Alternative phase transformation path in cryogenically treated AISI D2 tool steel

This study based on dilatometry test and microstructural observations showed a different phase transformation path during conventional heat treatment of AISI D2 tool steel instead of the traditionally expected fully martensitic transformation. Transformation started with carbide precipitation and continued with allotriomorphic ferrite, Widmanstätten ferrite, acicular ferrite, and finally bainite formation. This microstructure showed microhardness value of 730 HV, which is very high for conventionally cooled samples. They are not expected to have gone under martensitic transformation. The results indicated that bainitic transformation take place even at temperatures near 173 K and to obtain fully martensitic microstructure, cooling rates in the range of 50 K s−1must be utilized.