Kazuhiro Kawasaki

The Tribological and Fatigue Properties of Steel modified by Hybrid Surface Modification Combining Super Rapid Induction Heating and Quenching with DLC Coating

In recent years, with the background of weight saving and downsizing in machines and devices, it is important subject to downscale mechanical parts which are contained in them. So in this study, in order to achieve power transmission parts like a compact gearwheel which indicates high performance properties, hybrid surface modification was performed by combining Super Rapid Induction Heating and Quenching (SRIQ) which creates high fatigue strength and Diamond Like Carbon (DLC) coating which are well known for their high hardness, low friction and excellent wear resistance. And, in order to prevent the base material from decreasing its fatigue strength, DLC was coated by using Unbalanced Magnetron Sputtering (UBMS) method which can coat at low temperature. Rotational bending fatigue tests and friction-wear tests were carried out. It was clear that it is possible to keep high fatigue strength and to create excellent tribological properties at specimen surface by performed hybrid surface modification. Furthermore, it is possible to prevent increase in handling process because tempering process after quenching can be performed by heat process during DLC coating process.

Numerical Simulation of Inclusion Size Distribution Responsible for Fatigue Crack Initiation.

Numerlcal simulations considering spatial and size distributions of nonmetallic inclusions in the material have been carried out for the purpose of initiating a discussion concerning the critical size of the inclusion responsible for the fatigue failure of high-strength steel. Results show that the geometrically largest inclusion within a critical volume of the specimen is not necessarily responsible for the fatigue failure of such material, and that the most dangerous inclusion for fatigue failure is that of the near surface having the largest relative inclusion size calculated from the spatial distribution of inclusions within the critical volume of the specimen.

Fatigue strength of Ferritic Ductile Cast Iron Hardened by Super Rapid Induction Heating and Quenching

To clarify the effects of Super Rapid Induction Heating and Quenching (SRIHQ) on fatigue properties of Ferritic Ductile Cast Iron (FDI), rotational bending fatigue tests were carried out on specimens treated with four types of heating cycle. Results showed that; (i) the SRIHQ process generated a thin dark gray area around the graphite. This dark area was composed of a martensite structure (ringed martensite). (ii) The ringed martensite generated a compressive residual stress field at the surface hardened layer. Two types of compressive residual stress generative mechanisms were observed. One was a microscopic residual stress generative process due to the formation of ringed martensite and the other was a macroscopic residual stress generative process due to the expansion of the surface hardened layer. (iii) The fatigue strength of SRIHQ treated FDI specimen was higher than that of the untreated one. This was because the compressive residual stress field generated by the ringed martensite suppressed initiation and propagation of fatigue cracks.

Effect of fine-grained microstructure induced by induction-heating fine particle peening treatment on fatigue properties of structural steel (0.45%C)

To improve the fatigue properties of structural steel, a novel surface modification process which combines high-frequency induction heating (IH) with fine particle peening (FPP) was developed. IH-FPP treatment was performed on the surface of structural steel specimens (0.45%C) at temperatures from 600 to 750 °C, with peening times of 60 and 120 s. To determine the characteristics of the treated surfaces, the microstructure was observed using an optical microscope and a scanning electron microscope. Vickers hardness and residual stress distributions were also measured. The characteristics of fine-grained microstructures were examined by electron backscatter diffraction. Furthermore, in order to investigate the effect of the grain refinement achieved by IH-FPP treatment, rotational bending fatigue tests were performed on treated specimens. Results showed that IH-FPP treatment created fine-grained microstructures beneath the surfaces of steel samples. The average ferrite grain size was 4.06 μm for a treatment temperature of 700 °C, and finally 0.76 μm for 600 °C . This was due to dynamic recrystallization in the processed region. IH-FPP treated specimens exhibited a higher fatigue strength than untreated specimens. As almost no compressive residual stress was measured in the treated or untreated specimens, the increase in fatigue strength resulting from IH-FPP treatment was due solely to grain refinement.

A Study of Fatigue Fracture Properties of High Strength Notched Steel in High Cycle Fatigue Region with Special Focus on the Statistical Variation of the Local Fatigue Strength at the Notch Bottom

This study was conducted to examine how and why a difference of fatigue strengths between unnotched and notched specimens, in terms of maximum stress amplitude at the notch bottom, depends on the change of hardness level in steel. As a result, the local fatigue strength of the notch bottom in the notched specimen became higher than that of the un-notched one with increase of notch sharpness and this was remarkable in high strength steel. This is because the transition of the fatigue fracture mode occurs from a slip induced crack initiation mode to a defect controled one with increasing hardness in steel. In addition, using a new experimental approach combined with a simulation technique based on the concept of risk competition of defects, a computer simulation was conducted on an effect of inclusion on the local fatigue strength at the notch bottom of high strength steel. It was clarified that the difference of fatigue strength between notched and un-notched specimens in high strength steel was caused by the statistical variation of the largest inclusion size contained in the critical volume at the notch bottom.

Fatigue properties of steel with super rapid induction localized quenching

The aim of the present study is to make clear the factors controlling the fatigue strength of the locally hardened machine parts. Rotational bending fatigue tests were performed with special focus on the effect of a peak tensile residual stress generated at the quenching boundary on the fatigue properties of the super rapid induction hardened steel. Results are summarized as follows. (1) The final failure starts at the position of the specimen surface where the peak tensile residual stress has been generated near the quenching boundary. (2) The fatigue strength at 107 cycles of the locally hardened specimen is inferior to that of the non-hardened one because of such a peak tensile residual stress. (3) The effect of the peak tensile residual stress on the fatigue strength becomes smaller at higher stress amplitude because the relaxation of the residual stress occurs under stress cycling at such high stress levels. (4) Reduction of the fatigue strength due to localized hardening can be removed by introducing the cold working of the specimen surface after quenching.

Fatigue Properties of Nitrided Pure Titanium in a Hydrochloric Environment.

A Study has been made to clarify the effect of gas-nitriding on the corrosion resistance and the fatigue properties of pure titanium in a hydrochloric environment. The results are summarized as follows : ( 1 ) most of the fatigue life of nitrided pure titanium is spent in the period of the destruction of the TiN layer associated with pit formation due to hydrochloric acid attack ; ( 2 ) the fatigue strength of annealed pure titanium is greatly reduced in a hydrochloric environment because a fatigue crack initiation along the grain boundary is promoted by hydrochloric acid attack.