Effect of tempering temperature on precipitate and mechanical properties of an anti-sulfur, drill pipe steel in H ₂S containing environments

Precipitates and mechanical properties of an anti-sulfur drill pipe steel tempered at different temperatures were investigated by means of optical microscopy (OM), transmission electron microscopy (TEM), hardness and tensile tests. The results show that rod-shaped precipitates with micron size and spherical-shaped precipitates with nanometer size are observed in the matrix of ferrite for the steel tempered at 580°C. As tempering temperature increases to 625°C, the volume fraction of precipitate increases remarkably, and its shape is evolved into sphere gradually. Consequently, strength and toughness of the steel improves simultaneously, with the tensile strength higher than 1000 MPa. With increasing tempering temperature to 680°C, precipitation occurrs and segregates along grain boundary, which results in degradation in strength. Finally, the competitive mechanism between precipitate-dislocation interaction and harmonizing precipitate-matrix deformation, which is responsible for optimial match between strength and ductility, is discussed.