A. Each of the terms represent a metallurgical phase associated with the alloy.

Ferritic stainless steels must have more than 10-12% chromium to be considered stainless. This provides the ferrite phase, or structure, that is strong and hard. However, this structure cannot be significantly hardened by further cold-work or heat treatment.

Martensitic stainless steels have higher carbon and chromium contents than the ferritic stainless steel, plus manganese and silicon, which allow the formation of hard acicular (spiky) structures. These alloys can be hardened, or tempered, by heat treatment.

Austenitic stainless steel have chromium and nickel contents, which provide the material with excellent corrosion and malleable (workable) properties at room temperature.

Duplex stainless steels have controlled amounts of ferrite and austenite in their structures, which allow these alloys to exhibit higher strengths than their austenitic counterparts, while retaining the corrosion resistance of the austenitic materials. These benefits are achieved through balanced 50% austenitic and 50% ferrite structures. They offer good resistance to chloride ion stress corrosion cracking. back to main menu

2. Why is this series of materials called stainless?

A. Stainless steel must have at least 10% chromium in its content to be called stainless.

Below this value, the alloy steels will experience increasing corrosion rates and will begin to rust in water environments.

18-8 stainless steel (typically 304), which has 18% chromium, will exhibit low or negligible general corrosion rates in water. back to main menu

3. What are the 18-8's?

A. This characterizes those materials having 18% chromium and 8% nickel. These are known as the 300 series of austenitic stainless steels.

The more common of these materials is 304, 304L, 316 and 316L stainless steels, which represent over 50% of all stainless steels used today. back to main menu

4. What is the difference between 304 and 304L?

A. The "L" stands for the low carbon grade (>.03% carbon). This is the grade of austenitic stainless steel that should be selected when welding with these austenitic grades of material. back to main menu

5. What makes stainless, stainless?

A. The corrosion resistance of the austenitic stainless steels is realized through the tight, adherent oxide scale that forms naturally on stainless steel surfaces. Even small amounts of oxygen in the operating environment will allow stainless to form this protective scale. It is very thin (angstroms thick) and consists of chromium and nickel oxides. back to main menu

6. Can stainless steel rust?

A. Austenitic stainless steel does not rust. The presence of rusting on stainless steel components is due to iron particle contamination - from lifting gear; welding spatter; shop dirt; and general lack of cleanliness. When exposed to moisture or humidity, the iron particles will corrode (rust) leaving an undesirable surface condition on the stainless steel surface, which under the most severe conditions could lead to pitting of the stainless steel surface. 

However, this condition can be readily treated with a number of acids, followed by thorough washing to remove the excess acid. (reference ASTM Specification A380 for different approaches to cleaning).
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7. Can stainless steel be magnetic?

A. The ferritic, martensitic and duplex stainless steels will be magnetic.

A completely austenitic stainless steel (type 300 series) will be non-magnetic.

In the case of welded austenitic stainless steel (type 304L and 316L), a small amount of ferrite (<5%) may be desired in the weld in order to eliminate hot short cracking of the weld. In these cases, a slight magnetic response of the weld may be identified.

Also, in severely worked conditions, some martensitic phases can be produced which will also provide a magnetic response. This may be typical in the precipitation hardened, austenitic stainless steels.
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8. What does the S stand for in schedule 10S pipe sizes?