Stainless Steel Facts – Getting Technical
Stainless steel appears everywhere in our daily environment, but did you know there are numerous types of stainless steel grades that are used for different purposes? For residential use, grade 304 is the most popular and what we use for our stainless steel kitchen countertops and cabinets. For more detail of the facts on stainless steel, we have listed out the various grades and their technical properties for a comparison.
400 Series Martensitic – Typical grade: 410
Straight chromium (12-18%), magnetic and can be hardened by heat treatment. Typical use: Fasteners, pump shafts
400 Series Ferritic – Typical grade: 430
Straight chromium (12-18%), “low” carbon, magnetic, but not heat treatable. Typical use: Appliance trim, cooking utensils.
200/300 Series Austenitic – Typical grade: 304
Chromium (17-25%)/Nickel (8-25%), non-magnetic, not heat treatable. Can develop high strength by cold work. Additions of molybdenum (up to 7%) can increase the corrosion resistance. Typical use: Food equipment, chemical equipment, architectural applications
Corrosion-resistant steel of a wide variety that always contains more than 10% chromium, with or without other alloying elements. Stainless steel resists corrosion attack by organic acids, weak mineral acids, and atmospheric oxidation, keeps its strength at high temperatures, and is easily maintained. The most common grades of stainless steel are: Type 304, austenitic (chromium-nickel); Type 316, austenitic with 2%-3% molybdenum; Type 409, ferritic (low chromium) for high-temperature use; Type 410, heat-treatable martensitic (medium chromium) with a high strength level Type 430, ferritic general-purpose grade with some corrosion resistance.
Austenitic Stainless Steels
This family of stainless steels is made up of two groups of materials: Chromium-Manganese-Nickel types, or 200 Series, and the Chromium-Nickel types, or 300 Series.
The 200 Series alloys possess mechanical and corrosion resisting properties similar to their corresponding 300 Series materials. They also exhibit higher hardness and yield strength as well as excellent ductility and superior creep properties at elevated temperatures and are usually non-magnetic. These alloys were originally developed to conserve nickel as compared to the 300 Series by replacing nickel with manganese at a ratio of 2% manganese for each percent of nickel replaced.
Typical end uses include dairy equipment, food processing industries, cooking utensils and kitchen equipment, hospital wares, railroad car and truck trailer structurals, tanks and pressure vessels, hose clamps, cryogenic applications, and deep drawn parts
They are classified as austenitic, and are hardenable only by cold working methods. These grades of stainless have chromium (approx. 18 to 30%) and nickel (approx. 6 to 20%) as their major alloying additions. Type 304 (also known as 18-8) is the most widely used alloy of all stainless steels.
Their nickel content changes their fundamental structure and nature, lowers their thermal conductivity, doubles their coefficient of expansion and makes them non-magnetic as compared to straight chromium 400 Series alloys. They provide high resistance to corrosion and possess great strength and oxidation resistance at elevated temperature, yet retain good ductility at extremely cold temperatures. In the annealed state, 300 Series stainless steels possess unusual ductility and formability, high impact strength and high tensile strength compared to mild carbon steels.
The 300 Series alloys have excellent fabrication characteristics, including good weldability.
The 300 Series stainless steels can meet a wide variety of physical and mechanical requirements making them excellent materials for applications including auto molding and trim, wheel covers, kitchen equipment, hose clamps, springs, truck bodies, exhaust manifolds, stainless flatware, storage tanks, pressure vessels, and piping.
Ferritic Stainless Steels, 400 Series Straight Chromium, Non-Hardenable
These grades of stainless have 11 to 30% chromium as the major alloy addition and are low carbon. Ductility and formability are less than that of the austenitic grades. The corrosion resistance competes with the austenitic grades for certain applications. Thermal conductivity is about half that of carbon steels.
Ferritic stainless steels are magnetic, and resistance to high-temperature corrosion is better than that of martensitic types. They generally have good ductility and can be welded or fabricated without difficulty. These grades can be processed to develop an aesthetically pleasing, bright finish and, hence, are often used for automotive trim and appliance molding. They also find use in functional applications where cost is a major factor, e.g., automotive exhaust systems, catalytic converters, radiator caps, and chimney liners.
End uses for ferritic types include appliances, hot water tanks, automotive applications, and deep drawn parts such as cookware.
Martensitic Stainless Steels, 400 Series, Straight Chromium, Hardenable
These grades of stainless steel have chromium in the range of 11 to 17% as the major alloying addition, but the carbon levels are in amounts from .10 to .65%. This radically changes the behavior of the martensitic alloys relative to the ferritic 400 Series alloys. The high carbon enables the material to be hardened by heating to a high temperature, followed by rapid cooling (quenching). Martensitic types offer the ideal combination of corrosion resistance and superior mechanical properties, as produced by heat treatment to develop maximum hardness, strength and resistance to abrasion and erosion.
The martensitic grades are usually sold in the soft state. This allows the customers to cut or form the parts before they are thermally hardened. End uses include cutlery, scissors, surgical instruments, wear plates, garbage disposal shredder lugs, and industrial knives.