41xx steel is a family of SAE steel grades, as specified by the Society of Automotive Engineers (SAE). Alloying elements include chromium and molybdenum, and as a result these materials are often informally referred to as chromoly steel (common variant stylings include chrome-moly, CrMo, CRMO, CR-MOLY, and similar). They have an excellent strength to weight ratio and are considerably stronger and harder than standard 1020 steel, but are not easily welded (need pre and post weld thermal treatment to avoid cold cracking).
Examples of applications for 4130, 4140 and 4145 include structural tubing, bicycle frames, tubes for transportation of pressurized gases, firearms receivers, clutch and flywheel components, and roll cages. 4150 stands out as being one of the steels accepted for use in M16 rifle and M4 carbine barrels by the United States military. These steels are also used in aircraft parts and therefore 41xx grade structural tubing is sometimes referred to as "aircraft tubing".
|SAE grade||% Cr||% Mo||% C *||% Mn||% P (max)||% S (max)||% Si|
|* The carbon composition of the alloy is denoted by the last two digits of the SAE specification number, in hundredths of a percent|
|Material||Condition||Tensile strength [psi (MPa)]||Yield strength [psi (MPa)]||Elongation in 2" [%]||Hardness (Rockwell)|
|4130||Cold drawn—normalized||85,000–110,000 psi (590–760 MPa)||70,000–85,000 psi (480–590 MPa)||20–30||B 90–96|
|4142||Hot rolled—annealed||90,000–100,000 psi (620–690 MPa)||60,000–70,000 psi (410–480 MPa)||20–30||B 90–95|
|Cold drawn—annealed||105,000–120,000 psi (720–830 MPa)||85,000–95,000 psi (590–660 MPa)||15–25||B 96–100|
|4150||Hot rolled—annealed||90,000–110,000 psi (620–760 MPa)||65,000–75,000 psi (450–520 MPa)||20–30||B 90–96|
Other characteristics 
One of the characteristics of this class of steel is the ability to be case hardened by carburization of the surface. The core of the material retains its bulk properties, while the outer surface is significantly hardened to reduce wear and tear. This makes this grade of steel an excellent material for such uses as gears, piston pins, crankshafts, and bike frames.
- Central Steel & Wire Company Catalog (2006-2008 ed.), p. 246. Note: "For bar products; plate, sheet and tubing may be slightly different."
- Central Steel & Wire Company Catalog (2006-2008 ed.), p. 260.