Research On Weldability And Welding Materials Of/for High Performance Pipeline Steels

Systematical researches were carried out on the weldability and welding materials of/for the newly developed low sulphur, fine grain and high performance X70 steel and X80 steel by using differnent kinds of steels, developing several grades of welding wires and taking various welding procedures.For the cracking susceptibility of pipeline steels,a calculation formula was proposed for the prediction of the maximum hardness on heat affected zone for modern high strength pipeline steels. The formula was checked according to the results of the tests, showing good accordance and good adaptability. Maximum hardness HV₁₀ on heat affected zone for either X70 steel or X80 steel, from room temperature to 100℃preheat, is below 240, indicating low hardenability of the steels and good H₂S corrosion resistance of the weld joints as well.The current high performance SAW and GMAW wires were used for the welding tests of X70 steel,X80 steel and low CE high strength steel equivilent to X80. The present 10# SAW wire and GMAW wire WER60 for X70 steel and GMAW wire WER70 for X80 can meet the current technical requirements, but 10# SAW wire for X80 steel can't,so new SAW wires need developing.Based on the commonly used wall thicknesses and welding procedures of pipeline steel, the ratios of exotic filling metal to the weld metal in SAW and GMAW were analyzed respectively, and practical calculation formulas for the ratios were set up. According to the calculation results, for SAW, a curve describing the relationship between the ratio of exotic filling metal to the weld metal and the ratio of plate thickness to face root was made; for GMAW, a curve describing the relationship between the ratio of exotic filling metal to the weld metal and the plate thickness was made. A method for a quick prediction of weld strength was proposed with the aid of Excel software,which paved the way for accurate control of the goal composition and the strength of the weld metal.By using orthordonal designing,with the strength of weld metal as controlling objective, with C content, Mn content and modified combination of Ni, Cr and Mo,etc. as testing factors set at two levels, and with the addition of Ti and B at micro level, SAW wires were manufactured and studied. Standard SAW welding test was carried out to examine the properties of the weld deposits by use of CHF101 flux and CHF105 flux. The strength of deposit weld metal is from 560MPa to 690MPa and found correspondant well to the CE value of the wire.The newly developed SAW welding materials were employed for lab flat SAW and pipe SAW experiments on X70 steel and X80 with different welding procedures and the chemistry of the wire with best mechanical properties was obtained. The compositions of the SAW wires are innovative,and Cr which is abundant in the earth is a main alloying element in these wires. The transition temperatures of half energy of the upper shelf for weld metal and heat affected zone were respectively around -80℃, and the average Charpy impact energy at -20℃generally reach to 200J. The average impact energies in various zones of the joint of X80 steel at -20℃were not lower than 127J,and the strengths of weld metals were from 710 to 740MPa,and the hardnesses of the joints were within the required range. The high speed SAW welding procedure is producible to the improvement of the weld metal toughness. After the weld metal subjected to post welding heat treatment,the strength obviously increased for Cr bearing weld metal. X80 steel and its welding wire exhibit good weldability and good adaptability.These wires have better properties than the current wires,reaching to an internationally advanced level.When the procedures vary from wire depositing welding to pipe steel plate butt welding, the strength of the weld metals will also vary.The relationship between the strength variation and the CE difference between the wire and the base metal was set up. For X70 steel and X80 steel examined, if the minimum strengths of deposit metals of the welding wires are 560MPa and 620MPa respectively, they can meet the strength requirements of the butt welds too.There exist obvious influences of types of alloying elements on low temperature toughness. Multi-alloyed with Ni and/or Cr and/or Mo and micro-alloyed with Ti and B, weld metals feature dominant acicular ferrite microstructure, and have high strength and high toughness. Not only the composition but also the microstructure of base metal exerts influence on the microstructure and properties of weld metal resulting in variations in toughness of weld metals produced in different welding directions or with different base metals.The SAW joints exhibit softened zones especially for low CE high strength steel, But the softening of GMAW joint is acceptable even for low CE high strength steel.The microstructures of the welded joint of X80 steel are bainite in coarse-grained HAZ, mix of bainite and ferrite in normalized zone and incompletely normalized zone as well and mix of acicular ferrite and minor prior ferrite in weld metal. The grain size in CGHAZ is coarser in SAW than in GMAW and also coarser in higher linear energy, but significantly smaller than that of traditional steel.EBSD analysis shows that X80 weld metal has obvious finer grain with more uniformly distributed orientation and the acicular ferrite coarsen after heat treatment. In the tough dimples of high toughness fracture surface there are fine particles sized 0.5¹.0μm in diameter, mainly composed of Ti, Ca ,Si and so on. In the dimples of SAW weld fracture surface there are particles sized 2⁵μm in diameter, mainly composed of Al, Si, and the weld metal has low toughness. Reletively high Ti and low Als is the key to the formation of Ti-dominant fine particles in weld metal.