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In order to develop a rapid nitriding process for titanium alloy at a low temperature, fine particle peening (FPP) was performed prior to gas blow induction heating (GBIH) nitriding. The surface microstructures of a Ti-6Al-4V alloy treated by FPP and GBIH nitriding were characterized using X-ray diffraction, optical microscopy, scanning electron microscopy, laser microscopy and micro-Vickers hardness testing. The results indicate that nitrogen diffusion into titanium during GBIH nitriding was accelerated by the pre-treatment with FPP, and a nitrided layer was formed on the surface of the Ti-6Al-4V alloy at a relatively low temperature (923 K), which grain coarsening and phase transformation did not occur and a nitrided layer was not formed only by GBIH nitriding. This is because a fine-grained surface layer was generated by the pre-treatment with FPP. Reciprocating ball-on-disk wear tests were carried out to examine the wear resistance of the prepared specimens. GBIH nitriding at a low temperature improved the wear resistance of the Ti-6Al-4V alloy following the pre-treatment with FPP, due to the formation of a hard nitrogen compound layer at the surface. The results show that the proposed surface treatment process was effective at modifying the surface properties of a titanium alloy at a low temperature and within a short span of time while avoiding grain coarsening and phase transformation.

Atmospheric-controlled induction-heating fine particle peening (AIH-FPP) using Cr and Mo shot particles was introduced to improve the electrochemical characteristics of AISI 1045 medium carbon steel in NaCl solution. AIH-FPP can form transferred and diffused layers of shot particles on the surface of carbon steel. The corrosion potential of the AIH-FPP-treated steel was higher than that of the untreated steel in a NaCl environment. Furthermore, a passivation region was observed in the carbon steel treated with AIH-FPP using Cr shot particles after peening with Mo shot particles through the formation of a surface-modified layer with high Cr and Mo concentrations in 20% NaCl solution.

The surface hardened steel is used in a large-sized ball bearing part or the gear part in a large-sized heavy industrial machine, etc. The inspection of the surface hardened depth is important in the intensity or the guarantee of quality of these parts. Especially, since the size of the hardened steel parts in a large-sized heavy industrial machine is large, the destructive inspection using the Vickers hardness tester, etc. is made difficult. Therefore, the non-destructive inspection method is needed for the evaluation of its hardened depth. The maximum permeability and the conductivity of the hardened domain are smaller than the non-hardening domain inside the steel [1]. Therefore, the electromagnetic non-destructive methods for detecting the surface hardened depth by the difference of flux density inside the surface hardened steel are proposed [2], [3]. In this paper, the high sensitivity inspection method using the detecting of the magnetic field on the surface of the hardened steel is investigated. The detection sensitivity of this proposal technique is higher than the method [3]of detecting the flux density inside the hardened steel. The magnetic field is estimated by 3-D nonlinear finite element method (FEM) taking account of the electromagnetic characteristics of the layers with and without hardening. Moreover, the optimal design of the search coil in the proposal sensor for raising detection sensitivity is carried out using the evolution strategy method [4]. The usefulness of this proposal inspection method is shown also from comparison with an experimental verification. Inspection method and modeling of magnetic properties Fig.1shows the 1/2 domain of the proposed the inspection model for detecting the surface hardened depth inside the hardened SCM440 steel plate. The proposal electromagnetic sensor is composed of a magnetic yoke of lamination of silicon steel plates with an alternating exciting coil, and a search coil for the detecting of the magnetic field on the surface of the hardened steel. As for a search coil in this sensor, the $x -$direction of the magnetic field $( B_{x})$on the surface of the steel plate is detected. The width $( Sw)$, length $( Sl)$and height $( Sh)$of the search coil are 9.4mm, 6mm and 1.2mm, respectively. The distance (lift-off: $L_{o})$between the yoke of the sensor and the surface of the hardened steel is equal to 0.5mm. The exciting frequency and ampere-turns are 15Hz and 126AT, respectively. Fig.2shows the initial magnetization curves of the layer with and without hardening inside the SCM440 steel. And, since the conductivities of the steel plate with and without hardening are $3.61\mathrm {x} 10 ^{6}\mathrm {S} /\mathrm {m}$and $3.98\mathrm {x} 10 ^{6}\mathrm {S} /\mathrm {m}$, the permeability and conductivity are decreased with the increase of the hardness. The flux density and eddy current density are analyzed by 3-D electromagnetic FEM taking account of the initial magnetic curves and conductivities of the layers with and without hardening inside the steel. Moreover, the initial magnetic curve and conductivity in the layer between the hardened layer and non-hardened layer in the steel are calculated by interpolation using the electromagnetic property of each layer. Inspection of surface hardened depth in steel plate Fig.3shows the distribution of flux density inside surface hardened steel plate when the hardened depth is 0mm and 3mm, respectively. This figure denotes that since the permeability and conductivity in surface hardened domain is decreased the flux density in the surface domain in the steel plate is decreased when the hardened depth is increased. Fig.4shows the effect of the surface hardened depth on the value of the change rate of $B_{x}$in the search coil. The figure denotes that $B_{x}$is increased when the hardened depth is increased. This is, because the $B_{x}$from the impression magnetic field is increased since the permeability and conductivity in surface domain of the steel plate is decreased when the hardened depth is increased. As for $B_{x}$in the search coil, the calculated result is in agreement with measurement. In this research, comparison of the detection sensitivity of the inspection method [3]by the detecting of the flux density $( B_{z})$inside the magnetic yoke as shown in Fig.5, and this proposal method by the detecting of the magnetic field $( B_{x})$on the steel is investigated. In this model of Fig.5, a magnetic closed loop is formed between the sensor and the steel, and the flux density in the steel is detected with a search coil of the yoke. Fig.6shows the comparison result of absolute values of $B_{x}$and $B_{z}$for the detecting of the hardened depth of the surface hardened steel by experiment. This figure denotes that the detection sensitivity of the proposal method by $\vert B_{x}\vert $is higher than the method by $\vert B_{z}\vert $. Optimal design of search coil The optimal dimension of a search coil for detecting the magnetic field $B_{x}$in the proposal sensor is determined using the evolution strategy [4]. Fig.7shows the inspection result in the optimal search coil form determined by the evolution strategy. The width $( Sw)$, length $( Sl)$and height $( Sh)$of the optimal search coil are 20.36mm, 6mm and 1.8mm, respectively. The inspection result by the initial search coil model as shown in Fig.1is also shown in this figure. This figure denotes that the detection sensitivity of the optimal search coil is increased from the initial model. The optimal search coil is the form which detects more magnetic fields on the steel plate distributed between the magnetic poles of the magnetic yoke.

A time-of-flight Bragg-edge neutron transmission imaging was used to investigate the microstructure and strain distributions in a gear hardened by a newly developed two-step induction-heating method: precursor (Sample 1) and final product (Sample 2). The edge-position and edge-broadening were determined and mapped with high resolution, which enabled us to confirm the two-dimensional distributions of the microstructure and residual strain. A deep hardened layer was made for Sample 1 in which martensite was formed on the entire teeth and the outer peripheral portion of the gear body. Sample 2 was subjected to double induction-hardening, where a tempered martensite was formed as the thermal refined microstructure between a fine-grained martensite at the tooth surface and a ferrite-pearlite microstructure at the core. The relationship between edge-broadening and the Vickers hardness described by a linear equation was employed to derive the elastic residual strain. The residual strain map for Sample 2 revealed that a steep compressive strain was introduced into the fine-grained martensite at the tooth surface by the super rapid induction-heating and quenching process. The reversal of tension was speculated to occur below 2 mm from the tooth tip, and the strain was almost zero in the core region.

This work characterized the surface-modified layers generated on a titanium alloy during gas blow induction heating (GBIH), so as to elucidate the rapid nitriding mechanism. The surface microstructures of GBIH nitrided specimens were analyzed by X-ray diffraction, micro-Vickers hardness testing, scanning electron microscopy and X-ray photoelectron spectroscopy. The data indicate that applying this process for only several minutes can forms nitrided layers on Ti-6Al-4V alloy surfaces, the characteristics of which are similar to those of layers produced by ordinary gas nitriding for several hours. The temperature inside the material was determined to be higher than that on the surface during GBIH nitriding. However, the thickness of the nitrided layer exceeded the estimated value based on the diffusion coefficient at this high internal temperature. This especially thick nitrided layer can possibly be attributed to eddy currents generated in the metal by the application of a high-frequency current. Passivation films on the surface of the Ti-6Al-4V alloy were found to be removed during GBIH nitriding, which is ascribed to the diffusion of oxygen atoms in the passivation film into the substrate. Thus, the data show that GBIH nitriding is able to modify a Ti-6Al-4V alloy within a short period of time based on the effect of an eddy current and the disappearance of the passivation film.

In order to improve corrosion resistance of carbon steel by AIH-FPP treatment using Cr/Ni hybridized particles, the specimens was peened in an argon atmosphere , at 1200°C , for 30 s, varying post heating time from 0 s to 300 s. Cr and Ni particles were mixed by means of a planetary ball mill to obtain hybridized particles, and it was used as shot particles. Results showed that changing the post heating time strongly affects the characteristics of modified layer at the specimen surface. Results of EDX analysis showed that the Cr and Ni diffused layer was formed at the post heating time of 300 s. Corrosion tests were performed using a three-electrode electrochemical cell connected to a computer driven potentiostat. The specimens with Cr and Ni diffused layers showed a wide passive region and almost the same pitting corrosion resistance of Stainless Steel (X30Cr13). This is because of the existence of Cr passive films on treated surfaces.

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.

In order to increase surface hardness of AISI 1045 steel, a new hybrid surface modification; combination of atmospheric-controlled induction-heating fine particle peening (AIH-FPP) and plasma nitriding, was developed. Surface microstructures of plasma nitrided specimens pre-treated with AIH-FPP using Cr shot particles were characterized by an optical microscope, a scanning electron microscope (SEM), an energy dispersive X-ray spectrometer (EDX) and X-ray diffraction analysis (XRD). As results, the nitrided layer was formed at the surface of the specimen with Cr diffused layer induced by AIH-FPP. This nitrided layer showed higher hardness than that of without AIH-FPP specimen. This was because CrN was formed at the surface of the AIH-FPP/Plasma nitriding treated specimen. And, the surface hardness of the nitrided layer tended to increase as the Cr concentration in the surface layer decreased. It was clarified that the pore formation during the AIH-FPP/Plasma nitriding treatment was inhibited by decreasing Cr concentration. These results suggest that the proposed hybrid surface treatment remarkably increases surface hardness of AISI 1045 steel. [doi:10.2320/matertrans.MBW201205]

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.

Basic mechanisms of generating a surface modified layer formed by a previously developed atmospheric controlled IH-FPP system, consisting of induction heating system and fine particle peening system, were investigated. Three types of specimen treated at 900°C in argon with Cr, Al2O3 and Cu shot particles were prepared. Surface microstructures of the treated commercial-grade pure iron were characterized with a scanning electron microscope (SEM), an energy dispersive X-ray spectrometer (EDX) and an X-ray diffraction analysis (XRD). When treatment was carried out using Cr particles, the transferred layer and the diffused layer of the shot-particle element formed at the treated surface. This was because the radius of Cr atom was almost the same as that of the Fe, thus forming a solid solution in the substrate. Conversely, when the treatment was applied using chemically stable Al2O3 particles, a transferred layer only was observed. In the case of Cu particles, adhesion occurred due to the low melting temperature of Cu particles, resulting in the formation of a thick adhesive layer such as that generated by thermal spraying. These results suggest that the characteristics of the modified layer, generated by the atmospheric controlled IH-FPP treatment system, can be controlled by varying the shot particles and other processing parameters.

To evaluate the influence of hybrid surface modification processing on the rolling contact fatigue life, we used a roller pitting test comprising a combination of high strengthening by Super Rapid Induction heating and Quenching (SRIQ) and improvement of the tribological quality by a Diamond Like Carbon (DLC) membrane. The applicability of the process to a small gear was examined. Results show that the rolling contact fatigue life of the SRIQ+DLC specimen was longer than that of SRIQ specimen at all loads. Subsequent SEM observation of fractures showed that many micro-surface cracks and pittings along with pitting caused fracturing of the SRIQ specimen. However, the SRIQ+DLC specimen showed few micro-surface cracks aside from the pitting causing fracture. Many internal cracks were found. Based on these results, we infer that improvement of the rolling contact fatigue life is introduced by retarding of the generation of surface damage attributable to the decrease of tangential stress brought by superior low friction and excellent wear resistance of the DLC membrane. Therefore, SRIQ+DLC compound surface modification, which improves tribological and fatigue properties, is suggested as effective for improving rolling contact fatigue life. Regarding SRIQ, it is possible also for the discharge of carbon dioxide to produce a substantial decrease in comparison to other heat treatment processes. It can contribute also to protection of the terrestrial environment. It might be said that a new hybrid surface modification process has been produced.

A new surface treatment system (IH-FPPsystem) combined with an induction heating and a fine particle peening machine were developed. FPP treatments were carried out on structural steel discs by using Cr shot particle at room temperature, 400°C, 600°C and 900°C. After characterizing the surfaces by SEM, EDX and XRD, corrosion tests were performed by a three electrode method using a computer driven potentiostat. No noticeable differences were observed on the specimen treated at RT, 400°C and 600°C. In the case of the specimen treated at 900°C, however, higher corrosion resistance which is almost the same as FPP treated stainless steel (SUS316L) was observed. This was because Cr diffused layer covered with relatively smooth oxidized surface prevented a corrosion reaction in 3%NaCl environment.

It has been well known that mechanical properties of steels are improved by gain refinement, smaller cementite particle and finely dispersing cementite particle, It is enabled by super rapid induction tempering the the smaller cementite particle and the finely dispersing cementite particle. In this study, mechanical properties of induction quenching and tempering carbon steel using super rapid induction heating system were investigated. Dtail, the effect of quenching time on tensile properties is examined. The quenching time was changed in four steps of 0.44 s (1163 K), 10 s (1233 K) +10 s (1153 K), 20 s (1273 K) and 20 s (1433 K). As a result, it was found that the tensile strength of QT steel indicated an increase tendency with hardness. The yield strength was in proportion to a square root of grain size. However, the ductility improvement effected by super rapid induction quenching was not found, reduction of area did not depend on grain size.

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.

It seems to be possible to heat treat the gears having higher fatigue strength with lower distortion by applying newly developed super rapid induction heating and quenching (SRIHQ) technology. In this previous research, rotating bending fatigue properties of S 45 C tempered martensite structure plane specimens heat treated by SRIHQ having shallower hardened layer and higher compressive residual stress were studied, and it was observed that the fatigue strength was not improved due to the fatigue cracks initiation just beneath the shallow hardened layer. In the present research, rotating bending fatigue properties of same specimens with notch (α=2.3) heat treated by SRIHQ were studied. As a result, on the contrary of previous work, much higher fatigue strength was obtained in spite of shallow hardened layer. The compressive residual stress formed at the bottom of notch is extremely high (2000 MPa) due to the effect of stress concentration.

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.

Four kinds of surface hardened-specimens (ordinary structural steel with carbon content of 0.45% C) having hardened thicknesses of 0.7–1.8 mm were prepared using a ‘super-rapid induction heating (SRIH) system’. Rotation bending fatigue tests were performed with special focus on the effect of a hardened thickness on fatigue properties. Measurement of residual stress and observation of the fracture surface were also carried out to investigate the fracture mechanism of the specimen with a shallow hardened layer. It was found that there is not much improvement of fatigue strength at 10 7 cycles for specimens with shallow hardened layers in spite of having a high compressive residual stress of about 1000 MPa. This is because the fatigue crack originating from inside the hardened layer leads to the final fracture of the specimen (internal fracture mode). Improvement of fatigue strength has been achieved on the specimen with thick hardened layers, such as those about 1.8 mm thick. In this case, fatigue cracks originate from inclusions located in hardened layers, which leads to final fracture (hardened-layer fracture mode).

To improve the fatigue reliability for Mn-Si-C Steel by an environmentally sound surface modification, the influence of the modification on fracture mechanism was studied experimentally. The fatigue properties of Mn-Si-C Steel are found to be obviously improved by a Super-Rapid Induction Quenching, so that they can no longer be estimated by using a concept of "S-N Globe" which has successfully been applied to many surface-treated materials. The detailed observations have revealed that fatigue crack initiation in a Super-Rapid Induction Quenched specimen occurs in a hardened region near surface not in a subsurface region with virgin microstructure. The fatigue crack was induced by slip in the ferritic matrix. And the fatigue crack propagation in the hardened reagion was restricted by the presence of the martensite phase and compressive residual stress. Such a crack initiation mechanism is different from that previously observed in other surface treated materials, explaining the reason why the S-N Globe concept can not be applied to the present case.

In order to estimate fatigue reliability of super-rapid induction quenched spheroidal graphite cast iron, FCD500, FCD700, the influence of complex surface microstructure on the fatigue strength was examined through fractography. The main results obtained are as follows : (1) Fatigue strength of quenched FCD500 was superior to that of quenched FCD700 in the long-life region (N> 5.0×105), in spite of tensile strength of FCD500 was lower than that of FCD700. (2) Fatigue property of super-rapid induction quenched spheroidal graphite cast iron could be estimated using a concept of "S-N globe". (3) Initial stress intensity factor at crack initiation under the surface of quenched FCD500 was comparable to that of quenched FCD700, therefore the improvement in fatigue strength of FCD500 depended on the crack propagation behavior through the complex surface microstructure, not the crack initiation behavior. (4) The prevention of crack propagation through the complex surface microstructure gave and appearance of high level fatigue reliability of FCD500.

Recently, a new super rapid heating induction hardening technique which uses MICROPULSE SYSTEM has been developed. The new induction hardening technique differs from the former induction hardning in that the new technique is very useful for controlling case-hardened depth exactly, and is attendant on little deformation after hardening.In this study, in order to clarify the fatigue strength of the notched specimens of carbon steel (S45C, α=1.6) whose effective case-hardened depth are about 0.70mm and 0.45mm, uniaxial loading fatigue tests were conducted. It was found that the fatigue strength of the hardened specimens was 670MPa for the hardning depth 0.70mm and 630MPa for the hardening depth 0.45mm. Micro pulse induction hardening improved the fatigue strength about two times higher than that of the S45C base metal. The “Fish-eye” which indicates that fatigue cracks initiated from the inside of a specimen appeared on many fracture surfaces. And the position of the “Fish-eye” coincided nearly with that of the case-hardened depth.