The microstructure， fatigue fracture morphology， mechanical

properties， and torsional fatigue life of the traditional seamless steel pipe 26CrMo for non-dig drilling pipe and the economical high-strength seamless steel pipe AGCY27 for non-dig drilling pipe were analyzed using optical microscopy， scanning electron microscopy， transmission electron microscopy， and mechanical tests. In terms of composition design， compared with traditional drilling pipe， economical drilling pipe removes precious metal Mo and reduce Cr content， appropriately increases Ti alloy， and increases Si content. The results show that the Ac

3

point of the traditional drill pipe is 803 ℃， and the Ac

3

point of the economical drilling pipe is 860 ℃. Through the heat treatment process of quenching+490 ℃ tempering， both meet the requirements of API 5D-2020 standard for 135 ksi grade drilling pipe. Under the heat treatment process in this experiment， compared with traditional drill pipe， economical drilling pipe has better impact toughness and fatigue performance， with an impact energy of over 109 J and an average fatigue cycle of 36 107 times. The microstructure of traditional drilling pipe is tempered martensite， while the microstructure of economical drilling pipe is tempered sorbite. The proportion of small angle grain boundaries in economical drilling pipe is 21%， while that in traditional drill pipe is 29%. Economical drill pipe has more large angle grain boundaries， and their microstructure is conducive to obtaining better comprehensive mechanical properties. The precipitation phase of traditional drill pipe is Cr alloy cementite， which is beneficial for improving their mechanical properties. The precipitation phases of economical drilling pipe is mainly TiCN， TiC， and TiO， which play a role in constraining the growth of austenite grains and improving the comprehensive mechanical properties of the product.