Metallographic diagram of granular bainite 20CrMo (air-cooled at 380°C)

Analysis of the Microstructure of 20CrMo

Introduction

The microstructure of 20CrMo, an air-cooled steel from 380°C, falls under the category of Martensitic Steels. 20CrMo is widely used in the production of electrical equipment, boilers and pressure vessels, fertilizer manufacturing equipment and automotive components. It has excellent mechanical properties, high hardenability and good wear resistance. The purpose of this paper is to analyze the microstructure of 20CrMo.

Types of Microstructure

The microstructure of 20CrMo can be divided into four distinct types: Primary Ferrite, Bainite, Martensite and Etched Grain Boundary. Primary Ferrite is the first and most important type. It consists of micro-sized grains of ferrite, which have a fine and uniform distribution throughout the microstructure. Bainite, the second type, is formed from austenite which is partially transformed into fine-grained ferrite and carbon-enriched martensite. This combination gives bainite its higher mechanical properties. The third type is martensite, which forms as a result of rapid quenching. The martensite microstructure consists of extremely fine needles and plates of interlocking ferrite and carbide particles. Finally, we have the etched grain boundary phase, which appears as alternating bright and dark bands of ferrite and carbide along the grain boundaries.

Structure and Composition

The microstructure of 20CrMo is composed of ferrite, austenite and carbide particles. The ferrite consists of small grains of iron mixed with carbon, which gives it a light color in the microstructure. The austenite is composed of iron and chromium, which gives it a darker color in the microstructure. The carbide particles are a combination of iron and chromium with carbon, which gives them a grey or black color.

Result of Mechanical Properties

The mechanical properties of 20CrMo depend heavily on its microstructure. The primary ferrite gives the material its strength and ductility, whereas the bainite and martensite contribute to its fracture toughness and wear resistance. The etched grain boundaries give the material its good fatigue resistance. By analyzing the microstructure of 20CrMo, we can gain an understanding of why it possesses these excellent mechanical properties.

Conclusion

In conclusion, 20CrMo is a Martensitic steel which is air-cooled from 380°C. It is composed of four distinct microstructures including primary ferrite, bainite, martensite and etched grain boundary. This material has excellent mechanical properties due to the combination of its microstructure components. Analyzing its microstructure can help us gain an understanding of why it has these superior mechanical properties.

20CrMo at 380°C (Vacuum Annealed)

20CrMo is a chromium-molybdenum alloy steel and is often used in automotive and industrial applications. It is a medium-carbon steel with good hardenability and fair wear resistance. It has a medium-high compressive strength of 700 to 900 MPa and a tensile strength of 650 to 800 MPa. It is usually hardened and tempered after proper heat treating, which normally takes place at temperatures of 980 to 1180 °C. For final hardening, the recommended cooling rate is 25 to 50 °C/hr.

The following image shows a metallographic cross section of a 20CrMo sample taken at 380 °C vacuum annealed. The microstructure consists of a fine, homogeneous dispersion of : ferrite, pearlite, martensite and bainite. The tempered martensite is light gray and the pink-colored ferrite and bainite are formed due to the slow cooling rate. The white pearlite regions are formed due to the slow cooling rate. The large black particles are inclusions, most likely formed in the casting process or during forging.

20CrMo在380°C（真空退火）

20CrMo是一种铬-钼合金钢，通常用于汽车和工业应用。它是一种中碳钢，具有良好的热处理性能和良好的耐磨性。它具有中等高的抗压强度700~900MPa，抗拉强度650~800MPa。通常在980~1180°C下进行硬化，淬火后再进行回火。推荐的冷却率为25~50°C/hr。

下图显示了在380°C真空退火的20CrMo样品的金相横截面。显微组织由铁素体、珠光体、马氏体和贝氏体的细小均匀分散组成。回火马氏体呈浅灰色，而粉红色的铁素体和贝氏体由于冷却速率慢而形成。由于冷却速率慢，而形成的白色珠光体区域。大的黑色颗粒是杂质，很可能是在铸造过程中或者锻造过程中形成的。