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This research was performed to study the effect of surface roughness on the fatigue limit in martensitic stainless steel (STS 410). After heat treatment of STS 410 steel, mechanical property tests and rotary bending fatigue tests were performed by varying the surface roughness conditions of A-polishing, A-grinding, and A-#60 of STS 410 steel, respectively. The fatigue fracture surface was observed and analyzed using SEM and EDS. The fatigue limit of STS 410 steel decreased by 0.98% from 509 to 504 MPa when the surface roughness increased about 3 times from 0.226 to 0.664 μm. However, when it increased about 9 times from 0.226 to 2.053 μm, it showed a significant decrease of 7.66% from 509 to 470 MPa. The ratio of fatigue limit to tensile strength (fatigue ratio) of STS 410 steel decreased from 54.9 to 50.5% as the surface roughness increased from 0.226 to 2.053 μm. Beach marks, a typical shape of a fatigue fracture surface, were observed on the fracture surface near the start point of fatigue failure, and brittle (Fe, Cr, Mn, Si) based non-metallic inclusions that promote crack propagation and reduce fatigue limit existed. [ABSTRACT FROM AUTHOR]

Threshold stress intensity factor ( ΔK), one of the important parameters in plant design, is the value below which fatigue crack growth becomes unrecognizably slow. Although studies on the each effect of various factors such as temperature, frequency, stress ratio, oxidation and grain size on the ΔK have been reported widely, only few research works were reported on the complex effect of the plural factors acting simultaneously. In this study, the analysis of the complex effect of temperature and oxidation on ΔK of 12Cr steel, the turbine rotor material of the USC (ultra super critical) power plant, has been carried out. Two types of fatigue tests were performed in the air and in the nitrogen gas conditions. Experimental results showed that ΔK value increases with temperature increase and formation of oxidation scale. However it was observed that the magnitude of each effect is reduced when two factors are introduced simultaneously. [ABSTRACT FROM AUTHOR]