EFFECTS OF SURFACE FINISHING AND TEMPERING TEMPERATURE ON VERY HIGH CYCLE FATIGUE PROPERTY IN HIGH STRENGTH STEELS

摘要

In order to investigate effects of surface finishing and tempering temperature on the fatigue properties of high strength steels in very high cycle regime to over 10{sup}7 cycles, cantilever type rotating bending fatigue tests were carried out for nickel-chrome-molybdenum (JIS SNCM 439) high strength steels which were finished by the electro-polishing on the steel surface and/or tempered at four kinds of temperature of 160°C, 300°C, 500°C and 620°C. The fracture surface observed with a field emission type scanning electron microscope (FE-SEM) after the fatigue tests in order to specify the fracture origin sites. As results of fatigue tests, the electro-polished steels showed the same typical duplex S-N curve characteristics as surface ground steels. The steels tempered at 160°C showed the typical duplex S-N curve characteristics. The steel tempered at 300°C slightly showed the duplex S-N curve characteristics and the fatigue crack origin of this was surface crack origin type and internal crack origin type. But the all steels tempered at 500°C and 620°C do not show the duplex S-N curve characteristics and the fatigue crack origin was the surface crack origin type.