Modeling of Case Hardening

GUSTAVO SÁNCHEZ SARMIENTO; MARÍA VICTORIA BONGIOVANNI

Handbook of Thermal Process Modeling of Steels

CRC Press Taylor & Francis Group

Lugar: Boca Raton; Año: 2009; p. 627 - 672

INTRODUCTION Many metallic components require different properties near the surface than in the bulk of the material. Surface engineering is rapidly developing to enable the properties of the surface to be customized (by modifying the near-surface material or by application of coating), without damaging the bulk properties of the component.The design requirements for surfaces fall into four main categories: mechanical, electrical, chemical, and physical=optical (aesthetic). Consequently, there is a wide variety of surface treatments available, and new techniques are continually being developed. Shercliff and Beresford presented in Refs. (1,2) a classification tree for surface treatments of steels (reproduced in Figure 12.1) that provide resistance to wear and corrosion.Within those surface treatments, this chapter deals with thermochemical diffusion process methods, which are characterized by diffusion of carbon and nitrogen, oxygen, or boron into the material surface, after which the parts are thermally treated to form a hardened case.  Thermochemical processes include carburizing, carbonitriding, nitriding, ferritic nitrocarburizing, and boronizing (3). The typical process conditions for these methodologies are compared in Figure 12.2 (from Ref.(4) and reproduced in Ref. (3)). Attention will be given here to high-temperature diffusion of carbon and nitrogen in steels, which leads to an increase in hardness near the surface, as well as to local compressive residual stresses. Both kinds of thermochemical processes contribute to an increase of the lifetime of the component (5).Modeling diffusion can be considered as relatively straightforward, since the governing equations are well established. However, one must account for the fact that during diffusion, precipitation may occur and modify the kinetics of diffusion. For example, in the case of steel, carbides and nitrides can form during the diffusion of carbon and nitrogen. This precipitation contributes to the improvement of mechanical properties of the treated component (5). An overview of the significant progress that has been made up to 2002 in research focused on the assessment of residual stresses effects in carburized, carbonitrided, and case-hardening components was presented by T. Réti (6). The following sections provide an updated overview on the computer modeling of those thermochemical processes for steels, mainly focused on the residual stress and distortion of the treated parts. After reviewing the most employed case-hardening techniques based in carburizing, nitriding, and carbonitriding, the coupled fields present in those processes and the techniques for their computer modeling are described. The relationship between the residual stresses and the properties of the treated parts are also reviewed. Some typical applications recently published in the literature are summarized here.