Solid State Phenomena Vol. 118

Abstract: It is important to suppress grain coarsening during the carburizing treatment of automobile components such as gears and shafts so that high strength is maintained and heat treatment distortion is minimized. Two manufacturing methods are often used for the cost reduction. One is the increasing carburizing temperature. The other is the adoption of cold forging instead of hot forging. These methods are likely to result in grain coarsening in the case of conventional steels. It is well known that the key factors to control the grain growth are the initial austenite grain size and the volume and mean size of precipitates. In this study, the grain growth property of Ti-modified steel was investigated to confirm the influence of precipitates, and compared with those of Nb-modified and conventional steels. The influence of forging temperature on the grain growth property in a Ti-modified steel was also investigated.

Abstract: It is well known that the durability of tool steel could be improved by deep cryogenic treatment. It has been assumed that the increase of service life of tool steel caused by decrease of retained austenite and/or by formation of nano-scale fine η-carbide. But the principles of deep cryogenic treatment remain unclear yet. In this research, to manifest the effect of deep cryogenic treatment on wear resistance, the specimen was emerged in liquid nitrogen for 20 hours for deep cryogenic treatment after austenitizing and the following tempering temperature was varied. The microstructure of specimens was observed using TEM and the mechanical properties and wear resistance were examined. As the tempering temperature increased, the carbides became larger and fine carbides were formed above certain temperature. In the case of deep cryogenic treated specimen, the number of carbides increased while the carbides size was decreased, furthermore, the fine carbide forming temperature was lowered also. It was considered that the deep cryogenic treatment increased the driving force for the nucleation of carbides. As tempering temperature increased, hardness decreased while wear resistance and impact energy increased. The deep cryogenic treated specimens showed this tendency more clearly. It was considered that the wear resistance is affected not only to the hardness but also to the precipitation of fine carbides, and this carbide evolution can be optimized through the deep cryogenic treatment.

Abstract: The microstructures and mechanical properties of 17-4 PH stainless steel at each steps of heat treatment, such as homogenizing, solid solution treatment followed by aging treatment, longterm aging at 400 °C, and recovery treatment, in order to obtain a better understanding of the embrittlement phenomena on aging, was investigated. As the homogenizing treatment time increased, the length of δ-ferrite decreased and elongated shape of δ-ferrite turned to sphere shape with the decrease of volume fraction. The solution treated specimen mainly consists of lath martensite with a small fraction of elongated δ-ferrite. The spherical particles existed a little in the martensite matrix, while no precipitates were found in the δ-ferrite at the solution treated specimen. As the aging treatment temperature increased, the strength decreased while the toughness increased. The fcc Cu-rich particles precipitated in the δ-ferrite during the long-term aging at 400 °C after the solution heat treatment. This precipitation causes the aged hardening after long-term aging accompanied by decreases in elongation and charpy V-notch energy absorption. The strength and elongation was restored after recovery treatment and the fcc-Cu precipitated were almost dissolved into the δ-ferrite matrix.

Abstract: The life limit of the standard hardness block of HRC64 made from JIS-SUJ2 steel was evaluated. The HRC64 specimens were annealed at temperatures ranging from 55 to 200
C for about 104 h in maximum. The micro-Vickeres hardness decreased with the annealing time at all temperatures after some incubation periods. Transmission electron microscopy (TEM) showed that the microstructure of the specimen before annealing was composed of primary cementite particles, carbide and martensite matrix. After annealing, fine cementite particles of about few tens of nanometers in size were observed. The decrease in hardness during annealing was caused by the precipitation of the cementite. The life limit keeping the standard hardness value at ambient temperature was estimated to be about 3.5-4 years based on extrapolation of the annealing curves at various temperatures.

Abstract: The microstructural change with drawing and subsequent annealing for a patented pearlite steel was investigated by means of neutron diffraction. The dissolution of cementite plates with drawing and re-precipitation of spherical cementite particles with annealing after sever drawing were observed. In situ neutron diffraction during tensile loading was performed and it is revealed that the strengthening mechanism of the specimen without cementite differs from that for a ferrite-cementite steel where the load transfer is a main mechanism. The possible strengthening mechanism for the heavily drawn specimen is proposed.

Abstract: The effects of annealing temperature and annealing time on mechanical properties of cold drawn pearlitic steel wires containing 0.84wt% of silicon were investigated. Annealing treatment was performed on cold drawn steel wires for the temperature range of 200°C to 450°C with the different annealing time of 30sec, 1min, 15min and 1hr. The increase of tensile strength at the low annealing temperatures would be related with strain ageing behavior, while the decrease of tensile strength at the high annealing temperature is due to the spheroidization of cementite plates and the occurrence of recovery of the lamellar ferrite in the pearlite.

Abstract: This paper focuses on the variations of static and dynamic properties of tungsten heavy alloy with heat treatment. The matrix phase of 93W-4.9Ni-2.1Fe (weight percent) has been penetrated into W/W grain boundaries during a cyclic heat treatment which consists of repeated isothermal holdings at 1150 °C and water quenching between them. By applying the cyclic heat treatment, the impact energy of tungsten heavy alloy is increased about three times from 57 to 170 J. When the tungsten heavy alloy is cyclically heat treated at 1150 °C and then re-sintered at 1485 °C, W/matrix interface is changed from round to undulated shape. The irregularity of the interface is increased with increasing the number of heat treatment cycles. From the measurement of the residual stress of W grains by X-ray diffraction, it is found that the irregularity of the interface is closely related with strain energy stemmed from the difference of thermal expansion coefficient between W particles and matrix phase. From dynamic ballistic test, it is found that the tungsten heavy alloy with undulated W grains forms many narrow fracture bands which are preferential for the self sharpening effect, thus, for the improvement of the penetration performance.

Abstract: The Cu-Ag-Cr alloy is a kind of aging hardening copper alloy and has excellent combination properties of high strength and good electrical and thermal conductivity. In the present investigation, the aging precipitation behavior of Cu-Ag-Cr alloy is studied, and the effects of aging processes on the microstructure and properties are discussed. Emphasis is on the correlation between the coherency and coarsening behavior of the Cr precipitates. When the alloy aging at low annealing temperatures, the homogeneous dispersed Cr precipitates are observed to be approximately spherical and keep coherent with Cu matrix in the Cu-Ag-Cr alloy, the coherency is lost between at 450°C~520°C, coherency on coarsening process of Cr precipitates at high annealing temperatures, the radium for coherent / semi-coherent transition of the Cr precipitates is determined from TEM micrographs as 15-45nm. Aging in the intermediate stage, coherent and semi-coherent particles can co-exist(15

41