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The transformation behavior and microstructural characteristics of API X65 pipeline steel were investigated by dilatometry and microstructural observation. Microhardness measurements were used to verify the observed microstructures. The test steel is imported from abroad and is used extensively in Iran natural gas transmission projects. The continuous cooling transformation curves of the test steel were constructed. The results showed that with increasing the cooling rate from 0.5 to 40°C/s, the microstructure changes from polygonal ferrite, quasi-polygonal ferrite-pearlite to acicular ferrite. The microstructure was dominated by acicular ferrite in cooling rates higher than 5°C/s. The results can be used to design the optimum thermo-mechanical control process (through the selection of proper cooling rate) in domestic manufacturing process of the test steel.

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The API X70 steel is a high-strength low alloy steel which is used in construction of high-pressure long-distance gas and oil transportation pipelines with large-diameter. The pipe used in this study has 1422mm outside diameter and 19.8mm wall thickness, formed by spiral welding. As this steel is totally imported from abroad, the study of continues cooling transformation behavior and the optimum design of thermo-mechanical control processes are important for its domestic production. In this study, dilatometry examination was conducted on API X70 steel in a wide range of cooling rates from 0.5 to 40 °C/s. The optical microscopy observation and microhardness measurement were used to verify the observed microstructures. From the experimental results, the continuous cooling transformation curves (CCT) were constructed for API X70 pipeline steel. Different microstructures including granular bainite, pearlite, acicular ferrite and bainitic ferrite were observed depending on the cooling rate of tested samples. The observed dominant microstructure in 5 and 7.5 °C/s cooling rate was acicular ferrite which is the desired microstructure in energy transportation pipeline steels. These results can be used to design the optimum thermo-mechanical control process (through the selection of proper cooling rate) in domestic manufacturing of the API X70 steel.

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In submerged-arc welding, flux is produced through bonding so that alloying element can be added to Weld Metal. In this method, mineral ingredients and alloying elements are milled and mixed with glue in appropriate proportions. Once the drying of the pellets is complete in air, they are baked at 350 degree centigrade and broken up by using a sieve to attain the desired particle size (0.3-1 mm). The various content of Cr, Mo and Cr-Mo was added to bonded flux. Addition of alloying elements was done through flux and slag-weld metal reactions. Mechanical properties were studied by means of Longitudinal Tensile, Hardness and Charpy V-notch tests. Microstructure was studied by means of Optical and Scanning Electron Microscope. The addition of 0.4 wt. % Mo increased the volume fraction of Acicular Ferrite (AF) to 87%. The Ultimate Tensile Strength (UTS) increased by 20% and Impact Toughness (IT) decreased by 25%. Cr affected AF content less than Mo. The addition of 0.4 wt. % Cr increased the volume fraction of AF to 57%. The UTS almost did not change and IT decreased by 35%. Further increase in Cr content led to increase of Ferrite with Second Alloyed phase that strongly impaired IT (60%). The highest proportion of AF (95%) obtained in 0.28 wt. %Cr and 0.35 wt. % Mo. In this specimen UTS increased by 20% (100 Mpa) and Impact Toughness was decreased by 15% (20 j).