The Study On Microstructures&Properties Of Heat-Affected Zone Of High Grade Pipeline Steels

In this paper, the microstructures and properties of heat-affected zone (HAZ) of high-grade pipeline steels in the welding process were studied by welding thermal simulation techniques, mechanical properties test and material microscopic analysis method. The microstructure-property of different parts of HAZ, the influences of once thermal cycle and twice thermal cycle on microstructure-property, the fracture characteristics of welding local brittle zones, and the control and prevention of local embrittlement were analysed and discussed respectively.The research indicated that the welding heating process has a harm impact on toughness of HAZ and the toughness of Coarse-Grained HAZ (CGHAZ) was the worst. It is found that coarse grain size and M-A constituents are major reasons of CGHAZ embrittlement.CGHAZ of experimental steels can achieve higher toughness because of the multidirectional acicular ferrite and fine effective grain size when welding heat input is5～20kJ/cm in single pass welding. While welding heat input is up to30kJ/cm, the toughness of CGHAZ is greatly decreased due to grain coarsening and the formation of polygonal ferrite.The toughness of intercritically reheated coarse-grained HAZ (ICCGHAZ) of experimental steels was the lowest when the second peak temperature was in two phase (α-γ) temperature during multi-pass welding, showing ICCGHAZ local embrittlement. It is found that coarse grain and M-A constituents result in local embrittlement of ICCGHAZ.Combined with experimental observation and two-dimensional FE analysis, the fracture mechanism of HAZ of high-grade pipeline steel was investigated. The results show that slender M-A constituent is apter to crack than blocky one. Therefore, the toughness in the welding HAZ can be improved by controlling the shape of M-A constituent.Based on the research results of welding embrittlement mechanism of high-grade pipeline steel, the ways of controlling and preventing welding local embrittlement were proposed, such as alloy design, welding heat input, welding preheating temperature, welding interpass temperature, post-heat treatment and so on.