Reserch On Electrochemically Indused Surface Annealing Of Semi-Austenitic Precipitation Hardening Stainless Steel

Electrochemically induced surface annealing （EISA） is a kind of phasetransformation phenomenon in stainless steels by alternative pulses treatmentwhich can improve their pitting resistance. But EISA treatment has never beenreported in any steel other than metastable-austensite stainless steel typed ofAISI304L steel.A semi-austenitic precipitation hardening stainless steel of17-7PH typewas electrochemically treated with anodic/cathodic alternative pulses in8Msodium nitrite solution at80°C, and the treatment condition of EISA of17-7PH stainless steel was also investigated. EISA phase transformation wasachieved in a kind of steel other than metastable austenite stainless steel withthe pitting resistance promoted. With treatment time increasing, the martensitedecreased more obviously. The relationship between EISA treatmenttemperature and treatment effect was parabola. The anodic and cathodicpotential also had great effect on EISA. Only when the anodic and cathodicpotential reached a certain value, the martensite decrement could achieve the maximum value. In every cycle when cathodic time was too short, thedecrease of martensite content was high, but the EISA treatment was notefficient. Whereas when the cathodic time was too long, the decrement ofmartensite content could not reach a higher level. Besides NaNO₂, the EISAphenomenon was only observed in NaHCO₃. And after the electrochemicalpulses treatment in NaHCO3, the pitting resistance of the semi-austeniticprecipitation hardening stainless steel was promoted. In other media, such asNaOH, Na₂CO₃and Na₂SiO₃, the EISA phenomenon was not observed, buthydrogen-induced martensite phenomenon occurred evidently which couldpromote the hardness of surface, and the samples could be corroded.