Annealing of Steels
Annealing of Steels
When a metal is cold worked (deformed at room temperature), the microstructure becomes severely distorted because of an increased dislocation density resulting from the deformation. Cold working is also referred to as work hardening or strain hardening. As a metal is cold worked, the strength and hardness increase while ductility decreases. Eventually, it is necessary to anneal the piece to allow further forming operations without the risk of breaking it. In addition, some steels are strengthened primarily by cold working. In this case, it is important that the steel not soften appreciably when placed in service.
Cold-worked steels with highly distorted microstructures are in a high-energy state and are thermodynamically unstable. Annealing is the heat treatment process which softens a metal that has been hardened by cold working. Annealing consists of three distinct process stages namely (i) recovery, (ii) recrystallization, and (iii) grain growth. Although a reduction in stored energy provides the driving force, annealing normally does not spontaneously occur at room temperature. This is because the reduction in stored energy occurs by diffusion and the activation energy needed to start the diffusion process is normally insufficient at room temperature. Hence, heating is necessary to provide the thermal activation energy needed to transform the material to a lower-energy state. As the internal lattice strains are relieved during annealing, the strength decreases while the ductility increases.
Recovery – During recovery, there is a rearrangement of internal defects, known as dislocations, into lower-energy configurations. However, the grain shape and orientation remain the same. There is also a significant reduction in residual stresses, but the strength and ductility are largely unaffected. Because there is a large decrease in residual stress during recovery, recovery-type processes are normally conducted to reduce residual stresses, often to prevent stress-corrosion cracking or minimize distortion. During stress-relief operations, the temperature and time are controlled so there is not a major reduction in strength or hardness.
Recrystallization – It is characterized by the nucleation and growth of strain-free grains out of the matrix of the cold-worked metal. During recrystallization, the badly deformed cold-worked grains are replaced by new, strain-free grains. New orientations, new grain sizes, and new grain morphologies are formed during recrystallization. The driving force for recrystallization is the remaining stored energy which is not expended during recovery. The strength reduces and the ductility increases to levels similar to those of the metal before cold working.
Recrystallization is considered complete when the mechanical properties of the recrystallized metal approach those of the metal before it was cold worked. Recrystallization and the resulting mechanical softening completely cancel the effects of cold working on the mechanical properties of the work piece. An annealing curve for an alloy, such as a typical steel, show minimal changes in mechanical properties during recovery and large changes in properties which occur during recrystallization. Mechanical properties, such as hardness, yield strength, tensile strength, percent elongation, and reduction in area, change drastically over a very small temperature range. Although physical properties, such as electrical conductivity, undergo large increases during recovery, they also continue to increase during recrystallization.
Grain growth – It is the growth of some recrystallized grains, and it can only happen at the expense of other recrystallized grains. Because fine grain size leads to the best combination of strength and ductility, in almost all cases, grain growth is an undesirable process. Although excessive grain growth can occur by holding the material for too long at the annealing temperature, it is normally a result of heating at too high a temperature.
Annealing is a generic term denoting a treatment which consists of heating to and holding at a suitable temperature followed by cooling at an appropriate rate, primarily for the softening of metallic materials. It is a process involving heating and cooling, normally applied to produce softening. The term also refers to treatments intended to alter mechanical or physical properties, produce a definite microstructure, or remove gases. The temperature of the operation and the rate of cooling depend upon the material being annealed and the purpose of the treatment.
Generally, in plain carbon (C) steels, annealing produces a ferritic-pearlitic microstructure (Fig. 1). Steels can be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability. The choice of an annealing treatment which provides an adequate combination of such properties at minimum expense often involves a compromise. Terms used to denote specific types of annealing applied to steels are descriptive of the method used, the equipment used, or the condition of the material after treatment. Fig 1 shows microstructures of steel showing the effect of annealing.
Fig 1 Microstructures of steel showing effect of annealing
The iron-carbon binary phase diagram (Fig 2) can be used to better understand annealing processes. Although no annealing process ever achieves true equilibrium conditions, it can closely parallel these conditions. In defining the various types of annealing, the transformation temperatures or critical temperatures are usually used.
Fig 2 Iron-carbon binary phase diagram
Critical temperatures – The critical temperatures which are to be considered in discussing annealing of steel are those which define the onset and completion of the transformation to or from austenite. For given steel, the critical temperatures depend on whether the steel is being heated or cooled. Critical temperatures for the start and completion of the transformation to austenite during heating are denoted, respectively, by Ac1 and Ac3 for hypo-eutectoid steels and by Ac1 and Accm (or simply Acm) for hyper-eutectoid steels. These temperatures are higher than the corresponding critical temperatures for the start and completion of the transformation from austenite during cooling, which are denoted, respectively, by Ar3 and Ar1 for hypo-eutectoid steels and by Arcm and Ar1 for hyper-eutectoid steels. (The ‘c’ and ‘r’ in the symbols are derived from the French words ‘chauffage’ for heating and ‘refroidissement’ for cooling). These critical temperatures converge to the equilibrium values Ae1, Ae3, and Aecm as the rates of heating or cooling become infinitely slow. The positions of the Ae1, Ae3, and Aecm lines are close to the more general (that is, near equilibrium) A1, A3, and Acm lines on the iron-carbon binary phase diagram shown in Fig 2.
Different alloying elements distinctly affect these critical temperatures. As an example, chromium (Cr) raises the eutectoid temperature, A1 temperature, and manganese (Mn) lowers it. It is possible to calculate upper and lower critical temperatures using the actual chemical composition of the steel. The equations which give an approximate critical temperature for a hypo-eutectoid steel are (i) Ac1 (deg C) = 723 – 20.7(% Mn) – 16.9(% Ni) + 29.1(% Si) – 16.9(% Cr) with a standard deviation of +/- 11.5 deg C, and (ii) Ac3 (deg C) = 910 – 203 % C – 15.2(% Ni) + 44.7(% Si) + 104(% V) + 31.5(% Mo) with a standard deviation of +/- 16.7 deg C.
The presence of other alloying elements also has distinct effects on these critical temperatures. The equilibrium critical temperatures normally lie about midway between those for heating and cooling at equal rates. Since annealing can involve various ranges of heating and cooling rates in combination with isothermal treatments, the specific terms A1, A3, and Acm temperatures are used lesser while discussing the basic concepts.
Annealing cycles – In practice, specific thermal cycles of an almost infinite variety are used to achieve the various goals of annealing. These cycles fall into several broad categories which can be classified according to the temperature to which the steel is heated and the method of cooling used. The maximum temperature can be (i) below the lower critical temperature, A1 temperature (sub-critical annealing), (ii) above A1 temperature but below the upper critical temperature, A3 temperature in hypo-eutectoid steels, or Acm in hyper-eutectoid steels (inter-critical annealing), or (iii) above A3 temperature (full annealing).
Since some austenite is present at temperatures above A1 temperature, cooling practice through transformation is a crucial factor in achieving desired microstructure and properties. Accordingly, steels heated above A1 are subjected either to slow continuous cooling or to isothermal treatment at some temperature below A1 temperature at which transformation to the desired microstructure can occur in a reasonable amount of time.
Under certain conditions, two or more such cycles can be combined or used in succession to achieve the desired results. The success of any annealing operation depends on the proper choice and control of the thermal cycle, based on the metallurgical principles.
Sub-critical annealing does not involve formation of austenite. The prior condition of the steel is modified by such thermally activated processes as recovery, recrystallization, grain growth, and agglomeration of carbides. The prior history of the steel is, hence, an important factor. In as-rolled or as forged hypo-eutectoid steels containing ferrite and pearlite, sub-critical annealing can adjust the hardnesses of both the constituents, but excessively long times at temperature can be needed for considerable softening.
The sub-critical treatment is most effective when applied to hardened or cold-worked steels, which recrystallize readily to form new ferrite grains. The rate of softening increases rapidly as the annealing temperature approaches A1 temperature. Cooling practice from the sub-critical annealing temperature has very little effect on the established microstructure and resultant properties.
Austenite begins to form when the temperature of the steel exceeds A1 temperature. The solubility of C increases suddenly (nearly 1 %) near the A1 temperature. In hypo-eutectoid steels, the equilibrium structure in the inter-critical range between A1 temperature and A3 temperature consists of ferrite and austenite, and above A3 temperature the structure becomes completely austenitic. However, the equilibrium mixture of ferrite and austenite is not achieved immediately. As an example, the rate of solution for typical eutectoid steel is shown in Fig 3.
Fig 3 Austenitizing rate temperature curves for plain C eutectoid steel
Undissolved carbides can persist, especially if the austenitizing time is short or the temperature is near A1, causing the austenite to be in-homogeneous. In hyper-eutectoid steels, carbide and austenite coexist in the inter-critical range between A1 temperature and Acm temperature, and the homogeneity of the austenite depends on time and temperature. The degree of homogeneity in the structure at the austenitizing temperature is an important consideration in the development of annealed structures and properties. The more homogeneous structures developed at higher austenitizing temperatures tend to promote lamellar carbide structures on cooling, whereas lower austenitizing temperatures in the inter-critical range result in less homogeneous austenite, which promotes formation of spheroidal carbides.
Austenite formed when steel is heated above the A1 temperature transforms back to ferrite and carbide when the steel is slowly cooled below A1 temperature. The rate of austenite decomposition and the tendency of the carbide structure to be either lamellar or spheroidal depend largely on the temperature of transformation. If the austenite transforms just below A1 temperature, it decomposes slowly. The product then can contain relatively coarse spheroidal carbides or coarse lamellar pearlite, depending on the composition of the steel and the austenitizing temperature. This product tends to be very soft.
However, the low rate of transformation at temperatures just below A1 temperature necessitates long holding times in isothermal treatments, or very slow cooling rates in continuous cooling, if maximum softness is desired. Isothermal treatments are more efficient than slow continuous cooling in terms of achieving desired structures and softness in the minimum amount of time. Sometimes, however, the available equipment or the mass of the steel part being annealed can make slow continuous cooling the only feasible alternative. As the transformation temperature decreases, austenite normally decomposes more rapidly, and the transformation product is harder, more lamellar, and less coarse than the product formed just below A1 temperature. At still lower transformation temperatures, the product becomes a much harder mixture of ferrite and carbide, and the time necessary for complete isothermal transformation can again increase.
Temperature-time plots showing the progress of austenite transformation under isothermal transformation (IT) or continuous transformation (CT) conditions for many steels demonstrate the above principles. These IT or CT diagrams can be helpful in design of the annealing treatments for specific grades of steel, but their usefulness is limited since most published diagrams represent transformation from a fully austenitized, relatively homogeneous condition, which is not always desirable or obtainable in annealing.
In the continuous annealing process, an inter-critical annealing practice is used to develop dual-phase and tri-phase microstructures. In this practice, the steel is rapidly cooled from the inter-critical temperature. The rapid cooling causes the transformation of the pools of austenite to martensite.The final microstructure consists of islands of martensite in a ferritic matrix. Depending upon the alloy content of the austenite pools and the cooling conditions, the austenite cannot fully transform and the microstructure consists of martensite / retained austenite regions in a ferritic matrix.
Cooling after full transformation
After the austenite has been completely transformed, little else of metallurgical change can occur during cooling to room temperature. Extremely slow cooling can cause some agglomeration of carbides, and consequently, some slight further softening of the steel, but in this regard such slow cooling is less effective than high temperature transformation. Hence, there is no metallurgical reason for slow cooling after transformation has been completed, and the steel can be cooled from the transformation temperature as rapidly as feasible in order to minimize the total time required for the operation.
If transformation by slow continuous cooling has been used, the temperature at which controlled cooling can be stopped depends on the transformation characteristics of the steel. However, the mass of the steel or the need to avoid oxidation are practical considerations which can need retarded cooling to be continued below the temperature at which the austenite transformation ceases.
Effect of prior structure – The finer and more evenly distributed are the carbides in the prior structure, the faster is the rate at which austenite formed above A1 temperature approaches complete homogeneity. The prior structure, hence, can affect the response to annealing. When spheroidal carbides are desired in the annealed structure, preheating at temperatures just below A1 temperature sometimes is used to agglomerate the prior carbides in order to increase their resistance to solution in the austenite on subsequent heating. The presence of undissolved carbides or concentration gradients in the austenite promotes formation of a spheroidal, rather than lamellar, structure when the austenite is transformed. Preheating to enhance spheroidization is applicable mainly to hypo-eutectoid steels but also is useful for some hypereutectoid low-alloy steels.
Super-critical or full annealing
A common annealing practice is to heat hypo-eutectoid steels above the upper critical temperature (A3) to attain full austenitization. The process is called full annealing. In hypo-eutectoid steels (under 0.77 % C), super-critical annealing (that is, above the A3 temperature) takes place in the austenite region (the steel is fully austenitic at the annealing temperature). However, in hyper-eutectoid steels (above 0.77 % C), the annealing takes place above the A1 temperature, which is the dual-phase austenite-cementite region. Figure 4 shows the annealing temperature range for full annealing superimposed in the iron-carbon binary phase diagram from Fig 2. In general, an annealing temperature 50 deg C above the A3 temperature for hypo-eutectic steels and A1 temperature for hyper-eutectoid steels is adequate.
Fig 4 Iron-carbon binary phase diagram showing region of temperatures for full annealing
Austenitizing time and dead-soft steel – Hyper-eutectoid steels can be made extremely soft by holding for long periods of time at the austenitizing temperature. Although the time at the austenitizing temperature can have only a small effect on actual hardnesses (such as a change from 241 HB to 229 HB), its effect on machinability or cold-forming properties can be appreciable. Long-term austenitizing is effective in hyper-eutectoid steels since it produces agglomeration of residual carbides in the austenite. Coarser carbides promote a softer final product. In low C steels, carbides are unstable at temperatures above A1 and tend to dissolve in the austenite, although the dissolution can be slow.
Steels which have approximately eutectoid C contents normally form a lamellar transformation product if austenitized for very long periods of time. Long term holding at a temperature just above the A1 temperature can be as effective in dissolving carbides and dissipating C concentration gradients as is short term holding at a higher temperature.
Guiding principles for annealing
The metallurgical principles discussed above have been incorporated into the following seven number of rules, which can be used as guidelines for development of successful and efficient annealing schedules.
Rule number 1 – The more homogeneous is the structure of the as austenitized steel, the more completely lamellar is the structure of the annealed steel. Conversely, the more heterogeneous is the structure of the as austenitized steel, the more nearly spheroidal is the annealed carbide structure.
Rule number 2 – The softest condition in the steel is normally developed by austenitizing at a temperature less than 55 deg C above A1 temperature and transforming at a temperature (normally) less than 55 deg C below A1 temperature.
Rule number 3 – Since very long times can be needed for complete transformation at temperatures less than 55 deg C below A1 temperature, allow most of the transformation to take place at the higher temperature, where a soft product is formed, and finish the transformation at a lower temperature, where the time needed for completion of transformation is short.
Rule number 4 – After the steel has been austenitized, cool to the transformation temperature as rapidly as feasible in order to minimize the total duration of the annealing operation.
Rule number 5 – After the steel has been completely transformed, at a temperature which produces the desired microstructure and hardness, cool to room temperature as rapidly as feasible to decrease further the total time of annealing.
Rule 6 – For ensuring a minimum of lamellar pearlite in the structures of annealed 0.7 % C to 0.9 % C tool steels and other low alloy medium C steels, preheating is done for several hours at a temperature around 28 deg C below the lower critical temperature (A1) before austenitizing and transforming as usual.
Rule number 7 – For obtaining minimum hardness in annealed hypereutectoid alloy tool steels, heating is at the austenitizing temperature for a long time (around 10 hours to 15 hours), then transforming as usual.
These rules are applied most effectively when the critical temperatures and transformation characteristics of the steel have been established and when transformation by isothermal treatment is feasible.
From a practical point of view, most annealing practices have been established from experience. For many annealing applications, it is sufficient simply to specify that the steel be cooled in the furnace from a designated annealing (austenitizing) temperature.
Heating cycles which utilize austenitizing temperatures in the upper ends of the normal ranges results in pearlitic structures. Predominantly spheroidize structures are obtained when lower temperatures are used. When alloy steel is annealed to get a specific microstructure, greater precision is needed in specifying temperatures and cooling conditions for annealing.
For the majority of steels, annealing can be done by heating to the austenitizing temperature and then either cooling in the furnace at a controlled rate or cooling rapidly to, and holding at, a lower temperature for isothermal transformation. Both procedures result in virtually the same hardness. However, considerably less time is needed for isothermal transformation.
Uniformity of temperature
One potential contribution to the failure of an annealing operation is a lack of knowledge of the temperature distribution within the charge of steel in the furnace. Furnaces large enough to anneal around 20 tons of steel at a time are not uncommon. In some large forging shops, work pieces can weigh in excess of 300 tons. The larger the furnace, the more difficult it is to establish and maintain uniform temperature conditions throughout the charge, and the more difficult it is to change the temperature of the steel during either heating or cooling.
Furnace thermocouples indicate the temperature of the space above, below, or beside the charge, but this temperature can differ by 28 deg C or more from the temperature of the steel itself, especially when the steel is in a pipe or box, or when bar or strip is packed in a dense charge in a neutral atmosphere. When these conditions exist, the distribution of temperature throughout the load during heating and cooling is to be established by placing thermocouples among the bars, forgings, coils, and so on. A good practice is to spot weld a thermocouple to the work piece or to use embedded thermocouples (thermocouples placed in holes drilled into the work piece). Regulation of the furnace during the annealing operation is to be based on the temperatures indicated by these thermocouples, which are in actual contact with the steel, rather than on the temperatures indicated by the furnace thermocouples.
The majority of all spheroidizing activity is performed for improving the cold formability of steels. It is also performed to improve the machinability of hyper-eutectoid steels, as well as tool steels. A spheroidized microstructure is desirable for cold forming since it lowers the flow stress of the material. The flow stress is determined by the proportion and distribution of ferrite and carbides. The strength of the ferrite depends on its grain size and the rate of cooling. Whether the carbides are present as lamellae in pearlite or spheroids thoroughly affects the formability of steel. Steels can be spheroidized, that is, heated and cooled to produce a structure of globular carbides in a ferritic matrix.
Figure 5 shows microstructure of a eutectoid steel containing 0.77 % C with all cementite in the spheroidal form. Spheroidization can take place by the many methods namely (i) prolonged holding at a temperature just below Ae1, (ii) heating and cooling alternately between temperatures which are just above Ac1 and just below Ar1, (iii) heating to a temperature just above Ac1, and then either cooling very slowly in the furnace or holding at a temperature just below Ar1, (iv) cooling at a suitable rate from the minimum temperature at which all carbide is dissolved to prevent reformation of a carbide network, and then reheating in accordance with the first or second methods above (applicable to hypereutectoid steel containing a carbide network). Fig 5 shows the range of temperatures used for spheroidization of hypo-eutectoid and hyper-eutectoid steels. The rates of spheroidizing provided by these methods depend somewhat on prior microstructure, being greatest for quenched structures in which the carbide phase is fine and dispersed. Prior cold work also increases the rate of the spheroidizing reaction in a sub-critical spheroidizing treatment.
It is to be noted that it is difficult to establish consistent designations for critical temperatures. In discussions about heating with prolonged holding, the critical temperatures of interest are to be the equilibrium temperatures Ae1 and Ae3. Terminology becomes more arbitrary in discussions of heating and cooling at unspecified rates and for unspecified holding times.
Fig 5 Spheroidized microstructure and the iron-carbon binary phase diagram showing region of temperatures for spheroidizing
The effect of prior microstructure on spheroidization can be seen by giving the same time / temperature heating cycle to two samples (one with a prior martensitic microstructure, and the second with a prior ferrite-pearlite microstructure) and holding both the samples for 21 hours at 700 deg C. It can be seen that the spheroidization has occurs in the steel sample with the prior martensitic microstructure. On the other hand, it has just begun in the same steel with the prior ferrite-pearlite microstructure. It is seen that after 200 hours at 700 deg C, the spheroidization process is almost completed in the prior ferrite-pearlite steel. However, traces of the pearlitic areas can still be seen.
For full spheroidizing, austenitizing temperatures either slightly above the Ac1 temperature or around midway between Ac1 and Ac3 are used. If a temperature slightly above Ac1 is to be used, good loading characteristics and accurate temperature controls are needed for proper results, otherwise, it is conceivable that Ac1 cannot be reached and that austenitization cannot occur.
Low C steels are seldom spheroidized for machining, because in the spheroidized condition they are excessively soft and ‘sticky’, cutting with long, tough chips. When low C steels are spheroidized, it is generally to permit severe deformation. For example, when 0.2 % C steel pipe is being produced by cold drawing in two or three passes, a spheroidized structure is achieved if the material is annealed for 0.5 hours to 1 hour at 690 deg C after each pass. The final product has a hardness of about 163 HB. Pipes in this condition are able to withstand severe deformation during subsequent cold forming. As with many other types of heat treatment, hardness after spheroidizing depends on C and alloy content. Increasing the C or alloy content, or both, results in an increase in the as spheroidized hardness, which generally ranges from 163 HB to 212 HB.
As the hardness of steel increases during cold working, ductility decreases and additional cold reduction becomes so difficult that the steel material is to be annealed to restore its ductility. Such annealing between processing steps is referred to as in-process or simply process annealing. It can consist of any appropriate treatment. In most cases, however, a sub-critical treatment is adequate and least costly, and the term ‘process annealing; without further qualification normally refers to an in-process sub-critical anneal. Fig 6 shows the range of temperatures typically used for process annealing. It is frequently necessary to specify process annealing for parts which are cold formed by stamping, heading, or extrusion. Hot worked high C and alloy steels also are process annealed to prevent them from cracking and to soften them for shearing, turning, or straightening.
Fig 6 Iron-carbon binary phase diagram showing region of temperature for process annealing
Process annealing normally consists of heating to a temperature below Ae1, soaking for an appropriate time and then cooling, usually in air. In the majority of the cases, heating to a temperature between 10 deg C and 20 deg C below Ae1 produces the best combination of microstructure hardness, and mechanical properties. Temperature controls are necessary only to prevent heating the material above Ae1 and thus defeating the purpose of annealing.
When process annealing is performed merely to soften a material for such operations as cold sawing and cold shearing, temperatures well below Ae1 normally are used and close controls are unnecessary.
In the wire industry, process annealing is used as an intermediate treatment between the drawing of the wire to a size slightly larger than the desired finished size and the drawing of a light reduction to the finished size. Wire thus made is known as annealed in process wire. Process annealing is used also in the production of wire sufficiently soft for severe upsetting and to permit drawing the smaller sizes of low C and medium C steel wire which cannot be drawn to the desired small size directly from the hot rolled rod. Process annealing is more satisfactory than spheroidize annealing for a material which, because of its composition or size (or both), cannot be drawn to finished size because it either lacks ductility or does not meet physical requirements. Also, material which is cold sheared during processing is process annealed to raise the ductility of the sheared surface to a level suitable for further processing.
Annealed structures for machining
Different combinations of microstructure and hardness, considered together, are significant in terms of machinability. Based on many observations, optimum microstructure for machining steels of various carbon contents are given in Tab 1.
|Tab 1 Optimum microstructure for machining steels of various carbon contents
|As-rolled (most economical)
|Under 75 mm diameter – normalized, 75 mm diameter and over – as-rolled
|Annealed, to produce coarse pearlite, minimum ferrite
|Coarse lamellar pearlite to coarse spheroidized carbides
|100 % spheroidized carbides, coarse to fine
The type of machining operation is also a factor. For example, certain gears are made from 5160 grade steel pipe by the dual operation of machining in automatic screw machines and broaching of cross slots. The screw-machine operations are easiest with thoroughly spheroidized material, but a pearlitic structure is more suitable for broaching. A semi-spheroidized structure proved to be a satisfactory compromise.
Semi-spheroidized structures can be achieved by austenitizing at lower temperatures, and sometimes at higher cooling rates, than those used for achieving pearlitic structures. The semi-spheroidized structure of the 5160 grade steel pipe mentioned above is achieved by heating to 790 deg C and cooling at 28 deg C/hour to 650 deg C. For this steel, austenitizing at a temperature of around 775 deg C results in more spheroidization and less pearlite.
Medium C steels are much more difficult to fully spheroidize than are high C steels such as grades 1095 and 52100. In the absence of excess carbides to nucleate and promote the spheroidizing reaction, it is more difficult to achieve complete freedom from pearlite in practical heat-treating cycles. At lower C levels, structures consisting of coarse pearlite in a ferrite matrix frequently are found to be the most machinable. In some alloy steels, this type of structure can best be achieved by heating to temperatures well above Ac3 to establish a coarse austenite grain size, then holding below Ar1 to allow coarse, lamellar pearlite to form. This process sometimes is referred to as cycle annealing or lamellar annealing. For example, forged 4620 grade steel gears are heated rapidly in a 5 zone furnace to 980 deg C, cooled to 625 deg C to 640 deg C in a water-cooled zone, and held at that temperature for 120 minutes to 150 minutes. The resulting structure is coarse, lamellar pearlite in a ferrite matrix and has a hardness of 140 HB to 146 HB.