Research on the source of internal fault defects in 40Mn2 oil drill pipe Fang Gao, Yan-ping Bao, Li-dong Xing, Da-zhi Wang and Chao Gu Metall. Res. Technol., 121 6 (2024) 606 Published online: 01 November 2024 DOI: 10.1051/metal/2024089