The chromium equivalents of ferrite stabilizers in commercial stainless steels
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number of components in the system. The term X is atomic fraction, and L is the thermodynamic interaction parameter. The term ““G is the magnetic contribution to the Gibbs energy. I11The binary and ternary thermodynamic and magnetic interaction parameters given in Reference 1 were used as required. Additional binary interaction parameters[3-91listed in Table II were also used. The nonlinear equations obtained by equating the chemical potentials of the components in the two phases were solved by the Newton-Raphson iterative method. The simulated compositions listed in Table I were obtained as follows. The concentrations of all other alloying elements (except chromium in austenite) were fixed at the values given in the specification. The chromium concentration in austenite and the concentration of all alloying elements, including Cr in ferrite, were treated as unknowns and provided guess values to set up the chemical potential equalities. In this way, the number of unknowns to be determined was automatically set equal to the number of simultaneous equations to be solved by iteration in any multicomponent system. As the computation was to be done in atomic fractions, some trial and error calculations were necessary to match exactly the weight percents of elements (except Cr) in the specified and simulated compositions listed in Table I. The weight percents of Cr for simulated compositions in Table I are computed values corresponding to an annealing temperature of 1100 “C. As before,“] the Cr equivalent at 1100 “C is evaluated as the amount of Cr substituted by 1 wt pet of the ferrite stabilizer at the (y/y + a) phase boundary. To derive this value, the ferrite stabilizing element in the simulated compositions was set to zero and the computation was repeated for a system with one component less. From the difference in Cr concentrations in austenite between the two results, the equivalent was derived as an average over that concentration of the ferrite stabilizer. Where more than one ferrite stabilizer was present, this procedure was repeated separately for each ferrite stabilizer. Table III lists the chromium equivalents for 304, 316, 321, and 347 types for the simulated compositions in Table I. These are compared with the equivalents derived from quatemary phase equilibria.“] To rule out any effect of averaging over different concentrations, the equivalents from the quaternary phase equilibria were recomputed for the concentrations given in Table I. However, there was a negligible difference between these and the equivalents reported earlier.[‘l As seen in Table III, the equivalents obtained from
The Chromium Equivalents of Ferrite Stabilizers in Commercial Stainless Steels V. RAGHAVAN
and DARA P. ANTIA
In a recent article,“] the chromium equivalents of X (X = Al, MO, Nb, Si, Ti, V, or W) were reported, as derived from the computed quaternary phase equilibria in Fe-Cr-Ni-X systems. Commercial stainless steels usually contain some residual alloying elements other than the ferrite stabilizer X. In order to take into
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