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General about stainless steel - Inox

General about stainless steel - Inox

Inox and prokrom are synonyms and common commercial names for stainless steel.

Stainless steel or corrosion resistant steel is an alloy consisting of iron and at least 12% of chrome, whereas modern stainless steels contain up to 30% of chrome. Besides alloying with at least 12% of chrome, to make steel corrosion resistant (passive), one more prerequisite has to be met, namely the existence of a homogeneous single-phase ferrite, austenitic or marten site microstructure. This eliminates the risk of development of areas with electric potential different from the one of the basic mass.

Besides adding chrome, corrosion resistance may be increased through the addition of nickel. By combining the alloying with chrome and nickel, the 18/8 (18% Cr i 8% Ni) type steels have been developed, with austenite microstructure resistant to influence of acids. Molybdenum alloying enables easier passivation and increases the corrosion resistance and pitting corrosion resistance of Cr-Ni steels. Through alloying with strong carbide-forming materials, such as titanium, niobium, the risk of development of intergranular corrosion is being eliminated. In general, the group of corrosion resistant steels has to contain:

  • ferrite-forming elements – Cr, Si, Al, Mo, Nb, Ti
  • austenite-forming elements – Ni, Mn, (Co), Cu, N.

Stainless steels or corrosion resistant steels are divided by the generated microstructure: ferrite, austenitic, austenitic-ferrite (duplex) and martensite steels.

Ferrite stainless steels

Ferrite stainless steels contain 12 - 18% of chrome and less than 0.1% of carbon.

Characteristics of ferrite stainless steels: magnetic, relatively soft, poor weldability, prone to developing "embrittlement 475", prone to developing brittle sigma phase, poor deformability, good machining features (better than in austenite ones), bad resistance to chloride solutions (e.g. seawater), not sensitive to development of stress corrosion, through the addition of molybdenum their resistance to pitting corrosion is enhanced, more cost-effective than other stainless steels, prone to cracking at low temperature.

The elimination or reduction of the above-mentioned shortcomings can be achieved through an increased share of chrome, a reduced share of carbon and by alloying with molybdenum and nickel (possibly with titanium and niobium). Significantly improved features are achieved through increased purity of ferrite steels, i.e. by obtaining very low shares of impurities and admixtures by means of modern refinement methods.

An example:

  • o AISI 430 (X6 Cr 17), tensile strength 450 - 600 N/mm², elastic limit 270 N/mm², elongation up to 20%, applied in corrosion resistant cutlery (except for knife blades), household appliances components, parts of nitric acid and soap manufacturing devices, in petrochemical industry, etc.
Austenitic stainless steels

Austenitic stainless steels mostly contain 0.02 – 0.15% of carbon, 16 - 24% of chrome, 8 - 20% of nickel, with possible additions of certain quantities of molybdenum, titanium, niobium, tantalum, nitrogen. The main advantage of this group of steels is their resistance to intergranular corrosion and the main disadvantage of austenitic steels is the lower elastic limit due to lower share of carbon.

Characteristics of austenitic steels: non-magnetic, no possibility of grain refinement, higher stress and deformation during welding than with ferrite steels, excellent plasticity; through alloying with molybdenum, tungsten and vanadium good creep resistance is being achieved at temperatures over 600°C, high toughness, oxydation and corrosion resistance, high strength/mass ratio, good features at low temperatures, stable austenitic structure from „solidus“ temperature to below room temperature, cubic flat centred crystal grid enabling high deformability, not prone to crystal grain increase in the area of thermal influence during welding.

These are applied in food, chemical, pharmaceutical, textile, film and photo industries, cellulose and paper industries, manufacture of household appliances, catering equipment, surgical and medical instruments, etc.

Most often applied austenitic steels are: AISI 304, AISI 304L, AISI 316, AISI 316L, AISI 316Ti, AISI 321, AISI 310S.

Austenitic-ferrite (duplex) stainless steels

Duplex steels containing 22 - 24% chrome and 6 - 8% nickel possess a two-phase microstructure with 40 - 60% ferrite. Due to their two-phase austenitic-ferrite microstructure, duplex steels demonstrate less resistance to general corrosion, but on the other hand have a significantly increased resistance to stress corrosion in chloride solutions, as well as in H2S atmosphere.

One of the main goals of duplex steels alloying is to maintain a sufficiently high share of austenite, being particularly important during welding (in the zone of welded joint melting). Excessive share of ferrite may cause a diminished corrosion resistance and the development of brittleness. The implementation of duplex steels at higher temperatures is possible, but due to restricted implementation to max. 250°C - 350°C, its implementation is significantly reduced.

Duplex steels are most often implemented in oil and gas industries (gas stations, desulphurizers, distillers, desalinators, valves, pipings, pumps), in the petrochemical industry (PVC film extrusion tools, absorbers, separators, heat exchangers), in the chemical processing industry (in the production of acids, working with HF to HNO3 solutions, H2SO4 devices, nozzles), shipbuilding (thruster shaft, rudder, pumps, heaters, bearings), paper industry (valves, regenerator furnace pipes, mixer shafts, water purification), transport (tanks).

Duplex steels: W.Nr.1.4162, W.Nr.1.4362, W.Nr.1.4462, W.Nr.1.4501, W.Nr.1.4410.

Martensite stainless steels

Martensite stainless steels have a higher share of carbon (0.2 – 1.0%), over 12% of chrome (up to 18%), and may contain up to 1.3% molybdenum and 2.5% nickel. Optimum mechanical features and corrosion resistance of this group of steels is achieved by forced air or oil quenching and later tempering Martensite stainless steels may be divided into two subgroups: construction steels (containing up to ≈ 0.25% C, being improved) and tool steels ((>0.3% C, after quenching, low tempering). In the case of construction steels, particular focus is dedicated to corrosion resistance, whereas in the case of tool steels there is an additional requirement of resistance to abrasive wear. For this reason, tool steels have two-phase microstructure (martensite + carbides), having the corrosion resistance lower that the single-phase martensite microstructure.

An example:

  • AISI 420 (X20 Cr 13), particularly resistant to water and water vapour, organic acids: acetic, lactic, fruit. Implemented for surgical instruments (pliers, tweezers), cutlery: forks, spoons; machine parts: shafts, piston rods, valve discs, nozzle pins, turbine blades, „Dutch knives“ (for paper).