CORRELATION OF MICROSTRUCTURES WITH MICRO-MECHANICAL PROPERTIES OF AUSTENITIC STAINLESS STEEL WELD OVERLAYS IN LOW ALLOY STEEL

Weldment sections from AWS E-309 stainless weld overlays made by submerged arc welding process on standard and modified 2 1/4Cr-1Mo steel plates for the fabrication of pressure vessels were microstructurally and micro-mechanically investigated.;The postweld heat treatment (PWHT) on E-309/2.25Cr-1Mo steel greatly affected the HAZ microstructure. With increasing exposure time, the amount of decarburization of the HAZ adjacent to the fusion boundary is greater in the standard low alloy steel than in the modified low alloy steel.;The PWHT also affected the overlay weld by increasing its microhardness and ductility parameter. In addition, the PWHT induced the formation of sigma phase whose volume fraction depended upon the exposure time and temperature.;The standard 2 1/4Cr-1Mo steel overlayed with 309 stainless steel showed a very distinct hardness discontinuity, while the modified 2 1/4Cr-1Mo steel due primarily to the addition of boron and to the precipitation of titanium and vanadium carbides.;The modified steel consists mainly of tempered bainite with low ferrite volume fraction and the standard steel has more polygonal ferrite. Observations on the weld zone indicated a high ferrite content (15%) with mostly typical vermicular morphology and would therefore be susceptible to hot cracking. The overlay weld showed large columnar grains as a result of high heat input.;A hardness peak was observed in the HAZ approximately 2 mm down from the transition zone. While this observation was made for the as-welded condition, it was later found that this peak, after PWHT, moved up into the transition zone. The reason for this translation was the diffusion of carbon from the 2 1/4Cr-1Mo grains adjacent to the transition zone into this zone.;Ductility parameters obtained from hot hardness indentations on weld and HAZ held a good correlation with hot tensile uniform elongation. The HAZ hot hardness values dropped with increasing temperature and the ductility parameter increased, indicating that the uniform elongation decreased. The standard steel with wide HAZ has shown small initial deformation on loading and low saturation level, whereas the modified steel with narrow HAZ has shown large initial deformation on loading and high saturation level. (Abstract shortened with permission of author.)...