The characteristics of microstructure phase transformation and mechanical properties in GPM A30 powder metallurgy high speed steel

The effect of tempering on the decomposition of retained austenite in a powder metallurgy (PM) high-speed steel, GPM A30, has been monitored with a high-speed dilatometer. The corresponding microstructures of specimens with different tempering cycles have been investigated by a combination of scanning electron microscopy and analytical transformation electron microscopy. The as-quenched structure of the steel studied is composed of retained austenite, untempered martensite and carbides. The results indicate that the complete transformation of retained austenite can be more nearly accomplished by two or triple tempering cycles than by a single long-time cycle. The possible transformation mechanism for the decomposition of retained austenite during multiple tempering cycles is attributed to the invariant-plane-strain of the prior martensitic transformation extending accommodation defects to the adjacent retained austenite, which favors further transformations in the subsequent tempering operations.

There has been intensive research work on retained austenite and lath martensite in low-carbon alloy steels. However, little TEM research work has been carried out on twinned plate martensite with retained austenite in high-carbon alloy steels . It is naturally very difficult to produce electron-transparent samples for TEM (because the high-carbon alloys are quite brittle), but TEM investigation continues to assume greater significance in research.

GPM A30 is a high-carbon grade in commercial P/M high speed steels, and is widely used in such applications as metal cutting tools and metal forming dies. In this work, dilatometric experiments were performed to investigate the tempering response in quench-treated specimens of GPM A30 high speed steel. The resulting scanning electron micrographs and transmission electron micrographs were examined to elucidate the microstructural evolution.