Metallographic diagram of Cr12MoV (tempered three times at 520°C after vacuum quenching at 1020°C)

Microstructure of Cr12MoV after Vacuum Heat Treatment and Reheating

Cr12MoV is a kind of steel with excellent wear and corrosion resistance, making it a popular material for use in various technologies and engineering research. In order to maximize its performance, it is often heat treated under a vacuum and reheated a few times to change its microstructure and harden its surface. This article investigates the microstructure of Cr12MoV steel after it has been subjected to a vacuum heat treatment at 1020°C and reheated 520°C three times.

During the vacuum heat treatment, the materials surface layer remains at a temperature of 1020°C while the core of the material, which is exposed to atmospheric pressure, remains below 900°C. This results in a surface layer with a more homogenous microstructure, as well as a reduction in cracks and porosity, compared to when the entire material is treated at 1020°C.

At the same time, the rapid cooling that is associated with vacuum heat treatment results in increased hardness and greater mechanical strength. Through dynamic simulation, it was observed that during the vacuum heat treatment, the surface layer of Cr12MoV is significantly more hardened than the interior. This can be attributed to the rapid cooling effect of the vacuum heat treatment which produces an increased volume percentage ratio of martensite and lower austenite grain boundary boundary fraction. This significantly increases the hardness of the material, making it more wear and corrosion resistant.

Reheating is then done to further refine the microstructure and make the steel stronger. When Cr12MoV is reheated to 520°C three times, the iron-carbon austenite phase loses some of its carbon content, resulting in a larger grain size but with much finer grain boundaries. This in turn, enables the steel to better absorb and disperse large impacts, and maintain its hardness while reducing its brittleness.

Using scanning electron microscopy (SEM), it can be seen that the overall microstructure of Cr12MoV after vacuum heat treatment and reheating is composed of two distinct parts. The surface layer mainly has martensite, which is composed of large amount of primary carbides during the cooling process. The interior, on the other hand, has a well-refined austenite phase, where small amounts of intragranular and intervinular ferrite are also present.

Overall, the microstructure of Cr12MoV is remarkably different when subjected to vacuum heat treatment and reheating. The surface layer has a higher percentage of martensite, due to the rapid cooling effect, resulting in increased hardness and greater mechanical strength, while the interior has a well-refined austenite phase consisting of small amounts of intragranular and intervinular ferrite. Through this process, the steel is able to better absorb and disperse large impacts, while maintaining its hardness and reducing its brittleness. This makes it an even more effective material to use in various technological and engineering applications.

Introduction

Cr12MoV is a high-chromium, high-carbon, high-molybdenum tool steel, which contains robust and high-strength wear resistance and is used for high-precision parts. To improve the performance of Cr12MoV, different heat treatment processes including vacuum hardening and tempering are adopted. After 1020℃ vacuum hardening, Cr12MoV was tempered at 520℃ for three times.

Metal Structure

The microstructure of Cr12MoV consists of martensite and retained austenite. Upon tempering, the martensite retained Austenite transforms to martensite and offers higher strength with improved toughness. The carbon and chromium atoms have the same size and occupy specific sites. The alloy can provide a uniform and austenite structure due to large interatomic distance.

Metallographic Properties

After vacuum hardening and tempering at 520℃ for three times, Cr12MoV shows a homogeneous, distribution of ferrite, pearlite and retained austenite. Ferrite and pearlite are evenly dispersed in the alloy matrix. A single face-centered cubic crystal structure may form due to dislocation in the lath at the grain boundaries of the alloy. The retained austenite is uniformly distributed and the morphology is not changed. It can be observed in the form of spheroidization embedded in the matrix of ferrite and pearlite.

Conclusion

After vacuum hardening and tempering at 520℃ for three times, Cr12MoV presents a homogeneous, distribution of ferrite, pearlite, and retained austenite. The alloy provides a uniform and austenite structure due to larger interatomic distance. High-chromium, high-carbon, and high-molybdenum elements have their definite sites and carbon and chromium atoms have the same size. The element of martensite will retain after tempering and then offer higher strength and improved toughness.