Effect of Multiple Weld Thermal Cycles on the Microstructure and Mechanical Properties of HSLA-100 Steel

Abstract 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. Much work has been done to determine the properties in the heat affected zone (HAZ) for HSLA-100 but less results are available to understand microstructural evolution and resultant properties of the HAZ under multi-pass welding conditions. The objective of this study is to understand the effect of multiple thermal cycles on the blast resistance of the HAZ, as measured by the hardness and low temperature impact toughness.First, the large variations in the HAZ hardness were observed and were shown to be associated with compositional banding. These bands were analyzed, and it was found that the compositional gradients were due to micro segregation that is rolled out during processing. Dictra simulations of a typical heat treatment demonstrated that carbon preferentially segregates to the higher composition sections, increasing the hardness. This difference in composition also leads to changes in the transformation temperatures, leading to significant differences in the microstructure.The hardness and toughness of the different parts of the HAZ were then tested. In the HAZ simulations of HSLA-100, the hardness increases from the base metal (BM) value of approximately 260 HV. It reaches a maximum with a peak temperature of 900°C. The impact toughness in general was equal or higher than the BM, except for the coarse grained HAZ (CGHAZ), which was slightly below the acceptable minimum for the BM. However, the CGHAZ does not degrade in toughness after multiple thermal cycles. There is an increase in hardness and decrease in toughness in the inter-critical region with increasing weld passes, which is attributed to increasing amount of austenite transformation during heating. It was found that the transformation kinetics on heating were too slow to allow the sample to reach the equilibrium concentration of ferrite and austenite. A fully transformed sample was tested, which has comparable hardness and toughness to the multipass intercritical HAZ. The reheated CGHAZ showed a rejuvenation in toughness for the 810 and 900°C peak temperature samples while demonstrating no local brittle zone phenomenon or the corresponding microstructural M–A constituent phases that follow this reduction in toughness.Additional samples underwent welding simulation testing after a 10% pre-strain to study how residual stress and weld repairs would influence the resulting properties. The pre-strained FGHAZ and CGHAZ samples had comparable hardness and toughness to the single pass samples due to recovery, recrystallization, and grain growth. The pre-strained intercritical samples have comparably accelerated kinetics as compared to the single pass, possibly due to faster diffusion from the increased dislocation density.The work conducted to date demonstrates that HSLA-100 that has been exposed to multiple weld thermal cycles will maintain high toughness in the HAZ. The only exception to this observation is the CGHAZ, but it is only slightly below the minimum for the base metal, and simulated weld multipass or reheat condition will not lower the toughness significantly below this value. In fact, there is rejuvenation for some reheated CGHAZ samples. There is progressive phase transformation in the intercritical region, as seen with multiple weld passes, but the fully transformed samples still maintained excellent impact toughness and high hardness. It is important to note that the 10% cold working before the tests seem to accelerate these kinetics, leading to significant differences between the pre-strained and unaffected intercritical samples. The differences between the light and dark band, especially in the intercritical samples, is due to compositional differences which lead to a comparatively increased hardenability in the dark band. The autogenous welds confirm that this compositional banding has a greater effect on hardness than multiple weld passes.