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A new variable compression turbo (VC-Turbo) engine, which has a multi-link system for controlling the compression ratio from 8:1 to 14:1, requires high axial force for fastening the multi-links because of high input loads and the downsizing requirement. Therefore, it was necessary to develop a 1.6-GPa tensile strength bolt with plastic region tightening. One of the biggest technical concerns is delayed fracture. In this study, quenched and tempered alloy steels were chosen for the 1.6-GPa tensile strength bolt.

As trends of automobile engine exhaust gas temperature are reducing emissions, the material for the exhaust components have been changed from ductile irons to ferritic cast alloys or stainless steel, further to austenitic cast alloys for higher performance engines. The current austenitic alloys, however, have thermal fatigue failure over 1273K. The authors developed excellent thermal fatigue resistant austenitic cast alloys, by investigating the effects of alloying elements on strength and thermal expansion, which correlate with thermal fatigue property. Developed alloys are expected to apply to exhaust components at gas temperatures over 1273K.

Ferritic stainless steel gas metal arc welding (GMAW) wires have been widely using for automotive exhaust system components made of ferritic stainless steels. In order to enhance the high temperature strength of weld metal, it is necessary to make the microstructure of weld metal finer. In this study, the effect of the chemistry of ferritic stainless steel GMAW wire on the weld metal microstructure was investigated and new ferritic stainless steel GMAW wire providing fine grain microstructure of the weld metal was developed to improve high temperature mechanical properties, oxidation resistance, corrosion resistance of the weld metal and weldabiliy of the wire.

Improving the impact strength of the differential gears is one way to reduce the size and weight of the final drive unit. Previously, we developed high-strength steel for gear use by adding molybdenum and reducing impurities such as phosphorus and sulfur. However, additional improvement of impact strength is required these days due to higher engine torque and demands for further weight reductions. Toward that end, we focused on boron, which has been used as an element for improving hardenability, and analyzed what effect its addition would have on impact strength. Useful knowledge was obtained for improving impact strength through enhancement of grain boundary toughness. Various steels were then produced experimentally and used in gear strength tests. The results made it possible to improve impact strength while reducing the content of other alloys, resulting in the development of a chromium-molybdenum-boron case hardening steel with superior cold forgeabilty.

It was found that pitting resistance of gears is strongly influenced by resistance to temper softening of carburized steel. The investigation about the influence of chemical compositions on hardness after tempering revealed that silicon, chromium and molybdenum are effective elements to improve resistance to temper softening and pitting resistance. Considering the production of gears, molybdenum is unfavorable because it increases hardness of normalized or annealed condition. Developed new steel contains about 0.5 mass% of silicon and 2.7 mass% chromium. The new steel has excellent pitting resistance and wear resistance. Fatigue and impact strength are equivalent to conventional carburized steels. Cold-formability and machinability of the new steel are adequate for manufacturing gears because of its ordinary hardness before carburizing. The new steel has already been put to practical use in automatic transmission gears. Application test results are also reported.

Engine valve development trends are to first, reduce the costly metal content and secondly, increase strength or reduce weight. These developments can be used to reduce valve cost or fuel consumption or increase power. The authors developed a new strain age hardening type alloy, NCF2415C, which has both good cold forgeability and heat resistance. Its chemical composition is Fe-24Ni-15Cr-2.2Ti-1.5Al-0.5Nb-0.02C-.006B-2Cu. This new alloy and the establishment f cold forging technology made it possible to develop cold forged exhaust valves having durability equal or better than the conventional hot forged exhaust valves.

Honda has developed, in collaboration with Hitachi Metals and Daido Steel, two types of low-nickel heat-resistant alloys for exhaust valves which are more cost effective than the conventional nickel alloys. They are NCF4015 that contains approximately 40% nickel and NCF3015 with approximately 30% nickel content. The two types of new alloys were developed based on our unique alloy design concept. Both alloys feature superb high-temperature strength and are capable of maintaining favorable material properties, even after an high-temperature exposure. The NCF4015 is compatible with the conventional Inconel 751 and 60Ni alloys in terms of high-temperature strength. The NCF3015 falls slightly behind the two metals, but overwhelms the 21-4N (SUH35) in high-temperature strength. The exhaust valves made of the two alloys developed have been used for mass production engines.

A new high-performance and lightweight TiA1 intermetallic compound exhaust valve has been developed. The TiA1 valve can improve power output and fuel economy by contributing higher engine speeds and a reduction in valvetrain friction. It was achieved by developing a Ti-33.5A1-0.5Si-1Nb-0.5Cr (mass%) intermetallic compound, a precision casting method for TiA1 that provides a low-cost, high-quality process, and a plasma carburizing technique for assuring good wear resistance on the valve stem end, stem and face.

The application of both high strength gear steels and shot peening technology has succeeded in strengthening automotive transmission gears. This technology, though, improves mainly the fatigue strength at the tooth root, but not the pitting property at the tooth face. Therefore, demand has moved to the development of new gear steels with good pitting resistance. In order to improve pitting resistance, the authors studied super carburizing which is characterized by carbide dispersion in the case, especially processed with a plasma carburizing furnace. Firstly, the influence of the carburizing temperature and carburizing period on the carbide morphology was investigated and the optimum carburizing conditions were determined. Secondly, the fatigue strength and pitting resistance was evaluated using carbide dispersed specimens.

Some fundamental research on alloy design of the new titanium alloy and process design such as forging and surface treatment were carried out in order to develop new titanium alloy connecting rods. Free machining Ti-3Al-2V alloy is the best alloy for connecting rods because it has mechanical properties equivalent to quenched and tempered medium carbon steels, a popular material for connecting rods. The alloy can be machined at higher speed than the most popular titanium alloy Ti-6Al-4V. Forging in the β phase temperature range is desirable to enable one heat forging of connecting rods and to eliminate crack initiation. New technology such as induction heating in forging, pickling after forging and coating on large ends were developed. As a result of this research, new titanium alloy connecting rods which can be applicable to mass production have been developed.

Excellent fuel economy and high performance have been urgent in Japanese automobile industries. With increasing engine power, many of the power train components have to withstand higher loads. Differential pinion gear being one of those highly stressed parts, excellent fatigue and shock resistance have been demanded. At first the fundamental study on the fatigue and impact crack behavior of carburized components was studied and the new grade composed of 0.18%C-0.7%Mn-1.0%Cr-0.4%Mo was alloy designed. Furthermore, Si and P is reduced less than 0.15 and 0.015%, respectively aiming at the reduction of intergranular oxidation and improved case toughness. The differential gear assembly test has proved that the new grade shows three times as high impact strength as that of conventional steel, SCM418, and almost the same as that of SNCM420 containing 1.8%Ni.

JIS SCM440M (SAE4140H), heat treated to the strength level of 120 to 140 kgf/mm2(171 to 199 ksi) -ISO 12.9 class-, is currently used for cylinder head bolts of Japanese passenger cars. Lower alloy steels, such as SAE 1541 for example, have not been substituted for JIS SCM440H so far because of their high susceptibility to delayed fracture. Daido Steel has tackled this problem and succeeded in applying the lower alloy SAE 1541 steel to 12.9 class cylinder head bolts by enhancing the resistance to delayed fracture by reducing impurities, especially sulphur. In this paper mechanical properties and delayed fracture characteristics of SAE 1541-ULS (Ultra Low Sulphur) steel are reported. 1541-ULS (S<0.005%, S+P< 0.020%) shows outstanding resistance to delayed fracture compared to conventional steel. Furthermore, the amount of MnS inclusions decreases remarkably in ULS steel, which results in high toughness.

In order to save molybdenum (Mo) in chromium - molybdenum steels for automobile components, medium carbon - boron steels were investigated. Boron is not a new alloying element for structural steels, however, to date boron steels have not been widely used because of their unstable hardenability and poor machinability. Therefore, in this paper, the optimum content of boron was reexamined, and also the appropriate addition of titanium as a stabilizer of boron was investigated from the view point of hardenability. Furthermore the upper limit of manganese (Mn) content was studied to keep good machinability. The new steel grades, established on the basis of the above fundamental research, have been used on vital components of passenger cars.

For heavy duty gasoline or diesel engines, exhaust valves of 21-2N or 21-4N are generally coated by cobalt-base hard facing alloys, at faces. But the sluggish supply and the spiraling price of cobalt recently forced automakers to adopt valves of high-grade superalloys without hard facing. Candidate superalloys for high-performance exhaust valves are gamma-prime strengthened nickel-base alloys such as Inconel 751 and Nimonic 80A. Unfortunately above-mentioned alloys are too expensive for automobile components. So authors tried to develop a gamma-prime strengthened iron-base alloy, which bears basically 40% Ni-19% Cr-Al-Ti. To optimize Al and Ti contents the effect of the total amount and the ratio of them was examined thoroughly on hardness, strength and corrosion resistance of experimental alloys at elevated temperatures.