Influence of Post-Weld Heat Treatment on the Microstructure, Microhardness, and Toughness of a Weld Metal for Hot Bend

In this work, a weld metal in K65 pipeline steel pipe has been processed through self-designed post-weld heat treatments including reheating and tempering associated with hot bending. The microstructures and the corresponding toughness and microhardness of the weld metal subjected to the post-weld heat treatments have been investigated. Results show that with the increase in reheating temperature, austenite grain size increases and the main microstructures transition from fine polygonal ferrite (PF) to granular bainitic ferrite (GB). The density of the high angle boundary decreases at higher reheating temperature, leading to a loss of impact toughness. Lots of martensite/austenite (M/A) constituents are observed after reheating, and to a large extent transform into cementite after further tempering. At high reheating temperatures, the increased hardenability promotes the formation of large quantities of M/A constituents. After tempering, the cementite particles become denser and coarser, which considerably deteriorates the impact toughness. Additionally, microhardness has a good linear relation with the mean equivalent diameter of ferrite grain with a low boundary tolerance angle (2 degrees-8 degrees), which shows that the hardness is controlled by low misorientation grain boundaries for the weld metal.