Carbon Steel vs Stainless Steel

What does Carbon Steel actually mean?

“Carbon steel” has two meanings — a technical definition and a more general classification. The technical definition is very clear: According to the American Iron and Steel Institute (AISI), a steel must meet the following standards to match the technical definition of carbon steel:

• No minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium, or zirconium, or any other element to be added to obtain a desired alloying effect
• When the specified minimum for copper does not exceed 0.40 per cent
• When the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.
  

The technical definition, while complex, boils down to one simple constraint — true carbon steels must have almost no alloying elements, making them primarily comprised of two materials: iron and carbon. The amount of carbon can vary and there are a few acceptable alloying materials, but these steels are simple.

In addition to the precise definition, the term carbon steel is also used to refer to the broad group of alloy steels that are not stainless steels. Unlike carbon steels, low-alloy steels can contain small quantities of a wide variety of alloying elements, allowing them to be customized for a wider variety of applications. These steels, while not satisfying the technical requirements of carbon steel, signify the greater divide in steel: stainless steel vs everything else.

Carbon Steel vs Stainless Steel (by definition)

Carbon steel by definition is extremely simple. It’s Iron with some carbon, and limited alloying elements. In addition, any steel that requires alloying elements (like 4140 and 4340, for example) are not carbon steels. Within the carbon steel definition, materials can be defined as either low-carbon steel or high-carbon steel. Low-carbon steels are extremely common, while high-carbon steels are only used in high-strength, non-corrosive environments. 1020 Steel, a low-carbon steel, is one of the most popular steels produced today.

Carbon steel has varying mechanical properties based on carbon content. Low-carbon steels are weaker and softer, but can be machined and welded easily; while high-carbon steel is stronger, but significantly harder to process. All carbon steels are susceptible to rust, making them unfit for use in a wide variety of end-use applications. Overall, carbon steel is excellent if you’re looking for a low-cost metal, but generally unfit for high-quality or high-precision manufacturing operations.

Low-Alloy Steels (sometimes called carbon steels)

Low-alloy steels integrate one or more alloying elements (like chromium, cobalt, niobium, molybdenum, nickel, titanium, tungsten, vanadium, or zirconium) to improve on material properties of traditional carbon steels. They’re often stronger, stiffer, and slightly more resistant to corrosion than traditional carbon steels.

Alloy steels are defined by the primary alloying materials (in addition to carbon). 4140, one of the most common alloy steels, is a Chromium-Molybdenum alloy steel. This means that the primary alloying elements are chromium (which boosts corrosion resistance) and molybdenum (which boosts toughness). As a result, 4140 is used in high-wear applications and elevated temperatures.

Alloy steels are one of the most widely used steels in industry today. They’re machinable, affordable, readily available, and possess good mechanical properties. If a part doesn’t need to be corrosion-resistant, low-alloy steels offer the best bang for your buck.

The properties that make alloy steel advantageous to produce via conventional methods make it less valuable to 3D print. Because it’s easily machined and cheaply acquired, metal 3D printing’s higher inherent part costs make it economically untenable to print. A few metal printing companies offer low-alloy steels like 4140, but they’re generally rare.

Stainless Steels

Stainless steels are united around one key material property: excellent corrosion resistance, attributable to high Chromium content (>10.5% by mass) and low carbon content (<1.2% by mass). Beyond corrosion resistance, the mechanical properties of these steels can vary greatly.

Austenitic stainless steels are the most common type of stainless steel. They are corrosion resistant and can be both easily machined and welded, though they cannot be heat treated. 303 and 304 are the most common types of austenitic stainless steels, and 316L is a variant that maximizes corrosion resistance. These steels are used in a wide variety of operations — because they’re weatherproof, they work just about anywhere. Due to their higher costs, metal 3D printing can be a viable fabrication method for these parts.

Martensitic stainless steels offer better mechanical properties to austenitic steels at the cost of ductility. As a group, they lack the general versatility of austenitic steels — however, their high-strength hardness paired with corrosion resistance far superior to low alloy steels make them fit for any high-strength part that’s in an oxidizing environment. In addition, martensitic steels can be heat treated to further boost hardness, strength, and stiffness.

17-4 PH is a particularly useful type of martensitic stainless steel that can be heat treated to fit a variety of material properties. Due to its high hardness and extremely low machinability, it’s often cheaper to 3D print than painstakingly machine. If you’d like to learn more about 3D printing metal parts, check out the Markforged Metal X.

Carbon Steel vs Stainless Steel: Final Verdict

The debate of carbon steel vs stainless steel is a bit more complicated than originally thought, as carbon steel can refer to two different types of steel: traditional carbon steel and low-alloy steel.

Compared to low-carbon steel, stainless steel offers a massive upgrade in strength, hardness, and most importantly corrosion resistance. High carbon steel offers strength rivaling and sometimes exceeding stainless steel, but is largely a niche material in the manufacturing world. Unlike any carbon steel, stainless steel can survive and thrive, oxidation free, in corrosive or humid environments. That being said, carbon steel is much cheaper than stainless steel and better suited for large structural components, like tubes, beams, and rolled sheet steel.

Low-alloy steel is superior to carbon steel in most ways, but still lacks corrosion resistance. It can effectively match the material properties of stainless steel — as a result, alloys like 4140 and 4340 are often machined and used in many applications in which a little oxidation doesn’t hurt. Stainless steel is a higher grade material better used in industrial operations, where part quality can’t be compromised.