stainless steel

INTRODUCTION

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 1100�C.

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 STEELS CATEGORIES AND GRADES

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:

Martensitic stainless steel.
Ferritic stainless steel.
Austenitic stainless steel.

Martensitic steels are alloys of iron, chromium and carbon with typical relative contents of:

C " 1,10%

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:

C " 1,10%

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:

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.

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.

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.

PARAMETERS TO FOLLOW IN A PROJECT WITH STAINLESS STEEL

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:

Resistance to corrosion.
Mechanical properties .
Physical properties.
Elaboration and finishes.
Joining.
Costs.
Design.

None of these categories depend upon others, but all of them are necessary to achieve an overall concept of the project.

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.

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:

Martensitic Stainless Steels: 12% Cr
Ferritic Stainless Steels: 17%Cr
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.

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.

*Families of* *Stainless Steel*	*Martensitic Grades*	*Ferritic Grades*	*Austenitic Grades*	*Austenitic-ferritic Grades*
*Density*	7'7	7'7	7'9	7'8
*Modulus of Elasticity at 20�C*	215 000	220 000	200 000	200 000
*Coefficient of Thermical Expansion between 20�C and 200�C*	10'5.10^-6	10.10^-6	16.10^-6	13'0.10^-6
*Thermical Conductivity at 20�C*	30	25	15	15
*Specific Heat Capacity at 20�C*	460	460	500	500
*Electrical Resistivity at 20�C*	0'55	0'60	0'73	0'80

Table 3: Typical physical properties

ELABORATION AND FINISHES.

Elaboration

In this section, we will refer to eleborated products, beginning with metal sheet by deformation in the cold.

The most common techniques of transforming stainless steel are:

Stuffing.
Folding.
Curving.
Outlining.
Puncturing.

Evidently, the project developer, in the moment of design, will have to keep in mind that the characteristics of deformation are deeply related to the ways of action of the techniques mentioned, and to the technological aspects of different kinds of stainless steels.

During the conformation operations, caution must be kept in order to avoid contamination of the steel surface by small particles of iron or other materials, because if embedded, they might cause superficial punctures. If it is suspected that contamination has occurred, the components will have to be submerged in diluted nitric acid and rinsed with water to clean the surface without damaging the stainless steel.

Stainless Steel Finishes

Referring to the finishing of the surfaces of stainless steels, this is not the result of a final and superficial as in paintings, varnishes, chrome or nickel protection, but the result of a specific treatment, without the addition of any other material.

Depending on the dimensions (width, length, thickness), commercial products made of stainless steel present different superficial states that can be divided in two main families:

a) MILL FINISHES.

b) ABRASION FINISHES.

Among the standard finishes we will define those that are used mainly in construction and decoration, the common ones.

Finish 2B This is obtained by cold rolling, heat treatment and pickling and then a final light rolling using highly polished rolls gives the surface a smooth, reflective, grey sheen. Its mean rugosity is 0.08 m.

Finish BA This is obtained by cold rolling, annealing in an inert atmosphere and dry skinpass. Its aspect is mirror shiny and its rugosity is typically 0.04 m.

Nevertheless, in the majority of uses for decoration, more complex finishings are required for aesthetic reasons. Decorators, generally tend to go with satins of low reflectability; that is, the series of finishing touches obtained by abrasion, among which we will cite the following:

Finish n� 3 This is normally obtained from 2B by polishing with emering elements of grain 80 - 100. Its appearance is satin and rough.

Finish n� 4 This is obtained from n� 3 by succesive polishing with abrasive elements of grain 120 - 150. Its appearance is shiny satin. It is mainly used in visible places where an attractive surface is required for aestethic reasons.

Finish n� 6, 7 and 8 They are obtained by using abrassive elements of grain 400 - 800.

Actually, more finishes are being used with coloring or embossing. There is a wide range in the market (“Guide of finishing in stainless steel”, Euro Inox).

JOININGS.

One of the main items to consider in any given project is to assemble and join the different parts of a component by unions that are stable and do not move.

Welding.

Welding is, in general, the most used technique to join stainless steel, even with those of low thickness. Currently, the following types of welding are used:

Welding in an inert gas atmosphere, with a consumable electrode (Metal Inert Gas) and non-consumable (Tungsten Inert Gas).
Welding with a coated electrode.
Resistance welding (by dotting or rolling).

The austenitic type can be welded with or without welding materials of the same type. They present adecuate unions of optimal resistance with any kind of welding, either by voltaic arc or by resistance.

The ferritic type are normally welded by resistance welding, either by dotting or rolling. The same occurs with the martensinic type.

While choice of weld process will depend upon a number of factors, stainless steel is readily welded to stainless steel or to carbon steel. Cognisance of the higher thermical expansion and lower thermical conductivity values of stainless steel compared to carbon steel, should be taken into account during fabrication to minimise distortions.

The choice of surface finish should be taken into account when selecting the fabrication process and post weld clean-up to avoid damaging and mechanical finish. Restoration of directional finishes, for example, need to be considered at welded junctions.

Mechanical Unions.

Mechanical unions (by means of screws, nails, etc) are used in a normal way under the condition that the elements are galvanically compatible with the steel that is joined and have a degree of uprightness non inferior to the one by those very same steels when the process is performed in the presence of an electrolyte.

Stainless steel screws are extremely appropiate to join other metal surfaces without the need of special isolation:

Parts and alloys of aluminum..
Parts and sheets of copper and zinc.
Parts and sheets of galvanized steel.

In worst conditions of corrosion, as happens in the vicinity of chemical and marine industries where the risk of aspersion of salt water also exists, it is convenient to isolate the screw from the rest of the metalic elements. To achieve this, it is sufficient to cover the screw, during assembly, with neutral grease that lacks of acid substances, or with laquer, to avoid direct contact with other metals. Another system consists on isolating the parts by intercalation of components of plastic or rubber material.

Adhesive bonding.

Among the most advanced actual techniques of union, it is distinguished the use of structural adhesives. These, applied in cold or by heating them up to the relatively low temperatures of the surfaces to join, achieve unions with a mechanic resistance that equals the elements joined, provided the united surfaces have adecuate dimensions and design.

Selection of the appropriate adhesive (epoxy, resin, acrylic…) will depend upon a number of factors including; the material to be bonded to the stainless steel, the working environment of the composite construction, and the type of loading to be resisted.

In general, a coarse finish to the stainless steel will provide a key for the adhesive but a prebonding treatment may also be necessary, although modern adhesives are more tolerant of surface films and moisture.

COSTS.

The cost of sheets of stainless steel are related to the kind of composition of the alloy, to the thickness and to the finishing. Thickness, is particularly important in raising the cost along with its decrease.

In an ordered ranking, according with the kind of alloy, we would see that the cheapest ones are the martensinic steels, medium cost ones are ferritic and the highest value ones being austenitic steels.

In spite of the initial cost of stainless steel being higher than other materials, in this way, subtracting the cost of cleaning, painting and maintenance, it has to be considered that the excessive initial price of stainless steel is compensated.

DESIGN.

Finally, I would like to mention here this parameter because somehow it is going to sumarize and complete the rest.

Design, in the broadest meaning of the word and not as just a mere aesthetic component, modifies by itself and in a unitary way the following:

The possibility that the material is in the optimal conditions to minimize any corrosion attack.
The technological cycle of construction.
The possibility of an easy and precise composition and joining of both parts.
The possibility of performing tests while functioning and periodical tasks of maintenance.
The possibility that the component or manufacturing fulfills its function correctly.
The cost of the component or of the ensemble.

CHOICE OF A STAINLESS STEEL GRADE

They are also extensively used in transport equipment (railroad cars, wagons, truck tanks, refrigerated containers, bus bodies, etc…), in chemical and petrochemical engineering, in the oil industry, in electronics (non-magnetic components for electron guns, glass-metal fixing pins) and in the building and construction industry (curting, walling, elevator cages, escalators, roofs, fume ducts, town furnishing…). This list is by no means exhaustive and stainless steels are used for a large number of everyday objects, of which coinage is a good example.

On the basis of the above fundamental criteria, the following list of applications and appropriate steel grades has been drawn up, classified according to the five major families of stainless steel already described:

Austenitic stainless steels

(0.015 - 0.1% C, 17 - 20% Cr, 7 - 25% Ni, 0 - 4% Mo)

Milk storage tanks

X5CrNi18-10 AISI-304

White wine storage tanks

X2CrNiMo17-12-2 AISI-316L

Beer vats

X5CrNi18-10 AISI-304

Equipment for collective catering, hospitals, foodstuffs handling, etc.

X5CrNi18-10 AISI-304
X6CrNiMo17-12-2 AISI-316L
X2CrNi18-9 AISI-304L

Sink pans and complete sink units

X5CrNi18-10 AISI-304

Dishwasher tubs and door linings

X5CrNi18-10 AISI-304

Cooking utensils

X5CrNi18-10 AISI-304

Cutlery and dishes

X5CrNi18-7

Bus and coach bodies

X5CrNi18-10 AISI-304

Fume ducts

X5CrNi18-10 AISI-304
X2CrNiMo17-12-2 AISI-316L
X1NiCrMoCu25-20-5

Depending on the technology (rigid, flexible, single or double wall, with or without condensation, type of fuel, etc.)

Hot water tanks

X2CrNiMo17-12-2 AISI-316L
X6CrNiMoTi17-12-2

Stabilized Ferritic stainless steels

(0.02 - 0.06% C, 11 - 29% Cr)

Domestic appliances: washing machine and drier drums, dishwasher tank shells:

X6Cr17 AISI-430

Sinks and sink units:

X6Cr17 AISI-430
X3CrTi17 AISI-430Ti

Cutlery, dishes, pan lids:

X6Cr17 AISI-430

Automobile hose clamps�:

X6Cr17 AISI-430

Decorative automobile trimmings�:

X6Cr17 AISI-430
X6CrMo17-1 AISI-434
X6CrMoNb17-1

Washing machine tubs:

X3CrTi17 AISI-430Ti

Hot water tanks:

X2CrTi17
X2CrMoTi18-2 AISI-444

Automotive exhaust systems:

X2CrTi12 AISI-409
X2CrTiNb18

Drier-superheater tubes (electric power stations):

X3CrTi17 AISI-430Ti

Evaporator and reheater tubing and boilers for sugar refineries:

X3CrTi17 AISI-430Ti

Fume ducts:

X2CrMoTi18-2 AISI-444
X2CrMoCuTi29-4

Tubing for seawater desalination plants�:

X2CrMoCuTi29-4

Conveyor belt chains:

X6CrNi17-1

Structural elements, container frames, wagons, hoppers, bus and coach bodies:

X6CrNi12

Duplex austenitic - ferritic stainless steels

The most commonly used duplex grade is the 0.02% C 22% Cr - 5.5% Ni - 3% Mo alloy, whose standard European designation is X2CrNiMo22-5-3. Its principal applications are as follows:

Chemical engineering:

Heat exchanges for PVC plants.
Equipment for handling organic acids.

Papermaking:

Pressure vessels.
Pre-impregnators.
Boilers.

Offshore engineering:

Seamed spiral tubing.

Miscellaneous:

Precipitator plates.

Martensitic stainless steel

(>0.1% C, 12 -14% Cr)

Like many plain carbon steels, these alloys are used in the quenched and tempered condition, giving the end product a hardness perfectly adapted to the intended utilization. Depending on the grade considered, the principal applications are as follows:

Knife blades:

X20Cr13
X30Cr13 AISI-420
X46Cr13

Shear blades for the paper industry:

X30Cr13 AISI-420

Compressor membranes, Springs:

X20Cr13

Surgical instruments:

X30Cr13 AISI-420
X46Cr13

Heat resisting austenitic stainless steels

Furnace components,

Heat exchangers:

X12CrNi23-13 AISI-309-S
X8CrNi25-21 AISI-310-S

Burners:

X12CrNi23-13 AISI-309-S

Furnace bells

X15CrNiSi20-12

Automobile exhaust manifolds:

X15CrNiSi20-12

STANDARD DESIGNATION OF STAINLESS STEELS

The steel names and steel numbers were established in accordance with EN-10027. The European standard for stainless steels is EN-10028 and the designations systems adopted in this standard are the European Material Number and the material name.

EN 10088-1. Part 1 “List of stainless steels”

EN 10088-2. Part 2 “Technical delivery conditions for sheet/plate and strip for general purpose”

EN 10088-3. Part 3 “Technical delivery conditions for semi-finished products, bars, rods and sections for general purpose”

The material number comprises three parts, for example 1.4301 where 1 denotes steel, 43 denotes one group of stainless steels and 01 is the individual grade identification.

The material name system provides some indication of the alloy composition, for example C5CrNi 18-10, where X denotes high alloy steel, 5: 100 x % of carbon, CrNi main alloying elements and 18-10 percentage of main alloying elements.

PHYSICAL

PROPERTIES