Effect of Alloy Composition on Carburizing Performance of Steel

Abstract

This paper investigates the effect of alloy composition on the gas carburizing performance of AISI 1018, 4820, 5120, and 8620 steels. The mass-transfer coefficients and carbon diffusivities were calculated from experimental measurements using direct flux integration. Although steels with high concentration of austenite-stabilizing elements (Si, Ni) increased carbon diffusivity in austenite, they significantly reduced the kinetics of carbon transfer from the atmosphere to the steel surface and resulted in lower weight gain. Despite lowering the carbon diffusivities, steels alloyed with carbide-forming elements (Cr, Mo) significantly increased the mass-transfer coefficient in the atmosphere and enhanced the rate of carbon profile evolution. The experimentally determined carbon diffusivities were in good agreement with the carbon diffusivities obtained from the thermodynamic and kinetic databases in DICTRA. Overall, using the concentration dependent mass-transfer coefficient and carbon diffusivity in various alloy steels helped explain the experimentally observed variations in the carbon concentration profiles and the effective case depths. Recommendations are made to help achieve better case depth uniformity within a carburizing workload.

An Empirical Model for Carbon Diffusion in Austenite Incorporating Alloying Element Effects

“…Zhukov and Krishtal 18) indicated that that Cr attracts C and reduces the activity of C whereas Ni repulses C, resulting in an increase in C activity. Similarly, Rowan and Sisson 4) found that additions of Ni, which is an austenite-stabilizer, and Si, which is a ferrite stabilizer, increase the carbon diffusivity in austenite. Additions of carbide-forming elements such as Cr and Mo, on the other hand, decrease the carbon diffusivity by attracting C atoms.…”

“…Figure 6 compares carbon diffusivities predicted by Eq. (17) with measured 4,11,22,24) carbon diffusivities in various alloy steels. The temperature range of data used is between 850 and 1 075°C.…”

“…The diffusivity coefficient and the activation energy are constant for the equations from Table 1 However, as illustrated in multiple studies, alloy additions do alter carbon diffusion in austenite. 3,4,6,[15][16][17][18][19] Blanter (as cited in Krishtal 3) ) evaluated the effects of systematic modifications to ternary Fe-M-C alloys. Table 2.…”

“…1,2) At higher temperatures, carbon diffusion governs phase transformation kinetics during heating and cooling or during isothermal holding processes, and in several analyses, an accurate expression for carbon diffusivity in austenite is critical. As one example, carburizing models 3,4) for low alloy steels require a carbon diffusivity expression that incorporates a complete assessment of the effects of alloy additions on carbon diffusion in austenite.…”

The Gas Carburization of Linear Cellular Alloys as a Novel Alloy Development Tool

“…Specifically, Cr and Mo additions increase the mass transfer of carbon to the surface of the part over that of a plain carbon steel, and previous research has shown that the equilibrium surface concentration of carbon in Cr-and Mo-containing steels result in higher values than a plain carbon steel. [20] C. Heat Treatment and Mechanical Property Investigation Table V shows the average Vickers microhardness measurement for several samples carburized to carbon contents near 0.40 wt pct and cooled slowly to a pearlitic microstructure, prior to any subsequent heat treatment. Also included in Table V is the carbon content as determined through combustion analysis of these samples.…”