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Stainless Steel

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Stainless steel is an alloy, which is iron-based and contains various combinations of other elements to give it characteristics suitable for a wide range of applications. The element that makes stainless steel stainless is chromium. By definition, any steel, which contains a minimum of 11% chromium, is a stainless steel. The chromium content in the steel produces a rich, transparent oxide film, which coats the steel and protects it from corrosion and oxidation.

Stainless Steel Classifications

There are four basic classes of stainless steels, so designated for the metallurgical conditions of the steels:

Class I: Martensitic – Heat treatable, Straight” Chromium

This class is so named for the man, Martens, who first examined metals microscopically. It is referred to as “martensitic” because of its acicular or needle-like microstructure in the hardened condition. Its chief alloying agent is chromium, found in amounts from 11.5 to 18.0%. It contains from 0.08 to 1.10% carbon. It is magnetic and responds excellently to heat treating, producing a hard and strong stainless steel.

Class II: Ferritic – Non-Heat treatable, Straight Chromium

This class name is derived from the Latin word “ferrum” meaning iron. It is so named because its microstructure is very similar to that of low-carbon iron. It also utilizes chromium as its chief alloying agent, being found in amounts from 14.0 to 27.0%. It has a very low carbon content of .08 to .20%. Due to its high chromium and low carbon content, ferritic alloys do not generally harden in high temperatures. It is a magnetic alloy, and is soft and ductile.

Class III: Austenitic – Non-Heat treatable, Chromium-Nickel

The austenitic class derives its name from Roberts-Austen who first observed its charac­teristic banded grain structure. Its chief alloys are: chromium, found in amounts from 16.0 to 26.0%; and a appreciable nickel content from 6.0 to 22.0%. This alloy cannot be heat treated, but responds excellently to cold working. It is generally non-magnetic. In the an­nealed condition, this alloy is tough, strong, and extremely ductile. Austenite itself is soft and tough and remains ductile even at extreme low temperatures.

Class IV: Precipitation-Hardening

This is a relatively new class metallurgists have deemed necessary to group separately be­cause of its increasing popularity. These alloys have low hardening temperatures that produce precipitation hardening. This capability averts problems such as warping, cracking, and scaling. They can be hardened by simple heat treatments, require no stress-relief treat­ment and are available in all forms. These grades are easily fabricated, and are corrosion resistant without added treatment. They are also known for their high strength. An American Iron and Steel Institute (AISI) Type designation has not been issued these grades, as they are patented proprietary.

American Iron and Steel Institute (AISI) Designation System

300’s – Chromium-Nickel Stainless Steel this series is austenitic, non-heat treatable, and non-magnetic.

400’s – Chromium Stainless Steel this series is martensitic, heat treatable and magnetic. It also includes types, which are ferrite, non-heat treatable, and magnetic.

“L” at the end of the series number indicates low-carbon content. (Example: 304L)

“F” at the end of the series number indicates the addition of a “free-machining” element. (Example: 440F)

Other letters used at the end of the series number signify the symbol of the element added to the alloy. (Example: 440C – the C being the symbol for the Carbon additive)

Alloying Elements

Aluminum (Al) – acts as an active degasifier and deoxidizer. Controls inherent grain size.

Bismuth (Si) – acts to improve machinability.

Boron (B) – improves hardenability and increases depth of hardening. Usually found in amounts of .0005 to .003%.

Carbon (C) – improves hardenability, and increases tensile strength and response to heat treatment when added in amounts of 0.8 to 0.9%. If amount is further increased, heat and cold workability would greatly decrease, and the alloy would begin to exhibit the character­istics of cast iron.

Chromium (Cr) – gives stainless steel its stainless quality. Increases response to heat treatments and depth of hardness. In combination with nickel, greatly increases corrosion and oxidation resistance. Also increases toughness, tensile strength, and resistance to wear.

Cobalt (Co) – increases strength and hardenability of alloy. Improves effectiveness of other elements.

Columbium (Cb) – increases immunity to carbide precipitation and inter-granular corrosion.

Copper (Cu) – increases corrosion resistance and improves tensile and yield strengths with­out loss of ductility.

Iron (Fe) – this is the basic element of steel. Iron by itself lacks strength and does not respond to heat treatment; it is soft and ductile.

Lead (Pb) – greatly improves machinability in quantities of .15 to .35%.

Manganese (Mn) – normally present in all steel. Increases strength, hardness and response to heat treatment in amounts of .5 to 15%. It acts as a degasifier and deoxidizer, and increases the alloy’s resistance to wear. In combination with sulfur, improves forgeability.

Molybdenum (Me) – increases strength, hardness penetration, and machinability. Aids in resisting softening at high temperatures, and improves resistance to corrosion.

Nickel (Ni) – in amounts of 1.0 to 35% improves the strength and impact resistance without loss of ductility. Increases resistance to corrosion, but decreases work hardening. Improves machinability and fabricability.

Nitrogen (N) – can serve as a substitute for a portion of nickel in alloys. Improves machinability by producing a fine chip.

Phosphorus (P) – increases yield strength, hardness and machinability and greatly improves resistance to corrosion. Ductility is decreased at low temperatures.

Selenium (Se) – serves to improve machinability.

Silicon (Si) – it is a common degasifier and deoxidizer. Increases tensile strength, hardenability and forgeability. At high temperatures, resists corrosion and scaling.

Sulfur (S) – in amounts of .06 to .3%, increases machinability. It is not recommended for alloys used in hot forming. It decreases weldability and ductility.

Tantalum (Ta) – used primarily as a stabilizer. It also prevents localized carbon depletion. Tellurium (Te) – when added to leaded steels, greatly improves machinability.

Titanium (Ti) – serves to increase immunity to carbide precipitation and resistance to cor­rosion and oxidation.

Tungsten (W) – produces a fine, dense grain. Increases hardness in high-speed steel at high temperatures.

Vanadium (V) – increases shock resistance, strength, and hardness. Retards grain growth even after exposure to high temperatures.