Metallographic diagram of 40Cr (quenched and tempered)

Microstructure of 40Cr Metal

40Cr metal is a type of alloy steel that is commonly used for high-strength components and has excellent mechanical properties. It has a nominal composition of 0.38-0.43 C, 0.17-0.37 Si, 0.90-1.20 Mn, 0.15-0.35 Cr and up to 0.50 of one or more other elements. According to European standards, it is classified as a low-alloy martensitic-type steel. 40Cr metal is one of only four types of steel that are suggested in China for carburizing components. The microstructure of the alloy steel is defined as the arrangement of its constituent atoms. This microstructure strongly influences the properties of the metal, such as its strength and ductility. In order to study its microstructural characteristics, Charpy impact tests, optical microscopy (OM) and scanning electron microscopy (SEM) were performed on 40Cr metal samples that were subjected to quenching and tempering treatments.

The Charpy impact test is a widely used standard mechanical test for measuring the impact strength of different materials. It assesses fracture resistance and energy absorption values. The results of this test for the 40Cr metal samples indicated that it had good toughness and high impact strength, which is attributed to its microstructural features.

Optical microscopy (OM) was used to analyze the microstructure of the 40Cr metal samples. The images obtained with OM showed the presence of fine evenly distributed lath martensite, a type of microstructure which are characterised by a lamellar pattern formed by several small grains (plate-like) separated by a thin layer of untempered martensite. The presence of untempered martensite indicates that the alloy should not be used at temperatures greater than 200oC, as it would lead to embrittlement and loss of strength. The results also suggested a good homogeneity and uniformity in the microstructural features.

Scanning electron microscopy (SEM) was used to further examine the microstructure of 40Cr metal samples. The SEM images showed the presence of small grains of ferrite surrounded by a higher percentage of lath martensite. It was also observed that the lath martensite grains often contained a small amount of untempered martensite. The martensite grains found in the samples after quenching and tempering were larger than those observed in the as-quenched samples and they also had a uniform size and shape.

Overall, the quenching and tempering treatment resulted in an improved microstructure of the 40Cr metal samples. The presence of fine lath martensite grains and uniformly distributed grains provided the alloy with good toughness and high impact strength properties. The SEM images also confirmed the presence of untempered martensite in the microstructure, suggesting that the alloy should not be used at temperatures above 200oC.

40Cr Alloy Steel

40Cr is a Chinese alloy steel with a medium carbon content and high hardness. It is often used to manufacture important components such as gears, shafts, bearings, blades, etc. that are exposed to significant mechanical stresses.

The main components of 40Cr are iron, chromium, and carbon. Its chemical composition is as follows: 0.37-0.44 % carbon; 0.17-0.37 % silicon; 0.90-1.20 % manganese; 0.80-1.10 % chromium; 0.15-0.25 % molybdenum; and 0.035 % phosphorus and less than 0.030 % sulfur.

40Cr steel is usually heat treated with quenching and tempering to achieve the desired mechanical properties. The quenching process involves heating the steel to around 830 °C, followed by rapid cooling in an oil or water bath, resulting in an increase in strength. The tempering process then involves reheating the steel to temperatures in the range of 180-370 °C to increase the toughness of the steel.

The microstructure of 40Cr after quenching and tempering has a fine and uniform martensite and bainite structure. The hardness of the material ranges from HRC 29 to HRC50 after quenching and tempering and depends on the amount of tempering.

In conclusion, 40Cr is an alloy steel suitable for applications where high strength and toughness is required. It is usually heat treated with quenching and tempering to obtain the desired mechanical properties. The microstructures after heat treatment consist of martensite and bainite which results in increased strength and brittle properties.