Iron and the most usual iron alloys, steel, are from a corrosion viewpoint relatively poor materials since they rust in air, corrode in acids and scale in furnace atmospheres. In spite of this there is a group of iron-base alloys, the iron-chromium-nickel alloys known as stainless steels, which do not rust in sea water, are resistant to hot, concentrated acids and which do not scale up to 1100ºC.
It is this largely unique universal usefulness, in combination with good mechanical properties and manufacturing characteristics, which gives the stainless steels their reason to be and makes them an indispensable tool for the designer.
The usage of stainless steel is small compared with that of carbon steels but exhibits a steady growth, in contrast to the constructional steels. Stainless steels as a group are more homogenous than the constructional steels, but their properties are in many cases, relatively unknown.
Stainless steels are in some ways an unknown sphere but correct usage of these materials requires increased understanding of their basic properties.
Stainless steel is an alloy of iron, chromium and carbon, that sometimes is complemented with some other elements, mainly nickel.
The addition of chromium makes steel stainless. In environments with the capacity to oxidize, such as air, chromium forms a very compact and thin layer, that isolates the material from corrosion attack. The goal of using stainless steel must always be to keep the passive layer intact, which guarantees that these material behave well against corrosion.
Stainless steels are classified according to the different elements of their composition, and the relative amounts of each one. Generally speaking, there are three basic families:
û Martensitic stainless steel.
û Ferritic stainless steel.
û Austenitic stainless steel.
Martensitic steels are alloys of iron, chromium and carbon with typical relative contents of:
Cr: 12-14%
The kind of steel that characterizes this group is TP-420/EN-1.4028 according to the denominations of ASTM and EURONORMA.
Ferritic steels are also alloys of iron, chromium and carbon, with higher contents of chromium but less than martensinic steel. Typical values of these elements are:
Cr: 16-18%
The steel representative of this group of materials is TP-430/EN-1.4016.
Austenitic Steels are alloys of iron, chromium, nickel and carbon. The addition of nickel allows the modification of the structure of these materials, in such a way that remains austenitic at any given temperature.
The steel representative of this group is TP-304/EN-1.4301.
The main characteristics of these three families of stainless steel are the following ones:
a) Martensinic:
These suffer structural modifications with temperature, so they are usually submitted to thermical treatments of temper and swell. After this process they acquire good mechanical properties and resistance to corrosion. Its most characteristic application is in cutlery.
b) Ferritic:
These are steels with mechanical properties that allow making configurations of medium class. They weld well and are used often in applications where aesthetics are a very important factor. Resistance to corrosion is higher than in martensinic steels thanks to its higher content in chromium.
c) Austenitic:
These are a group of steels that have better applicability in relation to components and equipment manufacturing, as well as in service behaviour. They have excellent conformation properties, and high resistance to different kinds of corrosion that might appear.
From a general point of view, the parameters that the designer or project developer will have to consider and coordinate harmonically for the adecuate use of stainless steel, may be grouped into seven different categories, related among themselves:
û Resistance to corrosion.
û Mechanical properties .
û Physical properties.
û Elaboration and finishes.
û Joining.
û Costs.
û Design.
None of these categories depend upon others, but all of them are necessary to achieve an overall concept of the project.
3.1. Resistance to corrosion.
Stainless steels are resistant to corrosion because they have the ability to remain passivated in a large number of environments. On a passivated state, the metal is covered by a protective layer, which is extremely thin, invisible and of high stability.
This film is an oxide which protects the steel from attack in an aggressive environments. As chromium is added to a steel, a rapid reduction in corrosion rate is observed at around 10% because of the formation of this protective layer or passive film. In order to obtain a compact and continuous passive film, a chromium content of at least 11% is required. Passivity increases fairly rapidly with increasing chromium content up to about 17% chromium. This is the reason why so many stainless steels contain 17-18% Chromium.
The most important alloying element is therefore chromium, but a number of other elements such as molybdenum, nickel and nitrogen also contribute to the corrosion resistance of stainless steels.
Other alloying elements may contribute to corrosion resistance in particular environments, for example copper in sulphuric acid or silicon, cerium and aluminium in high temperature corrosion in some gases.
This layer has the essencial property of being able to selfrecuperate spontaneously if it suffers damage. This makes it different from protective covers such as: painting, varnish, smalt or any other metallic coverage, where any local damage will remain permanent, unless external intervention is applied.
This resistance to corrosion, which is inherent to stainless steels, is not the same for all of them, some are more resistant than others to corrosion. Faced with the dilemma of which stainless steel to choose, the first thing that we must be aware of is the environment where the product is going to remain, and if this will or will not be contaminated.
3.2. Mechanical properties.
Stainless steels are often selected on account of their corrosion resistance, but they are at the same time constructional materials. Mechanical properties such as strength, high-temperature, ductility and toughness, are thus also important properties.
As mentioned above, stainless steels are classified according to their structure and chemical composition, in:
û Martensitic Stainless Steels: 12% Cr
û Ferritic Stainless Steels: 17%Cr
û Austenitic Stainless Steels: 18%Cr – 8%Ni
The difference in the mechanical properties of different stainless steels is perhaps seen most clearly in the stress-strain curves in Figure 1. The high yield and tensile strengths but low ductility of the martensitic steels is apparent, as is the low yield strength and excellent ductility of the austenitic grades. Ferritic-austenitic and ferritic steels both lie somewhere between these two extremes.
Figure 1. Stress-strain curves for some stainless steels.
The ferritic steels generally have a somewhat higher yield strength than the austenitic steels, while the ferritic-austenitic steels have an appreciably higher yield strength than both austenitic and ferritic steels. The ductility of the ferritic and ferritic-austenitic steels are of the same order of magnitude, even if the latter are somewhat superior in this respect.
3.3. Physical properties.
These properties are of crucial importance because, having lower coeficients of lineal dilatation and thermical conductivity, they allow in some cases of use in gutters, rooves and structures ... to reduce or even eliminate dilatation joinings.
Martensitic Grades
Ferritic Grades
Austenitic Grades
Austenitic-ferritic Grades
Density
7’7
7’9
7’8
Modulus of Elasticity at 20ºC
215 000
220 000
200 000
Coefficient of Thermical Expansion between 20ºC and 200ºC
10’5.10-6
10.10-6
16.10-6
13’0.10-6
Thermical Conductivity at 20ºC
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