Many people commonly think that steel is simply a predetermined combination of iron and carbon, but in fact, there are over 3,500 different grades of steel. These grades can be compared by measuring the amount of carbon, additional alloys, and the methods employed by manufacturers in their processing.

The Four Main Types of Steel

1. Carbon Steel

Carbon steel contains minimal elements apart from carbon and iron. It is the most widely used type among the four steel grades and accounts for 90% of steel production.

Carbon steel is categorized into three major subgroups based on the amount of carbon present: low carbon steel/mild steel (up to 0.3% carbon), medium carbon steel (0.3–0.6% carbon), and high carbon steel (more than 0.6% carbon).

Due to its cost-effectiveness and sufficient strength for large-scale construction, carbon steel is often produced in bulk. It is typically used in structural components and machinery parts.

2. Alloy Steel

Alloy steel is created by adding alloying elements such as nickel, copper, chromium, and aluminum. These elements enhance the strength, ductility, corrosion resistance, and machinability of steel. It is primarily used in the manufacturing of automotive components and tools.

3. Stainless Steel

Stainless steel incorporates alloying elements, including 10-20% chromium and nickel, along with other elements like silicon, manganese, and carbon. This results in exceptional corrosion resistance and the ability to withstand harsh conditions, making it safe for exterior construction.

Stainless steel is widely used in various industries, including electrical equipment. For example, SUS304 is in high demand for its ability to withstand exposure to various elements while maintaining its properties.

4. Tool Steel

As the name suggests, tool steel is used for cutting and drilling equipment. It contains elements like tungsten, molybdenum, cobalt, and vanadium, which enhance its heat resistance and overall durability.

Tool steel is commonly used in hand and power tools, and its shape remains intact even after extensive use.

Grades of Steel

The steel grading system provides a method for classifying steel types based on various applications. For instance, the cooling rate at which steel is cooled by a manufacturer can impact its strength at the molecular level. The duration of time steel is held at critical temperatures during the cooling process also plays a crucial role. In practice, two steel plates with the same alloy composition can have different grades depending on their heat treatment processes.

ASTM vs SAE

What is ASTM?

ASTM International, formerly known as the American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services. 

The ASTM grading system assigns letter prefixes based on the broad category of each metal (with 'A' denoting specifications for ferrous materials like iron and steel) and sequential numbers assigned to specific properties of that metal. (Such as ASTM A242)

What is SAE?

SAE International, formerly known as the Society of Automotive Engineers, is another standards organization with a focus on automotive and aerospace standards. Similar to ASTM, SAE develops standards, but it has a narrower scope. 

The SAE grading system uses a four-digit number for classification (such as SAE 1045), where the first two digits indicate the type of steel and alloying elements, and the last two digits represent the carbon content of the metal.

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