Kai Miao

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