Chromoly, technically known as 4130 steel is an alloy steel containing molybdenum and chromium. The American Iron and Steel Institute has a four number code system that they use for naming these materials where the first digit indicates the type of steel (carbon, nickel, manganese, and in this case, chromium-molybdenum) the second indicates any alloying elements, and the last two digits indicate the percentage of the carbon content.
Chromoly is significantly stronger than common steel and as a result is used for high end bike frames, fuselages on small aircraft, and roll cages for race cars. It has both a high strength to weight ratio and high tensile strength. It is also more weldable, ductile, tough, and formable than other choices. It is much stronger and more durable than 1020 steel. Each of its applications uses chromoly to get different results.
Rather than emphasizing its strength, manufacturers use chromoly for bike frames to reduce weight. In other words, the strength of the bike frame is the same as if it were made of 1020 steel, but it is lighter because less chromoly is required to bring it to this strength. The stiffness of chromoly and 1020 steel are similar however, so chromoly bike frames are more flexible. Chromoly can also be hardened by a process known as carburization. As a result of carburization, the outside is hardened to reduce wear and tear while the core retains its properties. This property makes chromoly especially useful for gears, crankshafts, and piston pins.
The technique for welding chromoly is pretty much the same as it is for standard steel and stainless steel. TIG welding has been shown to be the best way to weld chromoly without risking the loss of its properties. Start off by preparing the area that is going to be welded. Make sure that joint surfaces are smooth and contaminant free. While preheating is not necessary, it won’t hurt. Preheat to a temperature appropriate for the ambient temperature. You will want to weld slower than you usually do, keeping in mind the cooling rate and preventing the chance of welded areas cooling too quickly. Make sure there is no breeze or draft in the area, and do not attempt to speed cool your new welds. Make sure you are using a TIG machine with a high frequency. This will eliminate arc strikes and therefore any chance of cracks or brittle spots. Also make sure you are using high quality tungsten electrodes. Also, use an undermatched filler metal like E70S2 TIG rod as it results in better properties and works better in general. Make sure to slowly taper off to eliminate the risk of craters that would eventually turn into cracks.
You can check for the presence of chromoly by looking at its spark when you grind it. Chromoly emits a dull orange spark that has a large, distinctive flair with a trailing tip. It sort of looks like a foxtail seed.