The Influence Of Microstructure Of Heat Affected Zone On Fatigue Crack Propagation Behavior Of Large Strain Pipeline Steel

The research of large strain pipeline steel started late in the country,so its production technology and application is not yet mature.Research and development of large strain pipeline steel can not only enable us to master the production technology and application,while reducing the dependence on foreign countries to improve our country's oil and gas resources and pipeline engineering stability and safety.In this paper,the microstructures and properties of X80 large strain pipeline steel in different regions of HAZ were studied by thermal simulation test,tensile test,hardness test,fatigue test and metallographic observation.The fatigue fracture was observed by SEM.Results show that: CGHAZ is of coarse grains,the prior austenite grain boundaries are clear and the dislocation density is high,so CGHAZ has a high tensile strength and yield strength;The microstructure of FGHAZ is uniform,mainly composed of PF and GB.The dislocation density is not high.FGHAZ has a good plasticity and fatigue properties;The ICHAZ is mainly composed of coarse PF and QF.Because the dislocation density is low and the microstructure is not uniform,the toughness is poor.But the comprehensive performance is good;The microstructure of SCHAZ is similar to base metal.Compared to base metal,yield strength of all HAZ is decreased in some degree and the hardness is increased execpt CGHAZ.The crack propagation rate test was carried out on the INSTRON8847 universal testing machine,and the experimental data were fitted and the EBSD analysis of the fatigue fracture was carried out.Results show that: there is a turning point in the fatigue crack growth rate curve of each region.According to the turning point,the curve is divided into AB and BC stages,and the crack growth rate decreases slightly in the second stage;The resistance of dislocation angle and the second crack to the main crack lead crack growth rate declining,FGHAZ and ICHAZ exist in a large number of secondary crack and dislocation Angle,and a big part of it is high angle,so the crack growth rate curve is of a greater degree of turning.Finally,according to the fatigue crack propagation rate curve,the material constants “C” and “m” in the paris equation are obtained,and thefatigue life prediction equation of the weld heat affected zone is obtained.