Austenitic Stainless Steel Diffusion Bonding Intergranular Corrosion Research

As an advanced materials joining technology, diffusion bonding has been widely used inaviation, aerospace and nuclear power, and other fields. It has also attracted extensiveattention in package in Micro-chemical Mechanical Systems. Austenitic stainless steelmaterial is preferable materials in miniature equipment manufacturing due to its goodcorrosion resistance and comprehensive mechanical properties.. The vacuum diffusionbonding process will have effect on corrosion properties for Micro-chemical mechanicalsystems of austenitic stainless steels Therefore, it is essential to investigate corrosion behaviorof the diffusion bonded joints of austenitic stainless steels for both optimization of bondingparameters and reliable package of engineering structures.In this thesis, 316LSS diffusion bonded joints were prepared by vacuum diffusion bonding.Both metallographic inspections and strength properties testing were carried out to determinethe optimum bonding parameters. In-situ tensile test was carried out to investigate theinterfacial void change and failure mechanism.Electrochemical Potentiokinetic Reactivation (EPR) is generally used for evaluation ofcorrosion austenitic stainless steel. It is non-destructive, rapid and quantitative features andcan be used for industrial on-site inspection of material sensitivity to intergranular corrosion.But single electrochemical method is still not sufficient enough to get reliable verdict. Ifcombined with the Electron Backscattered Diffraction (EBSD), they can provide enoughevidence about the intergranular corrosion of austenitic stainless steels. This paper includesthe following several aspects:(1) Sensitization treatment for both 316L stainless steel base materials and diffusionbonded joints were carried out. The results showed that: grains in diffusion bonded joints aremuch larger than those in base material. Diffusion bonding joint has a large number annealingtwins. No precipitates were found after 2 hours under sensitization temperature. As the timegoes up to 8 hours, few precipitates were observed in base materials. Surprisingly, noprecipitates were found in diffusion bonded joints. It can be explained that diffusion bondingis a thermal mechanical process in which large quantities of twin boundaries produced.Crystal boundary network increases the proportion of low-energy crystal boundaries, effectively blocking large-angle crystal boundaries depleted of chromium continuousdistribution and reducing the carbonization precipitated material possibilities.(2) Diffusion bonding of 316L stainless steel specimens in solution of H2SO4 + Na2S4O6corrosion sensitivity was investigated by using double-loop EPR. The result showed thatreactivation ratio of diffusion bonded joint is less than that of base material. That is, diffusionbonded specimen has more excellent intergranular corrosion resistance compared to the basematerial.(3) The frequency of coincident site lattice and grain boundary character distribution of316LSS diffusion bonding joints and base metal were examined by orientation imagingmicroscopy (OIM) EBSD. A large number of annealing twins were produced in thethermomechanical treatment and the material resistance to intergranular corrosion isdependent on large-angle grain level of community connectivity. The result showed that theannealing twins and the proportion of Coincidence Site Lattice(CSL) of diffusion bondingjoints are much higher than those in base material, indicating that the grain boundary energyin diffusion bonded joint is low. This is in agreement with the results in EPR test. From thegrain boundary engineering perspective, when the high angle grain boundary corrosionintersects the interface with high sensitization degree, it will cause corrosion of the ditchhampered progress, thereby improving the corrosion behavior of alloy grain boundaries andincreasing the alloy resistance to intergranular corrosion. Therefore, the diffusion bondingjoint has better resistance to intergranular corrosion than the base metal.