Yoshitaka Misaka

Proposal of Electromagnetic Inspection Method of Hardened Depth of Steel Using 3-D Nonlinear FEM Taking Account of Hysteresis

The inspection of the hardened depth of surface-hardening steel is important in the maintenance of various mechanical parts, and for other purposes as well. There is a difference in the electromagnetic properties in the layers with and without hardening. Therefore, the evaluation of the hardened depth is possible by detecting the difference in the electromagnetic characteristics. In this paper, the electromagnetic method for measuring the depth of a hardened layer using an alternating magnetic field of low frequency is proposed. The flux density in the surface-hardening steel is estimated by a 3-D nonlinear finite-element method taking account of the hysteresis characteristics of the layers with and without hardening. In addition, the experimental verification is also carried out.

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.

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. Fatigue Crack Propagation Paths Across the Interface of Macroscopic Hardness Discontinuity in Material.

The criterion determining the fatigue crack propagation path near or across the interface of hardness discontinuity in material inhomogeneous with respect to hardness was experimentally investigated. The Δδθ maximum criterion was used as a reference. Single-edge-cracked panels of S45C hard steel subjected to uniform pulsating axial stresses of R=0.1 were used as test specimens, which had in each a partial slant band-like or circular area hardened by induction-heat-treatment; the fatigue cracks were intended to propagate perpendicular to the specimen axis, if the Δδθ maximum criterion was valid. It was observed that the fatigue cracks propagated in all cases straight and perpendicularly to the specimen axis, crossing the interface of the hardness discontinuity, indicating that the Δδθ maximum criterion was valid in this case.