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Steels_es
FERRITIC STAINLESS STEEL

Ferritic stainless steels consist in Fe - Cr - C alloys and a minimum part of other elements – such as Mo – which do not have points of conversion A1 and A3.

A1 : austenit temperature equilibrium in comparison with ferrite and cementite.
A3 : austenit temperature equilibrium in comparison with ferrite.


It is a ferritic structure in which it is possible to find trace of precipitate carbides. The result consists in incapability to obtain hardening via heat treatment. It is though possible to improve resistance mechanical characteristics via a cold rolling hardening process. Within the AISI classification, they are classified as ‘series 400'.

The following are the most common types:

AISI 430
It is a ‘17% Chrome Steel' (used for the production of cooking pots and it is very bright), easily workable by cold rolling and low hardening attitude. It has good resistance to corrosion (at both room and high temperatures) and to dry sulphurous gases at warm temperatures

AISI 430F
It differs from the above because of the addition of Sulphur to improve the workability at the shearing machinery.

AISI 405
It contains Cr and Al. It has been created in order to obtain better toughness in welded structures. It is employed at relative high temperatures to produce tubes for heat exchangers and fractionating column components.

AISI 409
It is a ‘Muffler Grade' developed for the realisation of car mufflers. It is easily rolling workable and it origins tough welded joints.

AISI 410
It is the most used alloy because of its low cost. It is used when large quantities of product and a good resistance to corrosion are required. For example, it is used as filling material in fractioning columns to increase the useable surface in heat exchanges.


AUSTENITIC STAINLESS STELL

They do not have transformation points A1 and A3, therefore they have an austenitic structure during the whole existence. They can be divided into 2 groups:

Fe-C-Cr-Ni
Fe-C-Cr-Ni-Mo

Within the AISI classification they are identified as ‘series 300'.

AISI 301 - 302 - 302B - 304 - 305 - 308 - 384
They differ from any others because of the percentage of Ni content. An increase of Ni content means a decrease of hardening within the rolling process. Their mechanical characteristics are low at room temperature, but very good at extreme low temperatures and they also have high resistance to usage and low sensibility to notches.

AISI 309 - 309S - 310 - 314
They differ from any other alloys because of the presence of Nickel. These alloys are particular resistant to high temperature. They are also called ‘refractory steels', indicating the excellent characteristics about mechanical and corrosive resistance at high temperatures.

AISI 316 - 317
They differ from any other alloys because of the Mo content which provides a good resistance towards pitting corrosion and improves the resistance to stress corrosion. The presence of Mo with ferritization implies a major use of Ni to guarantee the austenite stability. These alloys perform better mechanical characteristics at high temperatures.

AISI 321 - 347 - 348
These alloys are a result of alloys 304 and 316 with Titanium and Niobium in addition. Their main characteristic is to avoid Cr carbides and therefore a loss in corrosion resistance. With quantities around 0.3-0.4%, titanium is the characterising element.

AISI 304L - 316L
These alloys are a slightly different version of alloys 304 and 316. Alloys 304L and 316L are characterised by a very low percentage of Carbon (C =0.03%) in order to allow welding processes without precipitation of carbides in the welded areas. Their characteristics are similar at room temperature but slightly less performing at high temperatures.

AISI 316SL (Basel Norm)
This alloy is mainly characterised by a high content of Mo (Mo = 2.5%). It has excellent performance against corrosion resistance.
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