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Phase Diagrams

Carburization of Carbon Steel

Carburization is a surface-hardening technique in which carbon is added to the surface of low-carbon steel at temperatures generally between 850 °C and 950 °C. In this temperature range, austenite — which has a relatively high solubility for carbon — is the thermodynamically stable phase (see the Fe-C phase diagram). Hardening is achieved when the high-carbon surface layer is quenched to form a hard, wear-resistant martensite on a tough, low-carbon steel core.

Case depth of carburized steel is a function of carburizing time and the available carbon potential at the surface. When prolonged carburizing times are used for deep case depths, a high carbon potential produces a high carbon content at the surface, which may result in excessive retained austenite and/or the precipitation of carbides. Both retained austenite and carbides have adverse effects on the distribution of residual stress in the case-hardened part. Consequently, a high carbon potential may be suitable for short carburizing times but not for prolonged carburizing.

Figure 1 shows the concentration profiles of carbon in a binary Fe-0.15C (wt. %) alloy after 5 minutes, 30 minutes, 2 hours, and 5 hours of carburization in a mixture of 40 % nitrogen and 60 % cracked methanol gas at the temperature of 900 °C.

The main carburizing reaction and the carbon activity, aC, are assumed to be CO + H2 = C + H2O and 0.64, respectively. A surface reaction controls the flux of carbon at the surface. The corresponding "carbon potential" and the mass-transfer coefficient for carbon are calculated to be 0.85 wt. % and 8.25·10-9 mol/s, respectively.

Figure 2 shows the activity of carbon at the surface of a binary Fe-0.15C (wt. %) alloy as a function of time in a mixture of 40 % nitrogen and 60 % cracked methanol gas at the temperature of 900 °C.

Concentration profiles of carbon as a function of time

Figure 1. Concentration profiles of carbon in a binary Fe-0.15C (wt. %) alloy after 5 minutes, 30 minutes, 2 hours, and 5 hours of carburization in a mixture of 40 % nitrogen and 60 % cracked methanol gas at the temperature of 900 °C. Calculated with DICTRA coupled with SSOL2 and MOB2 databases.

The activity of carbon at the surface as a function of time

Figure 2. Activity of carbon at the surface of a binary Fe-0.15C (wt. %) alloy as a function of time (expressed in seconds) in a mixture of 40 % nitrogen and 60 % cracked methanol gas at the temperature of 900 °C. Calculated with DICTRA coupled with SSOL2 and MOB2 databases.

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