| X80pipeline steel is maturely used in oil transportation, but because of the specialty of the welded joints, it becomes the safety hazard, especially in an environment containing destructive ions, so it’s necessary to conduct many tests to study the organization, mechanical properties and the corrosion resistance of X80steel and its weld joint to get an comprehensive understanding and grasp the regularity in order to avoid some accidents.In this paper,with the use of some laboratory instruments such as optical microscopy, tensile testing machine, slow strain tensile testing machine, micro hardness tester, autoclave, electrochemical workstation and scanning electrochemical microscope,the differences of the microstructures and properties between X80steel and its welded joints were studied by microstructure observation and the evaluation of mechanical properties,electrochemical properties, anti-stress corrosion properties and surface electrical activityMicrostructure results showed that:quenching and tempering both would change the microstructure of X80steel. The organization after quenching was primarily martensite surrounded by a small amount of ferrite and bainite particles; with tempering temperature rising,the transformation of martensite had occured,the compound that had been deposited became largergradually, the ferritewas uniform and the residual bainite gradually disappear. When the tempering temperature reached650℃,the organization was uniform polygonal ferrite. And the organizations of welded joints were mainly ferrite, bainite and pearlite. Their distribution in different regions of the welded joints affects the mechanical properties of welded joints.High temperature and pressure corrosion test results showed that:In corrosive media containing bicarbonate and chloride ions, the dense corrosion product on the X80steel and its weld joints was FeCO3and the chloride ions would have a destructive effect on it.Under this corrosion condition, the samples of X80after heat treatment had the almost same anti-corrosion property as the base material without heat treatment, which indicated that heat treatment would not improve the overall anti-corrosion property. And the corrosion rate of welded joints was the highest.Mechanical properties test results showed that:the hardness of base material had been greatly improved after quenching, the other mechanical properties did’t improve much.Some properties of the base material, such as tensile strength and impact performance, after quenching and tempering treatment, had improved.After tempering at about600℃the sample had better comprehensive mechanical properties.Weld joints had the worst mechanical properties among all the test samples.Electrochemical corrosion test results showed that:the corrosion product of X80steel and its weld joint in0.5mol/LNaHCO3solution had the protection property. However, to add chloride ions in the solution would undermine its protection, and with the increasing of the chloride ion concentration, the corrosion rate of X80steel base metal and welded joints had a tendency to increase. On the basis of the presence of chloride ions, and to add some sulfate ions would let the corrosion rate further increase. But with the concentration of sulfate ion increasing, the corrosion rate would decline slightly.Scanning electrochemical microscope test results showed that:the surface activation peak current density of X80steel and its weld joint elevated with the potential added on the substrate material increasing. The surface current peak was the highest when the potential was the pitting potential. The result was consistent with the analysis of the polarization curves.Slow Strain Tensile test results showed that:The welded joint of X80had a higher susceptibility to stress corrosion and was the potential hazard.The chloride and sulfate ion concentration both had some impact on the susceptibility of X80steel and its weld joint to stress corrosion. |
PDF Full Text Request