Optimization of Heat Treatments for Reversion of Strain-Induced Martensite in 304L SS Explosive Clad

Explosive clad joints of 304L SS and Ti-5Ta-2Nb alloy, fabricated for an important application in the spent nuclear fuel reprocessing industry showed formation of deformation induced metastable alpha' martensite and fcc Ti phase in SS and TiTaNb alloy respectively. A biphasic structure consisting of metastable phases is not preferred for industrial applications due to degradation of corrosion and mechanical properties of the structural materials during service. Hence, it is essential to carry out post cladding heat treatments. The results reported in this paper provide evidence for the presence of alpha' phase in 304L SS in 'as clad' joints and its reversion process during thermal exposure. The temperature window in the range of 400-700 degrees C and time was optimized based on complete transformation of the metastable phases to parent phases, and avoiding the formation of brittle Fe-Ti intermetallics at the interface. A systematic increase in the fraction of austenite phase associated with the reversion phenomena has been studied using electron back scattered diffraction and transmission electron microscopy. Orientation relationship between product fcc and parent bcc phases was found to obey the K-S relationship. The reverted gamma phase was found to nucleate within the martensite laths. A temperature of 550 degrees C for duration of about 10 h was found to be optimum for the post cladding treatments of the explosive clad joints.