超高强度钻杆低温断裂性能及裂纹扩展特征研究

超高强度钻杆在冬季低温环境下容易发生脆性断裂事故,为了从断裂力学角度揭示其冷脆现象的本质,采用ZBC2302-D型示波冲击试验机,获得了不同温度下超高强度钢在冲击断裂过程中的力-位移曲线,并对比分析了冲击功、起裂功和裂纹扩展功之间的变化关系。采用国家标准GB 4161—2007中的计算公式,计算了冲击试样的裂纹扩展量Δa和阻抗应力强度因子K_R;同时,引入K_R阻力曲线,得到超高强度钻杆在不同温度下的剩余强度图。结果表明,当冲击试样开始出现裂纹时,随温度降低,钻杆的初始起裂应力有所增加,出现硬化现象。当钻杆出现一定长度的裂纹时,随温度降低,阻抗应力强度因子越来越小,而且降低速度越来越快,剩余强度也表现出相同的变化趋势。随温度降低,材料抵抗裂纹扩展的能力逐渐下降,表明钻杆在低温下更容易断裂。

On the fracture properties and crack extension features of the ultra-high tough drill rod under low temperatures

The article is inclined to introduce its study on the fracture properties and crack propagation features of the ultra-high strength drill rod under low temperatures due to the cold brittleness of ultra-high strength drilling rod,which actually brings great challenges to the drilling operations.It comes from the aforementioned needs that have pushed this article to put its focus on the study of the cold brittleness of the drill rod from the point of view of fracture mechanics.In proceeding with our study,we have obtained the force-displacement curve of the fracture process for the materials to be drilled under different low temperatures with an instrumented impact tester (mode ZBC2302-D).At the same time,we have also analyzed and compared the relation between the overall fracture energy and the fracture initiation energy in addition to the crack propagation energy,whereas the crack extension △α and the fracture toughness KR have been worked out in accordance with the national standard of GB 4161-2007.What is more,we have also calculated the residual intensity of the drill rods at different temperatures by K-R resistance curve.The results of our study demonstrate that the initial resistance of the drill rod tends to increase with its toughness growing with the decrease of the temperature till the moment of its crack-down.And,later,when the crack of the rod reaches a certain extent,the resistance stress of the intensity factors may turn to get lowered with the drop of temperature.The same is true with the trend of the residual strength.It can also prove that the overall fracture intensity and that of the crack propagation tend to decrease with the decrease of temperature.However,since the fracture initial energy and the overall fracture energy should be destined in the opposition directions,the resistance to the initial crack growth is likely to get reduced with the lowering of the temperature,which should account for the drill rod crack due to its being subjected to the low temperature.The results of our study have thus provided basic experimental evidence for the field application of the ultra-high strength drill rod,and can also serve as a reliable guide to the manufacture of ultra-high strength drill rods.