| ASTM4130 steel in quenched and tempered (Q&T) condition is used for high-pressure pipelines such as cement and mud systems. However, welding this steel is difficult due to its high carbon content plus additions of chromium and molybdenum. The obvious problems occurred in welding are cold crack and embrittlement problems in the heat affected zone (HAZ). Therefore in this paper, firstly, implant test was used to evaluate the cold crack sensitivity of ASTM4130 steel. Secondly, welding simulation technique is used to replicate HAZ experiencing single and double welding thermal cycles, and effect of postweld heat treatment (PWHT) on HAZ microstructure and properties is further systematically studied. Finally, micro-shear test is used to investigate the HAZ performance of the actual joint.Implant test indicate that the the coarse-grained heat-affected zones are primarily composed of martensite and bainite under the conditions of no preheating, preheating 100℃and 200℃, and the content of martensite (bainite) decreases (increases) with increasing the preheat temperature. Without preheating and preheating temperature of 100℃, the fractured surface is the typical hydrogen-induced fracture with a delayed crack fracture. However, no hydrogen-induced delayed fracture crack characteristics is founded when the preheat temperature is 200℃. The critical fracture stress are 344.46 MPa, 642.55 MPa and 806.11 MPa corresponding to without preheat, preheat temperature 100℃and 200℃respectively. It is belived that the critical fracture stress is much higher than the yield strength of the base metal when the preheat temperature is 200℃, then the cold crack in ASTM4130 steel can be avoid.The simulated HAZ properties of ASTM4130 steel experising single and double thermal cycles were systematically investigated by thermal welding simulation technique. The results show that the embrittlement occurs in HAZ with single thermal cycle except for the peak temperature of 700℃. When the peak temperatures are 1200℃and 1350℃, the coarse-grained heat-affected zone (CGHAZ) has the lowest toughness, which is decreased by 94.5 % of the base metal. It is also found that serious embrittlemen occurs in the fine-grained heat-affected zone (FGHAZ) with its toughness decreased by 81.37 % compared with that of the base metal. When welding heat input is between 12 kJ/cm and 28 kJ/cm, its effect on mechanical properties of CGHAZ is not apparent. After the CGHAZ is rheated by the second thermal cycle, the low temperature impact toughness of CGHAZ is further decreased in addition to the peak temperature of 700℃, which indicates that the embrittlement of the overall reheated CGHAZ occurs. The intercritically reheated CGHAZ (IRCGHAZ) corrsponding to the peak temperature of 800℃becomes the local brittle zone for its toughness loss by 96.6 % compared with the base metal. Meanwhile, the toughness of the supercritically reheated CGHAZ (SCCGHAZ) decreases by 95.7 % of the base metal. This means that serious embrittlement also occurs in SCCGHAZ. Moreover, the phenomenon of"structure heredity"exsits in IRCGHAZ and SCCGHAZ.The effect of postweld heat treatment (PWHT) on HAZ microstructure and properties of ASTM4130 steel both single-pass welding and muti-pass welding has also been systematically investigated by means of welding simulation technique. It is found that, for single-pass welding undergoing 640℃/1.5 h PWHT, the toughness of the HAZ is significantly improved, and the hardness of the HAZ is remarkably decreased except the intercritical HAZ (ICHAZ). No temper embrittlement phenomenon is founded. When welding heat input is between 12 kJ/cm and 28 kJ/cm, its effect on mechanical properties of CGHAZ can meet the requirements. Thus, when the welding efficiency is taken into account, the larger welding heat input is preferred. However, 640℃/1.5 h PWHT can not restore the properties of the reheated CGHAZ significantly. In addition to the subcritically reheated CGHAZ (SCGHAZ), the toughness of other HAZ is improved, but the toughness of whole HAZ is far less than the base metal toughness. The"structure heredity"in IRCGHAZ and SCCGHAZ is not eliminated, so the embrittllement is still serious.Conventional mechanical properties of the weled joint shows that, when the temperature and soaking time of PWHT are 640℃, and 90 min, 120 min, 135 min and 165min respectively, impact energy and tensile strength can all meet the requirements, but hardness of soaking time of 165min can only meet the requirements. Microshear test shows that when the temperature of PWHT is 640℃, the microshear strength of the fusion line is significantly higher than that of the base metal, and it is not obviously influenced by the soaking time. The width of the softening HAZ and the degree of softening decreases with increasing the soaking time. When the soaking time is 165 min, the effect of PWHT temperature on the width and degree of softening HAZ is apparent. When the PWHT temperature is 590℃, the width and extent of softening HAZ is the highest. When the hardness, the width and degree of softening HAZ, and toughness are taken into account, 640℃×165 min PWHT get a good match of strength and toughness.
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