热处理温度对长期服役T23钢CGHAZ微观组织的影响

Effect of heat treatment temperature on CGHAZ microstructure of T23 steel in long-term service

以服役时间超过6×104 h的T23水冷壁管对接接头作为研究对象,对焊接接头进行不热处理(焊态)、520℃×1 h,580℃×1 h,650℃×1 h,720℃×1 h及760℃×1 h等6种不同参数的热处理试验。为研究不同热处理后粗晶热影响区(CGHAZ)微观组织变化,分析合金碳化物在晶界的分布规律,同时探索CGHAZ再热裂纹形成原因与析出物之间的内在联系,重点对粗晶区进行了显微硬度分析、金相组织分析、扫描电镜形貌分析及能谱分析。结果表明,焊态T23钢CGHAZ为粗大板条马氏体与贝氏体的混合组织,经测算最大晶粒尺寸约100μm,大部分晶粒尺寸30~50μm;在580℃热处理时,由于晶粒细化及析出物在晶内弥散强化作用,CGHAZ出现了二次硬化现象,根据再热裂纹形成理论,此时晶内强度与晶界强度差较大,应力松弛主要集中于晶界,CGHAZ产生再热裂纹的倾向较大;在720℃热处理时,在CGHAZ晶界位置出现了密集孔洞及合金碳化物,并且空洞与合金碳化物之间存在伴生关系,但短时间内孔洞在后续正常服役条件(560~600℃)下能否发展成为再热裂纹有待商榷。

Butt joints of T23 water-cooled wall pipes with a service time more than 6×104 h were taken as research object,and heat treatment tests with six different parameters were carried out on welded joints,namely no heat treatment(as welded),520℃×1 h,580℃×1 h,650℃×1 h,720℃×1 h,and 760℃×1 h.In order to study microstructure changes of CGHAZ after different heat treatments,analyze distribution law of alloy carbides in grain boundaries and explore internal relationship between formation of CGHAZ reheat crack and precipitates,microhardness analysis,metallographic structure analysis,scanning electron microscope morphology analysis,and energy spectrum analysis of coarse-grained region were emphasized.The results showed that as-welded T23 steel CGHAZ was a mixture of coarse lath martensite and bainite,the maximum grain size was about 100μm,and most of grain size was 30~50μm.During heat treatment at 580℃,due to grain refinement and dispersion strengthening effect of precipitates in the grain,CGHAZ hardness showed a secondary hardening phenomenon.According to theory of reheat crack formation,the difference between intragranular strength and grain boundary strength was large at this time,stress relaxation was mainly concentrated in the grain boundary,and CGHAZ tended to generate reheat cracks.During heat treatment at 720℃,dense voids and alloy carbides appeared at grain boundary of CGHAZ,and there was an associated relationship between voids and alloy carbides.However,whether the holes could develop into reheat cracks under normal service conditions(560~600℃)in a short period of time remained to be discussed.