Metallographic diagram of 20CrMo (air-cooled)

With the development of aviation industry, chromium-molybdenum alloy steel 20CrMo has been widely used in the manufacturing of aircraft engine parts and components. 20CrMo is a kind of low-alloy steel which contains both Chromium and Molybdenum.

The main alloying elements of 20CrMo are chromium and molybdenum with a content of 14-18% and 0.17-0.37% respectively. This makes 20CrMo a heat resistant alloy with a high strength-to-weight ratio and excellent fatigue resistance. On top of this, 20CrMo boasts outstanding low temperature performance and oxidation resistance, which makes it suitable for use in a wide range of industries, from aerospace engineering to automotive manufacturing.

20CrMo has been widely used for civil aircraft engines due to its excellent performance. This alloy shows good hot workability, and is easy to fabricate and machine, giving the designer more flexibility for component designs. 20CrMo is a heat treatable alloy and can be readily cold worked and welded, making it an ideal choice for aircraft parts.

To aid better understanding of the microstructure of 20CrMo, a number of metallurgical studies have been conducted. The microstructure of 20CrMo typically consists of a matrix phase of tempered martensite and fine carbides. The matrix structure can be further subdivided into two classes: primary Widmanstatten ferrite and secondary cementite. Mild cooling can also produce some untempered martensite as well as small amount of pearlite.

20CrMo also has good temperature and oxidation resistance, as well as a high level of corrosion resistance in a variety of different environments. This is due to the alloy’s chromium content, which increases its ability to resist corrosion. 20CrMo is also highly durable and fatigue resistant, making it suitable for use in the production of a variety of components including nuts and bolts, valve stems, shafts and other mechanical parts and components.

In order to get a better understanding of the microstructure of 20CrMo and its properties, it is important to analyze its metallurgical characteristics. To do this, a laboratory scale metallographic examination is usually conducted. This typically involves the mounting, grinding and polishing of test samples of 20CrMo, followed by examination on an optical, or scanning electron microscope.

The analysis of a metallographic sample of 20CrMo by a Scanning Electron Microscope (SEM) typically reveals a carbide dispersion network, which consists of mainly iron carbides of MC, M7C3 and M6C types, with a minor content of alloy carbides. In addition, a bright dispersed structure of martensite and Bainite types of tempering is also present. These structures result from the high levels of chromium, molybdenum and vanadium in the alloy.

The microstructure of 20CrMo also reveals various features of mechanical properties. This includes a pearlite-structured microstructure, whose main lattice planes are oriented in the plane perpendicular to the shaft. In addition, a network of carbides is present, which provides strength and shock resistance. The presence of the carbides increases the hardness of the alloy, while the bainite-structure brings improved ductility and toughness to the alloy.

The combination of 20CrMo’s excellent performance and its affordable cost makes it highly attractive to designers. 20CrMo is an ideal choice for applications requiring high toughness, fatigue and corrosion resistance. Its ability to withstand harsh climates also makes it suitable for a wide range of industries, from aerospace engineering to automotive manufacturing. In conclusion, 20CrMo is an excellent low-alloy steel for the manufacture of aircraft components and other mechanical parts.

The microstructure of 20CrMo (air-cooled) steel matrix consists of ferrite, carbide, carbide network and pearlite or bainite. Due to their different physical and chemical properties, their microstructure affects the mechanical properties of 20CrMo (air-cooled) steel. The ferrite matrix consists of granular lath-shaped ferrite, as well as a small amount of fine spheroidite, and the normal size is about 0.02~3.0µm. The number of ferrite in the structure decreased from the 70mm to the center of the 5mm. The ferrite in the 70mm and the center of the 5mm were more than 95%, and the ferrite distributed in the 30mm~5mm was about 90%. Retained austenite content in the 70mm~20mm part was slightly lower than that in the 20mm~5mm.

The carbides in the 20CrMo (air-cooled) steel can be divided into two kinds, big and small. The bigger ones are needle-shaped, granular and small pieces of spherical particles; the smaller ones are spherical particles, and their size is generally less than 0.5µm. The former are evenly distributed, and are spliced into a network with smaller particles, making the structure very compact. The content of big and small carbides was higher at the center of the 5mm and the 70mm, but the size of the big particles was much smaller at the center, and the size of the small particles did not change significantly.

The amount of pearlite or bainite in the 70mm to 5mm of 20CrMo (air cooled) steel is about 50%, which is higher than that of air cooling. There are a lot of fat and thin lamellae, the organization is neat and the size is fine, and the interface between the lamellae and the grains is relatively clear. Besides, the carbides in 20CrMo (air-cooled) steel are mostly located near the lamellae junction and are distributed evenly, making the structure more compact.