What Is Stainless Steel And Stainless Steel Used For?

What Is Stainless Steel?

Stainless steel is an alloy of Iron with a minimum of 10.5% Chromium. Chromium produces a thin layer of oxide on the surface of the steel known as the 'passive layer'. This prevents any further corrosion of the surface. Increasing the amount of Chromium gives an increased resistance to corrosion.

Stainless steel also contains varying amounts of Carbon, Silicon and Manganese. Other elements such as Nickel and Molybdenum may be added to impart other useful properties such as enhanced formability and increased corrosion resistance.

When was stainless steel discovered?

There is a widely held view that stainless steel was discovered in 1913 by Sheffield metallurgist Harry Brearley. He was experimenting with different types of steel for weapons and noticed that a 13% Chromium steel had not corroded after several months. However, the picture is much more complex than this.

What is stainless steel used for?

Stainless steels of various kinds are used in thousands of applications. The following gives a flavour of the full range:

Domestic – cutlery, sinks, saucepans, washing machine drums, microwave oven liners, razor blades
Architectural/Civil Engineering – cladding, handrails, door and window fittings, street furniture, structural sections, reinforcement bar, lighting columns, lintels, masonry supports
Transport – exhaust systems, car trim/grilles, road tankers, ship containers, ships chemical tankers, refuse vehicles
Chemical/Pharmaceutical – pressure vessels, process piping.
Oil and Gas – platform accommodation, cable trays, subsea pipelines, stainless steel flanges, stainless steel pipe fittings, stainless steel pipes.
Medical – Surgical instruments, surgical implants, MRI scanners.
Food and Drink – Catering equipment, brewing, distilling, food processing.
Water – Water and sewage treatment, water tubing, hot water tanks.
General – stainless steel springs, stainless steel fasteners (bolts, nuts and washers), stainless steel wire.

Is stainless steel non-magnetic?

It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1.

It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.

In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.

Does stainless steel corrode?

Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, in some circumstances it can corrode. It is 'stain-less' not 'stain-impossible'. In normal atmospheric or water based environments, stainless steel will not corrode as demonstrated by domestic sink units, cutlery, saucepans and work-surfaces.

In more aggressive conditions, the basic types of stainless steel may corrode and a more highly alloyed stainless steel can be used.

Can I use stainless steel at low temperatures?

Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.

Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.

Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.

In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C.

About Yaang Pipe Industry Co., Limited
Established in 2008, Yaang Pipe Industry Co., Limited is a professional organizer and one-stop-shop supplier for steel piping system products, including stainless steel pipes and stainless steel tubes, stainless steel forged flanges and butt-welding stainless steel pipe fittings, stainless steel pipe fittings, stainless steel elbows, stainless steel tees, stainless steel reducer, stainless steel stub end, stainless steel gaskets, stainless steel fasteners, stainless steel valves, Sanitary Services etc. in China. We have devoted to providing the best solutions of steel materials and industrial equipment for our respected customers.

Source: Yaang Pipe Industry Co., Limited (www.yaang.com)

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