Hardening mechanisms in a high wall thickness sour service pipe steel API 5L X65 before and after post-welding heat treatments

The strength of the pipe region opposite to the weld in a high wall thickness sour service pipeline steel API 5L X65, manufactured by the SAW UOE process, has been studied in detail. The fundamental hardening mechanisms that contribute to the yield strength of this pipe portion were determined according to its microstructural features before and after post-weld stress relieving heat treatment. The root mean square summation of the different strengthening mechanisms resulted in a reasonable estimate of the yield strength of this pipe portion before the heat treatment. The contributions of the different strengthening mechanisms proceeded at different length scales in this pipe portion before and after the heat treatment. The heat treatment at 650 °C (1.5 h) reduced the contributions of the strengthening mechanisms to the steel yield strength corresponding to this pipe portion. The dislocation strengthening contribution decreased by approximately 50%. The precipitation hardening and the solid solution and grain size strengthening reduced their contributions by approximately 14% and 3% respectively. •The individual strengthening mechanisms in this pipe steel were calculated before and after the stress relief heat treatments.•Precipitation hardening contributes to the yield strength approximately 11% in the as-received pipe steel.•Dislocation strengthening contribution was three times the corresponding precipitation hardening contribution.•The stress relieving heat treatments below the Ac1 temperature decreased the individual strengthening contributions at different length scales.•The dislocation strengthening contribution was the most affected after the heat treatments.