Junwan Li

Thermomechanical Analysis of Deep Cryogenic Treatment of Navy C-Ring Specimen

A thermomechanical coupling numerical model is built to reproduce the deep cryogenic treatment (DCT) of a cold work die steel Cr8Mo2SiV (SDC99) Navy C-ring specimen and explore the transient temperature distribution and microstructure evolution in specimen. Furthermore, the predicted results are validated by x-ray diffraction analysis and hardness measurement. The results indicate that for both the quenching treatment (QT) and DCT, the differences in cooling rate and temperature distribution between the gap and core regions of specimen are significant. The gap region of specimen shows a more rapid cooling rate, while the core region of specimen presents a slower cooling rate. There is an underlying risk of hardening crack at the gap region of specimen during the cooling process. Both the cooling rate and the temperature difference that occurred in the DCT process are markedly smaller than that in the QT process. After QT, about 15.5% of austenite will still remain, especially in the edge and corner of specimen, which is a potential factor for component failure. Subjected to DCT, the microstructure distribution of specimen demonstrates a distinct change and finally the volume fraction of retained austenite decreases to about 2.3%, which principally localizes at the gap region of specimen. The hardness of specimen after DCT has been a dramatic increase and shows a uniform distribution. In comparison with the experimental data, the predicted results show a quite good accuracy. It indicates that the thermomechanical couple model employed in this study can be used to optimal control of the DCT process.

Tempering stability of Fe–Cr–Mo–W–V hot forging die steels

The tempering stability of three Fe–Cr–Mo–W–V hot forging die steels (DM, H21, and H13) was investigated through hardness measurements and transmission electron microscopy (TEM) observations. Both dilatometer tests and TEM observations revealed that DM steel has a higher tempering stability than H21 and H13 steels because of its substantial amount of M2C (M represents metallic element) carbide precipitations. The activation energies of the M2C carbide precipitation processes in DM, H21, and H13 steels are 236.4, 212.0, and 228.9 kJ/mol, respectively. Furthermore, the results indicated that vanadium atoms both increase the activation energy and affect the evolution of M2C carbides, resulting in gradual dissolution rather than over-aging during tempering.