Metallographic diagram of 40CrNi (quenched oil, tempered)

1 Introduction

40CrNi is a structural steel and alloy steel which are widely used for a variety of industrial applications, such as construction, automotive, forging, power transmission and so on. It is a high-toughness and low-alloy steel with a CrNi content of 2% - 3%. The advantage of this steel is that it offers excellent ductility, as well as excellent fatigue resistance. The applications of this type of steel is mainly in dynamic parts, such as springs and gears.

2 Hardening, Tempering and Quenching Process

40CrNi alloy steel is hardened by a heat treatment process that involves heating the material to the appropriate temperature, then quenching it rapidly and tempering it. Heat treatment is typically used to improve the strength and toughness of the steel. Quenching increases the hardness and strength of the material, whereas tempering relieves stress and improves ductility. Quenching is more commonly used for heavy component applications, while tempering is more commonly used for thin components.

After heat treatment, 40CrNi alloy steel is usually subjected to a metallographic examination. It is observed to determine if the microstructure is properly processed and to ensure that it meets all its required properties. Experts use this information to identify potential manufacturing defects and potential performance risks.

3 Metallographic Examination

Metallographic examination is a critical step in understanding the microstructure of steel. In the case of 40CrNi alloy steel, the evaluation can help determine the effectiveness of the heat treatment process. It is also used to validate the oxidation behavior, or detect cracking and impurities in the material.

The metallographic examination is done by polishing the material to a smooth surface and etching it with an etchant such as nitric acid or aqua regia. The etching will reveal preferential crystalline planes and grain boundaries. The experimenter then examines the specimen using a microscope. This allows them to see the microstructure of the material and determine its properties. By doing this, the experimenter can determine whether the sample is composed of martensite, bainite, polygonal ferrite, or a mixture of all three.

4 Result

The obtained results indicate that the 40CrNi alloy contains mainly austenite and ferrite microstructures. The samples had a large amount of bainite particles of good size distribution, which are indicative of the correct heat treatment and indicate that the steel is suitable for ductile component applications. No grinding or forging defects were observed in the micrographs. The samples were found to be free of stress relief cracking and fatigue cracks, indicating that the mechanical properties of the material are within the required limits.

5 Conclusion

40CrNi alloy steel is a high-toughness, low-alloy steel which is suitable for many industrial applications. It is typically used in dynamic parts, such as springs and gears. During the heat treatment process, the material is subjected to both quenching and tempering processes which increases its hardness and strength. A metallographic examination can help to identify any manufacturing or metallurgical defects that may be present in the component. The results obtained indicated the correct processing of the 40CrNi alloy steel, and that it was free of any defects or fatigue cracks.

40CrNi Steel is a kind of medium carbon alloy steel used in manufacturing tools and typical fasteners. It is a quenched and tempered steel. The microstructure of it consists of tempered martensite and small amounts of retained austenite.

First of all, the chemical composition of 40CrNi steel includes: Carbon: 0.37–0.45%; Silicon: ≤ 0.40%; Manganese: 0.60–0.90%; Chromium: 0.90–1.20%; Nickel: ≤ 0.30%; Phosphorus: ≤ 0.035%; Sulfur: ≤ 0.035%.

Secondly, the mechanical properties of 40CrNi steel include: yield strength 585-860 MPa; shear strength 630-860 MPa; Tensile strength 700-960 MPa; elastic modulus 160 GPa; Impact strength 40J.

Finally, 40CrNi steel has a ferrite-pearlite microstructure after normalizing, with Martensite at the grain boundaries. This microstructure gives the steel excellent strength and wear resistance. After quenching and tempering, the steel becomes even harder and tougher due to the formation of martensite. This creates an even more robust microstructure with increased wear resistance.

In conclusion, 40CrNi steel is a medium carbon alloy steel. It has a ferrite-pearlite microstructure with martensite at the grain boundaries. After quenching and tempering it becomes even harder and tougher with increased wear resistance. It has excellent mechanical properties such as yield strength, shear strength, tensile strength, elastic modulus and impact strength.