Welding Performance Of High-strength Steel Grade EQ70for Offshore Platform

As marine structures gradually applied into the deep ocean, Steel plates used inmarine structures are made thicker and higher strength. So, the reasonable and reliablewelding process were badly needed to keep the quality of high strength and heavythickness marine platform steel and improve the construction efficiency as well.The purpose of this paper is to study weldability of high strength offshore platformsteel and to formulate reasonable welding process parameters for it. The microstructureand mechanical properties at the welding simulation area are studied using weldingthermal simulation technique for the high strength offshore platform steel gradeEQ70(hereinafter referred to as Marine steel) produced by Xinyu Steel. The continuouscooling transformation diagram (SH-CCT diagram) is drawn for the correspondingwelding area of this steel. The upper limit and lower limit of t8/5finally obtained toensure the performance of welding joint. Welding process parameters of steel gradeEQ70are calculated through the t8/5engineering calculating method and incombination with welding experiments. Preheating temperature range are gotten forwelding methods of automatic submerged arc welding and manual arc welding bystudying crack tests of inclined Y-groove welding. Welding materials are chosen forwelding according to the principle of "low strength match" and microstructure andproperties of welded joint are analyzed as well.Research results show that, t8/5in the welded joint of heavy plate grade EQ70isgenerally less than30s when welded by common process of submerged arc weldingand manual welding, and the heat affected zone is quite narrow, so no serious affectsappear in overall joint toughness.Cold crack sensitivity of manual welding is relatively high. If the preheattemperature is below100℃,crack rate of the weld root could be up to100%. So, the steel grade EQ70should be preheated over100℃, at about150℃, and continuouswelding should be carried out at the root pass.When submerged welding, the preheating temperature lies between50~80℃,surface crack rate is0, root crack rate is relatively high,it could be up to about60%,while the average section crack rate is below10%. When the preheating temperature is150℃, all crack rate is0.The process parameters of manual electric arc welding and automatic submergedarc welding: are as follows: for manual welding, preheating temperature is150℃,welding current is150~160A, welding voltage is22~26V, welding speed is13~14.5cm/min, and welding heat input is15KJ/cm. For submerged arc welding,preheating temperature is50~150℃,welding current is500~550A, welding voltage is28~30V, welding speed is33~35cm/min, and welding heat input is25KJ/cm. Theproduction practice proved that both of the welding technologies are feasible.Microstructure and performance test results in welding joint show that bainite+acicular ferrite, lath-shaped martensite+granular bainite, tempered bainiterespectively belong to the weld seam zone of weld joint, coarse crystal zone of HAZ(welding heat affected zone) and tempering zone of HAZ. The weak parts of thewelding joint is heat affected zone in welded joint, but the tensile strength of it is greaterthan770MPa, the impact toughness is greater than50J, which comply with therequirements of the enterprise production technology.