Feb_March_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 8 6 3 COPPER EVAPORATION DURING LOW PRESSURE CARBURIZING Stringent pressure control and gas species type both play an important role in minimizing the evaporation rate of not only copper, but other elements susceptible to evaporation in vacuum systems. Trevor Jones,* Virginia Osterman,* and Donald Jordan, FASM,* Solar Atmospheres Inc., Souderton, Pa. L ow pressure carburizing (LPC) in a vacuum furnace is increasingly the preferred method for case hardening aerospace gears, with some alloys like FerriumC61 and C64 designed specifically for LPC. Several industry process- es use acetylene as the principal carburizing gas. Each LPC cycle has a series of carburizing boost steps, immediately followed by an equal number of diffusion steps. A comput- erized control process using gasmass flow controllers allows for rapid buildup of carbon at the surface of parts during the boost stage. The time for each diffusion step increases throughout the cycle as a uniform case is formed, free of large blocky and networked carbides. The length and num- ber of each step is determined by the material type, tem- perature, and case depth requirement. During LPC, heating to the carburizing temperature can be performed in vacuum or using a partial pressure gas. The pressures used in LPC processing typically range from 0.1 Torr (13.3 Pa) to 11 Torr (1.5 kPa) [1] . With some of the high alloy steels specified for aerospace parts, hydrogen is often the preferred partial pressure gas to use during heat-up. Hydrogen reduces passive oxide films at the surface, readily activating it for efficient and consistent carburizing with no preoxidation treatment required. In comparison to the boost stages that use patented or proprietary acetylene/carrier gas mixtures for short periods, the total time for the diffusion stages is relatively long. Like the heat-up stage, the diffusion stage can be done in vacuum or using a partial pressure gas. Selective case hardening is common with gears, where certain sections of a part are masked to prevent carburiza- tion at those locations. The intent is for the masked areas to remain at the same carbon weight percentage as the core base metal (Fig. 1). For aerospace parts, masking is typically accomplished with copper electroplating, which in certain instances covers much of the part’s surface area. The low pressures and high temperatures used in LPC can lead to copper evaporation during the diffusion steps. Copper va- por then condenses on colder components in the hot zone, such as gas nozzles and ceramic insulators. In a worst-case scenario, copper deposits can cause arcing and melting of power feedthroughs—damaging the furnace and in some instances the parts as well (Fig. 2). The use of partial pressure gas to help suppress evap- oration is well established in the heat treating industry [2,3] . The present study investigates the effect of temperature, pressure, and carrier gas species on the amount of copper evaporation that occurs from copper foil test samples. EXPERIMENTAL DETAILS Oxygen-free high conductivity copper foil, 0.002 in. thick, was cut into 2-in. square coupons. Three couponswere prepared for each test process. Just prior to placing the cou- pons into the heat treat furnace, they were briefly immersed *Member of ASM International Fig. 1 — Copper plating shields the coated section from case hardening. Fig. 2 — Example of copper buildup on power feedthroughs. 13