CORROSION-FATIGUE BEHAVIOR OF THE TO HIGH DUCTILITY AGED PRECIPITATION-HARDENED STEEL PH-13-8-MO AND THE X20 CRMOV-121 STEAM-TURBINE BLADING STEEL IN COMPARISON

The corrosion fatigue behaviour of the martensitic stainless steels PH 13-8 Mo and X20 CrMoV 121 is compared. The investigation includes bot h electrochemical and corrosion fatigue tests. Also mechanical, metall ographical and SEM investigations were per formed. The corrosion fatig ue resistance at 150 degrees C of the precipitation hardened steel PH 13-8 Mo in the modification with the higher strength level (cast I) va ries between 100 and 135 MPa. The mean stress was 250 MPa. The influen ce of the pH-value is predominant to the Cl--concentration. The corros ion fatigue resistance of the lower strength modification of PH 13-8 M o (cast II) is significant lower than that of cast I. At the conventio nal 12% Cr-steel cracks are initiated from notches caused by corrosion under salt like deposits primarily at the grain boundaries, weakened by carbide precipitations. At the PH 13-8 Mo steel the crack initiatio n is located at zones of plastic deformation between the martensitic l aths. Corrosion attack occurred preferably by orthogonal orientation o f the laths to the surface.