原田 泰典

Ni-free austenitic steels containing high nitrogen have been developed to protect against earth resource. High nitrogen steels (HNS) have a lot of advantages, e. g., HNS have high strength, corrosion resistance, toughness, work hardening rate and large rocking parameter in the Hall-Petch equation. On the other hand, it is difficult to fabricate HNS by IM method under 0.1 MPa and to work at room temperature. We have tried to make HNS by combined use of metal injection molding method (MIM) and nitrogen absorption method. Powder compositions used was Fe-17Cr-12Mn-3Mo. The benefit of this method is to make metal parts in near net shape. In order to use this method, we should know the sintering heat schedule, timing for introducing nitrogen gas, gas pressure and setter material etc. Therefore, the shrinkage rate, density and the solution-treated microstructure of MIM compacts were examined to find out the optimum conditions.

Microstructure and mechanical properties of high nitrogen steels whose chemical composition were Fe-17Cr-12Mn-3Mo and that was produced by using metal injection molding method and nitrogen absorption methods were examined. A compact which is furnace cooled from 1573 K has a brittle surface layer with high chromium and nitrogen contents but the surface layer disappears when the compact is held at 1473 K. The compact which is furnace cooled at 1473 K is observed precipitates in the grains and the grain boundary, while the compact which is quenched at 1473 K shows homogeneous microstructure. In the heat treatments at 1473 K for 2, 5, and 10 h, the nitrogen content becomes higher with increasing of holding time. In the holding times of 5 and 10 h, the microstructure is austenite. In the tensile tests, tensile strength becomes larger with increasing of nitrogen content. In the specimen which is conducted the heat treatment at 1473 K for 10 h, tensile strength is about 1,000 MPa and elongation is 80%, which shows better balance of strength and elongation than SUS304 and SUS316 steels.

In this study, plastic flow joining using a shot peening process was investigated. Surface treatment is necessary to improve the surface properties. Shot peening is one of the surface treatments. Since the surface of substrate is hit repeatedly with a large number of shots, the substrate undergoes a large plastic deformation near its surface. Therefore, plastic flow characterized by a shear droop occurs at the edge of the substrate due to shot peening. If an implant made of a dissimilar material is set in a hollow space on the surface of the substrate and then shot-peened, it can be joined to the substrate due to the peening droop generated by the large plastic deformation during shot peening. In this method, the availability of the plastic flow, i.e., the peening droop makes the joining of the implant possible. In the experiment, a compressed-air-type shot peening machine was employed. To examine experimentally the influence of working temperature on bondability, equipment with a heating furnace was produced. The influence of processing conditions on the joining of the implant and the substrate was examined. The joint strength increased with the kinetic energy of shots and processing temperature. The improved implant with a step was effective in improving in bondability. The dissimilar material was also successfully joined to a thin sheet by using of the interaction of peening droops. It was found that the present method using the peening droop was effective for joining the dissimilar materials.

In the multi-stage deep drawing processes of a beta titanium alloy sheet, the formability has been investigated. The beta titanium alloy sheets have sufficient ductility at room temperature, whereas a seizure tends to occur during deep drawing due to high reactivity with other materials. To prevent the seizure, the beta titanium alloy sheet was treated by oxide coating heating, because the coated sheet was not in direct contact with the die during deep drawing due to the existence of the oxide layer. The blank used was the commercial beta titanium alloy Ti-15V-3Cr-3Sn-3AL The effect of the coating condition on the formability in the multi-stage deep drawing process was examined. It was found that long drawn cups with a height sixfold that of the diameter were successfully formed by oxide coating heating.

A lining process for carbon steel using shot peening was investigated. In the shot peening experiment, the dissimilar foil set on the carbon steel substrate is pelted with many shots at a high velocity. The foil is bonded to the surface of the substrate due to plastic deformation induced by the collision of the shots. In the experiment, an air-type shot peening machine with an electrical heater was employed. The substrates are commercially carbon steel S45C, and the foils are commercially pure aluminum, pure copper, and pure nickel. The effects of shot conditions and the heating temperature on the joinability were examined. To improve surface characteristics such as wear resistance and corrosion resistance, the surface alloying of the lined workpiece was also attempted. The metal foils were successfully joined to the surface of the substrate. It was found that surface properties of carbon steel could be improved by the shot lining process.

Particular attention is called to nickel-free or lower nickel austenitic stainless steels containing high nitrogen from the viewpoint of protections against earth resources and Ni allergy for human being. We have made high nitrogen steels (HNS) that were containing 0.7-2.0mass%N and 23Cr-2Mo by mechanically alloying method and the microstructure and mechanical properties have been examined. FINS containing 1.0-1.6mass%N have nearly full austenite phase and the mean grain diameter was about 2-3 mu m for every HNS. The strength and work hardening rate increased and the elongation decreased with increasing nitrogen content. Especially, HNS containing 1.0mass%N exhibited superior balance of strength-ductile properties, that is, yield strength of 1200MPa and fracture elongation of 36%. The yield point phenomena and ductile-brittle transition behavior can be seen in spite of austenitic stainless steels. The planar dislocation arrays were seen on the deformed structure. These results suggest that the formation of planar dislocation structure, high work hardening rate for high nitrogen steels and temperature dependence of mechanical properties might be caused by the formation of the Mo-N interstitial-substitutional atom couples.

The butt joining of dissimilar sheets using a shot peening process was investigated. Shot peening is a surface treatment and improves the performance of engineering components. In shot peening, the substrate undergoes a large plastic deformation near its surface due to a hit with many shots. Thus, plastic flow characterized by a shear droop occurs at the edge of the substrate due to shot peening. When the dissimilar sheets with the edge of the notch geometry are connected without level difference and shot-peened the connection, the sheets can be joined due to the plastic flow generated by the large plastic deformation during shot peening. In the experiment, a compressed-air-type shot peening machine was employed. The influences of processing conditions on the joining of the dissimilar sheets were examined. The joint strength increased with the kinetic energy of shots. It was found that the present method using shot peening process was effective in joining dissimilar sheets.

Shot peening is a surface treatment and improves the performance of engineering components. More recently, a new type of microshot has been developed to enhance peening effect. In the present study, the influence of microshot peening on the surface layer characteristics of cold tool steel was investigated. In the experiment, the microshot peening apparatus with a heating furnace was produced experimentally. The projective method of the microshot was a compressed air type. The peening microshots of 0.1mm diameter were cemented carbide and the workpiece was commercially cold tool steel SKD11. Surface roughness, compressive residual stress, and hardness in the peened workpiece were measured. The effect of microshot peening on the fatigue strength of cold tool steel was also examined. The use of hard microshot such as cemented carbide was found to cause a significantly enhanced peening effect for cold tool steel.

In this study, the grain refinement near the surface of metal workpiece using hot shot peening was investigated to improve the surface properties of the workpiece. In this process, the grains were refined due to plastic deformation generated by the collision of a lot of shots under hot working conditions. A model experiment using two shots was carried out to examine the effects of the amount of deformation, the processing temperature and the time interval of the collision on grain size. In the experiment, the workpieces were stainless steel SUS304 and commercially pure copper. It was found that the global surface layer successfully attained to the fine grains by means of hot shot peening.

The butt joining of dissimilar sheets using a shot peening process was investigated. Shot peening is a surface treatment and improves the performance of engineering components. In shot peening, the substrate undergoes a large plastic deformation near its surface due to a hit with many shots. Thus, plastic flow characterized by a shear droop occurs at the edge of the substrate due to shot peening. When the dissimilar sheets with the edge of the notch geometry are connected without level difference and shot-peened the connection, the sheets can be joined due to the plastic flow generated by the large plastic deformation during shot peening. In the experiment, a compressed-air-type shot peening machine was employed. The influences of processing conditions on the joining of the dissimilar sheets were examined. The joint strength increased with the kinetic energy of shots. It was found that the present method using shot peening process was effective in joining dissimilar sheets.

In this study, joining and shaping fit of dissimilar materials by the shot peening process was investigated. In this process, two kinds of shot peening machines, a compressed-air type and a centrifugal type, were employed. To join dissimilar materials efficiently on the surface of a cylindrical substrate, the shot peening equipment with a heating furnace and masking plate was fabricated. The influence of processing conditions on the joining between dissimilar material and the cylindrical substrate was examined. Joining of dissimilar materials using shaping fit was also performed. The bonding force increases with increasing kinetic energy of the shots and processing temperature. It was found that the present method using the peening droop was effective for joining dissimilar materials. (c) 2006 Elsevier B.V. All rights reserved.

Evolution of surface roughness and microstructure in Ti-50.9 mol%Ni shape memory alloy by shot peening were investigated by X-ray diffractometry and transmission electron microscopy. The crystallite size decreased significantly by shot peening of 30 s and remained essentially unchanged after longer peening. TEM observations of the deformed surface layer revealed the formation of amorphous and nanocrystalline in the samples after shot peening for 20 s. In the area with the lower extent of deformation, {114} B2 deformation twins and thin amorphous bands parallel to the twins were found. It implies that the amorphization occurs preferentially along the twin interfaces. ? 2006 The Japan Institute of Metals.

In this study, plastic flow joining using a shot peening process was investigated. The substrate used undergoes a large plastic deformation near its surface due to a hit with many shots. Therefore, plastic flow characterized by a shear droop occurs at the edge of the substrate due to shot peening. If an implant made of a dissimilar material is set in a hollow space on the surface of the substrate without any clearance and then shot-peened, it can be joined to the substrate due to the peening droop generated by the large plastic deformation during shot peening. In this method, the availability of the plastic flow, i.e., the peening droop makes the joining of the implant possible. In the experiment, a compressed-air-type shot peening machine was employed. To examine experimentally the influence of working temperature on bondability, equipment with a heating furnace was produced. The influence of processing conditions on the joining of the implant and the substrate was examined. The bond strength increased with the kinetic energy of shots and processing temperature. The improved implant with a step was effective in improving in the bondability. The dissimilar material was also successfully joined to a thin sheet by using of the interaction of peening droops. It was found that the present method using the peening droop was effective for joining dissimilar materials.

The influence of microshot peening on the surface layer characteristics of the steel for machine structural use was investigated. The microshot peening apparatus with a heating furnace was produced experimentally. The projective method of the microshot was a compressed air type. The peening microshots of 0.1mm diameter were cemented carbide, amorphous alloy and high carbon cast steel, and the workpieces were carbon steel, carburizing steel, spring steel and cold tool steel. The surface roughness, compressive residual stress and hardness in the peened workpieces were measured. The effect of processing temperature on the surface layer characteristics by warm shot peening and the evaluation of wear resistance were also examined. The use of hard microshots such as cemented carbide and amorphous alloy was found to cause a significantly enhanced peening effect for machine structural steel. It was also found that the surface of the workpiece peened by microshot was sufficient for the wear resistance.

In the multistage deep drawing processes of a beta titanium alloy sheet, the formability and the improvement of surface roughness have been investigated. The beta titanium alloy sheets have sufficient ductility at room temperature, whereas a seizure tends to occur during deep drawing due to high reactivity with other materials. To prevent the seizure, the beta titanium alloy sheet was treated by oxide coating heating, because the coated sheet was not in direct contact with the die during deep drawing due to the existence of the oxide layer. The blank used was the commercial beta titanium alloy Ti-15V-3Cr-3Sn-3Al. The effects of the coating conditions and lubricants on the formability in the multistage deep drawing processes were examined. Long drawn cups with a height sixfold that of the diameter were successfully formed by oxide coating heating. To improve the surface roughness of drawn cups, deep drawing and ironing as well as the grain refining by thermomechanical treatment were also attempted. It was found that the methods of ironing and grain refining were effective for improving the surface roughness of drawn cups.

In this study, deep drawing using plastic die was investigated to prevent seizure during forming. In the deep drawing process of aluminum and titanium sheets, seizure was found to occur during forming due to a high reactivity with other metals. For the prevention of seizure, plastic dies were used in this study. These plastic materials have a good resistant to seizing, since no seizure occurs in the contact surface of the metal and plastic material. Moreover, the effects of seizure and wearing die on the dimensional accuracy of drawn cups were examined using nylon die. The blanks used were pure aluminum, aluminum alloy A5052 and pure titanium. The blanks could be successfully formed with a water lubricant. To increase the dimensional accuracy of the drawn cups, deep drawing using fiber-glass-reinforced nylon and plastic composite steel die was also attempted. It was found that the use of plastic die was very effective in preventing of seizure.

In this study, we've used carburized-quenched-tempered steels that received shot-peening process with different shot grain sizes and evaluated variation of surface property of the steels. As a result of study, it is demonstrated that the fatigue life of carburized steels can be prolonged significantly by increasing surface hardness and compressive residual stress and by reducing surface roughness and abnormal carburized case depth at superficial layer, which we've investigated in the study.

In the present Study, lining of the metal with foils using shot peening was investigated to improve the surface layer characteristics. In the shot peening experiment, the foils set on the metal are pelted with hard particles traveling at a high velocity. The foils are bonded to the metal Surface due to plastic deformation induced by the collision of the particles. The foils and the metal are heated to heighten the bondability because of the reduction of flow stress. Lining the metal with the hard powder sandwiched between two aluminum foil sheets was also attempted. In this experiment, a centrifugal shot peening machine wite an electrical heater was employed. The metals are commercially aluminium alloys and magnesium alloys, and the foils are commercially aluminum, titanium and nickel. The effects of shot speed and the heating temperature on the bondability were examined. Wear resistance was also evaluated by grinding. The foils were Successfully bonded to the metal surface. It was found that the present method is effective in improving of surface layer characteristics.

Effects of processing temperature on the residual stress and hardness in the warm shot peening process of springs were investigated to determine the optimum shot peening conditions. Springs made of hard spring steel were heated to facilitate a shot peening operation by the decrease in flow stress. In a model experiment using shots of a large size, the shot peening apparatus was covered to keep the processing temperature constant, and the distributions of residual stress and hardness in the peened workpieces were measured. Although the flow stress of the workpiece is decreased by the increase in the processing temperature, the decrease of hardness and residual stress in the workpiece increases. The compressive residual stress and the hardness in the peened workpiece were the highest at a temperature of 200 degrees C. The optimum temperature was also evaluated in an experiment using an actual shot peening apparatus with shots of an average size. (c) 2005 Elsevier B.V. All rights reserved.

The role of hydrogen in the Si film during excimer laser annealing (ELA) has been successfully studied by using a novel sample structure, which is stacked by a-Si film and SiN film. Hydrogen contents in the Si films during ELA are changed by preparing samples with hydrogen content of 2.3-8.2 at.% in the SiN films with a use of catalytic (Cat)-CVD method. For the low concentration of hydrogens in the Si film, the grain size increases by decreasing hydrogen concentration in the Si film, and the internal stress of the film decreases as increasing the shot number. For the high concentration of hydrogens in the Si film, hydrogen burst was observed at 500 ml/cm2 and the dependence of the internal stress on the shot number becomes weak even at 318 mJ/cm 2. These phenomena can be understood basically using the secondary grain growth mechanism, which we have proposed. Copyright ? 2005 The Institute of Electronics, Information and Communication Engineers.

In the present study, a lining process for titanium alloys using shot peening was investigated. In the shot peening experiment, the foil set on the titanium alloy substrate is pelted with many shots at a high velocity. The foil is bonded to the surface of the substrate due to large plastic deformation induced by the collision of the shots. Lining of the substrate with the hard powders sandwiched between two aluminum sheets was also attempted. In the experiment, a centrifugal shot peening machine with an electrical heater was employed. The substrates are commercially pure titanium and titanium alloy Ti-6Al-4V, and the foils are commercially aluminum, titanium, nickel and sandwich foil. The effects of shot speed and the heating temperature on the bondability were examined. The wear resistance was also evaluated by grinding. It was found that the dissimilar material is successfully bonded to the surface of the substrate.

The ductile-brittle transition behavior of the rolled chromium was examined by means of the tensile test. The workpieces taken from the as-sintered block were repeatedly rolled with or without the intermediate annealing. The rolling was performed with a reduction of 10% in every pass up to 50 or 85% in total reduction with or without the intermediate annealing at 1273 K. Tensile specimens taken from the as-sintered block and the rolled sheets were tested in tension. Tensile axes for the specimens prepared from the sheets were at 0, 45 and 90degrees to the rolling direction. Some tensile specimens from the rolled sheets were annealed at 1473 K in vacuum to examine the effect of grain coarsening on the transition temperature. DBTT of the specimens prepared from the rolled sheets without the intermediate annealing was considerably higher than the temperature 500 K which was found in the as-sintered specimen. However, DBTT of the rolled specimens with the intermediate annealing was independent of rolling reduction and the tensile axis. It was found that DBTT of the rolled specimens with the intermediate annealing occurred at 400 K lower by 100 K than that of the as-sintered specimen. (C) 2004 Elsevier B.V. All rights reserved.

The ductility at room temperature was investigated with pure Cr, Cr-V and Cr-Mo alloys from the view of surface energy and shear modulus. The surface energy, gamma, was qualitatively evaluated by the creep method using thin plate specimens. The specimen contraction during creep was observed only when the low stress was applied to Cr-0.5 mol%V and Cr-0.2 mol%Mo alloys. This indicated that the gamma value was highest in these alloys among the other compositional alloys as well as pure Cr; although the reason still remained unknown. On the other hand, the shear modulus, G, was measured using the cantilever-type resonating method. It decreased monotonously with the V or Mo content in the alloys. Thus, the G/gamma ratio was supposed to be smaller in Cr-0.5 mol%V and Cr-0.2 mol%Mo alloys than in pure Cr. In response to this change in the G/gamma ratio, the ductility measured by the three-point bending test was high in these alloys. Nevertheless, the overall ductility change with alloying could not be explained by the G/gamma ratio alone. (C) 2004 Elsevier B.V. All rights reserved.

アモルファス-Si (a-Si)を固体YAG2ωグリーンレーザにより結晶化した低温多結晶シリコン(polycrystalline silicon, poly-Si)薄膜の物理解析を行った。グレインサイズ、表面に垂直方向の結晶化率、及び、表面ラフネスがエネルギー密度に関して極値をもつ。熱勾配の大きさにより、グレインの優先成長を生じるという機構を考える事によりこの現象を説明できる。再結晶化膜に関し、レーザスキャン方向、スキャン垂直方向に測定されたTOフオノンピークの標準偏差はほぼ1cm^<-1>であった。

アモルファス-Si (a-Si)を固体YAG2ωグリーンレーザにより結晶化した低温多結晶シリコン(polycrystalline silicon, poly-Si)薄膜の物理解析を行った。グレインサイズ、表面に垂直方向の結晶化率、及び、表面ラフネスがエネルギー密度に関して極値をもつ。熱勾配の大きさにより、グレインの優先成長を生じるという機構を考える事によりこの現象を説明できる。再結晶化膜に関し、レーザスキャン方向、スキャン垂直方向に測定されたTOフオノンピークの標準偏差はほぼ1cm^<-1>であった。

Nowadays, in the auto industry, the need for improvement of fuel efficiency is getting increased more and more in terms of the global warming, as well as the need of light-weighting of gears, transmission parts. Hence, we've studied the relation between the shot peening working conditions and factors for improvement of gear's fatigue life, and also the relation between the factors for improvement and the fatigue life, applying shot peening to a gear of SNCM220. Also, we examined the relation between the fatigue life and arc height that is utilized as a substitute characteristic for shot peening working conditions, adding its observation here.

Serration phenomenon of sintered and cast chromium was investigated by means of tension test at various strain rates. With increasing test temperature, tensile strength decreased gradually at first and showed a minimum at a certain temperature. Above this temperature, the strength continued to increase to a maximum value. Serrated flow appeared between the temperatures of the minimum and maximum tensile strengths. Tensile ductility minimum was also observed around the temperature of the maximum tensile strength. Relation between the strain rate and the temperature T of the maximum tensile strength could be well described by an Arrhenius equation. Activation energy of 101.3k/mol found in the present study is in good agreement with the activation energy for diffusion of nitrogen atom in chromium.

Shot peening process of inner surface of machine parts in which rebound shots collide with the inner surface was investigated. The jig used for rebound shot peening has the surface with a slope to the inner surface of machine parts, and the collision direction of shots was vertical to the inner surface. In this process, a compressed air type shot peening machine was employed. The influence of processing conditions was evaluated by the distributions of hardness, residual stress and surface roughness in the test pieces of pipes and coiled springs. The effect of shot peening increases with the increase of shots collision energy. A stress peening process for inner surface of coiled springs using rebound shots was also performed. The coiled spring was provided higher compressive residual stresses. The results of a fatigue test showed that the fatigue life of the spring was improved. The compressive residual stress reduces the tensile stress of coiled spring. It was found that the present process was suitable for improving the inner surface of machine parts.

The authors have proposed a lining process of metals with thin foils using shot peening. In this process, the foil is bonded to the surface of the workpiece due to plastic deformation generated by the hit of the shots. The foils such as steel, stainless steel, titanium and nickel were successfully bonded over the surfaces of the aluminium and steel workpieces. The lining process using shot peening is suitable for the bonding of thin and dissimilar foils required for the improvement of surface properties. In the present study, the lining process using shot peening was extended to the partial lining. The position of the collision of shots on the surface of the workpiece is limited by the masking plate. The foil is slightly larger than the masking plate, and it is fixed for the collision by the plate. A centrifugal type shot peening machine with an electrical heater was employed. The workpieces were aluminium alloys and magnesium alloy, and the foils were commercially pure nickel, commercially pure titanium and stainless steel SUS304 of 0.1mm. thickness. To assist the bonding, a pure aluminium foil was inserted between the metal foil and workpiece. The precision of the lined shape for the masking shape was investigated. The effects of the collision speed, the processing temperature and the thickness of foil on the bondability of the metal foil were examined. The partial lining of the circle and triangle on the surface of the workpiece succeeded. The bonding of the lined workpiece was confirmed to be sufficient by a bending test of the lined workpieces. It was found that the present method using the masking plate was very effective for the partial lining.

Resistance sintering characteristics of electrolytic and atomized copper powders mixed with graphite powder in steps of 10vol% up to 50vol% were experimentally investigated. In the experiments electrifying voltage and time, and applied pressure were varied. Post-electrification pressurization and insertion of low thermal conductive austenitic stainless steel discs between the sintering powder cake and electrodes were attempted to promote the sintering. The effects of the operational parameters including the two devices on sintering performance are discussed. Sintering strength of the resistance-sintered composites is also discussed.

固相成長(solid phase crystallization, SPC)の後エキシマ・レーザ・アニーリング(excimer laser anneal, ELA)、及びELAの後SPCを行うことにより形成された多結晶Si(polycrystalline-Si, poly-Si)の結晶性について調べた。内部応力の緩和がELAの前後にSPCを行うことにより生じた。また、欠陥密度はELAのみELAの後SPC、SPCの後ELAの順で小さくなった。590℃のSPCを300分行った後ELAを行うことにより低内部応力かつ低欠陥密度のpoly-Siを実現した。

Feasibility of sinter-joining method making use of resistance heating with an adiabatic feature was examined. Modeled experiments were conducted using pure metal powders of chromium, iron, nickel, copper and aluminum with a wide variety of melting points or sintering temperatures. All pairs of green compacts were sinter-joined varying compacting pressure and electrifying voltage at fixed electrifying time of 1 s, and their sinter-joining performances were rated by visual inspection and optical microscope observation of vertical section across the joining interface. The results were as follows. Most of the pairs were successfully sintered and joined under properly arranged conditions of electrifying voltage and compacting pressures, but in the combination of chromium and aluminum with a large melting point difference, the aluminum compact melted and infiltrated into the openings of the chromium compact, and in the combinations of chromium and iron with copper with high thermal conductivity, the sintered compacts of chromium and iron had both a porous structure near the joining interface. Those undesirable phenomena were, however, suppressed considerably by employing more rapid beating measures. It was concluded that the sinter-joining method using resistance heating has not a little feasibility. (C) 2003 Elsevier Science B.V. All rights reserved.

A sandwich-rolling, in which the workpiece sandwiched between two mild steel sheets is rolled, is tried to enhance the rolling workability for sintered compacts of pure chromium. In a conventional rolling, many thin cracks occur on the surfaces of workpiece rolled at higher temperatures than 1373 K. These cracks are supposed to occur by the rapid cooling of the workpiece surfaces in contact with the cold work rolls. The sandwich-rolling is very effective to prevent the occurrence of surface cracks since the mild steel sheets on both sides of the rolling workpiece interrupts the temperature decrease of the surfaces. The ductile-to-brittle transition temperature (DBTT) of sintered chromium was strongly lowered by a combination of large rolling reduction and high temperature annealing. (C) 2003 Published by Elsevier Science B.V.

Sintered molybdenum plate was rolled in one pass at 873 K to specimens with four degrees of 0, 10, 25 and 50% in thickness reduction. Tensile tests for the smooth and notched specimens were carried out to examine the effects of the level of plastic deformation and the edge notch on the change of ductile-brittle transition temperature (DBTT). The specimens were pulled at various temperatures between 163 K and 473 K and at a strain rate of 5.5×10<SUP>-3</SUP>s<SUP>-1</SUP>. Consequently, molybdenum could be deformed without any detectable defect up to 50% reduction in thickness by the rolling. For both of the smooth and notched specimens, the ductile-brittle transition temperatures were monotonously lowered with increasing the thickness reduction. The presence of an edge notch caused an increase of transition temperature by about 120 K to 160 K in a thickness reduction range of 0% to 50%.

Sandwich rolling, in which the workpiece is sandwiched between two mild steel sheets, is tried to enhance the rolling workability for sintered compacts of pure chromium. In conventional rolling, many thin cracks occur on the surfaces of workpiece rolled at higher temperatures than 1373 K. These cracks are supposed to occur by the rapid cooling of the workpiece surfaces in contact with the cold work rolls. Sandwich rolling is very effective in preventing the occurrence of surface cracks, since the mild steel sheets on both sides of the rolling workpiece interrupt the temperature decrease of the surfaces. This method is also very superior in workability to conventional rolling. The behavior of ductile-to-brittle transition and recrystallization were also examined in the rolled specimens. The ductile-to-brittle transition temperature is lowered in a specimen taken from a workpiece subjected to a large rolling reduction. The Vickers hardness of a specimen rolled at 973 K is considerably greater than that rolled at 773 K. The recrystallization temperature of the sintered compacts is almost the same to that of cast material. (C) 2002 Elsevier Science B.V. All rights reserved.