Materials Week '97: Monday PM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Monday afternoon, September 15.

MODELING CARBURIZING, NITRIDING AND RELATED DIFFUSIONAL SURFACE TREATMENTS: Session III: Microstructure and Properties I

Session Chair: M.A. Howes, IIT Research Institute, Chicago, Illinois

A HISTORICAL OVERVIEW OF THE MODELING AND PREDICTING OF CONCENTRATION AND HARDNESS PROFILES IN CARBURIZED AND NITRIDED STEELS: R.D. Sisson Jr., R.R. Biederman, M.M. Makhlouf, Worcester Polytechnic Institute, 100 Institute Rd., Worcester, MA 01609

The progress in the ability to model, predict, and control the hardness and composition profiles in carburized and nitrided steels will be presented from an historical perspective. Starting with the rituals of the ancient blacksmiths, progressing through a variety of graphical and empirical techniques, to the use of sophisticated computer models and the improvements in the ability to model, predict, and control to carburization and nitriding of steels will be discussed.

2:00 pm

FINITE ELEMENT SIMULATION OF THE "CORNER EFFECT": J.E. Morral, University of Connecticut, Dept. of Metallurgy and Materials Engineering, Storrs, CT 06269-3136; B. Dupen, WIX, P.O. Box 1967, Gastonia, NC 28053-1967

When compared with flat surfaces, the case depth of carburized parts is greater at external corners and smaller at internal corners. A finite element model that simulates vacuum carburizing was applied to corners in a plain carbon steel treated at 1040°C. The results showed that when the radius of curvature of a corner was similar in dimension to the case depth, the "corner effect" was negligible. Also, the model showed that the corner effect was amplified by two step, boost-diffuse heat treatments yielding significant variations in case depth regardless of corner radius.

2:30 pm

SIMULATION OF CARBURIZATION IN SECONDARY HARDENING STEELS: C.J. Kuehmann, BIRL Industrial Research Laboratory, 1801 Maple Ave., Evanston, IL 60201; J.P. Wise, C.E. Campbell, G.B. Olson, Northwestern University, Materials Science and Engineering Dept., 2225 N. Campus Dr., Evanston, IL 60208

The ThermoCalc and DICTRA codes have allowed the simulation of carburizing behavior of advanced secondary hardening gear and bearing steels. These simulations used in conjunction with materials systems design techniques and other computational tools, produced high-performance gradient structures for gear and bearing applications. Process simulations consider the 1-D multicomponent diffusion and simulataneous carbide precipitation from local equilibrium conditions during carburizing. Using the simulated carbon profile and a model for the heterogeneous precipitation of coherent M2C carbides, coupled with calculations for the precipitation strengthening behavior, the resulting hardness profile has been calculated and compared with experimental observations. Due to the high strengthening efficiency achieved during secondary hardening, these steels can attain surface hardness of greater that 60 Rockwell C at carbon levels below 0.6 wt%. Additionally, these properties can be achieved without the presence of primary carbides in the case microstructure. Preliminary bending fatigue results indicate this can significantly increase bending fatigue life.

3:00 pm BREAK

MODELING CARBURIZING, NITRIDING AND RELATED DIFFUSIONAL SURFACE TREATMENTS: Session IV: Microstructure and Properties II

Session Chair: R.D. Sisson Jr., Worcester Polytechnic Institute, Worcester, MA

MODELING THE KINETICS OF NITRIDING/NITROCARBURIZING OF IRON: M.A.J. Somers, E.J. Mittemeijer, Delft University of Technology, Laboratory of Materials Science, Rotterdamseweg 137, NL-2628 AL Delft, The Netherlands

The kinetics and the morphological and compositional development of the compound layer during nitriding and nitrocarburizing of pure iron has been investigated for various temperatures and various combinations of imposed nitrogen and carbon activities. The results indicate that no local equilibrium occurs at the gas/solid interface during nitriding/ nitrocarburizing, due to the slow kinetics of ammonia dissociation, the development of N₂ and the fast initial carbon uptake. The kinetics of gaseous nitriding of pure iron can be adequately described by a model that considers local equilibrium at the solid/solid interfaces and a composition-weighted intrinsic diffusion coefficient of nitrogen in the Fe-N phases. For nitrocarburizing modelling appears not possible yet, due to changing compositions at gas/solid and solid/solid interfaces and the lacking of diffusion coefficients of C in Fe-N-C phases.

4:00 pm

ELASTIC LAYERED MODEL OF GAS/PLASMA NITRIDED FE-CR SYSTEM BY ACOUSTIC SPECTRO MICROSCOPY: T. Aizawa, University of Tokyo, Dept. of Metallurgy, 7-3-1 Hongo, 113 Tokyo, Japan; H. Kuwahara, Institute of Applied Science, Kyoto 606, Japan

An Fe-Cr alloy for various mole content of Cr was gas/plasma nitrided with different temperature and processing time. The nitrided specimens were analyzed by XRD and observed by microscope to investigate the microstructure of iron and chromium nitrides distributed from the surface in the direction of thickness. Acoustic spectro-microscopy was further utilized to measure the surface wave velocity dispersion and to determine the variation of elastic constants in thickness. The effect of nitridation conditions on the layered elastic model was discussed both for gas and plasma nitridation.

4:30 pm

MODELING THE FORMATION, BUILD-UP AND GROWTH OF CARBONITRIDE PHASES AND LAYERS IN THE CARBONIT PROCESS: R. Roussev, S. Malinov, P. Petrov, Technical University of Varna, Studentska str. 2 dep. MTM, 9010-Varna, Bulgaria

Abstract not available.