Effect of Multiple Weld Thermal Cycles on HSLA-100 Steel The microstructure and mechanical properties of high-strength low-alloy steel during multipass welding was analyzed

High-strength low-alloy (HSLA)-100 is a precipitate-strengthened low-alloy steel that is often used for its good combination of high yield strength and impact toughness. During initial fabrication and service, multipass welds and weld repairs often need to be made. Work has been done to determine the properties in the heat-affected zone (HAZ) for HSLA-100, but fewer results are available to understand microstructural evolution and the resultant properties of the HAZ under multipass welding conditions. Variations in the HAZ hardness were observed and shown to be associated with compositional banding. These differences between the enriched and depleted solute bands led to differences in hardenability. Thermodynamic and diffusion simulations demonstrated that carbon preferentially segregates to the regions of higher concentration of substitutional elements, thus increasing the hardness. This difference in composition also led to changes in the transformation temperatures and caused local differences in the HAZ microstructure. Multiple-pass autogenous welds confirmed that this compositional banding has a greater effect on hardness than multiple weld thermal cycles. HAZ simulations showed the hardness in all regions of the HAZ was higher than that of the base metal (BM). The impact toughness of the HAZ was equal to or higher than the BM, except for the coarse-grain HAZ (CG-HAZ), which was slightly below the acceptable minimum for the BM. However, the CGHAZ toughness did not degrade further after three weld thermal cycles. The reheated CGHAZ showed a rejuvenation in toughness for subsequent thermal cycles with peak temperatures of 810 degrees and 900 degrees C. The phase transformations in the intercritical HAZ (ICHAZ) region were still unfinished after three weld thermal cycles, and the progressive transformation with each successive pass increased the hardness and decreased the toughness. However, fully transformed ICHAZ samples still maintained excellent impact toughness and high hardness. Additional samples underwent weld simulations after a 10% prestrain to study how plastic strain from residual stress and service would influence the resulting properties. Except for the CGHAZ, all regions of the strained BM and HAZ still exhibited toughness values above the minimum requirements.