What is Stainless Steel?

What is Stainless Steel?

Stainless steel is a generic term for a family of corrosion resistant iron-based alloy steels containing varying amounts of such elements as chromium, nickel, molybdenum and carbon. All stainless steels have a high resistance to corrosion. This resistance is due to the naturally occurring chromium-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 environments. The film is also rapidly self-repairing in the presence of oxygen, and damage by abrasion, cutting or machining is quickly repaired. The RotaBBQ is 100% stainless steel to ensure a long-lasting product that will not rust and will retain a quality finish for the life of the product.

Stainless steel can be classified into four basic types based on the metals they are alloyed with. These stainless steel families are austenitic, ferritic, martensitic and duplex and within these family groups, there is a range of grades based on the range of their composition and defined by standards that vary by country.

Austenitic

Austenitic stainless steels are perhaps the most common kind of stainless steel, as they have superior corrosion resistance to both ferritic and martensitic stainless steels. The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. Austenitic grades are the most commonly used stainless steels accounting for more than 70% of production; type 304 is the most commonly specified grade by far and is the grade used for all steel elements of the RotaBBQ.

Sometimes other alloying elements like molybdenum are used and surgical steel, for example, or “SAE 304 stainless steel,” is the most familiar of this family, containing 18–20% chromium and 8–10% nickel. Austenitic grade stainless steels do not exhibit a yield point, are extremely formable, and tough in extremely cold temperatures. They do not conduct heat well and are nonmagnetic. The non-conduction of heat is an advantage with the RotaBBQ to help prevent the Rota Arm from getting too hot and continuing to allow height adjustment through the cooking process.

Basic properties

  • Excellent corrosion resistance
  • Excellent weldability (all processes)
  • Excellent formability, fabricability and ductility
  • Excellent cleanability, and hygiene characteristics
  • Good high and excellent low temperature properties
  • Nonmagnetic (if annealed)
  • Hardenable by cold work only

Common uses

  • Computer keyboard key springs (301)
  • Kitchen sinks (304D)
  • Food processing equipment
  • Architectural applications
  • Chemical plant and equipment

Common austenitic grades and their uses

301         General purpose stainless steel with good corrosion resistance for most applications. Used where the high work-hardening exponent is desirable. Can be supplied cold worked to give high strength. Used for structural applications such as rail carriages and wagons.

304         General purpose stainless steel with good corrosion resistance for most applications. Used for architecture, food processing, domestic sinks and tubs and deep drawing applications.

304L       Chemical plant and food processing equipment where freedom from sensitisation is required in plate thicknesses.

304H       Higher carbon content than 304L, for increased strength, particularly at elevated temperatures.

316         Used where higher corrosion resistance is required, eg. marine equipment. Can be welded up to 3mm without subsequent heat treatment.

316L       A low carbon modification of 316 where heavy section weldments are required without the risk of intergranular corrosion.

Ferritic

These are plain chromium stainless steels with varying chromium content between 12 and 18%. Not only is ferritic steel not hardenable by heat treatment, there is also a very low carbon content, together resulting in steel with limited strength. This strength limitation renders it a poor weld steel when welded in thick sections compared to other stainless families. It is also unacceptable for sub-zero temperatures, as it becomes brittle. Despite its comparatively low strength, ferritic steel’s ferromagnetic properties make it useful for other applications. It also has superior corrosion resistance to martensitic stainless steels.

Basic properties

  • Moderate to good corrosion resistance increasing with chromium content
  • Not hardenable by heat treatment and always used in the annealed condition magnetic
  • Weldability is poor
  • Formability not as good as the austenitics

Common uses

  • Automotive trim (430)
  • Automotive exhausts (409)
  • Colliery equipment (3CR12)
  • Hot water tanks (444)

Common ferritic grades and their uses

409         Heat resistant steel, easily formed and welded. Mainly used for automotive exhausts or welded applications where superior performance to glavanised steel is required.

430         Interior architectural components, stove and automotive trim, dishwasher and clothes dryer lines. Fusion welds tend to be brittle

444         Heat exchanger and hot water tanks, and in chloride-containing waters. Not prone to chloride stress corrosion - superior resistance to pitting, crevice and intergranular corrosion. Possesses excellent deep drawing properties.

Martensitic

Martensitic stainless steels were the first stainless steels commercially developed (as cutlery) and have relatively high carbon content (0.1 - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12 and 18% chromium. The relatively high carbon content allows martensitic stainless steels to be hardened and strengthened with heat treatment. Increasing the content of carbon in martensitic steels increases its resistance to scratching but decreases its toughness, or ability to absorb impact energy. With added nitrogen and nickel, and lowered carbon, martensitic steels have improved toughness, weldability, and corrosion resistance.

Basic properties

  • moderate corrosion resistance
  • can be hardened by heat treatment and therefore high strength and hardness levels can be achieved
  • poor weldability
  • magnetic

Common uses

  • knife blades
  • surgical instruments
  • shafts
  • spindles
  • pins

Common martensitic grades and their uses

410         General purpose grade for use in mildly corrosive environments.

416         Free machining variation of 410.

420         General engineering uses, such as pump and valve shafts.

420C       Developed for high hardness after heat treatment. Used for cutting tools, surgical knives, etc.

440C       Capable of being hardened to 60HRC. Highest hardness and abrasion resistance of all the stainless steels. Corrosion resistance similar to 410.

Duplex

Duplex stainless steels are alloys of chromium, nickel, molybdenum, copper, and iron. They contain relatively high amounts of chromium (between 18 and 28%) and moderate amounts of nickel (between 4.5 and 8%) and molybdenum in a range of 2.5% - 4%. The nickel content is insufficient to generate a fully austenitic structure and the resulting combination of ferritic and austenitic structures is called duplex. Their microstructure consists of austenite and ferrite, matching the corrosion resistance of austenitic steels with the strength of ferritic steels. It is possible to weld duplex steels, but a balance of austenite and ferrite must be maintained, and forming duplex steels is possible but requires greater force than for austenitic stainless steels.

Basic properties

  • High resistance to stress corrosion cracking
  • Increased resistance to chloride ion attack
  • Higher tensile and yield strength than austenitic or ferritic steels
  • Good weldability and formability

Common uses

  • Marine applications, particularly at slightly elevated temperatures
  • Desalination plant
  • Heat exchangers
  • Petrochemical plant