Stainless steels

                         Stainless steels

 Stainless steel is a family of alloys of iron that contains at least 10.5% Chromium and a maximum of 1.2 % carbon which is essential of ensuring formation of a self healing surface passive layer. This passive layer provides the corrosion resistance. These characteristics make stainless steels totally different from mild steels.

The stainless steel was discovered between 1900 and 1915. In 1904, Leon Guillet discovered alloys with composition similar to steel grades 410, 420, 442, 446 and 440-C. In 1906 he also discovered an iron-nickel-chromium alloy which was similar to the 300 series of stainless steel. In 1909 Giesen researched on the chromium-nickel (austenitic 300 series) stainless steels. In Germany, in 1908, Monnartz & Borchers found that a relationship exists between a minimum level of chromium (10.5%) on corrosion resistance as well as the importance of low carbon content and the role of molybdenum in increasing corrosion resistance to chlorides.

 Stainless steel production process

Stainless steel is produced in an electric arc furnace where carbon electrodes contact recycled stainless scrap and various alloys of chromium, nickel and molybdenum etc. depending on the type of stainless steel. A current is passed through the electrode and the temperature increases to a point where the scrap and alloys melt. The liquid steel can also be produced in LD converter using hot metal as a major input material. The liquid steel from the electric arc furnace or LD converter is then transferred into an AOD (Argon Oxygen Decarbonization) converter, where the carbon levels are reduced and the final alloy additions are carried out to achieve the desired chemistry.  The liquid steel is either cast into ingots or continually cast into slabs or billets. The slabs or billets are either hot rolled or forged into the final shape. Sometimes hot rolled strips are further processed by cold rolling to further reduce the thickness as in sheets and some materials are further drawn into smaller diameters as in rods and wire. Most stainless steels are annealed and pickled with acid to remove furnace scale from annealing. This helps promoting the passive surface film that naturally occurs.

 Characteristics of stainless steels

Stainless steels are characterized by corrosion resistance, aesthetic appeal, heat resistance, low life cycle cost, full recyclability, biological neutrality, ease of fabrication, cleanability and good strength to weight ratio.  Fig 1 gives a view of various stainless steels. The family of stainless steels can be grouped into four types as given below. Each of these types has specific properties and a basic grade.

  • Austenitic
  • Martensitic
  • Ferritic
  • Duplex stainless steels.

Type of Stainless steels

Fig 1 Types of stainless steels

Austenitic stainless steels – These are iron based alloys with nickel content ranging 3.5% to 32%, chromium content ranging 16% to 28% and carbon content not more than 0.1%. 65% of global consumption of stainless steel is of this type. The most common grades are 18/8 and 18/10. The microstructure of these types of steels is austenite. These steels are non magnetic and can not be hardened by heat treatment but can be hardened by cold working. These steels have better corrosion resistance and can be welded. These steels have good ductility and toughness. These steels are having good hygienic properties and cleanability. Stainless steels of this type are having good resistance to low (cryogenic) and high (melting point of the alloy) temperatures since they retain the austenitic structure. The common uses of these steels are kitchen sinks, architectural applications such as roofs and gutters, doors and windows, tubular frames, food processing equipment, food preparation areas, chemical vessels, ovens, heat exchangers etc. These steels are designated by the following three different systems

  • Metallurgical structure – Austenitic
  • Grades: Such as 304 (most commonly used), 310 (mainly for high temperature), 316 (for better corrosion resistance) and 317 (for better corrosion resistance)
  • Unified numbering system (UNS): Such as S30400, S31000, S31600, S31700

Austenitic stainless steels have the following limitations:

i)                 The maximum temperature under oxidizing conditions is 925 deg C.

ii)                They are suitable only for low concentrations of reducing acid.

iii)               In cervices and shielded areas, there might not be enough oxygen to maintain the passive oxide film and crevice corrosion might occur.

iv)               Presence of very high levels of halide ions, especially the chloride ions, can result into the breakdown of the passive surface film.

Martensitic stainless steels – These iron based alloys have chromium content ranging 11.5% to 18% and carbon content 0.15% to 1.2%. Molybdenum can also be used in type of stainless steels. These stainless steels are magnetic in nature, can be hardened by heat treatment for strength and hardness and have poor welding characteristics. The main uses for this type of stainless steels are knife blades, surgical instruments, fasteners, shafts and springs etc. These types of stainless steels are designated by the following three systems:

  • Metallurgical structure – Martensitic
  • Grades: Such as 410 (most used), 420      (cutlery), 440C (for very high hardness)
  • Unified numbering system (UNS): Such      as S41000, S42000, S44004

Ferritic stainless steels -In ferritic grades of stainless steels carbon content is kept low (less than 0.08%) and chromium content can range from 10.5% to 30%. Some ferritic grades of stainless steels contain molybdenum up to 4%. Chromium is the main alloying element in these grades. Because of low carbon content these grades have a different metallurgical structure. These steels are magnetic in nature and cannot be hardened by heat treatment. They are always used in the annealed or softened condition. The weldability of these steels is poor. These steels are chosen when toughness is not the primary need but corrosion resistance especially to chloride stress corrosion cracking is important.The most common uses of these steels are automotive exhaust and fuel lines, architectural trim, cooking utensils, hot water tanks and bank vaults etc. These stainless steels are designated by the following three systems:

  • Metallurgical      structure – Ferritic
  • Grades:      Such as 409 (high temperature), 430 (major uses)
  • Unified      numbering system (UNS): Such as S40900, S43000

Duplex stainless steels – In duplex steels carbon is kept at a very low level (less than 0.3%) and the chromium content is kept high (21 to 26%). The nickel content in these steels is kept low (3.5% to 8%). These steels may contain molybdenum up to 4.5%. These steels have a mixed structure which is called duplex and is a combination of both ferritic structure (50%) and austenitic structure (50%). These steels have physical properties which reflect this structure. These steels have high resistance to stress corrosion cracking, increased resistance to chloride ion attack, good weldability and have higher tensile and yield strengths as compared to austenitic and ferritic stainless steels.The main uses of these steels are marine applications, heat exchangers, desalination plants, petrochemical plants, paper plants and food pickling plants etc.These steels are designated by the following three systems:

  • Metallurgical structure – Duplex
  • Grade: Such as 2205
  • Unified numbering system (UNS): Such as S31803