Fatigue Properties and Fracture Surface Analysis of Plazma-nitrided Cold Work Tool Steel

Abstract

Rotating fatigue properties of plazma-nitrided cold work tool steel have been investigated using modified SKD11 (Mod.SKD11), which contains 0.8C-8Cr-2Mo-0.5V. Specimens were tempered at 470, 520 and 540°C after quenching from 1030°C. Plazma-nitiriding was conducted at 450°C for 3 h after tempering. These tempering temperatures were decided by the following reasons: the hardness is same for the materials tempered at 470 and 540°C, the material tempered at 520°C has the highest hardness, and the material tempered at 470°C contains 15 vol% retained austenite which has a possibility of toughening the nitrided hard layer. Fatigue lives of nitrided specimens was five times longer than non-nitrided specimens, especially at higher stress amplitude. It was considered that crack initiation sites for nitrided specimens were shifted to interior of specimens because of generation of hardened layer and compressive stress near specimen surface by nitriding, while fatigued cracks generated at or near surface for non-nitrided specimens. At crack initiation sites, carbides or inclusions were observed in all specimens. Fatigue strength of nitrided specimens tempered at 470°C was higher than other nitrided specimens. It was considered that stabilized γ_R was contributed to suppress cracking of the carbides, which act as fatigue crack initiation sites.