Hydrogen And Residual Stress In Stainless Steels During Electrochemically Induced Annealing

The deformation induced martensite in metastable austenite stainless steels affects their physical and chemical properties. The martensite can be removed by electrochemically induced annealing (EIA), whereas the high hardness of cold-deformed stainless steel can be maintained. Although it is found that EIA treatment can promote the corrosion resistance of stainless steels, the mechanism of EIA has no convincing explanation. With microhardness distribution method, based on the model of unsteady diffusion in one dimension, the diffusion coefficient of hydrogen during EIA treatment is calculated out being 0.303×10^-13 m²s^-1 at 60℃. The result not only accord with the theoretical deduction, but also with the diffusion coefficient measurement results obtained with other methods. Therefore the method can be used to test the hydrogen diffusion coefficient in EIA treatment. The diffusion coefficients of nitrogen in the steel are calculated out by theoreticaldeduction respectively. They are 1.11 × 10^-28 m²s^-1 in austenite and 2.92×10^-17m²s^-1 in martensite, far below that of hydrogen during EIA treatment. It is found that the phenomenon of EIA treatment also exists in NaCO₃ solution containing no nitrogen. All these facts prove that nitrogen does not make major contribution to the EIA treatment. Residual stress in the stainless steel is tested by X-rays diffraction method. The results exhibit that residual stress after EIA treatment decreases compared with the residual stress before EIA treatment, and 78.73%⁹3.1% residual stress is maintained in the stainless steel. The pitting sensitivity of stainless steel to acid is studied and result shows that the EIA treatment promotes the corrosion resistance of the 304L stainless steel to acid.