Effect of Microstructure on Pitting and Corrosion Fatigue of 17-4 PH Turbine Blade Steel in Chloride Environments

Depending on its heat treatment, 17-4 PH stainless steel may contain significant levels of reformed austenite and untempered martensite in a matrix of tempered martensite. Shot peening can cause changes in the microstructure of the surface layers by transforming the austenite to untempered martensite. The effect of these microstructural variations on the resistance of 17-4 PH stainless steel to pitting and corrosion fatigue has been determined in chloride-containing environments. Several of these environments simulate conditions found in low pressure steam turbines. The results of two electrochemical tests [large amplitude cyclic voltammetry and the pit propagation rate (PPR) test] indicate that tempering temperature and shot peening have only minor effects on resistance to pit initiation and propagation in any one of three aqueous chloride environments. However, the susceptibility of this stainless steel to corrosion fatigue in one of these environments (6 Wt% FeCl3) was reduced by increasing the tempering temperature from 538 C (1000 F) to 649 C (1200 F). These results were interpreted in terms of reduced rates of crack growth in the steel with the lower strength level (tempered at 649 C). Shot peening had little effect on corrosion fatigue susceptibility; only in specimens tempered at 649 C did shot peening cause a small, but measurable, decrease in susceptibility to corrosion fatigue.