Selecting Stainless Steel
The selection of a particular type of stainless steel will depend on what requirements a particular application poses. Environment, expected part life and extent of acceptable corrosion all help determine what type of stainless to use. In most cases, the primary factor is corrosion resistance, followed by tarnish and oxidation resistance. Other factors include the ability to withstand pitting, crevice corrosion and inter-granular attack. The austenitic/higher chromium stainless steels, usually required in very high or very low temperatures, are generally more corrosion resistant than the lower chromium ferritic or martensitic stainlesses.
Austenitic - Alloys: 301, 302, 303, 304, 305, 308, 309, 310, 314, 316, 317, 321, 330, 347, 384
Austenitic stainless steels are the most weldable of the stainlesses and can be divided rather loosely into three groups: common chromium-nickel (300 series), manganese-chromium-nickel-nitrogen (200 series) and specialty alloys. Austenitic is the most popular stainless steel group and is used for numerous industrial and consumer applications, such as in chemical plants, power plants, food processing and dairy equipment. Austenitic stainless steels have a face-centered cubic structure. Though generally very weldable, some grades can be prone to sensitization of the weld heat-affected zone and weld metal hot cracking.
Ferritic - Alloys: 405, 409, 429, 430, 434, 436, 442, 446
Ferritic stainless steel consists of iron-chromium alloys with body-centered cubic crystal structures. They can have good ductility and formability, but high-temperature strengths are relatively poor when compared to austenitic grades. Some ferritic stainlesses (such as types 409 and 405) used, for example, in mufflers, exhaust systems, kitchen counters and sinks, cost less than other stainless steels. Other more highly alloyed steels low in C and N (such as types 444 and 261) are more costly, but are highly resistant to chlorides.
Martensitic - Alloys: 403, 410, 414, 416, 420, 422, 431, 440
Martensitic stainless steels, such as types 403, 410, 410NiMo and 420, are similar in composition to the ferrite group, but contain a balance of C and Ni vs. Cr and Mo; hence, austenite at high temperatures transforms to martensite at low temperatures. Like ferrite, they also have a body-centered cubic crystal structure in the hardened condition. The carbon content of these hardenable steels affects forming and welding. To obtain useful properties and prevent cracking, the weldable martensitics usually require preheating and postweld heat treatment.
Duplex - Alloys: 329, 2205, 2304, 2507, 3RE60
Primarily used in chemical plants and piping applications, the duplex stainless steels are developing rapidly today and have a microstructure of approximately equal amounts of ferrite and austenite. Duplex stainless steels typically contain approximately 22-25% chromium and 5% nickel with molybdenum and nitrogen. Although duplex and some austenitics do have similar alloying elements, duplexes have higher yield strength and greater stress corrosion cracking resistance to chloride than austenitic stainless steels.
Precipitation Hardening - Alloys: 13-8, 15-5, 15-7, 17-4, 17-7, A286
Precipitation-hardening stainless steels are chromium-nickel stainlesses, which contain alloying additions such as aluminum, copper or titanium that allow them to be hardened by a solution and aging heat treatment. They can be either austenitic or martensitic in the aged condition. Precipitation- hardening stainless steels are grouped into three types: martensitic, semiaustenitic and austenitic. The martensitic (such as Type 630) and semiaustenitic (such as Type 631) can provide higher strength than the austenitic (such as Type 660, also known as A286).