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Steel Standards Explained

When selecting or specifying steel for an application, automotive engineers tend to default to using Society of Automotive Engineers (SAE) standards. However, other sectors typically prefer the American Society for Testing and Materials (ASTM) standards. This article delves into the reasons for having two (apparently competing) sets of steel standards and how the standards relate to the four primary types of steel.

Steel Standards & Mechanical Properties

The two steel standards prioritize different aspects of performance. Both the SAE and ASTM standards define chemical composition but diverge from there.

SAE standards emphasize mechanical properties, especially those relevant to automotive applications, bringing factors like tensile and fatigue strength, hardness and wear resistance, and toughness and machinability to the fore.

In contrast, ASTM steel standards are more about physical characteristics and where the steel will be used. Properties like density, coefficient of thermal expansion, corrosion resistance, and weldability are prioritized.

How Steels Are Classified Within SAE & ASTM Systems

With some exceptions, the SAE steel standard gives every steel a four-digit designation. The first two digits indicate the primary alloying elements and the last two digits show the carbon content, in one-hundredths of a percent.

An illustrated diagram showing the SAE steel grading system. The first box at the top says 'SAE Steel Grading System' and there are two arrows pointing down from the box. The arrow on the left is pointing to another box titled 'Material Type.' The arrow to the right is pointing to another box titled 'Carbon Presence.' Both the 'Material Type' and 'Carbon Presence' boxes have one arrow coming out of them, pointing to separate boxes that say 'XX'

Comparatively, ASTM standards cover a far wider range of materials than just steel and use a prefix letter to indicate the type of material covered. Their steel standards all begin with the letter “A” and are often followed by a “Grade” reference. This grade makes it possible to differentiate between steels that meet a particular specification, but the numbers used to represent grades don’t necessarily have a specific meaning.

A look at how the two standards relate to the four main types of steel will help explain.

Carbon Steel

Carbon steel is composed of iron with just a small proportion of carbon.

Because its only alloying element is carbon, it’s designed as 10XX in the SAE system. So, SAE 1010, a widely used low-carbon steel, indicates that there is 0.10% carbon within this grade.

In the ASTM steel standard, an example of a similar low-carbon steel is A36. As noted, though, the ‘36’ has no special significance.

Alloy Steels

An alloy steel is one with measurable quantities of other elements besides iron and carbon. Metal steel is often alloyed with nickel, chromium, and molybdenum. Alloying is performed to enhance the physical and/or mechanical properties of the metal.

In the SAE steel standard, these alloying additions are shown by the first two digits of the code. To give some common examples:

  • SAE 31XX: Nickel-chromium steels.
  • SAE 41XX: Chromium-molybdenum (chromoly) steels.
  • SAE 8620: Nickel-chromium-molybdenum steels.

Widely used ASTM alloy steels include:

  • ASTM A193: Chromium-manganese alloy steels.
  • ASTM A182: Chromium-molybdenum alloy steels.
  • ASTM A572: Low-carbon steel with vanadium, niobium, and manganese.

Stainless Steel

Technically a class of alloy steel, stainless steels are known for their strength and corrosion resistance, both of which result from their naturally high levels of chromium and nickel, along with other alloys added to enhance specific properties.

Perhaps confusingly, the SAE steel standard uses a three-digit identification system for stainless steel that’s different from the designations used for carbon and alloy steel. Some highlights of this include:

  • SAE 2XX: Austenitic chromium and manganese steels.
  • SAE 3XX: Formable and corrosion-resistant stainless steels.
  • SAE 303: A chromium-nickel-manganese alloy where a small addition of sulfur improves machinability.

Two widely used ASTM stainless steels are:

  • ASTM A240: A chromium-nickel alloy used for food processing equipment, kitchen appliances, and construction and chemical applications.
  • ASTM A240: A chromium-nickel-molybdenum alloy with enhanced saltwater corrosion resistance.

Tool Steel

Like stainless, tool steels are a class of alloy steels. Tool steels are formulated primarily for strength and hardness. Main alloying elements include tungsten, cobalt, molybdenum, and vanadium.

The SAE standard for tool steels follows an alphanumeric format starting with a letter indicating the type of steel. This is done in terms of properties and hardening method rather than composition. The letter is followed by a numeric suffix signifying a specific grade within that type. Two examples are:

  • SAE M2: A high-speed steel used for cutting tools.
  • SAE O1: A general-purpose tool steel used for punches and dies.

As with other types of steel, the ASTM standard uses an alphanumeric code for tool steels, starting with “A” followed by an assigned number. ASTM A681 is the general specification and covers steels such as SAE M2 and SAE O1.

Have Questions About Steel Standards?

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