4140 Alloy Steel
4140 is a chromium-molybdenum alloy steel that combines good strength and toughness with the ability to be heat treated, making it one of the most widely used shaft and fixture materials in machining.
- Family /
- Alloy Steels
- Machinability /
- ~65%
- Condition /
- Annealed or pre-hardened (Q&T, ~28–32 HRC)
- Also called /
- forty one forty
4140 is a chromium-molybdenum alloy steel that combines good strength and toughness with the ability to be heat treated, making it one of the most widely used shaft and fixture materials in machining.
Machinability is approximate and rated against free-machining 1212 steel at 100%. Higher is easier to cut.
What it is
The chromium and molybdenum give it hardenability well below the surface, so a thick section can be through-hardened rather than just case-hardened. It is commonly bought pre-hardened and tempered — sold as 4140 HT or Q&T, typically around 28–32 HRC — which means it arrives ready to machine and already at strength.
How it cuts
In the annealed condition it cuts predictably and takes a good finish. Pre-hardened at 28–32 HRC it is noticeably tougher on tooling and wants rigid setups, positive rake and a real depth of cut rather than light passes. It produces manageable chips and does not work-harden the way stainless does.
What catches people out
Buying it pre-hardened avoids post-machining distortion but means every feature is cut in hard material. If a part needs to be hardened after machining, expect it to move and plan a finishing operation after heat treat.
4140 Alloy Steel — Common Questions
Is 4140 hard to machine?
- Not particularly, in the annealed condition — it cuts predictably and finishes well. Pre-hardened 4140 at 28–32 HRC is harder on tooling and needs a rigid setup and a genuine depth of cut, but it is routine work for a shop set up for it.
Should I machine 4140 before or after heat treatment?
- It depends on the tolerances. Machining annealed material and hardening afterwards gives the best tool life, but the part will move during heat treat and usually needs finishing afterwards. Starting from pre-hardened stock avoids the movement entirely, which is why most 4140 shaft work is done that way.
What is the difference between 4140 and 4340?
- 4340 adds nickel, which improves hardenability in heavy sections and raises toughness at a given strength. For most shaft and fixture work 4140 is sufficient and cheaper; 4340 is chosen for larger cross sections or higher shock loading.
Other alloy steels
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