Stainless Steels ClassificationsStainless steels are commonly grouped into martensitic stainless steels, ferritic stainless steels, austenitic stainless steels, duplex (ferritic-austenitic) stainless steels, and precipitation-hardening stainless steels

Sponsored LinksStainless steels are in general grouped into

martensitic stainless steels ferritic stainless steels austenitic stainless steels duplex (ferritic-austenitic) stainless steels precipitation-hardening stainless steels

Alloying metallic elements added during the making of the steel increase corrosion resistance, hardness, or strength. The metals used most commonly as alloying elements in stainless steel include chromium, nickel, and molybdenum.

Stainless steels are available in the form of

plate sheet strip foil bar wire pipes tubes

Stainless steels are a iron-based alloy containing at between 10.5% to 30% Cr. Stainless steel achieve its stainless characteristic through the formation of an invisible and adherent chromium-rich oxide surface film.

Other alloying elements added to improve the characteristics of the stainless steel include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulphur, and selenium.

Carbon is normally in amounts from 0.03% to more than 1.0% in some martensitic grades.

Selection of stainless steels are in general based on

corrosion resistance fabrication characteristics availability mechanical properties for specific temperature ranges product cost

Since stainless steel resists corrosion, maintains its strength at high temperatures, and is easily maintained, it is widely used in items such as automotive and food processing products, as well as medical and health equipment. The most common US grades of stainless steel are:

TYPE 304

The most commonly specified austenitic (chromium-nickel stainless class) stainless steel, accounting for more than half of the stainless steel produced in the world. This grade withstands ordinary corrosion in architecture, is durable in typical food processing environments, and resists most chemicals. Type 304 is available in virtually all product forms and finishes.

TYPE 316Austenitic (chromium-nickel stainless class) stainless steel containing 2%-3% molybdenum (whereas 304 has none). The inclusion of molybdenum gives 316 greater resistance to various forms of deterioration.

TYPE 409Ferritic (plain chromium stainless category) stainless steel suitable for high temperatures. This grade has the lowest chromium content of all stainless steels and thus is the least expensive.

TYPE 410The most widely used martensitic (plain chromium stainless class with exceptional strength) stainless steel, featuring the high level of strength conferred by the martensitics. It is a low-cost, heat-treatable grade suitable for non-severe corrosion applications.

TYPE 430The most widely used ferritic (plain chromium stainless category) stainless steel, offering general-purpose corrosion resistance, often in decorative applications.

AUSTENITIC STAINLESS STEELS

TYPE Equivalent UNS

201 S20100

202 S20200

205 S20500

301 S30100

302 S30200

302B S30215

303 S30300

303Se S30323

304 S30400

304L S30403

302HQ S30430

304N S30451

305 S30500

308 S30800

309 S30900

309S S30908

310 S31000

310S S31008

314 S31400

316 S31600

316L S31603

316F S31620

316N S31651

317 S31700

317L S31703

317LMN S31726

321 S32100

330 NO8330

347 S34700

348 S34800

384 S38400

FERRITIC STAINLESS STEELS

TYPE Equivalent UNS

405 S40500

409 S40900

429 S42900

430 S43000

430F S43020

430FSe S43023

434 S43400

436 S43600

442 S44200

446 S44600

MARTENSITIC STAINLESS STEELS

TYPE Equivalent UNS

403 S40300

410 S41000

414 S41400

416 S41600

416Se S41623

420 S42000

420F S42020

422 S42200

431 S43100

440A S44002

440B S44003

440C S44004

What is the difference between 316 v. 316L?Stainless Steel 316L is the low carbon content version of 316 Stainless Steel. Stainless Steel 316L is used in heavy-gauge welded products (over approximately 6mm). One advantage

of Stainless Steel 316L over 316 Stainless Steel is that it is more resistant to carbide precipitation and can be used in the 425-860 degrees Centigrade range, if certain types of corrosion resistance are an important factor. If there is the threat of corrosion in the heat-affected areas of weldments, Stainless Steel 316L should be used.

Comparison of 304 or 316 and 304L or 316L type compositions and effect on corrosion resistanceIntroduction - composition ranges

As American AISI basic grades, the only practical difference between 304 or 316 and 304L or 316L is carbon content.The carbon ranges are 0.08% maximum for 304 and 316 and 0.030% maximum for the 304L and 316L types.All other element ranges are essentially the same (nickel range for 304 is 8.00-10.50% and for 304L 8.00-12.00%). There are two European steels of the '304L' type, 1.4306 and 1.4307. The 1.4307 is the variant most commonly offered, outside Germany. The 1.4301 (304) and 1.4307 (304L) have carbon ranges of 0.07% maximum and 0.030% maximum, respectively. The chromium and nickel ranges are similar, nickel for both grades having an 8% minimum. 1.4306 is essentially a German

grade and has 10% minimum Ni. This reduces the ferrite content of the steel and has found to be necessary for some chemical processes.The European grades for the 316 and 316L types, 1.4401 and 1.4404, match on all elements with carbon ranges of 0.07% maximum for 1.4401 and 0.030% maximum for 1.4404. There are also high Mo versions (2.5% minimum Ni) of 316 and 316L in the EN system, 1.4436 and 1.4432 respectively. To further complicate mattters, there is also grade 1.4435 which is both high in Mo (2.5% minimum) and in Ni (12.5% minimum).Effect of carbon on corrosion resistance

The lower carbon 'variants' (316L) were established as alternatives to the 'standards' (316) carbon range grade to overcome the risk of intercrystalline corrosion (weld decay), which was identified as a problem in the early days of the application of these steels. This can result if the steel is held in a temperature range 450 to 850°C for periods of several minutes, depending on the temperature and subsequently exposed to aggressive corrosive environments. Corrosion then takes place next to grain boundaries.

If the carbon level is below 0.030% then this intercrystalline corrosion does not take place following exposure to these temperatures, especially for the sort of times normally experienced in the heat affected zone of welds in 'thick' sections of steel.

Effect of carbon level on weldability

There is a view that the low carbon types are easier to weld than the standard carbon types.

There does not seem to be a clear reason for this and the differences are probably associated with the lower strength of the low carbon type. The low carbon type may be easier to shape and form, which in turn may also affect the levels of residual stress left

the steel after is forming and fitting up for welding. This may result in the 'standard' carbon types needing more force to hold them in position once fitted-up for welding, with more of a tendency to spring-back if not properly held in place.

The welding consumables for both types are based on a low carbon composition, to avoid intercrystalline corrosion risk in the solidified weld nugget or from the diffusion of carbon into the parent (surrounding) metal.

Dual-certification of low carbon composition steels

Commercially produced steels, using current steelmaking methods, are often produced as the low carbon type as a matter of course due to the improved control in modern steelmaking. Consequently finished steel products are often offered to the market 'dual certified' to both grade designations as they can then be used for fabrications specifying either grade, within a particular standard.

For example for coil, sheet or plate 304 Types

BS EN 10088-2 1.4301 / 1.4307 to the European standard.

ASTM A240 304 / 304L OR ASTM A240 / ASME SA240 304 / 304L to the American pressure vessel standards.

316 Types

BS EN 10088-2 1.4401 / 1.4404 to the European standard.

ASTM A240 316 / 316L OR ASTM A240 / ASME SA240 316 / 316L, to the American pressure vessel standards.