A significant increase in the research activity dedicated to high manganese TWIP steels has occurred during the past five years, motivated by the breakthrough combination of strength and ductility possessed by these alloys. Here a review of the relations between microstructure and mechanical properties is presented focusing on plasticity mechanisms, strain-hardening, yield stress, texture, fracture and fatigue. This summarized knowledge explains why TWIP steel metallurgy is currently a topic of great practical interest and fundamental importance. Finally, this publication indicates some of the main avenues for future investigations required in order to sustain the quality and the dynamism in this field.

High manganese austenitic twinning induced plasticity steels: A review of the microstructure properties relationships

Twinning-induced plasticity (TWIP) steels

The physical metallurgy of steels

A novel concept for the heat treatment of martensite, different to customary quenching and tempering, is described. This involves quenching to below the martensite-start temperature and directly ageing, either at, or above, the initial quench temperature. If competing reactions, principally carbide precipitation, are suppressed by appropriate alloying, the carbon partitions from the supersaturated martensite phase to the untransformed austenite phase, thereby increasing the stability of the residual austenite upon subsequent cooling to room temperature. This novel treatment has been termed ‘quenching and partitioning’ (Q&P), to distinguish it from quenching and tempering, and can be used to generate microstructures with martensite/austenite combinations giving attractive properties. Another approach that has been used to produce austenite-containing microstructures is by alloying to suppress carbide precipitation during the formation of bainitic structures, and interesting comparisons can be made between the two approaches. Moreover, formation of carbide-free bainite during the Q&P partitioning treatment may be a reaction competing for carbon, although this could also be used constructively as an additional stage of Q&P partitioning to form part of the final microstructure. Amongst the ferrous alloys examined so far are medium carbon bar steels and low carbon formable TRIP-assisted sheet steels.

Quenching and partitioning martensite-a novel steel heat treatment

A subregular solution thermodynamic model was used to calculate the stacking fault energies (SFEs) of high-manganese (10 to 35 wt pct) steels with carbon contents of 0 to 1.2 wt pct. Based on these calculations, composition-dependent diagrams were developed showing the regions of different SFE values for the mentioned composition range. These diagrams were called SFE maps. In addition, variations in the SFE maps were observed through increasing the temperature, aluminum content, and austenite grain size. These changes were seen either as an increasing trend of SFE caused by raising the temperature and aluminum content, or as a decreasing behavior caused by increasing the grain size. The SFE value of 20 mJ/m2 within these diagrams was introduced as the upper limit for the strain-induced martensite formation. The variations in this limit caused by increasing the temperature and aluminum content were mathematically evaluated to find out the minimum amount of manganese that was required to avoid the martensitic transformation. By introducing the isocarbon and isomanganese diagrams of the SFE, it was seen that both temperature and aluminum had a greater effect on the SFE when added to the steels with the lower manganese contents. Moreover, by adding more aluminum to the composition of the high-manganese steels, its influence on the SFE decreased continuously.

Derivation and Variation in Composition-Dependent Stacking Fault Energy Maps Based on Subregular Solution Model in High-Manganese Steels

When Hadfield manganese steel in the single-phase austenitic condition was strained in tension, in the temperature range - 25 to 300 °C, it exhibited jerky (serrated) flow, a negative (inverse) strain-rate dependence of flow stress and high work hardening, characteristic of dynamic strain aging. The strain rate-temperature regime of jerky flow was determined and the apparent activation energies for the appearance and disappearance of serrations were found to be 104 kJ/mol and 146 kJ/mol, respectively. The high work hardening cannot be a result of mechanical twinning because at -50 °C numerous twins were produced, but the work hardening was low and no twins were formed above 225 °C even though work hardening was high. The work hardening decreased above 300 °C because of the cessation of dynamic strain aging and increased again above 400 °C because of precipitation of carbides. An apparent activation energy of 138 kJ/mol was measured for static strain aging between 300 and 400 °C, corresponding closely to the activation energies for the disapperance of serrations and for the volume diffusion of carbon in Hadfield steel. Evidence from the present study, together with the known effect of manganese on the activity of carbon in austenite and previous internal friction studies of high-manganese steels, lead to the conclusion that dynamic strain aging, brought about by the reorientation of carbon members of C-Mn couples in the cores of dislocations, is the principal cause of rapid work hardening in Hadfield steel.

Mechanism of Work Hardening in Hadfield Manganese Steel.

A study has been made of the work-hardening and wear resistance of aluminum-modified Hadfield manganese steels ranging in composition from 1.00 to 1.75 Pct carbon and from 0.0 to 4.0 Pct aluminum. Aluminum additions reduced carbon activity and diffusivity in austenites of Hadfield’s composition, increasing the metastable solubility of carbon in Hadfield steel. Aluminum additions inhibited mechanical twinning and, by inference, increased the stacking fault energy of austenite. Increasing carbon in solution in austenite expanded the temperature range over which dynamic strain aging and rapid work hardening occurred. Simultaneous aluminum additions and increased carbon content increased the work-hardening rate and high-stress abrasion resistance of Hadfield steel, but there was an optimum aluminum content beyond which both declined. Maximum work-hardening rate was exhibited by an alloy containing nominally 1.75 Pct C, 13.5 Pct Mn, and 1.3 Pct Al. Improved high-stress abrasion resistance was also found in an alloy containing nominally 1.00 Pct C, 13.5 Pct Mn, and 4.0 Pct Al.

The effect of aluminum on the work hardening and wear resistance of hadfield manganese steel

High-purity Fe-C alloys containing up to 5.8 pct Al were quenched from 730°C, then aged at temperatures up to 400°C. Precipitation of a carbide transition phase (presumably e) and of cementite were followed by transmission electron microscopy, coercive force measurements, and changes in yield strength. The presence of aluminum increases the temperature at which cementite begins to precipitate during aging. This effect is similar to that produced by silicon in ferrite, and opposite to that produced by manganese. It is suggested that the effects of Si, Mn, and Al on cementite precipitation from ferrite arise from the effects of these elements on the stability of cementite, as reflected in the equilibrium partition of carbon between cementite and ferrite.

Precipitation of carbides in low-carbon Fe-AI-C alloys

Five carbon steels, AISI 1008, 1020, 1035, renitrogenized 1010 and 1522, were dynamically strain aged in the temperature range 100 to 600°C (373 to 873 K), with strains from 3 to 9 pct and strain rates from 2 × 10-4 to 2 × 10-2/s. Following this, changes in tensile, notch impact and fatigue properties were determined. The data obtained indicate that for a commercial combined straightening and strengthening operation on the cooling beds of a bar mill, the temperature during straining should be between 200 and 400° C, the strain should be less than 5 pct and the strain rate is unimportant, up to at least 10-1/s. After such treatment the tensile properties will be:
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Effects of dynamic strain aging on the subsequent mechanical properties of carbon steels

The Bauschinger effect was measured in the conventional tension-compression mode in five steels, AISI 1020, 1522, and 1035, ASTM A374 (Cor-Ten) and an HSLA Mn-Mo-Cb steel. The results were compared with those obtained by performing the forward strain at a temperature near the peak of dynamic strain aging for each of the steels and the subsequent reverse strain at room temperature. The dynamic strain aging process sharply reduced the extent of the Bauschinger effect and was particularly effective in re-storing the elastic portion of the flow curve during the reverse strain. We conclude that the straightening of steel parts after heat treatment should be done warm, in the dynamic strain aging range, whenever feasible.

W. C. Leslie

Papers

Mechanism of Work Hardening in Hadfield Manganese Steel.

The effect of aluminum on the work hardening and wear resistance of hadfield manganese steel

Precipitation of carbides in low-carbon Fe-AI-C alloys

Effects of dynamic strain aging on the subsequent mechanical properties of carbon steels

On minimizing the bauschinger effect in steels by dynamic strain aging