Effect of weld thermal cycle on microstructure and fracture toughness of simulated heat-affected zone for a 800 MPa grade high strength low alloy steel
Effect of weld thermal cycle on microstructure and fracture toughness of simulated heat-affected zone for a 800 MPa grade high strength low alloy steel
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DOI：
10.1016/j.jmatprotec.2007.12.049
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发表时间：
2008-10
影响因子：
6.3
通讯作者：
Yaowu Shi;Z. Han
中科院分区：
文献类型：
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作者：
Yaowu Shi;Z. Han
采用热模拟方法研究了焊接冷却时间和峰值温度对800 MPa级低合金高强钢特征断裂韧性和热影响区组织特征的影响。结果表明，当t8/5冷却时间为18 s时，模拟粗晶热影响区的断裂韧性最好。随着冷却时间的延长，奥氏体晶粒尺寸、贝氏体铁素体和M/A相的体积分数增加。但随着冷却时间的延长，马氏体的体积分数降低。随着奥氏体晶粒尺寸和M/A相体积分数的增加，韧性显著下降。此外，取向对焊接热循环试样的断裂韧性也有影响。L-T取向模拟热影响区的断裂韧性一般高于T-L取向。其原因可能与轧制过程中形成的带状组织在热模拟后仍保留下来有关。此外，研究表明，粗晶区的韧性高于细晶区的模拟热影响区。其原因可能与热影响区在模拟所用的完整热循环过程中的组织演变有关。对于细晶热影响区，较短的冷却时间t8/5可能不利于自回火和M/A组分的分解，从而使细晶热影响区的韧性降低。而对于粗晶热影响区，较长的冷却时间t8/5有利于M/A组分的分解，从而提高粗晶热影响区的韧性。部分相变热影响区断裂韧性的急剧下降可能与M/A组元呈网状分布的混合组织的形成有关。
In the present investigation, thermal simulated specimens were used to investigate the effect of welding cooling time and peak temperature on characteristic fracture toughness and microstructure feature of heat-affected zone (HAZ) for an 800MPa grade high strength low alloy (HSLA) steel. It is found that the fracture toughness is the best for the simulated coarse-grained HAZ, when the cooling time of t8/5is 18s. In addition, the size of prior austenite grain, and the volume fraction of bainitic ferrite and M/A constituent increase with increasing the cooling time. However, the volume fraction of martensite decreases with increasing the cooling time. Remarkable decrease of toughness is observed with increasing the size of austenite grain and the volume fraction of M/A constituent. Moreover, there exists the effect of orientation on fracture toughness for the specimens subjected to weld thermal cycle. Generally, the fracture toughness of simulated HAZ with L–T orientation is higher than that with T–L orientation. The reason may be related to that the strip structure formed during rolling is remained after the thermal simulation. Furthermore, the investigation shows that the toughness of coarse-grained zone is higher than that of fine-grained zone for the simulated HAZ. The reason may be related with the microstructure evolution of the HAZ during the complete thermal cycle used in the simulation. For the fine-grained HAZ, the shorter cooling time of t8/5may be not benefit for the self-tempering and decomposition of M/A constituents, then the toughness of the fine-grained HAZ is lower. For the coarse-grained HAZ, however, the longer cooling time of t8/5, may promote the decomposition of M/A constituents, then the toughness of coarse-grained HAZ is improved. The fracture toughness deteriorated drastically for the partly phase transformed HAZ may be related with the formation of mixture microstructure, in which the M/A constituent is distributed in shape of network.