Microstructure and fracture-toughness studies in weld heat-affected zone of martensitic precipitation-hardened stainless steel

Abstract

This study concerned the microstructure and fracture toughness of the weld heat-affected zone (HAZ) of the martensitic precipitation-hardened stainless steel FV520B. It included both air-melted and vacuum-remelted material and involved metal inert gas arc welding at a heat input of 1·78 kJ mm⁻¹. While work was carried out on welded joints, most of the investigation made use of simulated HAZ material. Two preweld conditions were employed, namely primary hardened and overaged, and post-weld aging was carried out at 450°,550°, and 620°C. In all cases, the toughness was measured by the crack-opening displacement technique. The fracture toughness measured as crack-opening displacement at crack initiation δ_i in 6mm thick plate varied from 0·04 to 0·076mm for air-melted material and 0·069 to 0·081 mm for vacuum-remelted material. The actual value was influenced by heat-treatment condition and, in the case of air-melted plate, by orientation to the rolling direction. Areas of property variation were detected in hardness surveys on the weld HAZ which corresponded to regions where the peak temperatures were 750°-900°C and 1150°-1400° C. At the lower temperature (785°C) the toughness values δ_i were generally equal to or slightly less than the parent plate in the same heat-treatment condition. The higher temperature (1250°C)gave toughness values δ_i equal to or slightly greater than the parent metal for the same heat-treatment condition. The significance of these results is discussed in terms of microstructural changes and selection of the pre weld and post-weld treatment in practice.