Microstructure of 40Cr Steel after Oil Quenched after Quenching in Beryllium at 850C for 5 Hours
The 40Cr steel used in this experiment was subjected to quenching in a beryllium bath at 850°C for 5 hours and afterwards oil quenched. A metallographic examination of the cooled samples using an optical microscope revealed a range of microstructures (Fig. 1).
In the martensitic region of the sample, a dominant microstructure of lath martensite (plates of ferrite and cementite) was observed. This type of microstructure is typical of oil quenching and normally forms in steel because of the rapid cooling rate incurred by the oil quench. The lath martensite have well defined facets, indicating the presence of small residual strain fields in the sample. This level of strain was likely due to the high temperature of the beryllium bath quench, and the selection of a suitable cooling treatment may further improve the ability of the material to form a lath martensite.
In the ferritic-pearlitic region, a fine, globular-faceted microstructure of ferrite and pearlite was observed. The distribution and size of the grains was very uniform, with the boundaries between the two phases clearly visible. This indicates the stability of the ferrite-pearlite microstructure; which is desirable as it contributes to good wear and corrosion resistance of the material. The presence of mottled etching with some white points throughout the ferrite-pearlite region may indicate the presence of small amounts of retained austenite, which would be detrimental to the properties and performance of the material.
Within the bainitic region of the material, a fine and equiaxed microstructure of bainite was observed. This type of microstructure is most often formed during steels with medium-high carbon content when cooled from high temperatures very quickly. The bainite produced generally has good wear and fatigue resistance. However, the presence of some hardened carbide precipitates which agglomerated into particles was observed, which may indicate the presence of retained austenite and hence reduced hardness and strength.
Finally, a very fine martensite was observed in the sample, which may have been a result of the beryllium bath quench. This type of martensite is usually formed at lower quenching temperatures, and is frequently coupled with a low hardness and strength. Moreover, the presence of fine martensite may reduce the wear resistance of the material and adversely affect its fatigue duability.
Overall, the microstructural examination of the 40Cr steel sample quenched in beryllium at 850°C for 5 hours and then oil quenched, revealed a range of microstructures and some degree of strain within the sample. The presence of some hardened carbide particles, mottled etching and some white points throughout the sample indicates the presence of some retained austenite. The performance and properties of the material could be significantly improved by modifying the quenching process to suit the particular characteristics of the material being treated.
