SteelWhat is 4140 steel and what is it used for? Complete guide
4140 steel is probably the most widely used alloy steel in the metalworking industry. It is the most versatile machinery grade steel: it offers a good combination of strength, toughness and wear resistance, with acceptable machinability. In this guide we explain what 4140 steel is, its composition, its properties, how it is heat treated, what it is used for and the sizes we stock.
What is 4140 steel?
4140 is a low-alloy chromium-molybdenum steel (known in industry as "chromoly"). It is normally delivered quenched and tempered, that is, hardened and tempered at the mill, which gives it good mechanical strength without losing toughness. Chromium provides hardenability and wear resistance, and molybdenum improves hot strength and prevents brittleness during tempering.
It is a very forgiving general-purpose steel: it machines well, welds with precautions, can be hardened by through-hardening or surface hardening, and performs well in parts subject to stress and fatigue. That is why it is the "workhorse" of workshops, machinery manufacturers and the automotive industry.
Chemical composition of 4140 steel
This is the typical composition of 4140 steel (AISI 4140 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) | 0.75 – 1.00 % | Hardenability and strength |
| Chromium (Cr) | 0.80 – 1.10 % | Hardenability and wear resistance |
| Molybdenum (Mo) | 0.15 – 0.25 % | Hot strength, avoids brittleness |
| Phosphorus (P) | 0.035 % max. | Impurity, kept low |
| Sulfur (S) | 0.040 % max. | Impurity, kept low |
4140 properties and hardness
In the quenched and tempered condition, 4140 steel has a typical hardness of 28 a 34 HRC (approximately 285 to 330 HB), with good tensile strength and toughness suited to parts under stress. It can be hardened further by quenching, or only the surface can be hardened by nitriding o induction hardening, leaving the core tough. If you want to better understand hardness scales, we explain them in our article on steel hardness.
Heat treatment of 4140 steel
4140 responds very well to heat treatment. For reference:
- Annealed: at about 815 °C, slow cooling, to soften the steel and machine it easily.
- Hardening: 845 to 870 °C with oil quench.
- Tempering: from 480 to 650 °C depending on the final hardness desired (the higher the tempering temperature, the lower the hardness and the greater the toughness).
It also supports nitriding and induction hardening to achieve a harder surface without affecting the core. You can see the full picture in our article on heat treatments for steel.
What is it for? Applications of 4140
Thanks to its balance of strength and toughness, 4140 is used in parts that work under stress, torsion and fatigue:
- Drive axles and industrial shafts
- Machinery gears
- Automotive crankshafts and connecting rods
- Industrial spindles
- High-strength fasteners and industrial bolts
- Torsion bars
Forms and sizes we carry
At NAT MAC we stock 4140 steel in:
| Package size | Sizes |
|---|---|
| Round bars | Ø 1/2" (13 mm) a 24" (610 mm) |
| Plates | 1/2" (13 mm) to 10" (254 mm) thick |
If you need a particular size, tell us which one and we will gladly source it for you.
4140 equivalents and standards
4140 steel is also identified by these standards: AISI 4140 / ASTM A29 / DIN 1.7225 (42CrMo4) / JIS SCM440. If you want to understand how to read these equivalents, we explain it in our article on steel standards.
In summary
- 4140 is a chrome-molybdenum steel, quenched and tempered, very versatile and the most widely used in machinery
- Typical composition: C 0.38–0.43 %, Cr 0.80–1.10 %, Mo 0.15–0.25 %
- Typical quenched and tempered hardness 28–34 HRC; accepts nitriding and induction hardening
- Ideal for shafts, axles, gears, crankshafts, fasteners and high-strength bolts
- We carry it in round bar (1/2"–24") and plate (1/2"–10"), cut to size
Why buy your 4140 steel at NAT MAC?
We have more than 54 years of experience as an industrial supplier and ISO 9001 certified quality. 4140 is our most requested machinery grade steel and we keep it in stock in the most common sizes.
- Quality 4140 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.
