What is Chromoly Steel?

All over BMX and cycle sites you see the term 'Chromoly' or 'Chromoly steel'; but what is it exactly? In BMX frames durability is an essential factor in making a good product, so the use of chromoly has become widespread amongst all good BMX bike manufacturers.

So what is it?

Chromoly steel is a type of engineering steel that gets its name from a combination of the words “chromium” and “molybdenum” – two of the major alloying elements. Chromoly steel is often used when more strength is required than that of mild carbon steel, though it often comes at an increase in cost. Chromoly falls under the the AISI 41xx designations, and it is important to be aware of the different properties and applications of chromoly steel when deciding which material you need.

AISI 4130

Chromoly steel is actually engineering steel grade 4130. The “30” at the end of the grade number designates that it has approximately 0.30% carbon by weight. The added chromium and molybdenum help to give the steel different properties from its mild steel counterpart, AISI 1030, even though they have the same percentage of carbon. The alloying elements found in AISI 4130 help to increase the strength to a level higher than that of AISI 1030, which can be increased even more using a proper hardening procedure. The added chromium helps to increase the steels hardenability and also the corrosion resistance; although other types of materials with more corrosion resisting properties should be considered if the material is to be used in a corrosive environment. The added molybdenum helps to increase the toughness. Other important benefits of AISI 4130 include the ability to be easily hardened by heat treating or work hardening, and the ability to be case hardened using a process called carburizing.

In its annealed state, AISI 4130 has good formability, machinability, and is quite weldable. For thick sections or low heat input welding processes, preheat may be necessary and is almost always a good idea.

Common Applications outside the cycle industry

The applications of Chromoly steels can be found in a variety of industries. They are very commonly used in automotive and heavy equipment parts, the oil and gas industry, metal production and forming equipment, and many other industries. Here are some more specific examples aside from the BMX biking world:

Pins

Furnace equipment

Crank shafts

Chain links

Drill collars

Machine shafts

Conveyors

Tyre rods

Miscellaneous tooling