Finite Element Modeling of the Effect of Different Post-Weld Heat Treatment on Strength of Welded AISI 4130 Steel Pipe

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Abstract AISI 4130 steel is a low-alloy strength steel and is a popular choice for fabrication of drilling fluid pipes in the oil and gas industry. One of the drawbacks of this low-alloy steel is that it can form martensite at relatively low cooling rates during welding, which is why the welds in this steel require post-weld heat treatment (PWHT) to temper the martensite in the heat-affected zone (HAZ). While tempering martensite in the HAZ, the heat treatment might also lead to weakening of the base material near the weld in cases where the heat-treatment temperature is too high. In 2014 the PWHT procedure for welded pipes in the ASME Code (ASME B31.3) was changed and the maximum allowed PWHT reduced. Hence, according to the new standard, older welds have been heat treated at temperatures that are too high. The present article investigates the strength of AISI 4130 steel undergone different PWHT and uses the finite element method to model pipe failure in order to investigate the possible effects of the old heat-treatment procedure on pipe strength. The effect of the different PWHT temperatures on the base material strength is investigated through tensile tests of samples undergone different heat treatments. Then, from the extracted material data from the tensile tests the pipe is modeled and analyzed through explicit finite element method to evaluate what effect the anticipated material weakening from the old PWHT procedure has with regard to pipe strength and integrity.