Stainless Steel  Finishes              Stainless Steel Classifiaction

History of Stainless Steel:

In metallurgy stainless steel, also known as inox steel or inox from French "inoxydable” is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass. Stainless Steel does not stain, corrode, or rust as easily as ordinary steel, but it is not stain-proof. It is also called corrosion-resistant steel.

Harry Brearley, who was born in Sheffield, England, in 1871, probably invented stainless steel. His father was a steel melter and through private study and night school he became an expert in the analysis of steel and its production. In 1908 Brearley was given the opportunity to set up the Brown Firth Laboratories, which was financed by the two leading Sheffield steel companies of the day. In 1912 Brearley was asked to help solve the problems being encountered by a small arms manufacturer, whereby the internal diameter of rifle barrels was eroding away too quickly because of the action of heating and discharge gases. Brearley was therefore looking for steel with better resistance to erosion, not corrosion. As a line of investigation he decided to experiment with steels containing chromium, as these were known to have a higher melting point than ordinary steels.

It was Ernest Stuart, the cutlery manager of Mosley\'s who first referred to the new knives as "stainless" after, in experiments, he had failed to stain them with vinegar. "Corrosion resisting" steel would be really the better term, as ordinary stainless steels do suffer corrosion in the long term in hostile environments.

What is Stainless Steel:

Stainless steel is the name given to a group of corrosion resistant and high temperature steels. Their remarkable resistance to corrosion is due to a chromium-rich oxide film which forms on the surface. When ordinary carbon steel is exposed to rain water, for example, it corrodes forming a brown iron oxide, commonly called rust, on the surface. This is not protective and eventually the entire piece of steel will corrode and be converted to rust. But when enough chromium (more than about 10%) is added to ordinary steel, the oxide on the surface is transformed - it is very thin, virtually invisible and protective in a wide range of corrosive media. This is what we call stainless steel and there are several different types, and many different grades.

Types and Grades of Stainless Steel:

The three main types of stainless steels are austenitic, ferritic, and martensitic. These three types of steels are identified by their microstructure or predominant crystal phase.

Austenitic: 
Austenitic steels have austenite as their primary phase (face centered cubic crystal). These are alloys containing chromium and nickel (sometimes manganese and nitrogen), structured around the Type 302 composition of iron, 18% chromium, and 8% nickel. Austenitic steels are not hardenable by heat treatment. The most familiar stainless steel is probably Type 304, sometimes called T304 or simply 304. Type 304 surgical stainless steel is an austenitic steel containing 18-20% chromium and 8-10% nickel.

Ferritic: 
Ferritic steels have ferrite (body centered cubic crystal) as their main phase. These steels contain iron and chromium, based on the Type 430 composition of 17% chromium. Ferritic steel is less ductile than austenitic steel and is not hardenable by heat treatment.

Martensitic: 
The characteristic orthorhombic martensite microstructure was first observed by German microscopist Adolf Martens around 1890. Martensitic steels are low carbon steels built around the Type 410 composition of iron, 12% chromium, and 0.12% carbon. They may be tempered and hardened. Martensite gives steel great hardness, but it also reduces its toughness and makes it brittle, so few steels are fully hardened.

There are also other grades of stainless steels, such as precipitation-hardened, duplex, and cast stainless steels. Stainless steel can be produced in a variety of finishes and textures and can be tinted over a broad spectrum of colors.

Mechanical and Physical Properties:

For most corrosion resistant applications, strength is not a key issue. There are exceptions, such as pressure vessels, and here the high strength of duplex grades can make them attractive. A characteristic of the austenitic stainless steels is that they work harden easily - that is, their strength increases rapidly when they are formed at ambient temperatures, such as in rolling or wire drawing operations. The accompanying table (below) shows the large increase in strength of Type 304 in the 1/2 hard condition. This characteristic is valuable in items like bolts and springs.

Two important physical properties are thermal conductivity and thermal expansion rate. The common austenitic stainless steels, such as Type 304, have lower thermal conductivity than carbon steels and this is useful in applications such as cappuccino cups and thermos flasks. Their rate of thermal expansion is also greater than ordinary steel (but less than materials such as aluminium) so care must be taken during welding to ensure that the recommended jigging and tacking procedures and welding sequences are followed.

Corrosion Resistance

When carbon steel rusts, it does so by uniform corrosion - the entire surface of the steel corrodes more or less uniformly. Except in special environments, such as strong acids, stainless steels do not corrode in this way. If corrosion does occur, it is normally by localised corrosion and the most common forms of this are as follows:

Pitting is localised corrosion at individual sites on the surface of stainless steel. Pitting starts at points of weakness in the protective oxide film, such as at manganese sulphide inclusions on the steel surface.

Crevice corrosion takes place where physical crevices are present, such as at the joint between two overlapping sheets of stainless steel, in the crevice between a stainless steel flange and a non-metallic gasket or under surface deposits.

The mechanisms of pitting and crevice corrosion are similar and both most commonly occur in chloride environments. But crevice corrosion occurs more readily since it is assisted by the existence of a physical crevice, whereas pitting has to initiate on a surface which is effectively flat. It is often not possible to control environmental factors such as the amount of chloride or the temperature, so it is usually necessary to choose a sufficiently corrosion resistant grade for the service. An indication of pitting and crevice corrosion resistance is given by the \'Pitting Resistance Equivalent\' (PRE) number:

PRE = %Cr 3.3%Mo 16%N

This formula shows the beneficial effect of chromium, molybdenum and nitrogen and illustrates why Type 316, with 2-3% Mo, has better resistance than Type 304 to marine environments. However, for resistance to corrosion when immersed in seawater on a longterm basis, it is necessary to move up to a grade with a relatively high PRE number, such as a super-austenitic 6% Mo grade or a super-duplex such as 2507.

Chloride stress corrosion cracking (SCC) can occur in chloride-containing solutions at elevated temperature, normally above 50 degrees C, when tensile stress is present. It particularly affects austenitic stainless steels, and a common failure observed in the field is cracking from the outside of tanks or pipes carrying hot fluids. For example, if a water leak occurs into insulation on the outside, chlorides can concentrate through evapouration, and SCC can take place because of the tensile stresses present in the outside surface of pipes and tanks.

Chloride SCC is most comonly overcome by using a duplex stainless steel, such as 2205, or a grade with a higher nickel content, such as a 6% Mo material or high-nickel alloys like Alloy 825 (N08825). Ferritic stainless steels are very resistant to SCC but grades with equivalent pitting resistance to the austenitic grades have other major drawbacks.

Intergranular corrosion (IGC) is preferential attack at the grain boundaries of a stainless steel and is commonly associated with welding.

If stainless steel is heated into a sensitising temperature range, such as can occur in the heat affected zone of a weld, then chromium can combine with carbon in the steel to form chromium carbides in the grain boundaries. Such a stainless steel is said to be \'sensitised\'. Around each chromium carbide particle is an area low in chromium so that, when the material is placed in a corrosive environment, attack of these low-chromium regions can occur. This is called IGC.

The most common way today to avoid IGC is to specify a low carbon \'L\' grade of stainless steel when welding, such as Type 304L . In the past, when it was difficult for mills to achieve low carbon levels, titanium (Ti) or niobium (Nb) were added since these elements preferentially combine with carbon and so leave the chromium unaffected. Grades containing these additions include Type 321 (S32100) containing Ti and Type 347 (S34700) containing Nb.

Galvanic corrosion can occur when different metals are in contact in an electrically conductive liquid. Stainless steel is not normally corroded in such a galvanic couple, since it is usually the more corrosion resistant of the two metals and acts as the cathode. When the other metal which is in contact with the stainless steel is less corrosion resistant, it acts as the anode and corrodes preferentially. The rate of corrosion of the second metal can be rapid if its surface area is small relative to the area of the stainless steel cathode with which it is in contact. An example of this would be the use of galvanised steel fasteners to hold stainless steel sheets, a poor design unless the system is always dry. Galvanic couples are not necessarily a problem and can, in fact, be used to benefit in some designs.

Scope of Stainless Steel:

Stainless steels find use in a very wide variety of applications. Some typical examples are:

Consumer goods: Applications here include domestic kitchenware and tableware, kitchen sinks, laundry equipment and electrical and electronic appliances.

Architecture, building & construction: Stainless steel has been used in numerous famous buildings. The ChryslerBuilding in New York, the world\'s tallest building when it was built in 1929, used Type 302 (similar to Type 304) for the roof and upper structure. Type 316 is used to clad the outside of Petronas Twin Towers in Kuala Lumpur, currently the world\'s tallest buildings, and Jin Mao Tower in Shanghai, the third tallest. More common applications are balustrades, column wraps, roofing and guttering, signage, curtain wall supports, light poles, elevator doors and public seating. Stainless steel rebar is used in bridges, barrier walls and decking to extend the life of critical areas of roadways and marine structures.

Food and beverage industry: Type 304 and, in more aggressive situations, Type 316 are widely used in this industry for food and beverage production (milk silos, cheese vats, beer and wine fermenters, fruit juice tanks and piping), storage (wine tanks, beer kegs), cooking (large commercial kitchens) and serving (display cabinets, bench tops). Stainless steel equipment is easy to clean (sanitize) and also preserves the purity of the food.

 Transportation: A wide range of both decorative and functional components are fabricated from stainless steel, such as automotive exhaust systems, fasteners, trim, wheel covers and windscreen wiper arms; passenger railcars, coal wagons, bus frames and milk tankers; and seagoing chemical tankers.

Chemical, petrochemical, oil and gas, pulp and paper industries, and power generation: This field represents a very diversified market for stainless steel with many specialised applications such as heat exchangers, vessels for various types of chemicals, pipe, fittings, valves, pumps, mixers, high temperature furnace equipment, components for nuclear reactors, and gas and water turbines.