What is Stainless Steel?

Stainless steel is the generic name for a number of different steels used primarily for their resistance to corrosion. Stainless steels are produced by adding at least 11% chromium to steel to produce a thin passive protective layer of Cr2Q3, which promotes corrosion resistance. This is improved by further increasing the chromium content.

What are the Types of Stainless Steel?

Four basic groups of stainless steel are available:

  1. Ferritic Stainless Steels: – This group contains between 16 and 30 per cent chromium, with a maximum of 0.1 per cent carbon. These materials can be welded with a preheat of 150°C and the use of the 25 per cent chromium and 20 per cent nickel core wire electrode. Sometimes combination welds are made, with the final layers being completed with electrodes of even higher chromium content (up to 30 per cent), in order to give the surface an extremely high resistance to corrosion. A post heat at 730°C should be performed immediately after welding to prevent brittleness.
  2. Martensitic Stainless Steels: – Martensitic grades contain similar amounts of chromium, but more carbon than ferritics and possibly other additions such as molybdenum to increase hardenability and strength. Can be heat treated. This group contains from 11.5 to 14 per cent chromium and from 0.2 to 0.4 per cent carbon. Such steels are difficult to weld because they can form the very hard martensitic structure regardless of the cooling rate. They are known as air-hardening steels. They can be welded by preheating to around 350°C and using a 25 per cent chromium and 20 per cent nickel flux coated electrode. A post-heat of 750°C usually ensures acceptable ductility.
  3. Austenitic Stainless Steels: – Austenitic grades contain between 17-25% Cr, 7-20% nickel and in some instances molybdenum. They are face-centred cubic in structure, nonmagnetic and can be formed and welded more easily than ferritics. Austenite (gamma) phase field is extended to room temperature. Most corrosion resistant.
  • Typical composition: 18% chromium, 8% nickel, 0.15% carbon. Remainder: Fe (S. & P. kept below 0.045%), Ti or Nb.
  • Effect of elements on properties: The nickel is added to give toughness and may be increased to 11.5% to prevent work hardening (for rivets and fastenings). The chromium is added to give corrosion resistance. In addition titanium or niobium is often added to prevent inter-granular corrosion (weld decay) in the following proportions: Ti – 5 x C content, Nb = 10 x C content.
Approximate melting temperature 1420°C to 1395°C
Hardness 170 Hv (water quench from 1000°C)
Tensile strength Approx. 620 N/mm²
Yield point Approx. 280 N/mm²
Mass 7.92 g/cm³ at +20°C
Coefficient of linear expansion 0.02           °C
  • How to identify: This iron-based alloy has a characteristic silver lustre imparted to it by the amount of chromium and nickel. It is the chromium which combines with oxygen and rapidly forms a very thin oxide which is continuous and stable and impervious to further attack by the atmosphere. The thickness of this oxide film increases with the degree of polish to give a mirror finish. These steels give a dull red spark when touched on a grinding wheel.
  • Corrosion: The corrosion resistance is excellent in most environments but solutions of nitric, hydrofluoric and sulphuric acid will attack 18/8 stainless steel. These acid solutions are used to remove the oxide scale which result from strongly heating the material in air. The term used for this treatment is pickling.
  • Types of Corrosion:
  1. Inter Crystalline Corrosion (Weld Decay)
  2. Pitting Corrosion
  3. Stress Corrosion
  1. Duplex Stainless Steels: – Duplex stainless steels were developed to provide the strength of ferritics, but with improved corrosion resistance. They contain about 22% Cr, 5% Ni, and possibly molybdenum. Ferrite + Austenite
  2. Precipitation-Hardening (PH) Stainless Steels: – Ultra high strength due to precipitation hardening.

Applications of Stainless Steel

Stainless steels are used extensively in food and drink production and the chemical and energy industries; martensitics are used for cutlery and other cutting tool manufacture.

Applications of Stainless Steel

Properties of Stainless Steel

  • Aesthetic qualities: it can be polished to a satin or mirror finish;
  • “Dry corrosion” occurs to steel at higher temperatures where it oxidises or scales up. Stainless steel is far more resistant to this than ordinary carbon steel and grades such as 310 (25% chromium 20% nickel) were specifically developed for use at high temperatures;
  • Non-contamination of the liquids stainless comes into contact with, because there is no coating to break down and dissolve;
  • Weight savings; as thinner sections and more innovative design structures can be used, with cost savings on foundations and platform weights;
  • Many anti-corrosion coatings are fire hazards or the materials themselves have a low melting point.

Advantages of Stainless Steel

  • Does not react or influence other materials.
  • Used extensively in the food/catering divisions.
  • Used widely in the medical world. E.g. Plates and screws to repair bones.
  • Aesthetic Qualities: It can be polished to a satin or mirror finish.
  • Dry Corrosion: Stainless steel is far more resistant to this than ordinary carbon steel.
  • Grades such as 310 (25% chromium 20% nickel) were specifically developed for use at high temperatures.
  • Non-contamination: Because there is no coating to break down and dissolve there is no contamination of liquids that stainless steel comes in contact with.
  • Weight: As thinner sections and more innovative design structures can be used, giving cost savings on foundations and platform weights.

What are the Various Grades of Stainless Steel?

Austenitic Stainless Steels
301 High strength for roll formed structural components
304, 304L, 304H Standard 18/8 grades
310, 310S, 310H High temperature resistant grades
316, 316L, 316H Improved resistance to pitting corrosion in chloride environments
321, 321H, 347 Stabilized grades for heavy section welding and high temperature applications
904L High resistance to general corrosion, pitting and stress corrosion cracking
Ferritic Stainless Steels
409 Automotive exhaust grade – weld stabilized
430, 430F Resistant to mildly corrosive environments
439 Resistant to mildly corrosive environments – weld stabilized
444 A ferritic alternative to grade 316 / 316L – weld stabilized
Duplex Stainless Steels
2101 Lean duplex – economical alternative to 304 and 316
2304 Duplex alternative to grade 316
2205 Standard duplex stainless steel – high resistance to pitting and stress corrosion
2507 Super duplex with very high resistance to pitting and stress corrosion
Martensitic Stainless Steels
410 Standard martensitic grade for low-duty hardened applications
416 Free-machining bar grade
420 Higher hardness martensitic grade for cutlery, cutting tools and dies
431 High hardness and toughness grade, primarily for shafting
440A, 440B, 440C Very high hardness grades used in cutting tools
Precipitation Hardening Stainless Steel
630 (17-4PH) High strength shafting grade

What are the Maximum Operating Temperatures of Stainless Steel Grades?

Stainless steels have good strength and good resistance to corrosion and oxidation at elevated temperatures. Stainless steels are used at temperatures up to 1700° F for 304 and 316 and up to 2000 F for the high temperature stainless grade 309(S) and up to 2100° F for 310(S).

Maximum Operating Temperatures of Stainless Steel Grades

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Influence of Alloying Elements on the Properties of Stainless Steel

Element Effect on Stainless Steel
Chromium Forms a passive film with oxygen that prevent the further diffusion of oxygen into the surface
Composition needs to contain at least 10.5% to be a stainless steel
Nickel Increases ductility and toughness. Increase corrosion resistance to acids
Additon creates non-magnetic structure
Molybdenum Increases pitting and crevice corrosin resistance. Increase resistance to chlorides
Copper Increase corrosion resistance to sulfuric acid
Manganese Substitute for nickel (200 series)
Titanium/Niobium Ties up carbon and prevents inter-granular corrosion in welded zone of ferritic grades
Nitrogen Increase strength and corrosion resistance in austenitic and duplex grades
Silicon Improves resistance to high temperature scaling
Sulfur Usually kept low excet for “free-machining” grades
Carbon Usually kept low.  Used in martensitic grades to increase strength and hardness

Comparative International Grades of Stainless Steel

British French German Italian Japanese Swedish USA
304S21 Z12CN17.08 1.431 X12CrNi 17 07 SUS301 14 23 31 301
304S31 SUS302 14 23 32 302
304S15304S16 Z8CN18.09 1.4301 X5CrNi 18 10 SUS301 14 23 33 304
304S11 Z2CN18.10 1.4306 X2CrNi 18 11 SUS304L 14 23 52 304L
305S19 Z8CN18.12 X8CrNi 18 12 SUS305 305
309S24 Z15CN24.13 X16CrNi 23 14 SUS309 309
310S24 Z12CN25.20 1.4845 Z22CrNi 25 20 SUS310S 14 23 61 310
315S16 14 23 40
316S31316S33 Z6CND17.11 1.44011.4436 X8CrNiMo 17 13 SUS316 14 23 4314 23 47 316
316S11316S13 Z2CND17.12 1.44041.4435 X2CrNiMo 17 12 SUS316L 14 23 5314 23 48 316L
317S12 Z2CND19.15 1.4435 X2CrNiMo 18 16 SUS317L 14 23 67 317L
317S16 1.4436 SUS317 14 23 66 317
320S31320S33 Z8CND17.12 1.45711.4573 14 23 50
321S31 Z6CNT18.12 1.4541 X6CrNiTi 18 11 SUS321 14 23 37 321
347S31 Z6CNNb18.11 1.4558 X6CrNiNb 18 11X8CrNiNb 18 11 SUS347 14 23 38 347
403S17 Z6C13 1.4 X6Cr13 SUS403 14 23 01 403
405S17 Z6CA13 1.4002 Z6CrA1 13 SUS405 405
409S19 1.4512 409
430S17 Z8C17 1.4016 X8Cr 17 SUS430 14 23 20 430
434S17 Z8CD17.01 1.4113 X8CrMo 17 SUS434 14 23 25 434
410S21 Z12C13 1.40061.4024 X12Cr 13 SUS410 13 23 02 410
410S45 Z30C13