Thermodynamics And Kinetics Analysis Of Vacuum Brazing For Pure Fe, Ni, Mo, Cr

Stainless steel materials are widely used in petrochemical field for their goodcorrosion resistance. In recent years, research related with stainless steel vacuum brazingtechnology made certain progress. At present, it has been widely used in industrialmanufacturing that vacuum brazing of stainless steel was performed using nickel-basedfiller metal or composite nickel-based filler metal. However, there often exists defectsduring brazing such as many interfacial compounds, intercrystalline corrosion andinterfacial voids.In order to better understand the cause of the above defects and design morereasonable formula for brazing filler metal, it is essential to study the influence of elementsin stainless steel such as Fe, Ni, Mo, Cr on the microstructure and performance of brazingjoints. Therefore, vacuum brazing of pure Fe, Ni, Mo, Cr metals was performed usingBNi-2+40%BNi-5composite filler with brazing temperature as1100℃, brazing clearanceas30m and100m respectively, holding time as5min,30min and60min respectively. Byadjusting the brazing technological parameters, the microstructure of the brazed joints, theelement distribution character, phase composition and microhardness of the joints indifferent conditions were investigated using metallographic analyzer, scanning electronmicroscopy(SEM), energy dispersive spectrometry(EDS), electron probe, X-ray diffractionanalysis(XRD) and microhardness testing. Some intermetallics, calculated by phasediagram calculation software called Thermo-Calc, may form in the brazing joints todetermine phase composition. It is to explain the experimental results that using DICTRAsoftware to simulate the diffusion behaviors of elements in brazing filler metal. Toinvestigate the infiuence of technological parameters such as holding time and brazingclearance etc. on the brazing joints, organization and phase composition of the brazingjoints were compared and analyzed. The results indicated that, in the same brazing conditions, melting pointdepressants in the brazing joints of pure Fe and Cr had not easily spreaded, so it was veryeasy to generate brittle compounds in the joints. Compounds were relatively few in thebrazing joints of pure Ni and Mo, but there were voids generating in the interface ofbrazing joints for pure Ni. What’s more, diffusion layer was formed only in the brazingprocesses for pure Mo and Cr. When brazing clearance was small, it can effectively reducecompounds in brazing joints of pure Fe by increasing holding time. But Fe2B educt withlow hardness appeared in the grain boundary as holding time increased. Larger the brazingclearance was, more the compounds were produced. In this condtion, increasing holdingtime still cannot make B element fully spread, illustrating that Fe element may suppressdiffusion of B element. Therefore, in order to eliminate brittle compounds in the brazingjoints, gap control was more effective than holding time prolonging. When brazingclearance was small, microstructure of joints for pure Ni entirely consists of solid solution.However, voids appeared in the grain boundary of joints and porosity grew higher as theholding time was longer. When the brazing clearance was larger, more compounds wereproduced in the joints. Compounds were reduced effectively as holding time increased, butinterfacial cavity grew bigger and more. So brittle compounds should be reduced in thejoints by controlling brazing clearance. When brazing clearance was small, compounds inthe joints of pure Mo disappeared gradually by increasing holding time. However, too longholding time can lead to formation of precipitates which were likely to damage mechanicalperformance of the joints. When the brazing clearance was large, joints filling of thebrazing filler metal became unsatisfactory, declaring Mo element did harm to fluidity ofthe brazing filler metal. The larger the brazing clearance was, the more the compoundswere in the joints of pure Cr. And increasing holding time can reduce compoundsformation. Consequently, it can effectively eliminate compounds in the brazing joints byboth controlling clearance and prolonging holding time. The results, concerning the phasecomposition and the diffusion behaviors of elements during the solidification processes,calculated by Thermo-Calc and DICTRA software, coincide with the experimental resultsbasically.