Transmission electron microstructures of stainless steel strip overlay joints

The microstruction alterations and transformations in the as welded condition and after post-weld heat treatment (PWHT) around the transition zones of 2.25 Cr-1 Mo steel with stainless steel strip overlay joints were observed mainly by transmission election microscopy (TEM). In an as welded joint, the microstructural changes from the overlay weld metal across the transition zone to the base metal were: austenite plus some delta ferrite and deformation twins of weld metal-austenite(A)/martensite(M) weld interface-martensite layer-granular bainite of heat affected zone (HAZ)-ferrite plus granular pearlite of base metal. Across the PWHT joint, the general microstructural variations were similar to those across the as wielded joint. However, some major morphological transformations had occurred: carbides had precipitated along the austenite/delta ferrite grain boundary of the overlap; cal bides had precipitated in the overlay weld metal zone adjacent to the A/M interface; carbides had precipitated in the martensite layer; and there was a new martensite/ferrite (of the carbon depleted zone) weld interface. These results may be useful in understanding the disbonding mechanism of this type of joint.