原田 泰典

ショットピーニングを施した機械部品の中には使用中における熱や負荷応力などの影響によって，付与した圧縮残留応力の緩和を生じる場合がある．本研究では，ショットピーニングによって十分に加工処理した材料に対して低温熱処理で材料強度を高めた後，再びショットピーニングを施す一種の加工熱処理を行った. 低温熱処理および再ショットピーニングを施した材料において，おもに表面特性と疲労限度に及ぼす加工熱処理の影響について調べた．供試材料は焼鈍処理した中炭素鋼で，また投射材は鋳鋼製である．空気式機械を用い，投射圧力0.6MPaと投射時間100sの加工条件で行った．その結果，低温熱処理を施した試験片の場合，加熱に伴うひずみ時効によって最表面の硬化はショットピーニングによって生じる加工硬化より進むことがわかった．また，疲労限度はひずみ時効によって表面硬さの増加が見られた試験片において高くなることがわかった．再ショットピーニングを施した試験片の場合，付与された最大硬さは加工軟化によって低温熱処理したままの試験片の最大硬さに比べて低下した．また，表面近傍の圧縮残留応力値と疲労限度は高くなることがわかった．

The formation of an Fe-Al intermetallic compound film on carbon steel by shot lining and heat treatment was investigated. In the experiment, a centrifugal-type peening machine with an electrical heater was employed. The shot medium was high-carbon cast steel. The substrate was a commercial carbon steel, and the sheet was commercially available pure aluminum. The shot lining process of carbon steel with an aluminum sheet was carried out at 300 °C in air using a peening machine. Heat treatment was performed at diffusion temperatures from 600 to 1050 °C in vacuum. The lined substrates exhibited a harder layer of Al-rich intermetallics, such as Fe 2Al 5, in the diffusion temperature range of 650 to 900 °C. When the temperature of the lined substrates was more than 930 °C, the surface was covered with thicker and highly anticorrosive layers of Fe-rich intermetallics, such as FeAl and Fe 3Al. We found that the present method could be used for the formation of functional films on carbon steel. © (2012) Trans Tech Publications, Switzerland.

The formability of the magnesium-lithium alloy was examined by multi-stage deep drawing. Long cups of magnesium alloy were formed at ambient temperatures by multi-stage deep drawing processes. In multi-stage deep drawing, the magnesium-lithium alloy sheets were employed and a flat sheet blank is formed into a cylindrical by a punch. Various cups were drawn by exchanging the punch and ringed die. The die was flat in the first stage, and was taper without a blankholder in the subsequent stages. The effects of the ratio of blank diameter to punch diameter and blank thickness on the deep drawability were examined. It was confirmed that the Mg-Li alloy long-cups were successfully formed by a multi-stage deep drawing operation in cold.

The cold joining of thin sheets using a shot peening process was investigated. In the joined section, the edge shape of two sheets is a concavo-convex. In this method, the convex edges of the sheet are overlapped the other sheet. When the connection is peened, the material of the convex area undergoes large plastic deformation near the surface due to the collision of shots. In this process, particularly noteworthy is the plastic flow near surface layer. The convex edges of the sheet are joined to the other sheet, thus two sheets can be joined each other. In the experiment, the shot peening treatment was performed by using an air-type peening machine. The shots used were made of high carbon cast steel. Air pressure was 0.6MPa and peening time was in the range of 30-150s. The peening conditions were controlled in the experiment. The metal sheets were commercial low-carbon steel, stainless steel, pure aluminium, pure copper, and magnesium alloy. The effects of processing conditions on the joinability were mainly examined. The joint strength increased with the kinetic energy of shots. It was found that the present method was effective for joining of thin metal sheets. ? 2011 IBF (RWTH Aachen) & IUL (TU Dortmund).

The formability in multi-stage deep drawing process of pure titanium sheet was Investigated. Pure titanium has very good ductility In cold forming. The normal anisotropy of pure titanium Is very high, about 5 or 6 of the r-value. Therefore, the property is suitable to the sheet metal forming, especially deep drawing. However, It Is very well known that the occurrence of seizure becomes remarkable in severe forming operations such as deep drawing and ironing. For the prevention, pure titanium sheets were treated by heat oxide coating. The fresh and clean titanium Is not in direct contact with the die during the forming due to the existence of the oxide layer. In multi-stage deep drawing, pure titanium sheets of the JIS grade 2 were employed. Various cups were drawn by exchanging the punch and ringed die. The effects of the coating conditions on the occurrence of seizure In multi-stage deep drawing were examined. It was found that the coating of titanium sheet has sufficient ability In preventing the seizure In multi-stage deep drawing processes. A long cup was successfully drawn by a multi-stage deep drawing operation with intermediate annealing. It was confirmed that the pure titanium cups were successfully formed by a multi-stage deep drawing operation. ? 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Weinheim.

In general, shot peening is a surface treatment that improves the performance of engineering components, since application of surface compressive stresses reduces the tensile component of stress. In the shot peening process, the medium consists of small spheres, which are usually made of high-carbon cast steel; the diameter of the spheres is in the range from 0.3 to 1.2 mm. More recently, a new type of microshot has been developed to enhance the peening effect. The diameter of the new spheres is in the range from 0.02 to 0.15 mm. The effect of microshot peening on the residual stress of spring steel was investigated. The projective method of the microshot was of the compressed air type. The microshot of 0.1 mm diameter was high-carbon cast steel and cemented carbide, and the workpiece used was the commercially spring steel JIS-SUP10. The effect that process variables such as shot speed and peening time have primarily on residual stress was studied. The surface layer of the workpieces was sufficiently deformed by microshot peening. The residual stress was observed near the surface. At a large number of cycles to fracture, microshot peening can more effectively enhance the fatigue strength. The use of hard microshots such as cemented carbide was found to cause a significantly enhanced peening effect for spring steel.

The shot peening process is one of the surface treatments. In this process the peening effects are characterized by the fact that the surface layer undergoes large plastic deformation due to the collision of shots. The effects are greatly influenced by the processing history or the thermal history of material. Little is known about the relation between hardness of the shot peened surface and the processing history of materials. In the present study, the effect of heating and subsequent re-shot peening on the surface characteristics of the shot-peened carbon steel was investigated. Shot peening was performed with an air-type machine using cast steel balls. Hardness and compressive residual stress in the re-shot peened workpieces were measured. When the heat treated workpiece was re-processed by shot peening, surface hardness and fatigue life of carbon steel were improved. It was found that the surface characteristics of the peened carbon steel were improved by heating and subsequent re-shot peening.

To improve the surface properties, lining of magnesium alloys with hard powders by shot peening was carried out in order. The hard powders were tried to bond to the workpiece surface due to the collision of many shots. In order to fix the hard powders to the surface of the workpiece, the powders were set on an uneven surface. To easily facilitate fixing of powders, lining of the workpiece with the powder sandwiched between two aluminum foil sheets was also attempted. In this experiment, a centrifugal shot peening machine with an electrical heater was employed. The workpieces were magnesium alloys AZ31B and AZ91D, and the hard powders were commercial cemented carbide, alumina, and zirconia. The joinability of hard powders near the lined surface was observed by a optical microscope. The wear resistance was also evaluated by a wear test. The hard powders were successfully bonded to the surface of workpieces by the shot lining process. The results show that the present method is effective in wear resistance of the magnesium alloys.

The formation of an Fe-Al intermetallic compound film on carbon steel by shot lining and heat treatment was investigated. In the experiment, a centrifugal-type peening machine with an electrical heater was employed. The shot medium was high-carbon cast steel. The substrate was a commercial medium, namely, the carbon steel S45C, and the sheet was commercially available pure aluminum. The shot lining process of carbon steel with an aluminum sheet was carried out at 300 °C in air using a peening machine. Heat treatment was performed at diffusion temperatures from 600 to 1050 °C in vacuum. The lined substrates exhibited a harder layer of aluminum-rich intermetallics, such as Fe₂Al₅, in the diffusion temperature range of 650 to 900 °C. When the temperature of the lined substrates was more than 930 °C, the surface was covered with thicker and highly anticorrosive layers of iron-rich intermetallics, such as FeAl and Fe₃Al. We found that the present method could be used for the formation of functional films on carbon steel.

Shot peening is a mechanical surface treatment that is widely used in the automotive industry to improve the fatigue life and surface characteristics of machine parts. To enhance the peening effect, warm peening and double peening are effective. These processes have recently received considerable attention. Our aim in this study is to investigate the effect of warm double peening on the surface layer characteristics of high speed steel. This peening process is a combination of double peening and warm peening. The first peening was conducted in the warm state using large steel media. Then, the second peening was conducted at room temperature using microshot media. In the present study, a compressed-air-type shot peening apparatus with a heating furnace was produced experimentally. The workpiece was made of the high speed steel JIS-SKH51. The surface roughness, hardness, compressive residual stress, and fatigue strength of the peened workpieces were measured. We found that the warm double-peening method causes a significantly enhanced peening effect for high speed steel.

This paper deals with the effect of temperature on plastic buckling strength of cylindrical pipe of some magnesium alloys subjected to shot peening process. Workpieces are three cylindrical pipes of AZ31, AZ6 I and AZ80 magnesium alloys. The shape of pipe is 30mm in height, 16mm and 14mm in outer and inner diameters. The shot peening treatment was given on the surface of cylindrical pipe using an air-type peening machine. Plastic buckling strength test was performed under axial compression in a case of both ends with fixed condition, at several temperatures between 293K and 573K and at a crosshead speed of 1 mm/min using an lnstron-type testing machine. From the experimental result, buckling stresses for all kinds of AZ31, AZ61 and AZ80 alloy pipe were strengthened after the shot peening treatment, but a few of differences for the increasing ratio. The buckling stress of the shot-peened pipe was kept higher than that of as received one up to the test temperature of 473K, however it tended to be around equal at 573K.

The pure titanium long-cups were formed by preventing the seizing in cold multi-stage deep drawing processes. For the prevention, pure titanium sheets were treated by heating oxide coating. The coating is very effective in preventing the occurrence of seizure and galling. The fresh and clean titanium is not in direct contact with the die during the forming due to the existence of the oxide layer. In multi-stage deep drawing, pure titanium sheets of the JIS grade 2 was employed. Various cups were drawn by exchanging the punch and ringed die. The die was flat in the first stage, and was taped without a blankholder in the subsequent stages. The effects of the coating conditions on the occurrence of seizure in multi-stage deep drawing were examined. It was found that the coating titanium sheet has sufficient ability in preventing the seizure in multi-stage deep drawing processes. The pure titanium long-cups were successfully formed by a multi-stage deep drawing operation in cold.

Shot peening is a surface treatment that improves the performance of engineering components. In conventional shot peening, the medium consists of small spheres, which are usually made of high-carbon cast steel; the diameter of the spheres is in the range from 0.3 to 1.2mm. More recently, however, a new type of microshot has been developed to enhance the peening effect. The diameter of the spheres in the new medium is in the range from 0.02 to 0.15mm. In the present study, the effect of microshot peening on the surface characteristics of spring steel was investigated. The injection method of the microshot was of the compressed air type. The microshots of 0.1mm diameter were high-carbon cast steel and cemented carbide, and the workpiece used was the commercially spring steel JIS-SUP10. The surface roughness, hardness and compressive residual stress of the peened workpieces were measured. The surface layer of the workpieces was sufficiently deformed by microshot peening. A high hardness or residual stress was observed near the surface. The use of hard microshots such as cemented carbide was found to cause a significantly enhanced peening effect for spring steel.

The cold joining of dissimilar metal sheets using a shot peening process was investigated. In shot peening the substrate undergoes large plastic deformation near the surface due to the hit with shots. Consequently, plastic flow areas formed by cold working may form the surface layer. The dissimilar sheets with the concavo-convex edge are connected, and then the contact area is shot-peened. In this joining, the convex edges of the sheet are laid on the other sheet. Namely, in the joining area, the two sheets are superimposed. In the experiment, the shot peening treatment was performed by using an air-type peening machine. The shots used were made of high carbon cast steel. Air pressure was 0.6MPa and peening time was in the range of 30-150s. The peening conditions were controlled in the experiment. The sheets were commercial low-carbon steel, stainless steel, pure aluminum, pure titanium, pure copper, and magnesium alloy. The effects of processing conditions on the joinability were mainly examined. The joint strength increased with the kinetic energy of shots. It was found that the present method was effective for cold joining of dissimilar metal sheets.

Shot peening is a surface treatment that improves the performance of engineering components. In conventional shot peening, the medium consists of small spheres, which are usually made of high-carbon cast steel; the diameter of the spheres is in the range from 0.3 to 1.2 mm. More recently, however, a new type of microshot has been developed to enhance the peening effect. The diameter of the spheres in the new medium is in the range from 0.02 to 0.15 mm. In this study, the effects of microshot peening on the surface characteristics and fatigue strength of cold tool steel were investigated. The projective method of the microshot was of the compressed air type. The peening microshots of 0.1 mm diameter were cemented carbide, and the workpiece used was the commercially cold tool steel JIS-SKD11. The surface roughness, hardness, compressive residual stress and fatigue strength of the peened workpieces were measured. The surface layer of the workpieces was sufficiently deformed by microshot peening. A high hardness or residual stress was observed near the surface. Also, the process improved the fatigue life of the workpieces at all stress amplitudes. The use of hard microshots such as cemented carbide was found to cause a significantly enhanced peening effect for cold tool steel.

In the shot peening process, the substrate undergoes large plastic deformation near the surface due to the hit with many shots. A large plastic deformation characterized by a shear droop occurs at the edge of the substrate. When the dissimilar sheets with the edge of the notch geometry are connected without level difference and then the contact area are shot-peened, the sheets can be joined due to the plastic flow generated by a large plastic deformation during shot peening. This method is similar to joining by caulking. The aim of this paper is to investigate the butt joining of high strength steel and dissimilar metal sheets using a shot peening process. The shot velocity and the coverage were controlled in the experiment. The shots used were made of high carbon cast steel and cemented carbide with an average diameter of 0.1 mm. The sheets were high strength steel and aluminum alloys. The influences of processing conditions on the joinability were mainly examined. The joint strength increased with the kinetic energy of shots. Tensile test was also examined to evaluate bond strength. It was found that the present method can be used to enhance the butt joining of high strength steel and dissimilar metal sheets.

In shot peening the substrate undergoes large plastic deformation near the surface due to the hit with shots. The plastic flow areas formed by cold working may form the surface layer. Authors have recently proposed new joining methods using shot peening, shot lining and shot caulking. Our approach has been applied to the butt joining of the dissimilar metal sheets. In the present study, joining of thin metal sheets using a shot peening process was investigated to improve the joinability. In the joined section, the edge of sheets is the equally-spaced slits. In this method, the convex edges of the sheet are laid on top of the other sheet. Namely, the two sheets are superimposed in the joining area. When the connection is shot-peened, the material of the convex area undergoes large plastic deformation near the surface due to the collision of shots. In this process, particularly noteworthy is the plastic flow near surface layer. The convex edges of the sheet can be joined to the other sheet, thus two sheets are joined each other. In the experiment, the shot peening treatment was performed by using an air-type peening machine. The shots used were made of high carbon cast steel. Air pressure was 0.6MPa and peening time was in the range of 30-150s. The peening conditions were controlled in the experiment. The thin sheets were commercial low-carbon steel, stainless steel, pure aluminum, and aluminium alloy. The effects of processing conditions on the joinability were mainly examined. The joint strength increased with the kinetic energy of shots. It was found that the present method was effective for joining of thin metal sheets.

The shot peening process is one of the surface treatments. The peening effects are characterized by the fact that the surface layer undergoes large plastic deformation due to the collision of shots. This action imparts compressive residual stress on the surface, thus improving the fatigue life of the component. Therefore, this process has been utilized in order to improve the performance of engineering components. Researchers have been found a number of new phenomena in the shot peening process. It is well known that the peening effects are greatly influenced by the processing history or the thermal history of material. The hardness near the surface of the deformed material or heat treated material was often decreased by shot peening. Presently, little is known about the relation between hardness of the shot peened surface and the processing history of materials. In the present study, the effects of shot peening conditions on the surface characteristics of medium carbon steels with different heat treatments were investigated. In the experiment, the shot peening process was performed with an air-type machine using cast steel balls. Air pressure is in the range from 0.4 to 0.8 MPa and coverage is from 200 to 2000 %. The workpiece was used the commercial medium carbon steel JIS-S45C. These are annealed at 900 degrees C for 7.2 ks and quenched and tempered from 850 degrees C in oil. Hardness, surface roughness, and compressive residual stress in the peened workpieces were measured. When the tempered workpiece was shot peened, the hardness of the surface was considerably lowered. The hardness distribution shows work softening near the surface. This amount increases with increasing coverage. This is due to the influence of processing heat generated by the plastic deformation during shot peening. The maximum residual stress appears about 840 MPa at about 0.180 mm in depth from the surface. It was found that the difference of the thermal history of the workpiece influences the hardness of the peened surface.

Microstructure and mechanical properties of high nitrogen steels containing l7Cr-l2Mn-3Mo produced by means of metal injection moulding and nitrogen absorption methods were examined. The compacts were sintered at 1300°C, then furnace-cooled to 1200°C and were held at that temperature for 2 to 20 hrs. Subsequently, they were furnace-cooled. Thereafter, solution treatment was done at 1200°C. The solution treated compacts held at 1200°C for above 5 hrs had homogeneous microstructure. Increasing the holding time at 1200°C, ferrite phase disappeared due to the increment of nitrogen content and grain size became larger. Lattice constant of austenite phase also increased. Yield and tensile strength increased with increasing nitrogen content. Yield and tensile strength of the compact held at 1200°C for 10 hrs were 758 MPa, 1172 MPa respectively and elongation was 41 %. However, ductile-brittle transition (DBT) behaviour was seen, and the DBT temperature of every compact was about -20 °C. From the fracture surface analysis, brittle fracture surface was identified as (111).

High nitrogen stainless steel (Fe-l7Cr-l2Mn-3Mo-IN) fabricated by metal injection molding (MIM) method has prominent strength and ductility. However, the tensile properties are unexplainable only by the influence of nitrogen solid solution strengthening. In this study, to clarify it, we have examined tensile-deformed microstructure. Strength of high nitrogen stainless steel was three times as high as that of SUS3 I 6L, without losing ductility, and micro-deformation twinnings were observed for the steels tensile-fractured at room temperature. It is thought that high strength with high ductility of this steel is attributed to solid solution strengthening of nitrogen and the formation of deformation twinning.

High-speed steels are generally used for the cutting of other hard materials. These are hard materials, and can be used at high temperatures. Therefore, some of them are used for warm metal forming such as forging. However, in the tools used in hot working, an excellent hot hardness and long-life fatigue are strongly required. In the present study, the influence of shot peening on the surface characteristics of high-speed steels was investigated. Shot peening imparts compressive residual stresses on the metal surface, thus improving the fatigue life of the machine parts. In the experiment, the shot peening treatment was performed using an air-type shot peening machine. The shots made of cemented carbide were used. The workpieces were two types, W-type and Mo-type alloys. Surface roughness, compressive residual stress, and hardness of the peened work-pieces were measured. It was found that shot peening using the hard shot media was effective in improving the surface characteristics of high-speed steels.

Aluminum and magnesium materials are very attractive for light weight applications. However, their use is complicated by the fact that dissimilar metals are joined by fusion welding. In the present study, the cold butt joining of light metal sheet with dissimilar material sheet by shot peening was investigated. The shot peening process is widely used to improve the performance of engineering components. In this process the substrate undergoes a large plastic deformation near its surface when hit by many shots. The substrate material close to the Surface flows during shot peening. When the dissimilar metal sheets with notched edges are connected without a level difference and then the connection is shot peened, the sheets can be joined by the plastic flow generated by the large plastic deformation during shot peening. In this experiment, an air-type shot peening machine was used. The influences of peening time and shot material on joinability were mainly examined. The joinability was evaluated by tensile test. The joint strength increased with the amount of plastic flow. It was found that the present method can be used to enhance the butt joining of the light metal sheets with the dissimilar material sheets.

Various mechanical properties of magnesium alloys have been reported by many researchers. Some researchers have studied the plastic buckling behaviors of magnesium alloys, but the effect of shot peening on plastic buckling strength of magnesium alloys has not been researched yet. In this paper, we describe the effect of shot peening on the plastic buckling behavior of a cylindrical pipe of AZ31(Mg-3%Al-1%Zn) magnesium alloy subjected to axial compressive load. The pipe was exposed to three types of shot peening, and a buckling test on the magnesium alloy pipe with a 38 mm outer diameter, a 35.6 mm inner diameter and a 100 mm gauge height was carried out with both ends of the pipe fixed. The main experimental results are summarized as follows: the buckling stress of the pipe exposed to shot peening is higher than that of the as-received one at 293 K, and is almost the same as that of the as-received one at 573 K.

Aluminum and magnesium materials are very attractive for light weight applications. However, their use is complicated by the fact that dissimilar metals are joined by fusion welding. In the present study, the cold butt joining of light metal sheet with dissimilar material sheet by shot peening was investigated. The shot peening process is widely used to improve the performance of engineering components. In this process the substrate undergoes a large plastic deformation near its surface when hit by many shots. The substrate material close to the Surface flows during shot peening. When the dissimilar metal sheets with notched edges are connected without a level difference and then the connection is shot peened, the sheets can be joined by the plastic flow generated by the large plastic deformation during shot peening. In this experiment, an air-type shot peening machine was used. The influences of peening time and shot material on joinability were mainly examined. The joinability was evaluated by tensile test. The joint strength increased with the amount of plastic flow. It was found that the present method can be used to enhance the butt joining of the light metal sheets with the dissimilar material sheets.

Many machine parts are used in manufacturing industry, such as automobile and aircraft. In association with the reduced size, the need for higher strength and fatigue life of parts becomes increasingly severe. One of these high demands on automotive engineering is the use of advanced materials in combination with lightweight. Nowadays an increasingly wide variety of the engineering materials is available. Quite a few of science and technology researchers have developed new materials, which have many superior properties. On the other hand, in the machine parts surface treatment is necessary to improve resistance to wear and corrosion. The surface characteristics can be modified and improved by various surface treatments. Several processes are used for surface treatments. The shot peening process is one of the surface treatments. In this process the peening effects are characterized by the fact that the surface layer undergoes large plastic deformation due to the collision of shots. The effects are greatly influenced by the processing history or the thermal history of material. For example, the hardness near the surface treated by the deformation or heat treatment is often decreased by shot peening. It is well known to cause work softening. However, little is known about the relation between hardness of the shot peened surface and the processing history of materials. In the present study, the influence of shot peening on the surface characteristics of carbon steel JIS-S45C was investigated. Shot peening was performed with an air-type machine using cast steel balls. Hardness, surface roughness, and compressive residual stress in the peened workpieces were measured. When the tempered workpiece was shot peened, the hardness of the surface considerably fell down. It was found that the hardness of the peened surface was influenced by the different thermal history of the workpiece.

Shot peening is one of the surface treatments, and improves the performance of engineering components. In a conventional shot peening process, the peening media consists of small spheres. The shot media is usually made of high carbon cast steel; the diameter is in the range from 0.3 to 1.2 mm. More recently, a new type of shot media has been developed to enhance peening effect. The peening media called microshot or fine-particle are smaller and harder than the conventional one. These media are made of cemented carbide, amorphous alloy, and high speed steel. The diameter of new media is in the range from 0.02 to 0.15 mm. The use of new media is effective for the hard material such as tool steel. Many studies have been carried out to investigate the effect of new media on the fatigue life of the tool steel. However, little is known about the relation between the surface characteristics of tool steels and shot peening by using new media. In the present study, the influence of shot peening on the surface layer characteristics of tool steel was investigated. The projective method of the shot was a compressed air type. The peening shots of 0.1mm diameter were cemented carbide and the workpiece was commercially tool steel JIS-SKD11. Surface roughness, compressive residual stress and hardness in the peened workpiece were measured. The effect of shot peening on the fatigue strength characteristics of tool steel was also examined. The use of hard shot such as cemented carbide was found to cause a significantly enhanced peening effect for tool steel.

Aluminum and magnesium materials are very attractive for light weight applications. However, their use is complicated by the fact that dissimilar metals are joined by fusion welding. In the present study, the cold butt joining of light metal sheet with dissimilar material sheet by shot peening was investigated. The shot peening process is widely used to improve the performance of engineering components. In this process the substrate undergoes a large plastic deformation near its surface when hit by many shots. The substrate material close to the surface flows during shot peening. When the dissimilar metal sheets with notched edges are connected without a level difference and then the connection is shot peened, the sheets can be joined by the plastic flow generated by the large plastic deformation during shot peening. In this experiment, an air-type shot peening machine was used. The influences of peening time and shot material on joinability were mainly examined. The joinability was evaluated by tensile test. The joint strength increased with the amount of plastic flow. It was found that the present method can be used to enhance the butt joining of the light metal sheets with the dissimilar material sheets.

High-speed steels are generally used for the cutting of other hard materials. These are hard materials, and can be used at high temperatures. Therefore, some of them are used for warm metal forming such as forging. However, in the tools used in hot working, an excellent hot hardness and long-life fatigue are strongly required. In the present study, the influence of shot peening on the surface characteristics of high-speed steels was investigated. Shot peening imparts compressive residual stresses on the metal surface, thus improving the fatigue life of the machine parts. In the experiment, the shot peening treatment was performed using an air-type shot peening machine. The shots made of cemented carbide were used. The workpieces were two types, W-type and Mo-type alloys. Surface roughness, compressive residual stress, and hardness of the peened workpieces were measured. It was found that shot peening using the hard shot media was effective in improving the surface characteristics of high-speed steels.

To investigate the role of stress-induced martensitic transformations in the TRIP effect, stress-strain curves and TRIP effects in two types of metastable austenite steel (JIS-SUS304 and JIS-SUS301L) were compared. In static tensile tests, the tensile strength (TS) of SUS301L steel was larger than that of SUS304, and the 0.2% proof stress and uniform elongation (U.El) were both similar for the two steels. The transformation rate and strength of martensite in the stress-induced martensitic transformation were compared by X-ray diffraction and Vickers hardness tests. At the same strain, SUS301L contained a larger volume fraction of martensite than did SUS304. Both steels showed similar values of the Vickers hardness. The difference between SUS301L and SUS304 is mainly due to the transformation rate of stress-induced martensite. The effects of the transformation rate and martensite strength on the TS/U.El balance in metastable austenite steels were demonstrated by means of the Weng secant method, based on a micromechanical model. ? 2008 The Japan Institute of Metals.

Stress-strain curves for ferrite-cementite (FC) steels with ferrite grain sizes between 0.47 and 13.6 mu m were studied by tensile tests with strain rates of 10(3), 10(0), and 3.3 x 10(-4) s(-1) at 296 K. The stress-strain curves for the FC steels are categorized into two different types. In one type, the Luders deformation is interrupted due to the onset of necking, and in the other type, the Luders band propagates throughout the gage section of a tensile specimen followed by work-hardening. The lower yield and flow stresses increase while uniform and total elongations decrease with a decrease in ferrite grain size. The effect of ferrite grain size on flow stress is hardly dependent on strain rate. These experimental results reveal that the grain refinement strengthening contributes mainly to an increase in the athermal stress component. (C) 2007 Elsevier B.V. All rights reserved.

Magnesium and its alloys are very attractive for light weight applications. However, their use is complicated by the fact that dissimilar metals are joined by fusion welding. In the present study, the ability of shot peening to enhance butt joining of a magnesium alloy sheet with dissimilar material sheets was investigated. Shot peening improves the performance of the engineering components. In this process the substrate undergoes a large plastic deformation near its surface when hit by many shots. The substrate material close to the surface flows during shot peening. This plastic flow is characterized by a shear droop at the edge of the substrate, namely, the peened material overflows at the edge. When the dissimilar metal sheets with notched edges are connected without a level difference and then the connection is shot-peened, the sheets can be joined by the plastic flow generated by the large plastic deformation during shot peening. In this experiment, an air shot peening machine was used. The influences of air pressure and peening time on the joinability were examined. The joint strength increased with peening time, i.e., the amount of plastic flow. It was found that the present method can be used to enhance the butt joining of a magnesium alloy sheet with a dissimilar material sheet.

The effects of microshot peening on the surface characteristics of high-speed tool steel were investigated. Shot peening is widely used in automotive industry to enhance fatigue life and the surface characteristics of machine parts. More recently, new shot media have been developed to enhance the peening effect. New media is smaller and harder than the conventional one. The use of new media is effective in improving the surface of tool steels. The aim of this paper is to present the enhanced peening effect for the high-speed tool steel. In the present study, a compressed air-type microshot peening apparatus with a heating furnace was produced experimentally. Peening microshot was made of cemented carbide, and the workpiece was the high-speed tool steel. Surface roughness, compressive residual stress, and hardness of the peened workpieces were measured. The distribution of residual stress in the thickness direction was obtained from the X-ray diffraction method. The effects of processing temperature on the surface characteristics and the effect of microshot peening on fatigue characteristic were also examined. The use of hard microshots was shown to cause a significantly enhanced peening effect for high-speed tool steel. (C) 2007 Elsevier B.V. All rights reserved.

The features of sheared edges in warm and hot punching of an ultra high strength steel sheet using resistance heating were examined. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased. The rollover depth and burr height of the sheared edge markedly increased from about 800°C. The roughness level of the fracture surface became high above 650°C, whereas that of the burnished surface was almost constant. The punching load was considerably reduced by the heating, i.e., 40% of the cold punching load at 650°C and 15% at 1070°C. The effects of the punching speed and the clearance between the punch and the die on the surface quality of the sheared edge became marked at high temperatures. It was found that the warm and hot shearing of ultra high strength steel sheets is effective for improving the surface quality of the sheared edge and in reducing the shearing load.

Microstructure and mechanical properties of high nitrogen steels whose chemical composition were Fe-17Cr-12Mn-3Mo-0.2N and that was produced by using metal injection moulding method and nitrogen absorption methods were examined. Especially, we have paid careful attention for nitrogen gas introducing temperature, also we investigated the effect of the grain size on hardness. Sintering was performed at the temperature range of 1050 to 1250 in a vacuum, and then nitrogen gas was introduced. Specimens were held at 1300 for 3hrs under nitrogen atmosphere, and then furnace cooled to 1200. Specimens were held at that temperature for 5hrs to absorb nitrogen and give homogeneous microstructure. After held at 1200, specimens were furnacecooled Finally, they were solution-treated at 1200. To refine grain size, reverse-transformation heat treatment was performed. Porosity of compacts was deeply influenced by gas introducing temperature, and increasing the temperature, porosity decreased and hardness of compacts increased. The strength and elongation of the compact to which nitrogen gas was introduced at 1250was about 1050MPa and 60% , respectively. The SUS316L which has relatively high density similar to the high nitrogen compact and was made by means of MIM showed much lower tensile strength and elongation.

In the present study, a joining process for carbon steel using shot peening was investigated to improve surface characteristics such as corrosion and oxidation resistance. Shot peening treatment was performed by using an air-type peening machine. The apparatus with a heating furnace was fabricated to examine the effect of the working temperature. The shot velocity and the coverage are controlled in the experiment. The substrate was the commercial carbon steel S45C, and the foils were the commercial pure aluminium, pure copper, and pure nickel. The effects of shot conditions and the heating temperature on the joinability were examined. The metal foils were successfully bonded to the surface of the substrate. The dissimilar laminated foils were joined to the substrate without cracks and voids at the interface. It was found that surface properties of carbon steel could be improved by the joining process using shot peening.