Influence of weld thermal cycle and post weld heat treatment on the microstructure of MarBN steel
Influence of weld thermal cycle and post weld heat treatment on the microstructure of MarBN steel
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DOI：
10.1016/j.ijpvp.2019.05.010
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发表时间：
2019-07-01
影响因子：
3
通讯作者：
Thomson, R. C.
中科院分区：
文献类型：
--
作者：
Guo, J.;Xu, X.;Thomson, R. C.
添加硼氮强化马氏体钢（MarBN）是一种用于电厂高温、高应力应用的钢，已开发多年，被认为是替代传统91/92级钢的有希望的候选钢。在本研究中，对物理模拟热影响区（HAZ） MarBN材料进行了广泛的显微组织观察，并进行了膨胀模拟，将显微组织的变化与焊接热循环联系起来。随着焊接热循环峰值温度的降低，MarBN热影响区内的微观组织从精细的等轴组织转变为由分布在已有的先验奥氏体晶界（pagb）上的精细晶粒组成的双相组织。双相晶粒结构形成的温度范围与M23C6碳化物溶解的温度范围一致。焊后热处理（PWHT）后，M23C6碳化物在马氏体亚组织内分布均匀，有利于焊缝热影响区蠕变性能的提高。MX沉淀更耐热暴露，直到峰值温度达到1573 K（1300℃）才完全溶解。在焊接模拟和PWHT后均以富铌的MX相为主。在适当的条件下进行PWHT后，经历不同峰值温度热循环的材料之间的硬度没有明显变化，这可能会减轻HAZ内局部区域的不利应力状况。
Martensitic steels strengthened by Boron and Nitrogen additions (MarBN) were developed for high temperature/high stress service in power plant for periods of many years and are being considered as a promising candidate for the replacement of the more conventional Grade 91/92 steels. In the present study, extensive microstructural observation of physically simulated Heat Affected Zone (HAZ) MarBN material has been carried out after dilatometry simulations to link the variation in microstructure with weld thermal cycles. The microstructure in the MarBN HAZ has been observed to vary from a refined equiaxed morphology to a duplex microstructure consisting of refined grains distributed on the pre-existing Prior Austenite Grain Boundaries (PAGBs) as the peak temperature of the weld thermal cycle decreases. The temperature range corresponding to the formation of the duplex grain structure coincides with the temperature regime for the dissolution of the pre-existing M23C6 carbides. An even distribution of the M23C6 carbides within the martensitic substructure was also observed after Post Weld Heat Treatment (PWHT), which is beneficial for the creep performance of the weld HAZ. The MX precipitates are more resistant to thermal exposure and are not completely dissolved until the peak temperature reaches 1573 K (1300 degrees C). The Nb-rich MX precipitates are the predominant type observed both after weld simulations and PWHT. The hardness between the materials experienced with the thermal cycles with different peak temperature does not significantly vary after PWHT conducted in an appropriate condition, which is likely to mitigate an unfavoured stress condition in the localised area within the HAZ.