Steel is the Brady Bunch of fabrication metals. There’s a lot of them and they’re all related in one way or another. Plus, each type of steel has its own unique quirks and peculiarities. Many fans of the TV show had a favorite actor they followed through all five seasons. That’s okay for TV characters but it’s not a strategy for fabrication. Pick a steel unsuited to the task and your project could run into trouble.
While steel comes in many forms, (much like Brady Bunch kids,) the two types most often used in fabrication are stainless vs. carbon steel. Let’s think of them as Jan and Marcia. One is practical and down-to-earth, the other pretty but maybe also a little difficult. Here’s how to choose between them.
All steel is essentially iron with no more than 2% carbon. Go over that threshold and you get cast iron. That’s a useful metal for some applications but it’s not much used in fabrication.
When carbon content is below 1% the result is plain carbon steel. These steels have trace amounts of sulfur, manganese, phosphorus, and silicon to alter their properties. A lower carbon content makes the metal more ductile, machinable and easier to weld.
Stainless steels are plain carbon steels with a large serving of chromium. Technically speaking, stainless has a minimum of 10.5% chromium. Chromium has a strong affinity for oxygen and this is what gives stainless its corrosion-resistance. In plain carbon steel surface iron bonds to oxygen. This is what causes rust. With stainless chromium-oxygen bonds cover the surface and prevent rust from forming. Better still, scratch the surface and the newly exposed chromium creates more rust-preventing oxygen bonds.
What to Consider When Selecting a Stainless Steel vs. Carbon for Fabrication
Perhaps this is obvious, but the start point is, what does the application need? Strength? An attractive appearance? Or just the lowest cost? The key factors are listed below, but their relative importance will depend on what you’re trying to achieve.
Let’s go through each one of these, comparing carbon and stainless steel as we go.
Carbon steels need a coating to stop them rusting. Powder coating and painting are both popular choices. Surface preparation is needed for both, and paint in particular may need periodic touching-up.
Stainless needs no such protection, unless it’s to be used in a strongly chlorine atmosphere. Swimming pools and sea air are environments where some chlorine-induced corrosion is possible. If a fabrication will go into a situation like those it’s best to pick a more chlorine-resistant grade of stainless such as 316.
A good thing about stainless is that it can be given a range of finishes. Brushing is a popular choice, as used on many household appliances, but for other applications it can be polished. Our fire truck bumpers are great example, which we polish to a mirror-like shine. An alternative, often seen in architectural applications, is a satin finish.
Ductility refers to how readily a metal will bend and stretch without cracking. Bending operations, like those performed on a press brake, require a lot of ductility in the metal. Less ductile metals may be radiused but will often crack if bent through sharp angles.
Low carbon steels have excellent ductility. In stainless steel ductility depends very much on composition. Austentic stainless steels, which contain significant nickel in addition to chromium, have good ductility. The most common of these is the 304 grade.
Stainless steels with higher carbon content, such as the 420 and 440 grades have very low ductility. As a result they are rarely used in fabrication.
As a rule, strength is inversely proportional to ductility. Thus stronger metals are harder to form. Any grade of stainless steel is stronger than plain low carbon steels.
Plain carbon steels become harder to weld as carbon content rises. Preheating is often needed to ensure a good weld when carbon content exceeds 1%.
Stainless is harder to weld because thermal conductivity is lower than for plain carbon steels. That said, austentic stainless steels weld well when conductivity differences are taken into account. A particularly useful feature of austentic stainless is that it doesn’t harden when heated. That means when welding there’s no hardening in the heat-affected zone. However, welding can increase susceptibility to corrosion.
Pound for pound, stainless is significantly more expensive than carbon steel. However, there are a couple of mitigating factors to consider. First, since stainless is stronger you may be able to use less of it.
And second, its corrosion resistance means no ongoing painting expense. As a result, over the lifetime of a fabrication, especially one that will live outdoors, the cost differential could narrow significantly.
Get Help With Choosing Your Steel
The performance, durability and cost of your next fabrication project is strongly influenced by the type of metal you specify. Steel is often the default choice, thanks to a combination of low cost and formability, but remember that one word covers a large family of closely-related metals. If The Brady Bunch taught us anything about big families, it’s that individual members can behave very differently, so choose your favorite wisely!
If you’d like help in selecting the best steel for your next project, please discuss it with us. As fabricators with many years experience we can advise you on what will bend and weld easily. We’ll also be happy to take you through the various final appearance options so you get a fabrication that really meets your needs.