Iron and carbon together make steel. But the amount of carbon (plus other types of metal additives or even impurities) defines the main types of steel and how they differ. The precise composition of the steel, as well as factors like how quickly the metal cools during manufacturing, affects steel strength, durability, corrosion resistance, and ductility.
Although there are many different grades of steel, they all fall generally into one of four main groups, and they’re all used for different purposes.
While all steel contains carbon, this type of steel contains the purest combination of carbon and iron only, with just traces of other elements. The vast majority of steel produced is carbon steel. Carbon steel comes in three main varieties, based on how much carbon it actually contains:
- Low-carbon steel, also called “mild” steel, contains up to 0.3% carbon
- Medium-carbon steel has from 0.3 to 0.6% carbon
- High-carbon steel contains more than 0.6% carbon
A higher content of carbon makes the steel harder and stronger, but this content also makes the steel more difficult to work with, because high-carbon steel becomes brittle with handling. Carbon steel is the base for various products, like sheet metal, strapping, structural beams, and steel automobile wheels. Carbon steel can oxidize if not protected with appropriate paint or coatings.
Steel with added tungsten, cobalt, molybdenum, or vanadium has added heat resistance. It is harder steel that is useful for tools that cut or form other metals—hence, the name.
Stainless steel appears in many more objects than dining utensils. Stainless steel has a high amount of chromium added, and chromium is exceptionally corrosion-resistant. Medical tools and food handling equipment are often made of stainless steel. Certain types of plumbing fixtures are also made of stainless steel. Buyers of stainless-steel refrigerators may be surprised to discover that fridge magnets don’t stick to certain stainless steel appliances. This is due to the presence of around 8% nickel. This “austenitic” stainless steel is also not heat-treatable, in contrast to “martensitic” stainless steel, with a very low amount of nickel and a higher amount of carbon. Martensitic stainless steel is magnetic and can be heat-treated. This makes it good for knives, cutting tools, and medical and dental equipment.
Various proportions of metals like aluminum, copper, chromium, titanium, or nickel added to steel create differing characteristics. Alloys vary in ductility, machinability, strength, and resistance to corrosion. Alloys appear in auto parts, electric motors, and other applications that require steel more amenable to manipulation and forming by heat or mechanical means.
There are thousands of variations among these main types of steel, and the specific intended use will dictate the precise grade and type of metal necessary for each job.