Alloy Steel AISI/SAE 4140, 4140 HT Steel
AISI/SAE 4140 is a low alloy steel that contains chromium, molybdenum, and manganese. Known for its impact and abrasion resistance, Alloy Steel 4140 also offers users outstanding fatigue and torsional strength. The addition of these alloying elements and it’s relatively high carbon content (around 0.40%) provide this low-alloy steel with excellent strength to weight ratio and make it a lot stronger and harder than standard 1018 and 1020 steel.
4140 is typically supplied in either the hardened and tempered condition (referred to as 4140HT) and in the annealed condition. The prehardened condition is heat treated to a medium hardness (HRC 28/32) and is designated as 4140 HT. 4140 steel annealed is supplied in the soft condition and can be hardened to RC 54. 4140 steel annealed is designated as 4140 Ann or simply 4140. 4140 HT is supplied in many forms such as hot rolled or turned, ground and polished rounds and block or DCF plate to be used as a holder material for high strength structural components.
Welding:
Welding is possible through most popular techniques, and it has a high level of ductility. Machinability is possible in the annealed condition, while Alloy Steel 4140 takes cold working, heating, and quenching well.
For welding it is recommended that you choose 4140 steel in tempered condition. Welding on 4140 HT steel is not recommended due to the high susceptibility for cracking, even when using preheat. If you must weld 4140HT it is absolutely necessary that you stress relieve the part at a temperature 15C below the original tempering temperature. Typically, if you have to weld on 4140HT you are repairing something that broke.