Formation Mechanism And Processing Of Hastelloy C-276/Stainless Steel 304 Dissimilar Metals Laser Welding With Filler Wire

As the only rotating part in CAP 1400 nuclear power plant,the safe operation of nuclear reactor coolant pump is very important to the whole nuclear power system.And the reliability of the end sealing of the stator shield can of the nuclear reactor coolant pump is directly related to the safe operation of the nuclear reactor coolant pump.At present,the argon arc welding technology is adopt to end sealing of the stator shield can of the nuclear reactor coolant pump imported from the United States,but there are some problems in argon arc welding,such as poor stability,coarse microstructure and easy to be burned through.Therefore,a new technology of laser sealing welding is proposed for stator shield can(Hastelloy C-276)and flange(stainless steel 304)of nuclear reactor coolant pump.In this paper,the laser wire filling welding of Hastelloy C-276/stainless steel 304 was analyzed to study the evolution law of weld morphology,and the dilution level model of dissimilar metal pulsed laser welding with filler wire was established to study the evolution mechanism of dilution level.The laser welding process window was obtained based on optimization of dilution level.The microstructure of the laser weld was analyzed.and the effect of weld morphology and weld microstructure on the properties was studied.A new technology of low-temperature cooling was put forward to control microstructure in laser welding with wire filling,so as to realize high quality laser wire filling welding of Hastelloy C-276/stainless steel 304.This study provides technical reserves and theoretical basis for the end sealing of the stator shield can of the CAP 1400 nuclear reactor coolant pump.Specific research contents and conclusions are as follows:(1)The single factor experiment was used to study the laser wire filling welding process,and the relationship between laser process parameters and weld morphology was analyzed.Meanwhile,the dilution level model was established to study the evolution process of dilution level in laser welding.The results show that laser process parameters have a great influence on the weld penetration,but have little influence on the weld width.Meanwhile,the weld reinforcement is mainly related to the relative wire feeding.The weld penetration increases with the increase of single pulse energy,but decreases with the increase of pulse width,wire feeding speed and welding speed.And the weld penetration first increases and then decreases with the increase of defocusing distance and laser offset.The dilution level of stainless steel 304 increases with the increase of single pulse energy,welding speed and laser offset,but decreases with the increase of pulse width and wire feeding speed.Meanwhile,the dilution level of stainless steel 304 increases first and then decreases with the increase of defocusing amount.(2)The microstructure morphology,phase composition,grain orientation and element distribution were analyzed.The results show that there are unmixed zone and precipitated phase in the weld.The main elements of weld are uniformly distributed and grain angle of weld is dominated by small angle grain boundary,and there is no obvious heat affected zone near the fusion line of weld.The precipitation of precipitated phase is mainly determined by the microsegregation and the nominal content of Mo in the weld.The precipitated phase is composed of p phase and ? phase.And the p phase precipitates during solidification of weld,and ? phase is transformed from p phase.The coherent relationship exists between p phase and? phase.(3)The mechanical properties of welded joints were analyzed and evaluated by tensiletear test and microhardness test,and the corrosion properties of welded joints were analyzed and evaluated by electrochemical testing method.The research shows that the transverse and longitudinal microhardness distribution of the weld is uniform,and there is no "softening"feature of the heat affected zone in the weld.The anti-peel property of welded joint is closely related to the penetration.With the increase of penetration,the anti-peel property of welded joint increases gradually.The corrosion resistance of welded joint is closely related to the dilution level of SS304 base metal,as the increase of dilution level of SS304 base metal,the general corrosion resistance property of welded joint decreases,while the sensitivity of intergranular corrosion property of welded joint increases first and then decreases.(4)The welding experimental platform of low-temperature cooling was built,and the experiments with different cooling temperature were carried out to study the microstructure control mechanism of low-temperature cooling in welding.The results show that as the decrease of cooling temperature,the cooling rate and solidification rate of increase,the volume fraction of precipitated phase reduces,the width of unmixed zone decreases,and the grain size of the weld decreases.At the same time the proportion of small angle grain boundaries increases and the content of anti-corrosion elements increases gradually.When the cooling temperature is10?,the average micro-hardness and anti-peel property of the welded joint are improved by 11.7%and 10.3%,respectively.Similarly,the self corrosion potential of welded joint increases by 12.7%,and the self corrosion current density decreases by 38.9%.Meanwhile,the sensitivity of intergranular corrosion of the welded joint is reduced.Therefore,the low-temperature cooling can improve the mechanical properties and corrosion resistance of the welded joints.