Nov_Dec_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 6 4 T his summer, the Center for Heat Treating Excellence (CHTE), located at Worcester Polytechnic Institute (WPI) in Worcester, Mass., introduced a new simula- tion software tool called NitrideTool. This software has the unique ability to allow users to simulate the gas nitriding of steels so that they can better understand nitriding potential, temperature, time, and surface condition during the gas ni- triding process. Accurate process control is themost effective way to ensure the properties produced by the gas nitriding process are reliable and repeatable. Prior to CHTE’s work in this area, the heat treat industry could only simulate the gas nitriding of pure iron [1] . NITRIDING BACKGROUND Gas nitriding is an important surface heat treatment for steels. The process can considerably improve the wear and corrosion resistance, along with the fatigue endur- ance, of steel parts. The performance of these parts, such as gears, shafts, and rods are critical to end users in the au- tomotive, aerospace, and other manufacturing industries. It was for this reason that the team focused their energies on understanding and simulating the gas nitriding process to determine the process parameters for achieving the re- quired properties. Nitriding is a thermochemical surface treatment in which nitrogen is transferred from an ammonia atmo- sphere into the steel within the ferritic region. Being a low temperature process, nitriding minimizes distortion and deformation of the heat-treated parts. Traditionally, the industry used trial and error testing to determine the nitriding process parameters. This meth- od is expensive, time consuming, and hard to control. But now, the industry will be able to use this software to simu- late the gas nitriding process to meet pre-defined specifi- cations of the nitride part. HOW NITRIDETOOL WORKS NitrideTool is based on the combination of a mass transfer and diffusion model [2] and compound layer growth model [3] . NitrideTool can predict the nitrogen con- centration profile of selected steels with user-defined pro- cess parameters including process time, temperature, and nitriding potential. Because of the low solubility of nitrogen in ferrite, the nitrides precipitate during the nitriding process. A com- pound layer and an underlying diffusion zone are formed near the surface of the steel after nitriding. The compound layer, also known as the white layer, consists of nitrides and can greatly improve wear and corrosion resistance. The hardened diffusion zone is also responsible for a con- siderable enhancement of the fatigue endurance. Figure 1 shows the nitrogen concentration profile in compound layer and diffusion zone of the nitrided case. N s is the surface nitrogen concentration, which is assumed to be a constant when the thermodynamic equilibrium is reached at the gas-steel interface. N c and N d are the nitro- gen concentration at the compound layer-diffusion zone interface on the compound layer side and the diffusion zone side, respectively. N s , N c , and N d can be determined with the alloy specific Lehrer diagram [4] . N 0 is initial nitro- gen concentration in the steel. Figure 2 shows the flow chart of the algorithm of Ni- trideTool. The user-defined parameters include material selection, nitriding temperature, nitriding time, base ni- CHTE INTRODUCES SIMULATION SOFTWARE FOR GAS NITRIDING OF STEELS NitrideTool software simulates the gas nitriding of steels to understand nitriding potential, temperature, time, and surface condition. Richard D. Sisson, FASM,* Mei Yang,* and Haoxing You Center for Heat Treating Excellence, Worcester Polytechnic Institute, Massachusetts *Member of ASM International Fig. 1 — Schematic of gas Nitriding process. 10