What is stainless steel?

"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.

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.

Corrosion and oxidation resistance of stainless steels

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.

Families of stainless steels

There are several families of stainless steel: FERRITIC, MARTENSITIC, AUSTENITIC and DUPLEX. These names are derived from the crystal structure of the steels, which governs their metallurgical behaviour.

FERRITIC stainless steels are magnetic, have a low carbon content and contain chromium as the main alloying element, typically between 13% and 17%.They are not hardenable by heat treatment.

MARTENSITIC stainless steels are magnetic, containing typically 12% chromium with a higher carbon content than the ferritic types. They are hardenable by quenching and tempering like plain carbon steels and find their main application in cutlery, aerospace and general engineering.

AUSTENITIC stainless steels are non-magnetic and, in addition to chromium typically around 18%, contain nickel. This enhances their corrosion resistance and modifies the structure from ferritic to austenitic. They are the most widely used group of stainless steels. They are not hardenable by heat treatment.

DUPLEX stainless steels are used where combinations of higher strength and corrosion resistance are needed. They have a mixed structure of austenite and ferrite, hence the term "duplex". They are not hardenable by heat treatment.

PRECIPITATION HARDENING stainless steels, like the martensitic types, can be strengthened (ie hardened) by heat treatment. The mechanism is metallurgically different to the process in the martensitic types. This means that either martensitic or austenitic precipitation hardening structures can be produced.

"Super" austenitic or "super" duplex grades have enhance pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types. This is due the further additions of chromium, molybdenum and nitrogen to these grades.

Benefits and properties of stainless steels

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.

Stainless steel and the environment

The main source of raw material for making stainless steels is re-cycled scrap metal. This re-cycling route has been established for many years and the economics of the stainless steel making industry depend on recycling. Over 90% of new stainless steel is produced from recycled scrap.

The steel is melted electrically and in most cases refined by using inert air distilled gases, such as argon. Great care is taken to minimise fume and dust emissions. Some plants are equipped to re-cycle dust into the steel making process.

Most of the steel processing consumable materials, including cooling water, lubricating oils, pickling acids and "inter-leaving" paper are re-cycled in the plant or by specialist contractors. Stainless steel fabricators and processors re-cycle their scrap arisings and in-process consumables, including "caking" pickling acid residues for re-cycling.

As stainless steels are corrosion resistant alloys their life expectancy is usually long. A minimum of maintenance is needed and so, although more expensive initially, they offer attractive "life-cycle cost" benefits over alternatives such as carbon steels.

Stainless steels are easily cleansible and so an obvious choice for food and beverage manufacturing industries and catering equipment. There are no proven health risks from the normal use of stainless steels. The possible risks from alloying elements such as nickel and chromium are under constant review by experts.

More about stainless steel

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