| In order to meet the production requirements, transmission pipelines in oil, chemical and energy industries were working in high temperature, corrosion conditions. This required a specific raw materials that could meet the above working environments, generally used 1Cr5 Mo steel in chemical, oil and energy industry. Welding was the main method of connecting 1Cr5 Mo steel pipelines, Which was easy to form the defects of gas holes, shrinkage, and the strength of the welded joints were decreased. At present, the study of the heat-resistant steel welded joint after laser heat treatment(LHT) were lack of in-depth research. In this article, the surface of 1Cr5 Mo heat-resistant steel welding joints were heat treated with CO2 laser, the mechanical properties and corrosion resistance were studied, the formation mechanism of strengthening layer in the welded joint surface after laser heat treatment was discussed, and provided a safe and feasible heat treatment method on welded joints. The main conclusions and innovations are as follows:(1) The mechanism of grain refinement and micro reinforcement of 1Cr5 Mo steel welded joint after LHT were discussed. The effect of laser heat treatment on the microstructure, grain size, residual stress and residual austenite in heat affected zone of 1Cr5 Mo steel welded joints were analyzed. The results show that the grains of 1Cr5 Mo steel welded joints are refined by LHT, and the microstructure uniformity is improved significantly, the grain levels of welded zone, fusion zone, overheated zone and normalized zone are increased from level 9, level 9.8, level 8 and level 10.7 to level 10, level 10.2, level 8.5 and level 11. The tensile residual stress in the welded joint surface is eliminated by LHT and a layer of compressive residual stress with thickness of about 0.28 mm is formed. The residual austenite content in the welded joint surface increases after LHT, the distribution is more uniform and conducive to improving the mechanical properties.(2) 1Cr5 Mo heat-resistant steel as pyrolysis pipelines not only withstood the high temperature of working environment, but also resisted the structural stress introduced during installation process. High temperature tensile tests were used to simulate the real production situation. The results show that the yield strength, tensile strength and elongation of laser processed sample at 200 increased by 32.5%, 22.5% and 4.6% respectively, while that o℃ f LHT sample at 300 ℃only increased by 6.6% and 6.5% 3.1%, respectively. The tensile fractures before and after LHT are ductile rupture, the dimple size of LHT sample is bigger than that of original sample increasing with the temperature. The grain refinement and compressive residual stress after LHT are the main mechanisms of improving the tensile properties.(3) In view of transmitting the high salt content material in 1Cr5 Mo steel pipelines, salt spray corrosion test was carried out. The effects of laser heat treatment on the corrosion resistance were studied by means of SEM, EDS and XRD. The results show that the corrosion products on the welded joint surface are loose in the original state, while those on the welded joint surface by LHT are compact, which protects the substrate from destroying further by the corrosion media. The corrosion potential of the welded joint has increased by LHT, making the corrosion hardly to perform, in which the grain refining and compressive residual stress by LHT are the main mechanism of improving the corrosion capability.(4) The influence mechanism of NACE solution on stress corrosion cracking, stress corrosion fracture morphology and fracture mode of 1Cr5 Mo heat-resistant steel welded joints were studied by means of slow strain rate test. Moreover, the influence of laser scanning speeds on the stress corrosion resistance of the welded joints was also discussed. The results show that the anti stress corrosion properties are reduced after LHT, and the sensitivity index of stress corrosion decreased from 33.9% to below 23%. The laser scanning speed has somewhat effect on stress corrosion properties of 1Cr5 Mo heat-resistant steel welded joints, the stress corrosion sensitivity index is the least at the laser scanning speed of 400mm/min.In summary, this article provides experimental basis for improving the mechanical properties and corrosion resistance of 1Cr5 Mo heat-resistant steel welded joints by LHT. It has great academic significance and important engineering application value to the pipeline construction in China, and provides a technical basis for the application of laser heat treatment. |
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