Annealing of Steel: 7 Types | Methods | Heat Treatment | Metallurgy

The following points highlight the seven main types of annealing of steels.They are: 1. Full Annealing 2. Homogenising (Diffusion) Annealing 3. Process Annealing 4. Spheroidisation Annealing 5. Partial Annealing 6. Bright Annealing 7. Stress-Relieving Annealing.

Type # 1. Full Annealing:

Full annealing, or annealing consists of heating the steel to a temperature above its upper critical temperature, soaking there for sufficient time to obtain homogeneous austenite and left to cool in the furnace (normally 50°C/hr) i.e., the furnace is switched off.

Sometimes, the part may be submerged in a heap of ash, lime, etc., i.e., in a good heat insulating material. The austenitising temperature, as illustrated in Fig. 5.1 for full annealing is a function of carbon content of the steel (also shown in table 5.2) and is-

For hypo-eutectoid steels = Ac₃ + (20 – 40C)(to get single phase austenite)

For hyper-eutectoid steels = Ac₁ + (20 – 40C)(to get austenite + Fe₃C)

Heating hypo-eutectoid steels slightly above Ac₃ temperature results in fine grains of austenite (Fig. 5.2 a3), which on slow furnace cooling (annealing) results in coarse grains of ferrite and pearlite.

Heating this steel to very high temperature (Fig. 5.2 a7) causes grain growth of austenite, which on annealing produces very coarse ferrite and pearlite (Fig. 5.2 a9) at extra cost of heating, time, more scale formation and decarburisation. Heating 0.2%C steel up to only between Ac₁ and Ac₃ from room temperature, does not refine the original coarse ferrite grains (Fig. 5.2 a2), which on slow cooling (annealing) would impair the properties.

For annealing, hypereutectoid steels are heated to slightly above Ac₁ temperature only; as then, very fine grains of austenite are obtained (96% of structure in 1.0%C steel) with spheroidised Fe₃C (i.e., network of Fe₃C is broken) as illustrated in Fig. 5.2 (b2), which on furnace cooling produces fine grains (compared to original) of pearlite and spheroidised cementite, (Fig. 5.2 b3).

The driving force for spheroidization of Fe₃C is the reduction in austenite-cementite interface area, and thus, the reduction in interfacial energy accompanies spheroidization. Had the steel been heated to slightly above A_cm temperature (Fig. 5.2 b4) to get single phase, just formed fine grains of austenite, it is liable to fast grain coarsening as the proeutectoid Fe₃C had got dissolved.

This Fe₃C had been earlier restricting grain coarsening of austenite. Apart from grain growth, more time, energy (heat), less productivity, more scale and decarburisation occur as the temperature is very high.

Even if, not much grain growth has occurred, such steel on slow cooling (annealing) again gets proeutectoid Fe₃C formed at the grain boundaries of austenite, or pearlite (at room temp.) as a network. Such a Fe₃C network provides easy fracture path and renders the steel brittle during forming, or in service. Thus, heating is avoided in such ranges for annealing.

Not only is the temperature range of heating an important part of full annealing, but slow cooling rate associated with full-annealing is also a vital part of the process, as the austenite should decompose at a small undercooling (i.e. almost just below, or at A₁ temperature) to obtain equiaxed and relatively coarse grained ferrite as well as pearlite with coarse inter-lamellar spacing to induce softness and ductility (lowering the hardness and strength) in steels.

The presence of alloying elements shifts the CCT curve to longer times, and thus, alloy steels may be cooled more slowly than carbon steels to get ductility (i.e., the similar microstructures with cooling rate 30 – 50°C/hr). The cooling rate can be adjusted by opening, or closing the furnace doors, controlling the heating process, or by using special cooling chambers.

Once the austenite has fully transformed (by about 500-600°C), the cooling rate could be increased to reduce the time of annealing, and thus increase productivity by putting the articles in open air, provided the risk of developing thermal-stresses is not much. The cooling in the furnace should be continued to room temperature, if annealing is aimed at reducing stresses, particularly in critical and intricate-shaped parts.

On (full) annealing, whether a steel develops fine pearlitic structure, or a coarse pearlitic structure, it is relative to the original structure of steel, because with appropriate temperature of heating and almost the same temperature of transformation (due to slow furnace cooling) of austenite to pearlite (at or slightly below A₁) and proeutectoid product, the pearlitic interlamellar spacing is almost constant, i.e. the product is almost similar in all cases. It is fine, or coarse pearlitic as compared to the original micro-structure, which could.be coarse, or fine pearlitic respectively.

Full annealing is done with one, or more of the following aims:

1. To Refine the Grain Size of Steel Castings or of Hot Worked Steels:

Steels castings have invariably coarse austenite grains, which result in coarse ferrite grains, or widmanstatten type of structure with poor impact strength as illustrated in left side of Fig. 5.3 (a) and the fine micro-structure developed by full annealing on right side (schematic).

Fig. 5.3 (b) illustrates a widmanstatten and 5.3 (c) a fine grained annealed micro-structure:

Even the hot worked products, such as rolled or forged parts, where hot working had been completed at excessively high temperatures, resulting in austenite grain coarsening, or even widmanstatten structure in the normalised state. Even the welded parts may have micro-structure similar to the cast structure with coarse grains in the heat-affected zone (HAZ).

Even some heat treatment cycles like homogenising annealing leave the steel with coarse grains, as high soaking temperature of 1100°C to 1200°C had been used over a long soaking period. Full annealing at appropriate temperatures of the steels and slow cooling, or even others, produce fine grains of ferrite and pearlite with, simultaneous improvement in mechanical properties.

2. To Relieve Internal Stresses:

If the steel castings are intricate shaped, or are critical parts in which internal stresses are to be completely removed, then full annealing is done.

3. To Soften Some Steels Particularly before Machining:

Steels containing 0.3 to 0.4% carbon have better machinability in annealed state than normalised state. Annealing produces coarser pearlite and ferrite to improve softness and ductility, to improve machinability.

4. To Remove Micro-Structural Defects Produced during Casting, or Hot Working:

The sulphide inclusions aligned along ferrite bands in hot worked steels cannot be changed by usual full annealing. Double annealing is done, the first step being to heat the steel to a temperature considerably above Ac₃ temperature, and then cooling rapidly, to a temperature below the lower critical temperature, and then immediately reheating to the normal full annealing temperature followed by slow cooling.

The first heating coalesces the sulphide films in ferrite and produces homogeneity by rapid diffusion. The quick cooling prevents the formation of coarse ferrite grains. The second step refines the coarse grains and leaves the steel in a soft state. Even the banded structure improves in becoming more uniform by this treatment, though normalising does the trick better as explained in normalising.

Type # 2. Homogenising (Diffusion) Annealing:

Normally, when the carbon steel ingot, after teeming, has solidified, its structure is inhomogeneous. The subsequent heating, soaking and hot working homogenises the structure to a large extent.

This is so, because the diffusion of carbon is very fast at high temperatures, and the simultaneous plastic deformation breaks the dendrites with different portions moving in relation to each other, which facilitates the diffusion process to homogenise the structure quickly.

Dendrites and inter-dendritic segregation, if present, increase the susceptibility to brittle failure develops anisotropic properties and other defects such as low ductility and toughness, different hardenability in adjacent sections.

Chemical heterogeneity can be removed by homogenising (diffusion) annealing. As diffusion of substitutional solid solution forming elements is much slower than carbon at any temperature, the alloy steels ingots are usually homogenised at 1150°C to 1200°C for 10-20 hours followed by slow cooling. Alloy steel castings are also given in similar cycle. Slow cooling may at least be done up to 800-850°C followed by air cooling. The pearlitic classes of hypoeutectoid inhomogeneous alloy steels are held at 1000°C for 1-2 hours, whereas hypereutectoid alloy steels are held for 5-6 hours.

Homogenisation causes grain coarsening of austenite impairing the properties. Thus, steels after this heat treatment undergo either normalising, or full annealing (which avoids residual stresses too) to refine the over­heated structure. Homogenisation also produces thick scales on the surface of the steels.

As homogenisation itself is expensive with loss of metal as scale, and as it requires subsequent treatment for refining the grain structure, it is used in very special cases.

Type # 3. Process Annealing:

These are similar sub-critical annealing heat treatments commonly done to restore ductility to cold worked steel products of variety of shapes. As the temperature of heating (650-680°C) is below Ac₁ tempera­ture, i.e. below the lower critical temperature of Fe-Fe₃C diagram and, as no phase change takes place on heating as well in later cooling, it is called sub-critical annealing. (Fig. 5.1)

When a low carbon steel is cold-worked, work-hardening takes place, i.e. hardness and strength increase, but ductility decreases. The ductility of the steel may be restored by the full annealing operation, but more commonly, recrystallisation annealing is done.

Recrystallisation annealing consists of heating a cold worked steel above its recrystallisation temperature, soaking at this temperature and then cooling thereafter. The final structure after the treatment consists of strain-free, equi-axed grains of ferrite produced at the expense of deformed elongated ferrite grains.

Recrystallisation temperature on an average is given by:

T_r = (0.3 – 0.5) T_m.p.

where, T_r is recrystallisation temperature in Kelvin scale, and T_m.p. is the melting temperature in Kelvin scale. Though, the recrystallisation temperature of pure iron is about 450°C, but it increases with increasing alloy content and inclusions, increasing original grain size, with decreasing amount of prior deformation, increasing temperature of deformation and with decreasing holding time.

Thus, commonly, recrystallisation annealing of carbon steels is done at 650°C to 680°C, whereas of high carbon alloy steels (Cr, Cr-Si, etc.) is done at around 730°C for 0.5 to 1.5 hours. Coarse grained steels may be refined to produce fine-grained steels by heavy cold-working and recrystallisation-annealing.

Annealing for recrystallisation is most commonly applied to cold-rolled low-carbon sheet or strip steels. Generally, the microstructure of low-carbon steels, before the cold-working, consists of largely equiaxed ferrite grains with small amount of pearlite.

Medium, and atleast high carbon steels have normally sphe-iodized pearlite. Both are highly ductile micro-structures. Cold-working work-hardens the ferrite, elongating the ferrite grains in the direction of cold-working and introducing a high density of crystal defects, particularly dislocations. On heating during annealing, first recovery and then, recrystallisation occurs.

In the latter process, new, strain-free, equiaxed ferrite grains nucleate and grow in deformed ferrite. In addition, annealing leads to coalescence and spheroidisation of cementite, if not present already. Normally, grain growth of ferrite grains does not occur due to the presence of cementite globules unless, heated to very high temperatures.

The microstructure now has high ductility again, ready to undergo large cold deformation. Thus annealing may be done intermittently, to restore ductility every time for further processing a sheet, or strip, or wire, and thus are given different names. Stainless steels (for example 18/8), or Had- field-Mn steels are also given recrystallisation annealing quite commonly.

Recrystallisation annealing has some advantages over full-annealing as, little scaling, or decarburisation of steel surface takes place due to lower temperatures used. However in some cases, an undesirable phenomenon may occur during recrystallisation annealing. If the steel had been given light working or skin rolling, there is a region of critical deformation (5-10% reduction), which on recrystallisation (Fig. 5.5) annealing develops large grains, even of gigantic size with poor properties.

To avoid this phenomenon, either he prior cold work should be increased in excess of critical deformation, and if it is impracticable, then full-annealing is used instead of recrystallisation annealing. Full annealing produces lamellar pearlite too. Recrystallisation annealing is used both as intermediate operation and as a final operation.

Type # 4. Spheroidisation Annealing:

It is the annealing to obtain maximum softness particularly in high carbon steels and in high alloy tool steels to improve the machinability (as well as ductility). Ideally, the microstructure consists of coarse spheroidised cementite (or alloy carbides) particles embedded in ferrite matrix. Fully spheroidised condition is preferred for high alloy tool steels. For carbon and low alloy structural steels, the optimum machinability corresponds to 50% spheroidised and 50% lamellar carbide in structure.

A good machinable metal is the one which permits the removal of the metal with satisfactory finish at lowest cost.

The main factors in machining are:

(a) Cutting speed,

(b) Cutting force,

(c) Finish of the machined surface.

Thus, the higher the cutting speed possible in machining, then smaller is the cutting force needed, and better is the quality of the finished surface, and thus, better is the machinability of the metal.

Fig. 5.6. illustrates the effect of ductility and hardness on machinability of a material, and how the change in the microstructure changes the machinability of that material. Hardened steels have poor machinability as high cutting force is needed for the tools to cut in the steel being machined. High forces blunt the cutting tool edge, requiring still more cutting force, and thus, the cutting speed has to be reduced. The machine surface is notched and dull.

Highly ductile soft steel too, is difficult to machine because the long continuous turnings form without easy breakage. The machined surface is rough, uneven and torn, i.e. the quality of the surface is poor. The continuous turnings also wear off the cutting tool easily Low ductility promotes easy breakage of the chips as discontinuous chips.

Low carbon steels (up to 0.3% C) as well as low alloy low carbon steels are very soft and highly ductile in annealed state. Normalising of these steels decreases impact strength and increases slightly the hardness to cause easy chip-breaking to improve machinability, or slight cold-working too improves the machinability.

Medium carbon steels have good machinability in the annealed state. High carbon tool steels (too hard) as well as all alloy tool steels including high speed steels, ball bearing steels have highest machinability’ when the microstructure is spheroidised, or globular cementite (Fig. 5.7 a). Table 5.3 summarises the best state of steels for good machinability.

The softest and most ductile state of any pearlitic steel is when its microstructure consists of spherical coarse carbide particles embedded uniformly in a ferritic matrix, because in lamellar pearlite the movement of dislocations is easily blocked by cementite lamellae, but they by pass them in globular pearlite.

The relative good ductility of spheroidised structure with low hardness makes high carbon steels and alloy tool steels to have high machinability. Because of increased ductility, medium and high carbon steels are cold worked, invariably when in spheroidised state.

The globular microstructure has the lowest energy because of smaller ferrite/cementite interfacial area of cementite spheres in ferrite matrix as compared to large area in lamellar pearlite, and thus is the most stable microstructure.

A steel with any prior microstructure should change to globular microstructure by diffusion, if it is heated to high temperatures and for long times to reduce the interfacial area (and thus energy). Cementite lamellae or plates in lamellar pearlite break up into smaller particles, which eventually take spherical (Fig. 5.10) shapes at 650°C.

Once the lamellae have broken up, small particles dissolve to increase size of larger spherical particles due to further reduction in interfacial energy, resulting in fewer particles in number and more widely spaced. The time of spheroidisation is approximately logarithmically related to temperature. The rate of spheroidisation is inversely related to the lamellar spacing of the pearlite. The presence of either proeutectoid product, does not effect the rate of spheroidisation, i.e., carbon content has no effect.

Alloying elements decrease the spheroidisation process, as either they reduce the diffusion of carbon, or themselves diffuse slowly to form their own spheroidised carbide particles. Al-killed steels spheroidise at somewhat faster rate than do Si-killed steels.

As the interface between cementite and ferrite in pearlite is a low-energy interface, the lamellae of pearlite do spheroidise, but do so extremely slowly even at temperatures close to A₁ temperature, requiring more than 200 hours. Plastic deformation prior to heating, or during heating, increases the rate of spheroidisation. Cold rolling causes cementite plates to kink, or to rotate to become parallel to the rolling plane.

As the orientation changes, the energy of the cementite/ferrite interface increases to speed up the process of spheroidisation, which is faster, higher is the amount of cold work. A steel (.61% C, 0.6% Mn, 0.08% Si) after 75% cold rolling, got spheroidised by heating for 32 hrs at 650°. Fig. 5.8 illustrates effect of cold work in 0.60% carbon and 0.8% carbon normalised -steels on time and temperature of spheroidisation.

Spheroidisation rate of pearlite of hyper-eutectoid steel is similar to that of pearlite in hypoeutectoid steels, but spheroidisation of proeutectoid cementite occurs at a much faster rate due to irrational interface. However, widmanstatten plates of proeutectoid cementite take more time than the cementite of pearlite.

Spheroidisation is a very slow process when pearlitic structure is heated to just below A₁ temperature. The rate of spheroidisation is fast if carbide is present as discrete particles such as in bainitic structure, or the carbide particles obtained by tempering of martensite.

Also, if on heating to slightly above Ac₁ temperature, austenite is allowed to have a good degree of heterogeneity either by heating to lowest austenitising temperature so that inhomogeneous austenite has a large number of undissolved cementite nuclei on which precipitating cementite can grow readily during slow cooling; or, first heating to slightly below Ac₁ temperature so that some spheroids of cementite are formed, which on heating to slightly above Ac₁ temperature resist dissolution, and thus help in the spheroidisation of precipitating cementite when the heterogeneous austenite is cooled slowly through Ar₁ temperature.

The rate of cooling in both cases should be slow enough to ensure that the transformation occurs at temperatures only slightly below the A₁ temperature. The austenitisation temperature is another critical factor in these methods. Fig. 5.9 illustrates the range of austenitisation temperature which can produce spheroidal product, or a mixture of spheroidal and lamellar product, or a lamellar product, which varies with the carbon content of steel.

Hyper-eutectoid steels when heated slightly above A₁ temperature and cooled very slowly through A₁, show spheroidised eutectoid cementite with large spheroidised particles of proeutectoid cementite. Increasing austenitisation temperature results in plates of eutectoid cementite with increasingly larger plates of proeutectoid cementite. Heating to temperatures above A_cm and slow cooling results in lamellar eutectoid cementite with proeutectoid cementite as film (network) surrounding the austenite grain size.

Summarising the Spheroidisation Methods:

1. Heating the steel (C > 0.3%) to a temperature just below Ac₁ temperature, holding at this temperature for a very long period followed by slow cooling, transforms lamellar to spheroidised pearlite. Fig. 5.10 illustrates the process with the cycle. It takes very long time particularly with coarse pearlite.

2. An important rule to get industrially the spheroidised structure is:

Austenitise the steel at a temperature not more than 50°C above A₁ and cool very slowly through A₁ to transform inhomogeneous austenite at a temperature not more than 50°C below A₁ temperature.

Thus, two critical temperatures are:

(1) Temperature of austenitisation, the importance of which is illustrated in Fig. 5.9;

(2) Temperature of transformation below A₁. Closer the temperature to A₁, more coarse and soft is the spheroidised structure, but if transformation occurs much further below A₁, then the product is finer, more lamellar and harder pearlite.

Normally, austenitising temperatures are:

Eutectoid steel: 750 – 760°C

Hypo-eutectoid steel: 770 – 790°C

Hyper-eutectoid steel: 770 – 820°C

High speed steel: 875°C

For example, steel En 19 C having A₁ temperature about 750°C, is given spheroidisation annealing as:

i.. Incomplete austenitisation at 775°C for 2 hours.

ii. Cooling slowly at 10°C/h to 725°C in 5 hours.

iii. Cool in air to room temperature.

3. Steel after austenitisation is cooled slowly 30-50°C/h to 680-620°C and then held isothermally at this temperature. This method takes lesser time of 1-3 hours to get spheroidised structure.

4. Hypereutectoid steels should be first normalised to possibly prevent the formation of network of cementite but as fine dispersion of cementite. It is then heated to 770-820°C and cooled very slowly.

5. Carbon steels and low alloy steels having carbon between 0.5 to 0.77%, may be first given a pre-annealing at about 25°C below A₁ temperature, so that some spheroidisation of cementite takes place. Steel is then heated above Ac₁ ( < 50°C) and then cooled very slowly.

This process takes 2-6 hours to produce spheroidised structure as schematically illustrated in Fig. 5.11:

6. Pendulum Heating:

The steel is heated to 750°C and held at this temperature for a short time, then cooled in another furnace to 680-700°C. After holding for a short time heated again to 750°C and again cooled. These steps are repeated several times in succession to obtain spheroidised pearlite. This process is more difficult to perform, but takes less time.

During heating at 750°C, inhomogeneous austenite is obtained. On cooling, the precipitating cementite deposits on carbide nuclei in inhomogeneous austenite as spheroidal particles. This process continues. On heating again, the dissolution of spheroidised cementite is resisted. The precipitating cementite deposits on these undissolved cementite particles on cooling.

7. Spheroidised pearlite can be obtained by hardening and tempering at high temperatures.

Type # 5. Partial Annealing:

Partial annealing of hypo-eutectoid steels consists of heating the steels in the critical range, i.e., between Ac₃ and Ac₁ temperatures. The pearlite of the steel gets transformed to fine grained austenite, but the shape and the size of the proeutectoid ferrite almost remain as it was in the original micro-structure (Fig. 5.2 a2), i.e. the steel is partially annealed. As in annealing, the steel cools slowly, austenite transforms at, or close to A₁ to coarse and soft pearlite, while the ferrite grains stay as they were, but ferrite it-self is very soft phase.

Partial annealing thus produces softness required for machining. Though it is not a perfect method but is a good practical method reducing the cost of processing and the time to improve machinability and/or ductility if the original pearlite of the steel is fine and hard.

Type # 6. Bright Annealing:

The name itself suggests that it is an annealing treatment after which the surface remains as bright and lustrous as it was before the treatment, i.e., the surface remains free of discoloration and oxides. The surface is protected by using a protective medium (atmosphere) in the annealing furnace.

The atmosphere used depends on the type of steel. Commonly used atmospheres are; argon, or nitrogen, pure hydrogen, cracked ammonia, or a reducing gas atmosphere such as having 15% H₂, 10% CO, 5% CO₂, 1.5% CH₄ and remainder N₂.

Type # 7. Stress-Relieving Annealing:

Internal stresses (residual stresses or locked-in stresses) are stresses which remain in a part even after its source has been removed, i.e., these stresses exist in a part in the absence of external stresses.

These stresses could be developed during:

1. Cold Deformation Operations:

For example, when a metal strip is rolled, the central section of the strip gets greater reduction (elongates more) than the surface layers. As the longer central section pulls with it the surface layers, the tensile internal stresses in the surface layers and the compressive internal stresses in the central section are developed.

2. Machining:

Heavy machining especially leaves behind cold-worked surfaces which induce internal stresses, which may even cause cracking during subsequent heat treatment.

3. Heat Treatment:

Fast heating during heat treatment results in temperature gradient which causes differential expansion across the section of the part, resulting in compressive stresses in the surface layers and the tensile stresses in the interior. Fast cooling (without phase transformation) results in reverse nature of stresses than above.

Solid state phase change during cooling of steel leads to increase of specific volume and is a source of development of large residual stresses if the rate of cooling is high. Quenching stresses cause even development of cracks. Phase change and thermal non-uniform contraction can produce complicated stress patterns in the part. Even, the variation of composition of surface layers such as in carburising causes differential volume change to induce stresses.

4. Casting:

Stresses are invariably present in castings due to non-uniform cooling of the surface as compared to the centre of the castings (due to the different cooling rates between various sections).

5. Welding:

Due to differential expansion and contraction of the heat affected zone (HAZ), and the weld itself.

In every instant, the cause of the retention of these internal stresses is the occurrence of inhomogeneous plastic deformation, which may be due to unequal deformations in various portions of the body, or due to different changes of specific volumes in various sections of the part. Thus, the internal stresses may be thermal, structural, or both.

Residual stresses of different origins are algebraically added together and may form complicated patterns. Tensile residual stresses particularly in surface layers are most dangerous, as these get added to cause warpage or even cracks, even at low, or without external tensile stresses. Such stresses are especially dangerous in parts subjected to alternating stresses as these tensile residual stresses promote fatigue cracks. Residual stresses also promote inter-crystalline corrosion (such as season cracking in brasses).

Residual stresses may induce distortion (warping, etc.) of the shape and dimensional changes in components during its application, or during storage. A component warps (changes its shape and size) if the stress becomes higher than its yield stress; or cracks when it becomes higher than its tensile strength, the stress may be the internal tensile stress. During storage, a gradual redistribution of residual stresses occurs in the components through relaxation, to become very high at some time.

Quenched idle steel roll was found to fracture with a loud crash with pieces flying a few meters away. Catastrophic failures, of welded bridges and of almost all welded ships, have been attributed to residual stresses which became of large magnitude with the passage of their use as residual stresses of different origins got added through relaxation.

As is well known that, a metal if stressed beyond its yield point, gets plastically deformed releasing the stress above its yield stress value. The residual stresses are due to regions of elastic deformations of different signs in the component. If local plastic deformation can be initiated in each region of the elastic deformations in the component, then it can be made to relieve completely or partially the residual stresses.

By this process, there is no change in the dimensions of the components as the extent of elastic deformation in each region is replaced by same amount of plastic deformation. It is also well known, that yield stress of a metal decreases sharply with the rise of its temperature.

Thus, when a metal with residual stresses is heated, then beyond a definite temperature, the yield point becomes lower than the residual stresses. The local plastic deformation then takes place causing the residual stresses to decrease to the value of its yield stress at that temperature.

The residual stresses first decrease quickly due to large multiplication and slip of dislocations, to its value of yield point, and then the mechanism of plastic deformation becomes as in the processes of creep, which results in gradual stress relaxation which decreases with time.

Rosenstein’s results on a steel (C = 0.18%, Cr = 1.65%, Ni = 2.91%, Mn = 0.42%, hardened and tempered at 620°C) as illustrated in Fig. 5.13, indicate that stress-relaxation occurs initially very rapidly, but after which it slows down considerably, i.e. after a certain time at a temperature, it is fruitless to increase the time. It also indicates that higher the temperature of stress-relieving, lower is the remaining residual stresses. In fact, Rosenstein uses Hollomon and Jaffe tempering parameter (also called Larson-Miller parameter) to get stress-relaxation temperature and time for stress-relief.

The quick release of internal stresses by heating quickly to higher temperatures disturbs the equilibrium of internal stresses and thus, may produce warpage. By stress-relieving annealing, intention is to have a slow local plastic deformation, which increases gradually with simultaneous equal reduction of elastic deformation so that linear dimensions of a part do not change.

In many cases, stress relieving is a secondary process, i.e., it occurs alongwith other prime intended heat treatment process. For example, when prime aim in to do recrystallisation annealing, then the casting and welding stresses too are relieved. Tempering, done to get sorbite, relieves almost all the quenching stresses.

When the steel (or any metal, or alloy) is heated as a separate operation of heat treatment to eliminate the residual stresses, it is then called stress-relieving annealing. It is an annealing heat treatment to relieve the stresses induced in parts to reduce the chances of warpage during subsequent heat treatment with no chance of crack formation. Here, it may be required not to have undesirable structural and phase change on heating, which determines thus, the upper limit of temperature range of stress-relieving.

For example, stress-relieving if done above recrystallisation temperature, eliminates the residual stresses left after cold working, but it also removes the strengthening effect produced by cold working which may be inadmissible in most cases.

The rate of heating as well as cooling must be low. Faster heating may aggravate the stress-concentration to cause warpage, or even cracks during heating. Faster cooling may develop new thermal stresses in the component.

Although full annealing is able to relieve internal stresses in castings and forgings, but slow heating to 600°C, when no recrystallisation occurs particularly in steels up to 0.3% carbon is commonly used. Fig. 5.14 shows that at 600°C, almost all stresses are relieved.

As welded structures of steel may distort under its own weight if given full annealing, the following cycle in commonly used for them:

1. Slow heating in a furnace at a rate of 100-150°C/h up to 650°C.

2. Soaking at this temperature for a definite time based on maximum thickness at the rate of 3-4 minutes/mm to attain uniformity of temperature.

3. Slow cooling of 50-100°C/h to at least 300°C and then cooled in air to room temperature.

Tools which develop internal stresses during the application are many times relieved of these stresses by heating them to tem­perature 25°C below the tempering temperature.

Thus, stress-relieving annealing is done aiming:

1. To remove harmful tensile residual stresses to allow higher external loads to be applied.

2. Increase fatigue life.

3. Increase impact resistance.

4. Lower susceptibility to brittle fracture.

5. To avoid inter-crystalline corrosion and fatigue.

6. To stabilise the dimensions.

7. Prevent warpage.

In some articles, residual stresses are created to increase certain operating properties. Shot-blasting, carburising and nitriding increase fatigue life.