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.
Kloeckner Metals is a full-line stainless steel supplier and service center. Download our stainless steel spec sheet today and check what Kloeckner Metals routinely stocks.
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.
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 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:
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.
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.
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.
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.
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.
Kloeckner Metals is a full-line stainless steel supplier and service center. Kloeckner Metals brings a national footprint with the latest fabrication and processing technologies and most innovative customer service solutions.
Stainless steel can be broken down into three major groups – Austenitic, Ferritic and Martensitic
Austenitic Grades
Suncor Stainless mainly uses grade 304 and 316 stainless steel from the 300 series. Grade 304 and 316 are the most important members of the Austenitic family and are the most widely used. The Austenitic grades are generally nonmagnetic and hardenable only by cold working. However, they can be magnetic to a small degree, especially in formed parts. They can be softened by “annealing” heat treating.
Ferritic Grades
Suncor Stainless does not typically use ferritic grades of stainless steel. Ferritic grades are magnetic and nonhardenable.
Martensitic Grades
The Martensitic grades are hardenable by heat treatment and are magnetic. Varieties such as grade 440 find major applications as products that must resist atmospheric oxidation, mildly corrosive chemicals and wet or dry corrosion. This type contains up to 1% carbon. Suncor uses this type in some products such as Quick Attach Wedges, however, these parts have a plating to protect against further corrosion.
Typical Contents of Stainless Steel
Types and Grades of Stainless Steel
Austenitic
Type 301: contains less chromium and nickel than 302 for more work hardening.
Type 302: is the basic type of the 300 series, 18% chromium – 8% nickel group. It is the renowned 188 stainless and is the most widely used of the chromium nickel stainless and heat resisting steels.
Type 303: contains added phosphorus and sulfur for better machining characteristics. Corrosion resistance is slightly less than 302/304.
Type 303Se: contains Se and P added to improve machinability.
Type 304/304L: chromium and nickel increased and carbon lowered to reduce carbide precipitation and increase corrosion resistance. Carbon is lowest in 304L for welding applications.
Type 305: has increased nickel to lower work hardening properties.
Type 309/309S: have added chromium and nickel for more corrosion resistance and high temperature scaling resistance. 309S contains less carbon to minimize carbide precipitation.
Type 310S: have higher nickel content than 309/309S to further increase scaling resistance. 310S contains less carbon than 310 to minimize carbide precipitation.
Type 316 (Cast 316 is called CF8M) and 316L (Cast 316L is called CF3M): contain substantial molybdenum additions for improved corrosion resistance (particularly pit type corrosion), creep resistance and high temperature strength. The carbon content in 316L is low for welding purposes and also reduced corrosion.
Type 321: contains titanium to tie up the carbon and avoid chromium carbide precipitation in welding.
Type 330: ultra high nickel content provides best corrosion resistance to most furnace atmospheres. This grade has a low coefficient of expansion, excellent ductility and high strength.
Type 347 – 348: have columbium tantalum added to tie up the carbon and avoid chromium carbide precipitation in welding. Use for temperatures from 800 to 1,650 degrees Fahrenheit.
Ferritic
Type 405: contains 12% chromium with aluminum added to prevent hardening.
Type 430: is the basic type in the ferritic group, possessing good ductility and excellent resistance to atmospheric corrosion. Its scaling resistance is higher than 302 in intermittent service and somewhat lower in continuous use.
Type 430F/430Se: have sulfur and selenium (respectively) added for increased machinability.
Type 442: has added chromium for improved resistance to scaling.
Type 446: has higher chromium content (27%) for added scaling resistance and is the highest of the standard straight chromium types. Alloys with over 30% chromium become too brittle to process.
Martensitic
Type 410: is the basic Martensitic type. It is the general purpose corrosion and heat resisting chromium stainless steel. It can be hardened by thermal treatment to a wide range of mechanical properties. It can be annealed soft for cold drawing and forming. This grade is always magnetic.
Type 403: is a special high quality steel made for blades and buckets for steam turbine and jet engine compressors. This grade is eminently suited for very highly stressed parts. This material is magnetic in all conditions.
Type 416/416Se: are modifications of type 410, being the free machining, non-seizing, non-galling alloys. These properties are obtained by the addition of sulfur or selenium to Type 410. This is a heat treatable grade with corrosion resistance and other characteristics closely approaching those of type 410.
Type 420: is a chromium stainless steel capable of heat treatment to a maximum hardness of approximately 500 Brinell. It has a maximum corrosion resistance only in the fully hardened condition. Type 420 is magnetic in all conditions.
Type 431: is a nickel bearing (1.252.00%) chromium stainless steel which may be heat treated to high mechanical properties. It is magnetic in all conditions of use. It has superior corrosion resistance to types 410, 416, 420, 430 and 440 stainless steels.
Type 440C: is the stainless steel that can be heat treated to the highest hardness of any of the stainless steels.