THERMAL MECHANISMS OF GRAIN AND PACKET REFINEMENT IN A LATH MARTENSITIC STEEL

This work was done to clarify the metallurgical mechanisms that are us ed in thermal treatments to refine the effective grain size of lath ma rtensitic steels. The alloy chosen for this work was Fe-12Ni-0.25Ti, w hich provides a well-defined lath martensite structure in the as-quenc hed condition. The alloy was given four prototypic heat treatments: (1 ) an anneal-and-quench treatment, (2) an intercritical anneal, (3) a s ingle or double anneal in the austenite range, and (4) a single or dou ble reversion to austenite at a rapid heating rate. Two effective grai n sizes were identified for each heat treatment: the packet size, or d imension over which adjacent martensite laths have a common crystallog raphic orientation, and the prior austenite grain size. The former con trols the ductile-brittle transition temperature (DBTT), the latter co ntrols intergranular embrittlement. The different heat treatments supp ress the DBTT and increase resistance to intergranular embrittlement t o the extent that they decrease the relevant grain size. Rapid reversi on is the most successful of the heat treatments since it destroys the alignment of laths within a packet. Rapid reversion readily yields an effective grain size near 1 mu m, leading to a very low ductile-britt le transition temperature and a substantial resistance to temper or hy drogen embrittlement.