The coarse grain HAZ microstructure and property of X80 pipeline steel with different carbon content was investigated. The weld thermal simulation test was carried out on Gleeble 1500 thermal mechanical test machine. The Charpy tests were completed at -20 ℃ for evaluating the toughness of coarse grain heat affected zone (CGHAZ). The microstructure was examined by optical microscope (OM) and transmission electron microscopy (TEM), and the austenite constituent was quantified by X-ray diffraction. The results showed that the ultra-low carbon can improve the toughness of CGHAZ by suppressing the formation of carbide, decreasing the martensite austenite (M-A) constituent and increasing the residual austenite in the M-A.
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