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What is Chromoly?

Chromoly steel is a type of low-alloy steel which is stronger and more durable, weight for weight, than mild carbon steel. The name is derived from the two major alloying elements contained – chromium and molybdenum. The additional chrome increases the steel’s hardenability and resistance to corrosion; the added molybdenum increases the steel’s toughness and resistance to temperature fluctuations. Chromoly steel is often used in applications when high strength is needed as it is more durable and considerably stronger than standard steel.

Chromoly in manufacturing.

Chromoly’s excellent strength-to-weight ratio and tensile strength makes it highly suitable for use in the manufacture of aircraft fuselages, high-end bicycle frames, and racing car chassis. Due to its high tensile strength, manufacturers can use less of the steel to reduce weight, without compromising the strength or integrity of the final component. With high-end bikes and race cars, this reduction in weight can convert to an all-important increase in speed and performance, however, the stiffness of the steel remains approximately the same, so with less steel used, the chromoly bike frame is more flexible than one manufactured from standard steel. In the USA, nearly all chassis and roll-cages in NASCAR racing are made from chromoly because it offers better strength-to-weight ratio and higher ductility than carbon steels; it is also a more cost-effective material than carbon fibre or titanium.

Chromoly has been used widely in the aeronautical industry for over a hundred years and has been used in the automotive industry since the 1950s. The use of chromoly in the cycling industry is more recent, though is now very widespread, and is also in common usage in construction, and the oil and gas industries.

Characteristics of chromoly

An important characteristic of chromoly is that it can be easily hardened by heat treating or work hardening. It can also be case-hardened by a process known as carburisation, where the outside of the metal is significantly hardened whilst the material retains its chemical properties. Hardening is highly beneficial as it reduces wear and tear over the lifetime of use.  Carburisation is a process that enables the steel to absorb carbon while the metal is heated in a carbon-bearing material, charcoal or carbon monoxide for example. When the steel is quenched the carbon on the surface becomes hard; depending on the temperature  and length of the carburisation cycle, the treated area will vary in carbon content and therefore hardness.  In general, the longer the metal is treated for at a higher temperature, the more carbon diffusion will take place which will increase the hardness. The ability to increase the hardness through heat or work hardening makes chromoly an excellent material for manufacturing components such as pistons and crankshafts that undergo tough usage.

Welding chromoly

There are a few extra considerations when welding chromoly, though technique is not much different from welding mild steel or stainless steel. MIG or TIG welding techniques can be used, although industry feedback indicates that TIG welding is faster, more effective, and cleaner than MIG welding. Annealed chromoly has good machining and weldability properties.

A common belief is that chromoly needs to be preheated prior to welding, but this is not usually necessary if the sheet or tubing is less than 3 mm thickness. If the material is more than 3 mm in thickness, preheating should be done gradually. The material needs to be prepared by cleaning it thoroughly before welding as this will help prevent future problems with the weld. Welding should be carried out a little slower than normal which will slow the cooling rate, but it is not advisable to speed-cool the welds.

Corrosion properties

Whilst chromoly contains chromium, which is beneficial for increasing resistance to corrosion, the chromium content is not high enough to provide the level of corrosion resistance found in stainless steel; if the steel is to be used in a corrosive environment other types of materials with higher corrosion resistance are recommended.

Value for money

Chromoly steel does cost more than mild steel, but the strength and durability of chromoly combined with the ability to use less than its standard steel counterpart often balances and justifies this increase in cost.

Chromoly can also be considered against the much higher costs for carbon fibre and titanium as it is a much more attractively priced product giving great strength, durability, and flexibility.

 

Shop our range of EN24T chromoly steel round bar here.