Since stainless steel is aesthetically pleasing and corrosion resistant, it’s often used in manufacturing items such as cookware, kitchen sinks, and vehicles. The term stainless steel refers to any of several iron-based alloys that include the element chromium as at least 10.5% of their composition.
Although all stainless steel is defined by its crystalline structure, the metal comes in more than 100 grades. The American Iron and Steel Institute first developed a structure of grades for steel in the 1930s and 40s. Later, this system combined with one from the Society of Automotive Engineers (SAE) International to form a more standardized numbering system. Today, steel manufacturers may use more than one numbering system, however, which can complicate matters.
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The general idea behind grading the metal is that stainless steel can be divided into three broad categories — austenitic, ferritic, and martensitic — based on the crystalline structure of its iron atoms. Each category is further divided into series and grades. The grades reflect the specific alloy’s durability, quality, and temperature resistance. Numbers listed after the grade refer to the item’s chemical composition, specifically to it’s chromium and nickel percentages.
What are the various grading systems?
The Society of Automotive Engineers 3-Digit System
The SAE developed a four-digit grading system for carbon steel and a three-digit one for stainless steel. The first digit refers to the primary alloy element, the second and third digits to the alloy’s carbon percentage. Carbon steel needs four digits because there are more carbon steel alloys than stainless steel alloys.
The American Society for Testing and Materials (ASTM) 6-Digit System
The ASTM developed a six-digit system for categorizing stainless steel. Composed of both letters and numbers, this classification system begins with the letter A, which stands for any ferrous material, followed by a sequential series of numbers unrelated to the metal’s properties. The letter M, standing for “metric,” follows this number series. The last two digits refer to the year the standard for the grade was adopted or last revised.
Other organizations have also published grades and series of steel, including the British Standards (BS), German Standard (DIN), Chinese Standard (GB), European Standard (EN), Japanese Industrial Standards (JIS), and the International Organization for Standardization (IOS).
Each system functions entirely differently. For example, the SAE’s grade 304 steel would be graded as follows:
- UNS: S30400
- DIN: X5CrNi18-9, X5CrNi18-10, X5CrNi19-9
- EN number: 1.4305
- EN name: X8CrNiN18-9
- JIS: SUS 304, SUS 304-CSP
- BS: 304S 15, 304S 16, 304S 18, 304S 25, En58E
Regardless of its classification number, the specific alloy must meet the composition standards set for it by the system’s governing agency. To keep things as simple as possible, let’s use the SAE’s system to look at some of the most common grades of stainless steel.
What are the top series and what makes each grade unique?
Each grade of stainless steel has a slightly different chemical composition and consequently a distinct physical appearance.
The 300 series of stainless steel is the most common with 304 stainless steel and 316 stainless steel being the most prevalent grades. Two other common grades are 409 stainless steel and 430 stainless steel.
300 stainless series
Stainless steel grades in the 300 series are classified as austenitic with 18-30% chromium and 6-20% nickel as their major alloying additions. It is solid iron containing face-centered cubic crystals.
The composition of 300-series stainless steel grades makes it especially useful in the construction, automotive, and aerospace industries. Since it can withstand extremely high temperatures, series 300 stainless steel is hardenable only by cold working methods.
Attractive and malleable, grade 301 features high strength and corrosion-resistant composition. Since it is highly malleable and abrasion resistant, grade 301 stainless steel often gets used for decorative structural applications. Kloeckner Metals supplies 301 stainless steel sheet.
Grade 304 represents 50% of the steel produced around the world. Grade 304 contains 18% chromium and 8% nickel, which makes it aesthetically pleasing as well as durable. It’s one weakness is saline, so grade 304 can pit or crevice when used near salt water. We stock 304 stainless steel sheet, 304 stainless steel plate, 304 stainless steel bar, and 304 stainless steel tube.
The second most commonly produced steel, grade 316 contains 2-3% molybdenum. Consequently, it’s more resistant to saline and thus better for coastal environments. Many surgical supplies also get made from grade 316 stainless steel. Kloeckner Metals supplies the ultra low carbon variant of this grade stainless steel. It is available in 316L stainless steel sheet, 316L stainless steel plate, 316L stainless steel bar, and 316L stainless steel tube.
This grade provides high tensile strength at elevated temperatures. It is more resistant to corrosion and pitting than either grade 304 or grade 316 stainless steel. Due to its composition, grade 317 stainless steel is a more expensive alloy than most other 300-level grades.
400 stainless series
This series of stainless steel contains 11% chromium and 1% more manganese than 300-series grades, making it slightly magnetic. This steel also features a higher carbon content. As a result, 400-series stainless steel offers higher strength and more wear resistance, but it is also more easily corroded than 300-series grades. Heat-treating hardens 400-series stainless steel.
A ferritic steel, grade 430 forms easily but does not resist extreme temperatures or corrosion well. It’s often used for decorative purposes such as automotive trim. Grade 430 stainless steel is also commonly used in backyard barbecue grills. It is less expensive than grade 304 since it does not contain nickel. 430 stainless steel sheet is routinely stocked.
Among the most widely used ferritic steels, grade 434 is highly corrosion and oxidation resistant. It can also resist temperatures of up to 1500°F. Grade 434 stainless steel cannot be hardened by heat treatment and is generally cold formed like low carbon steel. It is typically used for automotive trim.
Grade 420 stainless steel is a very bright steel and one of the earliest carbon/iron alloys. Containing 12% chromium, this grade of steel is highly corrosion resistant. It is often called “blade grade steel” since manufacturers frequently use it to make surgical instruments.
904L stainless steel
A non-stabilized austenitic stainless steel, 904L grade products contain copper as part of the alloy. As a result, this grade of steel can resist corrosion from many acids. The high cost of this grade of stainless steel, however, has caused many manufacturers to turn to the lower-cost duplex stainless steel 2205 as an alternative.
Stainless steel comes in many series and grades. It’s important to choose the right kind of stainless steel for your product in your environment. Learn more about stainless steel on our website.
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