Low and High temperature thermomechanical treatments

Thermomechanical treatment involves the simultaneous application of heat and a deformation process to an alloy, in order to change its shape and refine the microstructure. Thus, hot-rolling of metals, a well-established industrial process, is a thermomechanical treatment which plays an important part in the processing of many steels from low carbon, mild steels to highly alloyed stainless steels. The traditional fabrication route involves the casting of ingots varying in size from 1 to 50 tones, which are soaked at very high temperatures (1200-1300°C), then progressively hot rolled to billets, bars and sheet. This leads to the breaking down of the original coarse cast structure by repeated recrystallization of the steel while in the austenitic condition, and by the gradual reduction of inhomogeneities of composition caused by segregation during casting. Also, the inevitable non-metallic inclusions, i.e. oxides, silicates, sulphides, are broken up, some deformed, and distributed throughout the steel in a more uniform manner.

Low temperature thermomechanical treatment -LTMT (Ausforming)

The process known as ausforming or low temperature thermomechanical treatment (LTMT), involves the deformation of austenite in the metastable bay between the ferrite and bainite curves of the TTT diagram. The treatment is shown schematically in Fig. 1a. Steel, with a sufficiently developed metastable austenite bay is quenched from the austenitizing temperature to this region, where substantial deformation is carried out, without allowing transformation to take place. The deformed steel is then transformed to martensite during quenching to room temperature, and the appropriate balance of mechanical properties achieved by subsequent tempering. This ausforming treatment can be contrasted with a high temperature thermomechanical treatment (HTMT), where the deformation is carried out in the stable austenite region (Fig. 1b), usually above Ac3 prior to quenching to form martensite. In a third process, isoforming (Fig. 1c), the steel is deformed in the metastable austenite region, but the deformation is continued until the transformation is complete at the intermediate temperature. The steel can then be slowly cooled to room temperature.

The ausforming process needs careful control to be successful and usually involves very substantial deformation. However, the attraction is that with appropriate steels dramatic increases in strength are achieved without adverse effect on ductility and toughness. Typically, a 4,7% Cr, 1.5%Mo, 0.4%V, 0.34%C steel has a tensile strength of about 2000 MPa after conventional quenching and tempering, whereas after ausforming the strength can be over 3000 MPa.

Steels, in which austenite transforms rapidly at subcritical temperatures, are not suitable for ausforming. It is necessary to add alloying elements which develop a deep metastable austenite bay by displacing the TTT curve to longer transformation times. The most useful elements in this respect are chromium, molybdenum, nickel and manganese, and allowance must be made for the fact that deformation of the austenite accelerates the transformation. Consequently, it is necessary to have sufficient alloying element present to slow down the reaction and avoid the formation of ferrite during cooling to the deformation temperature.

Figure 1. Schematic diagrams of thermochemical treatments:
a) ausforming-low temperature mechanical treatment;
b) high temperature mechanical treatment;
c) isoforming transformation.

The austenite grain size should be as tine as possible, not only to increase the dislocation density during deformation but also to minimize the martensite plate size on quenching from the metastable austenite bay.

Cooling from the austenitizing temperature to the metastable bay must be sufficiently rapid to avoid the formation of ferrite and, after deformation, the cooling should be fast enough to prevent the formation of bainite. The strength achieved as a result of ausforming increases as the deformation temperature is decreased, presumably because of the greater strain hardening induced in the austenite. In any case, the temperature chosen should be low enough to avoid recovery and recrystallization, but high enough to prevent bainite from forming during the deformation. Premature austenite decomposition has been found to be detrimental to mechanical properties.

The amount of deformation is a most important variable. There is a roughly linear relationship between the degree of working and the strength finally achieved, with increases between 4 and 8 MPa per percent deformation. One of the most significant trends is that for many steels the ductility actually increases with increasing deformation, although this only becomes significant at deformations above 30% reduction in thickness.

As might be expected, steels subjected to heavy deformation during ausforming exhibit very high dislocation densities (up to 10¹³cm^-2) formed partly during deformation and partly during the shear transformation to martensite. The deformation is usually carried out in the temperature range (500-600°C) in which alloy carbides would be expected to precipitate, so it is not surprising that fine alloy carbide dispersions have been detected by dark field electron microscopy.

On transforming the warm worked austenite to martensite, it is likely that at least part of the dislocation substructure, together with the fine carbide dispersion, is inherited by the martensite. The martensite plate size has been shown to be very substantially smaller than in similar steels given a straight quench from the austenitizing temperature.

Several factors must contribute to strength because anyone mechanism cannot fully account for the high degree of strengthening observed. However, it seems likely that the major contributions are from the very high dislocation density and the fine dispersion of alloy carbides associated with the dislocations. It should also be added that the fine precipitate particles can act as dislocation multiplication centers during plastic deformation. The martensitic transformation is an essential part of the strengthening process, as it substantially increases the dislocation density and divides each deformed austenite grain into a large number of martensitic plates, which are much smaller than those in conventional heat treatments. It is also likely that these small plates have inherited fine dislocation substructures from the deformed metastable austenite.

Isoforming

The process of isoforming involves deformation of metastable austenite, but the deformation is continued until the transformation of austenite is complete at the deformation temperature (Fig. 1c). This is because the lamellar morphology of pearlite leads to low toughness in ferrite/pearlite steels, the ductile/brittle transition temperature increasing with larger volume fraction of pearlite. However, by applying deformation during the phase transformation, instead of a ferrite/pearlite aggregate, the structure produced consists of fine ferrite subgrains (≈0.5μm diameter) with spheroidized cementite particles (≈25nm diameter) mainly located at subgrain triple points.

As in the case of steels for ausforming, the chosen steel must have a suitable TTT diagram. First, it is necessary to be able to deform the austenite prior to transformation, then the transformation must be complete before deformation has ceased. Only modest increases in strength are achieved. However, there can be a very substantial improvement in toughness due to the refinement of the ferrite grain size and the replacement of lamellar cementite by spheroidized particles. However, for significant gains in toughness, deformations in excess of 70% reduction in area are needed. Finally, care must be taken to restrict deformation to temperatures at which the ferrite and pearlite reactions take place as similar deformation in the bainitic region leads to marked reductions in toughness.

High temperature thermomechanical treatments (HTMT)

In high temperature thermomechanical treatments the deformation is carried out in the stable austenite range just above Ac₃ (Fig. 1b), and so can be performed in steels, which do not possess a suitable metastable austenite bay. The steel is then quenched to the martensitic state and tempered at an appropriate temperature. The strengthening achieved arises from austenite grain size refinement, typically from 10-60 μm to 3 μm, but optimum properties are often obtained if recryslallization of the austenite is avoided. As in ausforming strong carbide forming elements are beneficial, which suggests that alloy carbide precipitation occurs in the austenite during deformation. A particular advantage of this process is that optimum properties can be achieved at modest deformations (≈40%) so that deformation can be carried out more readily on existing equipment. The HTMT process does not yield as high strengths as in ausforming but the ductility and fatigue properties are usually superior.

Clearly, HTMT is a variant of controlled rolling. However, it is normally applied to steels with higher alloying contents which can then be transformed to martensite and tempered.

Industrial steels subjected to thermomechanical treatments

Ausforming has provided some of the strongest, toughest steels so far produced, with the added advantage of very good fatigue resistance. However, they usually have high concentrations of expensive alloying elements and must be subjected to large deformations, which impose heavy workloads on rolling mills. Nevertheless, these steels are particularly useful where a high strength to weight ratio is required and where cost is a secondary factor. Typical applications have included parts for undercarriages of aircraft, special springs and bolts.

The 12%Cr transformable steels respond readily to ausforming to the extent that tensile strengths of over 3000MPa can be obtained in appropriate compositions. 0.4C-6Mn-3Cr-1.5Si steel has been ausformed to a tensile strength of 3400 MPa, with an improvement in ductility over the conventional heat treatment. Similar high strength levels with good ductility have been reported for 0.4C-5Cr-1.3Mo-1.0Si-0.5V steel. All of these steels are sufficiently highly alloyed to allow adequate time for substantial deformation in the austenite bay of the TTT curve prior to transformation.