Abstract Scope |
Grade 91, or 9Cr-1Mo-V, is a ferritic-martensitic steel which is extensively used in petrochemical and thermal power generation industries for its high-temperature creep strength and oxidation resistance. Additionally, it has been recently considered as a candidate alloy for use at operating temperatures up to 540 °C in the cold sections (piping network connecting the cold thermal energy storage tank with the solar receiver and primary heat exchanger) for the new generation concentrated solar power systems (Gen3 CSP). During welding of Grade 91 steel, residual stress forms due to spatially non-uniform heating and cooling thermal cycles. Additionally, it has been found that the solid-state phase transformations, especially martensitic transformation, have an important impact on the residual stress. Many of the exiting models in the literature consider a two-dimensional (2D) simulation domain with either plane strain or axisymmetric assumption. In this paper a 3D finite element thermal-metallurgical-mechanical model is developed to calculate the residual stress and distortion in Grade 91 steel weldments. The solid-state phase transformations were modeled by tracking austenite, martensite and freshly-formed ferrite phase fractions over time. The effect of phase transformations on the mechanical process was represented quantitatively by the volume change and yield strength variation as a function of phase fraction. The arc heat flux was described by a well-established Goldak heat source equation whose parameters were carefully calibrated against the weld transverse cross-section. The material deposition was implemented by a novel progressive element activation method where elements belonging to weld were deactivated at the beginning and then activated gradually following the heat source motion. The model was validated by thermal history and distorted shape measurements of the welded plate. Another commonly used method for handling material deposition, where all elements belonging to one pass are activated at once, was evaluated and a 2D plane strain model was also tested. The simulation results from these two test cases were compared with those obtained by the 3D model with the progressive element activation method. Moreover, the effects of the bevel geometry, welding sequence, and processing parameters on the residual stress and distortion were studied numerically. |