为研究Nb含量对焊接热影响区微观组织和性能的影响,采用熔化极气体保护焊(gas metal arc welding,GMAW)和手工焊条电弧焊(shielded metal arc welding,SMAW)对 0.055%Nb和 0.075%Nb含量的X80钢管进行环焊.采用夏比冲击试验和金相分析方法,研究热影响区的微观组织差异和夏比冲击韧性.并借助扫描电镜和超高温激光共聚焦显微镜分析不同Nb含量X80管体的微观组织对热影响区性能的影响.结果表明,在 0℃和-20℃时,0.075%Nb和 0.055%Nb的X80钢管GMAW环焊接头热影响区均具有较高的冲击韧性,其平均冲击吸收能量均高于 150 J.其中 0.055%Nb略高于 0.075%Nb的GMAW环焊接头热影响区夏比冲击吸收能量;焊接热输入较低时,0.055%Nb低于 0.075%Nb的X80环焊接头粗晶区的韧脆转变温度,具有更好的低温韧性.焊接热输入较高时,0.075%Nb的X80环焊接头粗晶区具有更高的上平台冲击吸收能量,且上平台温度和韧脆转变温度也更低,其低温韧性也更优异;还发现了X80环焊接头热影响区的冲击韧性不仅与热输入量和热影响区马氏体-奥氏体组织(M-A)的形状、大小、分布有关,而且还受管体中Nb含量、原始的强度与韧性、微观组织状态的遗传影响.
Microstructure and toughness of heat-affected zone in girth welding of X80 steel pipe with different Nb content
In order to study the effect of Nb content on the microstructure and properties of heat affected zone(HAZ),the 0.055%Nb and 0.075%Nb content X80 steel pipes were girth welded by Gas metal arc welding(GMAW)and shielded metal arc welding(SMAW).Charpy impact test and metallographic analysis were used to study the impact toughness and micro-structure differences in the HAZ.The effects of the micro-structure of X80 pipe body with different Nb content on the properties of the heat-affected zone were analyzed by scanning electron microscope and confocal laser microscope with high temperature.The results show that at 0℃and-20℃,0.075%Nb and 0.055%Nb X80 steel pipes have high impact toughness in the HAZ of GMAW girth welded joints,and their average impact absorbed energy are higher than 150J.However,the Charpy impact absorbed energy in the HAZ of GMAW girth welded joint of X80 steel pipe with 0.055%Nb is higher than that of X80 steel pipe with 0.075%Nb.When GMAW is used,the ductile-brittle transition temperature(DBTT)of X80 girth welded joint with 0.055%Nb is lower than that of X80 with 0.075%Nb because of the low welding heat input.When SMAW is used,due to higher heat input,the coarse grain heat affected zone(CGHAZ)of X80 girth weld-ing joint with 0.075%Nb has higher impact absorbed energy on the upper-shelf and lower temperature of upper-shelf zone and DBTT,and its low temperature toughness is better.It is also discussed that the impact toughness of the HAZ of X80 girth welded joint is not only related to the heat input and the shape,size and distribution of M-A,but also genetically influ-enced by the content of Nb in the pipe body and the original strength,toughness and microstructure state.
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