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Stainless Steels:
Steels are ferrous alloys with a level of Carbon lower than 2%.
There are non-alloyed and alloyed steels (high-alloyed steel refers to elements in alloy higher or equal to 5%; low-alloyed have instead elements in alloy lower than 5%)
What are they:
Basically they are high-alloyed steels and more precisely alloys in Iron - Carbon – Chrome: Fe + Cr. (>10.5%) + C.
Main characteristics :
The key characteristic of Stainless Steel is that of having good resistance against corrosion, mainly thanks to the fact that these alloys form a slight chrome oxide layer on the surface which makes the steel resistant in sufficiently oxidising environments - this phenomenon is the so called passivation. This layer is mainly made of chrome oxides and metallic oxides, all of the Me (OH) type.
Kind of alloys:
Stainless steels are divided into 3 groups based on the structural characteristics of the alloy.
These groups are:
- Austenitic Steels : Fe + Cr (16÷28%) + Ni (6÷32%) + C (0.02÷0.1%)
- Ferritic Steels : Fe + Cr (10.5÷30%) + C (0.02÷0.1%)
- Martensitic Steels : Fe + Cr (12÷19%) + C (0.08÷1.2%)
The difference between these groups consists in the Chemical composition as well as in the characteristics of the alloys (mechanical characteristics and resistance to oxidation). In this brief introduction, we will consider only austenitic and ferritic steels, which are produced by Arinox S.r.l.
We would like to remind you that arguments treated below are not complete. They only relate to those aspects which are of major interest for what concerns the stainless steel, above all for what considers the description of Heat Treatments.
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.
Corrosion is a vast topic and it has to be discussed case by case, as corrosion could appear in various ways, depending on the original causes. The main corrosion types are therefore listed below.
General Corrosion
There are two types of general corrosion: regular and irregular, depending on their causes, but generally it reveals as a progressive aggression with constant speed. This is the less dangerous form of corrosion since it is possible to calculate quite precisely the piece's life time, the loss of weight or the reduction in thickness of the membrane in a certain period.
Electrolytic Corrosion
When two metallic elements are directly connected via continuous electricity at the presence of an electrolyte, the electrolytic corrosion reveals. The element which will oxidise quicker is the one which is more anodic.
The smaller the ratio between the anodic and cathode area, the quicker the more anodic material will oxidise. This phenomenon can reveal on a same metallic material whenever on its surface anodic and cathode zones appear because of the chemical and structural non- homogeneity (e.g. precipitated carbides or different stages).
Interstitial corrosion
It is also called ‘crevice corrosion'. It is a spotted corrosion which may reveal if an item shows gaps between the two coupled metallic parts' surfaces.
Pitting corrosion
It is a spotted type of corrosion, particularly hidden and dangerous as it reveals in deep and very small areas. It can be easily missed at visual checks although it can seriously damage a whole piece and even perforate it. For this reason it is very difficult to reveal the presence of this kind of corrosion.
Any pit of this kind of corrosion appears as a very small spot (Ø = 0.1 - 1 mm), surrounded by a small brighter area and a darker circle (the brighter area is the cathode zone). The ratio between the cathode and anodic (hole) area is extremely high and therefore it causes a high anodic electricity flux and, as a consequence, the ions pass quickly through the hole in solution. The motion direction is generally in gravity. Corrosion begins on the surface area where it is more difficult to create a condition of stable passivity. Pitting corrosion takes usually place in weak oxidising solutions containing Cl¯ or Br¯ ions. This corrosion can either penetrate in depth (see Figure A) or expand in a shape of a cave (see Figure B).
Two phases constitute the corrosion process:
• Incubation - aggressive ions attack the passive film and crack it.
• Pit growth –the corrosion process is auto-catalytic, which means that it creates all the conditions to allow a self-growth. Metallic ions are passing very fast at the bottom of the pit, while other cathode areas see an oxygen reduction due to the creation of OH¯ ions. At the bottom of the pit, there is also a sufficient quantity of Cl¯ which increases acidity in the solution (pH decreases) and accelerates the penetration attack.
The PI (Pitting Index) index is a rapid way to value the tendency of pitting. It is also called PRE (Pitting Resistance Equivalent).
- For austenitic steels: PRE = %Cr + 3.3(%Mo) + 16(%N)
- For ferritic steels: PRE=%Cr + 3.3(%Mo)
For AISI 316:
%Cr: 17
%Ni: 12
%Mo: 2.5
%N: 0.10
PRE: 27
For AISI 430:
%Cr: 17
%Ni
%Mo
%N
PRE: 17
It must be highlighted that PRE provides approximate information, and both environmental aggressiveness and working temperature should be always considered.
Stress Corrosion
It is a spotted type of corrosion. The corrosion process has specific characteristics, which differ from those occurring without either static or dynamic stress. This type of corrosion develops quickly within deep-narrow areas of the material with an approximate penetration speed of 1-2 mm/h. It is particularly threatening as it reveals without premonitions. This corrosion stops developing whether the stress ceases or its development causes a release of the tension. Generally, when a crack appears the material is already irreparably damaged because of the presence of deep cracks, which are usually branched off and inter-crystalline trended).
Fatigue corrosion
It is a spotted type of corrosion generated by a frequent cyclic stress within a certain period. Each alloy has a certain fatigue limit, without any stress (maximum cyclic stress within a certain time). Within an aggressive environment, this limit is considerably reduced.
Inter-crystalline corrosion
It is a selective type of corrosion as it attacks the grain boundary. In this particular condition each grain separates from the others and it can be taken away form any mechanical action. At this stage, the Cr percentage at the grain boundary drops because of the creation of Cr carbides, which are reducing the Cr percentage down to less than 12% (the steel loses its stainless characteristic under this limit). In these conditions, both cathode areas (inside the grain bodies) and anodic areas (areas with low percentage of Cr, near the grain boundary) are formed.
Passive material can be defined as material that is able to oxidise thermodynamically but the speed of this process is so low that the corrosion effect is almost negligible. The material is instead called active material if the corrosion process is thermodynamically possible at a certain speed.
Some metallic materials such as stainless steel can be either active or passive, depending on their environmental situation. The stainless steel change of condition is caused by the presence on its surface of a film defined as oxide. The passive condition can occur on the material via a spontaneous process, within a sufficiently oxidative environment, or via an induced process within a more oxidising environment.
Stainless steel corrosion resistance is caused by a passive surface film development, which needs to develop a Cr content = 10.5 %. The passive condition is due to a passive film, invisible and caused by the reaction between the metallic material and the environment. This continuous and sticking to the surface film is non-porous and insoluble, and it is able to form again in case of cracking when re-exposed to air or to oxidising environment. Stainless steel has the key characteristic of being able to switch from active to passive condition and vice versa.
Quantities characterising Stainless Steels behaviours are:
- Passivation current (I P ) value
- Corrosion current (I c,r ) value
- Passivation tension (E p ) value
- Trans-passivation tension (E t ) value
- (E p - E t ) value
Both the chemical composition of the alloy and the chemical composition of the solution they interact in, have great importance for the passivation condition. Generally, resistance to corrosion is good within medium-aggressive solutions, providing oxygen or other oxidising solutions presence. The passive film dissolves in reducing solutions (chloride acid, sulphur acid, concentrated organic acids, etc) and the steel is corroded at a similar speed as normal steel. The corrosion attack is anyway less dangerous since the thickness reduction is uniform and can be calculated.
Quench hardening is not applicable to austenitic or ferritic steels. Since the only way to improve their mechanical properties is by hard-rolling, it becomes necessary to reduce tensions caused by the hard-rolling process itself.
Ferritic Steel
Ferritic steels can be treated either through re-crystallisation annealing or a re-crystallisation process during rolling operations.
Re-crystallization
Treatment temperatures are different from one alloy to another, and for the same alloy they can be different depending on the way of rolling. The cooling process can be performed in air or water. The material cannot stay too long in the range of temperature of 400 -570°C. Consideration on alloy 430.
Two treatments are available for alloy 430:
- cooling process, as described above;
- annealing process 820°C - 920°C, after cold rolling operations
Re-crystallisation can cause a decrease of toughness in notch products. Moreover, staying at temperatures between 930-980°C, followed by a quick cooling process, may cause a poor inter-granular corrosion resistance.
Austenitic Steel
Austenitic steels can be divided into 3 groups, depending on the heat treatment:
- Normal austenitic: non-stabilised, non low carbon content and refractory materials
- Stabilised austenitic
- Low carbon austenitic
Solubilization
Solubilization is divided into three stages:
- annealing the steel to a sufficiently high temperature to remove structural alterations due to the rolling process
- keeping the material at such temperature so that all carbides become soluble (specifically Chrome carbides)
- cooling at sufficient speed to avoid carbides precipitation, which usually takes place at temperatures around 450 - 850°C.
This treatment achieves the maximum softening results and its three key factors are: temperature, duration of the treatment and cooling speed. Annealing temperature is around 1000-1100°C and the time in which the material is kept at such temperature mostly depends on the thickness of the material being treated. At this temperature austenitic steels are subject to surface re-carburisation with the risk of carbides precipitation (for this reason, the furnace atmosphere must not be carburising). Cooling must be quick to avoid any carbide precipitation (sensitisation). To be highlighted the fact that high temperature re-crystallisation leads to grain dimension bigger than the one achievable at lower temperatures. Bigger grains generally lead to more severe corrosions.
Inter-crystallizing corrosion sensitisation
This treatment is used only for laboratory purposes and it has to be avoided in practical applications to establish the sensitivity to inter-crystalline corrosion. Within a temperature range of 450- 850°C and in austenitic alloys, the precipitation of Cr carbides can take place causing a decrease of Cr concentration and therefore an increased probability of inter-crystalline corrosion. In order to avoid this problem, it is possible to use ‘stabilising elements' such as Titanium or Niobium which are likely to alloy with C rather than with Cr, avoiding the leaning out of free chromium in the alloy and therefore the sensitisation of the steel (in these conditions the sensitisation takes place at a much higher temperature: 1250-1300°C).
Stress Relieving
It aims is to eliminate, at least partially, the internal tensions built up during production. In case of extremely hard rolled material (e.g. springs) stress relieving is used to increase Tensile Strength in order to improve elastic limits. Stress relieving consists in warming the material up to a temperature just under its sensitisation level (T =450°C). The time required for achieving stress relieving is very short (just a few seconds) because this kind of material is usually very thin. Stress relieving materials with welded parts is much more difficult as treating the material at these temperatures can create, during cooling, tensions in the welded areas which will cause ‘Stress Corrosion'.
For about Stainless Steels:
Gabriele Di Caprio
Gli Acciai Inossidabili
3^ Ed.in lingua italiana
Hoepli - Milano - 1997
Gabriele Di Caprio
Los Aceros Inoxidables
2^ Ed. in lingua spagnola
Grupinox - Barcelona - 1999
Donald Peckner/I.M. Bernstein
Handbook of Stainless Steel
McGraw-Hill Book Company
Wiliam H. Cubberly/Paul M. Unterweiser / David Benjamin / Craig W. Kirkpatrick / Vicki Knoll / Kathy Nieman
Metals Handbook Ninth Edition
Volume 3
Properties and Selection:Stainless Steels, Tool Materials and Special-Purpose Metals American Society for Metals
Paul Schierhold
Nichtrostende Stähle
Verlag Stahleisen M.B.H.
P. Lacombe / B. Baroux / G. Berange
Les Aciers Inoxydables
Les éditions de physique
Our thanks to Prof. Ing. Gabriele Di Caprio, author of "Gli Acciai Inossidabili", for the information provided during the realisation of this section.
Thanks to STAINLESS STEEL CENTER of Milano for the specifications.