Thermal Forge Welding Simulations: A Weldability Study of X65 Pipeline Quality
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Shield active gas forge welding is a time-efficient welding technique for solid statewelding of pipelines. Pipes are first heated by induction or ohmic resistance, and areforged together using an active shielding gas to reduce surface oxides. The solid statewelding requires lower peak temperatures than fusion welding, reducing the graingrowth. The low peak temperature also makes this technique possible on oil rigs andvessels where the explosion risk is high. The cooling of the pipes after welding can beset to a specific rate, providing control over the microstructures obtained. It is alsopossible to add post welding heat treatments to the welding cycle to further controlthe microstructures and properties of the weld.Forge welding has two main components; a thermal aspect and a deformation component,obtained during forging. The effect of the deformation is not considered in thisthesis. The thermal aspect can be simulated using a weld simulator, and gives valuableinformation on the effect of thermal cycles on the material to be welded. Thisthesis considers an X65 steel, common in the oil and gas industry for pipelines.The goal of this thesis is to obtain information on the effect of different peak temperatures and cooling rates on the mechanical properties and the microstructure ofX65 steel, and to recommend a welding procedure where unwanted properties andmicrostructures can be avoided.First, three different heating times were investigated to evaluate the effect on ferrite grain size. Combinations of two different peak temperatures, 1150 °C and 1300 °C, and cooling rates in the interval 1 °C/sec - 60 °C/sec were then investigated to study the effect on the microstructure and mechanical properties of weld simulated specimens. Two continuous cooling transformation (CCT) diagrams, one for each peaktemperature, were sketched based on the dilatometer curves and microscopy imagesobtained. Based on the results from the mechanical testing, a study of the intercritical zone of the heat affected zone (HAZ) was carried out for two of the mostpromising cooling rates, 10 °C/sec and 60 °C/sec. This intercritical study gave detailed information on the mechanical properties and microstructures through theHAZ. Scanning electron microscope (SEM) analysis was done to analyze possiblemartensite/austenite (MA) phases, and to check the fracture surfaces of selected impacttoughness specimens.The results showed no appreciable effect of heating time on ferrite grain size. Resultsfrom all hardness, impact toughness and tensile testing for the intercritical studywere within the DNV-requirement . SEM-analysis revealed possible MA-phasesin the intercritical zone when using cooling rate 60 °C/sec. This might be the reasonfor the reduced impact toughness measured here. Microstructures obtained for peaktemperatures 1150 °C and 1300 °C were ferrite and pearlite at low cooling rates, andupper bainite at higher cooling rates. A peak temperature of 1300 °C followed by lowcooling rates 1 °C/sec - 5 °C/sec gave very brittle results and should be avoided. Using peak temperature 1300 °C with cooling rates 10 °C/sec and 60 °C/sec gave good hardness results, but with a certain scattering in the impact toughness measurements,which must be considered. Good results were obtained for cooling rates 3 °C/sec - 60°C/sec for peak temperature 1150 °C, and can be recommended.