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UBC Theses and Dissertations

Austenite decomposition in the coarse grain heat affected zone of X80 line pipe steel Roy, Sabyasachi


Pipelines are used worldwide to transport oil and natural gas from their reserves to refineries and end users. Microalloyed line pipe steels are used to build these pipelines. The construction of pipelines involves welding pipes along the girth to join them. Welding of steels results in microstructure changes in the heat affected zone (HAZ). The coarse grain heat affected zone (CGHAZ), closest to the weld pool, experiences temperatures close to the melting point and is regarded as a potential region of lower toughness. In the present work, austenite decomposition under continuous cooling conditions has been studied for simulated CGHAZ conditions in three different line pipe steels. Bulk samples were heat treated in a Gleeble 3500 thermo-mechanical simulator to replicate the austenite microstructure of the CGHAZ and subsequent cooling ranging from 3 to 100°C/s to simulate welding processes with different heat inputs. The investigated laboratory steels had systematically varied carbon and chromium contents. An increase in carbon content from 0.035 wt% to 0.061 wt% resulted in a reduction of the transformation start temperature by approximately 10°C at 3°C/s to 50°C at 100°C/s cooling rate. Further on, an increase in the chromium content from a residual amount to 0.24 wt% reduced the transformation temperature modestly by about 10°C for all cooling rates. Microstructure characterization and hardness testing confirmed that lower transformation temperatures are associated with finer bainitic microstructures and higher hardness values. Based on the experimental results a phenomenological model has been proposed to predict the transformation kinetics, microstructure and hardness as a function of cooling rate, and steel chemistry in terms of C and Cr content.

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