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

Abstract

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.

Appendix I: Determination of the Carbon Permeability Through the Ferrite Layer[30]:

In determining the equilibrium carbon concentration at the scale–steel interface, it is assumed that the dissolved carbon in steel can react with wustite, forming either CO and CO₂via the following reactions:

$$ \left[ {\text{C}} \right] + 2{\text{FeO}} = 2{\text{Fe}} + {\text{CO}}_{2} $$

(A1)

$$ \left[ {\text{C}} \right] + {\text{FeO}} = {\text{Fe}} + {\text{CO}} $$

(A2)

and the overall reaction of these reactions becomes:

$$ {\text{CO}} + {\text{FeO}} = {\text{Fe}} + {\text{CO}}_{2} $$

(A3)

Using the data given by Richardson and Jeffes[56] and quoted by Kubaschewski and Alcock,[57] the standard Gibbs free energy of formation for Reaction (A3) is given by

$$ \Delta G_{\text{A3}}^{o} = - 22800 + 24.267{\text{T}} \left( {{\text{J}}/{\text{mole of CO}}} \right) $$

(A4)

When the reaction (A3) reaches equilibrium,

$$ \frac{{P_{\text{CO}} }}{{P_{{{\text{CO}}_{2} }} }} = { \exp }\left( {\frac{{ - 22800 + 24.267{\text{T }}}}{\text{RT}}} \right) $$

(A5)

The $\frac{{P_{\text{CO}} }}{{P_{{{\text{CO}}_{2} }} }}$ thus obtained can be used to determine the equilibrium carbon activity at the interface assuming $P_{\text{CO}}$ + $P_{{{\text{CO}}_{2} }}$ = 1 atm from the following reaction:

$$ \left[ {\text{C}} \right] + {\text{CO}}_{2} \left( {\text{g}} \right) = 2{\text{CO}}\left( {\text{g}} \right) $$

(A6)

The standard Gibbs free energy of formation for this reaction is given by,[56,57]

$$ \Delta G_{\text{A6}}^{o} = - {\text{RT}}\ln \left[ {\frac{{P_{\text{CO}}^{2} }}{{a_{c} P_{{{\text{CO}}_{2} }} }}} \right] = 170,700 - 174.5{\text{T}}\, \left( {{\text{J}}/{\text{mole of C}}} \right) $$

(A7)

where $a_{c}$ is the equilibrium activity of carbon at the scale-steel interface with graphite being its standard state. From Eq. [A7], we obtain,

$$ a_{c} = \frac{{P_{\text{CO}}^{2} }}{{P_{{{\text{CO}}_{2} }} }}\exp \left[ {\frac{{\left( {170,700 - 174.5{\text{T}}} \right)}}{\text{RT}}} \right] $$

(A8)

After the equilibrium carbon activity at the interface is determined, the corresponding carbon concentration in the steel at the scale-steel interface can be calculated using the known relationships between carbon activities and carbon concentrations. For dissolved carbon in α-Fe, the relationship to express the activity coefficient of carbon in ferrite for carbon steel, $\varUpsilon_{\text{C}} \left( {\text{ferrite}} \right)$, is given by,[58]

$$ \log \varUpsilon_{C} \left( {\text{ferrite}} \right) = { \log }\left( {\frac{{a_{C} }}{{X_{C} }}} \right)\left( {\text{ferrite}} \right) = \frac{5846}{T\left( K \right)} - 2.687 $$

(A9)

where $X_{\text{C}}$ is the equilibrium molar fraction of carbon in ferrite at the scale-steel interface, which can be converted to carbon concentration in weight percent using the following equation,

$$ C_{\text{c}} \left( {{\text{wt}}\,{\text{pct}}} \right) = \frac{{100\,{\text{pct}} \cdot 12 \cdot X_{\text{c}} }}{{12 \cdot X_{\text{c}} + 55.85 \cdot \left( {1 - X_{\text{C}} } \right)}} $$

(A10)

When a ferrite layer formed on the steel surface and the alloy concentrations are constants across the ferrite layer, the difference in the carbon concentration between two interfaces of the ferrite layer, $\Delta C_{\text{C}}^{\alpha }$, provides a driving force for carbon to diffuse through the ferrite layer.

When the ferrite layer is thin, the carbon distribution in it can be approximated as having a linear concentration gradient and carbon diffusion through this layer can be described using the simplified Fick’s first law:

$$ J_{\text{C}}^{\alpha } = A \cdot D_{\text{C}}^{\alpha } \cdot \frac{{C_{{{\text{C}} {\text{in}} \alpha }}^{\alpha - \gamma } - C_{{{\text{C}} {\text{in}} \alpha }}^{{\alpha - {\text{FeO}}}} }}{M} {\text{moles cm}}^{ - 2} {\text{s}}^{ - 1} $$

(A11)

where $J_{\text{C}}^{\alpha }$ = the diffusion flux of carbon from the interface between the ferrite layer and the bulk of steel towards the scale-steel interface; $D_{\text{C}}^{\alpha }$ = the diffusion coefficient of carbon in ferrite; $C_{{{\text{C}} {\text{in}} \alpha }}^{\alpha - \gamma }$ = the carbon concentration on the ferrite side at the interface between the ferrite layer and the bulk of steel in wt pct ; $C_{{{\text{C in}} \alpha }}^{{\alpha - {\text{FeO}}}}$= the carbon concentration in ferrite at the steel-scale interface in wt pct ; $A$ = a constant used to convert the concentration of carbon from wt pct to mole/cm³; $M$ = the thickness of the ferrite layer.

From Eq. [A11], we can see that for a certain thickness of the ferrite layer, $X$, the rate of carbon diffusion is determined by the product of carbon diffusivity $D_{\text{C}}^{\alpha }$ and the carbon concentration difference between the two interfaces of the ferrite layer,$\cdot \Delta C_{\text{C}}^{\alpha } = C_{{{\text{C in}} \alpha }}^{\alpha - \gamma } - C_{{{\text{C}} {\text{in}} \alpha }}^{{\alpha - {\text{FeO}}}}$. Following the approach used by Smith,[28] the following product was defined as the permeability or relative permeability,[59] ($P_{\text{C}}^{\alpha }$) of carbon through the ferrite layer,

$$ P_{\text{C}}^{\alpha } = D_{\text{C}}^{\alpha } \cdot (C_{{{\text{C}} {\text{in}} \alpha }}^{\alpha - \gamma } - C_{{{\text{C}} {\text{in}} \alpha }}^{{\alpha - {\text{FeO}}}} ) = D_{\text{C}}^{\alpha } \cdot \Delta C_{\text{C}}^{\alpha } $$

(A12)

Substitution of Eq. [A12] in Eq. [A11] yields,

$$ J_{\text{C}}^{\alpha } = A \cdot \frac{{P_{\text{C}}^{\alpha } }}{M} {\text{moles cm}}^{ - 2} {\text{s}}^{ - 1} $$

(A13)

It can be seen that a greater permeability immediately leads to a greater carbon flux for a given ferrite layer thickness.

Strictly speaking, the alloying effect, particularly the effect of Si, should be considered in calculating the carbon concentration from the carbon activity data obtained from Eq. [A8]. However, as internal oxidation was inevitably observed, it was assumed that the dissolved alloying components (Si, Mn and Cr) had reacted with dissolved oxygen having diffused into the steel and therefore, the alloying effect from the remaining dissolved alloying components at the FeO-steel interface was considered to be negligible.