Yoshihito Kuroshima

Surface hardening of ductile cast iron using stainless steel

Abstract A surface hardening technique for improving the wear resistance of ductile cast iron (DI) is proposed by fabricating molten DI with stainless steel. The essential idea is concerned with using the chromium source in stainless steel and the carbon source in DI to form chromium carbides. It was demonstrated that the surface of the fabricated DI consisted of firstly a layer of high chromium white cast iron ((Cr,Fe) 7 C 3 eutectic carbides), which was originally stainless steel, and then a layer of medium low-chromium white cast iron ((Fe,Cr) 3 C eutectic carbides), which was originally DI. Thus the hardness of the surface layer became 2–3 times that of the baseline ductile cast iron. Applying austenitic stainless steel produces a much better hardening effect than applying ferritic stainless steel. The depth of the hardened surface layer depends on the pouring temperature of the iron melt, the wall thickness of the iron casting and the cooling rate of the casting mold. The present strategy is, in principle, possibly applied to complete as well as local surface hardening of various kinds of ductile iron castings.

Relieving of Welding Residual Stress by Applying Cyclic Load

A fundamental study of relieving or attenuating weld residual stress was done by applying cyclic load. Two kinds of tests were done. First, bead-on welding was done onto front surface of plate specimen. Then, the distribution of initial welding residual stress was measured in terms of X ray diffraction method. To relieve this initial residual stress, cyclic load was applied to the specimen in a vibrating machine. The change of stress cycle-dependent residual stress was measured. Secondary, this method was applied to a box-shaped rahmen structure. Both results showed that the initial welding residual stress can be relieved when the summation of stress component of the residual stress plus externally applied cyclic stress exceeds the yield stress of the material. Introduction Welding joint is one of the most useful force-transmitting tools in engineering practice. However, welding, itself, generally introduces welding residual stress in machine elements or structural components, which is to be noted detrimental. In this regard, post-welding heat treatments (PWHT) such as whole heat treatment, local heat treatment and low temperature heat treatment, over stressing, machining, peening, excited vibration using resonance [1], are done to weldment in engineering practice to relieve residual stress after welding. In spite of the knowledge that self-excited oscillation of cast product brings damping or attenuation of inherent residual stress produced during casting, no attempt can be seen in the literature to positively apply cyclic stressing to relieve welding residual stress in structures. Tensile or compressive welding residual stress is, in general, to be considered as a mean stress, in a sense of fatigue. This means that the tensile or compressive welding residual stress brings no effect to fatigue crack initiation process under cyclic loading condition. On the contrary, it will induce cyclic creep or thermal ratcheting. However, possibility of the occurrence of those phenomenon should differ depending on the level of cyclic stress. In this paper, two kinds of fatigue tests were carried out so as to examine the usage of the applied Key Engineering Materials Online: 2003-07-15 ISSN: 1662-9795, Vols. 243-244, pp 183-188 doi:10.4028/www.scientific.net/KEM.243-244.183

Quantitative Evaluation of Copper Clustering Process by Lattice Monte Carlo Simulation.

In this study, we simulate nanoscale copper precipitation process based on the vacancy jump model using Lattice Monte Carlo (LMC) method, where an activation energy is calculated from the first neighboring interaction model. We confirmed that status of copper clustering at temperature of 300 K is different from those of 600 K and 900 K at the same potential energy decrease, and copper clusters are formed more rapidly at higher temperature. We obtained a fact that process of copper clustering consists of the two phases which are formation phase, and coalescence and/or absorption phase.

Fatigue. Mode III Fatigue Crack Growth Behavior. Transition Condition of the Crack Growth Mode.

Mode III fatigue crack is usually observed at the root of circumferentially notched round bar. As the crack grows in the depth direction of specimen, the growth rate of crack decreases. In this respect, crack mode transition from mode III to mode I is understood responsible. However, quantitative evaluation on crack mode transition has been hardly done. Therefore, values of stress intensity factor KI under mode III fatigue crack growth tests on a low carbon steel were calculated with using principal stress, as crack mode transition began. Then KI values were compared with crack growth rate curve of mode I. As the result, it was observed that the condition of crack mode transition was given by the crack growth rate curve of mode I.

Evaluation of Fatigue Strength of Short Carbon Fiber Reinforced Thermo Plastics by Monitoring Temperature Rise in Fatigue Process.

Short carbon fiber reinforced thermo plastics (SCFRTP) fabricated with injection molding generally shows a large amount of scatter in fatigue strength. To find appropriate method of treating this scatter, fatigue tests were carried out. As the results, the scatter in fatigue strengh was estimated in terms of the initial volumetric Youngs modulus, which involves the effect of the fiber morphologies such as length distribution and orientation. Parallel to this, the heat generation during fatigue testing was measured on each specimen with different fiber morphologies. The results showed that monitoring terperature rise in specimen is a suitable way to evaluate the fatigue strength of SCFRTP. The validity of this method was demonstrated by fatigue tests with variable stress amplitude ad frequency.

Understanding Fatigue Damage Under High-Cycle Random Fatigue on the Basis of the Actual Fatigue Process.

In a previous paper, the present authors clarified that COD is a suitable parameter to interpret the fatigue damage on the basis of the fatigue crack growth in low-cycle random fatigue. Especially the COD parameter correlated well the fatigue damage counted using the rainflow method (RFM) with the crack growth life in LCF. To extend this idea to high-cycle random fatigue the present study has been done. The result showed that although the RFM is valid to evaluate total fatigue life, it is invalid to predict the crack growth life. This is due to the fact that the crack growth life is strongly affected by the effect of crack closure, which is more amplified in high-cycle fatigue than low-cycle fatigue.