Effects of post-weld heat treatment cycles on microstructure and mechanical properties of electric resistance welded pipe welds

Two post-weld heat treatment cycles of one-step normalizing and two-step quenching and tempering have been performed by Gleeble, a thermo-mechanical simulator, to improve the toughness of fine-grained electric resistance welded pipe welds. Comparison was made to API X65 grade steel, which is widely used for pipeline parts. Microstructural evolution was investigated by optical microscopy and transmission electron microscopy. Vickers hardness and Charpy V-notch impact toughness tests were used to evaluate the mechanical properties. While the mechanical properties of one-step normalizing heat treatment satisfied the API specification, the two-step quenching and tempering heat treatments were conditional upon tempering temperature for X65 grade and fine-grained steels. As a result, a one-step normalizing heat treatment was more effective for both steel pipes.

► For X65 grade steel, all heat treatment conditions gave satisfactory results of 47 J at −20 °C.
► For fine-grained steel, only normalizing condition and tempering at 600 °C condition meet required absorbed energy.
► One-step normalizing heat treatment was more effective for both X65 grade and fine-grained steels.