Metallographic diagram of 40Cr (forged)

Fig. 1 is the metallographic image of AISI 40Cr(Forging). It can be seen from the figure that the sample is made of carbide, pearlite and ferrite, as well as a small amount of martensite. The sample is sectioned from the center part of the forging with a plane perpendicular to the rolling direction.

The forged sample is composed of ferrite and pearlite. Ferrite accounters for the main part of the matrix with a diameter of about 0.5μm. The ferrite has a low carbon content and a large grain size, which is beneficial for obtaining good mechanical properties, such as good corrosion resistance and high strength. While pearlite, as a kind of microstructure, is composed of alternating layers of ferrite and cementite with a diameter of about 2μm. Its presence increases the strength and wear resistance of the sample by increasing its hardness.

In addition, there are some small amounts of martensite in the image, which are white precipitates in the center of the sample. Martensite is a subset of a special class of austenite, known as Widmanstatten ferrite, with a very fine grain structure. This shows that the sample has undergone a thermomechanical process and a bit of nitriding which allows martensite to nucleate and form within the grain boundaries of the ferrite and cementite. As a result, the hardness and strength of the sample are further enhanced.

To summarise, the sample of AISI 40Cr(Forging) examined in Fig. 1 consists of the components of ferrite, pearlite, and martensite with a diameter of less than 1μm, which possess excellent mechanical properties such as high strength and good corrosion resistance. This is due to the homogenous microstructure throughout the sample and the presence of a Widmanstatten ferrite, as well as a bit of nitriding, which induces the formation of martensite at the grain boundaries of the ferrite and cementite.

40Cr is a kind of quenched and tempered steel. After quenching and tempering treatment, the hardness of 40Cr can reach above HRC250 and the tensile strength can reach more than 900MPa, so it has a high strength, wear resistance and fatigue strength, good plasticity and impact toughness.

The microstructure of 40Cr is mainly composed of martensite and dispersed bainite, and the martensite is more and the bainite content will increase with the increase of tempering temperature. Sometimes there are lumps and sorbite in the matrix.

In a metallographic sample of 40Cr from casting, the microstructure is mainly composed of spherical solid solution tissue and coarse eutectic tissue, among which the fine spherical solid solution has a slightly larger particle size and a relatively uniform morphology. The carbon content in solid solution tissue can reach 0.5-1.2%. There are also a small number of non-metallic inclusions, mostly distributed along the grain boundary and the center of the dendrite.

In the metallographic sample of 40Cr obtained by forging, the microstructure is mainly composed of lath martensite, which is finer and uniform, and there are a small number of bainite or ferrite. The net lath morphology of lath martensite is relatively connected, which means that the prior-austenite grain size of the material is relatively fine. Generally, the size of prior-austenite grain is between 160 and 240μm, and the carbon content in microsome is 0.3-0.8%. There are also a small number of non-metallic inclusions, most of which are distributed along the grain boundary and some at the center of the dendrite.