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Fracture split steel connecting rod has been developed for new passenger diesel engines for its advantages in cost saving and better performance. The splitting type of steel con rod is made of high carbon micro-alloyed steel with no additional heat treatment after hot forging. This con rod blank is forged in one-die mold and later fracture splitted. Unlike the conventional types where the rods and caps are separately forged and machined, this steel split con rod needs no additional rod/cap contact face milling which means a substantial savings in machining cost. Besides, a firm contact between rod and cap improves stiffness and compatibility with other crank-train moving parts - a definite merit in engine performance. Our research work focuses on optimizing the two major technologies in this subject - microstructural analysis of controlled cooling (high carbon micro-alloyed steels) and detailed fracture splitting parameters and test results.

The service life of engine crankshaft is closely related to its material characteristics. Without any dimensional modification, one can significantly improve the fatigue life of crankshaft by applying various surface treatments. The objective of this research is to predict and to quantify the fatigue life of crankshaft at different alloy design and surface treatments - microalloyed steel vs. carbon steel as well as nitriding vs. fillet rolling. It is found that nitriding improves more than 60% in fatigue life depends upon matrix hardness. This study also demonstrates a relationship between fatigue life and fillet rolling force. We observed that the fillet rolled specimens with the applied force of 500∼900kgf exhibit 40∼80% improvement in fatigue life compared to non-treated sample. Consequently, methods are proposed to quantify and normalize fatigue life at various treatments to optimize crankshaft design for maximizing the engine performance.

A newly developed surface hardening process, supercarburizing, has been developed for the application of tappet shim to improve fuel economy. Supercarburizing has been introduced to increase resistance of wear and pitting performance and was designed to have supersaturated carbon surface layer and further to have spheroidized carbide morphology. In this presentation, the process variables, such as surface microstructure, morphology and distribution of carbide precipitation, will be discussed via the results of friction loss tests. At an entire speed range investigated, the application of supercarburized tappet shim improved fuel economy with 25∼30% in terms of valve train itself and with 4∼5% concerning on the gross engine performance. The fuel economy analysis showed that the improved surface hardening process of tappet shim increased fuel economy of vehicle about 1.4∼3.6%.

A new technique of heat treatment is developed for the bearings of automotive transmission and chassis to maximize their service life under contaminated and severe environments. This study demonstrates an improvement of the microstructure of bearing steels by applying special heat treatments. The microstructure is developed by optimizing various heat treating parameters (temperature, cycle time and gas atmosphere, etc.) as well as by modifying the quenching processes (double quenching and press quenching). We obtained a desirable microstructure of dense and fine martensite with optimum levels of retained austenite and compressive residual stress on the subsurface. The size and distribution of carbides and grains are found to be very fine and homogeneous. The endurance test results show that the specimens with new treatment have an excellent fatigue life compared with the conventional bearing samples.

The effects of tempering on carburized Cr-Mo gear steel were investigated through mechanical and fatigue tests. Specimens were carburized at 900°C for 180 minutes, and then oil quenched at 150°C for 10 minutes of holding time and cooled to room temperature. The subsequent tempering process was performed to 160°C for 90 minutes. Surface hardness and residual compressive stress were decreased by tempering treatment, whereas tensile strength, yield strength and impact energy were increased. Bending fatigue endurance limits for both tempered and untempered specimens were same as 779MPa. The strength of roller contact fatigue is also not greatly influenced by tempering treatment. Thermal distortion for carburized transfer driven gear before and after tempering exhibited a similar distribution. Microstructural changes during tempering were also discussed.

The banded microstructure of pearlite and ferrite in normalized alloy steel is susceptible to thermal distortion during carburizing process due to its unidirectional orientation parallel to rolling direction. The planetary gears with material of banded microstructure have been experienced in high thermal distortion during carburizing and quenching process and result in uneven surface hardness and effective case depth at the inside of pinion gear after honing. These defects played failure initiation site roles in durability test during development of new automatic transmission. The galling between the contacting components in severe lubricating system was the main failure mechanism. Double normalizing at 920 °C was designed to resolve the banded microstructure of normalized alloy steel. The microstructure and grain size of the double heated steel became equiaxed and fine due to homogenizing and recrystallization through double heat treatment.