Summary: Chromium-molybdenum alloy steel (or chrome moly), is an alloy employed for high pressure and temperature use. It’s used in construction, oil and gas, energy, and the automotive industries because of its corrosion resistance and high-temperature and tensile strength. The added reliability provided by chrome moly means it’s the material of choice for many applications, and this article outlines some of these applications, as well as the material’s properties.   

An Introduction to Chromium Molybdenum Steel

 

Chromium molybdenum steel – often shortened to chrome moly – is a type of low alloy steel used in various industries and applications. And as the name suggests, the two main alloying elements are chromium (Cr) and molybdenum (Mo). These alloys are typically sorted into one main group, with names such as chrome moly, croalloy, chromalloy, and CrMo often used [1]. Industries where the alloy is common include construction, oil and gas, energy, and automotive.

Why Cr and Mo?

Well, for years, Mo has been a standard alloying element used to make creep-resistant steel that could withstand temperatures up to 530 oC [2]. This is because Mo lowers the creep rate of steel successfully, and slows the coagulation and coarsening of carbides during high-temperature use. And this high-temperature suitability and creep-resistance mean the main application of Mo-based steel was in power generation and petrochemical plants.

But continually increasing the Mo content of the steel to further improve its properties doesn’t work, because creep ductility actually decreases with increasing Mo [2]. And another limitation was that graphitization (breaking down of iron carbides) occurs above 500 oC. These drawbacks limit the application of Mo-based steels.

But a solution was found by alloying chromium with molybdenum. This gives the steel many advantages not found in Mo-based alloys, and CrMo steels were the first to allow steam temperatures in power stations to exceed 500°C [2].

The reason this duo of alloying elements works so well is because of their combined properties (with a minimum Cr content of 9%, and a minimum Mo content of 1% [4]). For instance, Mo gives the steel added strength and higher working temperatures. And the Cr results in excellent oxidation, and helps the steel resist corrosion more effectively [1]. The Cr also provides good hardness penetration, and the Mo content ensures the hardness is uniform [3].

This added strength and corrosion resistance means that CrMo steel is used when the strength provided by mild carbon steel isn’t enough. These benefits give chrome moly added reliability, which is why it’s used in so many different applications.

 

Applications of Chrome Moly

 

For instance, the added tensile strength and extra corrosion resistance means chrome moly is ideal for environments with an elevated temperature level (beyond that of simple Mo-based steels) [4,5]. So any industries or applications that operate equipment under high temperatures can benefit from using chromium molybdenum alloys. These industries include oil and gas, energy, automotive, metal production, and forming equipment [5]. And with such a high temperature tensile strength and corrosion resistance, CrMo is also effective in salt-water applications [4].

Examples of equipment that use chrome moly include molds, bicycle tubing, crack shafts, chain links, drill collars, machine shafts and conveyors. The alloy’s properties also make it effective in construction and manufacturing. These properties include creep strength, rigidity, hardenability, wear resistance, good impact resistance, ease of fabrication, and the ability to be alloyed in ways that create “fitness for use” in certain applications [1].

 

Case study: A novel CrMo material using Cobalt

 

We’ve seen how adding Cr to Mo-alloys allowed them to be used in a variety of high-temperature applications, and provided benefits not found in Mo-based steel. This constant improvement of high-temperature steel is ongoing, and researchers are constantly looking for ways to boost the performance of chrome moly, and ensure it remains the material of choice in many industries.

And one example of this is alloying CrMo with cobalt (Co). Zaman et al. reviewed the machinability of cobalt chromium molybdenum (CoCrMo) alloy, an advanced material gaining widespread popularity in engineering and medical applications [6]. Generally, it’s difficult to machine this material because of its high strength, toughness, wear resistance and low thermal conductivity. This can mean rapid tool wear and shorter tool lifespan. The authors reviewed the characteristics and properties of CoCrMo, as well as how these properties contribute towards its machinability.

Machining these materials is challenging, and comes with many complications, mainly caused by the alloy’s high strength, toughness, high wear resistance and poor thermal conductivity. The authors conclude that based on the demand for cobalt-based CrMo alloys in many industries and applications, more studies are needed to overcome the problems of poor machinability. Right now, chrome moly is the steel of choice, but cobalt-based CrMo alloys may become the standard in the future, provided these machinability issues can be solved [6].

 

Conclusions and next step

 

It’s clear you need the right steel to ensure strong and long-lasting equipment. And chromium molybdenum alloy is ideal for many applications and industries, due to its hardness and corrosion resistance.

Masteel UK Limited has a global reputation for the supply of high-quality Pressure Vessel Steel, and they offer an extensive range of chrome moly. This range comes with benefits such as global shipping, a variety of plate widths, an extensive range of thicknesses, accurate ‘in-house’ cutting, and profiling, as well as high-quality steel from major European steel mills.

For further information about Masteel’s grades of chrome molybdenum steel plate, please visit https://masteel.co.uk/chrome-moly/

 

References

 

[1] http://www.thefabricator.com/article/metalsmaterials/chromiummolybdenumaits-uses-and-fabrication-requirements
[2] http://www.imoa.info/molybdenum-uses/molybdenum-grade-alloysteels-irons/high-temperature-steel.php
[3] https://www.azom.com/article.aspx?ArticleID=6116
[4] https://masteel.co.uk/chrome-moly/
[5] https://www.metalsupermarkets.com/what-is-chromoly/
[6] http://www.sciencedirect.com/science/article/pii/S2351978917303542