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What is Stainless?

What is Stainless?

In many situations galvanic protection or painting of a mild steel surface is impractical. This is where the special oxide film formed on chromium alloyed stainless steels becomes so useful.

What is 

Many types of stainless steels have been developed to resist different corrosion environments and working conditions ensuring that factories are safe, structures last longer and our food is hygienic. Stainless steel is even used for in systems to clean up the exhaust gases from cars and power stations.

Stainless steel is also recyclable: when scrapped, it can be re-melted to make something new.

Effect of Chromium

Stainless steels are chromium containing steel alloys. The minimum chromium content of the standardised stainless steels is 10.5%. Chromium makes the steel 'stainless'  this means improved corrosion resistance, as can be seen in the chart.

The better corrosion resistance is due to a chromium oxide film that is formed on the steel surface. This extremely thin layer, under the right conditions, is also self-repairing.

Besides chromium, typical alloying elements are molybdenum, nickel and nitrogen. Nickel is mostly alloyed to improve the formability and ductility of stainless steel. Alloying these elements brings out different crystal structures to enable different properties in machining, forming, welding etc.

The four major types of stainless steel are:

  • Austenitic
  • Ferritic
  • Ferritic-Austenitic (Duplex)
  • Martensitic
Compoenent

Austenitic is the most widely used type of stainless steel. It has a nickel content of at least of 7%, which makes the steel structure fully austenitic and gives it ductility, a large scale of service temperature, non-magnetic properties and good weldability. The range of applications of austenitic stainless steel includes housewares, containers, industrial piping and vessels, architectural facades and constructional structures.

Ferritic stainless steel has properties similar to mild steel but with the better corrosion resistance. The most common of these steels are 12% and 17% chromium containing steels, with 12% used mostly in structural applications and 17% in housewares, boilers, washing machines and indoor architecture.

Ferritic-Austenitic (Duplex) stainless steel has both ferritic and austenitic lattice structures - hence common name: duplex stainless steel. This steel has some nickel content for a partially austenitic lattice structure. The duplex structure delivers both strength and ductility. Duplex steels are mostly used in petrochemical, paper, pulp and shipbuilding industries.

Martensitic stainless steel contains mostly 11 to 13% chromium and is both strong and hard with moderate corrosion resistance. This steel is mostly used in turbine blades and in knives.

Source: Outokumpu

WHAT IS STAINLESS STEEL?

What is 

General Information
The many unique values provided by stainless steel make it a powerful candidate in materials selection. Engineers, specifiers and designers often underestimate or overlook these values because of what is viewed as the higher initial cost of stainless steel. However, over the total life of a project, stainless is often the best value option.

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.

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 and 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.

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 50% of new stainless steel comes from old remelted stainless steel scrap, thereby completing the full life cycle.

What is Stainless Steel?

Stainless steel is the term used to identify a family of steels with a minimum chromium content of 11%, which thereby makes it highly resistant to corrosion.


Distributed homogeneously throughout the entire stainless steel structure, chromium - on making contact with oxygen - forms a fine, all-encompassing and resistant layer of oxide on the steel surface, protecting it against the corrosive attacks of nature.

Despite being invisible and very thin, this oxide film adheres extremely well to the stainless steel base, and its resistance increases as more chromium is added to the compound. Even when the steel is scratched, dented or cut, oxygen from the air immediately combines with the chromium to reform the protective layer.

Other elements added to stainless steel - nickel, molybdenum, vanadium and tungsten - also increase corrosion resistance, besides making steel suitable for multiple applications. The correct selection of the stainless steel type and of its surface finish are important in ensuring its long life.

The ABC of Stainless Steel

Main Characteristics of Stainless Steel

  • High resistance to corrosion.
  • High mechanical resistance.
  • Easy cleansing / Low surface roughness.
  • Hygienic appearance.
  • Inert material.
  • Easy conformability.
  • Easy joining.
  • It maintains its properties even when submitted to very high or low temperatures (cryogenic properties).
  • Variety of surface finishes and forms.
  • Strong visual appeal (modernity, lightness, prestige).
  • Favorable cost/benefit relationship.
  • Low maintenance costs.
  • Recyclable material.

Important Considerations when Working with Stainless Steel

  • Having a good knowledge of the material.
  • Knowing about the families of material.
  • Knowing the project in which stainless steel will be applied.
  • Knowing about the available surface finishes.
  • Maintaining organized and clean facilities.
  • Maintaining the identification of the material.
  • Adopting production planning.
  • Apply the guidelines explained in the Acesita training courses and manuals.

Source: SSINA

What is Steel:

Steel is a metal alloy formed by several chemical elements, having iron and carbon as its main components.

What is Stainless Steel:

It is a type of steel with a chromium content of at least 11% and a balanced chemical composition which makes it highly corrosion-resistant.

What is Corrosion:

It is the natural enemy of metals. Common steels react with the oxygen in the air, forming a surface layer of iron oxide. This layer is extremely porous and allows for the continuous oxidation of the steel, causing corrosion, popularly known as "rust."

Passive layer:

This is an extremely fine, all-encompassing, stable and resistant layer formed on the surface of stainless steel by the combination of the oxygen in the air with the chromium contained in the steel alloy. This layer protects stainless steel against corrosive effects of nature.

Formation and Characteristics of the Passive Layer: The passive layer appears spontaneously in the presence of chromium and oxygen. The formation is instantaneous (chromium and oxygen have a high affinity). It is present on the entire surface of a piece of steel and is very stable. It will not come off. It is not porous and therefore blocks corrosion. It is also virtually invisible.

Stainless Steel's Main Families are:

Austenitic - Ferritic - Martensitic

When stainless steel is appropriately specified, the Return-on-Incurred-Costs rate is quite favorable.

Cleaning Routine:

The best friends of stainless steel are soap and gentle and/or neutral detergents and ammonia solutions diluted with lukewarm water. Apply with a soft cloth or a sponge of fine nylon, rinse well with water, and dry with a soft cloth.

For the Maintenance of stainless steel articles, always use the gentlest cleaning method possible. Be patient, and before turning to more severe cleaning methods, repeat the operation several times.

Source: Acesita