In large number of commercial heat treatment, the transformations occur during continuous cooling of the steel, and the information from TTT diagrams is not directly useful to these heat treatments.
However, TTT diagrams are the basis of certain heat treatments in which transformations occur isothermally, such as:
1. Martempering:
It is a hardening process with an objective to minimise distortion and cracking. In normal hardening operation, there develops considerable temperature difference between the surface and the centre of the part when heated steel is plunged in coolant. This produces considerable volumetric internal stresses (called thermal stresses) due to differential volume contraction to cause distortion, or cracking.
Internal stresses also develop (called structural stresses), when expansion lakes place due to the transformation of austenite to martensite, whose magnitude becomes much larger if the transformation to martensite occurs at differential times from the surface to the centre due to attainment of martensitic range at different times.
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If both these types of, internal stresses develop simultaneously, stresses become quite complex. At places, the total magnitude of the stresses may become large enough to cause distortion or even cracks. If the internal stresses of tensile nature exceed the yield point of the material, the steel distorts, but if exceed the tensile strength, the steel develops cracks.
Martempering consists of quenching the heated (austenitised) steel to a temperature above Ms, usually by plunging in a salt bath (maintained at 20-30°C above Ms i.e. 180°C to 250°C), holding for a time sufficient for the temperature to become uniform (from the surface to the centre of the part), and then air cooling through Ms to room temperature. Tempering is then done, as required for the part. Holding time should not be large, otherwise austenite may transform to unwanted bainite.
Fig. 3.8 illustrates schematically the process of martempering:
In martempering, as the bath temperature is above Ms (not the room temperature as in normal hardening), the steep temperature gradient is reduced; the magnitude of thermal stresses is reduced. The equalisation of temperature of the part prior to martensitic formation, so that martensite forms simultaneously across the section of the part, helps to reduce the transformation (structural) stresses as air-cooling does not produce much difference of temperature across the cross- section.
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When the part is removed from the salt bath for air-cooling, it being in austenitic stale, is ductile, and can be straightened in presses (usually) to remove any warpage specially of thin and long articles. As martensite forms during air-cooling (slow cooling), more austenite is retained, resulting in lesser volume- change. Thus, overall stresses are minimal to prevent warpage with the least danger of crack formation.
The rate of cooling from austenitising temperature should avoid formation of pearlite and bainite. Thus, the success of martempering depends on the incubation period at the pearlitic nose, and also at the bainitic hay (bainite formation is to be avoided during holding), and thus, the steel suitable for martempering should have sufficient hardenability. Plain carbon steels with diameters more than 10 mm are difficult to martemper and thus, this process is restricted to alloy steels.
TIT curves are useful:
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(i) To fix the correct bath temperature,
(ii) To fix the isothermal holding time without bainite formation,
(iii) To find out the maximum size of the part of that steel which can be martempered.
2. Austempering:
Austempering is a hardening treatment in which austenite transforms isotliermally to lower-bainite (rather than martensite), and thus, objectively reduces distortion and cracks in high carbon steels. The austenitised-steel is quenched in molten salt-bath held at a temperature above Ms (at 300°C-400°C), and then, the steel is held at this temperature to let austenite transform to lower-bainite. No tempering is done. Fig. 3.9. illustrates schematic diagram for austempering:
The equalisation of temperature throughout the part prior to bainitic transformation minimises the stresses developed during austempering, which are negligible compared to stresses developed during through-hardening.
Steel should have sufficient harden- ability to avoid pearlite-formation, when quenched into heated molten bath. Alloy steels, used for increased harden- ability, might have too long a bainitic bay (‘S’ curves are much more on the right side) i.e., very long times may be required at holding temperature for the completion of bainitic transformation (If bainitic transformation is incomplete, the remaining austenite transforms to martensite while cooling to room temperature, which impairs the properties).
Thus, the major processing advantage of austempering, i.e., requiring no tempering may be offset due to increased holding-time required. AISI 9261 steel is not austempered as the holding-time Fig. 3.9. Schematic diagram for austempering superimposed on TTT diagram, may be greater than 24 hours. Carbon steel parts of only small size (5 mm for AISI 1080) are austempered due to very high cooling rates needed to avoid pearlitic transformation.
Austempered steels are superior to the quenched and tempered products when the hardness is in the range of HRC 50-55. Table 3.1 compares the properties for these two heat treatments in 0.75% carbon steel.
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The advantages of austempering are:
(i) Improved ductility at the same hardness,
(ii) Freedom from distortion and cracks
(iii) No tempering is needed,
(iv) Improved impact strength,
(v) Uniformity of properties,
(vi) High endurance limit.
TTT diagram is of great help in scheduling austempering as it fixes:
(i) Temperature of holding of the bath,
(ii) Duration of holding there,
(iii) Section which can be quenched to avoid pearlite formation,
(iv) Whether austempering is worthwhile, or not, as the time at the bay may be too long.
3. Isothermal Annealing:
Steel, austenitised at a temperature above Act3, is cooled quickly to the temperature of isothermal holding, (which is below A1 in the pearlitic range), held there for the required time so that austenite transforms completely, and then cooled in air. The time and the temperature of holding is given by the TTT curve of the steel.
As the isothermal annealing is normally done to soften the steel, the temperature of isothermal holding is very close to but below A1 (by 30- 100°C). The closer is the holding temperature to A1, the coarser is the inter- lamellar spacing of pearlite, softer is the steel, but longer is the time of isothermal transformation.
Isothermal annealing has two common advantages over normal annealing. As the cooling can be done in air, after the transformation has been completed at the isothermal temperature, the total time of heat treatment (particularly in some alloy steels), the cost of annealing is lesser with higher productivity of the furnace.
The microstructure obtained is uniform because of transformation occurring at a constant temperature as compared to normal annealing where transformation occurs over a range of temperatures (Fig. 3.10). Isothermal annealing may be done in two furnaces-one for austenitisation, and the other for constant isothermal transformation or a continuous conveyer type furnace may be used.
4. Patenting:
Patenting is an isothermal heat treatment process used for producing high strength ropes, springs and piano wires of normally 0.45% to 1.0% carbon steel. In fact, the strongest material in commercial quantities is the patented and cold drawn wire made from 0.80% to 1.0% carbon steel, (disregarding whisker filaments) containing no martensite. Wires having high ultimate tensile strength of 4830 MN/m2 with 20% elongation have been reported. Table 3.2 gives some values.
Patenting consists of austenitising steel in a continuous furnace to temperature 150-200°C above Ac3 (usually 870°C to 930°C to get a completely homogeneous austenite so that it transforms to pearlite at the desired undercooling and not earlier), then cooling rapidly in and holding in a lead, or salt bath maintained at a temperature of 450-550°C (near the pearlitic nose of its TTT curve, i.e. in the lower temperature limit of pearlitic transformation) for sufficient time for austenite to completely transform to finest pearlite (some upper bainite may also form), and then wound on to driven drum.
This treatment produces finest pearlite of constant interlamellar spacing in the whole section of the part with no chance of forming proeutectoid ferrite, or cementite (depending on the original carbon content of the steel). The eutectoid-steel after patenting may have interlamellar spacing as small as 40 nm with strength as 1240-1450 MPa.
It is then drawn-cold, with interlamellar spacing of 10 nm. Now-a-days, double-cascade quench process is used. The wire from lead austenitising bath is quenched by liquid salt to a temperature below Art and then passed into second cascade quench at a temperature 450-565°C and finally enters a holding furnace where transformation is completed. Patenting can be done repeatedly after being drawn between each patenting. Hall-Petch relationship for flow stress is applicable with smallest interlamellar spacing.
The process of patenting helps in two ways to obtain high strengths. Patented wires can be cold drawn by large extent (80-90%) without fracture as the soft, weak proeutectoid ferrite, or brittle cementite is absent and the interlamellar spacing is same everywhere.
As the drastic wire drawing decreases the interlamellar spacing further to very small values to block dislocation motion, strength increases drastically with high toughness in twisting and bending. (After true strain of 4, the cementite lamellae thickness is about 2 nm). It is well known that wire should be kept cool during drawing to prevent fracture.
TTT diagram helps to fix the time and temperature of holding. The speed of motion of the wire through the bath should be such that it remains in bath for a time slightly greater than the time of completion of pearlitic transformation to avoid any chance of transformation of untransformed austenite to bainite, or martensite later. When a thick wire passes through the bath, its heat may raise the temperature of bath. Then, the bath is maintained at the lower limit of 450-550°C.