The air-hardening steel alloy often used in high-end bicycle frames is Reynolds 853.
Reynolds 853 is a high-strength steel tubing grade made by Reynolds Technology, the Birmingham, England, manufacturer founded in 1898. It is used mainly as thin-walled frame tubing, rather than as a complete bicycle-frame material in bulk form.
Its air-hardening characteristic means the steel gains strength as it cools after welding. That helps frame builders preserve strength around welded joints without relying on a separate conventional quench-hardening step. The result is a strong, relatively light steel frame with the ride qualities many builders associate with quality steel.
A common mix-up is confusing 853 with Reynolds 531, the company's famous earlier manganese-molybdenum tubing, or with Columbus SL, a competing Italian tubing range. Chromoly is a broad family of chromium-molybdenum steels, not this particular named Reynolds grade. Reynolds 953 is a newer maraging stainless-steel tubing grade, so the numbers are not interchangeable.