Cr12 (both quenched and tempered) metallographic diagram

Introduction

Cr12 steel is a high chromium, air-cooled and heat-treated steel, which is widely used in many cutting tools, bearings and other industrial applications. It is a high carbon, low alloy steel with a relatively high hardenability due to its increased chromium content. The microstructure of Cr12 steel usually consists of a quenched face-centered cubic ferrite and a tempered martensite. Its excellent combination of strength, toughness and hardenability make it an ideal material for many tough applications.

Chemical Compositions

Cr12 steel is a high carbon, low alloy steel composed of the following alloying elements: 0.4 – 0.6% carbon (C), 0.2 – 0.6% manganese (Mn), 0.02 – 0.15% silicon (Si), 12 – 14% chromium (Cr), 0.3 – 0.6% molybdenum (Mo), and 0.7 – 1% nickel (Ni). It may contain other trace elements, such as sulfur (S), phosphorus (P), boron (B), vanadium (V), and aluminum (Al). The presence of these trace elements can potentially alter the physical, mechanical, and other properties of the steel.

Heat Treatments

Heat treatment is a process used to improve the properties of steel by controlling the cooling rate, as well as the grain structure, hardness and ductility of the material. To obtain the desirable Cr12 steel characteristics, it is subject to a quenching and tempering process. This involves carrying out both an austenitizing and austempering process. The austenitizing process is heated to the austenitic phase of the material, usually above 910 °C, and then rapidly quenched to establish the solution of the austenite matrix. This results in the formation of a hard, density and wear-resistant structure. The austempering process is then executed, which involves heating the steel to a lower temperature, such as 780 °C, and allowing it to cool to a temperature that is close to the operating temperature. This process promotes the meeting of prior austenite plates to form a Widmanstatten ferrite plate structure.

Microstructure of Cr12 Steel

The microstructure of Cr12 steel after the quenching and tempering process consists of a face-centered cubic (FCC) ferrites, tempered martensitic structures, and retained austenite. The microstructure of the tempering plainexhibits a uniform distribution of small tempered martensite with a white coloration and retained austenite with some carbides present in the grain boundaries. The high hardness and wear resistance of Cr12 steel are attributed to the presence of the tempered martensite structure in the microstructure of the steel.

Conclusion

Cr12 steel is a high carbon, low alloy steel that possesses excellent strength, toughness and hardenability. It is widely used for industrial applications, such as cutting tools, bearings, and other components that require high wear resistance. The heat treatment process of Cr12 steel involves the quenching and tempering process, leading to a microstructure of face-centered cubic ferrites, tempered martensitic structures, and retained austenite. As a result of this structure, Cr12 steel exhibits excellent mechanical properties, including its high hardness and wear resistance.

This is a micrograph of a sample of AISI D3 Cr12 steel, which has been treated with both quenching and tempering. This steel is a cold work tool steel, normally used for applications that require high wear resistance and toughness.

The micrograph consists of a bright ferrite matrix, with a mildly coarser Widmanstatten structure at the upper left. Hard martensite (white) is also present, which can be seen in the forward-facing martensite laths and the plates of retained austenite (dark coloration) along the lath boundaries. This indicates that the steel has been cooled quickly enough to maintain the martensite and prevent reversion to ferrite.

The sample also contains small amounts of pearlite (grey/blue), which indicates that the tempering temperature is lower than the upper critical temperature, but greater than the lower critical temperature.

The black regions show a variety of precipitations, including both M2C and M3C carbides, as well as some secondary carbides. The M2C carbides are more finely dispersed than the M3C carbides, indicating that the steel has been held at the tempering temperature for an appropriate timescale for M2C to form.

Overall, the micrograph shows that the steel has been heat treated properly, with adequate hardening and tempering having been performed. The steel has good wear resistance and toughness, making it suitable for use in cold work tooling applications.