In this Komacut guide, we’ll look at the different types of stainless steel. We’ll explain what the main families of stainless steel are and look in detail at some of the common grades used in manufacturing. We’ll also look at where different types of steel are used and how to choose the right steel for your project.

Stainless steel is the name given to steels that are made with chromium as an alloy at a minimum concentration of 10.5%.  Chromium adds corrosion resistance, strength and toughness, and is what makes stainless steel what it is.

Stainless steel is also alloyed with other elements to create different variants; the amount of chromium can also be altered as can the amounts of additional alloying elements.

There can also be secondary processing, such as annealing or heat treatments, to enhance certain physical properties.

Key Takeaways

• There are a total of five primary families of stainless steel
• These are organized primarily by the different alloys and percentage of chromium
• The most commonly used grade of stainless steel is grade 304, which has very good corrosion resistance and formability.
• Generally, the selection of grade is going to be down to the hardness

Table of Contents

• Introduction
• Key Takeaways
• The Stainless Steel Grading System
• The Stainless Steel Material Families
• Shared Characteristics of All Stainless Steels
• Stainless Steels Commonly Used in Fabrication
• How to Select the Right Material Grade
• The Best Grades By Requirement
• Summary

The Stainless Steel Grading System

The different grades of stainless steel are classified according to a numbering system created by SAE International. The numbering system features three numbers and sometimes, letters added at the end.

The first number indicates the family to which each steel type belongs:

Some stainless steel grades include letter suffixes to indicate specific modifications:

• L – Low carbon (e.g., 304L): Improved weldability and resistance to intergranular corrosion.
• H – High carbon (e.g., 304H): Increased strength at high temperatures.
• N – Nitrogen added (e.g., 304N): Improved strength and resistance to pitting.
• F, Ti, Nb, etc. – Added elements for improved machining, stabilization, or strength.

Duplex and precipitation-hardened stainless steels often follow alternative or proprietary naming conventions. For example:

• UNS (Unified Numbering System): e.g., S31803 (Duplex)
• EN / DIN (European): e.g., 1.4462
• GB/T (China): e.g., 022Cr19Ni10

Here are some answers to some of the questions commonly asked about the stainless steel grade identification system:

What are the differences between grades?

Different grades of stainless steel are classified according to which alloying elements are used and the concentration of each element. Variation creates grades of stainless steel with widely varying physical properties.

Individual grades are also subject to modifications to enhance certain characteristics. Grade 420 is heat treated, for example, to create stainless steels with much higher strength and hardness.

What is 301 vs 304

Stainless steel 301 has a higher carbon content than stainless steel 304 but is made with less chromium and nickel. 304 is much more corrosion resistant, while 301 is much stronger.

What does the L signify in 304L (304 vs 304L)

The L in 304L indicates that the metal is a modification of standard 304 stainless steel that has been made with a lower carbon content.

The Stainless Steel Material Families

Stainless steel is a ferrous alloy that contains chromium at a minimum concentration of 10.5%. The primary role of chromium is making stainless steel corrosion resistant, to improve strength and durability and to improve at high temperatures.

There are five main families of stainless steel, which we’ll explore here.

Austenitic

Austenitic stainless steel is the most commonly used group of stainless steels. They are identified by a 200 or 300 series designation (e.g. 201, 304, 316…). What does that number signify? Hint:

Composition

• Austenitic stainless steels are made with at least 16% chromium. T
• he most common types of austenitic stainless steel are also made with nickel (at least 6%).
• It can also be made with manganese and nitrogen as alloy elements.

Physical Characteristics

• The main strengths of austenitic stainless steel are excellent corrosion resistance,
• high ductility
• high formability
• good weldability

Disadvantages

• Lower hardness
• Lower wear resistance
• Higher cost

Ferritic

Ferritic stainless steel is made with widely varying amounts of chromium and either very small amounts of nickel or no nickel. The amount of chromium used varies between 10.5% and 30%. Ferritic stainless steels are also made with a higher carbon content than austenitic stainless steels and are hardened by cold working.

The main strengths of ferritic stainless steel are good corrosion resistance, good performance at high temperatures, magnetism and lower cost. Weaknesses are lower toughness, reduced ductility, reduced weldability and lower performance in saltwater environments.

Ferritic stainless steels are identified by a 400 series designation (e.g. 420, 430…)

Martensitic

Martensitic stainless steel is made with 12% - 18% chromium and, in some cases, with small amounts of other alloying elements. They are also hardened by heat treating and cold working.

There are many different types of martensitic stainless steels, and the different types offer a wide range of different properties.

The different types are grouped into four main categories. The basic category is made with chromium and offers very good wear resistance. Nickel is added in the next type to increase corrosion resistance and hardness. Martensitic hardening and precipitation hardening is used in the next type to increase strength and toughness. In the fourth type, niobium, vanadium, boron and cobalt are added to increase strength and creep resistance.

Martensitic stainless steels are also identified by a 400 series designation.

Duplex

Duplex stainless steels are made with a mixed microstructure of austenite and ferrite. They also have a high chromium content (19 – 32%), contain up to 5% molybdenum and only have small amounts of nickel.

Duplex stainless steels are mainly produced for their exceptional corrosion resistance (especially to pitting and stress corrosion cracking) and high strength. They also have good weldability and are cost effective. The main weaknesses are brittleness at very low temperatures (below -50°C) and risk of sigma phase formation.

Precipitation Hardened

Precipitation hardened stainless steel is stainless steel that undergoes a heat treatment process called precipitation hardening.

Precipitation hardening significantly increases the strength of the material. Precipitation hardened stainless steels are typically 3 - 4 times harder than basic austenitic stainless steels. Besides being extra high strength, these steels are also highly corrosion resistant. The main disadvantage is the higher cost.

The three types of precipitation hardened stainless steel are:

• Martensitic precipitation hardened
• Semi-austenitic precipitation hardened
• Austenitic precipitation hardened

Shared Characteristics of All Stainless Steels

There are certain positive characteristics that all stainless steels possess when compared to alternative metals.

All stainless steels have good:

• Corrosion resistance: The chromium in stainless steel forms a passive oxide layer on the surface of the material, protecting it from corrosion.
• Strength and durability: All stainless steels have very high tensile strength and excellent wear resistance. Some grades of stainless steel have the highest tensile and yield strengths of all manufacturing metals.
• Hygienic properties: Stainless steel is highly cleanable, making it suitable for food-grade applications.
• Temperature resistance: Stainless steels perform well in high-temperature environments.
• Aesthetic appeal: Stainless steel has a nice appearance. It is often used in architectural applications and in jewelry.
• Recyclability: 100% recyclable, with quality retained.

Stainless Steels Commonly Used in Fabrication

In this section, we’ll look at some of the grades of stainless steel most commonly used in manufacturing.

Austenitic

Material Type 2: SS304

Stainless steel 304 contains 18 - 20% chromium and 8 - 10.5% nickel. It also contains small amounts of carbon, manganese, silicon and traces of phosphorus/sulfur.

• Mechanical characteristics: Excellent corrosion resistance and moderate strength). Variants with nitrogen also have higher strength.
• Processing characteristics: Good machinability and formability.
• Performance: Widely used in food, beverage and chemical processing industries.

Common Applications

304 is often used for food processing equipment because of its hygienic and corrosion-resistant properties.

It is often found in brewing equipment, dairies, sinks, food processing equipment, water tanks and kitchen appliances.

Material Type 4: SS316

Stainless steel 316 contains 16 - 18% chromium, 10 - 14% nickel and 2 - 3% molybdenum. It also contains small concentrations of carbon, manganese and silicon and traces of phosphorus/sulfur.

• Mechanical characteristics: Highly enhanced corrosion resistance due to the addition of molybdenum.
• Processing characteristics: Good machinability (including for sheet metal bending) and weldability.
• Performance: Common in marine and chemical environments.

Common Applications

Grade 316 is often used for machinery components and parts requiring good corrosion resistance. Some common examples of where it’s used are exhaust manifolds, sterile storage tanks, heat exchangers, valves, pump parts, chemical process equipment and jewelry.

Ferritic

Material Type 6: SS420

Stainless steel 420 is a martensitic steel with 12 - 14% chromium. It also contains small amounts of carbon, manganese and silicon and traces of phosphorus/sulfur. The higher the carbon content of 420, the better the material’s wear resistance

• Mechanical characteristics: Good hardness and wear resistance but limited corrosion resistance.
• Processing characteristics: Excellent machinability.
• Performance: Often used in cutlery, surgical tools and wear-resistant components.

Common Applications

• 420 is often used in applications requiring high wear resistance and moderate corrosion resistance.
• Surgical tools, knives, scissors and cutlery are examples of products that are often made with grade 420. 420 isn’t usually used in food grade applications because it is not corrosion resistant enough.

Summary Table

Here’s a summary of the stainless steel grades discussed above:

How to Select the Right Material Grade

In this section, we’ll look at the most important things to consider when you’re choosing a grade of stainless steel.

The most important factors that determine any choice of material are:

• 1. How it will be used
• 2. How it will be fabricated
• 3. Project cost sensitivity

What Factors Inform Your Decision?

Here are the elements to consider when you’re making your decision:

• Strength and durability requirements - You should accurately ascertain what tensile strength, yield strength and hardness requirements your component needs to meet and choose a material that at least meets your minimum requirements.
• Budget constraints – Select a material that meets your requirements within the production budget.
• Processing requirements – You need to understand exactly what fabrication processes your part will need to go through and choose a material that matches your requirements. This is often a good thing to discuss with your manufacturer.
• Corrosion resistance – Different grades of stainless steel vary significantly in the level of corrosion resistance they offer. Austenitic and duplex alloys such as 304, 304L, 316 and 316L are usually best. You should accurately ascertain what exposure your component will experience.
• High temperature resistance – Grades with high chromium, silicon, nitrogen and rare earth elements are usually more heat resistant. Austenitic grades, such as 310, are often the best choice.
• Low temperature resistance - Austenitic steel grades are also ideal for low-temperature or cryogenic environments. Common grades for low-temperature environments include 304, 304LN, 310 and 316.

The Best Grades By Requirement

Here’s a simple table to identify good grades for particular requirements.

Summary

Stainless steel is a far more diverse material than is widely known, filling a huge range of manufacturing roles. It is used in everything from low-end products to advanced engineering applications in fields such as aerospace. In many cases, it is not clear what the best choice of material is.

All stainless steels offer good corrosion resistance, strength and hardness. It’s important, however, to make a refined choice when you choose a type for a manufacturing project. The differences between grades vary widely, even within family groups. There are also sub-grades to consider for each grade.