"Stainless" is a term coined early in the development of these steels for cutlery applications. It was adopted as a generic name for these steels and now covers a wide range of steel types and grades for corrosion or oxidation resistant applications.

Stainless steels are iron alloys with a minimum of 10.5% chromium. Other alloying elements are added to enhance their structure and properties such as formability, strength and cryogenic toughness. These include metals such as:

• Nickel
• Molybdenum
• Titanium
• Copper

Non-metal additions are also made, the main ones being:

• Carbon
• Nitrogen

The main requirement for stainless steels is that they should be corrosion resistant for a specified application or environment. The selection of a particular "type" and "grade" of stainless steel must initially meet the corrosion resistance requirements. Additional mechanical or physical properties may also need to be considered to achieve the overall service performance requirements.
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Why is stainless steel "stainless"?

The corrosion resistance of stainless steel arises from a "passive", chromium-rich, oxide film that forms naturally on the surface of the steel. Although extremely thin at 1-5 nanometres (i.e. 1-5 x 10-9 metres) thick, this protective film is strongly adherent, and chemically stable (i.e. passive) under conditions which provide sufficient oxygen to the surface.

The key to the durability of the corrosion resistance of stainless steels is that if the film is damaged it will normally self repair (provided there is sufficient oxygen available). In contrast to other steel types which suffer from "general" corrosion where large areas of the surface are affected, stainless steels in the "passive state", are normally resistant to this form of attack.

Stainless steels cannot be considered "indestructible", however. The passive state can be broken down under certain conditions and corrosion can result. This is why it is important to select carefully the appropriate grade for a particular application.

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BENEFITS OF STAINLESS STEEL

Corrosion resistance
--lower alloyed grades resist corrosion in atmospheric and pure water environments, while high-alloyed grades can resist corrosion in most acids, alkaline solutions, and chlorine bearing environments, properties which are utilized in process plants.

Fire & heat resistance
--special high chromium and nickel-alloyed grades resist scaling and retain strength at high temperatures.

Hygiene
--the easy cleaning ability of stainless makes it the first choice for strict hygiene conditions, such as hospitals, kitchens, abattoirs and other food processing plants.

Aesthetic appearance
--the bright, easily maintained surface of stainless steel provides a modern and attractive appearance.

Strength-to-weight advantage
--the work-hardening property of austenitic grades, that results in a significant strengthening of the material from cold-working alone, and the high strength duplex grades, allow reduced material thickness over conventional grades, therefore cost savings.

Ease of fabrication
--modern steel-making techniques mean that stainless can be cut, welded, formed, machined, and fabricated as readily as traditional steels.

Impact resistance
--the austenitic microstructure of the 300 series provides high toughness, from elevated temperatures to far below freezing, making these steels particularly suited to cryogenic applications.

Long term value
--when the total life cycle costs are considered, stainless is often the least expensive material option.
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In general the corrosion and oxidation resistance of stainless steels improves as the chromium content increases. The addition of nickel to create the austenitic stainless steel grades strengthens the oxide film and raises their performance in more aggressive conditions. The addition of molybdenum to either the ferritic or austenitic stainless steels improves their pitting corrosion resistance.

The austenitic stainless steels are resistant to the wide range of rural and industrial atmospheres encountered in the United Kingdom, resulting in extensive use in architectural, structural, and street furniture applications. Their resistance to attack by acids, alkalis and other chemicals, has led to a wide use in the chemical and process plant industries.

The ferritic stainless steels are used in the more mildly corrosive environments, being often used in trim work and somewhat less demanding applications.

Martensitic stainless steels have similar corrosion resistance to the ferritic types, whilst that of the precipitation hardening stainless steels is claimed to be similar to the 304 (1.4301) austenitic type stainless steel.

Duplex stainless steels are alloys designed to have improved localised corrosion resistance, specifically to stress corrosion cracking, crevice and pitting corrosion.

Corrosion attacks at the surface of a material. It is important therefore to ensure that the surface finish is suitable and that the surface is clean and uncontaminated (particularly from non-stainless steel contact). This enables the "inherent" corrosion resistance conferred by the additions of chromium, nickel, molybdenum etc. to be fully exploited.
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In economic terms stainless steels can compete with higher cost engineering metals and alloys based on nickel or titanium, whilst offering a range of corrosion resisting properties suitable for a wide range of applications. They have better strength than polymer products such as GRP. Stainless steels can be manipulated and fabricated using a wide range of commonly available engineering techniques and are fully "recyclable" at the end of their useful life.

In addition to their corrosion resistance, stainless steels also offer other useful properties, depending on their "family".

The austenitics, in the fully annealed heat-treated condition, are:

• Fracture tough at cryogenic temperatures
• Para-magnetic with relative magnetic permeabilities around 1.05

The martensitic and precipitation hardening families are hardenable by heat treatment.

The duplex stainless steels are stronger than the austenitics in the annealed condition and so can be used in thinner sections to save weight and cost.

The ferritics are lower cost stainless steels.
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