In this article we will discuss about:- 1. Meaning of Hardening 2. Objectives of Hardening 3. Austenitising Temperature for Different Classes of Steels 4. Heating Time 5. Components 6.Internal Stresses during Quenching.
Contents:
- Meaning of Hardening
- Objectives of Hardening
- Austenitising Temperature for Different Classes of Steels
- Heating Time for Hardening
- Components for Hardening
- Internal Stresses during Quenching
1. Meaning of Hardening:
Martensite is the hardest micro-structure that can be produced in any carbon steel, but it can be, produced only if the transformation of austenite to mixtures of ferrite and carbide is avoided by faster cooling (quenching) the steel.
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Hardening consists of heating the steel to proper austenitising temperature, soaking at this temperature to get fine-grained and homogeneous-austenite, and then cooling the steel at a rate faster than its critical cooling rate (Fig. 6.1 b). Such cooling is called quenching. Normally, carbon steels are quenched in water, alloy steels in oil (as critical cooling rate of alloy steels is much less), etc.
Martensite having the BCT (body-centred tetragonal) structure is hard and brittle. Higher hardness of martensite relative to ferrite-pearlite, or spheroidised microstructure for common range of carbon steels. Hardness of hardened steel, depends on the formation of 100% martensite in it and the hardness of the martensite depends on the carbon content of the steel.
Hardening is done of steels containing more than 0.3% carbon as the gains in hardness are most substantial in these steels. Mild steels (< 0.3% carbon) tend to be difficult to harden (with not much increase of hardness), because critical cooling rate is attained with difficulty, and that too in very thin sections by using drastic cooling, which may cause distortion and cracks.
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2. Objectives of Hardening of Steels:
Hardening is done to all tools, heavy-duty carbon steel machine parts and almost all machine parts made of alloy steels.
The aims are:
1. Main aim of hardening tools is to induce high hardness. The cutting property of the tool is directly proportional to the hardness of the steel.
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2. Many machine parts and all tools are also hardened to achieve high wear resistance. Higher is the hardness, higher is the wear and abrasion resistance. For examples, spindles, gears, shafts, cams, etc.
3. The main objective of hardening the machine components made of structural steels of the pearlitic class is, to develop high, yield strength with good toughness and ductility, so that higher working stresses are allowed. But higher yield strength (and tensile strength) with good toughness and ductility are achieved not in the hardened state, but after high temperature tempering of hardened steels, i.e., hardening is done of structural steels, to prepare the structure for certain transformations which take place during tempering. Tempered structures have high toughness and ductility, the value of which in the hardened state is nearly zero.
3. Austenitising Temperature for Different Classes of Steels:
I. Austenitising Temperature for Pearlitic Class of Steels:
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One of the basic requirements for hardening a steel is, to first heat, to transform the steel to a homogeneous and fine-grained austenite. The austenitising temperature depends on the composition (carbon as well as alloying elements in steel) and section thickness (higher value of the specified range of temperature is used for thicker sections).
Figure 6.1 (a) illustrate that austenitising temperature depends on carbon content and is generalised as:
For hypo-eutectoid steels = Ac3 + (20 – 40°C)
For hyper-eutectoid steels and eutectoid steel = Ac1 + (20 – 40°C)
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The above range of austenitising temperature for hypo-eutectoid steels, results in single phase, fine grained and homogeneous austenite, which on quenching transforms to fine-grained (very fine needles/plates), hard martensite, which is desired to be obtained in hardening.
Heating hypoeutectoid steels only into the critical range, i.e., above Ac1 but below Av3 is avoided in practice, as the steel then has austenite and ferrite grains. On quenching, the austenite transforms to martensite, but no transformation occurs in ferrite grains, i.e. incomplete hardening occurs.
If hardening was aimed for high hardness, then the presence of soft ferrite does not permit to achieve high hardness, i.e. the hardness shall be low. If hardening was aimed to prepare structure to obtain high tensile and yield strengths by tempering, then the presence of ferrite, which has very low tensile strength and yield strength, does not permit to achieve them.
In fact, ferrite forms the easy path to fracture. Quenching of hypo-eutectoid steels from temperatures much above the required temperatures (Fig. 6.1 a), when grain coarsening of austenite has occurred, results in coarse acicular form of martensite (Fig. 6.2 a). Such a martensite has high brittleness and a unit, or two lower in hardness. The increased brittleness makes the steel to have low impact strength even after tempering, and more prone to quench-warping and cracking.
Hyper-eutectoid steels, when heated in the above range, i.e., just above Ac1 have fine grains of austenite and small nodules of proeutectoid cementite (the network of cementite has been assumed to be broken). On quenching, austenite transforms to fine martensite but the undissolved nodules of cementite remain unchanged. As the hardness of cementite (≈ 800 BHN) is more than that of martensite (650 – 750 BHN), such incomplete hardening results in a structure which has higher hardness, wear resistance as compared to only martensitic structure.
If the temperature of austenitising of hyper-eutectoid steels is increased, but still below Acm temperature, correspondingly increased amount of cementite is dissolved in austenite (whose carbon content then becomes higher than 0.77%), grain growth of austenite may occur, as the cementite barriers to the motion of grain boundaries essential for grain growth have largely dissolved. On quenching, coarse grained martensite with little amount of undissolved cementite, and a large amount of retained austenite are obtained.
The resultant as-quenched hardness of the steel is less, because of:
1. Lesser amount of hard cementite (undissolved) is present.
2. Large amount of retained austenite is obtained as Ms and Mf temperatures are lowered due to increased dissolved carbon in austenite. Austenite is a much softer phase than martensite.
3. Coarse grained martensite is more brittle and 1-2 units less hard.
If hyper-eutectoid steels are austenitised at a temperature above Acm, then the steel has 100% austenite. Dissolution of cementite leads to very rapid grain growth of austenite. The resultant martensite is more coarsely acicular, which is much more brittle, with increased tendency to warp and even crack. More so because much higher thermal stresses are induced due to quenching from a much higher temperature.
The as-quenched hardness, too, is low due to:
(i) Absence of harder cementite,
(ii) Much more retained austenite,
(iii) Martensite is much coarser.
II. Austenitising Temperature for Highly Alloyed Steels:
1. Austenitic class of steels,
2. Carbide class of steels.
1. Austenitic Class of Steels:
In these alloy steels, austenite is a stable phase from room temperature to high temperatures, i.e., austenite does not undergo phase transformation; neither on heating, nor on cooling, i.e., no grain refinement is possible by phase change.
The main aim of heating is to obtain single-phase homogeneous austenite at room temperature, and the heat treatment, called quench-annealing is limited only to austenitic class of steels. This treatment is, in fact the homogenizing annealing, or in some cases recrystallisation annealing. These steels on slow cooling as in castings or even on heating (to 500° – 800°C) precipitate carbides, generally on the grain boundaries of austenite.
Fig. 6.3 (a) illustrates structure of Hadfield manganese steel casting having intense carbide precipitation formed on cooling in mould from casting temperature:
Such carbide precipitation causes:
(i) Decrease in ductility and impact strength.
(ii) Depletes the regions close to grain boundaries of, for example, chromium in stainless steels (18/8: Cr/Ni) (Fig. 6.4 b), decreasing the corrosion resistance of the regions causing intergranular corrosion (Fig. 6.4 c).
(iii) Presence of double phase, instead of single phase austenite, further accelerates corrosion by forming micro-galvanic cells.
Fig. 6.4 (a) illustrates carbon solubility in stainless steels. As the solubility of carbon decreases markedly with the decrease of temperature, carbon precipitates as carbide if cooling is not rapid (Fig. 6.4 b).
As the presence of carbides in austenitic class of steels is always undesirable and detrimental to properties, the carbides are eliminated by heating the steel to higher temperatures (Fig. 6.4 a) to dissolve these carbides, and obtain homogeneous austenite at that temperature.
Hadfield manganese steel is usually heated around 1000-1100°C (commonly 1080°C), and then quenched in water. Air cooling too results, in good structure in thin sections. The fast cooling prevents precipitation again of carbides from austenite.
As no grain refinement occurs, the solutioning-treatment may cause some grain coarsening of austenite, which is retained at room temperature by water quenching. There is increase of toughness and impact strength as compared to double-phase structure.
Such treatment in ‘Hadfield’ Mn steel is many times called ‘water- toughening treatment’. The cold worked austenitic stainless steels by this treatment recrystallise to result in low hardness but with good corrosion resistance. The critically cold worked stainless steels may develop undesirable, very coarse grains of austenite on recrystallisation.
2. Carbide Class of Steels:
These steels are mostly alloy tool steels such as, high speed steel having Fe-0.75% C, 18% W, 4% Cr, 1% V. Such a steel, bases its high red hardness on secondary hardness in which the magnitude of increased hardness depends on the fine and uniform dispersion of as much of alloy carbides as possible to block the motion of dislocations.
The advantages of adding alloying elements in these steels are derived, when almost all alloying elements are dissolved in austenite at high austenitising temperature (1260-1290°C), leaving some vanadium carbide in undissolved state (but finely dispersed, which is made possible by forging etc.) to inhibit grain growth, and then precipitating them as fine and uniformly dispersed alloy carbides during high temperature tempering (540-560°C).
The as-cast condition of these steels have carbides segregated as eutectic (such steels have ledeburitic structure-check 0.75% carbon in Fig. 6.5). Forging breaks down the segregation to make the carbide present more uniformly in globular form (this state is good for shaping by machining).
These steels also undergo phase transformation, and thus, are heat treated to get martensite. The low rate and low degree of dissolution of carbides of alloying elements need, heating the steels to very high temperatures (1260-1290°C).
If austenitising temperature is kept slightly above Ac1 (as in pearlitic class), says 850°C, and then quenched, steel has a hardness of 45 Rc, that is characteristic of martensite having 0.22% carbon in it.
High temperature is thus needed to put more carbon in solution in austenite to obtain high carbon hard martensite. Such steels cannot be austenitised at temperatures above Acm/eutectic temperature (as in austenitic class of steels), because these steels shall then, burn and melt as these are ledeburitic steels.
Though higher the austenitising temperature, more amount of alloying elements are dissolved to be precipitated later during tempering as fine alloy carbides. In, fact, heating close to the eutectic temperature is done but for a few minutes (step heating with first step at 850°C is done) to avoid large temperature gradient, oxidation, decarburisation and grain growth.
As it is impossible to dissolve all the carbides in austenite, some finely dispersed carbide (such as vanadium carbide) are allowed to remain undissolved intentionally to inhibit austenitic grain growth at such high temperatures of austenitising. In hardened state, such steels have alloyed martensite, large amount of retained austenite (alloyed)—35 to 40% and little undissolved alloy carbides.
4. Heating Time for Hardening:
The total heating time should be just enough to attain uniform temperature through the section of the part to enable not only the completion of phase transformation, but also to obtain homogeneous austenite. It should not be longer to cause grain growth, oxidation, and decarburisation. The total heating time includes the soaking time too.
During initial heating-up stage, the surface of the steel is at a higher temperature than the centre. The closer the temperature of the steel becomes to the present temperature (of furnace), the smaller is this temperature difference, i.e., in actual practice, it can be assumed that when the surface has reached the temperature of the furnace, the steel is heated right through.
Based on calculated values, heating time to hardening temperature of 850°C in an oil-fired muffle furnace can be obtained from Fig. 6.6. This diagram is good for plain carbon and low alloy steels.
The time to heat to the temperature depends on the shape and size of the parts, the composition and structure of the steel, arrangement of parts in the furnace and the type of the furnace.
The soaking time depends mainly on the composition of the steel and its original structure. Soaking time depends on the desired degree of carbide dissolution. Since the amount of carbide is different in different types of steels, the soaking time thus depends on the grade of the steel.
The Table 6.6. give experimentally determined total heating time to 800-850°C in different types of furnaces:
A practical guide of time is when the component has attained throughout the required temperature, the colour of the part is indistinguishable from that of the furnace wall (otherwise the part is darker).
The soaking time begins when the surface has attained the present temperature. In salt bath too, the colour of the part is matched with the colour of the transparent liquid salt. Plain carbon and low alloy structural steels contain easily soluble carbides, and thus need a soaking time of 5-15 minutes.
Table 6.7 gives soaking time of some steels:
5. Components for Hardening:
The surfaces of the tools and components should be clean and smooth, and should be free of the foreign materials such as scale, sand etc. by cleaning with wire brushes, or in sand blasting machine, as their presence interferes with the quenching process and decreases hardness. The cleaning process has special significance for components requiring development of uniform and high surface hardness. Oil, grease, or wax, etc. can be removed by rinsing in caustic soda added hot water.
Holes in components and tools are increase tendency to cracking, particularly when water-quenched, (as hardening occurs first there). Such holes may be packed with wet asbestos, clay, or steel inserts to avoid hardening inside them. Threaded holes are blocked by screwing plugs in them.
Many times, special fixtures are made to hold the heated parts to be immersed in cooling tank to avoid distortion. Small-sized parts are often put in pans, or on iron-sheets to be heated and then simply poured into the cooling tank, which already has immersed netted basket, for easy withdrawal from the cooling tank.
The components having small cross-sectional area with long slender length, such as small tool bits, screw taps, etc. require quenching to be done in exactly vertical position, and need to be fixed in fixtures such as one illustrated in Fig. 6.11 (a).
Special tongs with sharp hits, or centre punches are used for withdrawing large-sized parts from the furnace and putting them in quenching tank. Normal tongs, if used, may not only produce soft spots, hut in some cases, even cracks at the contact areas due to large difference in cooling rates.
The degree of roughness of the machined surface appreciably effects the quenching results due to variations in the adherence of gas/vapour evolved, because gas bubbles have stronger tendency to cling to the rough surface and effects the wettability of the steel to the coolant. The adhering film of gas/vapour appreciably reduces the cooling process and results in general decrease in hardness, or may result in soft spots as compared to ground parts.
Components having large holes may be tied around with wires, or in some cases special hooks or suspensions may be used to immerse the components in the quenching tank. Springs of long length may be tightly fitted on hollow mandrels (made of thin-walled pipes) and then quenched.
6. Internal Stresses during Quenching:
Internal stresses are always produced due to non-uniform plastic deformation. In quenching of steels, the non-uniform plastic deformation may be caused by thermal stresses, or structural stresses, but usually by the combination of both factors. Internal stresses development is a very serious problem in hardening heat treatment, since they often result in distortion, or cracking, or even, premature failure of part in service.
If a steel is cooled slowly, the temperature distribution across the section of the part can be regarded to be uniform; thermal and structural volume change then, occur uniformly and simultaneously throughout the section. Hence, no internal stress is set up.
Cooling in quenching takes place non-uniformly, i.e., causes temperature gradient across the section. Surface layers contract more than central part and at different times, which leads to non-uniform volumetric changes.
The contraction of the surface layers is resisted by the central part. It puts the central part under compressive stresses and surface layers in tension. If the stress level becomes more than yield stress of steel (at that temperature), non-uniform plastic deformation occurs. The plastic deformation is neither simultaneous, nor the same throughout the layers of the cross- section.
As the central part is still contracting, the stresses may become smaller. At times, the surface layers may come under compressive stresses after reaching zero level, while the central part be under tensile stresses. The stresses that develop in a quenched part, as a result of unequal cooling, which causes temperature gradient and resultant non-uniform volume changes, are called thermal stresses.
Structural stresses are the stresses, which develop due to non-uniform volumetric expansion, due to phase change (mainly austenite to martensite) and at different times, when the steel is rapidly cooled.
Structural stresses are developed due to two main reasons:
(i) Austenite and its transformation products have unequal specific volumes, leading to a change in volume when transformation occurs.
(ii) The phase changes occur at different times in surface and in centre, and even to different amounts.
Table 6.8 gives specific volumes of different phases with approximate % change in volume and % change in length when austenite transforms to phases indicated there. The austenite to martensite leads to largest expansion. This expansion will be greater lower is the Ms temperature of the steel.
Under right conditions, both type of stresses get superimposed to become larger than the yield strength to cause warping, but when tensile stresses become larger than tensile strength, quench cracks can occur. When an austenitised cylindrical steel piece is quenched, the steel contracts thermally till Ms temperature is reached.
Then, between Ms and Mf temperature, expansion occurs due to austenite to martensite change. After Mf temperature, martensite undergoes normal contraction. The surface and the centre, undergo these changes to varying extent and at different times.
Three cases are considered:
I. Through-hardened steel,
II. Shallow hardening steel in which transformation occurs simultaneously at the surface and the centre,
III. Shallow-hardening steel in which transformation to pearlite occurs earlier in the centre, than martensite at the surface. Let the steel be eutectoid steel-0.77% carbon.
I. Through Hardening Steels:
Fig. 6.7 illustrates cooling of surface and centre of a cylinder superimposed on CCT curve of Steel (0.77% C). As the cooling rate even in the centre exceeds the critical cooling rate, the part is completely hardened up to centre. Fig. 6.8 illustrates the volumetric changes in the piece and the distribution of stresses from the surface to the centre at different stages in cooling.
In stage I, surface and centre are cooled rapidly to result in temperature gradient. Only thermal stresses are produced as the surface is prevented from contracting as much as it should by the centre, putting surface in tension and centre tinder compression as illustrated in 6.8 b-I. In stage II, surface having reached M, temperature, transforms to martensite and expands while centre is still contracting due to cooling, which leads to slight decrease in stresses as illustrated in b-II.
In stage II, under the stress, the centre may get plastically deformed as it is still ductile austenite. After stage II, brittle and hard martensite in surface thermally contracts, while centre is still contracting. This leads to slight increase in stress levels as shown in b-III.
At the beginning of stage IV, centre has attained Ms temperature and begins to expand, forming martensite, while surface is still slowly contracting. The centre, as it expands puts the surface in tension and stress levels are considerably (probably maximum) increased.
The surface has little chance of plastic deformation as it has brittle martensite (unyielding). It is during this stage, the greatest danger of cracking exists (that is why, a thumb rule is used in industry: put the piece in tempering furnace to minimise danger of cracking as tempering induces ductility in surface before centre transforms to martensite). An important conclusion is that internal stresses are highest, not in the beginning, or after it has been cooled completely, but when the centre is transforming to martensite.
Fig.6.8 Volumetric Changes on Quenching and the Distribution of Stresses from Surface to Centre.
In stage V, the centre is contracting thermally and the surface is almost at the room temperature, which leads to decrease in stress levels, and many tines it may even reverse (b-VI). These graphs are oversimplified as the actual distribution of internal stresses at different moments of cooling are more complicated. The stress difference particularly in stage IV increases, as the dimensions of the part and the rate of cooling are increased (provided the piece is through-hardened).
II. Shallow-Hardening Steels:
In it, transformation takes place simultaneously to martensite in surface and to pearlite in the centre in stage II. In stage I. thermal contraction of surface and the centre leads to surface in tension and the centre in compression. In stage II, entire piece is expanding but as expansion is more of the surface layers due to its transformation to martensite, i.e., surface tends to expand more than the centre. It puts the centre in tension and surface comes under compression.
In stage III, as the centre is thermally contracting, the surface (martensite formed) is almost at room temperature, prevents the contraction as much as it should. Higher tensile stresses develop in the centre which is pearlitic, of relatively low tensile strength. The greatest danger now is to produce a tensile crack in the internal central part, but cannot come to surface because of prevalent compressive stresses in the surface.
III. Shallow-Hardening Steels:
In this, the transformation has completed in the centre to 100% pearlite before the surface starts to transform to martensite. The centre has expanded in stage II. In stage III, martensite starts forming in the surface, i.e. expansion occurs of the surface layers. The final result is that compressive stresses increase considerably at the surface, while the centre is under tensile stresses.
Of the three cases, the maximum stresses are developed when the steel is through hardened for the same size of part.
Several factors effect the magnitude of internal stresses developed. Expansion occurs when austenite transforms to martensite, but it occurs over a range of temperature (Ms – Mf), and higher is the temperature of transformation, lesser is the expansion, due to corresponding change in lattice parameters of austenite and martensite, i.e. higher is the Ms temperature of the steel, the specific volume changes are smaller, and thus, there is reduced danger of quench cracking.
Increase of carbon and alloying elements lower the Ms temperature, make the steel more prone to quench-cracking. The presence of high carbon, not only aggravates by lowering Ms temperature, but also increases the brittleness of martensite, increasing the tendency to quench cracking.
The time of holding the quenched steel part between, room temperature and 100°C, if increased, then quench-crack tendency increases. The probable reason is, additional strain produced due to formation of martensite by isothermal transformation of retained austenite.
The development of internal stresses during hardening cannot be fully avoided but can be reduced by using different methods of cooling such as martempering etc. Industrial practice, wherever practicable, prefers surface-hardening, or not through hardening of tools and the machine parts if it can give good life in applications.
This is because, the usually compressive nature of internal stresses obtained in these methods, make the surface layers to bear greater amount of tensile stresses, i.e., the strength of the surface is increased. This advantage, which increases, thus, the life of the part, is used for example in leaf springs by shot pinning the surfaces to induce compressive stresses before putting them in actual service.