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Material Properties of Stainless Steel

What is stainless steel?

The designation stainless steel covers a wide range of alloys with different properties. One property common to all stainless steels is that they contain at least 12% chromium. The stainless steels can be divided into three main groups and a few mixed types according to the structure of the steel:

  • Austenitic stainless steel
  • Ferritic stainless steel
  • Martensitic stainless steel

Austenitic stainless steel is the most important, representing approx. 90% of total stainless steel consumption. Austenitic steel is also the only stainless steel suitable for drainage installations, and it is, of course, the type used by BLÜCHER.

Importance of alloying elements Austenitic stainless steel contains at least 18% chromium and 8% nickel – thus the well-known designation »18/8« steel. Corrosion resistance generally increases with increasing content of chromium. In alloys with 12-13% chromium, the passive layer is strong enough to prevent the steel from corroding in normal or mildly aggressive media. The main effect of the alloying element nickel is on the structure of the steel and its mechanical properties. The steel’s structure is austenitic with an adequate content of nickel. In contrast to the pure chromium steels (ferritic stainless steel), this results in significant changes in the mechanical properties, such as increased workability and ductility, better resistance to thermal stress and improved weldability. The austenitic structure also results in a change in the physical properties of the steel. For example, the steel is not magnetic and has higher thermal conductivity.

Nickel also increases resistance to corrosion caused by certain media. Molybdenum has the same effect on the structure as chromium, but it also has a strongly positive influence on corrosion resistance. Molybdenum-containing steel is normally designated »acid-resistant« because of the resistance of these steels to certain types of acids. But acid-resistant stainless steel will also have limited resistance to some media such as chlorine-containing media (see table of resistance).


Why is steel "stainless"?

The addition of chromium to the steel results in the formation of a passivating oxide film with a high content of chromium oxides. This oxide film protects the surface of the steel against oxygen in air and water. An outstanding property of stainless steel is that the chromium oxide film automatically regenerates if the surface of the steel is exposed.

This restitution of the oxide film can only occur if the surface of the steel is completely clean and free of tempering agents and slag from welding processes and residues from tools made from ordinary carbon steel.

If this surface contamination is not removed, the steel may ultimately corrode. To prevent this, the steel surfaces should be cleaned after welding and processing, e.g. by means of so-called acid pickling of the stainless steel.

The pickling effectively removes all impurities from the surface of the steel and permits the reestablishment of a strong, uniform chromium oxide film. The pickling bath normally consists of 0.5-5% v/v HF (hydrofluoric acid) and 8-20% v/v HNO3 (nitric acid) at a temperature of 25-60°C. This acid bath removes residues, the existing chromium oxide film and traces of iron, leaving the clean steel surface. The restitution of a strong chromium oxide film starts in the subsequent rinsing in water.


Material Specification

Material AISI 316 L 1.4404 AISI 304 1.4301
Analysis    
Carbon (C %) Max. 0.03 Max. 0.07
Chromium (Cr %) 16.5 - 18.5 17.0 - 19.0
Nickel (Ni %) 11.0 - 14.0 8.5 - 10.5
Molybdenum (Mo %) 2.0 - 2.5 -
Manganese (Mn %) Max. 2.0 Max. 2.0
Silicium (Si %) Max. 1.0 Max. 1.0
Sulphur (S %) Max. 0.030 Max. 0.030

Physical Properties

Structure Austenitic (nonmagnetic) Austenitic (nonmagnetic)
State Non-annealed
Specific gravity (g/cm3) 7.98 7.9
Melting point (°C) Ca. 1400 Ca. 1400
Decortication temperature in air (°C) 800 - 860 800 - 860
Expansion coefficient 20 - 100 °C (m/m . °C) 16.5 x 10-6 16.5 x 10-6
Specific resistance (20° C) (Ohm . mm2/m) 0.75 0.73
Heat conductivity (20°C) (W/°C-m) 15 15
Specific heat (J/g . k) 0.5 0.5

 

Mechanical Properties

Ultimate tensile strength (Rm) (N/mm2) 490 - 690 500 - 700
Yield point (Rpo2) (N/mm2) 190 195
Modulus of elasticity (E) (20° C) (N/mm2) 2.0 x 105 2.0 x 105
Hardness Brinell (HB) (N/mm2) 120 - 180 130 - 180