Metallographic diagram of 40CrNi (annealed state)

The alloy 40CrNi (heat-treated) is a medium-carbon nickel-chromium alloy which is commonly used for the production of highly stressed components in the automotive and aerospace industries. The alloy is capable of being heat treated to achieve a wide range of strength and hardness. It has good impact toughness and is also corrosion resistant.

The microstructure of the alloy 40CrNi (heat-treated) can be seen clearly with the help of a metallurgical microscope. At the lower magnification level (up to 40X), the alloy 40CrNi displays a fine, uniform grain structure. With different heat treatments, the grain size can be manipulated to produce a matrix of alloy grains ranging in size including ultra-fine, fine, medium, and coarse. The majority of the grain size is medium with scattered sight of some ultra-fine and fine grains.

Under higher magnification (up to 400X), the microstructure of the alloy 40CrNi can be further strengthened and featured. When viewed under this level, the alloy displays a mixture of ferrite, pearlite, and martensite structures. The ferrite is present in a plate-like structure which can also be seen scattered in between other structures, while the pearlite is in a spongy pattern. The martensite is seen within the ferrite as fine grains, and serves as the strengthening component of the alloy. The presence of small amounts of carbides (MC and M7C3) can also be seen scattered across the microstructure.

In the toughness test carried out on the alloy 40CrNi (heat treated), the results indicated an acceptable level of toughness and stability. The alloy has a good resistance to fractures, due to the combined effects of stress concentration, mechanical strength and toughness. The alloy also has good machinability as well as resistance to corrosion and abrasion.

Overall, the alloy 40CrNi (heat-treated) has many excellent properties. It is capable of being heat treated to achieve an even wider range of strength and hardness. The microstructure is finely divided and the presence of the ferrite, pearlite, and martensite structures serves to make the alloy even stronger against abrasion and corrosion. The alloy also displays a good level of toughness and resistance to fractures. This makes the alloy suitable for many applications where high levels of stress and strength are needed such as in the automotive and aerospace industries.

40CrNi is a kind of bearing steel with high carbon and chrome content, which has excellent machinability, case hardening properties and high strength. It is a type of steel used in high temperature bearing parts.

The microstructure of 40CrNi steel can be observed under an optical microscope. The microstructure comprises austenite and ferrite, carbonitride and nitride, and drops of liquefied steel. After annealing treatment, most of the cementite precipitates in the intergranular boundaries between ferrite grains, forming “black spots”. The microstructure of 40CrNi is characterized by a large number of ferritic grains, which appear in distinct shapes. The surface of the ferrite grains is smooth and the edges are round.

The microstructure of 40CrNi after annealing treatment changes into pearlite. The pearlite grains are located around the austenite grains in the microstructure and appear in a cellular structure. The pearlite grains are distributed in leaf shape, and the ferrite is distributed in short thin strips around the pearlite grains. The surface of the pearlite grains is smooth, while the edges are slightly raised and curved due to the diffraction of light.

The 40CrNi steel after heat treatment also includes martensite distribution with light etching area. The martensite has finer grains and has a higher hardness and higher strength than pearlite. The martensite is distributed around the pearlite in a network shape, which can improve the fracture toughness of the material.