Decarburization of 60Si2MnA in 20 Pct H₂O-N₂ at 700 °C to 900 °C

The decarburization behavior of a spring steel 60Si2MnA at 700 °C to 900 °C was examined. It was observed that after holding for 20 minutes in 20 pct H₂O-N₂, thick ferrite layers developed within 750 °C to 877 °C with a maximum thickness of about 100 μm observed at 805 °C to 825 °C, while the ferrite layers were much thinner at 900 °C and 700 °C. Carbon permeability analysis and theoretical calculation were conducted to assess the possibility of forming a ferrite layer. In the permeability analysis, several factors were considered: (1) carbon concentration at the steel surface, which was very likely determined by reaction equilibrium between FeO and dissolved carbon in steel, (2) carbon solubility in ferrite which had a maximum value at about 715 °C, and (3) carbon diffusivity in the ferrite phase. In the ferrite layer thickness calculation, the contribution from carbon diffusion in the austenite phase was also taken into account. While the carbon permeability analysis and ferrite layer thickness calculation showed good successes in predicting the pattern of ferrite layer thickness change with temperature, under the assumption of FeO-ferrite equilibrium the calculated ferrite layer thicknesses at 780 °C to 840 °C did not match the observed values well. Factors contributing to the discrepancy were discussed.