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Effects of nitrogen, ferrite morphology, and texture evolution on impact toughness in heat-affected zone simulations of V-microalloyed HSLA steel welds
Church, Adam J.
Church, Adam J.
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2025
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Microalloyed HSLA steels are used in lower-operating temperature structural applications, including bridges and pipelines, due to the wide range of achievable strength and toughness values obtained in the as-rolled products. However, the low temperature toughness associated with welds in high-impact applications remains an opportunity for improvement due to the tendency to form lower-toughness microconstituents such as bainite. Acicular ferrite (AF) is being investigated as a microconstituent to improve low temperature impact toughness in the coarse-grained heat affected zone of welds while maintaining required tensile strength levels for such applications. In this study, peak temperature and cooling rate were varied to simulate different HAZ regions in two experimental V-microalloyed steels differing in N content to produce microstructures with various ferritic microconstituents. Two alloys of different N contents of 80 and 200 ppm were utilized to change the volume fraction of VN precipitates in the microstructure. Optical microscopy and crystallographic analysis using EBSD were utilized to differentiate between the ferritic microconstituents and quantify microstructural evolution with processing parameters.
A shift from primary ferrite to martensite/bainite microstructures was observed with increasing cooling rate at both peak temperatures selected for continuous cooling behavior analysis. Greater N content was associated with more grain-boundary ferrite. Crystallographic analysis further supported a shift towards a more displacive nature of the developed ferrite microconstituent with increasing cooling rate. The hardness increased as cooling rate increased due to less primary ferrite, with the alloy containing more N producing greater hardness values in all conditions as a result of grain refinement and a higher fraction of high dislocation density microconstituents, such as bainite. A higher N content produced finer prior-austenite grains at every cooling rate, yielding greater amounts of grain-boundary ferrite morphologies. Charpy impact toughness at -20 °C increased as cooling rate increased for both alloys in AF-predominant conditions due to the reduction in grain-boundary ferrite amount and effective ferrite grain size. A lower N content was associated with enhanced low-temperature toughness at every cooling rate due to less grain-boundary ferrite.
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