To those unfamiliar with metals, the term “carbon steel” may seem mysterious. But all steel is an alloy of carbon and iron. Adding carbon transforms soft or brittle iron into a strong metal that can support structures like bridges and skyscrapers. So, in theory, carbon and high-carbon steels should be stronger than other steels, while mid-carbon steels should be average. And in certain contexts, that’s true: carbon substantially increases steel hardness. However, beyond a certain point, it also makes the steel brittle.

Mild steels retain some flexibility, allowing them to perform better under twisting or compressive forces. Ultimately, whether mild steel or carbon steel is better depends on the intended use.

The Types of Carbon Steel

It wouldn’t be inaccurate to say that our civilization is built of steel. It is the material of choice for reinforcing concrete, structural steel beams, and the pipelines that bring fuel to our cities. It is also the material of choice for wrenches, blades, some types of machine bits, and the heavy stamps that press metals into shape for body panels and other constructions. The reason that steel finds use in so many roles is that it is an incredibly versatile alloy whose mechanical properties can be adjusted with relative ease, and a major part of those adjustments is the steel’s carbon contents. It is more than mild steel vs carbon steel, with there being at least four categories of steel based on carbon content alone.

Types of Carbon Steel	Characteristics
Low Carbon Steels	Typically, low carbon steel is defined as having between 0.05 to 0.15% carbon. This is a highly ductile steel. As such, it is very suitable for stamping machines and produces auto parts like body panels and guards. It can also be used for piping, tubes, and some structural materials like angle iron and c-channel. There is some overlap at higher carbon levels in this class of steels with medium carbon steel or mild steel, which are sometimes called mild steels.
Medium Carbon or Mild Steels	Medium carbon or mild steels can have a carbon content in the range of 0.1 to 1.29% this gives these steels a good mix of ductility and strength, because of this blend of properties these steels are some of the most commonly used steels with widespread use in structures, fabrication of non-pressurized tanks, and fabrication of pressure vessels. Due to their carbon content these steels are capable of additional hardening through heat treatment which expands their use still further.
High Carbon Steels	The carbon content of high carbon steels ranges from 0.100 to 3.40% and this high carbon content makes the metal significantly harder than other steels. This steel is suitable for cutting other materials, and one of the most common places that high carbon steels can be seen is in kitchen and chef knives. This steel is hard enough that it can be used to cut low and medium carbon steels, and another common place it is seen is as blades in industrial equipment used to cut sheet steel.
Ultra-High Carbon Steels	Ultra-high carbon steels are very hard and potentially very brittle. They can be found in some very high quality and very expensive knife sets. However, they are far more commonly found in tooling, where they are used to machine other softer metals or to cut fine details—like threads—in other metals like aluminum, steel, or even stainless steel. These steels are very difficult to make through traditional methods, but methods like powder sintering are making ultra-high carbon steels more widely available.

The definitions for each category of carbon steel are loose. The astute reader may have noticed that there is a significant amount of overlap between categories. Low-carbon steel variants that are at the higher end of carbon content for the category are actually edging into the percentages that classify them as mild steels. The same is true across all categories, with some mild steels edging into high-carbon steels and some high-carbon steels being on the border of ultra-high-carbon steels.

There are a few reasons for this apparent inconsistency. The first is the sheer versatility of steel. Hardness is not solely determined by carbon content. Heat treatment can increase the hardness of mild steel to rival higher carbon steel. The addition of other metals like chromium or molybdenum to the alloy can likewise increase the hardness of the steel, so it is possible to find mild steel used where carbon steel may be expected. The second is that mild steel vs carbon steel is a largely informal designation used by metal workers, and what that means can change depending on which role they fill and in what industry they fill it.

Mild Steel vs. Carbon Steel in Architecture and Industry