Effect of simulated thermal cycles on the microstructure of the heat-affected zone in HSLA-80 and HSLA-100 steel plates
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Effect of Simulated Thermal Cycles on the Microstructure of the Heat-Affected Zone in HSLA-80 and HSLA-100 Steel Plates M. SHOME, O.P. GUPTA, and O.N. MOHANTY The influence of weld thermal simulation on the transformation kinetics and heat-affected zone (HAZ) microstructure of two high-strength low-alloy (HSLA) steels, HSLA-80 and HSLA-100, has been investigated. Heat inputs of 10 kJ/cm (fast cooling) and 40 kJ/cm (slow cooling) were used to generate singlepass thermal cycles with peak temperatures in the range of 750 °C to 1400 °C. The prior-austenite grain size is found to grow rapidly beyond 1100 °C in both the steels, primarily with the dissolution of niobium carbonitride (Nb(CN)) precipitates. Dilatation studies on HSLA-80 steel indicate transformation start temperatures (Ts) of 550 °C to 560 °C while cooling from a peak temperature (Tp) of 1000 °C. Transmission electron microscopy studies show here the presence of accicular ferrite in the HAZ. The Ts value is lowered to 470 °C and below when cooled from a peak temperature of 1200 °C and beyond, with almost complete transformation to lath martensite. In HSLA-100 steel, the Ts value for accicular ferrite is found to be 470 °C to 490 °C when cooled from a peak temperature of 1000 °C, but is lowered below 450 °C when cooled from 1200 °C and beyond, with correspondingly higher austenite grain sizes. The transformation kinetics appears to be relatively faster in the fine-grained austenite than in the coarse-grained austenite, where the niobium is in complete solid solution. A mixed microstructure consisting of accicular ferrite and lath martensite is observed for practically all HAZ treatments. The coarse-grained HAZ (CGHAZ) of HSLA-80 steel shows a higher volume fraction of lath martensite in the final microstructure and is harder than the CGHAZ of HSLA-100 steel.
I. INTRODUCTION
RAPID heating and cooling with negligible hold time at peak temperature usually characterizes the weld thermal cycle in thick plates.[1] For a given steel chemistry, the phases formed and, therefore, the mechanical properties in the heat-affected zone (HAZ) depend on the nature of the thermal cycle.[2,3,4] The duration of the austenite regime experienced during welding decides the degree of homogeneity and grain size of austenite. In high-strength low-alloy (HSLA) steels, the austenite grain growth in the HAZ is additionally influenced by the type and stability of the microalloy precipitates, which are controlled, in turn, by the heat input and peak temperature (Tp).[5] A number of previous studies on HSLA-80 and HSLA100 steels have been directed toward interpreting the microstructure as a function of the thermal cycle.[6,7,8] For instance, Krishnadev’s work showed the formation of lath martensite under slow cooling rates corresponding to heat inputs of 40 kJ/cm in simulated samples.[6,7] Subsequently, Bhole and Fox established the presence of accicular ferrite along with lath martensite in similar steels under real welding conditions.[9] In a few other inve
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