Development of Ni-modified Alloy Steel for Power Transmission Gear Material and Investigation of its Fatigue Failure
No Thumbnail Available
Date
2021-11
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Transmission gears have been working under severe working situations of loads and rotations, due to these situations, the properties and qualities of gear materials are greatly influenced. Consequently, contact fatigue failure is instigated. So, improving the mechanical properties of the existing gear material is very vital since these properties have a direct correlation with gear fatigue failure. Investigations were carried out to determine the mechanical properties of Ni-modified alloy steels by adding 1.55%, 1.75%, and 1.95% of the Ni-content to the existing Cr-Mo alloy steel, by ANN modeling that correlates the complex relationship of the input and output parameters and verified by experimental test. The investigation of these material properties with ANN modeling and experimental tests shows that the more Ni-content added to Cr-Mo alloy steel, the higher the ultimate and yield strength achieved. Likewise, fracture toughness, impact toughness, and percent of retained austenite of these materials were also investigated thoroughly. Thus, results showed that 1.55 % Ni- modified Cr-Mo alloy steel has a higher value on both impact toughness and fracture toughness without sacrificing yield strength compared with other Ni-modified counterparts. Therefore, based on both ANN and experimental results, 1.55 % of Ni- modified Cr-Mo alloy steel showed a better fatigue failure resistance. To address the behavior of lower alloying Ni-contents, the study includes 1.15% and 1.35% Ni-contents and explored with the ANN. Then, the results still indicate that a 1.55% Ni-content has better fatigue failure resistance.
After exploring the best Ni-modified Cr-Mo alloy steel (1.55% of Ni-modified Cr-Mo alloy), based on ANN and experimental approaches, design and fatigue analysis of a single-speed transmission gear was carried out for further confirmation. The methods of design and analysis employed were KISSsoft gear simulation software and AGMA standard. Explorations have been done by altering gear parameters like helix angle, face-width, and input torque to get fitting safety factors, fatigue stresses, and smooth operation of the transmission gear pair. Comparing the two materials, Ni-modified Cr-Mo alloy steel has a higher impact load-carrying capacity manifested by bending safety factor compared to existing gear material. However, there are no significant differences in terms of contact load-carrying capacity expressed in contact safety factors between the sample materials.So, further investigation was needed to identify the difference in surface durability and to verify the mechanical property results.
Then, Rolling contact fatigue (RCF) experimental test was carried out between 1.55% Ni-modified Cr-Mo alloy steel and existing material. In the RCF test of gears, micropitting was found to be the most vital damage property to characterize the rolled surface of disc samples. In this experiment, RCF tests were done on disc samples, two materials of Cr-Mo alloy steel, and 1.55%Ni-modified Cr-Mo alloy steels to evaluate the surface damage and topography of these materials. The methods utilized to determine the existence of micropitting on these materials were done on an adapted twin-disc machine. It is intended through this test to simulate asperities contact on surfaces of mating gear flanks using disc samples. The disc samples used in this experiment were contained low-speed specimens that are cylindrical-shaped discs, and high-speed specimens consist of crowned-shaped discs to attain a minimum effective contact area of 8.5mm. Thereafter, completing the RCF test, surface topographies were examined by employing SEM and OM for each measure of changes in surface topography and morphological alterations. As revealed from the post-processed micrographs, the 1.55% Ni-modified Cr-Mo alloy steel has better contact fatigue failure resistance compared with the Cr-Mo alloy steel in terms of Micropitted area ratio, pitted depth, and the number of pits. As an additional justification, verifying the result with the previous related literature of having a higher Ni-content (2%) was also compared with the 1.55% Ni-modified Cr-Mo alloy steel under the same conditions owes a 5 % micropitting ratio, and this indicates 1.55% Ni-modified has better surface durability. Thus, the 1.55% Ni-modified Cr-Mo alloy steel is recommended to use for transmission gears with high RCF damage suspicious.
Description
Keywords
Ni-modified, contact fatigue failure, rolling contact fatigue, twin-disc test rig, micropitting