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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 3 as the case develops. Acetylene (C 2 H 2 ) is a common carbon source gas, and it has a much better deep hole penetration capability than propane (C 3 H 8 ) gas. EXPERIMENTAL RESULTS The normal chemical composition of Ferrium C64 alloy steel is 0.11 C-16.3 Co-3.5 Cr-7.5 Ni-1.75 Mo-0.2 W-0.02 V-bal Fe. LPC trials were run using ∼ 100-mm diameter Ferrium C64 cylinders. LPC tests were conducted following different boost and diffuse schedules to generate different carbon and hardness profiles in the cylinders, with carburization temperatures of 940° and 1000°C. Table 1 lists an example of an LPC schedule used to characterize material properties, including carbon diffusivity, carbide forming and decompos- ing rates, but is not an optimized LPC recipe targeting any specific surface carbon and effective case depth. The carbon profile was determined by machining 0.05-mm layers from the cylinder using a sharp single point lathe cutter and collecting the chips from each layer to mea- sure carbon content by LECO carbon testing. It is important to avoid overheating of chips and/or bar during machining, which would result in lower measured carbon content for the chip sample. Microindentation hardness measurements weremade onmounted sections to compare against the car- bon weight percentages. Hardness and carbon profiles determined from four different trial runs are plotted in Fig. 2. The relationship be- tween hardness and carbon level for C64 tempered marten- site is determined from the experimental results. Runs 1 to 3 (Fig. 2) were carburized at 1000°C, and run 4 was at 940°C. Total boost time was the same for all runs, but total diffuse time increased from run 1 to 3, with diffuse time for run 4 the same as for run 2. Figure 2 shows that longer diffusion times result in deeper carbon penetration, and higher carburiza- tion temperature results in a deeper case. These results are more easily observed from the hardness data. The carbon profiles show the general trend of high carbon content at the surface becoming lower with increasing depth from the sur- face, but there is noticeable variation. Complicating the issue is the possibility of carbide formation and interference with pure diffusion of carbon through the austenite lattice dur- ing carburization. LPC process modeling and recipe design for specific surface carbon and case depth must take into account car- bon saturation of austenite, possible carbide formation and growth, carbide dissolution, and carbon diffusion, in ad- dition to the dissociation of the carbon carrier gas. Figure 3 shows a framework for LPC process design to determine a proper boost/diffuse schedule. Material property data needed to simulate LPC include steel grade base chemistry, carbon diffusion coefficient data, and austenite saturation versus temperature. A carburization specification for a part has at aminimum the required case depth (usually hardness at a specified depth), surface hardness, and core hardness. 11 TABLE 1 — TRIAL LPC SCHEDULE Step Step time, min Total time, min Boost 1 1.25 1.25 Diffuse 1 18.0 19.25 Boost 2 0.75 20.0 Diffuse 2 20.0 40.0 Boost 3 0.75 40.75 Diffuse 3 25.0 65.75 Boost 4 0.75 66.50 Diffuse 4 40.0 106.50 Boost 5 0.75 107.25 Diffuse 5 55.0 162.25 Boost 6 0.75 163.0 Final diffuse 25.0 188.0 Fig. 2 — Hardness and carbon profiles measured for an LPC trial.