根据焊接热影响区中不同亚区的热循环特征对低碳贝氏体钢进行了焊接热模拟实验.采用示波器载荷冲击试验机检测焊接热模拟试样的冲击韧性, 结合OM, SEM,TEM以及EBSD技术对模拟显微组织的观察, 分析了不同亚区的显微组织特征与冲击韧性之间的关系. 结果表明, 当冷却时间t8/5=30 s时,各亚区的裂纹形核功相差并不太大, 其值在40-70 J之间. 细晶区(FGHAZ)具有良好的止裂能力, 裂纹扩展功高达122 J; 而部分相变区(ICHAZ)和粗晶区(CGHAZ)的裂纹扩展功较小, 分别为51.8和17 J. 随t8/5的延长,各亚区的裂纹形核功和扩展功均下降, 其中CGHAZ的裂纹形核功和FGHAZ的裂纹扩展功的下降最为显著. 不同冷却速率下, M--A组元尺寸和形态的变化是影响裂纹形核功的重要因素. 对于裂纹扩展功来说, 高冷却速率下,具有高密度大角晶界的FGHAZ具有良好的抗裂纹迅速扩展的能力,但当冷却速率降低, 由于原始奥氏体晶粒长大而使裂纹扩展功下降.ICHAZ有效晶粒尺寸不均匀, 并随冷却速率的降低, 晶粒尺寸明显增大,裂纹扩展功下降. 而在CGHAZ中原始奥氏体晶粒显著粗化, 大角晶界密度的下降导致裂纹扩展功降低; 随冷却速率的降低, 原始奥氏体晶粒内的取向变得更为单一,裂纹扩展功进一步降低.
It is generally recognized that welding heat affected zone (WHAZ) is the poorest toughness region in the welded joint of low carbon bainitic steels. The thermomechanical simulator was employed to simulate the welding thermal cycle processes of different sub–regions in WHAZ of low carbon bainitic steel in this work. The toughness of simulated specimens were tested on the instrumented drop weight impact tester with oscilloscope, and miscrostructure features were observed by means of OM, SEM, TEM and EBSD. The results showed that when cooling time (t8/5) was 30 s, the crack initiation energy of various sub–regions was similar, and the range of their values was between 40 and 70 J. However, fine grained heat affected zone (FGHAZ) exhibited excellent crack arrest properties because the impact load–time curve included wide crack ductile propagation and crack brittle propagation stages. By contrast, the crack propagation energy of intercritical heat affected zone (ICHAZ) and coarse grained heat affected zone (CGHAZ) obviously deteriorated. With the increase in cooling time, both crack initiation energy and crack propagation energy of various sub–regions decreased, in which the crack initiation energy of CGHAZ and the crack propagation energy of FGHAZ decreased notably. Under different cooling rates, the variation of morphology and size of M–A constituents was mainly responsible for the deterioration of crack initiation energy. As for crack propagation energy, the FGHAZ had a good resistance to crack propagation due to high density of high angle grain boundary. Therefore, its crack propagation energy was far superior to other sub–regions. There was uneven effective grain size in the ICHAZ and ferrite grain grew with the decease in cooling rate, which decreased the crack propagation energy. In the CGHAZ, prior austenite grains coarsened and the density of high angle grain boundaries decreased greatly, which resulted in the decrease in crack propagation energy.
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