Stainless Steel

Stainless Steel Division

As the name implies, stainless steels are more resistant to rusting and staining than are plain carbon and lower allooy steels. This superior corrosion resistance is brought about by addition of the element chromium to alloys of iron and carbon.

The minimum amount of chromium necessary to confer this superior corrosion resistance depends upon the corroding agent. The American Iron and Steel Institute has chosen 10 per cent chromium as the dividing line between “alloy” steel and “stainless” steel. Most of these types are available in the main product forms such as plates, bars, shapes, sheet, strip, and tubes.

In 1912 Harry Brearley, head of the Brown – Firth Research Laboratory in England, while attempting to develop steels to resist the fouling and corrosion encountered in gun barrels, reported that a composition of 12.8 per cent chromium and 0.24 per cent carbon was quite resistant to corrosion.

The austenitic iron-chromium-nickel alloys were developed in Germany during the years 1909-1912 by Benno Strauss and Edward Maurer . Further work by Strauss and others ultimately led to the versatile 18 per cent chromium, 8 per cent nickel steels popularly called 18-8 which are used so widely today.

The resistance to attack is due to the naturally occurring chromiun rich oxide film formed on the surface of the steel. Although extremely thin, this invisible, inert film is tightly adherent to the metal and extremely protective in a wide range of corrosive media. This film is rapidly self reparing in the presence of oxygen, and damage by abrassion, cutting or machining is quickly repaired.

Benefits of Stainless Steel are:

  • Corrosion resistance
  • Ease of fabrication
  • Aesthetic and higienic appeal
  • High and low temperature resistance
  • Low maintenance cost
  • Life cycle characteristics
    100% recyclable

In addition to chromium, nickel, molybdenum, titanium, niobium and other elements may also be added to stainless steels in varaying quantities to produce a range of sainless steel grades, each with different properties.

Austenitic

austenticos 2

Ferritic

Ferríticos

Martensitic

Martensíticos

Duplex

Duplex

Austenitic Stainless Steel

When nickel is added to stainless steel in sufficient amount, the structure changes to “austenite”.
The basic composition of austenitic stainless steels is 18% chromium and 8% nickel.

Austenitic grades are the most commonly used stainless steel accounting for 50% to 70% of production, being the 304 grade the most commonly specified by far.

Common uses

Kitchen sinks.
Food processing equipment.
Chemical and plant equipment.
Architectural applications.

Common grades

304 General purpose steel with good corrosion resistance for most applications.

310 Furnace part equipment. Resistant to temperature 900°C to 1100°C.

316 Used where high corrosion resistance is required, i.e. marine equipment.

321 Contains titanium, heavy wellments in chemical and other industries. Suitable for heat resisting applications to 800°C.

Basic properties

  • Excellent corrosion resistance.
  • Excellent weldability.
  • Excellent formability and ductibility.
  • Excellent cleanability and hygiene characteristics.
  • Good high temperature properties.
  • Non magnetic.
austenticos

Ferritic Stainless Steel

These are plain chromium stainless steels with varying chroumium content between 12% and 18% but low carbon content.

Common uses

  • Cooking utensils.
  • Automotive exhaust.
  • Dishwashers and cloth dryer liners.
  • Ornamental goods.

Basic properties

  • Moderate to good corrosion resistance depending on chromium content.
  • Not hardenable by heat treatment.
  • Poor weldability.
  • Poor formability, not as good as austenitics.
  • Magnetic.

Common grades

409 Heat resistant steel, easily formed and welded. Mainly used for automative exhaust or welded applications where superior performance to galvanized steel is required.

430 Cooking utensils. Ornamental goods. Dishwashers and cloth dryer liners.

Martensitic Stainless Steel

Where the first commercially developed (as cutlery) and have relatively high carbon content (0,1% to 1,2%) compared to other stainless steels. They are plain chromium steels containing between 12% and 18% chromium.

 

Common uses

  • Knife blades.
  • Surgical instruments.
  • Springs.
  • Shafts.

Common grades

410 General purpose grade for use in mildly corrosive environment.

420 Hardenable steel used for cutting tools, surgical knives, etc.

Basic properties

  • They can be hardened by heat treatment.
  • Moderate corrosion resistance.
  • Poor weldability.
  • Magnetic.

Duplex Stainless Steel

These are stainless steels containing relatively high chromium (between 18% and 28%) and moderate amounts of nickel (between 4.5% and 8%). The nickel content is sufficient to generate a fully austenitic structure and the resulting combination of ferritic and austenitic structures is called duplex. Most duplex steels contain molybdenum in a range of 2.5% to 4%.

Basic properties

  • Increased resistance to chloride ion attack.
  • Higher tensile and yeld strenth than austenitic and ferritic steels.
  • Good weldability and formability.

Common uses

  • Marine applications particularly at slightly elevated temperatures.
  • Desalination plants.
  • Heat exchangers.
  • Petrochemical plants.

Common grades

2205 Superior corrosion resistance to 316L used in heat exchangers and marine and refinery applications.