involve variations of carbon and manganese, with or without additional alloys such as chromium, nickel, molybdenum, vanadium, titanium, and bismuth.

The available assortment of wrought grades is smaller and usually approximates ASTM composition B-3. Some wrought grades contain about 0.8% C and either 3% Ni or 1% Mo. Large heat orders are usually required for the production of wrought grades, while cast grades and their modifications are more easily obtained in small lots. A manganese steel foundry may have several dozen modified grades on its production list. Modified grades are usually produced to meet the requirements of application, section size, casting size, cost, and weldability considerations.

The mechanical properties of austenitic manganese steel vary with both carbon and manganese content. As carbon is increased it becomes increasingly difficult to retain all of the carbon in solid solution, which may account for reductions in tensile strength and ductility.

Nevertheless, because abrasion resistance tends to increase with carbon, carbon content higher than the 1.2% midrange of grade A may be preferred even when ductility is lowered. Carbon content above 1.4% is seldom used because of the difficulty of obtaining an austenitic structure sufficiently free of grain, boundary carbides, which are detrimental to strength and ductility. The effect can also be observed in 13% Mn steels containing less than 1.4% C because segregation may result in local variations of ±17% (±0.2%C) from the average carbon level determined by chemical analysis.

The low carbon content (0.7% C minimum) of grades D and E-1 may be used to minimize carbide precipitation in heavy castings or in weldments, and similar low carbon contents are specified for welding filler metal.

Carbides form in castings that are cooled slowly in the molds. In fact, carbides form in practically all as cast grades containing more than 1.0% C, regardless of mold cooling rates. They form in heavy-section castings during heat treatment if quenching is ineffective in producing rapid cooling throughout the entire section thickness. Carbides can form during welding or during service at temperatures above about 275°C. If carbon and manganese are lowered together, for instance to 0.53% C with 8.3% Mn or 0.62% C with 8.1% Mn, the work-hardening rate is increased because of the formation of strain-induced a (body-centered-cubic, or bcc) martensite. However, this does not provide enhanced abrasion resistance (at least to high-stress grinding abrasion) as is often hoped.

Titanium can reduce carbon in austenite by forming very stable carbides. The resulting properties may simulate those of lower-carbon grade. Titanium may also somewhat neutralize the effect of excessive phosphorus; some European practice is apparently based on this idea. Microalloying additions (<0.1%) of titanium, vanadium, boron, zirconium and nitrogen have been reported to promote grain refinement in manganese steels. The effect, however, is inconsistent. Higher level of these elements can result in serious losses in ductility. Nitrogen in amounts greater than 0.20% can cause gas porosity in castings. An overall reduction in grain size lowers the susceptibility of the steel to hot tearing.

Sulfur. The sulfur content in manganese steels seldom influences its properties, because the scavenging effect of manganese operates to eliminate sulfur by fixing it in the form of innocuous, rounded, sulfide inclusions. The elongation of these inclusions in wrought steels may contribute in directional properties; in cast steels such inclusions are harmless. However, it is best to keep sulfur as low as is practically possible to minimize the number of inclusions in the microstructure that would be potential sites for the nucleation of fatigue cracks in service.

It is a long-standing tradition to discuss the various alloying elements in terms of the properties they confer on steel. For example, the rule was that Chromium (Cr) makes steel hard whereas Nickel (Ni) and Manganese (Mn) make it tough. In saying this, one had certain types of steel in mind and transferred the properties of particular steel to the alloying element that was thought to have the greatest influence on the steel under consideration. This method of reasoning can give false impressions and the following examples will illustrate this point.

When we say that Cr makes steel hard and wear-resisting we probably associate this with the 2% C, 12% Cr tool steel grade, which on hardening does in fact become very hard and hard-wearing. But if, on the other hand, we choose a steel containing 0,10% C and 12% Cr, the hardness obtained on hardening is very modest.

It is quite true that Mn increases steel toughness if we have in mind the 13% manganese steel, so-called Hadfield steel. In concentrations between l% and 5%, however, Mn can produce a variable effect on the properties of the steel it is alloyed with. The toughness may either increase or decrease.

A property of great importance is the ability of alloying elements to promote the formation of a certain phase or to stabilize it. These elements are grouped as austenite-forming, ferrite-forming, carbide-forming and nitride-forming elements.
Austenite-forming elements

The elements C, Ni and Mn are the most important ones in this group. Sufficiently large amounts of Ni or Mn render a steel austenitic even at room temperature. An example of this is the so-called Hadfield steel which contains 13% Mn, 1,2% Cr and l% C. In this steel both the Mn and C take part in stabilizing the austenite. Another example is austenitic stainless steel containing 18% Cr and 8% Ni.

The equilibrium diagram for iron-nickel, Figure 1, shows how the range of stability of austenite increases with increasing Ni-content.
An alloy containing 10% Ni becomes wholly austenitic if heated to 700°C. On cooling, transformation from g to a takes place in the temperature range 700-300°C.
Ferrite-forming elements

The most important elements in this group are Cr, Si, Mo, W and Al. The range of stability of ferrite in iron-chromium alloys is shown in Figure 2. Fe-Cr alloys in the solid state containing more than 13% Cr are ferritic at all temperatures up to incipient melting. Another instance of ferritic steel is one that is used as transformer sheet material. This is a low-carbon steel containing about 3% Si.
Multi-alloyed steels

The great majority of steels contain at least three components. The constitution of such steels can be deduced from ternary phase diagrams (3 components). The interpretation of these diagrams is relatively difficult and they are of limited value to people dealing with practical heat treatment since they represent equilibrium conditions only. Furthermore, since most alloys contain more than three components it is necessary to look for other ways of assessing the effect produced by the alloying elements on the structural transformations occurring during heat treatment.

One approach that is quite good is the use of Schaeffler diagrams (see Figure 3). Here the austenite formers are set out along the ordinate and the ferrite formers along the abscissa. The original diagram contained only Ni and Cr but the modified diagram includes other elements and gives them coefficients that reduce them to the equivalents of Ni or Cr respectively. The diagram holds good for the rates of cooling which result from welding.

Figure 3. Modified Schaeffler diagram
A 12% Cr steel containing 0,3% C is martensitic, the 0,3% C gives the steel a nickel equivalent of 9. An 18/8 steel (18% Cr, 8% Ni) is austenitic if it contains 0-0,5% C and 2% Mn. The Ni content of such steels is usually kept between 9% and 10%.

Hadfield steel with 13% Mn (mentioned above) is austenitic due to its high carbon content. Should this be reduced to about 0,20% the steel becomes martensitic.
Carbide-forming elements

Several ferrite formers also function as carbide formers. The majority of carbide formers are also ferrite formers with respect to Fe. The affinity of the elements in the line below for carbon increases from left to right.

Cr, W, Mo, V, Ti, Nb, Ta, Zr.

Some carbides may be referred to as special carbides, i.e. non-iron-containing carbides, such as Cr7C3 W2C, VC, Mo2C. Double or complex carbides contain both Fe and a carbide-forming element, for example Fe4W2C.

High-speed and hot-work tool steels normally contain three types of carbides, which are usually designated M6C, M23C6 and MC. The letter M represents collectively all the metal atoms. Thus M6C represents Fe4W2C or Fe4Mo2C; M23C6 represents Cr23C6 and MC represents VC or V4C3.

Hardening of steel is obtained by a suitable quench from within or above the critical range. The temperatures are the same as those given for full annealing. The soaking time in air furnaces should be 1,2 min for each mm of cross-section or 0,6 min in salt or lead baths. Uneven heating, overheating and excessive scaling should be avoided.

The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite, and to cause a partial decomposition at such a low temperature to produce martensite. To obtain this, steel requires a critical cooling velocity, which is greatly reduced by the presence of alloying elements, which therefore cause hardening with mild quenching (e.g. oil and hardening steels).

Steels with less than 0,3 % carbon cannot be hardened effectively, while the maximum effect is obtained at about 0,7 % due to an increased tendency to retain austenite in high carbon steels Fig. 1.

Figure 1. Variation of hardness of martensite and bainite with carbon content

Water is one of the most efficient quenching media where maximum hardness is required, but it is liable to cause distortion and cracking of the article. Where hardness can be sacrificed, whale, cotton seed and mineral oils are used. These tend to oxidise and form sludge with consequent lowering of efficiency.

The quenching velocity of oil is much less than water. Ferrite and troostite are formed even in small sections. Intermediate rates between water and oil can be obtained with water containing 10-30 % Ucon, a substance with an inverse solubility which therefore deposits on the object to slow rate of cooling. To minimise distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. To prevent steam bubbles forming soft spots, a water quenching bath should be agitated.

Fully hardened and tempered steels develop the best combination of strength and notch-ductility.
Tempering and toughening

The martensite of quenched tool steel is exceedingly brittle and highly stressed. Consequently cracking and distortion of the object are liable to occur after quenching. Retained austenite is unstable and as it changes dimensions may alter, e.g. dies may alter 0,012 mm.

It is necessary, therefore, to warm the steel below the critical range in order to relieve stresses and to allow the arrested reaction of cementite precipitation to take place. This is known as tempering.

*
150-250°C. The object is heated in an oil bath, immediately after quenching, to prevent related cracking, to relieve internal stress and to decompose austenite without much softening.
*
200-450°C. Used to toughen the steel at the expense of hardness. Brinell hardness is 350-450.
*
450-700°C. The precipitated cementite coalesces into larger masses and the steel becomes softer. The structure is known as sorbite, which at the higher temperatures becomes coarsely spheroidised. It etches more slowly than troostite and has a Brinell hardness of 220-350. Sorbite is commonly found in heat-treated constructional steels, such as axles, shafts and crankshafts subjected to dynamic stresses. A treatment of quenching and tempering in this temperature range is frequently referred to as toughening, and it produces an increase in the ratio of the elastic limit to the ultimate tensile strength.

The reactions in tempering occur slowly. Reaction time as well as temperature of heating is important. Tempering is carried out to an increasing extent under pyrometric control in oil, salt (e.g. equal parts sodium and potassium nitrates for 200-600°C) or lead baths and also in furnaces in which the air is circulated by fans. After the tempering, the objects may be cooled either rapidly or slowly, except for steels susceptible to temper brittleness.

Temper colours formed on a cleaned surface are still used occasionally as a guide to temperature. They exist due to the interference effects of thin films of oxide formed during tempering, and they act similarly to oil films on water. Alloys such as stainless steel form thinner films than do carbon steels for a given temperature and hence produce a colour lower in the series. For example, pale straw corresponds to 300°C, instead of 230°C (Table 1).

Table 1.

Temper Colour	Temperature °C	Objects
Pale straw	230	Planing and slotting tools
Dark straw	240	Milling cutters, drills
Brown	250	Taps, shear blades for metals
Brownish-purple	260	Punches, cups, snaps, twist drills, reamers
Purple	270	Press tools, axes
Dark purple	280	Cold chisels, setts for steel
Blue	300	Saws for wood, springs
Blue	450-650	Toughening for constructional steels

For turning, planing, shaping tools and chisels, only the cutting parts need hardening. This is frequently carried out in engineering works by heating the tool to 730°C, followed by quenching the cutting end vertically. When cutting end gets cold, it is cleaned with the stone and the heat from the shank of the tool is allowed to temper the cutting edge to the correct colour. Then the whole tool is quenched. Oxidation can be reduced by coating the tool with charcoal and oil.

The solid-solution hardening of carbon has a major effect on the strength of martensite, but ductility can only be obtained at low carbon levels. Although alloying elements affect hardenability, they have a minor effect on hardness except to reduce it at high carbon levels by causing austenite to be retained.

Alternative ways of improving the strength of alloy steels are:

(1) Grain refinement, which increases strength and ductility. This can be developed by severely curtailing the time after the cessation of forging at some low temperature of austenite stability or by rapid heating, coupled with a short austenitising period. Fine grain is produced in 9% Ni steel by tempering fine lath martensite.

(2) Precipitation hardening by carbide, nitride or intermetallic compounds.
(a) By secondary hardening, e.g. 12% Cr steel with additions.
(b) Age hardening a low carbon Fe-Ni lath martensite supersaturated with substitutional elements, e.g. maraging.
(c) Age hardening of austenite, e.g. stainless steels. Phosphorus and titanium are common additions. Stacking faults are often associated with fine carbide precipitates, and strength can be raised by increasing the number of stacking faults (i.e. lower fault energy).
(d) Controlled transformation 18/8 austenite steels in which transformation to martensite is induced by refrigeration or by strain.

(3) Thermomechanical treatments which may be classified into three main groups:
(a) Deformation of austenite prior to the transformation.
Ausforming consists of steel deforming in a metastable austenitic condition between Ac1 and Ms (e.g. 500°C called LT) followed by transformation to martensite and light tempering (Fig. 1). This results in increased dislocation density in the martensite and a finer carbon precipitation on tempering. Strengths up to 1800 MPa can be obtained without impairing the ductility (~6 % deformation). Steels must possess a TTT-curve with a large bay of stable austenite, e.g. 826 M40. Typical application is for leaf springs.

Figure 1. Methods of thermomechanical treatment

Deformation of stable austenite just above Ac3 before cooling (called HT). The properties are somewhat inferior to those produced by ausforming.
Deformation induced transformation originally used in Hadfield 13% Mn steel, but can be adapted to metastable austenitic stainless steels. The fully austenitic steel is severely warm-worked above the lowest temperature at which martensite is produced during the straining. The distinctive property is the high rate of straining hardening, which increases ductility.

(b) Deformation of austenite during the transformation
Isoforming is the deformation of a steel (e.g. 1% Cr) during the isothermal transformation to pearlite, which refines the structure and improves fracture toughness (Fig. 1). A somewhat similar thermomechanical process can be used in the bainitic region, producing bainite and martensite.
Zerolling consists in forming martensite by deformation at subzero temperatures to strengthen 18/8 austenitic steels. The amount of martensite is influenced by alloy composition and increased with deformation and lowering of the temperature.

(c) Deformation after the transformation of austenite
Marforming consists of deforming the maraging steel in the soft martensitic condition, generally cold. There is a pronounced increase in strength of the subsequent maraged product. With other steels, considerable increases in strength can be obtained by straining martensite (~3 %) either in the untempered or tempered condition. A strengthening effect also occurs on re-tempering, probably due to the resolution and reprecipitation of the carbides in a more finely dispersed form.
Strain tempering and dynamic strain ageing
Both processes involve about 5% deformation at the room temperature between two stages of tempering -- strain tempering -- while in dynamic strain ageing deformation is concurrent with tempering.

Manganese

All commercial steels contain 0,3-0,8% manganese, to reduce oxides and to counteract the harmful influence of iron sulfide. Any manganese in excess of these requirements partially dissolves in the iron and partly form Mn3C which occurs with the Fe3C. There is a tendency nowadays to increase the manganese content and reduce the carbon content in order to get a steel with an equal tensile strength but improved ductility

If the manganese is increased above 1,8% the steel tends to become airhardened, with resultant impairing of the ductility. Up to this quantity, manganese has a beneficial effect on the mechanical properties of oil hardened and tempered 0,4% carbon steel. The manganese content is also increased in certain alloy steels, with a reduction or elimination of expensive nickel, in order to reduce costs. Steels with 0,3-0,4% carbon, 1,3-1,6% manganese and 0,3% molybdenum have replaced 3% nickel steel for some purposes.

Non-shrinking tool steel contains up to 2% manganese, with 0,8-0,9% carbon. Steels with 5 to 12% manganese are martensitic after slow cooling and have little commercial importance.

Hadfield`s manganese steel contains 12 to 14% of manganese and 1,0% of carbon. It is characterized by a great resistance to wear and is therefore used for railway points, rock drills and stone crushers. Austenite is completely retained by quenching the steel from 1000°C, in which soft condition it is used, but abrasion raises the hardness of the surface layer from 200 to 600 VPN (with no magnetic change), while the underlying material remains rough. Annealing embrittles the steel by the formation of carbides at the grain boundaries. Nickel is added to electrodes for welding manganese steel and 2% Mo sometimes added, with a prior carbide dispersion treatment at 600°C, to minimize initial distortion and spreading.
Nickel

Nickel and manganese are very similar in behavior and both lower the eutectoid temperature. This change point on heating is lowered progressively with increase of nickel (approximately 10°C for 1% of nickel), but the lowering of the change on cooling is greater and irregular. The temperature of this change (Ar1) is plotted for different nickel contents for 0,2% carbon steels in Fig. 1, and it will be seen that the curve takes a sudden plunge round about 8% nickel. A steel with 12% nickel begins to transform below 300°C on cooling, but on reheating the reverse change does not occur until about 650°C. Such steels are said to exhibit pronounced lag or hysteresis and are called irreversible steels. This characteristic is made use of in maraging steels and 9% Ni cryogenic steel.

The addition of nickel acts similarly to increasing the rate of cooling of a carbon steel. Thus with a constant rate of cooling the 5-8% nickel steels become troostitic; at 8-10% nickel, where the sudden drop appears, the structure is martensitic, while above 24% nickel the critical point is depressed below room temperature and austenite remains. The lines of demarcation are not so sharp as indicated by Fig. 1, but a gradual transition occurs from one constituent to another.

The mechanical properties change accordingly as shown in the lower part of Fig. 1. Steels with 0,5% nickel are similar to carbon steel, but are stronger, on account of the finer pearlite formed and the presence of nickel in solution in the ferrite. When 10% nickel is exceeded the steels have a high tensile strength, great hardness, but are brittle, as shown by the Izod and elongation curves. When the nickel is sufficient to produce austenite the steels become non-magnetic, ductile, tough and workable, with a drop in strength and elastic limit.

Carbon intensives the action of nickel and the change points shown in Fig. 1 will vary according to the carbon content. The influence of carbon and nickel on the structure are shown in the small inset (Guillet) diagram in Fig. 1, for one rate of cooling. Steels containing 2 to 5% nickel and about 0,1% carbon are used for case hardening; those containing 0,25 to 0,40% carbon are used for crankshafts, axles and connecting rods.

The superior properties of low nickel steels are best brought out by quenching and tempering (550-650°C). Since the Ac3 point is lowered, a lower hardening temperature than for carbon steels is permissible and also a wider range of hardening temperatures above Ac3 without excessive grain growth, which is hindered by the slow rate of diffusion of the nickel. Martensitic nickel steels are not utilized and the austenitic alloys cannot compete with similar manganese steels owing to the higher cost. Maraging steels have fulfilled a high tensile requirement in aero and space fields. High nickel alloys are used for special purposes, owing to the marked influence of nickel on the coefficient of expansion of the metal. With 36% nickel, 0,2% carbon, 0,5% manganese, the coefficient is practically zero between 0° and 100°C. This alloy ages with time, but this can be minimized by heating at 100°C for several days. The alloy is called Inver and it is used extensively in clocks, tapes and wire measures, differential expansion regulators, and in aluminium pistons with a split skirt in order to give an expansion approximating to that of cast iron.

A carbon-free alloy containing 78,5% nickel and 21,5% iron has a high permeability in small magnetic fields.
Chromium

Chromium can dissolve in either alpha- or gama-iron, but, in the presence of carbon, the carbides formed are cementite (FeCr)3C in which chromium may rise to more than 15%; chromium carbides (CrFe)3C2 (CrFe)7C3 (CrFe)4C, in which chromium may be replaced by a few per cent, by a maximum of 55% and by 25% respectively. Stainless steels contain Cr4C. The pearlitic chromium steels with, say, 2% chromium are extremely sensitive to rate of cooling and temperature of heating before quenching; for example:

Temp. of Initial Heating, °C


Critical Hardening Rate
(Mins to cool from 836° to 546°C)

836


3,5

1010


6,5

1200


13

The reason is that the chromium carbides are not readily dissolved in the austenite, but the amount increases with increase of temperature. The effect of the dissolved chromium is to raise the critical points on heating (Ac) and also on cooling (Ar) when the rate is slow. Faster rates of cooling quickly depress the Ar points with consequent hardening of the steel. Chromium imparts a characteristic form of the upper portion of the isothermal transformation curve.

The percentage of carbon in the pearlite is lowered. Hence the proportion of free cementite (hardest constituent) is increased in high carbon steel and, when the steel is properly heat-treated, it occurs in the spheroidised form which is more suitable when the steel is used for ball bearings. The pearlite is rendered fine.

When the chromium exceeds 1,1% in low-carbon steels an inert passive film is formed on the surface which resists attack by oxidizing reagents. Still higher chromium contents are found in heat-resisting steel.

Chromium steels are easier to machine than nickel steels of similar tensile strength. The steels of higher chromium contents are susceptible to temper brittleness if slowly cooled from the tempering temperature through the range 550/450°C. These steels are also liable to form surface markings, generally referred to as "chrome lines".

The chrome steels are used wherever extreme hardness is required, such as in dies, ball bearings, plates for safes, rolls, files and tools. High chromium content is also found in certain permanent magnets.
Nickel and chromium

Nickel steels are noted for their strength, ductility and toughness, while chromium steels are characterized by their hardness and resistance to wear. The combination of nickel and chromium produces steels having all these properties, some intensified, without the disadvantages associated with the simple alloys. The depth of hardening is increased, and with 4,5% nickel, 1,25% chromium and 0,35% carbon the steel can be hardened simply by cooling in air.

Low nickel-chromium steels with small carbon content are used for casehardening, while for most constructional purposes the carbon content is 0,25-0,35%, and the steels are heat-treated to give the desired properties. Considerable amounts of nickel and chromium are used in steel for resisting corrosion and oxidation at elevated temperatures.

Embattlement. The effects of tempering a nickel-chromium steel are shown in Fig. 2, from which it will be noticed that the Izod impact curve No. 1 reaches a dangerous minimum in the range 250-450°C in common with many other steels. This is known as 350°C embattlement. Phosphorus and nitrogen have a significant effect while other impurities (As, Sb, Sn) and manganese in larger quantity may also contribute to the embattlement.

Temper brittleness is usually used to describe the notch impact intergranular brittleness (Grain boundaries are revealed in temper brittle samples by etching in 1 gm cetyl trimethyl ammonium bromide; 20 gm picric acid; 100 cc distilled water, 100 cc ether. Shake mixture, allow to stand for 24 hrs; use portion of top layer and return to tube afterwards) induced in some steels by slow cooling after tempering above about 600°C and also from prolonged soaking of tough material between about 400° and 550°C.

Temper brittleness seems to be due to grain boundary enrichment with alloying elements-Mn, Cr, Mo-during austenitising which leads to enhanced segregation of embattling elements P, Sn, Sb, As-by chemical interaction on slow cooling from 600°C. The return to the tough condition, obtained by rehearing embattled steel to temperatures above 600°C and rapidly cooling, is due to the redistribution and retention in solution of the embattling segregation. Antimony (0-001 %), phosphorus (0-008 %), arsenic, tin, manganese increase, while molybdenum decreases the susceptibility of a steel to embattlement. 0-25 % molybdenum reduces the brittleness as shown by Izod curve No. 2. Table 1 illustrates the effect rate of cooling after tempering and the influence of an addition of 0-45 % molybdenum:

Table 1. Steel 0,3% C, 3,5 % Ni, 0,7%, Cr, tempered at 630°C

Steel	Cooling Rate	TS MPa	Elongation	RA	Izod ft lbf	Izod J
Ni-Cr	Oil	896	18	60	64	87
Ni-Cr	Furnance	880	18	60	19	25
Ni-Cr-Mo	Furnance	896	18	61	59	80

Molybdenum

Molybdenum dissolves in both alpha- and gama-iron and in the presence of carbon forms complex carbides (FeMo)6C, Fe21Mo2C6, Mo2C.

Molybdenum is similar to chromium in its effect on the shape of the TTT-curve but up to 0,5% appears to be more effective in retarding pearlite and increasing bainite formation. Additions of 0,5% molybdenum have been made to plain carbon steels to give increased strength at boiler temperatures of 400°C, but the element is mainly used in combination with other alloying elements.

Ni-Cr-Mo steels are widely used for ordnance, turbine rotors and other large articles, since molybdenum tends to minimize temper brittleness and reduces mass effect. Molybdenum is also a constituent in some high-speed steels, magnet alloys, heat-resisting and corrosion-resisting steels.
Vanadium

Vanadium acts as a scavenger for oxides, forms a carbide V,C, and has a beneficial effect on the mechanical properties of heat-treated steels, especially in the presence of other elements. It slows up tempering in the range of 500-600°C and can induce secondary hardening. Chromium-vanadium (0,15%) steels are used for locomotive forging, automobile axles, coil springs, torsion bars and creep resistance.
Tungsten

Tungsten dissolves in gama-iron and in alpha-iron. With carbon it forms WC and W2C, but in the presence of iron it forms Fe3W3C or Fe4W2C. A compound with iron-Fe3W2-provides an age-hardening system. Tungsten raises the critical points in steel and the carbides dissolve slowly over a range of temperature. When completely dissolved, the tungsten renders transformation sluggish, especially to tempering, and use is made of this in most hot-working tool ("high speed") and die steels. Tungsten refines the grain size and produces less tendency to decarburisation during working. Tungsten is also used in magnet, corrosion- and heat-resisting steels.
Silicon

Silicon dissolves in the ferrite, of which it is a fairly effective hardener, and raises the Ac change points and the Ar points when slowly cooled and also reduces the gama-alpha volume change.

Only three types of silicon steel are in common use-one in conjunction with manganese for springs; the second for electrical purposes, used in sheet form for the construction of transformer cores, and poles of dynamos and motors, that demand high magnetic permeability and electrical resistance; and the third is used for automobile valves.

C	Si	Mn
1. Silico-manganese	0,5	1,5	0,8
2. Silicon steel	0,07	4,3	0,09
3. Silichrome	0,4	3,5	8

It contributes oxidation resistance in heat-resisting steels and is a general purpose deoxidizes.

Other elements

Copper dissolves in the ferrite to a limited extent; not more than 3,5% is soluble in steels at normalizing temperatures, while at room temperature the ferrite is saturated at 0,35%. It lowers the critical points, but insufficiently to produce martensite by air cooling. The resistance to atmospheric corrosion is improved and copper steels can be temper hardened.

Cobalt has a high solubility in alpha- and gama-iron but a weak carbide-forming tendency. It decreases hardenability but sustains hardness during tempering. It is used in "Stellite" type alloys, gas turbine steel, magnets and as a bond in hard metal.

Boron. In recent years, especially in USA, 0,003-0,005% boron has been added to previously fully killed, fine-grain steel to increase the hardenability of the steel. The yield ratio and impact are definitely improved, provided advantage is taken of the increased hardenability obtained and the steel is fully hardened before tempering. In conjunction with molybdenum boron forms a useful group of high tensile bainitic steels. Boron is used in some hard facing alloys and for nuclear control rods.

Some examples of alloy steels used for shearing are given in Table.

High-Speed Steels

The evolution of high-speed cutting tools commenced with the production of Mushet`s self-hardening tungsten-manganese steel in 1860. The possibilities of such steels for increased rates of machining were not fully appreciated until 1900, when Taylor and White developed the forerunner of modern high-speed steels. In addition to tungsten, chromium was found to be essential and a high hardening temperature to be beneficial. The -steel resisted tempering up to 600°C. This allowed the tool to cut at speeds of 80-50 meters per minute with its nose at a dull red temperature and it was one of the astonishing exhibits at the Paris Exhibition of 1900.

Table . Shear blade Steel

Type of Work	C	Cr	V	W
Cold shearing for heavy materials	0,85		0,2
	0,55	Mn=0,8	Mn=0,8
Cold shearing for light materials	1,0	-	0,2
	0,7	0,9	0,2	-
	0,6	4	1	18
	2,2	12	-	-
Shears for hot work	0,5	1,2	0,2	2
	0,4	3,5	0,4	10

The main constituents in high-speed steel are 14 or 18% tungsten, 3 to 5% chromium and 0,6% carbon. Other elements are frequently added to modern steels which vary considerably in composition and cost. 0,09-0,15% sulphur is sometimes added to give free machining for unground form tools, e.g. gear hobs in 6,5×2 M2S.

Vanadium improves the cutting qualities of the tools and increases the tendency to air hardening. Cobalt, often added to the "super high-speed" steel, raises the temperature of the solidus and enables a higher hardening temperature to be used, with consequent greater solution of carbon. Secondary hardness is marked in such steels, and this permits the use of deep cuts at fast speeds. The molybdenum steel is susceptible to decarburisation. The high vanadium steel is somewhat brittle, but is excellent for cutting very abrasive materials.

The study of the structures of such highly alloyed steels is complex, but it can be simplified by converting the amounts of the various elements to an equivalent percentage of tungsten as regards the effect on the closed g-loop:

1% of	Mo	V	Cr
Equivalent percentage of tungsten	1,5	5,0	0,5

Hence 18 W, 4 Cr, 1 V is equivalent to 25% tungsten and the section of the FE-W-C equilibrium diagram is shown in Fig. 1.

Figure 1. Section of the Fe-W-C equilibrium diagram at 25% tungsten

In the ingot the structure is similar to cast iron, but the cementite consists of mixed carbides (Fe, W Cr, V),C with the balance of the elements in solution in the ferrite. In this condition the steel is extremely brittle and the eutectic net-work has to be broken up into small globules, evenly distributed by careful annealing, followed by forging. "Strings" or laminations of carbides should be avoided, otherwise cracks are liable to form during hardening.
Annealing

High-speed steel is softened by annealing at 850°C for about four hours, followed by slow cooling. The steel must be protected against oxidation. After forging, tools should be heated to 680°C for -If hour and air cooled before hardening in order to reduce risk of fracture. The annealed structure consists of carbide globules in a matrix of fine pearlite.
Hardening

From Fig. 1 it will be seen that on heating, austenite forms at about 800°C, but contains only 0-2% carbon (eutectoid E). Quenching produces martensite, which tempers readily and has no advantage over carbon tools. More carbide dissolves on heating, as indicated by line EB, and quenching produces structures of increasing red-hardness, due to the effect of the larger amounts of alloying elements in solution, which render the steel sluggish to tempering. Even at 1300°C, when melting occurs, only 0,4% carbon (B) is dissolved and the remainder exists as complex carbides. It will be seen, therefore, that to attain maximum cutting efficiency sufficient carbon and alloying elements must be dissolved in the austenite and this necessitates temperatures little short of fusion, usually 1150-1350°C.

Grain growth and oxidation occur rapidly at such temperatures. Hence the tools are carefully preheated up to 850°C, then heated rapidly to the hardening temperature and quenched in oil or cooled in an air blast without soaking. To reduce the severe stresses set up by quenching, the following modifications can be used to reduce the temperature gradient from outside to center prior to the austenite-martensite transformation:
a) cool in salt bath at 600°C until temperature is uniform; then quench in oil, or
b) oil quench to 425°C, then air cool to room temperature.
Tempering

When quenched from high temperatures high-speed steels contain an appreciable amount of retained austenite which is softer than martensite. This is decomposed by tempering, or by sub-zero cooling to -80°C. Multi-tempering is often more effective than a single temper of the same duration.
Tempering at 350-400°C slightly reduces the hardness but increases toughness. Tempering at 400-600°C increases the hardness, frequently to a value higher than that produced by quenching. This phenomenon is known as secondar hardening. The structure of the hardened high-speed steel consists of isolated spherical carbides embedded in an austenite-martensite matrix.
Dark etching grain boundaries are frequently evident. Tempering produces a general darkening of the matrix. "Stellite" type alloys consist of a cobalt base with about Cr, 30; W, 15 with other additions, including carbon. The structure consists of a cobalt matrix with complex tungsten-chromium carbides. lt has a high resistance to corrosion and to tempering and is used for tools, gauges, valve seatings and hard facing.

Standard BS 4659:1971 groups tool steels into six types:

1. high speed,
2. hot work,
3. cold work,
4. shock resisting,
5. special purpose and
6. water hardening.

The designations follow the AISI with the addition of B. Thus BTI and BMI designates high speed steel of tungsten and molybdenum grades respectively.
Non-Shrinking Steels

This term refers to steels which show little change in volume from the annealed state when hardened and tempered at low temperatures. Usually the following volume changes occur.

Pearlitic	austenitic state, contraction
austenitic	martensitic state, expansion
martensitic	sorbitic state, contraction

In non-shrinking steels the volume changes counterbalance each other, and such steels are required for master tools, gauges and dies which must not change size when hardened after machining in the annealed condition. The cheapest non-shrinkage steel contains 0,9% carbon and about 1,7% manganese. A better steel is,

C, 1.0; Mn, 0.95; W, 0.5; Cr, 0.75; V, 0.2

Both steels are oil quenched from 780° to 800°C and tempered 224-245°C. High carbon 5% and 12% chromium steels are also used for non-distortion.
Finishing Tool Steel

While high-speed steels are very efficient with heavy cuts and high speeds they are incapable, at slow speeds and lighter cuts, of holding the keen edge necessary for obtaining a very smooth finish on certain articles. Special steels have been produced for this purpose, known as finishing steels, which are capable of retaining a keen cutting edge for much longer periods than carbon steel used under similar conditions. The usual type has the approximate composition:

C, 1.1 to 1.4; W, 4; Cr, 0.7 to 1.5; V, 0.3

After preheating to 650°C it is water hardened at 820-840°C and immediately tempered at 150-180°C. Anneal at 750°C. Tungsten steels containing 1 to 5,5% and 1 to 1,3% carbon are used for twist drills, taps, milling cutters, drawing dies and also tools for rifling gun barrels, boring cylinders and expanding tubes, which require long continuous cutting without interruption for regrinding. They are tempered at 200-230°C.

Cold Die Steels

The standard oil hardening die steels contain 1 C, 1 Mn, 0,3-1,6 W, 0,5 Cr, hardened from 800°C and immediately tempered at 170-250°C. For cold obtrusion punches high-speed steels are satisfactory, e.g. 6W6 Mo.
High carbon-chromium (A)

C	Cr	Mn	Si	Harden °C	Temper °C
2	13	0-25	0-6	OQ 950 or AC 1000	480-2 hrs

This steel has good resistance to oxidation at elevated temperatures, high hardness and good wearing properties. lt is suitable for intricate sections, dies for blanking, coining, toller threading and drop forging hard materials. The structure is martensitic on cooling in air but the carbides can be precipitated and the steel softened by very slow cooling from 840°C.
High Tungsten-Chromium Steel

C	Mn	W	Cr	V	Mo	Harden,°C	Temper,°C	Anneal, °C
0,3	0,3	10	3	0,3	0,3	OQ 1150	570	850

This is the best type of steel for hot work except where resistance to scaling or oxidation is important. lt is used for hot-drawing, hot-forging, extrusion dies and dies for die casting aluminium, brass and zinc alloys. Die-casting die steels often fall through surface cracking caused by cyclic expansion and contraction, aggravated by the erosive action of the molten metal. Increased die life necessitates regular maintenance and careful preheating before use.

Sensitivity of die steels to distortion during heat-treatment is largely affected by directionality and particle size of the carbides in the microstructure. Expansion is greatest in the direction of carbide stringers. Fine random distribution of carbides are therefore desirable. For die casting and extrusion dies molybdenum containing 0,5 Ti + 0,08 Zr is useful in critical applications. Thermal conductivity, resistance to thermal shock and attack by molten metal is high and no heat treatment is required. Nimonic 80(a) and 90 have also been used satisfactorily for dies and inserts. Die block steels for drop forging have been standardised into four type. These are:
1) 0,6 carbon steel,
2) 1% nickel, 0,6 C,
3) 1,5 Ni, 0,7 Cr, 0,6 C,
4) 1,5 Ni, 0,7 Cr, 0,6 C, 0,25 Mo.

Hardness ranges from 425/455 for dies with shallow impressions to 298/355 for very large forgings.