In the high temperature thermo-mechanical treatment (HTMT), the steel is plastically deformed when austenite is present in its stable state just above Ae3 and then quenched to martensite state as schematically illustrated in Fig. 9.1 (a), followed by tempering at a suitable temperature. The increase in strength occurs due to refinement of austenite grains to 3-10µm. The optimum properties are obtained if the recrystallization of austenite is not allowed to take place.
This is aided by the presence of strong carbide forming elements like V, Nb, or Ti as the precipitation of alloy carbides occurs in the austenite during deformation. Before quenching, the structure of steel should have well developed polygonised structure in austenite. Thus, when martensite forms from such an austenite, the martensitic crystals are seen to be fragmented by sub-boundaries consisting of dislocations.
This results in higher yield and ultimate strength of steel without decreasing the ductility. The optimum properties are obtained at modest deformation of 30-40%, so that such deformation can be done easily on normally existing facilities.
This structure reduces the proneness of steels to temper embrittlement. Though HTMT process does not yield very high strengths but the ductility, toughness and fatigue properties are improved. Fig. 9.2 illustrates schematic diagram showing changes in stress-strain curve of such a steel. Normally HTMT is given to high alloy steels.
The strongly textured martensite obtained is tempered. The light armour plate of such steels increase the ballistic limits by about 15% with the ballistic limit increasing linearly with texture intensity. The ballistic limit is the maximum impact velocity of a standard projectile that the armour plate can withstand without penetration. This resistance is probably due to high modules of elasticity in a direction normal to the plate surface and to increased yield strength in compression in the through-thickness direction.
Types of Thermo-Mechanical Treatment:
It is a thermo-mechanical treatment in which the metastable austenite is deformed while it is transforming to ferrite- pearlite, or bainite as illustrated schematically in Fig. 9.1 (c) and (d) respectively. The deformation is continued until the transformation of metastable austenite is complete at the deformation temperature. It is called isoforming because deformation and transformation take place at a constant temperature.
The presence of lamellar pearlite in normal steels reduces the toughness and raises the ductile/brittle transition temperature with increasing amount of lamellar pearlite. Deforming the austenite while it is transforming, results in fine ferrite subgrains of around 0.5 µm diameter with fine spheroidised cementite particles of around 25 nm diameter located mainly at ferrite subgrain triple points.
Significant gain in toughness occurs when the deformation exceeds 70%, and polygonisation is completed with misorientation across the sub- boundaries of between 1° and 10°. Isoforming in pearlitic region causes modest increase in strength, however significant increase in toughness is obtained with decreased ductile/brittle transition temperature.
Isoforming can also be done while metastable austenite is transforming to bainite (Fig. 9.1 d). This results in marked increase in strength but seriously decreases ductility and toughness.
For isoforming to be performed, the steel should have suitable TTT diagram, i.e., after austenitisation, it should be possible to cool austenite without transformation to the deformation temperature. Then, it should be possible to deform the metastable austenite prior to the transformation, and also that the transformation should be complete before the deformation has been completed. The product, which can be fine pearlite, or bainite, must have properties that are highly anisotropic.
ii. Cryoforming or Zerolling:
It is a slight variation of isoforming in which for example, the metastable austenitic stainless steels such as AISI 301 and 304 are deformed at low temperatures, where some martensite forms during the straining. Due to the formation of some martensite, the plastic deformation of which also occurs alongwith austenite, high rate of work hardening takes place. The yield strength, tensile strength and hardness increase. It is one of the important methods of strengthening austenitic stainless steels (18/8) as well as semi-austenitic precipitation hardenable steels to a tensile strength of over 1550 MPa. It is more effective process for stainless steels than ausforming.
In steels, whose Ms temperature is raised by cold working and may become higher than the cold working temperature, some metastable austenite transforms to martensite Thus, the steels are austenitised, and quenched to sub-zero temperatures, where, then cold working is done.
As Ms temperature is raised by plastic deformation, the amount of martensite formed depends on the extent Ms is raised higher than the deformation temperature. Cold working is done at sub-zero temperatures as it has to be below the Md temperature of the steel as well as the amount of martensite formed depends on the difference between Ms-deformation temperature. If it is too low, austenite may completely transform to martensite during initial stages of rolling.
That is why it is called zerolling (rolling at 0°C). A crying sound is heard when rolling is done at these temperatures due to deformation and transformation occurring simultaneously.
At any given deformation temperature, the yield and tensile strengths increase rapidly with the amount of cold reduction, but the ductility decreases; more significant increase in tensile strength occurs at lower temperatures of deformation. At a constant strength, the ductility is higher at lower deformation temperature.
In zerolling, due to increasing amount of martensite formed, heavy amount of cold working has to be done to achieve high strength. Thus, such high strength steels can be obtained only in the form of strips and require very hard rolls.
The strength obtained by cold rolling of such steels is due to the combined effects of the following factors:
1. Strain hardening of retained austenite which achieves around 80% of the strength.
2. The amount of martensite in structure.
3. The strain hardening of martensite.
Zerolling invariably retains some austenite untransformed, which may transform later on during service causing brittleness as well as stresses in the steels.