Stainless

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The alloys detailed below represent a few of the most popular for architectural applications. There are literally hundreds of stainless alloys, so please inquire with us if you have a special requirement and do not see it listed here.

304

This austenitic alloy contains 18% chromium and 8% nickel. It is one of the most widely used alloys because it offers good corrosion resistance and has excellent forming characteristics. 304L is the low carbon version of this alloy and offers better welding and corrosion resistance without the need for post-welding annealing.

316

This austenitic alloy contains 16% chromium, 10% nickel, and 2% Molybdenum. The combination of a higher nickel content and the addition of Molybdenum make this allow much more corrosion resistant than 304. It is commonly used where contact with sea water or brine solutions will be encountered. 316L is the low carbon version of this alloy and offers better welding and corrosion resistance without the need for post-welding annealing.

430

This ferritic alloy contains 16% chromium and no nickel. It is highly magnetic and offers only very basic corrosion resistance, nowhere near as good as the 300 series alloys. It has limited forming and welding characteristics and it can not be hardened by heat treating.

Duplex

Duplex Stainless Steel alloys have a structure that contains both ferrite and austenite. Duplex alloys have higher strength and better stress corrosion cracking resistance than most austenitic alloys and greater toughness than ferritic alloys, especially at low temperatures. The corrosion resistance of duplex alloys depends primarily on their composition, especially the amount of chromium, molybdenum, and nitrogen they contain. Duplex alloys are often divided into three sub-classes: Lean Duplex, Standard Duplex, and Superduplex.

 

What is Stainless Steel?

Stainless steel is essentially a low carbon steel which contains chromium at 10% or more by weight. It is this addition of chromium that gives the steel its unique stainless, corrosion resisting properties.

The chromium content of the steel allows the formation of a rough, adherent, invisible, corrosion-resisting chromium oxide film on the steel surface. If damaged mechanically or chemically, this film is self-healing, providing that oxygen, even in very small amounts, is present. The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel and nitrogen.

There are more than 60 grades of stainless steel. However, the entire group can be divided into five classes. Each is identified by the alloying elements which affect their microstructure and for which each is named. The austenitic group (also referred to as the 300 series) is what this abstract will focus on, more specifically on grade 304/304L.

Life Cycle of Stainless Steel

To ensure a high quality of life, the materials that we use as consumers and manufacturers should meet not only technical performance standards, but have a Long Service Life, be usable in a great number of applications, and be environmentally friendly. Once their service is complete, they should be 100% Recyclable, thereby completing the life cycle to be used once again. Stainless Steel is such a material.

The longevity of stainless is the result of the alloying composition and, therefore, it has a natural corrosion resistance. Nothing is applied to the surface that could add additional material to the environment. It does not need additional systems to protect the base metal, the metal itself will last.

Stainless steel needs less maintenance and its hygienic qualities means that we do not have to use harsh cleaners to get a clean surface. There is little or nothing to dump into the drain that could have an environmental impact.

Stainless steel products complete their service life. There is less concern about disposal since this material is 100% recyclable. In fact, over 80% of new stainless steel comes from old remelted stainless steel scrap, thereby completing the full life cycle.

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Why is stainless "Stainless"?

Ordinary steels will rust if not completely protected from the atmosphere. We are all too familiar with this loose brown iron oxide film that destroys the surface, and in time, the usefulness of the metal.

Stainless steel also rusts, but in a much different manner than we are accustomed to. Instead of the unsightly and destructive iron oxide film, an invisible, tenacious and highly protective chrome oxide film forms on the surface. It is the presence of the element chromium in amounts in excess of 11.5 % that is basically responsible for the formation of this film, and the exceptional corrosion resistance properties of stainless steel. The chromium teams up with the oxygen in the atmosphere to form a chrome oxide film, and to turn a rust prone material of limited life into a metal with infinite longevity.

The role of nickel

We have described why chromium is the most important alloying element in stainless steel. The second is nickel, which we find combined with chromium in the 304 Series Stainless Steels. The presence of nickel in amounts over 7 per cent enhances the corrosion resistance of the corresponding straight chromium grade and at the same time, greatly improves its ductility.

One principal feature of this type of steel is that it can be "work hardened" and strengthened at room temperatures by cold rolling or "cold working" the steel. This combination of characteristics make the 304 grade one of the finest and popular of all stainless alloys, both from the standpoint of engineering properties and corrosion resistance.

About corrosion resistance

Corrosive attack on metals may come from many sources... the atmosphere, corrosive processing media and so on. Most do not present a serious challenge to stainless steel. Normally, it is simply a case of selecting a steel for the job from the numerous variations in composition, treatment and structure available.

Sometimes however, it is the environment that has to be altered. It will be recalled that it is oxygen from the surroundings that combines with chromium in the steel to form the all important chrome oxide armor against corrosive attack. Oxygen must be available to the surface of stainless steel for this film to form, and, if removed, to have the ability to reform immediately. Stainless steel, like man himself, needs oxygen to survive. Making oxygen available where it normally would not be present is not really difficult.

One such example is that an acid solution, which would normally corrode stainless, could simply be aerated to maintain the protective film... and thereby restore the steel's resistance to corrosion.

Other benefits

Stainless steels have outstanding permanency because of the inherent corrosion resistance of the metal. In design, no provision is required for corrosion, painting, plating or wear. In service, maintenance is minimal and, in most applications, the surface finish needs only an occasional soap and water wash to restore surface luster. In many exterior applications, natural rainwater performs an effective cleaning action.

Stainless steel finishes are not affected, in any way, by ultra-violet light. Nor will they change color under natural climatic conditions. In certain applications involving wear or frequent cleaning using abrasives, the original surface appearance may change but the inherent long-life aesthetic properties of the metal will not diminish.

Grade 304L

In some cases, the low carbon Type 304L alloy may show a lower corrosion rate than the higher carbon Type 304 alloy. Historically, the traditional test data for formic acid, sulfamic acid and sodium hydroxide tests support this claim. Otherwise, the Types 304, and 304L may be considered to perform equally in most corrosive environments.

A notable exception is in environments sufficiently corrosive to cause intergranular corrosion of welds and heat-affected zones on susceptible alloys. The Type 304L alloy is preferred for use in such media in the welded condition since the low carbon level enhances resistance to intergranular corrosion.

Post welding

In welding, inert gas processes are used. Scale or slag that forms from welding processes is correctly removed with a stainless steel wire brush. Normal carbon steel wire brushes will leave carbon steel particles in the surface which will eventually produce surface rusting. For more severe applications, welded areas should be treated with a descaling solution such as a mixture of nitric and hydrofluoric acids and these should be subsequently washed off.

The effect of heat

Exposure of type 304 stainless steel to temperatures in the 800°F to 1500°F (427°C to 816°C) range may cause precipitation of chromium carbides in grain boundaries. Stainless steels are sometimes "sensitized" and subject to intergranular corrosion when exposed to aggressive environments. The carbon content of Type 304 may allow sensitization to occur from thermal conditions experienced by the welds and heat-affected zones near the welds.

For this reason, the low carbon Type 304L alloy is preferred for applications in which the material is put into service in the as-welded condition. Low carbon content extends the time necessary to precipitate a harmful level of chromium carbides, but does not eliminate the precipitation reaction for material held for long times in the precipitation temperature range. The low carbon Type 304L material is the better choice for service in the stress relieved condition in environments which might cause intergranular corrosion.

But it's rusting!

When some level of rusting is discovered (other than at a weld), it is usually the presence of a contaminant such as free iron on the effected surface which is rusting, as opposed to the base metal. Free iron is not present on the surface of stainless steel, unless it has been subjected to some level of post production contamination. The most popular cause of this contamination is the piece coming in contact with carbon steel during secondary operations or handling. Hand grinders, shear blades, sheet lifters and even wire brushes have all been identified as common culprits.

Cleaning stainless steel

Stainless Steel requires periodic cleaning just as other materials do when in service. For materials utilized in interior, light industrial or milder service, minimum maintenance is required. Finger marks, deposits from tobacco smoke, stains from hard water, or food left to dry on counter tops, all detract from the original attractive appearance.

Stainless used in exterior architectural building components is subject to road salt spray at ground level (similar to automotive applications) and deposits from polluted urban air at higher levels. Virtually all exterior applications utilize natural rainwater (or car washes) to perform an effective cleaning action. Only sheltered areas require additional washing with mild detergent and/or a stream of pressurized water wherever possible.

Cleaning methods

Cleaning methods and the frequency with which they are employed vary considerably according to application. One of the outstanding features of stainless steel is the ease with which its fine appearance can be maintained. In many instances, only a periodic warm water wash is sufficient to restore the original "gleam". In other cases, such as stainless food handling equipment, cleansing with strong sterilizing solutions may be done several times each day. Between these two extremes are a very broad range of cleaning requirements and methods. In all cases, there are some important "do's and don'ts" that should be followed.

DO

  • Wash regularly with warm tap water and mild soap or detergent using a clean cloth or soft bristles.

  • Remove stubborn grime using recommended cleansers and methods.

  • Always clean in the direction of original polish lines.

  • Always rinse after cleaning and wipe dry.

DON'T

  • Don't use ordinary steel wool or other scrapers to remove stubborn dirt.
    Don't allow dirt to accumulate.

  • Don't allow protective coatings to remain on new building components. Remove all traces of adhesive as well as plaster, terrazzo, paint and other construction "splashes".

  • Do not use harsh, untested cleaners.

 


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