Zhijun Huang

Adaptability of X80 Steel and its SAW Welding Wire to the Welding Procedure

Taking into account cost and safety for pipelines transporting oil and natural gas, the strength of the steel used is getting higher and the plate wall thicker. After a good use of X70 steel in China’s East-to-West Gas Transmission Project, X80 steel is drawing more and more attention in the future pipelines, resulting in its successful production in Wuhan Iron and Steel (Group) Company, and also bringing some concerns about welding. With increasing the wall thickness of the pipeline, the welding heat input is expected to increase for an increased deposited rate and a quality weld configuration. However, it is likely for the thermo-mechanically controlled rolled fine-grained steel that the properties of the heat affected zone will get deteriorated after it is subjected to large weld heat inputs, so the influence of the weld linear energy on the weld configuration and the properties of the welded joint have been studied. It is evident that an increased weld heat input can significantly reduce the number of the welding runs, thereby increasing the production proficiency and favoring the welding quality control as well. Furthermore, for Ti, Nb micro alloyed X80 steel, almost no property deterioration was found in this experiment, indicating that X80 steel is adaptable to various welding heat inputs, which gives much possibility of selecting a desirable large heat input. X80 steel is characteristic of low carbon and low sulphur contents with the addition of Mn, Mo alloying elements, and correspondingly, its weld metal should have the similar features that need to be obtained with the welding materials. However, the welding wires for X80 steel aren’t currently and commercially available in domestic market, so developing new welding wires is an urgent task for the mass application of X80 steel. Two submerged arc welding wires, one used for X70 steel and the other one branded as WGX2 with a higher strength were used in this welding experiment on X80 steel. The former produced excellent average impact energy up to 172J at −20°C close to that of the base metal but practically under-matched strengths and unqualified bend properties. The welded joint produced with WGX2 wire practically has a little over-matched strengths, qualified bend properties and satisfactory impact energy as well.Copyright

Experimental Study on High Performance Welding Materials for Pipeline Steels

West-to-East Natural Gas Transmission Project is in great need of high quality steels of API grade X70 and suitable high quality welding materials. The weld of pipe should have high strength, high toughness and low sulphur and arsenic contents (less than or equal to 0.005 wt % respectively). The work performed in WISCO concerning welding wires of SAW, GMAW and SMAW and weldability of pipe steel is reported in this article. Steel grade X70 is a TMCP high strength and high toughness steel with low carbon equivalent. It has good weldability and is less susceptible to cold cracking in the heat-affected zone. Generally in order for weld to gain the same strength as the base metal, more alloying elements should be added into the weld. Therefore, it is likely that the weld is expected to be more susceptible to cold cracking. The mechanism of strengthening and toughening of the weld should be carefully investigated. WISCO has made great progress in both property improvement and manufacturing of welding materials. Through the addition of alloying elements, the influences of some alloying elements on the strength and toughness of welds, especially on the low temperature toughness were carried out. The results show that the upper shelves of the Charpy V transition curves for the weld metals remain high despite the different alloying elements. However, the influence of alloying degree on the low-temperature toughness is significant. The weld metals micro-alloyed with Ti,B and Ni, etc. have high acicular ferrite volume fractions in the metallographs, thereby possessing high strengths and high toughness. For SAW weld, the strength is more than 590MPa, the Charpy impact energy at −40°C above 180J; for the weld of gas metal arc welding, the Charpy impact energy at −30°C reaches 190J which are far better than some current specifications. If the welding is performed with caution, no cracks were found. HIC and SSCC test for corrosion resistance of the welds were also performed and the results fully met the requirements concerned. With respect to strength and toughness, chemistry and metallography, the HIC performance of welds was analysed.Copyright