**5.6.1 Stainless steel alloy**

Low C tenor alloy (except some martensitic steels), or better Fe-Cr or Fe-Cr-Ni alloy, with Cr percentage varying from the 12 to 30% and of Ni from the 0 to 35% (main characteristic is the corrosion resistance). Such characteristic is due to the passivation property that such steels, in oxidation conditions, have (that is capacity to create an oxide film of extremely thin dimensions whose characteristics remarkably according to change the chemical composition of the alloy, of the heat treatment, the structural composition, the superficial stress state) . Normally they are subdivided in following the three classes:


The martensitic steels have the critical points and are submitted to heat treatment; they have higher mechanical characteristics in comparison to the ferritic and austenitic ones, but lower corrosion resistance. The ferritic steels, instead, do not possess the critical points, can only be submitted to re crystallization annealing treatments; they have good resistance to oxidation at high temperature, as well as more elevated how much greater are the chromium content. the austenitic steel are, between the stainless steel, those mainly produced. They have elevated corrosion resistance in numerous atmospheres; they are submitted to heat volatilization treatment (hardening of austenitic steel) to melt, in the austenitic matrix, the Cr carbides (their presence decrease the corrosion material resistance). For this same reason the C tenor has to be held to low values. Besides these three classes are the stainless steel hardening by precipitation, with high mechanical characteristics, whose corrosion resistance, also being lower than the austenitic types, is higher than martensitic ones. Hardening process derives by the precipitation insoluble phases dispersed during the aging process. Between this type of steels we remember:

