| The marine economy will develop faster in the 21st century, the oceans are not only the vast treasure house of resources and energy, but also important base for sustainable development for mankind. It is predicted that offshore oil production will reach 36~39 million barrels per day by 2015, add up to 37~41 million barrels per day by 2020. The sea explorations and the exploitation of ocean resources are mainly based on ocean engineering platform and its equipment technology. Fixation of marine engineering platforms and equipments depends on high-level mooring chain. The R4s grade mooring chain is the highest-ranking marine engineering chain at present, weld joint was the weakest area of the entire chain link, which could affect the integrated performance of the entire chains directly. The properties of the material are depended on chemical compositions and microstructures. And heat treatment is an effective measure to alter microstructures and increase comprehensive properties, under the condition of the chemical composition unchanged. At present, the papers about mechanical properties of R4s level mooring chain flash welding joint are very little at home and abroad. It is very significance to study the properties of weld joint under Heat-Treatment, simultaneously provides the necessary conference for the following industrial production.According to the mooring chain actual working condition, this paper uses the cyclic quenching process to obtain better mechanical properties. The results are as followed: Compared with the normal quenching and tempering, after cycle quenching process, the tensile strength increases by 200MPa. After different tempered, the residual stress had different changing. Especially under 600℃tempered, the residual stress could reduce to 300MPa. With the same quenching temperature (920℃), after tempering at 560℃, 580℃, 600℃, 620℃, 640℃, and respectively tempering for 20min, 30min and 40min,It was found that the weld joint has the best comprehensive mechanics when tempered at 600℃for 30min. It still has tempered lath martensite phase even tempered at 640℃for 40min, which indicts that the material used in this experiment has good tempering stability. Tensile test and impact test showed that,the tensile strength can reach 1000MPa,and the impact energy at 0℃reached 100J. The fracture all contains large fiber zone which is characterized by ductile fracture. Especially under the 920℃×30min+920℃×30min+600℃×30min quenching and tempering treatment, the specimen has a large number of fine dimples, which has the best mechanical performance. Hanging patch test and electrochemical corrosion tests showed that the mainly corrosion of weld joint is pitting and intergranular corrosion. The corrosion mechanism is as followed: the micro-battery corrosion that consist of the low potential Fe-based and high potential second-phase particles happened. The process is constant corrosion with anodic dissolution and the cathodic protection, in which Fe-based as an anode, while the carburizing as the cathode body. Cyclic quenching and tempering treatment makes the organization and the corrosion resistance of the weld joint improved significantly. It also can be found from the polarization curves of the specimen, the material does not form an effective passivation film, so the anode protection law can not be used for this material. |
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