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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 4 Hardness is specified rather than carbon level because it is easier to measure. To simulate the LPC process, part geometry is used as an input, and the total surface area to be carburized is calculat- ed. After the carrier gas type is specified, calculations are made to determine time to saturate austenite at the carburization temperature (boost step) and the time needed to diffuse carbon away from the surface (diffuse step). The number of boost and diffuse steps can then be deter- mined to meet specified case re- quirements. Figure 4 shows modeling re- sults usingDANTEwithnineboost and diffuse steps. There are two curves each for two locations, po- sition 501 on the surface and po- sition 492 at a depth of 0.25 mm below the surface. The solid lines are predicted carbon weight frac- tions, and dashed lines are pre- dicted normalized carbide size. Using this boost/diffuse schedule, modeling results show forma- tion and growth during the LPC process. However, carbide size decreases during the final long diffuse step. The subsurface lo- cation is predicted to form carbides during the last diffuse step, but the carbides dissolve at the end. From the plot, car- bide formation is predicted to occur when the carbonweight fraction reaches a value of 0.010. To prevent any significant carbide growth during the LPC process, the boost/diffuse schedules should be carefully designed. SUMMARY Modeling carburization of higher alloy content steels, especially alloys containing a chromium content well above 1.0%, requires more than a simplified mass diffusion model. Carbide formation and carbide growth anddissolution affect the development of the carburized case. LPC adds the com- plication of rapid carbon buildup on the part surface due to direct dissociation of the carbon source gas on the hot part surface. However, LPC offers the ability to more rapidly de- velop a desired case and more flexibility to tailor the shape of the carbon profile. Validated heat treatment simulation software, such as DANTE, gives the heat treaters or part de- signers the ability to take advantage of LPC capabilitieswhile avoiding the possible detriments of excessive surface and grain boundary carbides or surface contamination by soot or tar. ~HTPro For more information: Zhichao (Charlie) Li, vice president, DANTE Solutions Inc., 7261 Engle Rd., Suite 105, Cleveland, OH 44130, 440.876.7578, charlie.li@dante-solutions.com, www.dante-solutions.com. Reference 1. O. Karabelchtchikova, Fundamentals of Mass Transfer in Gas Carburizing, Ph.D. thesis, Worcester Polytechnic Institute, 2007. 12 Fig. 3 — Framework for LPC process design using DANTE heat treatment software. Fig. 4 — LPC simulation results for a high-alloy steel showing carbide formation and dissolution during a nine-step boost-diffuse schedule.