Technical Information
Stainless Steel Introduction
Stainless steel is an alloy which is iron-based and contains various combination 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.
Doubtless, the family of stainless steels withstands a greater variety of service environment than any other metal. The following properties of stainless steels give this group of alloys a high degree of versatility rendering them useful and valuable for a wide range of applications:
- Corrosion resistance
- Heat-resistance
- Strength and toughness
- Cryogenic property
- Formability
- Appearance
- Ease of maintenance
The selection of stainless steel is however, based upon following three basic criteria in order of importance:
Corrosion Resistance: This is the prime reason for specifying stainless steel. The level of corrosion resistance required and the corrosive environment expected must be known. Corrosion data on various stainless steel are available in standard literature which may be referred to in this connection.
Mechanical Properties: Emphasis should be placed upon the alloy’s strength. This second criterion, along with the corrosion resistance, would designate in most of the cases the specific alloy for a particular application.
Fabricability: How the material is to be processed should also be known. This includes machining, welding, forming, cold heading etc.
Stainless Steel Classifications
There are three 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 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 "ferurn" meaning iron. It is so named because its micro-structure 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 Isw carbon content of 0.8 to 2.0%. Due to its high chromium and low carbon content, ferritic alloys do not generally harden in high temperatures. t is magnetic alloy and is soft and ductile.
Class III : Austenitic - Non-Heat Treatable, Chromium-Nickel
The austenitic class derives its name from R beFis-Austen who first observed its characteristic banded grain structure. Its chief alloys are: chromium, found in amounts from 16.0 to 26.0% and an 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.
Extra-Low-Carbon (ELC) Grades
These alloys contain only approximately 0.3% carbon which is low enough to permit elimination of carbide precipitation during welding. These grades are not generally recommended for high-temperature utilization.
American Iron and Steel Institute (AISI) Designation System
200's - Chromium - Nickel - Manganese Stainless Steel
This series is austenitic, non-heat treatable and non-magnetic.
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 ferritic, non-heat treatable, and magnetic.
"L" at the end of the series number indicates low-carbon content (not exceeding .03%) Example: 304L
"F" at the end of the series number indicates the addition of a "free-machining" element. (Example: 440F)
"S" at the end of 309 or 310 type indicate carbon not exceeding .08%
"Ti" at the end of the series number denotes addition if titanium. (Example: 316Ti)