4340 Alloy Steel
4340 is a nickel-chromium-molybdenum alloy steel used where high strength and toughness are needed in heavy sections, particularly under shock loading.
- Family /
- Alloy Steels
- Machinability /
- ~55%
- Condition /
- Annealed or quenched and tempered
- Also called /
- forty three forty
4340 is a nickel-chromium-molybdenum alloy steel used where high strength and toughness are needed in heavy sections, particularly under shock loading.
Machinability is approximate and rated against free-machining 1212 steel at 100%. Higher is easier to cut.
What it is
The nickel addition over 4140 improves hardenability, so large cross sections harden more uniformly through their thickness, and raises toughness at a given hardness. It is a standard aerospace and heavy-shaft material.
How it cuts
Similar in character to 4140 but tougher, so it is harder on tools and wants slower speeds with adequate feed. It is routinely supplied in the quenched and tempered condition, where it machines like a stronger, stickier 4140.
What catches people out
At high hardness 4340 is notch-sensitive. Sharp internal corners, tool marks and undercuts left in highly stressed areas become crack initiation sites, so surface finish in fillets matters as much as the dimension.
4340 Alloy Steel — Common Questions
When should I specify 4340 instead of 4140?
- When the section is thick enough that 4140 will not harden uniformly through it, or when the part sees shock loading and needs toughness at high strength. For most shafts under a few inches in diameter, 4140 does the job for less money.
Other alloy steels
Got a part for 4340 Alloy Steel?
Send the drawing or the sample. We will tell you straight what it will take to make.