TRANSMISSION ELECTRON MICROSTRUCTURES OF STAINLESS-STEEL STRIP OVERLAY JOINTS

The microstruction alterations and transformations in the as welded co ndition and after post-weld heat treatment (PWHT) around the transitio n zones of 2.25 Cr-1 Mo steel with stainless steel strip overlay joint s were observed mainly by transmission election microscopy (TEM). In a n as welded joint, the microstructural changes from the overlay weld m etal across the transition zone to the base metal were: austenite plus some delta ferrite and deformation twins of weld metal-austenite(A)/m artensite(M) weld interface-martensite layer-granular bainite of heat affected zone (HAZ)-ferrite plus granular pearlite of base metal. Acro ss the PWHT joint, the general microstructural variations were similar to those across the as wielded joint. However, some major morphologic al transformations had occurred: carbides had precipitated along the a ustenite/delta ferrite grain boundary of the overlap; cal bides had pr ecipitated in the overlay weld metal zone adjacent to the A/M interfac e; carbides had precipitated in the martensite layer; and there was a new martensite/ferrite (of the carbon depleted zone) weld interface. T hese results may be useful in understanding the disbonding mechanism o f this type of joint.