The effects of the nitrided case produced by gas nitriding processes on the mechanical and corrosion resistance properties of the JIS SACM 645 steel were studied in this paper. JIS SACM 645 steel specimens with different substrate hardness were gas nitrided at 530?C for various nitriding durations. Nitrided specimens were characterized by means of optical and scanning electron microscopy, X-ray diffraction, glow discharge optical spectrometry, microhardness profiling, wear test, torsion mode fatigue test as well as electrochemical corrosion test in an aerated 3.5% NaCl solution. The surface hardness values of the nitrided specimens with Fe3N and Fe4N phases precipitated in the case layer were observed higher than 1000 HV0.1. Mass loss measurement of the wear test showed increases of wear resistance of the nitrided specimens, and the mass losses of the specimens were strongly influenced by nitriding durations. Electrochemical measurements showed that corrosion current density of the specimens was significantly decreased after nitriding and the corrosion potential was shifted to the noble direction as the increase of the nitriding durations. The fatigue limit of the specimen nitrided for 96 h rose 44% to 600 MPa in exceeding the untreated specimen in this study
A novel method using induction heating equipment was employed to investigate the thermal fatigue behaviors of two hot-work steels, JIS SKD61 and modified JIS SKD61, vacuum hardened treated to 45 HRC. Selected specimens, austenitized at 1298 K for 25 min, gas quenched to room temperature and tempered at 873 K, were salt bath nitrocarburized at 843 K for 80 min and low pressure nitrided at 813 K for 6 h, respectively. Microstructure, microhardness, X-ray diffraction and thermal fatigue tests were conducted. The results show that the thermal fatigue properties of the 898 K tempered specimen were better than those of other treated specimens. The reason is that the hardened processes would give high tensile strength, which improved the tool material thermal fatigue resistance. The thermal fatigue properties of modified JIS SKD61 specimen, including mean crack length and crack distribution density, were better than those of JIS SKD61 specimen. Low pressure nitriding treatment with a homogeneous nitrided layer could better maintain thermal fatigue resistance than the nitrocarburized steel.
Thermal fatigue cracking is one of the most important failure mechanisms in hot work die steels. Shot-peening can be used in a much wider field to obtain higher static strength, as well as better fatigue resistance. This study investigates the shot-peening effect on the microstructure and mechanical properties of hardened JIS SKD61 hot-work steel. The thermal fatigue test is based on cyclic induction heating and water cooling. A non-peened specimen with a hardness of 510 HV 0.05 was used as the reference material. The scanning electron microscopic observations showed craters on the surface and a severely worked hardened area in the shot-peened specimen subsurface. The microhardness values of the shotpeened surface are about 720 and 750 HV 0.05 for the Almen intensity of 15 A (typical shot-peened specimen) and 18 A (severe shot-peened specimen), respectively. The work hardening depths of typical shot-peened and severe shot-peened specimens are about 20 and 30 µm, respectively. The thermal fatigue properties of shot-peened specimens, including the mean crack length and crack distribution density, are better than those of non-peened specimens. Only a slight improvement in thermal fatigue properties occurred for the severe shot-peened specimen compared with typical shot-peened specimen.
The wear behavior of carburized SNCM 220 steel subjected to cold treatment and carbide spray coating is investigated. WC/Co coatings in 15 « 5 µm thickness were deposited on SNCM 220 specimens in a high velocity oxy-fuel (HVOF) process. Specimens were subjected to wear test under normal loads of 95.9 N at 180 rpm on a dry block-on-roller tester and to two different counter-roller materials with hardness values of 42 and 60 HRC. Wear test result after 12 h showed that the cumulative mass loss measurement of carburized SNCM 220 under 188 K subzero treatment was 15 mg, improved from 37 mg mass loss of SNCM 220 steel without cold treatment. In addition, the cumulative mass loss of carburized SNCM 220 specimens with sprayed carbide coating decreased from 37 mg to 4 mg. Moreover, wear tested specimens with HVOF coatings countered with 42 HRC rollers have resulted higher mass losses than those countered with 60 HRC rollers. Analysis of the wear microstructures has revealed the effects of the cold treatment and HVOF coatings. It has shown that 42 HRC counter roller induced plastic deformation on the specimen surface, which is detrimental to the specimen wear resistance. The compatibility of counter-roller and specimen hardness becomes the major factor to improve the wear of SNCM 220 steel.
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