Study of the Origin of the Unexpected Pearlite during the Cooling Stage of Two Cast High-Speed Steels

Abstract

Two HSS grades (A and B) belonging to the complex system Fe-Cr-C-Si-X, where X is a strong carbide-forming element such as V, Mb or W, were studied. Samples in the as-received conditions came from an industrial spin casting process, with a varying cooling rate during processing. Chemical compositions of both alloys were closed to each other and were chosen to enhance their hardenability and to avoid less resistant phases such as pearlite and ferrite. Differential Thermal Analysis was performed on both alloys, in order to increase their crystallization behaviour. Light microscopy and SEM associated with EDS analyses were done to characterize the microstructure of both alloys in the as-received conditions and after DTA trials. The matrix of both HSS grades was composed of eutectic carbides, martensite and retained austenite, these phases exhibiting similar chemical compositions in both alloys. Unexpected pearlite was found in the as-cast HSS alloy B without W, this grade containing more Mo, more V and less Cr than the HSS grade A. It appeared from DTA tests that pearlite found in the alloy B arose more from the destabilisation of the Cr-rich retained austenite associated with the plate-like M2C carbide, than from the matrix itself. In fact, pearlite zones located in the vicinity of M2C are due to related isothermal solid phase transformations form the previous austenitic eutectic phase that is enriched with Cr and Mo.