Q. Carbon and Low Alloy Steels for Welding?

All welding of pressure retaining equipment must be completed as per the respective code, typically ANSI/ASME B31.1 or Section VIII of the Boiler and Pressure Vessel Code.

Thermal stress relief of carbon and low alloy steel weldments is presently mandatory unless certain conditions are met. Thickness limits, affecting the requirements for postweld heat treatment, vary between the two codes.

Thermal stress relief, of carbon and low alloy materials, is intended to reduce the residual stresses created during the welding process and possibly to temper the weldment and heat-affected zone (HAZ) microstructures. However, the metallurgical requirements, for materials in high temperature service, often conflict with those for optimum nil-ductility transition temperature performance. Fine-grained microstructures are typically specified to ensure low nil-ductility transition temperatures, whereas coarse grained microstructures are preferred for high temperature service. Studies have shown that the ‘as-welded' weld metal was stronger than the stress relieved condition and hence did not deform as readily. Therefore, there is some debate as to the benefits of PWHT for components in high temperature service.

Weld joint designs should be continuous and fully penetrating. All slag should be removed prior to going into service to prevent any corrosive effects.