Study On The Effect Of Heat Input On Q690qENH Low-alloy High-strength Steel GMAW Welded Joints Corrosion Resistance

High strength weathering steel has the advantages of high strength,good toughness and corrosion resistance.It is widely used in engineering fields such as construction machinery,mining equipment,bridge construction,offshore construction and vehicle ships.In order to improve performance,Cr,Mo,Cu,Ni and many other alloying elements are added to the material,which seriously affects the weldability of the material.How to ensure the performance of welded joints of high-strength weathering steel has become a key issue in the practical application of high-strength weathering steel.This thesis utilises XY-ER80 QNH weathering wire and uses Gas Metal Arc Welding(GMAW)to weld Q690 qENH low-alloy high-strength weathering steel.Under the premise of enhancing the corrosion resistance of welded joints,three groups of welding processes with different heat inputs are developed and optimised.The influence of welding heat input on the microstructure,electrochemical corrosion performance and stress corrosion performance of welded joints is studied by means of microstructure observation,electrochemical corrosion testing and four-point bending stress corrosion testing.The welded joint consists of weld seam,heat affected zone and base material.In the three into the heat input,the welded joint weld area for the needle ferrite and carburized body interlaced formation of the lower bainite,the lower the heat input,the longer the needle ferrite,the finer,more consistent orientation.The heat affected zone can be subdivided into coarse crystalline zone,fine crystalline zone,incomplete quenching zone and tempering zone.The coarse crystalline zone and fine crystalline zone are different sizes of slate-martensite and granular bainite.The lower the heat input,the smaller the width of the zone,the smaller the slate-martensite grain size,the more obvious and dense slate-like,the grain bainite precipitation reduced,more concentrated distribution;incomplete quenching zone and tempering zone consists of different densities of ferrite,carburite and slate-martensite,the lower the heat input,the amount of carbide and carburite precipitation decreased.Electrochemical corrosion test results show that the entire welded joint and the parent material corrosion potential difference is small,are about-0.620 V,but the corrosion current density is significantly higher than the parent material.Weld corrosion potential are much higher than the heat-affected zone,corrosion current density is lower than the heat-affected zone.Three kinds of heat input weld and heat-affected zone between the percentage difference in corrosion potential were: 36.7 %,22.7 %,70.9 %.In the entire welding joint corrosion process,the weld/heat-affected zone,the base material/heat-affected zone constitutes an electric couple pair,constituting an electric couple corrosion,three kinds of heat input under the weld,heat-affected zone are presented the lower the heat input,the smaller the corrosion tendency;the slowest corrosion rate under the heat input,the fastest corrosion rate under the high heat input.3.5 % Na Cl solution at room temperature after soaking for a period of time,the surface of each specimen are formed dense rust layer.Electrochemical corrosion performance tests show that the corrosion potential gap is reduced,the welded joint and the parent material corrosion current density gap is small,the influence of galvanic corrosion is reduced.Heat-affected area with rust specimen corrosion tendency is higher than the weld and the parent material area with rust specimen,corrosion current density is not a linear relationship,the smaller the heat input corrosion tendency is smaller,the heat input Q690 qENH welded joint electrochemical corrosion resistance is the best.Four-point bend stress corrosion test results analysis shows that the three heat input four-point bend constant strain stress corrosion soaked 720 h,three heat input joints appear under the different thickness of the rust layer,the heat-affected zone of the rust layer is the thickest.The base material rust layer was next and the rust layer in the weld was the thinnest.At low heat input,small pits were produced on the compressed surface in the weld area;the size of the pits in the heat affected zone was also smaller and the number of pits increased compared to the weld area.Under medium heat input the pits in each area increase and increase in size compared to low heat input.At high heat input,cracks appear in the heat affected zone of the drawn surface,with most of the cracks starting at the edges of the pits and a small proportion of the cracks being intergranular corrosion,with the pits in the heat affected zone overlapping each other to form an uneven surface.In the unstressed specimens,the three heat inputs produced a scattered distribution of pits,the overall shape tends to be circular.Through the study of Q690 qENH low-alloy high-strength weathering steel,it was found that using XY-ER80 QNH welding wire,GMAW welding,using a welding speed of 0.4 m/min,welding current of 230 A,welding voltage of 22.6 V and welding heat input of 779.4 J/mm welding parameters for the optimum corrosion resistance of Q690 qENH welded joints.