“CRMO” redirects here. For the autoimmune disease see Chronic recurrent multifocal osteomyelitis. 41xx steel is a family of steel grades, as specified by the Society of Automotive Engineers (SAE). Alloying elements include chromium andmolybdenum, and as a result these materials are often referred to as chromoly steel or CRMO. They have an excellent strength to weight ratio, are easily welded and are considerably stronger and harder than standard 1020 steel. 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 include structural tubing, bicycle frames, AK-47 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. They are also used in aircraft and therefore 41xx grade structural tubing is sometimes referred to as “aircraft tubing”.