軸受鋼の熱處理の研究(I)

熱處理による機械的性質と寸法安定性との關係

抄録

Specimens of the ball bearing steel, SUJ 2, were heat-treated by subzero-treatment, oil quenching and tempering, martempering-oil cooling, and martempering-air cooling. Some groups of these were tested in the static notched bending of charpy-type specimens and the compressive breaking of ring shaped one, and the other were studied as to the dimensional stability of retained austenite by various quenching methods. The quantitative determination by pointcounting and lineal analysis was carried out in microscopical methods.
The results of experiment were as follows:
1) In the ball bearing steel, SUJ 2, the coarsening temperature of austenite grain was at 870°C, duplex range was from 930°C to 1050°C, and quenching crack was generated by quenching from above 850°C austenitizing temperature. Considering above these, retained austenite and carbide solubility, the best austenitizing temperature was at 840°C±5°C.
2) In the comperessive breaking test the oil quenchihg and tempering was superior to subzero heat-trsatment, but there was the tempered brittleness at 180°C and 250°C tempering temperature in the static notched bending test. In sabzero-treatment there was no notched brittleness. Both martempering heat-treatment made the specimens generate the primary acicular bainite, and on account of it hardness was too low, but the mechanical properties were very good.
3) From dimensional stability at room temperature by various quenching the experimental equations were gained:
Sabzero treatment…
Oil quenching…
Interrupted quenching…
4) Lineal analysis and point-counting with microscopical photographs without the Hurlbut counter were inadequate for quantitative determination of this degree's retained austenite. Lineal analysis was more convenient in quantitaiive determination than point counting.