Metallographic diagram of 9Cr2Mo (martensite)

Five-stage heat treatment of 9Cr2Mo

Five-stage heat treatment of 9Cr2Mo is an important technique for improving the performance of 9Cr2Mo material. Heat treatment process of 9Cr2Mo usually includes five stages, which are interruption of water quenching, solution of retained austenite, tempering, aging, quenching tempering, and tempering. In order to understand the performance of 9Cr2Mo material after five-stage heat treatment, it is necessary to analyze the microstructure of 9Cr2Mo steel by optical microscopy, scanning electron microscopy and X-ray diffraction measurement.

The result of optical microscopy images of 9Cr2Mo steel shows that there are two kinds of microstructures exist on the steel surface, ferrite and martensite. The high magnification of optical microscopy reveals that there exist a lot of different second phase particles on the 9Cr2Mo steel, including spread and precipitated types. The spread particles are regular rectangular shapes which composed of a large number of layered arms and distributed on the matrix near surface of 30-40μm. The size of those precipitated particles ranges from 1μm to 3μm.

The results of scanning electron microscopy show that there are many kinds of microstructures exist on the 9Cr2Mo steel, including martensite, ferrite and various types of second phase particles. There exist both dispersed particles and precipitated particles, among which the type of dispersed particles is followed by the ferrite and martensite. The precipitates exist as small spheres, fibers, nets and lamella layers on the surface. The chemical compositions of dispersed particles and precipitated particles were analysed by the energy dispersive X-ray spectrometry, and their component concentrations were determined.

X-ray diffraction measurement reveals that the crystallographic structure of 9Cr2Mo steel has been changed after the five-stage heat treatment, which demonstrates that there is a significant difference between the as-cast and treated 9Cr2Mo steel. The result shows that the crystallographic structure changes from an FCC structure to a mixture of FCC and BCC structures, which exhibits a decent combination of micro-structures.

Finally, based on the above analysis and discussion of the five-stage heat treatment of 9Cr2Mo steel, it is concluded that this heat treatment method is an effective way to improve the microstructure and performance of 9Cr2Mo steel for different application fields. The 9Cr2Mo material has shown a considerable improvement in strength, hardness, wear resistance and impact value after the five-stage heat treatment.

9Cr2Mo is a creep-resistant alloy made of 9% Chromium, 0.2% Molybdenum for the balance of Iron. It belongs to the family of Martensitic stainless steels and it possesses good corrosion-resistant and strength-toughness combination coupled with excellent hardenability. Its machinability is affected by its high chromium content; however, its mechanical properties are well retained at elevated operating temperature.

This alloy is also known as “9Cr2Mo bearing steel”, “9Cr2Mo valve steel”, or “9Cr2Mo steels” depending on the application.

The ferrite microstructure of 9Cr2Mo is composed of small subunits of martensite and austenite that are usually not visible under the optical microscope. When 9Cr2Mo is tempered at 500-600°C, its martensite is converted to a bainitic ferrite. The microstructure also changes from fine, uniformly distributed, submicron particles to larger (approximately 2-4µm size range), more uniform particles. In addition, the grain size may be about 2-4 times larger than for anneals of the same material.

When heat treating 9Cr2Mo steel, a furnace temperature of 1350-1450°F (732-787°C) is usually recommended. This austenitization should produce a fairly low rate of cooling between 850-1010°F (454-545°C).It will generally be found that the higher the tempering temperature, the better the hardness and differential hardness are obtained in the product.

The finished microstructure of 9Cr2Mo consists of a combination of bainite, ferrite and carbides. The carbides will typically be found at the prior austenite grain boundaries and at the grain-size junctions. The types of carbides present vary with the heat treatment and alloy composition. The most common carbide found in 9Cr2Mo is M7C3, and this type is typically found preferentially at grain boundaries and intersections.

The 9Cr2Mo material is a good choice to provide a wear-resistant and creep-resistant material and offers good machinability. It is often used in valves and shafts that require wear-resistance and high-temperatures.