Metallographic diagram of 40Cr (1100℃×20min+500℃×4s water cooling)

The microstructure of AISI 40Cr steel after being heat treated at 1100°C for 20 min followed by oil quenching is relatively complex. The structure consists of upper and lower layers of martensite, a bainite/pearlite band in between them, carbide precipitation on the ferrite/austenite grain boundaries, and recrystallized grains.

The martensite layer at the upper part of the microstructure is characterized by the presence of lath or packet martensite with clear martensite/austenite boundaries. There is a distinct martensite/austenite network of equiaxed grains. The martensite lamellae show a sharp upper edge contrast, referred to as white etching layer. A modified wedge test indicates the presence of macroscopic martensite.

The bainite/pearlite band located between the martensite layers shows that a hard bainite/pearlite mixture with a layered distribution of carbon-rich bainitic ferrite and pearlite are present. The grains are also well defined, with a characteristic Widmanstätten microstructure indicating partial devitrification of the bainite/pearlite mixture. Carbon content in the bainitic ferrite has been measured to be 0.8-1.0 mass %.

Carbide precipitation on the ferrite/ austenite grain boundaries is characterized by a homogeneous distribution of finely dispersed carbides which is presumed to be a type of MC carbides. As the grain size of the ferrite/ austenite grains is relatively small, the carbide precipitates exist mainly on the grain boundaries. The carbide precipitates destabilizes the grain boundaries against recrystallization which accelerates the formation of lath or packet martensite in the microstructure, considerably increasing its hardness.

The recrystallized grains visible in the microstructure are mainly located at the bottom part of the sample. They are mostly composed of equiaxed grains with fine grains. The recrystallization results in an increase of grain size and a decrease in dislocation, leading to a decrease in strength and hardness as well as an increase in ductility.

Overall, AISI 40Cr steel after being heat treated at 1100°C for 20 min followed by oil quenching exhibits a complex microstructure composed of martensite, a bainite/ pearlite band in between, carbide precipitation on the ferrite/ austenite grain boundaries, and recrystallized grains. The microstructure displays high hardness due to carbide precipitation, but also good ductility due to recrystallization.

This metallographic analysis examines the microstructure of a SCr40Cr (1100℃x20min +500℃x4s water-cooling) alloy. The microstructure observes features such as microcrystal structure, many graphite networks, inclusions and microstructure-mechanical properties.

The microstructures observed mainly consist of chromium, iron and graphite. The iron is present in three forms: ferrite, martensite and pearlite. Ferrite is the most abundant micro constituent, with the majority of the grains being spheroidal in shape. Martensite is also present, mainly in the surface layer of the sample. It has small, distorted grain boundaries in which graphite is visible. Finally, very small grains of pearlite are present, as evidenced by its orderly arrangement.

In addition to the three microstructural components, there is a network of graphite inclusions. The graphite presents itself as small disks, flakes and fibers, randomly dispersed throughout the microstructure.

Inclusions are also present in the sample and can be seen in the optical micrographs. They consist of aluminium oxide, silicon oxide and titanium oxide, with a few small particles of unidentified material.

Overall, the metallographic analysis of the SCr40Cr (1100℃x20min +500℃x4s water-cooling) alloy shows that it has a complex microstructure that includes ferrite, martensite, pearlite, graphite and different kinds of inclusions. This combination of features is likely to produce a good combination of mechanical and physical properties in the sample. However, further testing and analysis are required to confirm this.