The effect of carbon on magnetic properties of an Fe-Cr-Co permanent magnet alloy

The effect of carbon on magnetic properties of an Fe- 24Cr-12Co-1Si-1Ti alloy with carbon content from 0.012 to 0.291 percent (by weight) was studied. It has been found that for a carbon content larger than 0.05 percent there is an abrupt degradation in magnetic properties for alloys treated according to a normal procedure to develop the magnetically optimum state. The reasons have been thoroughly explored by optical metallography, X-ray diffractometry, transmission electron microscopy, carbide extraction, and analysis and hardness measurements. Carbon stabilization of the γ phase, the amount of which increases with carbon content, is the main deteriorating factor found in this alloy. Abnormal spinodal decomposition between α phase and non-α-phase regions and Cr-rich carbide precipitated in or near α-phase impair magnetic properties further. It has been shown that prolonged solution treatment at temperatures near the solidus can compensate part of the carbon effect at the expense of (B-H)m value. Hence, for practical purposes, the 0.05 percent C is set to be the upper limit for ensuring (B-H)m value greater than 5.5 MGOe for this alloy, and the 0.05 percent C limitation is suggested to be common to all Fe-Cr-Co permanent magnet alloys.