Thermal simulation test and microstructure characterization method of heat affected zone of pipeline steel girth weld
The automatic girth welding process is widely used in high grade oil and gas long distance pipe-lines with large diameter.The microstructure and property of the heat affected zone of the girth weld are complicated.It is important to establish a systematic analysis method for the microstructure and property of the heat affected zone of the girth weld.In this study,the process route of weld thermal simulation test was optimized according to the phase change characteristics of high-grade pipeline steel.Gleeble 3500 ther-mal simulation test machine was used to conduct the weld thermal simulation test,and the thermal simula-tion samples were obtained under different heat input and peak temperature conditions.The microstructure and its substructure were analyzed by scanning electron microscope(SEM)and electron backscattered dif-fraction(EBSD),and the relationship between microstructure and Charpy impact toughness was estab-lished.The results showed that the microstructure characteristics closely related to the low temperature toughness of the heat affected zone of the girth weld included effective grain size,high angle grain bounda-ry ratio,and the morphology and size distribution of martensite/austenite(M/A)components.When the effective grain size was small,the proportion of high angle grain boundary was large,and the small size M/A component was granular,it had high low temperature toughness.When the large size M/A island chain structure appeared,the low temperature toughness was low.
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