The effect of rapid cooling of steels after ion-nitriding on hardness, tensile strength, fatigue life, fatigue limit under tension-tension, & respectively was investigated. Specimens of steels SS41 (KS, low carbon steel) & SCM4 (KS, alloy steel) were quenched after ion-nitriding by a chamber, which can perform both process.
The low carbon steel (SS41) showes the better hardenability than the alloy steel (SCM4) when the cooling rate is sufficiently high (rapid quenching). The fatigue limit in rotary bending test of SS41 is found to increase substantially if the steel is rapidly quenched in water after ion-nitriding, however, it increases a little if it is slowly cooled in furnace after ion-nitriding. In the same experiment of SCM4, the fatigue limit appeared the same regardless of the cooling rate after ion-nitriding. The fatigue life of both steels under tension-tension was improved by slow cooling after ion-nitriding, and the improvement was more significant in both steels when the specimens were rapidly quenched after ion-nitriding.
