Stainless Steel

   

In metallurgy, stainless steel, also known as inox steel or inox, is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel (it stains less, but it is not stain-proof). It is also called corrosion-resistant steel or CRES when the alloy type and grade are not detailed, particularly in the aviation industry. There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are cutlery and watch cases and bands.

Stainless steel differs from carbon steel by the amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide. Stainless steels have sufficient amounts of chromium present so that a passive film of chromium oxide forms which prevents further surface corrosion and blocks corrosion from spreading into the metal's internal structure.

Stainless steel's resistance to corrosion and staining, low maintenance, relatively low cost, and familiar luster make it an ideal base material for a host of commercial applications. There are over 150 grades of stainless steel, of which fifteen are most common. The alloy is milled into coils, sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment, and as an automotive and aerospace structural alloy and construction material in large buildings. Storage tanks and tankers used to transport orange juice and other food are often made of stainless steel, due to its corrosion resistance and antibacterial properties. This also influences its use in commercial kitchens and food processing plants, as it can be steam cleaned, sterilized, and does not need painting or application of other surface finishes.

Stainless steel is also used for jewellery and watches. The most common stainless steel alloy used for this is 316L. It can be re-finished by any jeweller and will not oxidize or turn black.

Some firearms incorporate stainless steel components as an alternative to blued or parkerized steel. Some handguns, such as the Smith & Wesson Model 60 and the Colt M1911 can be made entirely from stainless steel. This gives a high-luster finish similar in appearance to nickel plating; but, unlike plating, the finish is not subject to flaking, peeling, wear-off due to rubbing (as when repeatedly removed from a holster over the course of time), or rust when scratched.

Some automotive manufacturers use stainless steel as decorative highlights in their vehicles.

There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For greater hardness and strength, more carbon is added. When subjected to adequate heat treatment, these steels are used as razor blades, cutlery, tools, etc.

Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost.


Stainless Steel Grades:

The SAE steel grades are the most commonly used grading system in the US for stainless steel. Other steel grades include the UNS grades.

  • 100 Series—austenitic chromium-nickel-manganese alloys:
    • Type 101—austenitic that is hardenable through cold working for furniture.
    • Type 102—austenitic general purpose stainless steel working for furniture.
  • 200 Series—austenitic chromium-nickel-manganese alloys:
    • Type 201—austenitic that is hardenable through cold working.
    • Type 202—austenitic general purpose stainless steel.
  • 300 Series—austenitic chromium-nickel alloys:
    • Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
    • Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.
    • Type 303—free machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1″ in accordance with ISO 3506.
    • Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as "A2″ in accordance with ISO 3506.
    • Type 304L— same as the 304 grade but contains less carbon to increase weldability. Is slightly weaker than 304.
    • Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.
    • Type 308—used as the filler metal when welding 304.
    • Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel.
    • Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. 316 is often used for building nuclear reprocessing plants.
    • Type 316L—extra low carbon grade of 316, generally used in stainless steel watches and marine applications due to its high resistance to corrosion. Also referred to as "A4″ in accordance with ISO 3506.
    • Type 316Ti—includes titanium for heat resistance, therefore it is used in flexible chimney liners.
    • Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.
  • 400 Series—ferritic and martensitic chromium alloys:
    • Type 405— ferritic for welding applications.
    • Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
    • Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
    • Type 410—martensitic (high-strength iron/chromium). Wear-resistant, but less corrosion-resistant.
    • Type 416—easy to machine due to additional sulfur.
    • Type 420—Cutlery Grade martensitic; similar to the Brearley's original rustless steel. Excellent polishability.
    • Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.
    • Type 439—ferritic grade, a higher grade version of 409 used for catalytic converter exhaust sections. Increased chromium for improved high temperature corrosion/oxidation resistance.
    • Type 440—a higher grade of cutlery steel, with more carbon, allowing for much better edge retention when properly heat-treated. It can be hardened to above Rockwell 55 hardness, making it one of the hardest stainless steels. Due to its hardness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Available in four grades: 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered more desirable in knifemaking than 440A, except for diving or other salt-water applications.
    • Type 446—For elevated temperature service.
  • 500 Series—heat-resisting chromium alloys
  • 600 Series—martensitic precipitation hardening alloys:
    • Type 601 through 604: Martensitic low-alloy steels.
    • Type 610 through 613: Martensitic secondary hardening steels.
    • Type 614 through 619: Martensitic chromium steels.
    • Type 630 through 635: Semiaustenitic and martensitic precipitation-hardening stainless steels. Type 630 is most common PH stainless, better known as 17-4; 17% chromium, 4% nickel.
    • Type 650 through 653: Austenitic steels strengthened by hot/cold work.
    • Type 660 through 665: Austenitic superalloys; all grades except alloy 661 are strengthened by second-phase precipitation.
    • Type 2205— the most widely used duplex (ferritic/austenitic) stainless steel grade. It has both excellent corrosion resistance and high strength.

 

EN-standardSteel no. k.h.s DIN EN-standardSteel name SAE grade UNS
    440A S44002 
1.4112    440B S44003
1.4125   440C S44004
    440F S44020
1.4016 X6Cr17 430 S43000
1.4408 G-X 6 CrNiMo 18-10 316  
1.4512 X6CrTi12 409 S40900
    410 S41000
1.4310 X10CrNi18-8 301 S30100
1.4318 X2CrNiN18-7 301LN N/A
1.4307 X2CrNi18-9 304L S30403
1.4306 X2CrNi19-11 304L S30403
1.4311 X2CrNiN18-10 304LN S30453
1.4301 X5CrNi18-10 304 S30400
1.4948 X6CrNi18-11 304H S30409
1.4303 X5CrNi18-12 305 S30500
  X5CrNi30-9 312  
1.4541 X6CrNiTi18-10 321 S32100
1.4878 X12CrNiTi18-9 321H S32109
1.4404 X2CrNiMo17-12-2 316L S31603
1.4401 X5CrNiMo17-12-2 316 S31600
1.4406 X2CrNiMoN17-12-2 316LN S31653
1.4432 X2CrNiMo17-12-3 316L S31603
1.4435 X2CrNiMo18-14-3 316L S31603
1.4436 X3CrNiMo17-13-3 316 S31600
1.4571 X6CrNiMoTi17-12-2 316Ti S31635
1.4429 X2CrNiMoN17-13-3 316LN S31653
1.4438 X2CrNiMo18-15-4 317L S31703
1.4539 X1NiCrMoCu25-20-5 904L N08904
1.4547 X1CrNiMoCuN20-18-7 N/A S31254

 

 

 
 
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