Stainless steel is an alloy with a basis of iron, chrome and carbon in addition to other elements principally nickel (Ni) but also molybdenum (Mo), manganese (Mn), silicon (Si) and titanium (Ti).
The meaning ‘stainless’ does not answer to the real nature of these metals: they are precisely ‘oxidized’ as they have the possibility to ‘auto passively treat themselves’ that means they become covered by an extremely thin layer of invisible oxidation that protects the lower metal from corrosion attack. By law the quantity of chromium (Cr) in the composition of the alloy must be a minimum of 10,5 % with a maximum of 1,2% carbon, as required by EN 10020.
Besides the content of chromium, another important supposition for the passive film layer is the presence of an oxidant environment (such as the air that we breath or water) which promotes the spontaneous forming or reactivation process, in case of damage.
The passive film is fundamental for a good steel firmness in addition to contrasting adequately in the various cases of corrosion.
Both during the work phase and in use, it is necessary to allow the material to exchange a sufficient quantity of oxygen with the surrounding environmental. In this way the material gets the optimal passive film conditions.
Obviously, this passive layer can be more or less resistant and more or less anchored to the material depending on the present chromium concentration in the alloy and on the eventual presence of other elements. (ex. Molybdenum)
And it is obvious that different stages of oxidation and corrosion-resistance exist.
From the mechanical performance point of view, these materials are able to satisfy the most varied needs concerning tension properties, surface hardness and toughness at low temperatures.
The regular user is confronted with many performances and often demands to be able to chose the right material according to the use in order to avoid expensive ‘overstatements’ or dangerous ‘understatements’. At this point, it is necessary to show in a few words the large aggregation of the different steel types.
The martensitic stainless steels are alloys with chromium (from 11% – 18% approx.) containing small quantities of other materials, such as nickel. These are the only stainless steels that can be quenched and so increase their mechanical characteristics (ultimate tensile strength, yield strength, hardness) through thermal treatment. Excellent for plastic deformation such as during the heat process as well as in re- sulfurous versions giving a regular guarantee during machine processing.Iron based steels are also stainless steels of lower chromium (the content varies between 16% – 28%) but through thermal treatment they cannot raise their mechanical characteristics through thermal treatment. These are easily used for plastic deformation, both heat and cold working and can be used on machine tools (especially the re -sulfurous ones).
They present an excellent welding possibility, especially in the case of resistance welding (spot welding and rolling). Whereas the austenitic steels are chromium-nickel or chromium manganese based alloys and are by far the most popular. Again, these steels are not hardened, but can increment their tensile properties through the hardening effect consequent by plastic deformation (rolling, deep drawing etc). There are many uses: with low carbon content, stabilized, with nitrogen. These are excellent for working with plastic deformation such as deep drawing as well as welding. Moderate working performance is indicated for machine work with chip removal that increments in the versions ‘with improved workability.
The austenitic-iron based steels called also duplex or biphasic steels present a mixed structure of austenic and ferrites in virtue of an opportune balance of the austenitic elements (principally Ni, Mn, N) and iron based (principally Cr, Mo) that are present in the alloy. These materials are used when required for particular corrosion resistance characteristics (especial concerning stress-corrosion). Usually they have a better degree of welding and mechanical characteristics as the ferritics and austentic steels. Ultimately the ‘hardened by precipitation’: these present the possibility of considerably raising the mechanical characteristics with determinate particular ageing thermal treatment, which allows for plunging some composed elements into the metal matrix in order to up – grade the mechanical properties of the alloy.
Besides, those ‘ hardened by precipitation’ have a significant corrosion-resistance, certainly comparable to the classical austensic steels. Currently stainless steels have reached a significant differentiation. However, it has been decided to collect the most common with their indicative chemical compositions and the approximate correspondence among the unification of the different countries.
In many cases the customer decides to work and use a certain stainless-steel component always hoping in the magic word ‘stainless’ and expecting such material to be always resistant in many different environments and conditions of use. Besides, it is necessary to consider that there are not only ‘stainless steels’, but there exist, as already said before, many types and according to the various situations it is possible to chose suitable alloys in order not to incur disagreeable and unexpected situations. Besides, it is opportune that once the decision has been taken, it is necessary to follow up with particular sagacity the working, welding and installation in order to guarantee optimal holding in due course. Thus it can be seen, on the whole, how a corrosion action can be manifested, the main causes and the suggested alloy types which best withstand the process.
The involved parameters
It is always very hazardous, generally, the behavior in the course of time of certain metallic material if it comes into contact with certain environments. Stainless steels, thanks to their chemical composition have the possibility to withstand most different aggression conditions.
Especially the chemical composition is one of the indicative factors for resistance and corrosion, because this is connected to the ‘force’ of the passive layer and the material capacity to withstand corrosive attacks. As previously stated, the fundamental element is chromium (Cr): the more the content in the alloy, generally the more will be the corrosion resistance. The molybdenum (Mo) greatly aids the chrome to reinforce the passive layer.
With regards to the nitrose (N), while the austenic alloys and duplex increase the corrosion resistance increases, for the ferrite based steels it is better to keep the content on an extremely low level, (together with the carbon content) if the same result is required.
The parameters that intervene in triggering a corrosion phenomenon are many, including:
persistent agent (tipology, concentration, pH)
the temperature of the persistent agent
the metal surface finishing
the speed of the liquid on the walls of the material.
Generally, it can be said that the chlorides (CI-) the main ‘enemy’ of stainless steel because they are able to ‘break’ the passive layer and hinder its formation; the concentration of chloride ions and acidity (pH) together with the temperature are factors which must be looked into when choosing a stainless steel type.
The aspect of surface finishing is too often disregarded, when on the contrary it is absolutely fundamental to avoid the use of purer alloys. It is somewhat intuitive when the surface is ‘smooth’ the anchoring possibility of a persisting agent will diminish more.
Besides it should be remembered that the ‘defensive’ capacity of stainless steels is thanks to the passive layer. Such layers will be more easily formed and will be more stable with a better substratum surface. In the end, concerning the speed of the liquid, stagnation conditions are the most dangerous one.
They allow deposit formation, leaving corrosive agents to work undisturbed and do not favor the phenomenon of spontaneous passivity. In spite of everything, stainless steels can also be problematic. The most common problematic forms are: pitting, crevice corrosion, inter – granular corrosion, stress corrosion cracking and galvanic corrosion.
Regarding corrosion, most resistant steels are the austenic ones followed by the ferrite based steels while the last group are the martensitics; however this classification must be considered generally because of the existence of the austenitic group such as the range 200 which has less corrosion resistance with regards to certain ferro based steels as for example 441.
Technical annotation from the article ‘Stainless steels and corrosion resistance’ edited by V. Boneschi (Stainless Steel Centre, Milan) and M. Boniardi (School of Engineering at Milan). Published in the magazine “LAMIERA” (April 2008).
AISI type 304 belongs to the austenitic steel group and is the most used.
Mori 2A uses this steel type, because it offers good performance from the deforming point of view besides guaranteeing good corrosion resistance.
The strong volatility of the nickel price has in the last ten years contributed to steel diffusion in which nickel is partially or totally replaced with manganese (series 200).
These new steel types introduced on the market have a considerable price advantage, lower than the series 300 but also come with a series of problems than cannot be disregarded by the user.
The chrome content of 18% is not compatible with low nickel values without ferrite forming, for this reason, the chrome content in the steel at the range 200 is reduced 15%-16% and in certain cases 13%-14%, performing corrosion resistance not comparable to the 304 type and similar products, precisely the more the chrome reduction the higher the inter – granular corrosion risk because of carbides at grain boundaries. It must be remembered that manganese even if austentic it is not as much as nickel which is second only to nitrogen.
Concerning the acidity conditions, the re – passive properties of manganese are slowed as a result of acidity and the speed of dissolving steel in the series 200 is about 10 – 100 times higher as regards as the 304. Nickel, besides giving tenacity to steel, favors the auto passivity.
Often, these materials (series 200) are produced with systems that do not allow the control of the sulfur residual level, the carbon percentage and more seriously, it is not possible to trace the material and it can even be hidden. This last aspect reflects directly on the former, such as material recycling: if not declared, the chrome manganese could be a dangerous source of unexpected manganese rich rubbish.
Mori2A chooses AISI type 304 for its easy workability and considerable corrosion resistance capacity.
As regards other steels of the 200 series, the stainless steel used by Mori2A presents a very good deforming level, optimizing the working performances and guaranteeing an end-item with excellent price/quality ratio.
Besides the use of AISI type 304, Mori2A performs a particular thermal treatment (solution heat-treatment or anneling) for products derived from deep-drawing (where the work hardening is very high) which need to remove structural alterations and send carbides in solution. The solution heat-treatment consists in heating the steel at a sufficiently high temperature (1000-1100°) (1832 – 2012 °F) holding it for a certain time, stressed above all by the thickness of the treated piece, and in the chilling process with sufficient speed to prevent the precipitation of the carbides that happen on the average between 450°C – 850°C (842 – 1562°F). With this treatment steel arrives at the maximum soft coating.
The production cycle of these products also includes acid pickling and an accurate polishing (electrochemistry, vibro – polished, mechanical) which as previously mentioned is a fundamental aspect for corrosion-resistance.