镁处理对CGHAZ冲击韧性及裂纹扩展的影响

通过镁处理Q345GJ钢引入含镁的复合夹杂物，研究其对大线能量焊接(线能量为100 kJ/cm)粗晶热影响区(CGHAZ)组织和断裂韧性的影响。结果显示，试验钢经镁处理后CGHAZ韧性值明显提高(冲击功由56 J提高到108 J)。主要原因为，镁处理钢在CGHAZ焊接热循环冷却过程中优先形成含镁的复合夹杂物，作为高表面能的惰性基体，其周围形成的贫锰区和高能应变场共同促进针状铁素体(AF)形核，AF体积分数为(82.9±2.0)%,宽度为(0.96±0.1)μm(无镁处理钢对应为(32.4±1.5)%、(3.13±0.2)μm);研究了大线能量焊接后CGHAZ冲击试样中微孔的形成及裂纹扩展，两种试验钢夹杂物附近均存在大量位错，变形过程中位错通过夹杂物时形成位错环堆积引起局部应力集中；当应力积累到无法使相邻晶粒的位错源开启时，不利于滑移取向上的应力达到临界值导致微孔的产生，相邻微孔在外力作用下连接形成裂纹。无镁处理钢CGHAZ区域微孔更为密集，受力后更易形成微裂纹。镁处理钢CGHAZ区域大角度晶界比例为80.2%、几何必要位错密度(GND)为0.806、局部取向差分布值(KAM)为0.91...

Effect of Mg-treated on impact toughness and crack propagation of CGHAZ

Mg-bearing composite inclusions were introduced into Q345GJ steel by Mg-treated,and the effects of Mg-bearing composite inclusions on the microstructure and fracture toughness of coarse-grained heat affected zone (CGHAZ) with high heat input welding (energy input 100 kJ/cm) were studied. The results show that the CGHAZ toughness of test steel after Mg-treated is significantly improved (impact energy increases from 56 J to 108 J). Because of in the process of weld thermal cycle cooling,Mg-bearing composite inclusions are preferentially formed in the CGHAZ,the presence of Mn depletion zone and high-energy strain field around the mg-bearing inclusions with high surface energy promotes the nucleation of acicular ferrite (AF),with the AF ratio of (82.9±2.0)%(volume percent) and the width of (0.96±0.1) μm((32.4±1.5)% and (3.13±0.2) μm for the Mg-free steel). The formation of micropores and crack propagation in CGHAZ impact specimens after high heat input welding were investigated. There are a large number of dislocations near the inclusions in test steel. During the deformation process,stress concentration is caused by the accumulation of dislocation loops. When the dislocation source of stress concentration to adjacent grains cannot is activated. Micropores are formed on the orientation which is not conducive to slip and then connected to form cracks under external force. Micropores of Mg-free steel are denser,which provides favorable conditions for crack initiation. The proportion of high angle grain boundaries(HAGBS) in Mg-treated steel is 80.2%,the geometric necessary dislocation density (GND) is 0.806,and Kernel Average Misorientation (KAM) is 0.912. (with 71.9%,0.896 and 0.956 for the Mg-free steel). The small and self-locking AF in CGHAZ improves the impact toughness of Mg-treated steel due to hinders the expansion of micropores and inhibits the propagation of cracks. The crack propagates along the HAGBS,the crack propagation path is prolonged and the energy consumption increases when the crack encounters AF.