低碳微合金管材TIG焊热影响区软化成因分析
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摘要
针对低碳微合金管材在环焊对接中产生的热影响区(HAZ)软化问题,采用光学显微镜、EBSD技术和透射电镜对板材、管材及环焊HAZ的组织结构进行了对比研究。结果表明,板材的晶粒度在12～13级之间,属于超细晶粒,组织为粒状贝氏体;制管过程中由于形变强化,管材中小角度晶界所占比例上升,相比于板材增加了27.5%,变形晶粒比例由变形前的8.75%增至变形后的78.75%,位错密度由原先的4.2×10~(13)/m~2增加到1.9×10~(14)/m~2。随着环焊热循环的影响,热影响区中部分区域硬度大幅下降,出现了软化现象。研究发现,软化区小角度晶界减小11.8%,位错密度下降至3.9×10~(13)/m~2,与板材处于同一水平。因此,晶粒长大,位错密度下降,再结晶比例增加等是低碳微合金管材环焊接头产生软化的主要原因。
With aim to solve the softening problem of heat affected zone(HAZ) in girth welding of low carbon microalloyed tube, the microstructure of plate, tube and HAZ was analyzed by optical microscope(OM), EBSD and transmission electron microscope(TEM).The results show that the grain size of the plate is between 12 and 13 grade which belongs to ultrafine grain range, and the microstructure is granular bainite. Due to the deformation strengthening during the process of tubulation,the fraction of the low-angle grain boundary increase by 27.5% compared with plate metal. The fraction of the deformed grain increase from 8.75% to 78.75% after deformation. The dislocation density rise from the original 4.2×10~(13)/m~2 to 1.9×10~(14)/m~2 after deformation. With influence of the thermal cycle of girth welding, the hardness in the part of HAZ is greatly reduced, and softening occurs. Furthermore, in the softening region, the low angle grain boundary decreases by 11.8%, and the dislocation density decreases to 3.9×10~(13)/m~2, at the same level of plate. Therefore, the grain growth, the dislocation density decrease, and the recrystallization ratio increase are main reasons for the softening of girth welding joint of low carbon microalloyed tube.
With aim to solve the softening problem of heat affected zone(HAZ) in girth welding of low carbon microalloyed tube, the microstructure of plate, tube and HAZ was analyzed by optical microscope(OM), EBSD and transmission electron microscope(TEM).The results show that the grain size of the plate is between 12 and 13 grade which belongs to ultrafine grain range, and the microstructure is granular bainite. Due to the deformation strengthening during the process of tubulation,the fraction of the low-angle grain boundary increase by 27.5% compared with plate metal. The fraction of the deformed grain increase from 8.75% to 78.75% after deformation. The dislocation density rise from the original 4.2×10~(13)/m~2 to 1.9×10~(14)/m~2 after deformation. With influence of the thermal cycle of girth welding, the hardness in the part of HAZ is greatly reduced, and softening occurs. Furthermore, in the softening region, the low angle grain boundary decreases by 11.8%, and the dislocation density decreases to 3.9×10~(13)/m~2, at the same level of plate. Therefore, the grain growth, the dislocation density decrease, and the recrystallization ratio increase are main reasons for the softening of girth welding joint of low carbon microalloyed tube.
引文
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2毕宗岳,张昆,雷阿利,等.焊接学报,2011,32(4),29.
3 Mochizuki M,Shintomi T,Hashimoto Y,et al.Welding in the World,2004,48(9),2.
4 Hochhauser D I F,Rauch M R.Welding in the World,2012,56(5-6),77.
5董现春,张楠,陈延清,等.焊接学报,2012,33(11),72.
6 Mohandas T,Madhusudan Reddy G,Satish Kumar B.Journal of Materials Processing Technology,1999,88(1),284.
7牛靖,陈宏远,刘甲,等.热加工工艺,2016(21),176.
8毕宗岳.焊管,2012,35(9),5.
9朱小平.钻采机械,2000,23(6),51.
10李博锋,刘云,汪强,等.钢管,2018,47(3),16.
11李鸿美,张慧杰,孙力军,等.稀有金属,2010,34(s1),97.
12胡赓祥,蔡珣,戎咏华.材料科学基础(第三版),上海交通大学出版社,2010.
13 He W,Ma W,Pantleon W.Materials Science&Engineering A,2008,494(1-2),21.
14 Pantleon W.Scripta Materialia,2008,58(11),994.
15 Pantleon W.Metal Science Journal,2013,21(12),1392.