STUDIES ON INDUSTRIALIZATION OF MT PERMANENT MAGNET

On the Heat Treatment and Structure

Abstract

The optimum heat treatment and the structure of MT magnet was investigated on the specimens consisted of representative composition. This alloy is provided with poor magnetic properties when it was quenched from 1150°C or lower temperature. The critical temperature is therefore considered to be 1200°C, and those samples quenched from this temperature showed a uniform austenite solid solution. When holding time at quenching temperature was too long, magnetic properties, especially residual induction, was deteriorated by decarburisation. The most adequate period of holding time is from 20 to 30 minutes.
Cooling rate also strongly affects the magnetic properties in case of Fe-Al-C alloys. It is related to the formation of β martensite which is essential for retaining high coercive force. A favourable state for β martensite seems to be produced only by proper cooling rate. Among several cooling methods, oil-quenching gave the best result and water-quenching at 20°C gave the most inferior result.
Since the quenched specimen is consisted of austenitic phase, a part of its structure needs to be converted to ferromagnetic martensite. According to aging curve, the maximum magnetic properties was obtained by tempering for 60 or 120 minutes at 300°C. It is noteworthy that the second hardening occurs at around 500°C in the course of tempering, accordingly, the peak of magnetic properties are recognized at about 300°C and 500°C,
MT magnet turns to machinable by means of annealing. If the annealing temperature reaches 1, 000°C, deterioration of magnetic properties are unavoidable, The annealed specimen was consisted of α phase and double carbide.