| The components welded by dissimilar stainless steel and carbon steel have found an increasingly wide utilization in the oil drilling, oil and gas transportation and refining of crude oil industries. They possess both strong corrosion resistance of stainless steel and economy of carbon steel. The welding of316L stainless steel and carbon steel Q235dissimilar steel is often used in structural design and manufacturing. However,316L stainless steel and carbon steel Q235have big differences in terms of chemical composition, structure, and physical properties, which makes it difficult for welding process implementation and requires higher quality assurance of weldment.In order to optimize the316L/Q235dissimilar steel welding process, improve the microstructure and mechanical properties of welded joints, the welding process of316L/Q235dissimilar steel and post-weld aging heat treatment were studied by experimental study in this paper. The mechanical properties and corrosion resistance of welded joints with different welding process and aging treatment were investigated. In order to clarify the thermal process of316L/Q235dissimilar steel welding and the influence of the thermal process on the performance of welded joints, the three-dimensional temperature field of316L/Q235dissimilar steel welding process was simulated. The specific studies and the main conclusions are as follows:(1) with surfacing buffer layer welding process, selecting different overlay and filled welding materials to be a combination, the316L/Q235dissimilar steel plate butt welding experiment was conducted to study the influence of different welding materials matching on the structure and performance of316L/Q235dissimilar steel welding joints. The microstructure and properties of welded joints were analyzed by metallurgical analysis, tensile test, impact test, hardness test and the test results show that:A042welding material adopted as the Q235base groove overlay buffer layer material, surfacing about3-5mm, then A022welding material adopted as the filled welding materials between the buffer layer and316L base. By this method the mechanical properties of welded joints were optimal. So this combination of welding materials was the optimal welding consumables matching scheme.(2) In order to further improve the performance of welded joints, the316L/Q235dissimilar steel welded joints were treated respectively by different ageing technologies with temperature at300℃,500℃,700℃for2hours. The action mechanism of aging treatment to the ingredients and organizational structure of welded joints and the influence of aging temperature on the microstructure and properties of welded joints after aging were analyzed by metallurgical analysis, tensile test, impact test, hardness test. The comprehensive evaluation on the performance of welded joints with different temperatures aging treatment shows that aging at the temperature of300℃for2h, then being air-cooled at low temperature was more conducive to optimize the composition and microstructure of the welded joints, which can further improve the comprehensive mechanical properties of welded joints. (3) The316L/Q235dissimilar steel welded joints with no aging treatment and aging treatment at different temperature were corroded respectively in simulated formation water with corrosion temperature of60℃,90℃,120℃, Cl-concentration of0.08mol,0.10mol,0.12mol and CO2partial pressure of2MPa to carry out high temperature and high pressure corrosion test. The CO2corrosion behavior of316L/Q235dissimilar steel welded joints was studied and the influences of corrosion temperature, Cl-concentration and aging process on CO2corrosion of welded joints were investigated though corrosion rate, corrosion product film morphology and corrosion product film composition. The results showed that the CO2corrosion rate of316L/Q235dissimilar steel welded joints increased with the increasing of Cl" concentration; the CO2corrosion rate first increased, then decreased with the increasing of corrosion temperature from60℃to90℃and reached the peak value at90℃. Aging at low temperature of300℃can better improve the corrosion resistance of welded joints. The main CO2corrosion products of316L/Q235dissimilar steel welded joints were FeCO3and Cr2O3.(4) The three-dimensional dynamic thermal process of316L/Q235dissimilar steel plate butt welding was simulated by using the Ansys software. The temperature distribution of weldment at different times and the thermal cycle curve of different nodes on weldment were obtained. The temperature field of weldment, weld pool shape and thermal cycling curve variation under different welding current and welding speed were analyzed. The results showed that the temperature field simulation result was accord with the actual welding process temperature variation. The temperature of Q235base changed more quickly in welding process because that the thermal conductivity and convection coefficient of Q235steel are greater than that of316L steel, which is more conducive to heat conduction and spreading. The increase of the welding current enabled the heat input to increase in welding unit time, the temperature of overall welding to rise and the weld pool to be larger and deeper. With the welding speed decreasing, the welding time correspondingly increased and the total amount of welding heat input increased, so the temperature of weldment elevated, the size and the peak temperature of the weld pool increased and the residence time of high temperature zone been longer. |
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