Study on Strength and Plasticity of High Steel Grade Drill Pipe

Increasing drill pipe strength grade results in a change in plastic property so a high yield ratio has become the main problem in high-strength drill pipe popularization and application.Tensile tests at room temperature were conducted on X95,G105,S135,V150 and HL165 series of high-strength drill pipes.In addition,the influence of the strength and plastic characteristic parameter variation rules and yield ratio on the safety of drill pipes were analyzed by means of a true tress-true strain curve.Test results showed that with the promotion of steel grade and continual increase of drill pipe strength,engineering yield-ratio and true yield-ratio both increased,but the latter was 5.5%-7.0% less than the former.The engineering yield-ratio of a different grade of steel drill pipe had no corresponding relationship with its elongation,impact energy,strain capacity at plastic instability point,uniform deformation capacity,static toughness and other ductility indices.Although the yield ratio of V150 and HL165 series of ultra high strength drill pipes reached 0.953 and 0.941 respectively with slight decrease of deformation hardening capability,they still possessed a high plastic deformation capacity,high toughness level and high breaking strength.The study suggested that it was inappropriate to take engineering yield ratio as rigid index to measure high grade drill pipe quality and acomprehensive list of properties is the key to ensuring the safe use of drill pipes.