The low temperature magnetic properties of austenitic Fe-Cr-Ni alloys: 2. The prediction of Néel temperatures and maximum susceptibilities

Abstract

The compositional dependence of the Néel temperature has been studied, from data derived by different techniques and by various authors, for 30 austenitic stainless steels or special Fe-Cr-Ni alloys whose compositions fall near the range of the AISI 300 series. A linear relationship enables the predicted Néel temperature, T N to be evaluated with an rms deviation of 3.5 K on the basis of the wt % of alloy constituents. The effect of alloying elements in lowering T N ∗ increases in the order Cr, Ni, Mo, and Si, while Mn is unique in raising T N ∗ . By comparing this equation for T N ∗ with previous equations to predict the onset of a martensitic phase change at a temperature M s , it is concluded that isotherms for M s in ternary Fe-Cr-Ni alloys should also be parallel to the tangent to the boundary between fcc and bcc phases calculated from thermodynamic data. This conclusion throws doubt on the applicability of the Eichelman and Hull equation to the 300 series of stainless steels. If γ phase stability is achieved by adding Cr, Ni, Mo, or Si, the alloys are likely to be ferromagnetic at low temperatures, but if structural stability is due to C, N, or Mn, then the alloys should be antiferromagnetic or superparamagnetic. A logarithm plot of the maximum initial susceptibility versus Ni content can be used to estimate X v max in the AISI 300 series stainless steels. This relationship does not apply to alloys with Mn > 2 wt %, but in these the value of X v max is lowered. The significance of these observations is discussed in terms of the application of stainless steel in magnetic environments at low temperatures.