4140 and 4340 can perform similarly, but the best choice depends on your specific application. We talked to Craig Darragh, one of Sullivan’s consulting metallurgists, to clarify the differences between these two grades.
“It really depends on the components you're making,” summarizes Craig. “What they're going into, what the specs are and things like that. In short, the size, cost and engineering requirements of the application.”
The most notable difference in the chemical composition between these two alloy steels is nickel—4340 contains nickel, whereas 4140 does not.
| 4340 | 4140 | |
| % Carbon | 0.38 - 0.43 | 0.38 - 0.43 |
| % Manganese | 0.60 - 0.80 | 0.75 - 1.00 |
| % Silicon | 0.15 - 0.35 | 0.15 - 0.35 |
| % Chromium | 0.70 - 0.90 | 0.80 - 1.10 |
| % Nickel | 1.65 - 2.00 | - |
| % Molybdenum | 0.20 - 0.30 | 0.15 - 0.25 |
*Table values found in the updated Metallus Practical Data for Metallurgists Guide.
Although similar in composition, 4340 and 4140 have some differences in their material properties. These differences are generally greater as the cross-sectional size of a part increases.
Here's a generalized overview of how they compare:
| 4340 | 4140 | |
| Tensile Strength | Very good in larger sizes | Very good in smaller sizes |
| Toughness | Very good over larger size range | Very good in smaller sizes |
| Hardness | Very good over larger size range | Very good in smaller sizes |
| Hardenability | Excellent | Moderate |
| Machinability | Slightly more difficult | Easier |
As Craig puts it, “In 4140, there’s some hardenability kick from the manganese (Mn), chromium (Cr) and molybdenum (Mo). But in 4340, the nickel along with Mn, Cr and Mo gives you a lot more hardenability, and accordingly, more strength, ductility and toughness over a much larger size range.”
It’s important to note the difference between hardness and hardenability. “The hardness will tell you how hard a grade can get for a specific heat treatment,” says Craig, “but the hardenability tells the depth to which you can achieve a given hardness. It is what allows you to design a larger size part and still achieve the desired properties.”
In other words, a steel’s hardness is how hard it can get, while hardenability tells you how deep that hardness can go. This is why the size and dimensions of the finished component are part of the decision for which alloy to use.
Both 4340 and 4140 can be heat treated with various conditions; for example, normalized and tempered, quenched and tempered or normalized, quenched, and tempered. A tailored heat treatment procedure can help achieve the properties required for your application.
As 4340, on average, has more hardness and toughness than 4140, it can be more challenging to machine. If you’re making similar parts machined to tight tolerances, 4140 may be the better choice.
Both 4340 and 4140 are weldable, but, due to their carbon content (and overall carbon equivalents), they require particular welding procedures for a successful weld—more so than if you’re using a lower-carbon steel.
Both 4140 and 4340 are excellent grades for a wide range of industries due to their balance of strength, hardness, toughness and fatigue strength.
Craig points out, “When you have a component that has to take some shock loading and have good strength and toughness, 4340 is a wonderful grade.”
“4140 is a good grade that will do a cost-effective job for a lot of parts,” says Craig.
There is no single answer for choosing between 4340 and 4140. Chemical composition, heat treatment, machinability, finished part dimensions, cost, and more all play a part in deciding which alloy is best. Each grade allows you to tailor the material to the part.
Sullivan Steel’s experts support the steel we sell with professional, application-specific advice. We can help you determine which material will meet your performance requirements.
Deciding between 4140 and 4340? Start a Live Chat or contact us to get started.