Metallographic diagram of 20 steel (after carburizing and heat preservation at 960°C, cooling to 860°C and quenching)

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Introduction

Carbon Steels are the most common type of steels in modern times. They are used in a variety of applications, from large construction projects to everyday consumer goods. A specific type of this material is the 20 Steel. This steel is composed of three elements: carbon, iron, and chromium.

The 20 Steel is a type of steel meant to be used in high temperature and pressure applications. The steel is heated to a high temperature of 960°C and maintained at that temperature for a period of time to allow for the carbon to sinter and form a protective layer on both the surface and the interior of the steel. After this process is complete, the temperature is lowered to 860°C and allowed to be cooled in an annealing process.

Microstructure

At the microstructural level, the 20 Steel has a ferrite-pearlite composition composed of iron and carbon. The microstructure of the steel is comprised of grains that are bainitic in form. The grain boundaries, which are the boundaries between the grains, are made up of a mixture of bainite and ferrite. This mixture provides the steel with strength characteristics and good ductility.

On the other hand, the hierarchical structure of the 20 Steel is made up of two phases: ferrite and pearlite.The ferrite phase is the harder of the two, and it primarily comprises the grains. The pearlite phase comprises the grain boundaries and provides a ductile property to the steel.

Surface

When the steel is cooled to 860°C and the grain boundaries form, the steel becomes quite hard and the surface may present with some roughness and porosity. This porosity and roughness are present due to the heat treatment that is employed when cooling the steel from 960°C down to 860°C. This heat treatment can cause some of the finer features, such as small pores, which are characteristic of the 20 Steel.

Metallography

In order to analyze the microstructure of the 20 Steel, a metallographic analysis is employed. This analysis involves the use of an optical microscope to look at a sample of the steel. This sample is usually cut into thin slices and polished to a high gloss finish in order to reveal the details of the structure.

The metallographic analysis reveals that the 20 Steel has a large grain size and a very fine grain boundary network. It also reveals that the hardness of the steel is high, as well as its strength.

Conclusion

In conclusion, the 20 Steel is a high temperature and pressure material that is composed of ferrite, pearlite, and bainite. Its hierarchical structure is made up of two phases, ferrite and pearlite. It is quite strong and hardened due to the heat treatment employed when cooling the steel from 960°C to 860°C. Metallographic analysis of the steel reveals that it has a large grain size and a very fine grain boundary network. The 20 Steel is a material that is commonly used in many high temperature and pressure applications.

The microstructure of 15Cr20 steel after carburizing and tempering at 960°C and cooling to 860°C is mainly pearlite with a small amount of bainite, ferrite, tempered martensite and carbide. The pearlite is long and thin, and its lamellar may not be well developed due to its cooling rate. There also exist ferrite and Tempered Martensite islands distributed inside, but the proportion of Tempered Martensite islands is much less than that of ferrite islands. A small part of the residual austenite decomposed into carbides or Tempered Martensite, and further transformed into eutectoid ferrite and carbide after tempering at a lower temperature. The austenite grains which have not transformed still exist in a certain amount and some carbide grains distributed on the surface and in grain boundaries.