Rolling Contact Fatigue Technical Publications
188. Surface Densification Coupled with Higher Density ProcessesTargeting High-Performance Gearing: This paper will describe a powder and processing method that facilitates single press-single sintered densities approaching 7.5 g/cm³. At this sintered density, mechanical properties of the powder metal (PM) component are significantly improved over current PM technologies and begin to approach the performance of wrought steels. High performance gears have the added requirement of rolling contact fatigue durability that is dependent upon localized density and thermal processing. Combining high density processing of engineered PM materials with selective surface densification enables powder metal components to achieve rolling contact fatigue durability and mechanical property performance that satisfy the performance requirements of many high strength automotive transmission gears. Data will be presented that document PM part performance in comparison to conventional wrought steel grades.
175. Surface Densified PM Steel - Comparison With Wrought Steel Grades: The next major opportunity of PM steels is in automotive transmission gears. The stress levels in these types of gears arise from rolling contact of gear teeth with certain amount of sliding. RCF studies have been undertaken to study the fatigue properties of powder metal gears using ZF test rig. AISI 8620 grade steel was also tested. Results of these investigations suggest that P/M gears are competitive with wrought steel grades.
169. Rolling Contact Fatigue Performance Contrasting Surface Densified, Powder Forged, and Wrought Material: Previous experimental work demonstrated that rolling contact fatigue durability of high-density powder metallurgy samples was influenced by depth of surface densification (achieved via roll densification), sintering temperature, and heat treat practice. One observation of the previous work was reduced rolling contact fatigue life at high Hertz stress levels relative to wrought machined steel samples. There were also some questions regards the influence of nickel rich regions and how they affected rolling contact fatigue performance. In an effort to understand the influence of elemental nickel additions, FLN2-4405 samples were sintered at 2050 °F (1120 °C) and 2300 °F (1260 °C) and subsequently powder forged to full density. This experimental work was designed to clarify the effects of elemental nickel additions on rolling contact fatigue durability. Additionally, wrought AISI 8620 carburizing steel was machined into rolling contact fatigue samples, carburized and tested. Additionally, the AISI 8620 was evaluated for tensile, impact and fatigue characteristics in the quench and tempered condition.
160. Surface Densification Approach to High Density Gears: High performance components for automotive gearing applications have requirements incorporating high static strength, high bending fatigue, and lastly rolling contact fatigue durability. Advances in PM alloys and processing can produce as-sintered densities approaching 7.4 g/cm³ in complex helical gearing geometries. This high sintered density results in high static and fatigue resistance. However, to achieve the rolling contact fatigue properties required in high performance gears, fully dense surface and sub-surface conditions are necessary. This paper will investigate the effects of part processing and surface densification on the rolling contact fatigue properties of a high density FLN2-4405 material. Variable studied include depth of densification, sintering conditions, surface microstructure, and post densification heat treatment practices. The results will demonstrate the effects of residual porosity, carburizing practice, and the effects of soft-nickel rich regions (as influenced by the sintering practice) on the rolling contact fatigue properties. Metallographic analysis will examine the cause of the failures leading to future improvements.
137. Rolling Contact Fatigue of Surface Densified Material: Microstructural Aspects (Surface Densification Approach to High Density Gears): Automotive gearing applications have material requirements combining static strength, bending fatigue, and rolling contact fatigue durability. Advances in PM alloys and processing can produce as-sintered densities greater than 7.4 g/cm³ in complex gearing geometries. This high sintered density results in high static and fatigue resistance. However, at less than full density rolling contact fatigue performance is compromised. For high duty cycle gearing, pore free density is needed in the tooth contact region and in the area where the tooth flank intersects with the gear tooth root radius. This paper will investigate the effects of part processing and surface densification on the rolling contact fatigue properties of a high density FLN2-4405 material. Variables studied include: depth of densification, sintering conditions, surface microstructure, and post densification heat treatment practices. The results will demonstrate effects of residual porosity, case microstructure, and soft-nickel rich regions on rolling contact fatigue. Metallographic analysis will illustrated the cause of the failures associated with these variables.