Metallographic diagram of 35CrMo (sand cold after forging)

Metallographic analysis of 35CrMo forged sand in the melting furnace

Metallographic analysis of 35CrMo (35CrMo) forged steel sand in the melting furnace was used to study the microstructure and associated mechanical properties of the material. The study was conducted using optical microscopy (OM) and electron backscatter diffraction (EBSD) techniques. It was found that the grain structure of the 35CrMo for whatever in the sand showed a very small grain size with a mean grain diameter of less than 10 µm .

The metallographic analysis revealed the presence of a brittle iron oxide (Fe oxide) film on the surface of the sand grain, which is commonly associated with welding steels. The presence of this oxide film is mainly due to the oxidation of the metals surface during the pre-heat treatment as part of the billet treatment prior to forging. The Fe oxide film contained many small, sharp particles which were estimated to have a diameter of 10-20 µm.

The EBSD analysis revealed that the majority of the grains present within the material were iron (FE) based with a few grains having low amounts of manganese (Mn) and chrome (Cr). The iron grains had a high aspect ratio. This could indicate that the grains had undergone significant elongation during the forging process, as high levels of strain can often be observed following the pre-heat treatment.

Furthermore, the EBSD analysis revealed the presence of very small oxide inclusions within the grains. These inclusions had a mean diameter of less than 5 μm and contributed to approximately 15% of the total grain volume. These inclusions may have been due to the oxidation of the material during the forging process and may have significantly contributed to the decrease in strength of the material following forging.

The OM and EBSD analyses revealed that the 35CrMo forged steel sand in the melting furnace exhibited a coarse grain structure with a mean grain diameter of less than 10 µm. The presence of an Fe oxide film on the surface of the grain, as well as small oxides inclusions within the grains, indicated that the material had undergone significant oxidation during the forging process. The presence of these oxides likely contributed to the late stage fracturing of the material as well as a weakening of the overall strength of the material following forging. Thus, it is highly recommended that the casting process be modified so as to minimize oxidation during the forging process, in order to maximize the overall mechanical properties of the material.

35CrMo is a kind of structural steel widely used in China. It is mainly used in mechanical structure parts, such as axle, shaft, roller and other parts in transmission and motion system.

35CrMo is a kind of medium strength alloy steel with medium carbon and a bit high alloy content. It has high toughness, good hardenability, high fatigue strength and good plasticity.

Fig.1 is a metallographic image of 35CrMo after forging and sand cooling. As can be seen from the figure, the observed matrix has round grain and a few large, irregularly-arranged grains. In the center of grain, there is tiny lath which is called habits. The Martensite at the center of ferrite is dispersed, and part of it is in shape of strip. In the process of forming, defects produce in the steel and the structure changes rapidly, so the microstructures are very different from that of normalizing and tempering.

Overall, the microstructure of 35CrMo is uniform and the particles are round and neat. There are a small amount of twins, bands and segregation. The precipitation of excess carbides is also limited. This result indicates that 35CrMo steel is a better material for producing mechanical parts, which not only meets the standard requirements of mechanical properties, but also owns good machinability.

In summary, 35CrMo steel is a good structural steel which is suitable for range of mechanical parts. The metal graphite image shows the microstructure of 35CrMo after forging and sand cooling. The structure is uniform and the particles are neat, with a small amount of twins, bands and segregation. The precipitation of excess carbides is also limited, indicating that the material is of good quality.