Decarburisation of 60Si2MnA in wet air at 1000 – 1280 °C

The present study investigated the decarburisation behaviour of a Si-containing spring steel 60Si2MnA in wet air containing 24.8%H2O at 1000–1280°C. It was found that after the steel was oxidised for 20 minutes, the depth of the total decarburisation zone increased with increased temperature up to 1150 °C but then decreased with increased temperature to 1170 °C. At 1170–1280 °C, no decarburisation was observed. An oxide layer comprising Fe2SiO4 + FeO and porosity formed between the FeO layer and the steel. Thermodynamic calculation indicated that, if the interface equilibrium was determined by 2Fe + 4CO2 + (Si)→ Fe2SiO4 + 4CO, the interface carbon activity could be significantly higher than that when the interface equilibrium was determined by FeO + CO → Fe + CO2 and afforded a good protection against decarburisation. However, the experimental results indicated that the latter reaction prevailed at 1000–1150 °C, leading to severe decarburisation. Calculation of carbon diffusion in steel at 1000–1150 °C indicated that the diffusivity data given by Krishtal for quaternary Fe-C-Si-Cr steels allowed a good prediction to the decarburisation tendency of the steel, indicating that the combined effect of Si and Cr was significant. At temperatures above 1170 °C, the molten oxide phase formed on the steel surface had a shielding effect against decarburisation. This was attributed to the impermeable nature of the molten oxide to CO and CO2 gases as the potential decarburisation reaction products. However, molten oxide formation at the interface led to rapid consumption of the steel by encapsulating and eroding the steel piece by piece into the scale.