LNG储罐用9Ni钢-196℃焊缝断裂韧性及断口分析

Fracture toughness and fracture analysis of 9Ni steel weld at-196 ℃ for LNG storage tank

根据GB/T 21143—2014测试了厚度30 mm的9Ni钢板TT和SAW焊缝的-196℃裂纹尖端张开位移(CTOD),观察并分析了试样微观组织及其断口。结果表明,TT和SAW焊缝的δ平均值分别为0.434 mm和0.259 mm,两者的CTOD特征值均远高于工程标准要求的0.15 mm。TT和SAW焊缝金属的显微组织均以奥氏体柱状晶形式存在,TT焊缝的组织更加细小均匀,细小均匀分布的胞状晶组织能够增加裂纹扩展阻力,从而增加焊缝的低温断裂韧性,小的热输入也有利于9Ni钢焊缝的低温断裂韧性。SAW焊缝金属中的Al2O3能够限制焊缝中奥氏体的长大,起到细化晶粒的作用,从而提高焊缝金属的强度和韧性;TT焊缝晶粒更小,断面更加粗糙,断面存在更多的10μm左右的富铁和富钼的第二相颗粒也阻碍裂纹扩展,使得焊缝具有更好的低温韧性。

The crack tip opening displacement(CTOD) test of SAW and TT weld metal of 30 mm 9Ni steel plate was carried out at-196 ℃ according to GB/T 21143—2014.The microstructure and fracture of CTOD specimens were analyzed.The results show that the average δ values of TT and SAW welds are 0.434 mm and 0.259 mm respectively, and their characteristic values are much higher than 0.15 mm required by engineering standards.The microstructure of both weld metals is austenitic columnar crystals, but that of TT weld is finer and uniform, which can increase the crack propagation resistance of cellular crystal and low-temperature fracture toughness of weld.The small heat input is also beneficial to the low temperature fracture toughness of 9Ni steel weld.Al2O3in SAW weld metal may limit the growth of austenite in weld and refine grain, so as to improve the strength and toughness of weld metal.Smaller grain, rough section and more Fe or Mo rich second phase particles of about 10 μm size also prevent crack propagation, resulting in better low-temperature toughness in TT weld metal.