高温时效对T23钢粗晶热影响区显微组织及再热裂纹敏感性的影响  

Effect of High Temperature Ageing on Microstructure and Stress-Relief Cracking Susceptibility of Coarse Grain Heat Affected Zone in T23 steel

作　　者：王学 李勇[2,3] 王家庆[3] 胡磊 WANG Xue;LI Yong;WANG Jiaqing;HU Lei(Key Laboratory of Green Fabrication and Surface Technology of Advanced Metal Materials,Ministry of Education,Anhui University of Technology,Ma􀆳anshan 243032,China;School of Power and Mechanics,Wuhan University,Wuhan 430072,China;Da Tang Boiler Pressure Vessel Inspection Center Co.,Ltd.,Hefei 230088,China)

机构地区：[1]安徽工业大学先进金属材料绿色制备与表面技术教育部重点实验室,马鞍山243032 [2]武汉大学动力与机械学院,武汉430072 [3]大唐锅炉压力容器检验中心有限公司,合肥230088

出　　处：《金属学报》2021年第6期736-748,共13页Acta Metallurgica Sinica

基　　金：国家自然科学基金项目No.51574181;四川省科技计划项目No.2018JY0668。

摘　　要：利用热模拟机制备T23钢粗晶热影响区(CGHAZ)试样,对其进行650℃、0~48 h时效实验,对时效前后的试样进行高温短时蠕变破断实验,评价其再热裂纹敏感性,采用OM、SEM、TEM+EDS等手段分析CGHAZ在时效过程中的显微组织演变,对断口形貌及断口附近显微组织进行观察,分析合金元素在晶界附近的分布,揭示T23钢CGHAZ形成再热裂纹的机理,探究时效改善再热裂纹敏感性的原因。结果表明,T23钢CGHAZ在焊态下为马氏体/贝氏体混合组织,硬度较高;经650℃时效后组织发生回复及再结晶,位错密度下降,亚晶粒(板条)尺寸增大,M_(23)C_(6)、M_(7)C_(3)和MX碳(氮)化物等在晶内、晶界逐渐析出,硬度逐渐下降。焊态CGHAZ对再热裂纹敏感,时效后CGHAZ的再热裂纹敏感性下降;当时效时间超过24 h时,对再热裂纹不敏感。焊态CGHAZ产生再热裂纹主要是由于M23C6在晶界析出长大,导致晶界形成软化区,并促进孔洞的形成,减弱了晶间结合力。时效使不稳定的CGHAZ组织发生预先转变,碳化物大量析出,基体发生回复与再结晶,降低了晶内强度,同时晶界附近合金元素贫化消除,晶内和晶界强度的差异减小,塑性变形能力明显提升,故再热裂纹敏感性降低。CGHAZ时效后的硬度与再热裂纹敏感性有一定的对应关系,当硬度高于250 HB时对再热裂纹敏感,硬度低于250 HB时对再热裂纹不敏感。Owing to its high creep rupture strength,good weldability,and low costs,T23 steel is an ideal material for manufacturing the heating components of water walls,superheaters and reheaters in ultra-super critical plants.However,its coarse grain heat affected zone(CGHAZ)is prone to stress-relief cracking(SRC)during post-weld heat treatment or high-temperature service.The mechanism of SRC is controversial and an effective method for forecasting and preventing SRC in T23 components is currently lacking.Clarifying the mechanism of SRC in the CGHAZ of T23 steel,and developing a practical engineering technique for predicting and preventing SRC generation,are therefore essential.In this work,CGHAZ specimens of T23 steel were simulated in a thermo-mechanical simulator,and aged at 650℃for 0-48 h.After simulating the microstructure evolution of the as-welded CGHAZ during service,the SRC susceptibility of the CGHAZ was evaluated.The microstructural changes and carbide precipitation were observed by OM,SEM,TEM,and EDS.The as-welded CGHAZ of T23 steel was composed of mixed martensite and bainite with high hardness.After ageing at 650℃,the structure recovered and recrystallized with a lower dislocation density and larger sub-grains than the as-welded CGHAZ.Carbides such as M_(23)C_(6),M_(7)C_(3),and MX gradually precipitated inside the grains and grain boundaries,decreasing the hardness.The SRC susceptibility was high in the as-welded CGHAZ,but decreased with increasing ageing time.When the ageing time exceeded 24 h,the sample was SRC-resistant.The main cause of SRC in the CGHAZ was precipitation and growth of M23C6 on the grain boundaries,which induced the formation of softened zones in the matrix near the grain boundary,and promoted the formation of micro-voids.During ageing,the unstable microstructure in the as-welded CGHAZ transformed as carbides precipitated and the matrix recrystallized,thereby reducing the intragranular strength.Meanwhile,the depletion of alloy elements near the grain boundary was eliminated.The micros

关 键 词：T23 钢 粗晶热影响区 时效 再热裂纹 

分 类 号：TG404[金属学及工艺—焊接]