Microstructural Stability and Softening Resistance of a Novel Hot-Work Die Steel

A novel hot-work die steel, named 5Cr5Mo2, was designed to obtain superior thermal stability. The proposed alloy is evaluated in terms of its hardness, microstructure, and tempering kinetics. Compared with the commonly used H13 steel, the softening resistance of the designed steel is superior. Based on SEM and transmission electron microscopy (TEM) observations, a higher abundance of fine molybdenum carbides precipitate in 5Cr5Mo2 steel. Strikingly, the coarseness rate of the carbides is also relatively low during the tempering treatment. Moreover, owing to their pinning effect on dislocation slip, the dislocation density of the 5Cr5Mo2 steel decreases more slowly than that of the H13 steel. Furthermore, a mathematical softening model was successfully deduced and verified by analyzing the tempering kinetics. This model can be used to predict the hardness evolution of the die steels during the service period at high temperature.

Thermal Fatigue Properties of H13 Hot-Work Tool Steels Processed by Selective Laser Melting

Currently, selective laser melting (SLM) is gaining widespread popularity as an alternative manufacturing technique for complex and customized parts, especially for hot-work and injection molding applications. In the present study, as the major factors for the failure of H13 hot-work die steels during hot-working, thermal fatigue (TF) properties of H13 processed by SLM and a conventional technique were investigated. TF tests (650 °C/30 °C) were conducted on the as-selective laser melted (As-SLMed), thermally treated selective laser melted (T-SLMed), and forged (Forged) H13. Results show that the As-SLMed H13 exhibited the best TF resistance properties among the specimens herein (the shortest total crack length and highest hardness of 687 ± 12 HV5), whereas the Forged H13 exhibited the poorest TF resistance properties (the longest total crack length and lowest hardness of 590 ± 11 HV5) after TF tests. TF resistance properties were closely related to the initial and final hardness. Further microstructural investigations revealed that the typical cell-like substructures, increased amount of retained austenite, and most importantly, refined grain size were the main reasons for the improved TF resistance properties in the As-SLMed H13 compared to the Forged counterparts.