![Time and Temperature relationship in Tempering
Time and Temperature relationship in Tempering
For a given steel, a heat treater might like to choose some convenient tempering time, say
over night, otherwise different than 1 hour, and thus, wants to calculate the exact
temperature required to achieve the constant hardness.
Hollomon and Jaffe’s “tempering parameter” may be used for this purpose as it relates the
hardness, tempering temperature and tempering time. For a thermally activated process, the
usual rate equation is :
Where, t is the time of tempering to develop a given hardness, and Q is the ‘empirical
activation energy’ . ‘Q’ is not constant in the complex tempering processes but varies with
RT
Q
Ae
t
Rate /
1 −
=
=
activation energy’ . ‘Q’ is not constant in the complex tempering processes but varies with
hardness. Thus, hardness was assumed to be a function of time and temperature:
Interestingly, is a constant, and let it be t0. Equating activation energies of eq (1)
and (2) gives,
As t0 constant then
Where, C is a constant, whose value depends on the composition of austenite. The single
parameter which expresses two variables time and the temperature i.e., T (C + ln t) is called
the Hollomon and Jaffe tempering parameter. (hardness in vickers is preferable)
]
[ / RT
Q
te
f
H −
=
]
[ / RT
Q
te−
[ ] )
(
ln
ln 0 H
f
t
t
T
Q =
−
=
[ ]
)
ln
( t
C
T
f
H +
=
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Full material with slide
- 1. Avala Lava Kumar : D.K.Mishra : Dr.S.K.Badjena Department of Metallurgical & Materials Engineering (MME) Veer Surendra Sai University of Technology (VSSUT), Burla -768018 *E-mail : lavakumar.vssut@gmail.com 134
- 2. Classification Classification HEAT TREATMENT HEAT TREATMENT BULK Annealing Normalizing Hardening SURFACE Thermal Thermo-Chemical Tempering Diffusion Annealing Partial Annealing Process Annealing Full Annealing Recrystallization Annealing Stress Relief Annealing Spheroidization Annealing Flame Induction Laser Electron Beam Carburizing Nitriding Cyaniding Boronizing Chromizing 135
- 3. The steel is heated above A3 (for hypo-eutectoid steels) | A1 (for hyper-eutectoid steels) →(hold) →then the steel is furnace cooled to obtain Coarse Pearlite Coarse Pearlite has ↓ Hardness, ↑ Ductility Not above Acm →to avoid a continuous network of proeutectoid cementite along grain boundaries (→path for crack propagation) Full Annealing N N Annealing Annealing A1 A3 Acm T Wt% C 0.8 % 723oC 910oC Spheroidization Recrystallization Annealing Stress Relief Annealing Full Annealing Full Annealing Normalization Normalization A1 A3 Acm T Wt% C 0.8 % 723oC 910oC Spheroidization Recrystallization Annealing Stress Relief Annealing Full Annealing Full Annealing Normalization Normalization 136
- 4. Recrystallization Annealing The Heat below A1 → Sufficient time → Recrystallization Used in between processing steps (e.g. Sheet Rolling) Cold worked grains → New stress free grains No No Annealing Annealing A1 A3 Acm T Wt% C 0.8 % 723oC 910oC Spheroidization Recrystallization Annealing Stress Relief Annealing Full Annealing Full Annealing Normalization Normalization A1 A3 Acm T Wt% C 0.8 % 723oC 910oC Spheroidization Recrystallization Annealing Stress Relief Annealing Full Annealing Full Annealing Normalization Normalization 137
- 5. Stress Relief Annealing Annihilation of dislocations, polygonization Differential cooling Martensite formation Residual stresses → Heat below A1 → Recovery → → Annealing Annealing Welding Machining and cold working Martensite formation → → → A1 A3 Acm T Wt% C 0.8 % 723oC 910oC Spheroidization Recrystallization Annealing Stress Relief Annealing Full Annealing Full Annealing Normalization Normalization A1 A3 Acm T Wt% C 0.8 % 723oC 910oC Spheroidization Recrystallization Annealing Stress Relief Annealing Full Annealing Full Annealing Normalization Normalization 138
- 6. Spheroidization Annealing Heat below/above A1 (Prolonged holding*) Cementite plates → Cementite spheroids → ↑ Ductility Used in high carbon steel requiring extensive machining prior to final hardening and tempering Driving force is the reduction in interfacial energy Annealing Annealing The spheroidized structure is desirable when minimum hardness, maximum ductility, or(in high-carbon steels) maximum machinability is important. Low-carbon steels are seldom spheroidized for machining, because in the spheroidized condition they are excessively soft and “gummy”. Medium-carbon steels are sometimes spheroidization annealed to obtain maximum ductility. *If the steel is kept too long at the spheroidized-annealing temperature, the cementite particles will coalesce and become elongated thus reducing machinability 139
- 7. Diffusion Annealing Annealing Annealing This process also known as homogenizing annealing, is employed to remove any structural non-uniformity. Dendrites, columnar grains and chemical inhomogeneities are generally observed in the case of ingots, heavy plain carbon steel casting, and high alloy steel castings. These defects promote brittleness and reduce ductility and toughness of steel. In diffusion annealing treatment, steel is heated sufficiently above the upper critical temperature (say, 1000-1200°C), and is held at this temperature for prolonged periods, usually 10-20 hours, followed by slow cooling. usually 10-20 hours, followed by slow cooling. Segregated zones are eliminated and a chemically homogeneous coarse grain steel is obtained by this treatment as a result of diffusion. The coarse grained structure can be refined either by plastic working for ingots or by employing a second heat treatment for castings. Hypoeutectoid and eutectoid steel castings are given full annealing treatment, whereas hypereutectoid steel castings are either normalized or partially annealed for this purpose. 140
- 8. Annealing Annealing Diffusion annealing Partial Temperature ranges for various types of annealing processes Partial annealing Process annealing 141
- 9. Partial Annealing Annealing Annealing Partial annealing is also referred to as intercritical annealing or incomplete annealing. In this process, steel is heated between the A1 and the A3 or Acm. It is followed by slow cooling. Generally, hypereutectoid steels are subjected to this treatment. Resultant microstructure consists of fine pearlite and cementite. The reason for this is that grain refinement takes place at a temperature of about 10 to 30°C above Ac1 for hypereutectoid steels. As low temperature are involved in this process, so it is cost effective than full annealing Process Annealing Process Annealing In this treatment steel is heated to a temperature below the lower critical temperature, and is held at this temperature for sufficient time and then cooled. Since it is a subcritical annealing, cooling rate is of little importance The purpose of this treatment is to reduce hardness and to increase ductility of cold-worked steel so that further working may be carried out easily. It is an Intermediate operation and is sometimes referred to as in process annealing. The process is less expensive than Recrystallization annealing. 142
- 10. Heat above Heat above A A3 3 | | A Acm cm → Austenization → Air cooling → Fine Pearlite ( → Austenization → Air cooling → Fine Pearlite (Higher hardness) Higher hardness) To reduce segregation in casting or forgings Refine grain structure prior to hardening To harden the steel slightly Purposes A1 A3 Acm T Wt% C 0.8 % 723oC 910oC Spheroidization Recrystallization Annealing Stress Relief Annealing Full Annealing Full Annealing Normalization Normalization A1 A3 Acm T Wt% C 0.8 % 723oC 910oC Spheroidization Recrystallization Annealing Stress Relief Annealing Full Annealing Full Annealing Normalization Normalization In hypo-eutectoid steels normalizing is done 50oC above the annealing temperature Normalizing Normalizing In hypo-eutectoid steels normalizing is done 50 C above the annealing temperature In hyper-eutectoid steels normalizing done above Acm → due to faster cooling cementite does not form a continuous film along GB Annealed Normalized Less hardness, tensile strength and toughness Slightly more hardness, tensile strength and toughness Pearlite is coarse and usually gets resolved by the optical microscope Pearlite is fine and usually appears unresolved with optical microscope Grain size distribution is more uniform Grain size distribution is slightly less uniform Internal stresses are least Internal stresses are slightly more Annealed Vs Normalized 143
- 11. Heat above A3 | Acm → Austenization → Quench (higher than critical cooling rate) Certain applications demand high tensile strength and hardness values so that the components may be successfully used for heavy duty purposes. High tensile strength and hardness values can be obtained by a processes known as Hardening. hardening process consists of four steps. The first step involves heating the steel to above A3 temperature for hypoeutectoid steels and above A1 temperature for hypereutectoid steels by 500C. The second step involves holding the steel components for sufficient socking time for homogeneous austenization. Hardening Hardening homogeneous austenization. The third step involves cooling of hot steel components at a rate just exceeding the critical cooling rate of the steel to room temperature or below room temperature. The final step involves the tempering of the martensite to achieve the desired hardness. Detailed explanation about tempering is given in the subsequent sections. In this hardening process, the austenite transforms to martensite. This martensite structure improves the hardness. In the hardening process, which involves quenching and tempering. During quenching outer surface is cooled quicker than the center. In other words the transformation of the austenite is proceeding at different rates. Hence there is a limit to the overall size of the part in this hardening process. 144
- 12. A few salient features in hardening of steel Hardening Hardening Proper quenching medium should be used such that the component gets cooled at a rate just exceeding the critical cooling rate of that steel. Alloy steels have less critical cooling rate and hence some of the alloy steels can be hardened by simple air cooling. High carbon steels have slightly more critical cooling rate and has to be hardened by oil quenching. Medium carbon steels have still higher critical cooling rates and hence water or brine quenching is necessary. quenching is necessary. 145
- 13. Hardening Hardening Factors affecting Hardening Processes Chemical composition of steel Size and shape of the steel part Hardening cycle (heating/cooling rate, temp, soak time Homogeneity and grain size of austenite Quenching media Surface condition of steel part Hardening Methods Conventional or direct quenching Quenching in stages in sequence in different media Spray Quenching Quenching with self tempering Austempering or Isothermal Quenching Martempering 146
- 14. Retained Austenite Retained Austenite Austenite that is present in in the ferrous alloys even after the completion of the heat treatment process is referred to as retained austenite. In simple words, retained austenite is the untransformed austenite. Austenite transforms to martensite between Ms and Mf temperatures as it is essentially an athermal transformation. However, this transformation never goes to completion, i.e., 100% martensite (Mf temperature line is illustrated as dotted line in TTT diagrams). This is because at Mf , a small amount of (~1%) of austenite is present in highly stressed state along with ~99% martensite, and can not transform to martensite because unfavourable stress conditions. Both Ms and Mf temperatures decrease with increase in carbon content. Therefore amount of retained austenite in hardened steels increase with increase in carbon content. All alloying elements, except Al and Co, lower the Ms temperature and hence enhance the amount of retained austenite. Therefore, both high carbon steels and high alloy steels are more prone to the presence of retained austenite. The substructure of retained austenite differs from that of the original austenite as it has as a higher density of imperfections like dislocations, stacking faults, etc. which are created by local plastic deformation of the austenite by martensite crystals. Tool steels may have retained- austenite in the range of 5-35%. At the surface of a quenched steel, that restrains are minimum. R.A is less at surface than center of the part. 147
- 15. Retained Austenite Retained Austenite Advantages Ductility of austenite can help to relieve some internal stresses developed due to hardening, to reduce danger of distortion and cracks. 10% retained austenite along with martensite is desirable. The presence of 30-40% retained austenite makes straightening operation of the components possible after hardening. Straightening increases the hardness slightly. Non-distorting steels owe their existence to retained austenite. Here enough austenite is retained to balance the transformational contracting during heating, on the formation of austenite from ferrite carbide aggregate on the one hand, and the expansion corresponding disadvantages austenite from ferrite carbide aggregate on the one hand, and the expansion corresponding to the formation of martensite during cooling, on the other, Here, the basis of dimensional stability of non-distorting steels is the presence of retained austenite. The soft austenite if present, in large amounts, decreases the hardness of hardened steels. As retained austenite may transform to lower bainite, or to martensite, there takes place increase in dimensions of the part. Not only it creates problems in precision gauges, or dies, the neighboring parts may be put under stress by it. In the component itself, stresses may be created to cause distortion or cracking. Retained austenite decreases the magnetic properties of the steel. 148
- 16. Sub Sub- -Zero treatment Zero treatment The retained austenite is generally undesirable, sub-zero treatment is one of the method to eliminate retained austenite. As the room temperature lies between Ms and Mf temperatures of steel, quenching to room temperature results in retained austenite. Subzero treatment consists in cooling the hardened steel to a temperature below 0°C. The temperature of the sub zero treatment depends on the position of Mf temperature of the steel. A steel can be cooled much below the Mf temperature, but it, evidently achieves nothing, because it cannot bring about any additional increase of hardness, or any additional increase because it cannot bring about any additional increase of hardness, or any additional increase of martensite, because the Martensitic transformation ends at Mf temperature. Sub-zero treatment is more effective, if it is carried out immediately after quenching operation. Any lapse of time between hardening and the cold treatment causes the stabilization of austenite, makes the retained austenite resistant to further transformation. Most steels can be cooled by subzero treatment in a low cooling unit with one of the mediums as given in table (next page) . The low-cooling unit consists of two vessels, the interior one of copper, where the parts or tools to be deep frozen, are placed and the exterior one of steel provided with a good heat insulation. 149
- 17. Sub Sub- -Zero treatment Zero treatment Coolant Minimum temperature ° ° ° °C Dry ice (solid Co2) + Acetone -78 Ice + Salt (NaCl) -23 Ice + Salt (CaCl2) -55 Liquid air -183 Liquid Nitrogen -196 Table : Subzero Coolants with Temperature of Application Liquid Nitrogen -196 Liquid Pentane -129 Freon -111 Figure : Liquid cooled (liquid N2 ) system. Components are immersed in a bath of alcohol, or trichloro ethylene, which is cooled by a submerged liquid nitrogen spray (-150°C ), cooling rates can be controlled. 150
- 18. The space in between the vessels is filled with one of the chosen medium, or a system (figure in previous page) which is inexpensive and can be used. Usually the temperature range used is in range of -30°C to -150°C, and total time of cooling and holding at that temperature (Mf) varies from ½ - 1 hour. The hardness increased by 2-4 HRc. As the amount of martensite increases by sub-zero treatment, it increases hardness, abrasion resistance, fatigue resistance and eliminates the danger of developing grinding cracks. As the newly formed martensite may add further to unfavorable stresses to cause distortion and cracks, the complicated, or intricate shaped components may be first tempered at 150- Sub Sub- -Zero treatment Zero treatment and cracks, the complicated, or intricate shaped components may be first tempered at 150- 160°C immediately after first quenching and then given the sub-zero treatment. Alloyed tool steels – like high speed steel, which now shall need only single stage tempering. Tools and components which need exact dimensions – gauges Carburized steels, especially alloy steels (having elements like Ni in it) to increase their hardness and wear resistance Steels having 0.8 to 1.1%C as hardness increases by 1-3 HRc Sub-zero treatment has been most extensively used for…! 151
- 19. Tempering Tempering The hardened steel is not readily suitable for engineering applications. It possesses following three drawbacks. Martensite obtained after hardening is extremely brittle and will result in failure of engineering components by cracking. Formation of martensite from austenite by quenching produces high internal stresses in the hardened steel. Structures obtained after hardening consists of martensite and retained austenite. Both these phases are austenite. Both these phases are metastable and will change to stable phases with time which subsequently results in change in dimensions and properties of the steel in service. Tempering helps in reduce these problems. Tempering is the process of heating the hardened steel to a temperature maximum up to lower critical temperature (A1), soaking at this temperature, and then cooling, normally very slowly. Variation in properties with tempering temperature 152
- 20. Tempering Tempering Objective Relieve Internal stresses Restore ductility and toughness To improve dimensional stability To improve magnetic properties Structure in as Quenched state Highly supersaturated martensite Retained austenite Undissolved carbides Rods, or plates of carbide particles produced during ‘auto-tempering’ Segregation of carbon 153
- 21. Tempering of plain carbon steels Tempering of plain carbon steels First Stage of tempering temperature extends from room temperature to 200°C. The tempering reactions in steels, containing carbon less than 0.2%, differ somewhat from the steels containing more than 0.2% carbon. In the former, if carbon atoms have not yet segregated (during quenching) to dislocations, these diffuse and segregate around the dislocations and lath boundaries in the first stage of tempering. No ε-carbide forms as all the carbon gets locked up to the dislocations (defects). Martensite in steels with more than 0.2% carbon is highly unstable because of super saturation, and interstitial diffusion of carbon in BCT martensite can occur. Thus in the first First stage of Tempering saturation, and interstitial diffusion of carbon in BCT martensite can occur. Thus in the first stage of tempering, the decomposition of martensite into low-tetragonality martensite (containing ~0.2%C, c/a ~ 1.014) and ε-carbide, Fe2.4C occurs. (There are reports of precipitation of eta-carbide, Fe2C and Haggs carbide, Fe2.2C. ε-carbide is a separate phase and is not a preliminary step in the formation of cementite, but it nucleates and grows more rapidly than cementite. It has HCP structure with c = 4.33A°, a = 2.73A°, c/a = 1.58A° and forms as small (0.015-0.02 μm) platelets, or needles observed under electron microscope. The structure at this stage referred to as tempered martensite, which is double phase mixture of low tetragonal martensite and ε-carbide. In this stage volume ↓ because specific volume of martensite ↓ due to rejecting of C atoms. 154
- 22. Tempering of plain carbon steels Tempering of plain carbon steels Second stage of Tempering Second Stage of tempering temperature lies between 200-300°C. The amount of retained austenite in the as-quenched steel depends mainly on the composition of the steel, and the temperature to which steel is quenched. In the second stage of tempering retained austenite transforms to lower bainite (the carbide in bainite is ε-carbide). The matrix in lower bainite is cubic ferrite (c/a = 1), where as in tempered martensite, the low tetragonal martensite has c/a ~ 1.014 When retained austenite changes to lower bainite, their takes place increase in volume. Third stage of Tempering Third Stage of tempering temperature lies between 200-350°C. In this stage of tempering, ε-carbide dissolves in matrix, and low tetragonal martensite losses its completely its carbon and thus, the tetragonality to become ferrite . Cementite forms as rods at interfaces of ε-carbide and matrix, twin boundaries, interlath boundaries, or original austenite grain boundaries. During this stage, volume decreases just as in stage one, due to complete loss of tetragonality. In a 1% carbon steel , the total decrease in length in the first and third stages in around 0.25% 155
- 23. Tempering of plain carbon steels Tempering of plain carbon steels Fourth stage of Tempering Fourth Stage of tempering temperature lies between 350-700°C. Growth and spheroidisation of cementite, as well as recovery and Recrystallization of ferrite occur. Though the growth of cementite starts above 300°C, its spheroidisation starts above 400°C to 700°C. Spheroidisation takes place due to reduction in interfacial energy of ferrite-cementite interfaces. As quenched martensite has high concentration of lattice defects. Though their annealing out starts in the third stage of tempering, but the cementite precipitates retard the recovery processes. recovery processes. Substantial recovery processes starts occurring only above 400°C. original lathe boundaries are stable up to 600°C, but above this, these are replaced by equiaxed-ferrite grain boundaries – the process, which is best described as ‘Recrystallization’. In the end, the optical microstructure consists of equiaxed ferrite grains with coarse Spheroidal particles of cementite, and then the structure is called globular pearlite, or spheroidized cementite. The structure perhaps is the most stable of all ferrite- cementite aggregates, and is the softest with highest ductility with best machinability. 156
- 24. Effect of carbon on Tempering Effect of carbon on Tempering Effect of tempering temperature( 1 hour at each temperature) on hardness and reactions 157
- 25. Tempering of alloy steels Tempering of alloy steels The presence of alloying elements, steels can change their nature and properties effectively. Most common elements (except cobalt) shift the CCT curve to longer times, which essentially result in the increase of hardenability of the steels, so that pearlitic transformation can be avoided easily to obtain Martensitic structure, even at a slower trate of cooling and in thicker parts. Alloying elements also lower Ms and Mf temperatures, increasing further the amount of retained austenite. The decomposition of retained austenite on tempering, plays quite a significant role on the properties of tempered steels, specially having high carbon and high alloying elements. Some elements, that are not found in carbides, but are present as solid solution in ferrite, are Al, Cu, Si, P, Ni, and Zr. Some elements arranged in order of increasing tendency to form carbides are Mn, Cr, W, Mo, V and Ti. These carbide forming elements retard most effectively the rate of softening during tempering of the steel. The first stage of tempering does not appear to be effected by the presence of the alloying elements. However, most of the alloying elements in steels tends to increase the hardness after tempering than a plain carbon steel with the same carbon content. At smaller concentration, they merely retard the tempering processes hence the softening, particularly at higher temperature (> 500°C), where these elements have good diffusivity to take part in tempering reactions. 158
- 26. Tempering of alloy steels Tempering of alloy steels When alloy carbides are formed, the drop in hardness during tempering is not only retarded but is significantly increased. The steel is then said to secondary hardening. Thus, 0.5% chromium , or less than 0.5% Mo resists softening but secondary hardening is produced by either 12% chromium, or 2 % Mo.. Stronger the carbide, the more potent is the secondary hardening. Effect of increasing chromium in 0.35%C steel on tempering Effect of increasing Mo on tempering of as quenched 0.1%C steel 159
- 27. Tempering of alloy steels Tempering of alloy steels Element, such as, silicon dissolves in ε-carbide to stabilize it. Steels with 1-2% silicon have ε-carbide present even after tempering at 400°C, which means that the formation of cementite is delayed considerably, and thus, resisting the softening. Otherwise, the effect of silicon is essentially due to solid solution strengthening. Nickel has a small, but constant effect on tempered hardness at all temperatures due to solid solution strengthening as it is not a carbide former. Manganese has little effect on softening at low tempering temperatures, but at high temperatures, has a strong effect on due to its faster diffusion then, and thus, it resists cementite coarsening as it is present in cementite as (Fe, Mn)3C. Martensite in plain carbon steels losses its tetragonality by 300°C, but the tetragonality is seen at 450°C, or even at 500°C if the steels have elements like Cr, W, Mo, Ti, V and Si. The basic cause of steep softening in carbon steels on tempering above 400°C, is the coagulation of the cementite particles. Alloying elements notably Si, Cr, Mo, V, when present in steels, retard the coalescence and the coarsening of cementite particles., resulting in enhanced hardening over and above the solid solution hardening effect. Elements like Cr, Si, Mo, or W delay coarsening to temperature range of 500-550°C. up to tempering temperature 500°C, the carbides formed are of iron with proportional alloying elements in it, but above 500°C, alloying elements can form their own carbides and thus, coarse cementite particles are replaced by fine dispersion of more stable alloy carbides. 160
- 28. Tempering of alloy steels Tempering of alloy steels An Fe-10 Ni alloy shows constant hardness on tempering up to 450°C and then, there takes place some decrease in strength (curve I). Addition of 0.12% carbon increases the as quenched strength to almost double, and slow decrease of hardness occurs on tempering to fall to 0.7 GPa at 500°C. A 8% cobalt addition, which doesn’t enter the carbide, delays the softening to enter the carbide, delays the softening to have strength of 0.8 GPa at 500°C . Addition of 2% Cr almost continuously but slowly ↑ hardness to start falling at above ~450°C to become 1.1 GPa at 500°C by fine dispersion of Cr carbide. Addition of Mo causes secondary hardening, as it is very strong carbide forming element, to attain a hardness of 1.3 Gpa at 500°C Effect of C, Co, Cr, and Mo on tempering of Fe-10Ni steels 161
- 29. Tempering of alloy steels : Tempering of alloy steels : Secondary Hardening In alloy steels, having larger amounts of strong carbide forming elements like Mo, Ti, V, Nb, W, Cr etc., and carbon , a peculiar phenomena occurs, the hardness of the as-quenched martensite (called primary hardness) on tempering, decreases initially, as the tempering temperatures is raised, but starts increasing again to often become higher than the as quenched hardness, with much improved toughness, when tempered in the range of 500 to 600°C. This increase in hardness is called secondary hardness (also called red hardness). This is great importance in high speed steels, as these are able to continue machining, at high speeds (as these are able to resist fall in hardness and thus, the cutting property ) even when they become red hot. Secondary hardening is a process, similar to age hardening, in which coarse cementite Secondary hardening is a process, similar to age hardening, in which coarse cementite particles are replace by a new and much finer alloy carbide dispersion of V4C3, Mo2C, W2C (which normally form on dislocations). As in aging a critical dispersion causes a peak in the hardness and strength of the alloy, and as over aging takes place, i.e., carbide dispersion slowly coarsens, the hardness decreases. Secondary hardening is best shown in steels containing Mo, V, W, Ti and also in Cr steels at high chromium concentrations. The amount of secondary hardening in an alloy steel is directly proportional to the volume fraction of the alloy carbides, and thus is directly proportional to the concentration of strong carbide forming elements present in steels. The alloy carbides must precipitate as fine dispersion in ferrite matrix rather than massive carbide particles. 162
- 30. Time and Temperature relationship in Tempering Time and Temperature relationship in Tempering For a given steel, a heat treater might like to choose some convenient tempering time, say over night, otherwise different than 1 hour, and thus, wants to calculate the exact temperature required to achieve the constant hardness. Hollomon and Jaffe’s “tempering parameter” may be used for this purpose as it relates the hardness, tempering temperature and tempering time. For a thermally activated process, the usual rate equation is : Where, t is the time of tempering to develop a given hardness, and Q is the ‘empirical activation energy’ . ‘Q’ is not constant in the complex tempering processes but varies with RT Q Ae t Rate / 1 − = = activation energy’ . ‘Q’ is not constant in the complex tempering processes but varies with hardness. Thus, hardness was assumed to be a function of time and temperature: Interestingly, is a constant, and let it be t0. Equating activation energies of eq (1) and (2) gives, As t0 constant then Where, C is a constant, whose value depends on the composition of austenite. The single parameter which expresses two variables time and the temperature i.e., T (C + ln t) is called the Hollomon and Jaffe tempering parameter. (hardness in vickers is preferable) ] [ / RT Q te f H − = ] [ / RT Q te− [ ] ) ( ln ln 0 H f t t T Q = − = [ ] ) ln ( t C T f H + = 163
- 31. Temperature and colours for Heating and Tempering of Steel Colours of Hot Solid metal ° C Process of Heat treatment White 1500 Yellow white 1300 High speed steel hardening (1230-1300°C) Yellow 1100 Orange Red 1000 Alloy steel hardening (800-1100°C) Light-Cherry-Red 900 Cherry-red 800 Carbon steel hardening Dark-red 700 Vary dark-red 500 High speed steel tempering (500-600°C) Black red in dull light, or darkness 400 HEAT COLOURS Black red in dull light, or darkness 400 Steel gray 300 Carbon steel tempering (150-575°C) Colour of Oxide film ° C Parts Heat treated Steel Gray 327 Cannot be used for cutting tools Pale-light blue 310 For springs Purple 282 Spring and screw drivers Brown 270 Axes, wood cutting tools Gold 258 Shear blades, hammer faces, cold chisels Dark-straw-light-brown 240 Punches and Dies Light-Straw-Yellow 220 Steel cutting tools, files, paper cutters TEMPER COLOURS 164
- 32. Embrittlement during Tempering Embrittlement during Tempering Normally, as the tempering progresses, the hardness and the strength decreases, but the ductility and impact strength increase. But in certain steels, there is an unexpected decrease of the impact strength in certain ranges of temperatures. This indicates that there are two main types of embrittlement during tempering. Tempered Martensite Embrittlement (TME) Temper Embrittlement Both these embrittlement raise the impact transition temperature (ITT) to higher temperature. Figure (below) indicates the increase in impact transition temperature, Δ(ITT) due to TE in SAE 3140 steel. due to TE in SAE 3140 steel. unembrittled embrittled Temper-embrittlement in SAE 3140 steel shifts the impact transition temperature to higher temperature . Δ(TTT) indicates the amount of increase. 165
- 33. Embrittlement during Tempering Embrittlement during Tempering Normally, the degree of both type of embrittlement is expressed in terms of relative displacements of ITT i.e., by Δ (ITT) illustrated in figure (in previous slide). In both the transgranular fracture mode is replaced by an intergranular (IG) mode below the transition temperature, i.e., these show bright inter crystalline fracture along original austenite grain boundaries. Inspite of these similar effects, the two types of embrittlement are two separate phenomena, which occur in two different temperature ranges. Moreover, TME is a much faster process usually occurring in one hour, where as TE takes many hours. Temper embrittlement is of mush greater concern from practical point of view, as the rotors Temper embrittlement is of mush greater concern from practical point of view, as the rotors and shafts of power generating equipment even after tempering above 600°C, develop it when thick section cool very slowly through the range (450-600°C) Schematic illustration of effect of tempering temperature on impact strength in steels prone to embrittlement during tempering. 166
- 34. As TME develops after tempering in range 260°C to 370°C, it is called as “350°C” embrittlement, or 500°F embrittlement. It is called ‘one-step embrittlement’ as during heating only in this range, TME develops. It is also called ‘irreversible embrittlement because a steel embrittled by tempering in this range , if heated further to above 400°C (above the critical range), becomes tougher, and the tempered martensite embrittlement does not occur again if cooled down to or tempered in the range of 260°C to 370°C again. All steels, including the plain carbon steels are prone to irreversible embrittlement to some extent, and that is why tempering range of 260°C- 370°C is avoided in all steels, though it is a malady of low alloy steels. The embrittlement is associated with the change in the structure of carbide from epsilon (ε) Tempered Martensite embrittlement Tempered Martensite embrittlement The embrittlement is associated with the change in the structure of carbide from epsilon (ε) to cementite in the form of a film at the grain boundaries. On tempering at higher temperatures, this film disappears and can not be restored, on repeated heating in 260°C-370°C temperature range. Although, tempered martensite embrittlement is concurrent with the precipitation of cementite, but such precipitation is not in itself the cause of loss of impact toughness, as the embrittlement is does not occur if P, Sb, Sn, As, or N are not present in steel. Addition of sufficient silicon to the steel inhibits the formation of cementite in the critical range, as silicon dissolved in epsilon carbide, increases its stability, and thus embrittlement does not occur 167
- 35. Temper embrittlement Temper embrittlement The sickness of alloy steels occurs when they are tempered in the range 450°C to 600°C. it is also called reversible embrittlement (as well as two step embrittlement), because it occurs, when steels are tempered in this range, but gets removed, when heated to high temperatures, but occurs again on slow continuous cooling through this range from that high temperature (>600°C). The degree of embrittlement depends on the rate of cooling in the range 600-450°C. The phenomena of temper embrittlement results in loss of toughness as measured by notched impact test (without affecting very much the hardness, Y.S, UTS, elongation and fatigue properties), and a rise in ductile to brittle transition temperature occurs, with an intergranular (IG) fracture, below the transition temperature along the original austenitic intergranular (IG) fracture, below the transition temperature along the original austenitic grain boundaries. Carbon steels in general, but with less than 0.5% Mn, do not show temper embrittlement. Alloy steels of high purity do not show it. It is caused primarily by Sb and P and secondarily by Sn or As (even in amounts as low as 0.01%) in presence of elements like Ni, Mn, Cr, Si in steels. The highest effect is in Ni-Cr and Mn-Cr steels. Presence of elements like Mo, Ti, Zr delay, or remove embrittlement. The characteristic features of temper embrittlement are best explained by the concept of co- segregation. The impurity solutes are the surface active elements in iron, i.e., these reduce the grain boundary energy, and thus reduce the cohesion. Elements like Sb, P, As, Sn, interact with certain elements like Ni and Mn in steels. 168
- 36. Temper embrittlement Temper embrittlement These interactions leads to co-segregation of alloying elements and the impurity elements such as between Ni-Sb, Ni-P-Ni-Sn and Mn-Sb. The reason of co-segregation is the stronger interaction between them than, between either of these and iron. If the interaction is very strong then, co-segregation does not occur, but a scavenging effect is got, as happens between Mo-P, Ti-P, which is the cause of elimination of embrittlement by 0.5% Mo in such steels. If larger amount of Mo, Ti, Zr are present , then these elements slowly react with carbon to form stable carbides releasing the impurity atoms to segregate to the boundaries. Additional segregation may take place, when two alloying elements are present simultaneously, such as Ni and Cr. At high temperatures (>600°C), thermal vibrations make simultaneously, such as Ni and Cr. At high temperatures (>600°C), thermal vibrations make the equilibrium segregation low enough not to cause embrittlement., and at lower temperature (<450°C), the diffusion of the elements is too low to cause enough co- segregation with in the normal tempering time. The following methods are normally recommended to minimize the effect of Temper embrittlement Keep the impurities such as Sb, P, Sn, As as low as possible Alloy the steel with Mo (0.5-0.75%) Quench from tempering at higher temperatures 169
- 37. Heat Treatment Defects Heat Treatment Defects Heat treatment of steels or aluminum can lead to several defects. The principal types of defects found in quenching of steels are internal and external cracks in the work, distortion and warping. CRACK : When the internal tensile stresses exceed the resistance of the steel to separation the crack occurs. The insertion of the tools in the furnace without preheating for tempering is one of the main causes of crack propagation. The crack formation is reduced by preheating the tool between 2000Cto 3000C. DISTORTION : Distortion occurs due to uneven heating, too fast cooling, part incorrectly supported in furnace, incorrect dipping in quenching and stresses present before preheating. Distortion can be prevented by preheating the tool or check furnace capacity, reduce the Distortion can be prevented by preheating the tool or check furnace capacity, reduce the hardening temperature, and by reviewing the method of dipping. WARPING : Asymmetrical distortion of the work is often called warping in heat-treating practice. Warping is usually observed upon non-uniform heating or over heating for hardening. It also occurs when the work is quenched in the wrong position and when the cooling rate is too high in the temperature range of the martensite transformation. An elimination of these causes should subsequently reduce warping. The properties required in the heat treated part are obtained without the parts being distorted beyond the acceptable limits. The ideal design for a heat treatable part is the shape that when heated (or cooled) would have the same temperature at every point with in the part. 170