On the Mechanical Properties of Rapidly Heated and Quenched Nickel-Chromium-Molybdenum Carburizing Steels

Abstract

As the present author reported previously, the surface hardness and toughness of carburized Ni-Cr-Mo steel which was heated rapidly to the optimum austenitizing temperature and quenched afte holding at that temperature for a short time were higher than those of steels which was slowly heated and quenched after holding for a longer time.
To invcstigate the reason of the above result, static bending test, rolling-bending fatigue test, investigation of tempering characteristics as well as the metallographic examination in electron microscope were carried out.
The results are summarized as follows:(1) The optimum austenitizing conditions for JIS SNCM21 steel carburized to content of 0.73%C are the quenching temperature of 800°, the holding time of 5 minutes, and the minimum heating rate of 30° per minutes.(2) From the results of static bending test and rolling-bending fatigue test, it was found that the beneficial effect of rapid heating and short time holding treatment was very small in steels containing less than 0.50%C.(3) The rapid heating and short time holding result in lower carbon content in martensite, higher Ms temperature and finer austenitic grain size compared with the case of the slow heating and long time holding.(4) Lower carbon martensite with undissolved carbide and fine grain size are attributed to rapid heating and short time holding.
From these reasons and analysis of the thermal expansion curves of the specimens of different carbon content, it is clarified that the specimens treated by rapid heating and short time holding can be tempered at lower temperatures than those treated by slow heating and short time holding.