SteelWhat is 9840 steel and what is it used for? Complete guide
9840 steel is a chrome-nickel-molybdenum machinery grade that offers an excellent combination of strength and toughness, with very good fatigue resistance. It is widely used as an alternative to 4140 and 4340, and even as a substitute for mold steels. In this guide we explain what 9840 steel is, its composition, its properties, how it is heat treated, what it is used for and the sizes we stock.
What is 9840 steel?
9840 is a triple-alloy low-alloy steel, with chromium, nickel and molybdenum. It is delivered quenched and tempered and stands out for its balance of strength and toughness, with good fatigue resistance. It combines the virtues of 4140 and 4340, so it is used as an alternative to both depending on availability and application.
In addition, because of its hardness in the quenched and tempered condition and its good machinability, it is also used as a substitute for mold steels such as P20+Ni, in mold bases and tooling components.
Chemical composition of 9840 steel
This is the typical composition of 9840 steel (AISI/SAE 9840 standard):
| Element | Contents | What for? |
|---|---|---|
| Carbon (C) | 0.38 – 0.43 % | Provides hardness and strength |
| Silicon (Si) | 0.20 – 0.35 % | Deoxidizer, adds strength |
| Manganese (Mn) | 0.70 – 0.90 % | Hardenability and strength |
| Chromium (Cr) | 0.70 – 0.90 % | Hardenability and wear resistance |
| Nickel (Ni) | 0.85 – 1.15 % | Toughness and hardenability |
| Molybdenum (Mo) | 0.20 – 0.30 % | Hot strength, avoids brittleness |
| Phosphorus (P) | 0.040 % max. | Impurity, kept low |
| Sulfur (S) | 0.040 % max. | Impurity, kept low |
9840 properties and hardness
In the annealed condition it is around 190 HB, and it is usually delivered pre-hardened at 248 to 301 HB, ready to machine. Quenched and tempered it reaches approximately 322 HB (34 HRC), with a tensile strength on the order of 750 MPa and a yield strength close to 690 MPa. With hardening and tempering it can reach higher hardnesses, close to 50 HRC. Its main strength is fatigue resistance, superior to that of 4140.
Heat treatment of 9840 steel
9840 is mainly supplied in two conditions:
- Annealed or pre-hardened: for easy machining; it is often supplied pre-hardened and ready to work.
- Quenched and tempered (hardening and tempering): to obtain working strength and toughness; allows additional surface hardening.
You can get the big picture in our article on heat treatments for steel.
What is it for? Applications of 9840
9840 is used in machinery parts that demand strength and toughness:
- Axles and shafts
- Transmission gears
- High-load pinions
- Crankshafts
- Connecting rods
- High-strength bolts
- Industrial couplings
- Heavy machinery components
- Mold bases and molds
Forms and sizes we carry
At NAT MAC we stock 9840 steel in:
| Package size | Sizes |
|---|---|
| Plates | 1/2" (13 mm) to 10" (254 mm) thick |
| Round bars | Ø 1/2" (13 mm) a 5" (127 mm) |
If you need a particular size, tell us which one and we will gladly source it for you.
9840 equivalents and standards
9840 steel is also identified by these standards: AISI 9840 / EN 39NiCrMo3 (1.6510). You can see how to read them in our article on steel standards.
In summary
- 9840 is a triple-alloy Cr-Ni-Mo steel, quenched and tempered, with good strength and toughness
- Typical composition: C 0.38–0.43 %, Ni 0.85–1.15 %, Cr 0.70–0.90 %, Mo 0.20–0.30 %
- Quenched and tempered ~322 HB (34 HRC); with hardening it can approach 50 HRC; good fatigue strength
- Alternative to 4140 and 4340, and substitute for P20+Ni in mold bases
- We carry it in plate (1/2"–10") and round bar (1/2"–5"), cut to size
Why buy your 9840 steel at NAT MAC?
We have more than 54 years of experience as an industrial supplier and ISO 9001 certified quality. 9840 is a great alternative when you are looking for a balance of strength, toughness, and fatigue resistance; we help you compare it with 4140 and 4340.
- Quality 9840 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.
