HEAT TREATMENT OF DIFFERENT MATERIALS

Heat treatment of different materials

------->  Introduction:

Numerous plain carbon and alloy steels are in use today. Each one of them finds a wide range of applications depending on service requirements, fabricability and economy. For some of the well-defined groups, generalized heat treatment practice has been given in this chapter. And also some steels of commercial importance and their  heat treatment, alloy steels and its heat treatment, and heat treatment of some special steels also have been given.

1  Plain carbon steels:

Plain carbon steels are alloys of iron and carbon in which carbon varies from traces to about 2 percent by weight. The upper limit of carbon in plain carbon steels corresponds to the maximum solubility of carbon in austenite. Commercial steels, in addition to carbon, contain elements such as manganese, silicon, sulphur and phosphorus. These elements are always present in all steels. Sulphur and phosphorus are highly detrimental and are treated as undesirable elements. On the basis of carbon contents, plain carbon steels can be divided into three classes, namely, low carbon steels, medium carbon steels, and high carbon steels.

2  Low carbon steels:

Low carbon steels contain carbon up to 0.25 % (by weight). Low carbon steels are cheap and possess good formability and excellent weld­ability. These steels are extensively used as sheet and strip steel, structural steel, cold heading steel, free cutting steel and case hardening steels.

Low carbon steels exhibit poor response to heat treatment as a means for improving mechanical properties, particularly tensile and yield strength. The major micro constituent of these steels is ferrite. Hence marginal diffe­rences in mechanical properties are observed in annealed and normalized low carbon steels only. The magnitude of difference in mechanical properties increases with rise in carbon content. In low carbon steels, hardly any martensite is formed on quenching. Hence on hardening by quenching, there is no significant improvement in mechanical properties. But this does not mean that they are not subjected to heat treatment process at all. In fact, many heat treatment processes are an integral part of the manufacturing and fabrication processes of low carbon steels. Low carbon steels can be grouped into two classes: low carbon steels up to O. I 0 % carbon, and low carbon steels with carbon from 0.10 % to 0.25 %.

Low carbon steels with less than 0.10 percent carbon have excellent formability. For this reason, these steels are employed for general engineer­ing constructional work involving severe cold deformation such as bending, riveting and deep drawing. These steels, in the form of cold-rolled sheets, find applications in automobile, furniture and refrigerator industries. Tin cans, tin plates and galvanized sheets are some other applications of these cold-rolled sheets. Annealing plays an important role during the manufac­ture of cold-rolled sheets. The manufacturing process essentially consists of a combination of cold working and annealing treatment. In addition to using them as cold-rolled sheets, these steels are

Description
Composition	0.10 - 0.15% C	0.15 - 0.25% C
Annealing, Temperature (0C)	880 - 930	880 – 920
Normalizing, Temperature (0C)	880 - 930	880 – 920
Hardening, Temperature (0C)	770 - 800	770 – 800
Quenching media	Water	Water
Carburizing, Temperature (0C)	880 - 930	880 – 930
Refining treatment after carburizing	Heating to 760-7800C, followed by water quenching	Heating to 760-7800C, followed by water quenching
Tempering temp.,(0C)	150 - 200	150 - 200

 

employed extensively as soft magnetic material. Removal of last traces of carbon 'from steel to convert it into pure iron is a costly process. These steels find applications such as for yokes in electrical machines and armature in switchgears. Low carbon steels as magnetic material are always subjected to the annealing, which ensures the attainment of optimum magnetic properties. This anneal­ing differs from conventional annealing in the sense that heat treatment temperature is significantly high (I000-1200°C) and holding time is considerably longer in this case. Such an annealing treatment results in alignment of magnetic domains.

Steels with 0.10-0.25 % carbons possess high strength and tough­ness values. On case carburizing, these steels develop a hard and wear resis­tant case with a tough core. Carburized steel is always subjected to further heat treatment to refine case or core or both. These steels are very well suited for light duty general engineering

purposes such as welded structures, riveted structures, forgings and machined parts.

Some typical applications include ship plates, boilerplates, cams, and shafts, stay bolts, wheel hubs, brake housings and brake pedal levers. These steels are given normalizing heat treatment in order to improve machinability.

2  Medium carbon steels:

Steels with carbon varying from 0.25 % to 0.65 % are referr­ed to as medium carbon steels. these steels respond to heat treat­ment is much better manner than that of low carbon steels. In order to have maxi­mum advantage, these steels are always used in heat-treated condition. Depending on this response to heat treatment, they can be divided into two groups. The first group consists of steels with 0.25-0.35 % carbons, and the second group consists of steels with 0.35-0.65 % carbons. The response of the first group is not very encouraging but the second is excellent. It is for this reason that steels of the second group are generally used in hardened and tempered condition. The desired levels of strength, ductility and tough­ness can be obtained by a proper combination of hardening and tempering treatment for these steels. Machinability of the first group is improved by normalizing and second group by annealing.

 

Steels of the first group are suitable for moderately stressed components. Typical applications include railway couplings, driving rings and flanges, hand tools, sockets, levers, cams, and tubes for bicycle, automobiles and aircrafts. Steels of the second group can be subjected to relatively higher stresses. These steels can be surface hardened by flame or induction hardening. Such hardened steels, in addition to hard and wear resistant cases have very tough cores. Typical applications include spindles of machine tools, gears, bolts, shafts, axles, pinions, cylinders, cylinder liners, cams, crank shafts, keys, machine tools, rifle barrels and ball mill balls.

3  High carbon steels:

The carbon content of high carbon steels generally varies from 0.65 % to 1.5 %. The higher the carbon more is the strength and attendant brittleness. Therefore, steels with more than 1.3 percent carbon are rarely used in practice. These steels have poor fabricability, formability, machinability and weldability as compared to medium carbon steels. Machinability and formability can be improved to a great extent by a heat treatment process known as spheroidizing. The spheroidizing treatment results in reduction in strength and hardness values. However, the tendency towards brittleness is reduced considerably with attainment of a satisfactory level of ductility.

 

Steels with 0.65- 1.00 % carbon are frequently used for manufac­turing springs. Two common methods adopted for fabricating springs are hot rolling and cold drawing. Springs made by hot rolling process are sub­jected to hardening and tempering treatment.  A hardness of 56 to 60 HRC can be obtained by hardening and tempering treatment. These carbon steels are used for light springs. Tem­pering these steels at 200-250°C results in improvement in elastic limit. Higher tempering temperatures, say about 400°C, are required where ser­vice conditions demand enhanced toughness and ductility. In addition to springs, high carbon steels are also utilized for making gauges, machine knives, piston rings, saws, cutting tools, chisels and hand tools.

4  Alloy steels:

Alloy steels are superior to plain carbon steels in several respects. Depending on the nature and amount of alloying element(s), alloy steels, in general, possess (I) better strength, hardness and ductility; (ii) higher fatigue and impact strengths; (iii) better resistance to grain growth and softening during tempering; (iv) excellent high and low temperature pro­perties; (v) enhanced corrosion resistance and wear resistance; and (vi) better electrical and magnetic properties.

5  Manganese steels:

Steels with more than 1.65 % manganese are included in alloy steel group. Manganese in the range 1.65-1.90 % is added to improve tensile strength, hardenability and hot workability of steel. The presence of manganese also enhances the response of steel to heat treatment. Hardening and tempering treatment results in the best possible properties. Normalizing improves impact property and is frequently adopted for large forgings and castings.

Low carbon manganese steels (with manganese content ranging from 1.65 % to 1.90 %) are generally used for structural purposes in rolled condition without heat treatment. However, medium carbon steels with same range of manganese content are used in heat-treated condition. Conventional quenching and tempering treatment gives the best possible mechanical properties. Normalized steel possesses mechanical properties equivalent to hardened and tempered plain carbon steels of the same carbon content. Addition of small amounts of alloying elements such as vanadium or molybdenum imparts some additional desirable characteris­tics in these steels. A wide range of engineering components is manufac­tured from these steels. Applications include rails, gears, axles, connecting rods, crankshafts, bolts, nuts, studs, steering levers, aircraft fittings and gun barrels.

High carbon low manganese steels (with manganese content ranging from 1.65% to 2.0 %) constitute an important series of tool steels. This series is popularly known as non-deforming tool steels and has a general composition of 0.9 % carbon and1.8% manganese. The heat treatment given to these steels is oil quenching from 840°C,

followed by tempering at 230°C. These steels possess hard surface, greater depth of hard­ness, keen cutting edge, high impact strength, and are free from expansion or shrinkage as compared to plain carbon steels of equivalent carbon content.

6  Silicon steels:

Silicon is present in all steels. Only those steels, which contain more than 0.6 %silicon, are termed silicon steels. These steels are characterized by improved elastic properties, excellent electrical and magnetic properties with enhanced resistance to scaling at high temperatures. Since silicon imparts brittleness to the steels, in straight silicon steels, the silicon content is restricted to about 5 %. A steel with 3-4 % silicon and less than 0.5 % carbon is popularly known as electrical steel which is used for manufacturing cores of electric motors, generators and transformers. It is also known as trans­former steel. As carbon content decreases, properties are improved; carbon-free iron-silicon alloy has the best properties. Excellent properties are derived from a coarse grained and textured structure. The texture is obtained by repeated cold rolling and annealing. For obtaining coarse-grained structure, the steel is annealed at 1100-1200°C under hydrogen atmosphere. This heat treatment constitutes an important step in the manu­facturing of cores. Transformer steel is also known as grain-oriented silicon steel. Steel with 1-2 % silicon is referred to as dynamo steel.

      Steels with about 2.0 % silicon and I.0 %manganese with carbon varying from 0.50 % to 0.70 % are of great commercial importance. Properly heat-treated steels of this category have high elastic limit and fatigue strength. For this reason, these steels are well suited for manufacturing springs and are widely used as leaf springs, coiled springs, chisels and punches. The most frequently adopted heat treatment consists of austenitizing, oil quenching and tempering. Depending on the chemical composition, steel is oil quenched from a temperature range 840-930°C. Tempering is done within the temperature range 400-550°C.

7  Chromium steels:

Low carbon (0.1-0.2% C), low chromium (0.5, 0.75 and 1.0% Cr) steels are generally case carburized. Increasing chromium contents increases the wear resistance of case. Toughness in the core is somewhat reduced by increasing chromium contents. In order to have optimum pro­perties, these carburized steels are used in heat-treated condition. The purpose of post carburizing heat treatment is to refine the case or core or both.

Medium carbon (0.35% C), low chromium (0.5% (Cr) steel finds applica­tion in manufacturing gears, jaws of wrenches, machine gun barrels, axles and shafts. The most common heat treatment given to this steel is oil quenching/water quenching, followed by tempering. Heat-treated steel has excellent wear resistance and satisfactory level of toughness. The quenching medium depends on the size of the

component. Hardening is carried out at about 870°C. Steel with 0.5 % carbon and 1.5

% chromium is used for making springs and compressed air tools. This steel is oil quenched from 850°C and tempered at about 300°C (or at any suitable temperature) so as to get a hardness number of about 44 on Rockwell C-scale.

High carbon, low chromium steels find numerous applications as tool steels. All such steels are, in general, used in heat-treated condition. Steel with 0.9 percent carbon and 1.0 % chromium is used for making twist drills, hacksaw blades, knives, hammers and similar products. The heat treatment cycle for this steel consists of heating to 810°C, oil quenching and tempering in the temperature range 250-300°C.

Steel with 0.95-1.10 % carbon and 1.3-1.6 % chromium is comparatively an inexpensive variety used for making ball, and roller bearings. This steel is most popularly known as ball bearing steel. It is spheroidized annealed in order to improve machinability. For heat treatment, it is oil quenched from 830°C to 840°C, followed by tempering at 150-160°C in order to achieve a hardness of 62 HRC. In fully heat-treated condition, this steel has a high compressive strength and resistance to abrasion. Apart from ball and roller bearings, other applications include spindles, cold forming rolls and hardened machine parts. The purpose of chromium addition is to ensure attainment of required hardenability. Therefore, smaller sized components can be made of steels with lower chromium content. Two more popular grades of ball bearing steels contain 0.5 percent and 1.0 % chromium,­ respectively. High carbon, low chromium steels are also used as hard magnetic material.

The properties of low chromium steels can be improved significantly by the addition of alloying element(s) such as nickel, molybdenum, vanadium and tungsten. For this reason, complex steels like chromium-nickel, chromium-molybdenum, chromium-vanadium, and chromium-nickel-molyb­denum are more commonly used in practice than straight low chromium steels. Medium chromium and high chromium steels have a large number of applications, e.g. valve steels, tool steels, heat resisting steels and stainless steels.

8  Nickel steels:

As compared to plain carbon steels, nickel steels are characterized' by higher tensile strength and toughness values, improved fatigue strength, impact resistance and shear strength. Nickel steels require lower heat treat­ment temperatures than plain carbon steels, and hence less drastic quench­ing is needed for obtaining equivalent hardness values.

Low carbon (0.I-o.2%C), low nickel (2.5-3.5% Ni) steels are widely used for case carburizing as these steels have very good core toughness values. Another important group of carburizing nickel steel has slightly increased nickel contents, i.e. 3.5-5.0 %, whereas carbon level remains the same, viz. 0.1-0.2 percent. These steels have better toughness and strength than those of 3.0 % nickel steels but are more expensive.

They are used for manufacturing wrist pins, pinions, engine cams, transmission gears, and other parts, which are subjected to severe service conditions.

Medium carbon, low nickel (2.5-3.5% Ni) steels are widely used for structural applications. These steels, when properly heat-treated, have excel­lent shock resistance and tensile strength values. Optimum mechanical properties are obtained by hardening and tempering treatment. Hardening is carried out by quenching from 830°C to 860°C, using water or oil as quenching medium. Tempering is done in the range 550-650°C. Various applications of these steels include aero plane parts, crankshafts, pinion shafts, propeller shafts, turbine shafts, pins, studs and bolts.

High carbon nickel steels are not very common since nickel is a graphi­tizer. However, high carbon nickel steels can be used with strong carbide formers. For most of the applications, nickel is not added alone to the steel. It is generally added in conjunction with some other element(s). This is why nickel-chromium, nickel-molybdenum, and nickel-chromium-molybdenum steels are in more common use. This is not only for steels with low nickel contents but also for high nickel containing steels such as valve steels, stainless steels and heat resisting steels.

9  Molybdenum steels:

Properties of alloy steels can be improved significantly by the addition of molybdenum, which improves hardenability, ductility, toughness, and elevated temperature properties of the steel. Molybdenum inhibits grain growth and makes the steel less susceptible to temper brittleness. It forms complex carbides, which are more stable than cementite. Therefore, molyb­denum steels require higher heat-treating temperatures.

As molybdenum is an expensive element, straight molybdenum steels, in general, do not contain more than 0.70 % molybdenum. Three popular grades of molybdenum steels contain 0.15-0.30 %, 0.30-0.45 % and Q.45-O.70 % molybdenum respectively. Low carbon grades of these steels are generally employed for case carburiz­ing. In carburized steels, the main requirements are improved wear resis­tance of the case and good toughness of the core. These steels are used for shafts, transmission gears, bearing for gears, and bearing axles under moderate service conditions.

With higher carbon contents, these steels can be subjected to heavy-duty applications such as coil spring, leaf spring and various other automobile and aeroplane parts.

Heat treatment temperatures for molybdenum steels are generally 10­-20°C higher than carbon steels of same carbon contents.

Typical examples of this class of steel include nickel-molybdenum, chromium-molybdenum and nickel-chromium-molybdenum steels. Molyb­denum up to 2 % is present in low alloy creep resisting steels, and up to 2-4 % in austenitic stainless steels.

10  Chromium-molybdenum steels:

The beneficial effects arising from the presence of chromium in steels are enhanced by addition of molybdenum. For this reason, about 0.25- 0.65 % molybdenum is added to low chromium steels. Applications of chromium-molybdenum are similar to those of corresponding chromium steels. Chromium-molybdenum steels have the added advantages of improv­ed toughness and freedom from temper brittleness.

Low carbon, low chromium-molybdenum steels possess high tensile and creep strength. These steels are, in general, case nitrided. Applications of these steels include aircraft engine cylinders and lightly stressed wear resis­tant parts for aircraft and automobile construction.

Medium carbon, low chromium-molybdenum steels have high strength, ductility, and good shock resistance. These steels are used for making axles, shafts, connecting rods and other moderately stressed components. They can also be subjected to induction hardening.

High carbon, low chromium-molybdenum steels are used as cold-worked tool steels. Molybdenum steels find nume­rous applications. Low carbon and medium carbon, high chromium molybdenum steels are used as stainless, heat resisting, and valve steels. High carbon, high chromium-molybdenum steels are used extensively for making drawing dies, bushings, shear blades, punches and cold forming rolls.

11  Nickel-molybdenum steels:

Molybdenum, when added to nickel steels, improves hardenability, ductility and toughness. It also checks grain growth and reduces the tendency towards temper brittleness. Applications of nickel-molybdenum steels are similar to those of corresponding nickel steels. Heat treatment temperatures­ are marginally higher, say about 10ºC, than the corresponding nickel steels.

12  Nickel-chromium steels:

Nickel-chromium steels have almost all possible advantages and mini­mum possible disadvantages due to the presence of both the elements. Depending on the chemical composition, these steels exhibit enhanced mechanical properties, excellent corrosion and oxidation resistance, high temperature strength and unique physical properties.

Low carbon, 1.25 % nickel, 0.75 % chromium steels are case carburized. In this condition, it has a hard wear resistant case and a tough core. Typical applications are gears, shafts, levers, bearings and races. Low carbon, nickel-chromium steels with 2.0 % nickel and 0.90 % chromium have properties which are superior to low carbon, nickel-chro­mium steels containing 1.25 % nickel and 0.75 % chromium. Low carbon steels with about 3.50 % nickel and 1.50 % chromium have unusually high hardenability. These steels, when case carburized, possess very hard wear resistant surface with a very tough core. Mechanical properties can be varied over a sufficiently large range with varying heat treatment. The few applications include case hardened gears and pinions, heavy heat-treated parts and spline shafts.

Medium carbon, 1.25 % nickel, 0.75 % chromium steels are hardenable to greater depths than low carbon steels. These steels are used in heat-treated condition. One common heat treatment is oil quenching and tempering. Some applications of these steels include crankshafts, propeller shafts, drilling equipment parts and parts of earth moving equipments. Medium carbon, 2.0 % nickel, 0.90 % chromium steels have better properties than low carboI1-and medium carbon, 1.25 % nickel, 0.75 % chromium steels. Medium carbon steels with 3.50 % nickel and 1.50 % chromium have better mechanical properties because of their high alloy contents. Their properties can be improved by hardening and tempering treatment. In hardened and tempered condition, these steels possess extraordinarily high toughness. For this reason, these steels are employed for heavy-duty applications- such as highly stressed components in aircrafts and automobiles.

High carbon, nickel-chromium steels are not very common. High nickel chromium steels constitute the well-known series of austenitic stainless steels. High nickel-chromium steels are also used as heat resisting and valve steels.

13  Nickel-chromium-molybdenum steels:

Nickel-chromium-molybdenum steels are sometimes referred to as triple alloy steels. Due to the presence of three alloying elements, these steels have properties which are superior to corresponding double alloy steels, i.e. nickel-chromium, chromium-molybdenum or nickel-molybdenum steels. Low carbon grades of these steels are generally case carburized. Case carbu­rized steels are characterized by very high case hardness and core toughness. These steels are suitable for manufacturing gears, gudgeon pins, shafts, levers, and camshafts, drive wheels, clutch plates, collets and valve rockers.

Medium carbon, low alloy steels have good strength and ductility. The response of these steels to heat treatment is excellent and a wide range of mechanical properties can be attained with the help of suitable heat treat­ments. General engineering purpose steel has 0.35-0.45 % carbon, 1.30-1.80 % nickel, 0.9-1.4 % chromium, and 0.20 - 0.35% molybdenum. This steel possesses good strength, ductility, toughness and hardenability. Properly heat-treated steel has very good resistance to wear and shock. Typical uses include axle shafts, boIts and studs, high duty engine connecting rods and high temperature bolts and oil refining and steam installations.        .

Medium carbon steels with about 2.50 % nickel, 0.70 % chromium and 0.50 % molybdenum have high hardenability; good strength coupled with high ductility and toughness, fairly good low tempe­rature properties and resistance to shock. Increasing nickel contents to about 3.50 % (microstructure, Fig.) results in increase in depth of hardening and toughness. Therefore, fairly large sections of such steels can be heat treated in order to develop good mechanical properties.

14  Structural and tool steels:

Taking into account their uses, steels may be classified into three main classes, namely, structural steels, tool steels, and special purpose steels. In this section, based on their applications, the heat treatment" of some cate­gories of steels is discussed.

a) Structural steels:

Hot-rolled steels:

In general, hot-rolled structural sections and sheets are not subjected to heat treatment. Since hot rolling itself is a high temperature process, it takes care of a number of heat treatment such as homogenizing, stress relieving and breaking of cast structure.

Thin steel sheets used as dynamo and transformer steel are subjected to specific heat treatment for getting improved magnetic and electrical pro­perties. Low carbon steel sheets are annealed or normalized in order to attain a fine-grained structure. Medium carbon steel sheets are subjected to spheroi­dization annealing. High carbon steels in hot-rolled condition are spheroi­dized to improve the machinability. Alloy steels, especially high alloy steels, in hot-rolled condition are tempered at high temperatures in order to improve machinability.

Cold-drawn and cold-rolled steels:

In contrast to hot-rolled steels, cold-drawn/cold-rolled steels are always subjected to heat treatment, irrespective of the chemical compositions of the steels. While hot-rolled steels are heat treated after the hot rolling opera­tion, cold-drawn steels are heat treated prior to cold drawing/rolling, in between the cold drawing/rolling cycles and after completing the cold drawing/rolling operations.

For general engineering applications, the heat treatment given prior to cold drawing/cold rolling operation consists of annealing. It imparts maxi­mum softness to enable further cold working. Exception to this is hypereutectoid steels for which annealed structure is brittle because of the presence of cementite network at grain boundaries. Normalizing can eliminate such a network. Most of the medium carbon and high carbon steels are spheroidized prior to cold drawing/cold rolling. This imparts ductility coupled with strength. Spheroidizing constitutes an important treatment for objects, which are made from cold-drawn/cold-rolled products, especially by machining.

Intermediate heat treatment is essential as it eliminates the effect of strain hardening due to prior cold working and thus makes the product amenable to further processing (cold working) without failure. In general, recrystallization annealing is employed as an intermediate heat treatment.

Final heat treatment is chosen in accordance with the properties required in the material. By a proper combination of heat treatment and cold working, different

combinations of strength and ductility can be attained.

Structural steel castings:

Steel castings, in most of the cases, are heat treated to eliminate the cast structure. Cast structure is undesirable for the following reasons:

1.    It is generally coarse and is not desirable

2.    It has non-uniformity of chemical composition and of structure

3.    It has poor mechanical properties, especially dynamic properties.

4.    It has low machinability.

5.    It is always associated with internal stresses due to thermal gradient.

All these defects can be removed by heat treatment operations.

b) Tool steels:

Water hardening tool steels:

These are essentially high carbon steels. Carbon content of these steels is as high as 1.40 %. High carbon steels are characterized by high tensile strength and hardness levels but low ductility and toughness values. Hence these cannot be used where impacts loads are applicable. They are widely used for making tools as these are cheap and possess good machinability and high hardness.

Since machining is an important process in forming of tools, these steels are subjected to spheroidization in order to improve machinability. Spheroidized steel is subjected to water or brine quenching from hardening temperature. Sometimes this treatment results in distortion of the tool. Other problems encountered with these steels, which have hypereutectoid compositions, are of decarburization, possibility of retained austenite in hardened structure, and grain coarsening. These problems demand addition of strong carbide formers such as chromium, vanadium, and molybdenum. The presence of chromium improves both hardness and hardenability-the properties required in most tools. On the other hand, vanadium checks the tendency of steel towards grain coarsening. The struc­ture of hardened steel changes with tempering temperature. Therefore, tempering temperature has to be chosen carefully. In general, these steels are tempered in the range 170-220°C. Replacement of conventional water or brine hardening heat treatment by martempering leads to development of better mechanical properties in the tools.

Applications of water hardened tool steels include heavy forging ham­mers, forging dies,

 

large blanking tools, chisels, scissors, knife blades, hand hammers, hot and trimming tools, cutting dies, bending dies, drift punches, lathe centers, milling cutters, boring tools, watch maker's tools etc.

Shock-resisting tool steels:

As the name suggests, these tool steels are characterized by good tough­ness. For this reason, the carbon contents of these grades of steel are kept low as compared to water hardening steels. Most of the steels belonging to this group have carbon ranging from 0.5 percent to 0.6 percent. Another important property of tool, i.e. hardness, is imparted by alloying additions. Commonly added alloying elements are chromium, molybdenum and tungs­ten. These elements not only increase hardness but also improve harden­ability to a considerable extent. These steels are water or oil hardened. Tempering temperature depends on the final properties required in the tool. For an optimum combination of toughness and hardness, low temperature tempering is preferred. High temperature tempering is performed where toughness is of primary importance.

Silicon-manganese steels with about 0.55 percent carbon, 2.0 percent silicon and 1 percent

 

manganese, which have high toughness in hardened and tempered condition, are also included in this group. These steels are considerably cheaper than steels alloyed with tungsten, molybdenum and vanadium. Due to high silicon contents, decarburization and grain coarsening take place in hardened steels. Applications of these steels include, chisels, punches, shear blades and scarfing tools.

Cold work tool steels:

These steels are mainly employed for making tools intended for cold work applications. Sometimes, steels belonging to this group are referred to as non-deforming or non-distorting steels. But all steels exhibits some distortion or deformation, during hardening heat treatment, and steels of this group are also not exceptions to this. The chemical com­position and hardening heat treatment for these steels are so adjusted as to produce minimum possible deformation and, consequently, these are termed as non-deforming steels. These steels have been divided into three groups, namely, oil hardening, air hardening, and high carbon, high chromium type.

Oil hardening steels are basically carbon-manganese steels in which chromium and tungsten are added occasionally to improve hardness, hardenability and wear resistance. These steels are used in oil hardened and tempered condition. They possess good machinability as compared to other cold-worked tool steels, and this enables them to be shaped in complicated shapes by machining to a high degree of precision, such as reamers, taps, press blanking and stamping dies.

 

Air hardening steels are primarily chromium-manganese-molybdenum steels. All these elements considerably improve hardenability and impart air-hardening characteristics to the steel. Due to air hardening tendency, distortion of these steels is less than in oil hardening cold work tool steels. Applications are almost similar to those of oil hardening steels. These steels have improved wear resistance and non-deforming properties.

 

High carbon, high chromium steels contain minimum amount of 1.0 % and 12.0 % carbon and chromium, respectively. These steels do not exhibit grain coarsening up to about 1040°C. The high chromium contents enable the steels to develop martensitic structure on air-cooling and, because of this, distortion is much less. These steels are used for larger tools and tools with intricate shapes. In addition to air hardening charact­eristics, chromium imparts very high hardness and abrasion resistance to the tool due to the formation of chromium carbide. Properties of these steels can be further enhanced by the addition of molybdenum, vanadium and tungsten.

		Table  Heat treatment temperature for cold work High Carbon High Chromium tool steel

 

Hot work tool steels:

Depending on the principal alloying element, hot work tool steels (be grouped into three main classes, namely, chromium base, tungsten base and molybdenum base tool steels.

In contrast to cold work tool steels, these steels are employed for hot working applications such as hot forging and hot extrusion. They are also used for fabrication of die casting dies. Therefore, high temperature properties, such as red hardness, wear-resistance, erosion resistance, thermal cracking of reticular type (heat checking) due to severe thermal shocks, are the main considerations for such steels. Red hardness is imparted by tung­sten. The larger the tungsten content, the higher is the red hardness and stability of the steel. Chromium improves both hardness and oxidation resistance. Other beneficial alloying elements are molybdenum and vanadium for increasing hardness and high temperature properties. These steels require higher hardening temperatures due to the presence of strong carbide formers. This causes considerable distortion. Erosion resistance is improved by increasing the

carbide contents of the steel. Sometimes, cobalt is added to these steels as it improves the resistance to erosion as well as to heat checking during severe thermal shocks.

			Table Heat treatment temperature for hot work tool steel – Chromium base
				Heat treatment temperature for hot work tool steel – Molybdenum base

 High Speed Steels:

As the name indicates, these steels are well suited for manufacturing cutting tools, which can be operated at high speeds. The service conditions for such cutting tools demand high red hardness and elevated temperature wear resistance. Both these characteristics can be imparted in steel by alloying it with strong carbide forming elements such as tungsten, molyb­denum, chromium and vanadium. Alloying elements should be added in sufficient amounts so that all the carbon may combine with them to form alloy carbides. To some grades of high speed steel, cobalt is added as an alloying element in order to enhance cutting ability of the tool. Cobalt containing high speed steel tools possesses superior cutting power than those of cobalt-free high speed steel tools. Materials with poor machinability can be cut successfully with such tools. The total alloy contents, in general, vary from 20 % to 40 % in high-speed steels. Carbon varies from 0.70 % to 1.5 %. Low carbon grades of these steels are tougher than high carbon grades. However, high carbon grades are characterized by higher hardness and wear resistance. Typical applications include high speed cutting tools, heavy cut tools, milling cutters, reamers, deep hole drills, blanking dies, hot forming dies, lathe centers and wearing plates.

One of the most popular grades among all high-speed steels is designated as       18-4-1. It contains about 0.7 % carbon, 18 % tungsten, 4.0 % chromium, and 1.0 % vanadium.

		Heat treatment of 18-4-1 HSS

 

In view of its high cost and scarcity, tungsten has been replaced partly by molybdenum. Tungsten-molybdenum high-speed steels are relatively cheaper than tungsten high speed steels, and are in common use now-a-days. Even the classical grade of high-speed steel (18-4-1) has been replaced by tungsten-molybdenum steels. Thus, depending on the composition, high-speed steels can be classified into two classes namely tungsten base and molybdenum base high-speed steels.   

All high-speed steels are heated to the maximum possible temperature for hardening treatment. However, this temperature should not result in large scale grain coarsening. A high hardening temperature ensures dissolution pf all the carbon and alloying elements in the austenite. This highly alloyed austenite transforms to martensite of exactly similar composition on quenching. The martensite, thus formed, which is highly enriched in carbon and alloying elements, has high red hardness and structural stability.

Generally, the hardening temperature for high speed steels varies from 1150°C to 1350°C. Heating to such a high temperature poses certain problems like oxidation and decarburization in addition to grain growth. Added to these problems is the poor thermal conductivity of high-speed steel. These problems are minimized by heating high-speed steels to final hardening temperature in stages. Simpler shapes and smaller-sized tools are heated in two stages. The tool is preheated to about 800ºC and then quickly transferred to another furnace maintained at the final hardening temperature. Larger tools or intricate tools are generally heated in three stages. The first step consists of heating to about 400ºC, followed by second heating up to about 800ºC. The holding time up to 800ºC is calculated on the basis of 20-30 seconds per millimeter of diameter or section thickness of the tool. The holding time at the final hardening temperature is less and rarely exceeds 5 minutes. Salt bath furnaces are preferred in order to avoid oxidation or decarburization.

High-speed steels are either quenched in oil or in stream of air or in salt baths. Normally, direct oil quenching is not practiced. High-speed steel is first cooled to about 1000°C, and only then it is quenched in oil. This two-step oil quenching avoids formation of quench cracks. Quenched high-speed steels may have some retained austenite along with martensite. Such steels are subjected to sub-zero treatment. These steels, after sub-zero treatment, are immediately tempered. Tempering is carried out at about 550°C. It has been found that multistage tempering is much more beneficial than single tempering of same duration.

c) Special steels:

Stainless steels:

Stainless steels are high alloy steels and possess excellent corrosion and oxidation resistance. Due to these characteristics, these steels find numerous applications in nuclear plants; power generating units, pulp and paper manufacturing plants, food processing units and petrochemical industries.

Stainless steels can be classified into four groups, namely, austenitic, ferritic, martensitic, and precipitation hardening stainless steels. Martensitic stainless steels are straight chromium steels containing 11.5-18 % chromium. These are the cheapest among tae family of stainless steels. Ferri­tic stainless steels, which are similar to martensitic steels, are straight chromium steels containing 14-27 % chromium. These steels are superior to martensitic stainless steels in their corrosion resistance but are expensive. Austenitic stainless steels are chromium-nickel steels with minimum total chromium and nickel contents of 25 %. In general, minimum 8 percent nickel and 17 % chromium contents are essential to make the steel completely austenitic in the presence of low carbon contents. These steels, though costly, possess the best possible properties. Precipitation hardening stainless steels

contains nickel, chromium, molybdenum, copper and aluminium. These steels are strengthened by precipitation of intermetallic compounds. Depending on the matrix, they can be further divided into three groups, namely, austenitic, semi-austenitic, and martensi­tic steels.

Austenitic stainless steels possess optimum combi­nation of strength, ductility and toughness. Hence, they are the most widely used of all varieties of stainless steels. There is no phase transformation with temperature for this class of stainless steels. For this reason, they are not subjected to heat treatment operations in order to improve the properties. Cold working is the only strengthening mechanism for these stainless steels.

Annealing is done in order to facilitate the cold working process. Stress relieving is also employed on cold-worked and welded parts. When austeni­tic stainless steels are heated to a temperature range 425-870°C, chromium carbide precipitation takes place at austenitic grain boundaries. Precipitation of chromium carbide will result in depletion of chromium in the matrix adjacent to grain boundaries. Hence, corrosion resistance decreases sharply. Therefore, this temperature range is avoided during heat treatment/welding of austenitic stainless steels. This is also known as sensitization. It can be solved in three ways:

1.    By solution treatment,

2.    By controlling carbon content, and

3.    By stabilization.

Just as in austenitic stainless steels, there is no phase change in ferritic stainless steels with temperature. Therefore, these steels cannot be subjected to heat treatment processes for strengthening. The composition of these steels is so adjusted as to give ferritic structure at room temperature. High chromium ferritic stainless steels remain ferritic on cooling from high tempe­ratures. However, in the case of relatively low chromium ferritic stainless steels, there is always the possibility of formation of some martensite on cooling from high temperatures. This martensite, though small in amount, affects the mechanical properties, especially ductility and toughness. This is so because martensite is located at the grain boundaries. Hence, addition of strong ferrite formers to these grades of ferritic stainless steels is generally recommended. Properties of ferritic stainless steels are adversely affected by heating in the temperature range 400-450°C. This embrittIement seems to be due to the precipitation of sub-microscopic particles of iron-chromium compound at grain boundaries. Rapid cooling above 600°C can avoid this. The presence of sigma phase and

other intermetallic phases decreases toughness and corrosion resistance. Almost all ferritic stainless steels with more than 14 % chromium suffer from this embrittlement from inter­metallic phases. These phases appear on heating to a temperature range 600-1000ºC. These steels should not be selected for service conditions that demand exposure within this temperature range. Heating ferritic stainless steels above 1000ºC results in the dissolution of these phases.

In contrast to austenitic and ferritic stainless steels, martensitic stainless steels can be heat treated to improve mechanical properties. Carbon content of martensitic stainless steels is one of the important parameters, which controls the properties of heat-treated steels. Martensitic stainless steel can be hardened by oil quenching from austenitizing temperature, which depends on the chemical composition of steel. For most of the standard grades of martensitic stainless steels, austenitizing temperature lies between 925°C and I075°C. Due to their high hardenability, these steels can be hardened by air-cooling. However, some decrease in ductility and corrosion resistance has been observed in air-hardened steels. It is due to the precipitation of car­bides at grain boundaries on slow cooling within a temperature range, 870-540°C. Therefore, air cooling is performed, only for those components whose sizes or shapes are such that the problem of distortion is severe, on oil quenching. Martensitic stainless steels with high carbon contents or nickel contents pose an additional problem, namely, the presence of retained auste­nite in as-quenched structure. Such grades of martensitic stainless steels are subjected to sub-zero treatment in order to transform retained austenite to martensite. Hardened martensitic stainless steels arc subjected to tempering treatment. Depending on the final properties required, low or medium tem­perature tempering is carried out. Low temperature tempering results only in stress relieving and does not change the mechanical properties. It is carried out by heating to the temperature range 150-370°C. Medium tem­perature tempering results in change in mechanical properties. Occasionally, high temperature tempering is carried out. It imparts maximum softening.

Precipitation hardening stainless steels have either austenitic or marten­sitic matrix. Steels with martensitic matrix are more common in use than those with austenitic matrix. The austenitic precipitation hardening stainless steels are non-magnetic in nature and have a minimum nickel content of 10 percent. It ensures formation of austenitic matrix, which is strengthened by addition of molybdenum, copper, niobium and titanium. Heat treatment of these grades consists of solution treatment so as to get a single-phase structure. Solution treatment is carried out by heating the steel to about 1200°C.It is followed by quenching which results in the retention of high temperature phase at room temperature. Quenched steel is then aged at about 750°C. In the aged condition, the structure consists of austenite and intermetallic compounds. Due to high ageing temperature, these steels can be used successfully up to about 650ºC.

Martensitic grades of precipitation hardening stainless steels are alloyed with copper, molybdenum, aluminium, titanium, niobium and nitrogen. The heat treatment sequence is similar to that described for austenitic grades. Martensitic grades are generally aged at relatively lower temperatures (about 450°C) than those adopted for austenitic grades. High nickel steels, known as semi-austenitic precipitation hardening steels, have good formability at room temperature

Maraging steels

These steels are essentially high alloy, high strength steels possessing good forming characteristics, weldability, high yield strength to tensile strength ratio and fracture toughness. The chemical composition of a marag­ing steel is so adjusted that austenite to martensite transformation proceeds even on air-cooling. This results in practically insignificant distortion of the steel part. In maraging steel, there is no danger of decarburization on heating.

Maraging steels are basically alloys of iron and nickel. In these steels, minimum 18-20 % nickel is needed to impart air-hardening character­istics. In addition to nickel, cobalt, titanium, beryllium, aluminium, niobium, tungsten and molybdenum are also present in these steels. In maraging steels, carbon is present as an impurity and is generally kept below 0.03 percent. The martensite formed in maraging steels is almost carbon free. The iron-nickel martensite is soft by nature. This martensite has low strength and high ductility. The hardened structure is aged at elevated temperatures. Such an ageing treatment results in the precipitation of intermetallic compounds in a matrix of martensite. Such a structure is responsible for ultra-high strength of maraging steels.

Spring steels:

Steels possessing high elastic limit, toughness and fatigue strength are suitable for manufacturing springs. High carbon steels are the cheapest among all the grades of spring steels. These steels are used in either hardened and tempered condition or in patented and cold-drawn conditions.

High quality springs are made from chromium-vanadium steels. Heat-treated chromium-vanadium steels develop high elastic limit, toughness, resistance to fatigue and machinability as compared to high carbon steels. These steels are also used in oil hardened and tempered condition. Typical applications include automobile and aircraft engine valve springs and high quality laminated and coil springs for motorcars. A typical steel belonging to this class contains 0.50% C, 10% Cr, 0.20% V, 0.40% Si and 0.70% Mn. it is oil hardened from 860°C, followed by tempering at about 500°C.

For medium duty applications, leaf and helical springs are generally made from silicon-chromium steels. A representative steel of this class has 0.60% C, 0.70% Si, 0.60% Mn and 0.80% Cr. The heat treatment consists of oil hardening from 830°C, followed by tempering in the temperature range 550-600°C.

Most commonly employed steels for manufacturing springs are silicon manganese steels.  The use of chromium-manganese steels for making springs is a relatively recent introduction. These steels are being used increasingly for those appli­cations for which silicon-manganese steels were being used earlier. In other words, these steels provide an alternative to silicon-manganese steels. A steel belonging to this group contains 0.50% C, 0.40% Si (max), 0.80% Mn and 1.0% Cr. Heat treatment consists of oil hardening from 820°C. Temper­ing is performed at about 400-450°C.

Properties of chromium-manganese steels can be improved by the addition of 0.10-0.20 percent vanadium or 0.15~0.25 percent molybdenum, or both.

Valve steels:

High chromium-silicon (Cr + Si ~ 10%) steels are popularly known as valve steels and are extensively used for manufacturing automobile, aero and marine engine valves. Besides, these steels are also used for making furnace parts, nuts and bolts for high temperature service, tube bending mandrels, tube piercing points, super heater supports and gas turbine parts. These applications imply that chromium-silicon steels can be successfully used as heat resisting steels. In fact, these steels possess very good resistance to heat corrosion and scale formation. Parts made from these steels can be successfully used up to about 600-650°C. Properties of these steels can be further improved by addition to nickel and tungsten. Chromium-silicon-­nickel valve steel has better heat, corrosion, and scale resistance than chromium-silicon steel, and is well suited for making heavy-duty outlet valves. It is generally used in hardened and tempered condition.

In addition to the martensitic grades of chromium-silicon steels, com­plexly alloyed austenitic grades of chromium-nickel-silicon-tungsten steels are also used for manufacturing high duty exhaust valves for aircrafts and automobile engines. These austenitic grades cannot be hardened by heat treatment and are used in softened condition.

Heat resisting steels:

Heat resisting steels are also known as high temperature steels. Since parts made from these steels are exposed to elevated temperature service conditions, such steels should possess high temperature strength and good resistance to oxidation and/or scaling. In addition to these characteristics, dimensional as well as str1Jctural stability is of prime importance. Appli­cations of heat resisting steels include components of jet engines, rockets, gas turbines and boilers. A wide range of steels, from plain carbon steels to high alloy steels, can be used as heat resisting steels, depending on the service conditions to which they are exposed.

Plain carbon steels can be successfully subjected to loading up to about 300°C, as creep is not significant below this temperature. Low alloy steels are used at higher temperatures. A low alloy steel with about 0.50 % molybdenum is quite popular due to its low cost and better high tem­perature properties. The properties of this steel can be improved by addition of chromium or vanadium or both. These elements (Mo, Cr and V) raise the recrystallization temperature of ferrite and thus improve heat resistance. These steels are used for manufacturing steam pipe lines, super heaters, fittings, steam drum, fasteners and for other applications in the temperature range 500--600°C. Depending on the composition (mainly based on chromium contents), these low alloy steels .can be grouped in two classes, pearlitic and martensitic steels.     

Pearlitic steels have low chromium contents and are used in either nor­malized and tempered condition or in oil quenched and tempered condition. Pearlitic grades of low alloy steels cannot be employed at temperatures above 500-580°C. Martensitic grades of low alloy (Cr-Mo-V) steels are used in hardened and tempered condition. Tempering temperature should be more than the service temperature. Due to higher chromium contents, these steels (martensitic grades) have better heat resistance than pearlitic steels.

Raising the chromium content can increase the heat resistance of steel. This is why both martensitic and ferritic stainless steels are used frequently as heat resisting steels. Martensitic (chromium) steels alloyed with tungsten, vanadium, molybdenum, niobium and titanium are preferred to straight chromium steels because of the better properties that can be achieved in the former. These elements raise the recrystallization tempera­ture of the steel and form strong carbides. For these reasons, these complex­ly alloyed steels have better heat resistance. Heat resistance can be further improved by the addition of boron, zirconium, cerium and nitrogen. This class (martensitic) of high chromium steels in hardened and tempered condition offers opti­mum heat resistance.

Austenitic heat resistant chromium-nickel steels differ from austenitic stainless steels in the sense that heat-resisting steels have higher nickel and chromium contents and are alloyed with tungsten, vanadium, niobium and boron. Such complex steels possess enhanced heat resistance. Austenitic heat resisting steels are regarded as possessing the best heat resistance among all steels and can be used for making components operating at tem­peratures up to 750°C. Austenitic heat resisting steels are strengthened by precipitation of either carbides or intermetallic compounds.

Chromium-silicon steels also constitute a popular class of heat resisting steels. These steels are also known as valve steels

15  Heat treatment of aluminium and its alloys:

In aluminium alloys, the basic aim of heat treatment is to increase strength and hardness. Depending on the rate of heating, cooling and soaking time at given temperatures, the properties can be varied according to requirement. Main heat treatment processes used for aluminium alloys are as follows: (1) Annealing (for example, recovery, recrystallization, homogenization and high temperature annealing), (2) solution treatment, and (3) precipitation hardening.

Annealing:

Annealing of aluminium alloys is carried out to relieve internal stresses caused by cold working. Temperature control is necessary to avoid grain growth. During annealing, the precipitation of the alloying elements may also take place. This complete precipitation gives dimensional stability even at high temperature to the parts in service. There is no appreciable change in mechanical properties. It improves ductility

and resistance towards shock, while tensile and yield strength decreases to some extent.

This process can be divided into two subgroups, type 1 and type 2 annealing. Type 1 annealing includes recovery, recrystallization and homo­genization. Type 2 annealing is a high temperature annealing process, also termed phase recrystallization.

Type 1 Annealing:

The main purpose of this treatment is to convert the structure of metal to a stable form from an unstable one. This is a spon­taneous and irreversible process. A coarse grained structure produced by this process will not become fine grained only after further cold working. In heat treatable aluminium alloys, this process decreases the degree of supersaturation of solid solution. The deciding factors for this process are temperature and time of soaking, and not rate of heating and cooling.

Recovery treatment:

During recovery treatment, most of the elastic distortions in the metal, which are generated during cold working or during fast rate of solidifica­tion, are relieved. For this treatment, components are soaked at definite temperature (which varies for different alloys), but below recrystallization temperature of the alloy. Hardness and strength of the alloy decrease due to recovery treatment, while ductility and toughness increase.

Recrystallization annealing:

Recrystallization annealing treatment is given to wrought semi-finished aluminium alloys to change the structure, i.e. to change the directional grains into equiaxed grains. Thus, isotropy is maintained. As the degree of deformation is increased, there is decrease in grain size. Any decrease in the temperature of deformation has also similar effect on the grain size. The rapidity of recrystallization process increases with rise in tempera­ture and decrease in original grain size. For a complete and fast rate of recrystallization, the alloy structure should be thermodynamically unstable. With a critical deformation, there is formation of coarse-grained rim on the deformed material.

Industrially, the following steps are taken for obtaining a fine grained equiaxed structure in semi finished wrought products:

1. The recrystaIIization process is interrupted just after nucleation process. .
2. Fast heating rate is preferred for annealing temperature (for this, nitrate bath is used).
3. RecrystaIIization should be carried out at maximum possible tem­perature and in minimum time.

Homogenization treatment:

Homogenization treatment is given to distribute the alloying additions uniformly throughout the alloy. Thus the adverse effect of segregation is removed by this treatment. As cast structure is also modified. In so doing, the mechanical properties of the alloy also improve.

Homogenization annealing treatment is given to those semi-finished products, which have comparatively high hardness. The aim is to introduce good plasticity before cold and hot working of the ingot. Thus, workability of the alloy improves. This is the first treatment given to ingots.

Type 2 Annealing:

In this type of annealing, the alloy is heated above critical temperature and then cooled slowly. Due to this treatment, the shape and size of the grains and nature of distribution of particles of precipitating phases are modified considerably from original structure. We can obtain a fine-grained structure, which has great bearing on mechanical properties of the alloy. In this case, the change is of reversible type, i.e. the coarse grain­ed structure may change into fine grained, and the reverse of this is also true. The rate of cooling is a major factor besides temperature and soaking time.

Solution Heat Treatment

This involves heating of the alloy to a particular temperature for suffi­cient time so that alloying elements such as Cu, Si, Mg, and Zn go into solid solution and form single-phase solid solution. Overheating and under heating are to be avoided. Overheating may cause formation of surface blisters, excessive grain growth, and eutectic melting in some alloys. On the other hand, under heating may lead to incomplete solutionizing prior to precipita­tion. After solutionizing, the alloy is quenched in cold water or hot or boiling water or hot oil or fused salts. The factors, which affect the final properties of the alloy, are soaking temperature, soaking time and cooling rate. the soaking temperature is differ­ent for various alloys. It depends on the various phases, which are present in alloys. Each phase will dissolve in solid solution at different rates. The rate of dissolution increases with rise in temperature. Soaking time is based on the rate of dissolution of alloying elements into solid solution. Soaking

 

Precipitation Hardening

After quenching, precipitation of second-phase particles occurs. Precipi­tation with time at room temperature is called natural ageing, whereas precipitation at higher temperatures is referred to as artificial ageing. The hardness after solution treatment is comparatively low. The maximum hardness and strength develops when ahoy is aged at a suitable temperature which normaIIy ranges between 120°C and 200°C. In some cases the ageing temperature may be as high as 300°C. Ageing time may vary from 4 hours to 24 hours.

Spontaneous decomposition of supersaturated solid solution takes place during ageing treatment. Ageing temperature and degree of supersaturation playa great role on the final properties of the alloy. The higher the ageing temperature and higher the degree of supersaturation, more intensive will the ageing be. Higher temperature ageing is adopted when more stable phase is required together with dimensional stability. This process is also called stabilizing ageing. Besides mechanical properties, physical and chemical properties are also affected by ageing. This happens due to the metastable structures of the alloy, which are formed during ageing of supersaturated solid solution obtained by solution treatment. Rise in temperature changes the atomic positions with corresponding changes in the forces associated with inter atomic bonds. At the same time distribution of second phase particles also changes.

Following steps are associated with the process of precipitation harden­ing in most of the aluminium alloys:

1.    The first stage preceding the formation of particles of the precipi­tating phase consists of rearrangement of atoms within the crystal lattice. This constitutes formation of clusters and Guinier-Preston zones, During this process, mechanical properties are improved due to development of micro strains in the lattice.

2.    Formation of transition structures in the form of modified Guinier­Preston zones (e.g. GP-II zones) and intermediate phases. This may give rise to maximum strengthening in the alloy.

3.    Formation of stable phase from transition phases whose particles have common boundaries with the grains of the matrix.

4.    Growth of the certain larger particles at the expense of neighboring smaller particles. Due to this stress relief takes place in the lattice usually at higher ageing temperatures, which causes considerable decrease in strength and increase in ductility of the alloy.

16  Heat treatment of Copper and its alloys:

Copper and its alloys find wide application in industries as electrical conductor, fuel and oil lines, bearings, automobile radiators, pressure vessels, and so on. They are good electrical conductors, have reasonable strength coupled with good corrosion resistance, and are non-magnetic.

Heat treatment processes which are relevant to copper and copper alloys include homogenizing, annealing and stress relieving. Beryllium bronzes are precipitation hardenable alloys.

Homogenization is carried out at 50°C above upper annealing tempera­ture for 3-10 hours. This treatment is required specially for long freezing range alloys such as tin bronzes, copper-nickel and silicon bronzes. As a result of homogenization, segregation and coring are reduced. Thus, hot and cold ductility of the cast alloys is improved. Stress relieving which 'is carried out below annealing temperature relieves internal stresses. In gene­ral, for wrought copper alloys, stress-relieving temperature varies from 190°C to 260°C.

Heat treatment of copper:

In the case of pure copper, the heat treatment process, which is adopted­, is annealing. The purpose of annealing is to achieve the original ductility and softness in cold-worked copper. Pure copper is annealed at about 600°C. After holding at this temperature for some time, copper is quenched in cold water. Annealing temperature of copper is higher than its recrystal­lization temperature (270°C). Water quenching removes scale formation and gives clean surface of copper. Annealing above 600°C does not have any significant effect on copper except that grain coarsening occurs. Grain coarsening is undesirable since it reduces ductility.

Heat treatment of brasses:

Like pure copper, brasses are also subjected to recrystallization anneal­ing treatment after cold working. By annealing above recrystallization tem­perature, the original degree of ductility and toughness can be restored in the cold-worked brass. Annealing may be carried out by heating the cold worked brass at a temperature between 650°C and 700°C, followed by cooling at any suitable rate. Controlled atmosphere is often used while annealing to avoid excessive oxidation and discolouration of the surface of the brass. A low temperature (300°C) annealing heat treatment may be given to brass articles for about one hour to remove internal stresses. Such treatment reduces the tendency for season cracking. By low temperature annealing, tensile strength is maintained within acceptable limits.

Heat treatment of bronzes:

As in the case of aluminium alloys, beryllium bronzes (copper-beryllium alloys) respond to precipitation heat treatment. Other types of bronzes are not given any heat treatment except annealing at about 500°C. Beryllium bronzes contain 1.5-2.25 % beryllium. The solid solu­bility of beryllium in copper increases with temperature from less than 1 percent at room temperature to more than 2 % at 815°C. Due to decrease in solid solubility with decrease in temperature, coupled with the possibility of quenching supersaturated solid solution and formation of precipitates with coherency, these alloys are amenable to precipitation heat treatment which greatly improves mechanical properties such as strength and hardness. Copper-beryllium alloys are solutionized at 800°C for about half an hour, followed by quenching in water. They are aged at a temperature between 300-320°C for a period of 2-4 hours. Ageing treatment precipi­tates second phase particles (γ-phase) uniformly in the matrix. With this process, the hardness of the order of 200-400 BHN can be attained, depend­ing on ageing time. Tensile strength and yield strength also increase. By suitable heat treatment and cold working, tensile strength as high as 1400 MPa can be attained.