Valbruna AOD

Stainless steels are a group of iron-based, chromium alloys designed to be resistant to corrosion. They have excellent formability and mechanical properties at cryogenic and high temperatures, good resistance to the formation of calamine, surface oxidation and creep at specific temperatures. If Chromium is the element that gives stainless steel its resistance to corrosion, other additional elements in the alloy enhance its resistance in terms of oxidation, mechanical properties and the stabilization of particular structures.

Ferritic, Austenitic and Austenitic-Ferritic (Duplex) steels cannot be hardened by heat treatment but only by cold plastic deformation which increases resistance values thus influencing their resistance to specific forms of corrosion in certain environments. The dual-phase structure of Duplex steels assures increased resistance to corrosion under stress and tenacity at low temperatures.

In austenitic steels, increased strength is a result of the formation of Martensite induced by cold plastic deformation or cryogenic treatments. Martensitic steels behave similarly to Carbon and alloyed steels and can be annealed, quenched and tempered to give increased values of resistance and hardness. Similar results can be obtained using Martensitic precipitation hardening alloys.

In general, there is not one type of steel designed or available for all types of corrosive media, but specific grades are produced which excel in each type of corrosive environment. The incorrect use of a particular alloy can be avoided by reviewing information regarding its performance in certain environments.

Austenitic Stainless Steel

Steels with Chromium content between 16% and 26%, Nickel between 7% and 35% and Carbon (max. 0.15%), cannot be strengthened by quench hardening, but only through cold working (e.g. cold drawing). The main features of these steels are their resistance to corrosion, which is generally higher than other stainless steels, and their nonmagnetic behaviour.

Martensitic Stainless Steel

Steels with Chromium content between 10–18%, Carbon up to 2% and with the addition of other elements.

In order to improve both mechanical properties and corrosion resistance, they are heated to an appropriate temperature, 950°-1050°C, followed by suitable quenching and tempering.

Martensitic steels are ferromagnetic.

Precipitation Hardening

Steels with a chromium content greater than 10,0% characterized by excellent mechanical properties due to the ageing process at set temperatures.

This treatment generates a sub-microscopic phase precipitation of an element (e.g. Cu, Ti) consistent with the martensitic matrix of steel, thus enhancing the mechanical properties.

Ferritic Stainless Steel

Steels with a chromium content of more than 10,5% and less than 0,15% carbon. Unlike martensitic steels, these grades cannot be stiffened by quench hardening but only through cold working (e.g. Cold drawing). Ferritic steels are ferromagnetic.

Duplex Stainless Steel

These steels contain more than 16% Cr, 4-6% Ni and 1,5-3% Mo. Their dual austenitic and ferritic grain structure ( hence the term duplex) means that they have the good mechanical properties of austenitic stainless steels but with a superior resistance to stress corrosion and pitting.

Superduplex Stainless Steel

Compared to standard duplex steels, they are characterised by higher Cr, Mo and N content which increase resistance to corrosion. These alloys are particularly suitable for use in aggressive chlorides environments.