SteelWhat is 4340 steel and what is it used for? Complete guide
4340 steel is the high-strength grade par excellence among machinery grade steels. When a part has to withstand very high loads, torsion and fatigue without failing, 4340 is the benchmark. In this guide we explain what 4340 steel is, its composition, its properties, how it is heat treated, what it is used for and the sizes we stock.
What is 4340 steel?
4340 is a low-alloy chromium-nickel-molybdenum steel. The addition of nickel is what sets it apart from 4140: it gives greater toughness and deep hardenability, that is, it hardens uniformly even in large sections. It is delivered quenched and tempered and offers an outstanding combination of strength, toughness and fatigue resistance.
It is the steel chosen for the most demanding parts, where failure is not an option. That is why it is common in aerospace, high-load transmissions and critical structural components.
Chemical composition of 4340 steel
This is the typical composition of 4340 steel (AISI 4340 standard):
| Element | Contents | What for? |
|---|---|---|
| Carbon (C) | 0.38 – 0.43 % | Provides hardness and strength |
| Silicon (Si) | 0.15 – 0.35 % | Deoxidizer, adds strength |
| Manganese (Mn) | 0.60 – 0.80 % | Hardenability and strength |
| Chromium (Cr) | 0.70 – 0.90 % | Hardenability and wear resistance |
| Nickel (Ni) | 1.65 – 2.00 % | Toughness and deep hardenability |
| Molybdenum (Mo) | 0.20 – 0.30 % | Hot strength, avoids brittleness |
| Phosphorus (P) | 0.035 % max. | Impurity, kept low |
| Sulfur (S) | 0.040 % max. | Impurity, kept low |
4340 properties and hardness
In the annealed condition, 4340 is around 217 HB. Its real value shows up quenched and tempered: with hardening and tempering it reaches very high strength, with typical hardnesses of 38 to 54 HRC depending on the tempering temperature, while retaining good toughness. Its great advantage over 4140 is its deep hardenability and better fatigue resistance, which makes it ideal for large sections and heavily loaded parts.
Heat treatment of 4340 steel
4340 has an excellent response to heat treatment. For reference:
- Annealed: at about 844 °C, slow furnace cooling, for easy machining.
- Hardening: around 830 °C with oil quench.
- Tempering: adjusted to the desired hardness and toughness.
Thanks to the nickel, it hardens evenly even in large diameters. You can get the big picture in our article on heat treatments for steel.
What is it for? Applications of 4340
4340 is reserved for the most demanding parts:
- High-strength axles and drive shafts
- Crankshafts
- High-load gears
- Industrial spindles
- Engine connecting rods
- High-strength bolts
- Landing gear
- Heavy machinery components
Forms and sizes we carry
At NAT MAC we stock 4340 steel in:
| Package size | Sizes |
|---|---|
| Round bars | Ø 1/2" (13 mm) a 24" (610 mm) |
If you need a particular size, tell us which one and we will gladly source it for you.
4340 equivalents and standards
4340 steel is also identified by these standards: AISI 4340 / EN-DIN ≈ 1.6565 / JIS ≈ SNCM439. You can learn how to read them in our article on steel standards.
In summary
- 4340 is a chrome-nickel-molybdenum steel with high strength and deep hardenability
- Typical composition: C 0.38–0.43 %, Ni 1.65–2.00 %, Cr 0.70–0.90 %, Mo 0.20–0.30 %
- In the quenched and tempered condition it reaches hardness of 38–54 HRC with good toughness and fatigue resistance
- Ideal for crankshafts, high-load gears, landing gear and large sections
- We carry it in round bar (1/2"–24"), cut to size
Why buy your 4340 steel at NAT MAC?
We have more than 54 years of experience as an industrial supplier and ISO 9001 certified quality. 4340 is the best choice when the part cannot fail; we advise you on choosing the right form and treatment.
- Quality 4340 Steel, backed and traceable
- Material cut to the size you need
- Technical advice on choosing the right grade for your application
- On-time deliveries and stock in the most requested sizes
Tell us what part you are going to make and in what dimensions, and we will recommend the right grade and form. Request your free quote.
Sources: data sheet from our NAT MAC catalog and public industry references (Metal Supermarkets, AZoM, MatWeb, AoBo Steel and Serviacero). Composition and hardness values are for reference and may vary by standard, heat and treatment.
