Study On Microstructure And Properties Of Heat-affected Zone Of X80 Bend In Low Temperature Environment

In this paper,the structure and properties of the heat-affected zone of X80 elbow welding in low temperature station were studied by means of welding thermal simulation technique,microstructure observation and mechanical property test.Analyzed the organizational performance in different areas of the welding heat affected zone and reveals the welding heat input and secondary peak temperature on microstructure and low temperature toughness of coarse grained region influence law,on the basis of thermal heat treatment process was studied in low temperature toughness of coarse grain area,ultimately determine the best low temperature environment with X80 pipe bending roasted matching process.The results show that: in the case of single pass welding of X80 elbow in low temperature environment,when the peak temperature of a thermal simulation is at 650℃,950℃ and 1100℃,the subcritical zone and fine crystal zone have fine grain size and good low temperature toughness.When the peak temperature of a thermal simulation is at 850℃,there are non-uniformly distributed granular carbide in the tissue,resulting in a decrease in the low temperature toughness of the critical region.When the peak temperature of A thermal simulation is 1300℃,the austenite grains in the coarse grain area grow up,and there are large size and high carbon content m-a elements in the structure,which leads to the serious deterioration of its low temperature toughness,and becomes the weakest area in the welding heat affected area.In low temperature environment,the low temperature toughness of the thick crystal area of the X80 bend decreases with the increase of heat input and is lower than that of the base material.When the value of heat input is relatively small,the cross-distributed slat BF and GB in the coarse crystal structure can maintain its low temperature toughness at a relatively high level.However,when the heat input reaches more than 40KJ/cm,the coarse grain structure becomes coarser,and the large size of m-a elements and PF in the structure seriously reduces its toughness at low temperature.Low temperature environment with X80 pipe welding by both sides or multi-channel welding,when the secondary thermal simulation peak temperature at 800℃ of two phase zone(alpha gamma),apparent critical coarse grain zone(ICCGHAZ)local embrittlement phenomena,the cause of embrittlement mainly for the regional organizations did not occur after partial recrystallization refinement,at the same time have A chain around the austenite grain boundary brittle M-A island groups gathered.In low temperature environment,the low temperature toughness of the thick grain area of the X80 bending pipe was improved by heat treatment.During the process of quenching 920℃+ tempering 650℃,the coarse crystal area was mainly composed of homogeneous and fine bainite with multi-phase distribution,and the low temperature toughness improvement was the strongest.Therefore,from the perspective of actual production,it is recommended to choose quenching 920℃ water cooling,followed by 650℃ tempering and holding air cooling for 90 min as the best process matching in the process of X80 bending pipe heating.