JMEPEG (1996) 5:728-733 9 International Investigation into Nitrided Spur Gears B.S. Yilbas, A. Coban, J. Nickel, M. Sunar, M. Sami, and B.J. Abdul Aleem The cold forging method has been widely used in industry to produce machine parts. In general, gears are produced by shaping or hobbing. One of the shaping techniques is precision forging, which has several advantages over hobbing. In the present study, cold forging of spur gears from Ti-6AI-4V material is in- troduced. To improve the surface properties of the resulting gears, plasma nitriding was carried out. Nu- clear reaction analysis was carried out to obtain the nitrogen concentration, while the micro-PIXE technique was used to determine the elemental distribution in the matrix after forging and nitriding processes. Scanning electron microscopy and x-ray powder diffraction were used to investigate the met- allurgical changes and formation of nitride components in the surface region. Microhardness and fric- tion tests were carried out to measure the hardness depth profile and friction coefficient at the surface. Finally, scoring failure tests were conducted to determine the rotational speed at which the gears failed. Three distinct regions were obtained in the nitride region, and at the initial stages of the scoring tests, fail- ure in surface roughness was observed in the vicinity of the tip of the gear tooth. This occurred at a par- ticular rotational speed and work input. Keywords [ plasma nitriding, spur gear 1. Introduction THE COLD AND WARM forging of metals has gained impor- tance due to the increasing demand for mass production of pre- cision parts in industry. Depending on the temperature of the slug, the processes are called warm, hot. or cold forging (Ref 1). Warm forging may have several advantages, including bet- ter utilization of material, improved surface finish, and better dimensional accuracy when compared with hot forging, and re- duced press loads when compared with cold forging (Ref 2). On the other hand, die-tool wear is considerable in warm forg- ing. Alternatively, cold forging is a well-known process and is being developed continuously for producing geometries of ever-increasing complexity. It has been reported that cold forg- ing produces parts with excellent tolerances and surface finish due to lack of the thermal expansion and scale formation found in hot forming (Ref 3). Consequently, postforging operations can be eliminated. However, cold forging is limited in terms of the shaping options and the choice of workpiece materials, be- cause it results in relatively small deformation capability at room temperature. Therefore, in a multistage process the defor- mation capacity can be exhausted due to strain hardening be- fore the intended operations have been completed. To overcome this problem, intermediate treatments such as an- nealing and phosphating have become necessary (Ref 4). Gears are produced by either shaping or hobbing. To meet the high accuracy requirements and form complex geometric contours, several methods have been introduced and investi- gated: fine blanking, press shaving, roll forming, and precision forming. The requirements in these processes are low cost and finished-form end product. It has been suggested that fine blanking and press shaving are limited to a gear width of 6 mm B.S. Yilbas,* A. Coban,** J. Nickel,** M. Sunar,* M. Sami,* and B.J. Abdul Aleem,* King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia *Mechanical Engineering Department **Energy Research Laboratoryof the Research Institute (Ref 5) and that thicker gears can be produced through warm shaving. Roll forming has the disadvantages of low production rate and high cost (Ref 6). This process is also limited to ring gears. On the other hand, precision forging provides high accu- racy, reduces row material disposal, and improves production time. However, due to the tool cost, precision cold forging of gears is not feasible for small-volume production (Ref 7). The multistage sequential method was applied by Korner and Knodler to form spur gears (Ref 8). The forming load was're- duced considerably, but in some cases a further operation was required for the surface finish. Ti-6AI-4V is an alloy that has been used extensively in in- dustry due to its high strength and corrosion resistance at low specific weight. High friction coefficient and low wear resis- tance limit its widespread application, but many techniques are available to improve the tribologicai properties of the surface (Ref 9, 10). The formation of titanium nitride on the surface is one of the popular techniques for modifying the surface. Through the plasma nitriding process, considerable nitride layer thickness can be achieved (Ref 10). The present study introduces cold forging of spur gears us- ing a multiaction gear forming system. The gear materials se- lected was Ti-6AI-4V due to its high strength-to-weight ratio. In order to increase the wear properties of the spur gears pro- duced, a plasma nitriding process was carried out (Ref 11). Two major methods of titanium nitriding can generally be ap- plied: TiN molecules are formed in the gaseous phase and are then deposited on a substrate; or nitrogen atoms are allowed to diffuse into the titanium matrix. In the present study the second process was used to nitride the gear surfaces. Nuclear Reaction Analysis (NRA) and Particle Induced X-ray Emission (micro- PIXE) tests were conducted to determine the nitrogen concen- tration and elemental distribution in the vicinity of the surface of the gear material. The study was extended to include the in- vestigation of the wear properties of the resulting gears. 2. Experiment A multiaction cold forging system was used to form the gear samples. Forging was carried out in enclosed dies. Material 728----Volume 5(6) December 1996 Journal of Materials Engineering and Performance