Study On Welding Physical Metallurgy Behavior Of High Performance Offshore Engineering Steel

With the continuous development of marine structures from shallow sea to deep sea regions,the strength and thickness of the steel used for the construction of the marine structure increase gradually.Meanwhile,the construction of steel structure has more stringent requirements for the welding performance of the base material.But in the welding process,factors affecting the microstructure transformation included chemical composition of the base metal and welding material,welding procedures(heat input,interpass temperature,cooling rate and welding thermal cycle),post-weld heat treatment and others,are variable and less controllable.Thus,the microstructure transformed either in welding heat affected zone or weld metal is very complex.Nevertheless,for any complex process of the steel formation,it has its laws and can be effectively controlled on the premise of identifying its evolution process and influencing factors.Therefore,it is very important to study the physical metallurgy principle and welding performance of the offshore engineering steel.In this paper,the submerged arc welding wire with high strength and high toughness was developed to use for low temperature environment,and the effect of Mn,Ni and Mo proportion on microstructure and properties of weld metal was studied.The results showed that the microstructure of weld metal primarily comprised of fine acicular ferrite(AF),proeutectoid grain boundary ferrite(GBF),ferrite side plates(FSP)and small martensite/austenite(M/A)constituents.The weld metal with 0.2%Mo can effectively restrain the formation of GBF and FSP.The increased Mn and Ni enhanced the low temperature toughness of weld metal by increasing the fraction of acicular ferrite.However,the concentration of Mn and Ni should be control under a critical value,as much more Mn and Ni additions would promote the formation of martensite or other low temperature microstructural features,which is detrimental to low temperature toughness.For K65 pipeline steel,the optimum combination of alloying element content was 1.5%-2.0%Mn,0.9%-1.2%Ni,0.2%-0.25%Mo.Excellent strength and toughness can be obtained through replacing Ni by Mn in the terms of the concentration of Mn and Ni being above the Ms(martensite start)line.The influence of interpass temperature on the microstructure and mechanical properties of multi-pass weld joints(up to 36-mm thickness)in a 550MPa grade offshore engineering steel was studied.Moreover,the microstructural evolution in weld metal during thermal cycle was also investigated through thermal simulation,and then the qualitative relationship between various microstructure and impact toughness was established.The results showed that increasing the interpass temperature leads to the coarsen of ferrite.Moreover,the grain size of columnar grain and the volume fraction of M/A increase significantly.In addition,the width of reheated zone in weld metal and heat affected zone increase,accompanying by a decrease in strength and toughness.The low interpass temperature promotes a larger volume fraction of martensite to form in the cap,which,in turn,scatters the hardness and enhances the tendency of cold cracking.Optimal mechanical properties were obtained at the interpass temperature of?130?,while the average impact toughness is still less than 70J.Because the machined notch of actual impact sample contained one or more brittle reheated zones.In these brittle reheated zones,a large amount of M/A constituent necklacing prior austenite grains was formed in the reheated zone of all weld metals and was mainly responsible for lower toughness of the entire weld metal.Fortunately,this deterioration in toughness could be reduced as decomposition of necklace type M/A constituent occurred due to the later welding passes.To enhance the toughness of multi-pass weld joint,conventional tempering and new intercritical heat treatment were designed.The results suggested that there was insignificant effect on toughness through conventional tempering(tempering at 500-740? and holding for 30min),but obvious improvement through combination of quenching plus intercritical annealing and tempering.The intercritical heat treatments that promotes the reversion of necklace M/A constituent and formation of?5-6vol.%retained austenite by the enrichment of Mn and Ni in reversed austenite during intercritical tempering process,significantly enhance the low temperature(-40?)impact energy from 39.2J in weld metal to 97.7J or 83.5J.Moreover,the comprehensive performances of base metal and weld metal could be improved significantly through a reasonable post-weld heat treatment designed based on alloy compositions.Crystallographic study on the microstructure of coarse grained heat affected zone of offshore engineering steel indicated that crystallography is the intrinsic property of materials,and the crystallographic structure determines its impact fracture toughness.Decreasing the cooling rate or prior austenite grain size under a specific condition,will enhance the variant selection of bainite.Meanwhile,the mode of variant selection changes from CP(close-packed plane)grouping to Bain grouping,which significantly reduces the density of block boundary formed by V1/V2 variants.In addition,the establishment of the quantitative correlation between impact toughness and crystallographic structure suggested that the variation of impact toughness was mainly correlated to the block size.Furthermore,under certain cooling conditions,the uneven size of austenite grain will also lead to the diversification of variant selection modes,which,in turn,diversifies the mechanism of crack propagation and increases the impact toughness scatter.