Stainless steel is highly versatile (with dozens of specialized grades), durable, and extraordinarily resistant to corrosion. Chromium is the element that makes a steel “stainless”: it can polish to a stunning shine and won’t rust as easily as carbon or galvanized steel.
Surgical implants, turbine blades, cutlery, sailing ships, and a multitude of other industrial applications have steadily increased our reliance upon stainless steel in recent years. According to Statista, global stainless steel production increased from 2005 to 2017 by nearly 25,000 million metric tons (close to doubling in the process).
With ever-more-widespread use, but so many grades to choose from, it is imperative to understand the relative advantages and applications of each. Stainless steel can be subdivided into five primary categories:
- Precipitation-Hardening (PH)
Below is a brief overview of these categories and several of the most common grades for outdoor industrial use.
These chromium steels have no nickel, and are known for their hardness, strength, and wear resistance. Blades (like Swiss Army knives) and cutlery favor it because it can be polished to a shine and hold a sharp edge through heavy use.
Ferritic grades resist stress, cracking, and oxidation, but they can’t be hardened through heat treatment (like Martensitic grades). Overall, their corrosion resistance rests between the more vulnerable martensitic and more resistant austenitic grades. Decorative automotive trim, sinks, vehicle exhaust systems, and cooking utensils are all known for using ferritic stainless steels.
This non-magnetic subgroup is by far the most common class of stainless steels (including the widely used 304 and 316 grades). The high percentages of chromium and nickel give these steels superior corrosion resistance and excellent mechanical properties. Though heat treatment is not an option, they can be hardened by cold-working.
- Grade 304: The most common stainless steel, 304 is an 18% chromium, 8% nickel steel alloy with high corrosion resistance and machinability, preferred for welding, bending, and forming applications. It sees use in fasteners, piping, food/dairy and beverage processing equipment, the pulp and paper industry, and some marine applications.
- Grade 316: Also known as “surgical steel” or “marine-grade steel,” the extra molybdenum (2-3%, as opposed to trace amounts in 304) sets 316 apart by giving it unsurpassed corrosion resistance in the harshest environments. It’s preferred in nautical environments (which can involve harsh substances like chlorides), medical devices, refinery equipment, and chemical storage/processing equipment.
Unique fabric-ability and heat treatability typify PH stainless steels. They exhibit higher strength than martensitic steels with corrosion resistance in line with austenitic grades. These materials see use in turbine blades and aerospace components. Here are two of the more common grades:
- Type 17-4: This is the most common PH type. Copper in the alloy lends this steel to heat treatment in a variety of temperatures (leading to equally various properties). It can be made incredibly strong, which suits it to high strength posts in premier pulley designs. Since it doesn’t lose its strength at high temperatures or pressures, it’s an ideal choice for gas and windmill turbine blades.
- Type 15-5: With even more toughness and corrosion resistance than the older 17-4 formula, type 15-5PH stainless steel is chosen for applications that require top-shelf transverse properties and weatherability. It’s used in hardware like padeyes and snap shackles, in addition to aerospace fasteners and structural components. The nuclear industry also uses 15-5PH in its valves, gears, and pumps.
These kinds of stainless steel combine ferritic and austenitic material for toughness and ductility greater than the former but less than the latter. Their corrosion resistance is very close to austenitic grades like 304 and 316, with improved resistance to pitting and crevice corrosion.
- Type 2205: This type takes the durability in high-pressure, corrosive environments of PH steels and adds even more erosion fatigue, but sacrifices some heat resistance found in other varieties—extensive use in high temperatures can make it brittle. Overall, it has lower thermal expansion and higher thermal conductivity than austenitic grades do. Complex machined parts for the marine, paper, chemical, and petrochemical industries may use this type.