40Cr(Carbonitrided) Metallographic micrograph
Carbonitriding is a form of surface hardening that involves subjecting the material to a combination of nitrogen and carbon diffusion. The treatment of steel with carbonitriding is particularly effective for atmospheric hardening ferritic and martensitic steels, resulting in increased surface hardness and compressive stresses. This increased surface hardness lends itself to improved wear and fatigue resistance, making it a desirable method for many gear, shaft and bearing applications.
This micrograph shows a carbonitrided cross section of a steel sample made of 40Cr material. This material is a medium carbon alloy steel, perfect for hardening by carbonitriding. The micrograph shows that the carbon and nitrogen diffusion has increased the diffusion interlayer thickness to 0.22μ and has created a hardened layer at the surface that reaches up to 65 HRC at 500HV. The structural phase of the sample remains martensite and the structure is uniform, indicating a successful carbonitriding treatment. There are some oxidation spots present at the grain boundaries as a result of the high temperatures required during treatment.
The area at the top of the micrograph between the hardened layer and the original Martensite interlayer shows the formation of a semi-austenite layer as a result of the high nitrogen content of the carbonitriding permeate. This layer increases the frictional properties, making this treated material desirable for wear and fatigue applications. Furthermore, the majority of the structure remains Martensite with only small areas of Acicular Ferrite or Bainite present. This is beneficial as the Martensite phase provides greater strength and hardness compared with the other two.
The carbonitrided 40Cr material has achieved excellent hardness, uniformity, fatigue and wear resistance properties as a result of hardening treatment and can be confidently used for applications where these properties are required.
