【作者】 李立英；

【导师】 王勇；

【作者基本信息】 中国石油大学 ， 材料学， 2011， 博士

【摘要】 钻井平台高压水泥和泥浆系统材料ASTM4130钢属于中碳调质合金钢,含碳量高,以及合金元素的加入,焊接性较差。这种钢焊接存在的突出问题是冷裂纹和热影响区脆化。本文采用插销试验对ASTM4130钢的冷裂敏感性进行了评定,采用焊接热模拟技术研究了单次热循环和二次热循环焊后热处理前后热影响区组织和性能的变化规律,同时研究了实际接头热影响区的组织及常规和微剪切性能。插销试验表明,不预热、预热100℃和预热200℃三种情况下,粗晶区组织主要为马氏体和贝氏体,随预热温度升高,马氏体含量降低,贝氏体含量增加。不预热和预热100℃时,断口具有典型的氢致延迟裂纹断裂特征。预热200℃时,断口不存在氢致延迟裂纹特征。不预热、预热100℃、预热200℃时的临界断裂应力分别为344.46 MPa、642.55 MPa、806.11 MPa。预热温度为200℃,临界断裂应力远远高于母材的屈服强度,能避免ASTM4130钢产生焊接冷裂纹。热模拟试验结果表明,模拟ASTM4130钢单道焊接热影响区,除峰值温度为700℃的回火区外,均发生脆化现象。峰值温度为1200℃和1350℃的粗晶区冲击韧性损失为母材的94.5 %,成为单次热循环的韧性低谷。峰值温度为950℃的细晶区韧性损失为母材的81.37 %,脆化程度仅次于粗晶区。焊接热输入在12 kJ/cm~28 kJ/cm之间变化时,对粗晶区韧性影响不大。模拟再热粗晶区,除峰值温度为700℃的亚临界粗晶区外,粗晶区韧性进一步降低,发生热影响区整体脆化。峰值温度为800℃的临界粗晶区和峰值温度为950℃的过临界粗晶区的韧性损失分别为母材96.6 %和95.7 %,脆化最为严重,且两区存在“组织遗传”现象。热模拟试验结果表明,640℃×1.5 h焊后热处理可完全消除ASTM4130钢单道焊接热影响区脆化。除回火区外,热影响区其它区域的冲击韧性显著提高,硬度明显降低,未发现回火脆性现象。焊接热输入在12 kJ/cm～28 kJ/cm之间变化时,粗晶区的性能均能满足要求,为提高焊接效率,可采用稍大的热输入。640℃×1.5 h焊后热处理未能明显改善再热粗晶区性能。除亚临界粗晶区外,热影响区的冲击韧性虽得到一定程度的改善,但均远远低于母材的韧性。临界粗晶区和过临界粗晶区的“组织遗传”现象未能消除,脆化程度仍然严重。接头的常规力学性能试验表明,焊后热处理温度为640℃,保温时间为90 min、120 min、135 min和165 min,冲击功和抗拉强度均能满足要求,但只有保温时间165 min,硬度才能满足要求。接头的微型剪切试验表明,焊后热处理温度为640℃,熔合线处剪切强度均明显高于母材,受保温时间的影响不明显,随保温时间的延长,热影响区中的软化区宽度变窄,软化程度有所减弱。保温时间为165 min,焊后热处理温度对软化区宽度和程度影响明显,温度为590℃时,热影响区软化区最宽,失强率最大。从硬度、软化区宽度及程度、韧性三方面综合考虑,640℃×165 min焊后热处理可使接头获得良好的强韧性匹配。更多还原

【Abstract】 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.更多还原