Defects Behavior And Microstructures And Properties Of Nuclear Power Steel Welding Joints By Ultra-narrow-gap Laser Welding With Filler Wire

The big thickness SA508 Gr.3Cl.2 forgings are used in steam generator of nuclear power as the shell materials.Development of high efficiency and high quality of thick plate welding technology is one of the key factors to ensure the security of steam generator operation.The experiment research of SA508 Gr.3Cl.2 nuclear power steel by ultra-narrow-gap laser welding with filler wire is mainly discussed in this paper.Firstly,the effects of welding process measures on porosity and welds formation were studied.Then,the welding crack tendency was studied,and the types and formation mechanism of the crack was analyzed.Finally,the microstructures and mechanical properties characteristics of 50 mm thick welding joint of post welding heat treatment state were studied.The influence law of welding process measures on porosity rate was mainly studied.Results show that the influence of weld bead design on porosity rate is the most significant.When the number of weld passes decreases from 6.72 to 5.07,the porosity rate increases by about 8 times.In this experiment,choosing the wire feeding speed of 2.0 m/min and the laser power of 4.3 kW can effectively inhibit porosity defect.Porosity rate decreases with the increase of the preheating temperature and groove angle.Choosing the preheating temperature of 200 ? and groove angle of 8° is relatively appropriate.It is process porosity formed by laser keyhole instability in the experiment.Then,the effect of welding process measures on welds formation was studied.When the number of weld passes decreases or groove angle decreases,the weld tallness increases,the weld penetration decreases,and the side wall penetration remain unchanged.When the preheating temperature increases,the weld penetration and the side wall penetration increase,which can effectively inhibit the formation of incomplete fusion.Then the influence law of welding process parameters on crack rate was studied.Crack rate increases with increase of laser power and decrease of welding speed,and decreases with decrease of wire feeding speed.Cracks are in the weld surface and it was cracking along the center line of the weld.In the crack fracture source area,thermal crack characteristics are appeared such as dentrite morphology.In the end of the crack fracture,a small part of the cold crack induced by thermal cracking is appeared.In order to effectively prevent crystallization crack,laser power of 3.9~4.2 kW,welding speed of 0.42~0.48 m/min,and wire feeding speed of 3.0~3.5 m/min are chosen.Finally,the ultra-narrow-gap multi-layer laser welding with filler wire of 50 mm thick SA508 Gr.3Cl.2 nuclear power steel was carried on.The central organization of the welds in the middle of joints mainly consists of upper bainite and lower bainite.The coarse grain area in the heat affected zone near the base material consists of high-temperature tempered martensite,and the microhardness of this area is the highest,about 280 HV.In the tensile test at room temperature and high temperature of 360 ?,the tensile fracture position of welding joints is located beyond the welds,which indicates that the tensile strength of joints is good.The impact toughness of the welds in the top,middle and bottom area of joints are smaller than that of the base material,but the fracture morphology of the impact specimen extracted from the welds has certain characteristics of toughness fracture.