High-temperature strength of die steel with regulated austenitic transformation during exploitation after quenching and tempering

Steels with regulated austenite transformation during exploitation (RATE) represent a novel category of tungsten-free ferritic steels designed for hot forming [1; 2]. These steels fall into either single-phase (γ) or two-phase (γ + α) region and demonstrate a propensity to maintain high-temperature hardening [3]. This effect enables the hot pre-hardening of the steel. Before being put into operation, the steel undergoes austenitization at 1150 °C, followed by a cooling process down to 450 °C. The steel retains its austenitic structure because the supercooled austenite is exceptionally stable. Subsequently, the steel becomes ready for shaping, resulting in sufficient hardness maintenance within the temperature range of 750 – 800 °С [4].

It is worth noting that RATE steels can operate at high temperatures even without pre-hardening, relying solely on quenching and tempering [5]. However, the newer, advanced RATE steel grades have not been extensively investigated in this context. Therefore, the objective of this study is to assess the high-temperature strength of the new RATE die steel after quenching and tempering.

We subjected the 5Kh2G4N3М2FSТB steel to guenching (1100 °C for 20 min in oil) and double tempering (590 °C for 2 h + 560 °C for 2 h in air) to achieve a hardness of 55 HRC [6]. The image depicts the Zwick machine employed for testing threaded cylindrical samples measuring 20 mm in length and 4 mm in diameter:

– the samples were heated to 450 °C, held for 15 min, and subsequently elongated until failure;

– the samples were heated to 450 °C, held for 15 min, and then elongated to 5 %;

– the samples were heated to 750 °C, held for 15 min, and then elongated to 5 %.

The strain hardening factor (n) was calculated as outlined in [3].

At 450 °C, the steel displayed a yield offset and ultimate strength of 1277 and 1379 MPa, respectively. The relative elongation reached 9% (as shown in the figure). This suggests that plastic deformation at 450 °C leads to strain hardening of the steel (n = 0.03). Under the maximum load, deformation is concentrated ultimately leading to sample failure. When the sample is deformed at 450 °C to a 5 % relative elongation, the steel hardens to 1335 MPa. Subsequent heating to 750 °C results in a significant reduction in strength, with the offset yield strength during deformation measuring 311 MPa (a quarter of that at 750 °C). However, further deformation at 750 °C enhances the steel’s susceptibility to strain hardening (n = 0.06). The steel’s strength reaches 356 MPa at approximately 2 % relative elongation, attributed to the steel’s austenitization as a result of the α → γ transformation. The achieved hardening is maintained throughout further deformation, up to approximately 5 % relative elongation.

Conclusion

This study reveals that RATE steels experience strain hardening within the 450 – 750 °C operating temperature range, with higher strain hardening observed at 750 °C. The hot strength at 750 °C, with approximately 5 % relative elongation, reaches ~360 MPa. These results suggest that die tooling constructed from RATE steel can consistently operate at temperatures up to 750 °C.