Metallographic diagram of W2Mo9Cr4Co8 (M42) (quenching and tempering treatment)

Introduction

This article addresses the metallurgical structure of the W2Mo9Cr4Co8 steel alloy (M42), a high-speed tool steel alloy which has been extensively used in the manufacture of cutting tools due to its strength, wear resistance, and cost efficiency. It became popular shortly after World War II due to its ability to maintain cutting edge sharpness and endure long service life.

Due to its hardness and toughness, the W2Mo9Cr4Co8 steel alloy is usually subject to a two-step heat treatment sequence involving tempering and annealing. This article will outline the steps involved in these treatments, highlighting the subsequent changes in metallurgical structure and properties of the steel alloy in detail.

Overview

The W2Mo9Cr4Co8 steel alloy is a high-speed tool steel containing tungsten, molybdenum, chromium, and cobalt. The combination of these elements results in a tougher alloy as compared to other high-speed tool steels. It is weldable and relatively easy to machine and grind and it does not acquire a purple-blue hue from tempering at temperatures below 600˚C. The steel alloy is therefore an ideal choice for hot-machining applications, as well as for cutting and forming operations.

The W2Mo9Cr4Co8 steel alloy, which is primarily used for lathe cutting tools, is subject to a process known as heat treatment. This is a process which aims to improve the mechanical properties of the steel alloy by altering its microstructure through controlled heating and cooling. Heat treatment can be divided into two main categories, tempering and annealing.

Tempering

Tempering is a heat treatment process in which the steel alloy is heated in order to attain a desired hardness. The temperature at which the steel alloy is heated is determined by its composition and the subsequent required properties. The W2Mo9Cr4Co8 steel alloy is heated to between 540-680˚C, however, lower temperatures around 510-540˚C provide a better balance between hardness and ductility. The heated steel alloy is then cooled in the air or in oil, allowing the material to harden gradually.

After tempering, the W2Mo9Cr4Co8 steel alloy has more uniform hardness and higher strength, along with improved fatigue resistance and better ductility. A fine, grain-like structure can also be observed. It is important to note that although tempering is used to improve the mechanical properties of the steel alloy, the hardness of the material is reduced.

Annealing

Annealing is a form of heat treatment which is used to soften the steel alloy. In this process, the material is heated to temperatures around 790-870˚C and then cooled slowly to room temperature. This thermomechanical treatment improves the ductility of the material without sacrificing its strength, making it more suitable for machining operations.

The annealing process also increases the resistance to wear and fatigue in the steel alloy by positively affecting the microstructure. The heat-treated material acquires a uniform, fine-grained and somewhat homogenous structure, creating improved characteristics without increasing hardness.

Conclusion

The metallurgical structure of the W2Mo9Cr4Co8 steel alloy is heavily influenced by tempering and annealing. Tempering is used to increase the strength and wear resistance of the steel alloy, while annealing is used to improve the material’s ductility and fatigue strength. These heat treatments also result in a finer, more uniform grain structure, improving the mechanical properties of the steel alloy without sacrificing its overall hardness.

The purpose for this experiment was to investigate the microstructure of AISI 4340 steel after being heat treated. AISI 4340 is a nickel-chromium-molybdenum low-alloy steel known for its toughness and durability. It is a versatile steel alloy used in many applications such as automotive, aerospace, and general engineering.

First, a 300 grams sample of AISI 4340 was cut from a heat treated bar and placed in a heat-treated oven. The sample was heated to 845 °C (1553 °F) over a period of 2 hours and then quenched in a brine solution at an approximate temperature of 40°C (104°F). The quenched sample was tempered in the oven at 360°C (680°F) over a two-hour period. After annealing, the sample was inspected with a metallographic microscope to examine the microstructure.

The microstructure of the AISI 4340 steel exhibited several phases after heat treating, including ferrite and pearlite. The ferrite was visible as a homogeneous and lightly colored constituent, while the pearlite was characterized by a distinct dark gray pattern. The higher sulfur content of the AISI 4340 steel resulted in the presence of small amounts of plastically deformed martensite near the borders of ferrite, which was also observed under the microscope.

Overall, it may be concluded that the heat treatment of AISI 4340 steel results in a considerable amount of microstructural refinement. The main phases observed in this experiment were ferrite and pearlite, which are expected for this alloy due to its composition. The slight presence of martensite, likely resulting from the steel’s sulfur content, further confirms the effectiveness of the heat treating process.