What is Chromoly and what does 4130 mean?

This posting is a shortened version of a technical paper I wrote for work. I hope you enjoy, and learn in the process!

At one time, all bikes were made of steel.  Steel has been popular thanks to its strength to weight ratio and ease of use.  These factors as well as it’s low cost and availability make it ideal for use in bicycles.  The most common type of steel used for bikes is chromoly, and 4130 chromoly is the most widely recognized.  So what does chromoly mean and what do the numbers 4130 refer to?

Steel is an alloy of iron and other metals.  The term ‘alloy’ is commonly used to refer to aluminum but it actually refers to any metal that’s mixed with other elements in order to achieve the desired mechanical properties such as strength, weight, durability, hardness, weldablitly, etc.  'Chromoly' refers to a specific type of steel alloy that combines Iron with Chromium and Molybdenum (chro-moly, get it?) along with small amounts of other metals.  Changing these small amounts of other metals in the alloy creates grades of chromoly with different mechanical properties and costs.  These steel alloys are identified by four digit codes set forth by the American Iron & Steel Institute (AISI) and are defined by their approximate chemical composition. 

In ‘4130 Chromoly’,  the "41" means that it is a low alloy steel containing chromium and molybdenum (about 1% and 0.2% respectively). The "30" indicates a carbon content of 0.30%.* These small percentages don't seem like much, but changing the amounts only slightly will affect mechanical properties of the steel quite a bit. For comparison, in '4340 chromoly', '43' means that it contains 0.8% Chromium and 0.25% molybdenum. The '40' indicates a carbon content of 0.4%. 4340 is a much stronger and harder steel alloy that is more difficult to bend and cut, plus it's a bit heavier than 4130. It costs more as well.

Since most steel bikes constructed in similar fashion when they’re built, 4130 Chromoly happens to have the best mechanical properties for use in bicycle frame building, and the low cost makes it ideal.  There are other chromoly alloys that are stronger, but they are more difficult to cut, bend, shape, and weld than 4130, or they are more costly.

It is widely thought that companies like Reynolds, Columbus, Deda, and Tange (among others) have their own formulas for steel alloys.  Actually, name brand tubing is normally off the shelf 4130 chromoly tubing that they will then make into double or triple butted tubing with butted sections in different lengths.  They are then sold as tubing sets designed for specific ride qualities (and sometimes rider sizes).  Newer name brand steel tubing sets (Reynolds 853 for example) offer technology that relates to how the tubing is annealed or hardened - this offers more resistance to weld and brazing weakness than others.  This allows for lighter, stronger frames.

I hope this offers some useful information about what 4130 chromoly is and why it's used for bikes. Want the full document with more information about different types of steel, including name brand steel (Reynolds, Columbus, etc), cheap department store steel bikes, and a comparison between 4130 and other steel? Send me a message and I'll forward the entire document to you.

I originally posted this on my other blog but I thought it was a good way to start off this blog.