Research On The Effects Of Preheating And Heating Temperature On Weld Microstructure And Properties For Three High Frequency Resistance Welded Pipes

For petroleum and gas, pipeline is the most economic, secure and environment-friendly mode of transportation. In order to satisfy the strict requirement during the application, normalizing is applied on welding seam of high frequency resistance welding pipe to improve properties. In this study, we set three different parameters during normalizing process: preheating temperatures, peak temperatures and thermo insulation time. It will provide experimental basis for the best production process according to microstructure observation and performance analysis.The thermo-mechanical simulator, Gleeble-3500, has performed different heat treatment cycles. Results of preheating temperatures show that different preheating temperature has an extremely small effect on both of the microstructure and performance of experimental steels. In the relatively flat changing trend, experimental steels get their best comprehensive properties of strength and toughness at 730 ℃.Effect of peak temperature on the experimental steels was studied by using SEM fracture scanning, metallographic analysis, fatigue life test, Charpy impact test and so on. The results show that with the increasing of peak temperature, the trends of microstructure and properties of three experimental steels are different. X52 steel has the finest grain, the microstructure gets uniform size and dispersively distributed at 950 ℃; under the same condition, L415 steel has a larger amount of large-angle grain boundaries, ferrite grains are more isometric and uniform but less pearlite content. As the temperature increases, hardness and tensile strength of X52 and L415 steels have the same changing trends but they are opposite from the changing trends of toughness. When the peak temperature reached 950 ℃, their strength are slightly lower but toughness are better. According to the fatigue life tests, X52 steel obtained its longest fatigue life when it is normalized at 950 ℃. When the peak temperature reached 925 ℃, X65 steel obtained fine, uniform ferrite grains, high impact energy but less amount of pearlite. Its characteristic of toughness fracture is apparent, but has small changes in strength. In the practical production, setting normalizing temperature in the range of 925~950 ℃ according to the type of steel can improve the microstructure effectively and obtain the best comprehensive properties.According to the practical production, we set 11.7 s for thermal insulation at the peak temperature. From the results of thermal insulation, it found that the toughness of X52 steel decreased significantly, because of the coarse grain; microstructure and strength of X65 steel relatively have little change; the microstructure of L415 steel is refined caused by alloying elements, and then its strength increased and the toughness has been improved.