The STAINLESS STEEL is a ferrous alloy which is able to combine the mechanical properties, typical of steels, with the intrinsic characteristics of noble materials such as, typically, the remarkable qualities of resistance to corrosion. They consist essentially of iron, carbon and chromium, not less than 10.5%, and with the addition, as appropriate, of other elements such as nickel, manganese, molybdenum etc.
The key element is chromium, the higher its content the higher its resistance to corrosion.
The standard EN 10088 defines stainless steels as ferrous alloys containing chromium at a rate of, at least, 10.5%.
In addition to the corrosion resistance properties, there is a wide range of mechanical resistance which gives the possibility to choose between different types of performance, both at room temperature and at elevated temperatures.
The problem that often arises is precisely to be able to choose the right material according to the applications, there are many types with different features for various usages, in fact some types of stainless steel offer better performance in applications in marine and corrosive environments , others are more suited to high temperatures.
It's important to consider that there is not one "stainless steel", but there are many grades, depending on the condition it will be used, it's possible to choose the proper alloy and by following certain precautions in machining, welding and installation, an excellent durability can be obtained.
Stainless steel is not only resistant to rust and stains, but it is a strong material, hygienic and fully recyclable.
Stainless steels can be divided into three large families: austenitic, ferritic, martensitic.
AUSTENITIC STAINLESS STEEL:
is Chrome-Nickel alloy, with a content of chromium from 16 to 28% and nickel at the rate of 6 ÷ 32%, with very low carbon contents, below 0.15%. Shall be mentioned that there are also alloys in which part of the nickel is replaced by manganese, a more austenitic element.
These are the best-known stainless steels.
There are many different of them for various types of application. Even these steels can't be submitted to hardening process, great they have a attitude to cold plastic deformation processes, such as deep drawing and weldability, consequently they do not have high mechanical properties.
They have a high electrical resistence and a lower thermal conductivity than ferritic.
They are non-magnetic in the annealed condition even if in consequence of cold plastic deformation or other working process can be seen an increase in the magnetic permeability.
Very high is the elongation ability before breaking, have good resistance to fatigue and maintain high values of toughness even at very low temperatures.
is plain chromium alloys with a Cr variable content between 10.5% to 28%. Chromium is the essential element which ensures the corrosion resistance. They contain very low carbon levels, usually below 0,1%, while it is, in most part, absent the nickel.
Several types of ferritic contain stabilizing elements such as titanium and niobium, in order to prevent the intergranular corrosion phenomena. In fact the stabilization of ferritic also gives a lower grain enlargement susceptibility during the welding and simultaneously improves the cold formability (AISI 409 1.4512; AISI 439 1.4510, AISI 441 1.4509). The presence of molybdenum instead increases the resistance to corrosion (AISI 434 1.4113; AISI 436 1.4513, AISI 444 1.4521)
Ferritic steels are ferromagnetic, or are attracted by a magnet, for that reason they are instantly recognizable by austenitic.
They have a higher thermal conductivity compared to austenitic steels, while the coefficient of thermal expansion is significantly lower, closer to those of the carbon steels' most common values.
The mechanical properties of the ferritic steels are less efficient than the austenitic in terms of percentage elongation. The yield strength is generally a bit higher and are penalized in the ultimate loads.
They have good weldability, especially in the case of welding resistance (spot welding and rolling).
is chromium alloy (from about 11 to about 18%) containing small amounts of other elements, such as nickel, in quantities not exceeding 2.5% and with a carbon content of 0.1 ÷ 0,5% with higher values up to 1%. They are the only stainless steel that can harden and therefore increase their mechanical properties (tensile strenght, yield strength, hardness) by heat treatment. They have a attitude to plastic deformation machining, especially at high temperature and in resulfurized versions also give discrete guarantees of machinability.