Effect of tempering temperature on DWTT properties of X70Q pipeline steel

To investigate the effect of tempering temperature on the drop-weight tear test (DWTT) properties of X70Q pipeline steel, the influence of different tempering temperatures (300-600 ℃) on the microstructure and mechanical properties, especially the DWTT performance, of X70Q thick-walled pipeline steel and its underlying mechanisms were studied using metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), JMatPro simulation, transmission electron microscopy (TEM), and other methods. The results show that in the low-to medium-temperature tempering range of 300-400 ℃, dislocation recovery and sub-grain coalescence occur in the tested steel, leading to a significant increase in the proportion of high-angle grain boundaries (reaching 68.3% at 400 ℃). Meanwhile, the precipitation of fine M₃C carbides contributes to strengthening, resulting in an optimal combination of strength and toughness. During high-temperature tempering at 500-600 ℃, grain coarsening, brittle-phase precipitation, and decomposition of martensite/austenite (M/A) islands cause a remarkable deterioration in DWTT performance. The deterioration of DWTT performance during high-temperature tempering is mainly attributed to three key factors: grain coarsening and equiaxed, brittle phase precipitation along grain boundaries, and decomposition of small M/A islands. Grain coarsening and equiaxed refers to the transformation of needle shaped ferrite with small fracture units and large orientation differences into equiaxed polygonal ferrite after high-temperature tempering, resulting in an increase in effective grain size, especially the abnormal growth of 001 cleavage plane grains, which reduces crack propagation resistance. Brittle phase precipitation along grain boundaries refers to the precipitation of coarse M₂₃C₆ along grain boundaries after high-temperature tempering, resulting in chromium depletion near the grain boundaries and weakening the bonding strength of the grain boundaries. The decomposition of small M/A islands refers to the important structure that hinders crack propagation. After high-temperature tempering, the small M/A islands are basically completely decomposed, losing their beneficial effects. The research results provide reference for the production of X70Q heat treatment.