Steel Grading: Understanding the Types of Steel Grades

With many different types of steel grades available on the market, knowing which steel grade classification is best for your project can be challenging. Whether you need precise aerospace parts or strong construction components, not all steel grades are created equal.

In this guide, we’ll break down the different types of steel grades and their industry-use applications, helping you make an informed decision on which steel grade is the best option for your specific needs. By understanding the unique properties and strengths of various steel grades, you can made the right decisions on what materials to use for your specific needs.

Why Choosing the Correct Steel Grade Matters

Selecting the right steel grade for a project is crucial to the success and longevity of the final product. Different steel grades offer varying levels of strength, corrosion resistance, and weldability. For instance, using a lower-grade steel in a high-stress environment could lead to premature failure, resulting in costly repairs or replacements. On the other hand, choosing a steel grade that offers enhanced corrosion resistance can greatly extend the life of structures exposed to harsh weather conditions or chemicals. By understanding the specific requirements of your project, you can make an informed decision that balances cost with performance, ultimately leading to a safer and more durable product.

A Complete Guide to Steel Grades

There are many types of steel, so understanding their different types of steel grades is vital to picking the right materials for your application. First, we’ll answer, “What are steel grades?” to determine their classification.  

How Grades of Steel Are Classified: ASTM, SAE, and AISI Grading Systems 

Steel grades are measured on a letter-numeric rating system according to ASTM and SAE standards. It evaluates steel properties and classifies all variants to ensure safer use for manufacturers and consumers.  

Some factors that impact ASTM and SAE standards for steel grading are: 

• Physical properties – material, chemical, mechanical, and metallurgical composition 
• Applications and production – mechanical components, industrial parts, construction elements, etc.  
• Minimum ordinary strength – 34 ksi yield strength 
• Minimum higher strength – 46 ksi yield strength 
• Heat treatment/forming process – cold working (CDS), hot working (HR), quenching and tempering (Q&T), etc. 

Below, we’ll compare the differences between steel grading systems and their classification.

ASTM Steel Grading 

The American Society for Testing and Materials (ASTM) steel grading is based on a letter and numeric grading system. The letter classifies the general steel type category, and the number represents the variations based on physical properties.  

Common types of ordinary steel grade types in ASTM are Grade A and Grade B. They both categorize mild carbon steel and are subtle variations of each other. Grade A and Grade B steels share similar qualities in chemical composition and mechanical properties. The differences depend on your project’s rules or society’s specifications, as they have distinct applications.  

SAE-AISI Steel Grading

SAE International is another organization that sets standards for steel grades. The difference between SAE and ASTM grading is that SAE only classifies alloy steel grades through a four-digit numeric system.   

It’s important to note that SAE steel grading is often referred to as AISI (American Iron and Steel Institute) since they jointly designed their standards. So, you may encounter both classifications when referencing steel grades, such as “SAE 4140 or AISI 4140”.  

Here’s how SAE steel grades are classified: 

• First digit – main alloying elements 
• Second digit – top grade elements (tg) 
• Last two digits – carbon amount by weight basis points (wt%)  

There are instances where letter suffixes are added to SAE steel grading, like “H” for hardenability. Additionally, there are instances where letter prefixes are added, like “E” for electric arc furnace steel, to notate the steelmaking process.  

Now that you know all steel grading systems, the next step is understanding the different types of steel grades. The four main classifications of steel grades are alloy, carbon, stainless, and tool steel. Grades of steel can also be classified by heat treatment and physical strength, as mentioned above. Although there are other steel grading variables, these four steel types are primary categories.  

Below, we’ll break down the four different groups of steel grades with their grading variations.  

Alloy Steel Grades

Alloy steel grades are measured using both ASTM and SAE-AISI grading systems. These systems have a diverse range of alloy steel types, each with its own unique steel grades. The composition of these different types of steel grades includes, but is not limited to, chromium, nickel, and carbon. This diversity in alloy steel grades ensures that there is a suitable steel grade for a wide array of applications and industries.

A list of steel grade types used for alloy steel include: 

• AISI 316L – designed for corrosion-resistance 
• AISI 4140 – formulated for strength and toughness 
• AISI 4340 – manufactured for high tensile strength and wear resistance 

Grades of alloy steel have a flexible range of mechanical and physical properties, making them suitable for many industry applications. Common industry uses for alloy steel grades include automotive components, aerospace structures, oil and gas equipment, machine parts, and more. Alloy steel grades offer consumers the ability to meet diverse performance requirements.  

Carbon Steel Grades

Carbon steels are a type of steel grade classified according to the ASTM and SAE-AISI grading systems due to the presence of various alloying elements. While these grades are primarily composed of carbon, they are engineered by incorporating other alloying elements, such as manganese or silicon, to significantly enhance their strength, durability, and overall performance.

Some common types of carbon steel grades are: 

• ASTM A36 — low-carbon steel grade, easy weldability and ductility 
• AISI 1045 — medium-carbon steel grade, high strength and hardness 
• AISI 1095 — high-carbon steel grades known for superior hardness and wear-resistance  

Carbon steel grades are commonly used for construction, automotive, and machinery applications due to their cost-effectiveness, ease of fabrication, and versatility. Selecting a suitable carbon steel grade is crucial to ensure that its specific properties meet industry application performance requirements. 

Stainless Steel Grades

Stainless steel is another main classification containing varying grades within its category. Since there are over 3,500 different steel grade types, it can be difficult to pinpoint each stainless steel grade.  

For this reason, it’s crucial to refer to the relevant material specs and standards from their unique classification society for precise information on the properties and best usage. This ensures that the chosen stainless-steel grade meets the specific project requirements and complies with standard safety regulations. 

A list of the different types of steel grades usually referenced in stainless steel includes: 

• Austenitic stainless steel – 304 (most common), 316, 321 
• Ferritic stainless steel – 430 (often household appliances) 
• Martensitic stainless steel – 410 (typically surgical instruments)  

Each stainless-steel grade has a framework for specific applications based on factors such as corrosion resistance, strength, and temperature resistance. When selecting a stainless-steel grade, it’s crucial to consider the requirements and environmental conditions to ensure optimal performance and longevity. 

Tool Steel Grades

Tool steels are high-alloy steels that come in a variety of grades, each defined by the specific alloy elements used in their composition. These grades commonly include alloys such as chromium, vanadium, and molybdenum, which are carefully selected to provide unique properties like increased hardness, wear resistance, and heat tolerance.

A list of steel grades typically used in tool steels are: 

• AISI D2 – high-chromium tool, exceptional wear resistance and edge retention (ideal for cutting tools and cold work treatments) 
• AISI M2 – high-speed tool, great heat resistance and cutting performance (best for machining and drilling) 
• AISI A2 – a combination of toughness and wear-resistance (suitable for forming and punching tools) 

Tool steel grades provide the durability and precision required for various shaping and machining operations. Their ability to withstand high temperatures, pressures, and mechanical stresses makes them practical for metalworking, woodworking, and plastic molding tools. 

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