X80管线钢的冲击断裂行为

IMPACT FRACTURE BEHAVIOR OF X80 PIPELINE STEEL

在-20和-60℃对X80管线钢进行Charpy冲击实验,用SEM-EDS对断口进行形貌和夹杂物成分分析,并将断口切开进行金相和EBSD观察,研究X80管线钢的冲击断裂行为.结果表明:温度对X80管线钢最大冲击载荷影响较小,但随实验温度降低,裂纹形成功、裂纹扩展功和止裂后吸收的能量显著减小.在冲击断裂过程中强烈的拉应力作用使断口附近产生变形带,晶粒沿与主裂纹垂直的方向被拉长,原始奥氏体晶界几乎不变形,因而产生应力集中,并产生沿晶界的断裂.同时,钢中的脆性第二相粒子也是裂纹源.在裂纹扩展过程中,强烈的内应力使主裂纹附近的晶粒产生剧烈的变形,拉长直至破裂,并形成新晶粒;新形成的晶界表现为大角晶界.裂纹扩展区附近的晶粒尺寸较原始组织细小而大角晶界比例有所增加.-20℃冲击断口裂纹扩展区附近的晶粒尺寸比原始组织小得多,而-60℃断口裂纹扩展区附近的晶粒尺寸与原始组织相差不大.

Pipeline steels have been widely used in a long-distance transportation of large amounts of crude oil or natural gas under high pressure due to their high transportation efficiency,low energy loss and production cost.For high pressure gas transmission pipelines made from high-strength steels an important problem is to know their fracture behavior in a long running process.In this paper, fracture toughness of X80 pipeline steel was measured by V-notch Charpy impact test at -20 and -60℃.OM,SEM,EDS and EBSD were used to analyze its fracture mechanism.Experimental results show that the maximum impact load is slightly influenced by temperature,but with the decrease of temperature,crack forming work,crack propagating work and energy absorbed by crack arresting decrease significantly.During fracture process,the intensive tensile stress in the sample would induce the deformation bands formed around the fracture surface and grains elongated along the direction vertical to the main crack,but original austenite grain boundaries are hardly deformed,it would cause stress concentration and then produce intergranular fracture.Generally,brittle second phase particles could act as crack sources under intensive internal stress.In the crack propagation process,grains near the main crack are deformed and elongated tempestuously,resulting in their breaking and new grains with high angle grain boundary（HAGB） formed.Therefore,the grain size decreases and the fraction of HAGB increases in the crack propagation region.Temperature has great influence on the plastic deformation behavior of pipeline steel during facture process.At -20℃,the tested steel has good plasticity and the grains are refined during deformation,but at -60℃,they are difficult to deform and refine.Consequently,the grain size near crack propagation zones in the sample impacted at -20℃is much smaller than the original grain size,but in -60℃impacted sample the grain sizes has little difference.