February 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 | F E B R U A R Y / M A R C H 2 0 1 9 6 2 *Member of ASM International LOW PRESSURE CARBURIZING PROCESS DESIGN FOR HIGH-ALLOY STEELS The low pressure carburizing (LPC) process can be enhanced by using computer modeling to characterize the process and design boost/diffuse schedules. Zhichao (Charlie) Li,* B. Lynn Ferguson, FASM,* and Justin Sims* DANTE Solutions Inc., Cleveland H igh-hardenability steels with high alloy content typ- ically contain strong carbide-forming elements such as chromium and molybdenum. During low pres- sure carburization (LPC), strong carbide-forming elements can form stable carbides on or near the surface, which can effectively block carbon diffusion and retard the carburiza- tion process. To facilitate carbon penetration into the part and achieve the desired case depth, the LPC process must consist of a series of boost and diffusion steps to control the surface carbon content and the amount of carbides that are present. During a diffusion step, the near surface carbides can provide a carbon source as they decompose. This arti- cle discusses a computer modeling-based methodology, using the commercial finite element based heat treatment software DANTE, to characterize the LPC process and design boost/diffuse schedules for high-alloy steels. The power transmission gear industry, especially for aerospace applications, is being pushed to increase power density of the transmission for improved acceleration, load capacity, and extended life. In addition, the ability to per- form for extended time under poor lubrication conditions, where gear temperatures are high, is required in military applications. To meet these requirements, the gear indus- try turned to carburizing grades of ultrahigh-strength steels. These steels have high alloy content, relying on strong car- bide-forming elements such as chromium and molybde- num, which exhibit secondary hardening during tempering at relatively high temperatures (i.e., ∼ 500°C). Examples of these steels are Pyrowear 675, Ferrium C64, CSS-422L, and M50NiL, in which high amounts of strong carbide-forming elements affect the carburization process. CARBURIZING PRINCIPLES Figure 1 is a graphical representation of what occurs during gas carburization [1] . A carbonpotential, C P , existswith- in the furnace chamber due to the gas composition, which is at one bar pressure or just slightly over one bar to maintain positive pressure in the furnace. The base carbon of the steel part, C 0 , is lower than the gas carbon potential, so carbon is deposited on the part surface, where it then diffuses into the part. As the surface reaction continues, a boundary layer, β , exists as the neighboring gas carbon potential is reduced. D C is the diffusion coefficient of carbon in the steel. With time, surface carbon level, C S , increases, and the carbon level with- in the steel increases. The process typically takes more than six hours, depending on the desired depth of case, process temperature, and other factors. A crucial point is that atmo- sphere carbon potential does not exceed the maximum sol- ubility of carbon in the austenitized steel. While this process description is simplistic, the key is maintaining the surface carbon level low enough so no significant amount of car- bides will form. Typical gas carburizing temperatures are between 900° and 950°C (1650° and 1740°F), and the carbon potential of the gas atmosphere is about 0.8%. Low pressure carburization is conducted in a vacuum furnace at pressures of about 0.1 to 1 torr. During heating to the carburizing temperature, a nonreactive gas such as ni- trogen may be added to provide convection to speed heat- ing through the lower temperatures, where radiation is less effective. As the temperature increases, parts must be pro- tected from surface oxidation, so the vacuummust be below 0.1 to 0.3 torr. A partial pressure of a surface-cleaning gas such as hydrogen may be added during this stage to make the steelmore receptive to carbon absorption. After parts are heated to the desired temperature, carburization commenc- es by means of a series of boost and diffuse steps. Boost steps are short (typically <2min). Diffuse step times can start from one to several minutes, becoming progressively longer 10 Fig. 1 — Graphic representation of gas carburization [1] .

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