Stainless steel

1. Stainless steel is a ferrous alloy with a minimum of 11% chromium content which does not stain, corrode or rust as easily as ordinary steel.

Besides chromium, other typical alloying elements are:
• - Nickel
• - Molybdenum
• - Manganese
• - Titanium
• - Niobium

These elements are added to enhance structure and properties such as formability, ductility, strength and toughness. Stainless steel is 100% recyclable. In fact the main source of raw material for its production comes from re-melted scrap metal. Moreover, its life expectancy is usually long and only a minimum of maintenance is needed. This is why stainless steel is rendered as an environmental friendly material.

2. The chromium in the steel combines with the atmospheric oxygen to form a very thin (1-5 X 10-9m), chromium rich oxide layer called the "passive" film. This film is invisible meaning the metal stays shiny. However, it is strongly adherent and chemical stable and protects the metal beneath. The corrosion resistance durability of stainless steel arises from the ability of the passive film to self repair when scratched from the surface. Where there is sufficient oxygen available more oxide will quickly form and recover the exposed surface.

3. The main requirement for stainless steel is to be corrosion resistant for a specific application or environment. Also mechanical or physical properties may need to be considered to achieve the overall performance requirements. For these reasons it is important to select the appropriate type and grade of stainless steel.

4.Three of the main types of stainless steel identified by their microstructure crystal phase are: AUSTENITIC stainless steels contain a maximum of 0.15% carbon, 18 – 20% chromium, 8 – 10% nickel and/or manganese. It is the most widely used type of stainless steel. The added elements make the steel to retain an austenitic structure at all temperatures, give it ductility, good weldability and non-magnetic properties. Austenitic steels are not hardenable by heat treatment.

FERRITIC stainless steels are highly corrosion resistant, but less durable and ductile than austenitic grades. They are magnetic and can not be hardened by heat treatment. They have a low carbon content, 13 – 17% chromium, very little nickel if any and some compositions include molybdenum or titanium.

MARTENSITIC stainless steels are less corrosion resistant than the other two types, but they are extremely strong and tough. They are magnetic and can be hardened due to martensite which also reduces its toughness and makes it brittle. They contain 12% chromium, 0.2 – 1% molybdenum, no nickel and 0.1 – 1.5% carbon.

5.PROPERTY TABLES

DESIGNATIONS

PROPERTIES

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CHEMICAL COMPOSITION

MECHANICAL CHARACTERISTICS