Heat Treatment Processes for Steel
Heat Treatment Processes for Steel
Steels can be heat treated to produce a large range of microstructures and properties. Generally, heat treatment uses phase transformation during heating and cooling to change the microstructure in a solid state. In heat treatment, the processing is normally thermal and which modifies only the structure of the steel. In case of thermo-mechanical treatment process of steels, the shape and structure of the steel components also gets modified. In case of thermo-chemical process of steels, the surface chemistry and structure of the steel gets modified. Both the thermo-mechanical and thermo-chemical treatment processes are also important processing approaches for heat treatment of steel and these are being considered in the domain of heat treatment. Heat treatment processes requires close control over all the factors affecting the heating and cooling of the steel. The atmosphere of the heating furnace also affects the condition of the steel being heat-treated.
All the heat-treating processes consist of subjecting the steel to a definite time-temperature cycle. This time-temperature cycle has three components namely (i) heating, (ii) holding at particular temperature range (soaking), and (iii) cooling. Individual cases can differ, but certain fundamental objectives are there.
The heating rate of a part depends on several factors. These factors are (i) heat conductivity of the steel, (ii) the condition of the steel, and (iii) the size and the cross-section of the steel. The heat conductivity of the steel is an important factor. The steel with high-heat conductivity is heated up at a faster rate than one with a low conductivity. The rate of heating is not particularly important unless the steel is in a highly stressed condition, such as is imparted by severe cold working or prior hardening. In such cases the rate of heating is to be slow. Frequently this is not practicable, since the furnaces used for heating can be at operating temperatures and placing the cold steel in the hot furnace can cause distortion or even cracking. This danger can be minimized by the use of a preheating furnace which is maintained at a temperature below the A1 temperature in the iron-carbon phase diagram (Fig 1). The steel, preheated for a sufficient period, is then transferred to the furnace which is at the operating temperature. This procedure is also advantageous when treating steels having considerable variations in section thickness or which have very low thermal conductivity.
Fig 1 Iron-carbon phase diagram
After the steel section is heated to the proper temperature, it is held at that temperature until the desired internal structural changes take place. This process is called ‘soaking’. The length of time held at the proper temperature is called the ‘soaking period’. The objective of soaking is to ensure uniformity of temperature throughout its entire volume. Apparently, thin sections need not be soaked as long as thick sections, but if different thicknesses exist in the same steel piece, the period required to heat the thickest section uniformly decides the time at temperature. As a thumb rule around 30 minutes of soaking is needed for a section having 25 mm thickness.
After the steel section has been soaked, it is to be returned to room temperature to complete the heat-treating process. To cool the metal, a cooling medium can be used. The cooling medium can be composed of a gas, liquid, solid, or combination of these. The rate at which the steel section is cooled depends on the steel and the properties desired. The rate of cooling depends on the cooling medium, and hence, the choice of a cooling medium has an important influence on the properties required. The micro-structure and properties of steel depend upon the rate of cooling of the steel and this, in turn, is governed by such factors as mass, quenching medium, etc. It is to be understood that the thicker is the steel section, the slower is the rate of cooling regardless of the method of cooling used except in such operations as induction hardening.
The various types of heat-treating processes are similar because they all involve the heating and cooling of the steels. However, the processes differ in the heating temperatures and the cooling rates used and the final results. The normal processes used for the heat treatment of steels are (i) annealing, (ii) normalizing, (iii) hardening, and (iv) tempering.
Annealing is a heat treatment process which involves heating and cooling. The process is normally used for the softening of steel. The term also refers to treatments intended to alter the mechanical or the physical properties, to produce a definite microstructure, or to remove gases. The temperature of the operation and the rate of cooling depend upon the type of the steel being annealed and the purpose of the treatment. The different types of annealing processes are described below.
Full annealing – It is a softening process in which the steel section is heated to a temperature above the austenitic transformation range and after being held for a sufficient time at this temperature, is cooled slowly to a temperature below the transformation range. The steel is generally allowed to cool slowly in the furnace, although it can be removed and cooled in some cooling medium. Since the transformation temperatures are affected by the carbon content of the steel, it is obvious that the high carbon steels can be fully annealed at lower temperatures than the low carbon steels. The microstructure of the hypo-eutectoid steels which result after full annealing consists of ferrite and pearlite. Eutectoid and hyper-eutectoid steels often get spheroidize partially or completely on full annealing.
Process annealing – Process annealing is also frequently called as stress relief annealing. The process is generally used for the cold-worked low carbon steels (upto around 0.25 % of carbon) to soften the steel sufficiently so that further cold working can be done. The steel is normally heated close to, but below, the A1 temperature. If the steel is not to be further cold-worked, but relief of internal stresses is required, then a lower range of temperature is adequate (around 540 deg C). Here, rate of cooling is not important. This type of annealing causes recrystallization and softening of the cold-worked ferrite grains, but normally does not affect the relatively small amounts of cold-worked pearlite.
Spheroidizing is a process of heating and cooling steel which produces a rounded or globular form of carbide in a matrix of ferrite. It is generally carried out by prolonged heating at temperatures just below the A1 temperature, but can be facilitated by alternately heating to temperatures just above the A1 temperature and cooling to just below the A1 temperature. The final step, however, consists of holding at a temperature just below the critical temperature A1. The rate of cooling is not important after slowly cooling to around 540 deg C. The rate of spheroidization is affected by the initial structure. The finer the pearlite, the more readily spheroidization takes place. A martensitic structure is very receptive to spheroidization. This treatment is normally used for the high carbon steels (0.60 % of carbon and above). The purpose of this treatment is to improve the machinability of the steel. The process is also used to condition high-carbon steel for cold-drawing into wire.
Normalized treatment is frequently applied to the steel in order for the achievement of any one or more of these objectives, namely (i) to refine the grain structure, (ii) to obtain uniform structure, (iii) to decrease residual stresses, and (iv) to improve the machinability of the steel.
Normalizing is a process in which steel is heated, to a temperature above the A3 or the Acm temperatures and then cooled in atmospheric air. The purpose of the normalizing treatment is to remove the effects of any previous heat treatment (including the coarse grained structure sometimes resulting from high forging temperatures) or cold-working. The normalizing process is done to ensure a homogeneous austenite on reheating for hardening or full annealing. The resultant structures are pearlite or pearlite with excess ferrite or cementite, depending upon the composition of the steel.
The structures after normalizing are different from the structures resulting after annealing and the steels of the same carbon content in the hypo-eutectoid or hyper-eutectoid ranges, there is less excess ferrite or cementite and the pearlite is finer. These are the results of the more rapid cooling. Since the type of structure, and, hence, the mechanical properties, are affected by the rate of cooling, substantial variations can take place in normalized steels because of differences in section thickness of the shapes being normalized.
Steels can be hardened by the simple means of heating the steel to a temperature higher than the A3 transformation temperature, holding long enough to ensure the achievement of uniform temperature and solution of carbon in the austenite, and then cooling the steel rapidly (quenching). Complete hardening depends on cooling so rapidly that the austenite, which does not decompose on cooling through the A1 temperature and is maintained at relatively low temperatures. When this is accomplished, the austenite start transforming to martensite on cooling below the Ms temperature (around 220 deg C) and is completely transformed to martensite below Mf temperature. Rapid cooling is necessary only to the extent of lowering the temperature of the steel to well below the nose of the S curve (Fig 2). Once this is achieved then slow cooling from then on, either in oil or in air, is beneficial for avoiding distortion and cracking. Special treatments, such as time quenching and mar-tempering, are designed to bring about these conditions. As martensite is quite brittle, steel is rarely used in the as-quenched condition, that is, without tempering. The maximum hardness which can be achieved in completely hardened low-alloy steels and plain carbon structural steels depends primarily on the carbon content.
Fig 2 Time-temperature-transformation curve
Effect of mass – The mass of the steel has its influence on the formation of the martensite. It can be seen that even with a sample of relatively small dimensions, the rate of removal of heat is not uniform. Heat is always removed from the surface layers at a faster rate than from the inner potion. In a given cooling medium, the cooling rate of both the surface and inner portion decreases as the dimensions of a sample increase and the possibility of exceeding the critical cooling rate becomes less. To overcome this, the steel is required to be quenched in a medium which is having a very high rate of heat removal, such as iced brine, but, even so, many steels have a physical restriction on the maximum size responsive to complete hardening regardless of the quenching medium. The marked effect which the mass has upon the hardness of quenched steel can be shown by measuring the hardness distribution of different size rounds of the same steel quenched in the same medium.
Effect of carbon – Content of carbon in the plain carbon and low alloy steels influences the Ms transformation temperature. As the carbon content increases, the Ms temperature is reduced (Fig 3).
Fig 3 Influence of carbon on Ms temperature
Tempering (sometimes called drawing) is the process of reheating hardened (martensitic) or normalized steels to some temperature below the A1 temperature. The rate of cooling is not important except for some steels which are susceptible to temper brittleness. As the tempering temperature is increased, the martensite of the hardened steel passes through stages of tempered martensite and is gradually changed into a structure consisting of spheroids of cementite in a matrix of ferrite (formerly termed as sorbite). These changes are accompanied by a decreasing hardness and increasing toughness.
The tempering temperature depends upon the desired properties and the purpose for which the steel is to be used. If substantial hardness is essential, then the tempering temperature is to be low. On the other hand, if substantial toughness is needed, then the tempering temperature is to be high. Proper tempering of hardened steel needs a certain amount of time. At any selected tempering temperature, the hardness drops rapidly at first, gradually decreasing more slowly as the time is prolonged. Short tempering periods are normally undesirable and are to be avoided. Good practice needs at least 30 minutes (or preferably, 1 to 2 hours) at tempering temperature for any hardened steel.
The necessity for tempering the steel promptly after hardening cannot be overstressed. If fully hardened steel is allowed to cool to room temperature during hardening there is a danger of the cracking of the steel. Carbon steels and most of the low alloy steels are required to be tempered as soon as they are cool enough to be held comfortably in the bare hands. Steels are not to be tempered before they cool to this temperature because in some steels the Mf temperature is quite low and untransformed austenite can be present. Part of all of this residual austenite transforms to martensite on cooling from the tempering temperature so that the final structure consists of both tempered and untempered martensite. The brittle untempered martensite, together with the internal stresses caused by its formation, can easily cause failure of the heat-treated steel part. When it is possible that such a condition exists, a second tempering treatment (double tempering) is to be given to temper the fresh martensite formed on cooling after the initial tempering treatment.
If structural steels are to be used in the normalized condition, the normalizing operation is frequently followed by heating to a temperature of around 650 deg C to 700 deg C. The purpose of this treatment, which is also designated as tempering, is to relieve internal stresses resulting on cooling from the normalizing temperature and to improve the ductility of the steel.
Case hardening is a process of hardening a ferrous alloy so that the surface layer or case is made considerably harder than the interior or core. The chemical composition of the surface layer is altered during the treatment by the addition of carbon, nitrogen, or both. The most commonly used case-hardening processes are carburizing, cyaniding, carbo-nitriding, and nitriding.
Carburizing – Carburizing is a process which introduces carbon into a solid ferrous alloy by heating the metal in contact with a carbonaceous material to a temperature above the A3 temperature of the steel and holding at that temperature. The depth of penetration of carbon is dependent on the temperature, the time at the temperature, and the composition of the carburizing agent. As a rough indication, a carburized depth of around 0.75 mm to 1.25 mm can be achieved in around 4 hours at 930 deg C, depending upon the type of carburizing agent, which can be a solid, a liquid, or a gas. Since the primary objective of the carburizing is to get a hard case and a relatively soft, tough core, only low-carbon steels (upto a maximum of around 0.25 % of carbon), either with or without alloying elements (nickel, chromium, manganese, or molybdenum), are normally used. After carburizing, the steel has a high-carbon case graduating into the low-carbon core.
A variety of heat treatments can be used subsequent to carburizing, but all of them involve quenching the steel to harden the carburized surface layer. The simplest treatment consists of quenching the steel directly from the carburizing temperature. This treatment hardens both the case and core (in so far as the core is capable of being hardened). Another simple treatment, and perhaps the one most frequently used, consists of slowly cooling from the carburizing temperature, reheating to above the A3 temperature of the case (around 775 deg C),and quenching. This treatment hardens the case only. A more complex treatment is to double quench consisting of first quenching from above the A3 temperature of the core (around 900 deg C for low-carbon steel) and then from above the A3 temperature of the case (around 775 deg C). This treatment refines the core and hardens the case. The plain carbon steels are almost always quenched in water or brine while the alloy steels are usually quenched in oil.
Although tempering following hardening of carburized steel is sometimes omitted, a low-temperature tempering treatment at around 300 deg C is a good practice. It is sometimes desirable to carburize only certain parts of the surface. This can be done by covering the surface to be protected against carburizing with some material which prevents the passage of the carburizing agent. The most widely used method is copper plating of the surfaces to be protected. Several proprietary solutions or pastes, which are quite effective in preventing carburization, are also available. The commercial compounds commonly used for pack (solid) carburizing contain mixtures of carbonate (usually barium carbonate), coke (diluent), and hardwood charcoal, with oil, tar, or molasses as a binder. Mixtures of charred leather, bone, and charcoal are also used. The carburizing action of these compounds is diminished during use and it is necessary to add new material before the compound is reused. Addition of one part of unused to three to five parts of used compound is common practice.
The parts to be carburized are packed in boxes (or other suitable containers) made of heat-resistant alloys, although rolled or cast steel can also be used where long life of the box is not important. The top cover of the box is to be sealed with fire clay or some other refractory to help prevent escape of the carburizing gas generated at the carburizing temperature. The depth and uniformity of case is affected by the method of packing and design of the container. Liquid carburizing consists of case hardening steel or iron in molten salt baths which contain mixtures principally of cyanides (violently poisonous), chlorides, and carbonates. The case produced by this method contains both carbon and nitrogen, but principally the former. The temperatures used range from around 850 deg C to 900 deg C or higher, depending upon the composition of the bath and the desired depth of the case. At 900 deg C a case depth of around 0.25 mm to 0.4 mm can be achieved in 1 hour and around 0.5 mm to 0.75 mm can be achieved in 4 hours. Considerably deeper cases can be obtained at higher temperatures with longer periods of time.
After carburizing, the steel is to be quenched just as in solid carburizing, but it is usual done directly from the molten bath. In all present-day commercial gas carburizing, two or more hydrocarbons are used in combination for supplying the carbon to the steel. The hydrocarbons used are methane, ethane, propane, and oil vapours. The steel parts are placed in sealed containers through which the carburizing gases are circulated. The temperatures used are in the vicinity of 925 deg C. Average expectation for depth of case in gas-carburized steel is shown in Fig 4. After carburizing, the steel is to be quench-hardened.
Fig 4 Relation of time and temperature to carbon penetration in gas carburizing
Cyaniding – A hard, superficial case can be achieved speedily on low-carbon steels by cyaniding. This process involves the introduction of both carbon and nitrogen into the surface layers of the steel. Steel to be cyanided is normally heated in a molten bath of cyanide-carbonate-chloride salts (normally containing 30 % to 95 % of sodium cyanide) and then quenched in brine, water, or mineral oil. The temperature of operation is generally within the range of 850 deg C to 875 deg C. The depth of case is a function of time, temperature, and composition of the cyanide bath. The time of immersion is relatively short as compared with carburizing, generally varying from around 15 minutes to 2 hours. The maximum case depth is rarely more than around 0.5 mm and the average depth is considerably less.
Steels can be cyanided also by heating to the proper temperature and dipping in a powdered cyanide mixture or sprinkling the powder on the steel, followed by quenching. The case thus formed is extremely thin. Cyaniding salts are extremely poisonous, if allowed to come in contact with scratches or wounds. They are fatally poisonous if taken internally. Fatally poisonous fumes are evolved when cyanides are brought into contact with acids. Cyaniding baths are to be equipped with a hood for venting the gases evolved during heating and the work room is to be well ventilated. Molten cyanide is never to be permitted to come in contact with sodium or potassium nitrates normally used for baths for tempering as the mixtures are explosive. Also, care is necessary in preparing a salt bath and the steel is to be completely dry before placing in the molten bath. The recommendations of the salt manufacturers are to be sincerely followed in the operation and maintenance of salt baths.
Carbonitriding – Carbonitriding is also known as gas cyaniding, dry cyaniding, and ni-carbing. It is a process for case hardening a steel part in a gas-carburizing atmosphere which contains ammonia in controlled percentages. Carbonitriding is used mainly as a low-cost substitute for cyaniding. As in the cyaniding process, both carbon and nitrogen are added to the steel. The process is carried out above the A1 temperature of the steel, and is practical upto 925 deg C. Quenching in oil is sufficiently fast to attain maximum surface hardness. This moderate rate of cooling tends to minimize distortion. The depth to which carbon and nitrogen penetrate varies with the temperature and time. The penetration of carbon is roughly the same as that obtained in gas carburizing (Fig 3).
Nitriding – The nitriding process consists in subjecting machined and heat-treated steel, free from surface decarburization, to the action of a nitrogenous medium, generally ammonia gas, at a temperature of around 500 deg C to 540 deg C. A very hard surface is obtained by this process. The surface-hardening effect is due to the absorption of nitrogen and subsequent heat treatment of the steel becomes not necessary. The time required is relatively long, normally being 1 day to 2 days. The case, even after 2 days of nitriding, is generally less than 0.5 mm. And the highest hardness exists in the surface layers to a depth of only a few hundredth of a millimeter.
Special low-alloy steels have been developed for nitriding. These steels contain elements which combine readily with nitrogen to form nitrides. The most favourable of these elements are aluminum, chromium, and vanadium. Molybdenum and nickel are used in these steels to add strength and toughness. The carbon content usually is around 0.20 % to 0.50 %, although in some steels, where high core hardness is necessary, it can be as high as 1.3 %. Stainless steels can also be subjected to nitriding.
Since nitriding is carried out at a relatively low temperature, it is advantageous to use quenched and tempered steel as the base material. This gives a strong, tough core with an intensely hard wear-resistant case which is much harder, indeed, than can be obtained by quench hardening either carbonized or cyanided steel. Although warpage is not a problem during the nitriding, steels increase slightly in size during this treatment. Allowance can be made for this growth in the finished article. Protection against nitriding can be achieved by tin, copper, or bronze plating, or by the application of certain paints.
It is often necessary to harden only the surface of steels without altering the chemical composition of the surface layers. If the steel contains sufficient carbon to respond to hardening, it is possible to harden the surface layers only by very fast heating for a short period, thus conditioning the surface for hardening by quenching.
Induction hardening – In induction hardening, a high-frequency current is passed through a coil surrounding the steel, the surface layers of which are heated by electro-magnetic induction. The depth to which the heated zone extends depends on the frequency of the current (the lower frequencies giving the greater depths) and on the duration of the heating cycle. The time required to heat the surface layers to above A3 temperature is surprisingly small, frequently being a matter of only a few seconds. Selective heating (and hence hardening) is accomplished by suitable design of the coils or inductor blocks. At the end of the heating cycle, the steel is usually quenched by water jets passing through the inductor coils. Precise methods for controlling the operation, that is, rate of energy input, duration of heating, and rate of cooling, are necessary. These features are incorporated in induction hardening equipment, which is usually fully automatic in operation.
Flame hardening– It is a process of heating the surface layers of steel above the transformation temperature by means of a high-temperature flame and then quenching. In this process, the gas flames impinge directly on the steel surface to be hardened. The rate of heating is very fast, although not so rapid as with the induction heating. Plain carbon steels are usually quenched by a water spray, whereas the rate of cooling of alloy steels can be varied from a rapid water quench to a slow air cool depending on the composition. Any type of hardenable steel can be flame hardened. For best results, the carbon content is to be at least 0.35 %, the usual range being 0.40 % to 0.50 %.
Special treatment processes
Special treatment processes normally include austempering, martempering, and cold treatment
Austempering – Austempering is a trade name given to a patented heat-treating process. Basically, it consists of heating steel to above the A3 transformation temperature and then quenching into a hot bath held at a temperature below that at which fine pearlite gets formed (the nose of the S-curve, Fig 2), but above the Ms temperature as shown in Fig 3. The product of isothermal decomposition of austenite in this temperature region is bainite. This constituent combines relatively high toughness and hardness.
The austempering process has certain limitations which make it not practicable for use with many steels. In order to assure a uniform structure (and hence uniform properties), it is essential that the entire cross section of the steel is cooled rapidly enough so that even the centre escapes transformation at the nose of the S-curve. In carbon steels, the time required to start transformation at the nose of the S-curve is extremely short, so that only relatively small sections (around 10 mm maximum thickness) can be successfully hot quenched in austempering baths. The time required for transformation of the austenite of alloy steels to fine pearlite is usually longer, and hence larger sections can be successfully austempered (around 25 mm maximum). However, the time required for transformation to bainite often becomes inordinately long with many alloy steels and hence, the process of austempering is normally not practicable for these steels.
Martempering – Martempering consists of heating steel to above its A3 transformation temperature and then quenching into a bath held at a temperature approximately equal to that of its Ms temperature. The steel is maintained in the hot bath until its temperature is essentially uniform and then it is cooled in air. Severe internal stresses develop in the steel during hardening. Steel contracts during cooling but undergoes a marked expansion when the austenite transforms to martensite. Since the quenched steel is to cool from the surface inward, various portions transform at different times. The steel is thus subjected to a variety of differential expansions and contractions, resulting in considerable internal stresses. By equalizing the temperature throughout the section before transformation takes place, and then cooling slowly through the martensite (Ms-Mf) range, the internal stresses are considerably reduced, which is also the prime objective of martempering.
Cold treatment – The Mf temperature of many alloy steels is so low that complete transformation of austenite to martensite does not occur on quenching to room temperature or on cooling after tempering. This retained austenite can be partially or completely transformed by cooling below atmospheric temperatures and such treatment is called the ‘cold treatment’. The beneficial effects of cold treatment have not been fully explored. It is known that the retained austenite of highly alloyed steels is frequently difficult to transform. Cooling these steels to low temperatures (to the temperature of solid CO2 or even lower) immediately after the quenching is sometimes effective in transforming the retained austenite, but with the associated danger of cracking. When the cold treatment is applied after tempering, the retained austenite is considerably more resistant to transformation. If cold treatment is used, the steel is always to be tempered afterwards.
Repeated alternate heating to a temperature slightly below which is used in tempering and cooling to a subzero temperature in a refrigerated iced brine, carbon dioxide, liquid air, or liquid nitrogen is generally used for transforming the retained austenite (dimensional stabilization) of steel gauges, especially those of the ball-bearing type composition.