According to Wikipedia:
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 referred to as chromoly steel or CRMO. 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 treatement to avoid cold cracking).
While these grades of steel do contain chromium, it is not in great enough quantities to provide the corrosion resistance found in stainless steel.
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".
"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 outside is significantly hardened to reduce wear and tear on the part. This makes this grade of steel an excellent material for such uses as gears, piston pins, and crankshafts."[1]