Materials Science Forum Vols. 539-543

Abstract: The properties of ultrahigh carbon steels (UHCS) are strongly influenced by aluminum additions. Hardness studies of quenched UHCS-Al alloys reveal that the temperature for the start of transformation increases with increases in aluminum content. It is shown that this change is a function of the atomic percent of solute and of the valence state when comparisons are made with UHCSs containing silicon and tin as solutes. The thermal expansion of UHCSs with dilute aluminum additions shows no discontinuity in the vicinity of the ferrite-austenite transformation temperature. This is the result of a three phase region of ferrite, carbides and austenite. The slope of the expansion curve is higher in the austenite range than in the ferrite range as a result of the dissolution of carbon in austenite with temperature. Processing to achieve a fine grain size in UHCS-Al alloys was principally by hot and warm working (HWW) followed by isothermal warm working (IWW). The high temperature mechanical properties of a UHCS-10Al-1.5C material show nearly Newtonian-viscous behavior at 900 to 10000C. Tensile elongations of 1200% without failure were achieved in the 1.5%C material. The high oxidation corrosion resistance of the UHCS-10Al materials is described.

Abstract: The present requirements for the connecting rod are indicated and the fracture splitting (FS) technologies for constructing the big end boss are reviewed. Two possibilities of FS for a high strength Cr-Mo steel and Ti alloys were discussed. The carburized quench-temper FS connecting rod has a superior fatigue strength with a high dimensional accuracy at the big end boss. The possibility of using a titanium FS connecting rod was examined and proposed.

Abstract: There is a glaring need for omitting intermediate heat treatments in the manufacturing processes of carburized parts and increasing the carburizing temperature aimed at cost reduction. It is necessary to develop techniques to inhibit grain-coarsening since some austenite grains tend to grow abnormally during carburizing and coarse grains have negative effects on the properties of parts. Therefore, we developed new techniques to inhibit grain-coarsening by refining precipitates’ size and increasing their volume fraction based on the abnormal grain growth theory by Gladman. In this study, AlN and Nb(CN) were chosen as the precipitate particles for the pinning of austenite grain growth. And we investigated grain-coarsening behavior in several manufacturing processes of parts. As a result, we developed anti-coarsening steels for various manufacturing processes of carburized parts.

Abstract: In this research, the effect of the aluminum addition to cast iron on damping property was investigated by central vibration method, dynamic mechanical analysis, ultrasonic attenuation measurement and impact sound method. All measurement results indicated that the addition of aluminum improve the damping property of cast iron.

Abstract: Microstructures in a bearing steel, JIS SUJ2, have been observed and analyzed in detail by means of transmission electron microscope and synchrotron radiation diffraction in SPring-8. Spherodized carbides in the steel are generally recognized as spherical cementite particles, but some of them have been clearly shown to be M23C6 type of carbide in this study. The shapes and sizes of these two types of carbides are similar to each other. On the martensitic matrix of the steel, it is believed to be stable at relatively low temperature, but it is also shown to be decomposed to form cementite by prolonged aging at 383 K, which provides a reason for decrease in hardness in standard hardness blocks, previously reported by the authors.

Abstract: The effects of chemical composition, cold rolling and subsequent annealing parameters on the reversion of strain-induced martensite to austenite were investigated in three experimental Mn and Si-free Cr-Ni austenitic stainless steels and two commercial Type 301 and Type 301LN grades by optical and electron microscopy, X-ray diffraction and magnetic measurements. Hardness and tensile tests were performed to determine the mechanical properties achieved. In cold rolling, completely martensitic structure could be obtained in the experimental heats, but only partially in 301 and 301LN grades at reasonable reductions. Upon annealing, in 301LN the reversion took place by the nucleation and growth mechanism, and submicron austenite grains were formed within a few seconds at temperatures above 700°C. In the other steels, reversion took place by the shear mechanism, and ultra-fine grains were formed by the recrystallization of austenite at temperatures of 900°C or above. Partial reversion resulted in an excellent combination of yield strength and elongation in 301LN, and also in 301 such ones were attained in the reverted structure even before any profound formation of submicron grains.

Abstract: The effect of the degree of supersaturation in a coarse-grained h-ferrite matrix on the consequent isothermal transformation of h-ferrite to austenite at 1150‖ in a 2205 duplex stainless steel has been investigated. The results indicate that rapid cooling (at the cooling rate 100‖/s) to the ambient temperature after 135P‖ solution treatment of h-ferrite matrix leads to a very high supersaturated solid solution state, which significantly enhances the formation of the intragranularly nucleated austenite in the subsequent isothermal transformation at 1150‖. The feature of the intragranularly nucleated austenite has been revealed; it appears that after the initial nucleation events for intragranularly nucleated austenite, subsequent small particles of austenite can form sympathetically on the ferrite/austenite interface.

Abstract: A new method to improve oxidation resistance of ferritic stainless steel was studied. Increase in oxidation resistance of ferritic stainless steel with laves phase precipitation was succeeded in addition to the conventional method such as addition of rare-earth element. Ultra low carbon 20%Cr steels with various niobium content continuously oxidized at 1073-1173K in air. A mass gain with exposure at 1073K decreased with increasing niobium content and saturated more than 0.1% in niobium content. In steels contained niobium less than 0.1%, mass gain were also decreased with increase in initial ferritic grain diameter. In steels contained niobium more than 0.1%, laves phase was observed at the ferritic grain boundary by using transmission electron microscope. But then a mass gain with exposure at 1173K did not decreased with niobium content and laves phase was not observed. On the assumption that oxidation behaviors of steels contained niobium more than 0.1% obey the parabolic law, the parabolic rate constant obtained at 1173K is similar to that controlled by grain boundary diffusion, and parabolic rate constant obtained at 1073K is similar to that controlled by lattice diffusion. These results suggest that precipitation of laves phase retard growth of oxide film effectively with suppressing grain boundary diffusion of cation forming oxide film.

Abstract: Transformation of austenite to martensite during cold rolling operations is widely used to strengthen metastable austenitic stainless steel grades. Static strain aging (SSA) phenomena at low temperature, typically between 200°C and 400°C, can be used for additional increase in yield strength due to the presence of α’-martensite in the cold rolled metastable austenitic stainless steels. Indeed, SSA in austenitic stainless steel affects mainly in α’-martensite. The SSA response of three industrial stainless steel grades was investigated in order to understand the aspects of the aging phenomena at low temperature in metastable austenitic stainless steels. In this study, the optimization of, both, deformation and time-temperature parameters of the static aging treatment permitted an increase in yield strength up to 300 MPa while maintaining an acceptable total elongation in a commercial 301LN steel grade. Deformed metastable austenitic steels containing the “body-centered” α’-martensite are strengthened by the diffusion of interstitial solute atoms during aging at low temperature. Therefore, the carbon redistribution during aging at low temperature is explained in terms of the microstructural changes in austenite and martensite.