1018 vs. 4140 Steel: Choosing Between Two Great Metals

Posted On March 1, 2022 By Protolabs

Looking for the best steel grade for your next project? There are certainly loads of them to choose from. Cruise the website of any online metal supermarket or leaf through the pages of Machinery’s Handbook, however, and you might leave more confused than when you started. Not to worry. We’ve identified two of the more common and popular steel grades—1018 and 4140—both of which boast a host of desirable properties. Here’s everything you need to know about each.

steel material blocks for cnc milling

1080 Steel: The Lowdown on Low Carbon

A type of mild steel, 1018 is a go-to material for a range of everyday applications. Common uses include:

  • mounting plates and brackets
  • pump parts and motor shafts
  • tie rods
  • mounting plates
  • pins
  • bushings, and more

Like all low carbon steels (meaning those with less than 0.30%), 1018 is readily machined and welded, and because it is relatively soft, it can also be swaged, crimped, and bent.

It’s also among the least expensive of all steels, although this is rarely a deciding factor until automotive-like part production volumes are required. Perhaps the biggest difference between it and 4140 is its hardenability—where the latter can be heat-treated to 58 HRC or higher, 1018 is limited to case hardening—a.k.a. carburizing—a process that leaves low carbon steel parts with a hard exterior and relatively soft core. 

Fun Facts About 4140 Steel

It’s 4140’s higher carbon content that makes this possible. See those last two digits? That’s the carbon amount, which in the case of 4140 is 0.40%. 1018 steel, on the other hand, contains 0.18% carbon. It doesn't look like much, but it makes a huge difference to metallurgists. Take a look at the chart below. Aside from carbon content, it might seem that 1018 and 4140 are largely the same.

Yet this chart doesn’t tell the entire story. Like many other “alloy steels,” 4140 also contains small amounts of chromium and molybdenum (less than 1% or so), two elements that bring a great deal to the metallurgical table.

 Element  1018 Steel 4140 Steel
 Iron  98.81-99.26%  96.79-97.78 
 Carbon  0.18%  0.40% 
 Manganese  0.60-0.90%    0.75-1.00%
 Phosphorus (max)   0.04%  0.035% 
 Sulfur (max)  0.05%   0.040%


As with stainless steels and superalloys, chromium and molybdenum make metals stronger, harder, and more resistant to corrosion. For example, 4140’s ultimate tensile strength is roughly half-again that of 1018 steel, as is its Brinell hardness in the annealed state. In addition, 4140’s fatigue strength is two to three times that of 1018, an important consideration to anyone concerned with product life and durability. It’s called high-tensile chrome-molybdenum steel for a reason.

steel rod stock for cnc turning

This makes 4140 an excellent choice for parts requiring impact resistance and torsional strength. Connecting rods and crankshafts are often made of 4140. So are many parts used in the logging industry, oil and gas applications, machine tools, jigs, molds, and fixtures. It's also quite machinable, albeit not quite at 1018's level. In fact, if 4140 steel were as weldable and formable as its mild steel counterpart, there'd be little reason to choose the latter, but here's where 4140 comes up short. It can be welded but not as readily as 1018, and becomes brittle without the right procedures, such as a post-welding heat treatment.

Choices, Choices

So where does that leave product designers and manufacturers? As stated earlier, these two metals are popular for a reason. Both offer distinct benefits as well as some distinct differences that may affect the finished part's price and function. CNC milling or turning a 4140 workpiece will call for somewhat lower cutting speeds and feedrates than one made of 1018. Tool wear is also a bit greater when machining alloy steels, although any difference in finished price due to such machining considerations is surely secondary to the product’s functionality and performance.

As noted, 4140 is tougher and more hardenable than 1018. If your application requires a workpiece that is both “through hard” and extremely impact resistant, then 4140 is a clear choice. It is less cold-workable than 1018, however, making a swaged or crimped joint unlikely. And as mentioned, 1018 is a welder’s dream. The same can’t be said for 4140, although it does have a significant edge in corrosion, heat, and wear resistance thanks to its higher carbon content and the small amounts of molybdenum and chromium present in 4140 steel.

Still have questions? One way to resolve at least some of them is by uploading your part file to our website. Our automated quoting tools will provide a quote and identify potential design concerns. Another way is to give one of our customer service people a call. They are always available to help at 877-479-3680 or [email protected]. All have extensive experience in material selection and can help guide you to the right decision for your unique requirements (even if it means choosing one of our other metals).