Stainless Steel Numbering Principles
Stainless steel is a versatile and reliable material used in many applications across a range of industries, from medical tools to construction hardware. But before it can be used, stainless steel must be properly identified so that it can be properly employed in different tasks. To ensure that stainless steel is properly identified and its characteristics understood, stainless steel is given unique numerical designations. This article explains the numbering system for stainless steel in more detail.
The numbering system for stainless steel combines both chemical and mechanical properties of the material. It assigns a numerical code from 1 to 10 to a common classification that identifies the steel’s chemical composition. The higher the number, the higher the concentration of chromium and molybdenum that have been added to the steel. The higher the number, the greater the durability and corrosion resistance of the stainless steel.
The most common types of stainless steel are used in commercial applications and are classified by the American Iron and Steel Institute (AISI) by their numerical designation. The most commonly used stainless steels are numbered between 400 and 600. The numbers 400 through 409 are ferritic, 410 and 416 are martensitic, and 420 through 600 are austenitic.
The numerical designation for each type of stainless steel contains information about its tensile strength, which is the maximum strain or load at which the material will not fail. Within each numerical designation, there are classes with different tensile strength capabilities, as indicated by additional letters. AISI-numbered grade 304 is composed of 18-20% chromium, 8-10.5% nickel and 0.08% carbon. This grade of stainless steel is also commonly called 18-8 stainless steel. It has a tensile strength of 90,000 pounds and is commonly used for structural applications such as screws and fasteners. The letter “L” indicates a lower strength value, so the designation for a lower strength 304 stainless steel is 304L which denotes a tensile strength of 70,000 pounds.
The exact composition of a particular grade of stainless steel is defined by the American Society for Testing and Materials. The ASTM-numbered versions of these grade 304 stainless steels are 304 and 304L. This organization is able to give exact specifications of the chemical composition, allowing one to know not only the grade of steel, but also the exact amounts of chromium, carbon, nickel, and phosphorus.
There are a number of other numbering systems in use, though they are not as common. The Unified Numbering System (UNS) provides a standardized system for North American steel. The European numbering system (EN) uses a similar system, including the entire range of grades, and has specific terminology for each grade.
Stainless steel is an essential material in many industries, so it is important for users to understand its composition. The numbering system for stainless steel allows one to have a full and clear understanding of the material so that it can be applied correctly and with confidence.
