Types of Steel and How They’re Graded

At its very element, steel is a mix of iron and carbon. Carbon makes up 0.002% to 2.14% of the mix. Steel itself is most often mixed with other elements or metals to take advantage of certain characteristics of these substances. The combination of steel and added elements leads to a wide range of products, from tools to structural building components.

Knowing about the types of steel available and how steel is graded can help you select the right metal for your project.

Types of steel

What Are the Different Types Of Steel?

There are four types of steel: carbon, stainless, alloy, and tool. Each of these is comprised of a different mix of metals, creating a variety of end products.

Carbon steel is a mix of carbon and iron, with trace amounts of other elements. It’s classified by the carbon content:

• Low carbon steel – up to 0.3% carbon, used for structural components, wires and bolts
• Medium carbon steel – 0.3–0.6% carbon, used for machinery and railroad tracks
• High carbon steel – more than 0.6% carbon, used for steel bars and spring steel

It is the most popular type of steel produced, making up about 90% of steel production and is used in large scale construction projects. 

Stainless steel is created by adding 10-20% chromium, as well as other elements like nickel, silicon, or manganese, to steel. It’s most notable for its shiny finish and is corrosion resistant and strong. It is often mistaken for chrome, which is created by adding an outer layer of chromium to metal. Stainless steel is different from chrome because it includes chromium, nickel and other metals within the alloy. 

Types Of Stainless Steel

It’s common to see stainless steel in kitchen appliances, medical tools and automotive applications, but stainless steel is highly valued for other uses. Stainless steel is grouped into four subcategories that each serve a different purpose.

• Martensitic alloys: Toughness is a hallmark of martensitic alloys, but they’re prone to corrosion. Manufacturers form these alloys with a rapid-cooling process that makes them ideal for medical instruments, cutlery and pliers.
• Ferritic alloys: These are less-expensive steels with low amounts of carbon and nickel. Automotive manufacturers use ferritic alloys because of their chromium-induced strength and sheen.
• Austenitic alloys: Austenitic alloys have higher chromium and nickel contents, which improves their resistance to corrosion and causes them to be non-magnetic. They’re present in commercial kitchen appliances since they’re durable and easy to clean.
• Duplex alloys:A combination of austenitic and ferritic alloys results in a duplex alloy that inherits the properties of both while doubling strength. They’re also ductile and corrosion-resistant due to their fairly high chromium content. Duplex alloys are common among instruments and pipework used in gas, oil and chemical industries.

Alloy steel is a combination of steel and other metals in significant proportions. Two alloy steels you may recognize are copper and aluminum.

Tool steel is used to create steel tools and parts, like drill bits and screwdrivers. It is a high carbon steel with added tungsten, molybdenum, cobalt and vanadium that is hard and heat resistant.

What Is The Best Grade of Steel?

There is no universal “best” grade of steel, as the optimal steel grade for an application depends on many factors, such as the intended usage, mechanical and physical requirements, and financial limits.

Steel grades that are regularly used and deemed the top series from each type include:

• Carbon steels: A36, A529, A572, 1020, 1045, and 4130
  
• Alloy steels: 4140, 4150, 4340, 9310, and 52100
  
• Stainless steels: 304, 316, 410, and 420
  
• Tool steels: D2, H13, and M2
  

How Is Steel Graded?

There are two grading systems that are used to categorize each type of steel.

ASTM (American Society for Testing and Materials) uses an alphanumeric classification to determine a steel’s overall categorization and specific attributes. Each alloy is given a letter prefix based on its overall category (“A” is the designation for iron and steel materials), followed by a sequential number to correspond with that material’s specific properties.

SAE (Society of Automotive Engineers) uses a four-digit numerical classification that shows the steel type and carbon content, with the presence of other alloying elements. The first two digits give the steel type and element concentration and the last two indicate how much carbon is in the metal.

Grades are used by engineers and architects to specify the type of metal required and ensure quality and consistency.

Selecting The Right Steel For Your Project

There are several factors you should consider when selecting the right steel for your project:

• The requirements of the application (strength, aesthetic appearance, etc.)
• Environmental conditions
• Ductility, or how pliable the material needs to be
• Weldability, or how easy it is to weld or solder
• Machinability, or how easy it is to cut
• Tensile strength, or how much force is required to snap the metal
• Corrosion resistance, or how well can it withstand damage caused by oxidization or other chemical reactions.
• Cost

Steel is one of the most versatile building materials out there. It can be used for structural components, wiring, equipment, tools, and fixtures. Knowing the best steel for the job requires understanding the types of steel and how they are made and graded.