Chromoly is an abbreviation for "chromium-molybdenum steel". Chromoly is an alloy used to produce tubing for bicycle frames and race-car roll cages. It is not as lightweight as some steel alloys, but has the advantages of high tensile strength and malleability. It is also easily welded and is considerably stronger and more durable than standard (1020) steel tubing.
It is also known as 4130 steel, and is composed of 0.28 to 0.33% carbon, 0.4- to 0.6% manganese, 0.8 to 1.1% chromium, 0.15 to 0.25% molybdenum, 0.04% phosphorus, 0.04% sulfur, and 0.2 to 0.35% silicon, in addition to iron.