Effects of Tempering Process on Microstructure and Hardness of 110 ksi Grade Thick Wall Casing Steel Pipes

The effects of different tempering temperatures and holding times on the microstructure and hardness of thick wall sulfur-resistant casing steel， which exhibits martensite （M） structure at the outer wall and martensite + bainite （M+B） structure at the middle wall after quenching， were investigated. The results show that in the tempering temperature range of 695 ℃-720 ℃， Rockwell hardness fluctuation caused by microstructure differences is 0.8 - 1 HRC. Both types of microstructure show a decreasing trend in Rockwell hardness with the prolongation of tempering holding time. But when the tempering holding time is 75 min - 85 min， both groups have obvious secondary hardening phenomenon. Microstructure observation on samples subjected to different tempering process shows that， solid solution carbon content of martensite structure is high， and the precipitation of carbon atoms during tempering leads to relatively fast hardness attenuation and poor tempering stability. However， the higher solid solution carbon content increases the percentage of vanadium carbide （VC） microalloyed precipitates， and the secondary hardening phenomenon compensates for the hardness degradation caused by carbon dissolution. In actual production， the uniform microstructure should be obtained as far as possible to reduce the hardness vaiation in the wall thickness range of the pipe. At the same time， tempering treatment should avoid the secondary hardening interval， so that the hardness value can meet the requirements of mechanical properties of sulfur-resistant pipes.