Chromium-molybdenum alloy steel (or chrome moly), is an alloy used for high pressure and temperature use. It is used in oil and gas, energy, construction and the automotive industries because of its corrosion resistance and high-temperature and tensile strength.
The added reliability provided by chrome moly means that it is the material of choice for a number of applications, and this article outlines a few of these applications and also the material’s properties.
An Introduction to Chromium Molybdenum Steel
Chromium molybdenum steel – frequently shortened to chrome moly – is a kind of low alloy steel used in a number of applications and industries. As the name suggests, the two key alloying elements are molybdenum (Mo) and chromium (Cr). These alloys are normally sorted into one main group, with names such as chrome croalloy, chromalloy, moly and CrMo often used . Industries where the alloy is common include construction, energy, oil and gas, and automotive.
Why Cr and Mo?
For years, Mo has been a standard alloying element used to produce creep-resistant steel capable of withstanding temperatures up to 530 °C . This is because Mo decreases the creep rate of steel successfully, and also slows the coagulation and coarsening of carbides during high-temperature use. Furthermore, this high-temperature suitability and creep-resistance mean the key application of Mo-based steel was in power generation and petrochemical plants.
However, continually increasing the Mo content of the steel in order to further improve its properties does not work since creep ductility actually decreases with increasing Mo . Another limitation refers to the fact that graphitization (breaking down of iron carbides) takes place above 500 °C. These drawbacks hinder the application of Mo-based steels.
A solution was discovered by alloying chromium with molybdenum. This gives the steel a number of advantages not found in Mo-based alloys, and CrMo steels were the first to allow steam temperatures in power stations to exceed 500 °C .
The reason this duo of alloying elements works so well is due to their combined properties (with a minimum Cr content of 9%, and a minimum Mo content of 1% ). For example, Mo gives the steel higher working temperatures and added strength. Moreover, the Cr results in exceptional oxidation, and helps the steel resist corrosion in a more effective manner . The Cr also provides good hardness penetration, and the Mo content guarantees the hardness is uniform .
This added strength and corrosion resistance explains the fact that CrMo steel is employed when the strength provided by mild carbon steel is not enough. These benefits provide chrome moly added reliability, which is why it is used in so many different applications.
Applications of Chrome Moly
For example, the added tensile strength and extra corrosion resistance means chrome moly is perfect for environments with an elevated temperature level (beyond that of simple Mo-based steels) [4,5]. So any applications or industries that operate equipment under high temperatures can benefit from using chromium molybdenum alloys. These industries include energy, automotive, oil and gas, metal production, and forming equipment . With such a high temperature tensile strength and corrosion resistance, CrMo is also found to be effective in salt-water applications .
Examples of equipment that use chrome moly include crack shafts, molds, chain links, machine shafts, bicycle tubing, drill collars and conveyors. The alloy’s properties also make it effective in manufacturing and construction. These properties include creep strength, hardenability, wear resistance, rigidity, good impact resistance, ease of fabrication, and the ability to be alloyed in ways that develop “fitness for use” in specific applications .
Case Study: A Novel CrMo Material Using Cobalt
Adding Cr to Mo-alloys allowed them to be used in different high-temperature applications, and provided benefits that were not found in Mo-based steel. This constant advancement of high-temperature steel is continuing, and researchers are continually looking for ways that will help boost the performance of chrome moly, and ensure that it remains the material of choice in a number of industries.
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 extensive popularity in medical and engineering applications . Generally, it is difficult to machine this material due to its high strength, wear resistance, toughness and low thermal conductivity. This can mean shorter tool lifespan and rapid tool wear. The authors reviewed the properties and characteristics of CoCrMo, and also analyzed how these properties contribute towards its machinability.
Machining these materials is considered to be challenging, and comes with a number of complications, mainly brought about by the alloy’s high strength, high wear resistance, toughness and poor thermal conductivity. The authors conclude that based on the demand for cobalt-based CrMo alloys in several applications and industries, more studies are needed to overcome the problems of poor machinability. Presently, chrome moly is the steel of choice, but cobalt-based CrMo alloys could become the standard in the future, provided these machinability issues can be solved .
Conclusions and Next Step
It is obvious that one requires the right steel in order to ensure long-lasting and strong equipment. Moreover, chromium molybdenum alloy is perfect for a number of industries and applications, due to its corrosion resistance and hardness.
Masteel UK Limited has an international reputation for the supply of high-quality Pressure Vessel Steel, and they offer a wide range of chrome moly. This range is available with benefits such as global shipping, a wide range of plate widths, a variety of thicknesses, profiling and accurate ‘in-house’ cutting, and also superior-quality steel from major European steel mills.
This information has been sourced, reviewed and adapted from materials provided by Masteel UK Ltd.
For more information on this source, please visit Masteel UK Ltd.