High permeability of Fe-Ni-Cr with ultra-high purity

Effects of the balance of Ni, Cr, and impurities, particularly O and S, on magnetic properties of Fe-Ni-Cr soft magnetic alloys have been investigated. Initial permeability is shown to increase remarkably as Cr increases above 4%, at Ni content between 36% and 39%. Initial permeability increases drastically by a factor of two with decreasing O and S contents below critical threshold values. High permeability alloy can be obtained for Fe-38%Ni-8%Cr with O and S contents below critical threshold values (O<25 p.p.m., S<10 p.p.m.). The drastic decrease of coercive force is closely related to the growth of grain size. It is considered that there are a critical size and distribution of inclusions in the neighborhood of 10~20 p.p.m. of O and S contents, below which the growth of grain size and the motion of domain walls may not be disturbed by inclusions