Thermal Mechanisms of Grain and Packet Refinement in an Lath Martensitic Steel
This work was done to clarify the metallurgical mechanisms that are used in thermal treatments to refine the effective grain size of lath martensitic steels. The alloy chosen for this work was Fe–12Ni–0.25Ti, which provides a well-defined lath martensite structure in the as-quenched condition. The alloy was given four prototypic heat treatments: (1) an anneal-and-quench treatment, (2) an intercritical anneal, (3) a single or double anneal in the austenite range, and (4) a single or double reversion to austenite at a rapid heating rate. Two effective grain sizes were identified for each heat treatment: the packet size, or dimension over which adjacent martensite laths have a common crystallographic orientation, and the prior austenite grain size. The former controls the ductile–brittle transition temperature (DBTT), the latter controls intergranular embrittlement. The different heat treatments suppress the DBTT and increase resistance to intergranular embrittlement to the extent that they decrease the relevant grain size. Rapid reversion 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 μm, leading to a very low ductile–brittle transition temperature and a substantial resistance to temper or hydrogen embrittlement.
- ISIJ international
ISIJ international 38(11), 1277-1285, 1998-11-15
The Iron and Steel Institute of Japan