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STAINLESS STEEL SELECTION
Reproduced and adapted from the BSSA website as a supplement
to the notes about stainless steel and designing stainless tanks and vessels

To go to the Notes about stainless, click
To go to the notes about tank design, click

 

Stainless steel is an alloy of iron with a minimum of 10.5% chromium content.

Other alloying elements are added to enhance their structure and properties, but fundamentally, stainless steels are considered to be steels with corrosion resistant properties.

In economic terms they compete with higher cost engineering metals and alloys based on nickel or titanium, whilst offering a range of corrosion resisting properties suitable for a wide range of applications. They have better strength than most polymer products (GRP), are readily repaired and 'recyclable' at the end of their useful life.

The important features to consider are: -

    Corrosion (or oxidation) resistance

    Mechanical & physical properties

    The availability of forming, fabrication & joining techniques
    Environmental & material costs (including total life cycle cost)

The basic approach is to select a grade at as low cost as possible, with the required corrosion resistance. Other considerations such as strength and hardenability will be secondary.

Corrosion resistance

Chromium (Cr) content sets stainless steels apart from other steels.

The unique self-repairing 'passive' surface layer on the steel is a result the chromium.

Commercially available grades contain around 11% minimum chromium, and may be either ferritic or martensitic, depending on carbon range control.

More chromium enhances corrosion and oxidation resistance, so a 17% Cr 430 (1.4016) ferritic would be expected to be an improvement over the '410S' (1.4000) types.

Similarly martensitic 431 (1.4057) at 15% Cr can be expected to have better corrosion resistance than the 12% Cr 420 (1.4021 / 1.4028) types.

Chromium levels over 20% provide improved 'aqueous' corrosion resistance for the duplex and higher alloyed austenitics and also forms the basis of the good elevated temperature oxidation resistance of ferritic and austenitic heat resisting grades, such as the quite rare ferritic 446 (25% Cr) or the more widely used 25 % Cr, 20% nickel (Ni) austenitic 310 (1.4845) grade.

In addition to this basic 'rule', nickel (Ni) widens the scope of environments that stainless steels can handle.

The 2% Ni addition to the 431 (1.4057) martensitic type improves corrosion resistance marginally. Additions of between about 4.5% and 6.5% Ni are made in forming the duplex types. The austenitics have ranges from about 7% to over 20%.

The corrosion resistance is not only related to the nickel level however. It is incorrect for example, to assume that 304 (1.4301) with its 8% Ni has better corrosion resistance than 1.4462 duplex with 5% Ni.

More specific alloy additions are also made, to enhance corrosion resistance.

These include molybdenum (Mo) and nitrogen (N) for pitting and crevice corrosion resistance. The 316 types are the main Mo bearing austenitics. Many of the currently available duplex grades contain additions of both Mo and N.

Copper is also used to enhance corrosion resistance in some common, hazardous environments such as intermediate concentration ranges of sulphuric acid. Grades containing copper include the austenitic 904L (1.4539) type.

Mechanical and physical properties

Basic mechanical strength increases with alloy additions, but the atomic structure differences of the various groups of stainless steels has a more important effect.

Only the martensitic stainless steels are hardenable by heat treatment, like other alloy steels. Precipitation hardening stainless steels are strengthened by heat treatment, but use a different mechanism to the martensitic types.

The ferritic, austenitic and duplex types cannot be strengthened or hardened by heat treatment, but respond to varying degrees to cold working as a strengthening mechanism.

Ferritic types have useful mechanical properties at ambient temperatures, but have limited ductility, compared to the austenitics. They are not suitable for cryogenic applications and lose strength at elevated temperatures over about 600 C, although have been used for applications such as automotive exhaust systems very successfully.

Austenitic types, with their characteristic face centred cube 'fcc' atomic arrangement, have quite distinct properties. Mechanically they are more ductile and impact tough at cryogenic temperatures.

The main physical property difference from the other types of stainless steel is that they are 'non-magnetic' i.e. have low relative magnetic permeability's, provided they are fully softened. They also have lower thermal conductivity's and higher thermal expansion rates than the other stainless steel types.

Duplex types, which have a 'mixed' structure of austenite and ferrite, share some of the properties of those types, but, fundamentally are mechanically stronger than either ferritic or austenitic types.

Forming, fabrication and joining techniques

Depending on their type and heat-treated condition, wrought stainless steels are formable and machinable. Stainless steels can also be cast or forged into shape.

Most of the available types and grades can be joined by use of appropriate 'thermal' methods including soldering, brazing and welding.

Austenitics are suitable for a wide range of applications involving flat product forming (pressing, drawing, stretch forming, spinning etc)

Although ferritics and duplex types are also useful for these forming methods, the excellent ductility and work hardening characteristic of the austenitics make them a better choice.

Formability of the austenitic types is controlled through the nickel level.

The 301 (1.4310) grade which has a 'low' nickel content, around 7% and so work hardens when cold worked, enabling it to be use for pressed 'stiffening' panels.

In contrast nickel levels of around 8.5% make the steel ideally suited to deep drawing operations, for example in the manufacture of stainless steel sinks.

Martensitics are not readily formable, but are used extensively for blanking in the manufacture of cutting blades.

Most stainless steel types can be machined by conventional methods, provided allowance is made for their strength and work hardening characteristics.

Techniques involving control of feed and speed to undercut work hardening layers with good lubrication and cooling systems are usually sufficient.

Where high production volume systems are employed, machining enhanced grades may be needed.

In this respect, stainless steels are treated in similar ways to other alloy steels, sulphur additions being the traditional approach in grades like 303 (1.4305). Controlled cleanness types are now also available for enhanced machinability.

Most stainless steels can be soldered or brazed, provided care is taken in surface preparation and fluxes are selected to avoid the natural surface oxidising properties being a problem in these thermal processes.

The strength and corrosion resistance of such joints does not match the full potential of the stainless steel being joined, however.

To optimise joint strength and corrosion resistance, most stainless steels can be welded using a wide range of techniques.

The weldablity of the ferritic and duplex types is good, whilst the austenitic types are classed as excellent for welding. The lower carbon martensitics can be welded with care but grades such as the 17% Cr, 1% carbon, 440 types (1.4125) are not suitable for welding.

Summary of the main advantages of the various types

Type

Examples Advantages Disadvantages
Ferritic 410S, 430, 446 Low cost, moderate corrosion resistance & good formability Limited corrosion resistance, formabilty & elevated temperature strength compared to austenitics
Austenitic 304, 316 Widely available, good general corrosion resistance, good cryogenic toughness. Excellent formability & weldability Work hardening can limit formability & machinability. Limited resistance to stress corrosion cracking
Duplex 1.4462 Good stress corrosion cracking resistance, good mechanical strength in annealed condition Application temperature range more restricted than austenitics
Martensitic 420, 431 Hardenable by heat treatment Corrosion resistance compared to austenitics & formability compared to ferritics limited. Weldability is limited.
Precipitation Hardening 17/4PH Hardenable by heat treatment, but with better corrosion resistance than martensitics Limited availability, corrosion resistance, formability & weldability restricted compared to austenitics


Special grades with enhanced compositions have also been developed that minimise the short comings of any particular type. These include: Super Ferritics, Super Austenitics, Super Duplexes, Low Carbon Weldable Martensitics, Austenitic Precipitation Hardening Types.

The information above is reproduced and adapted by EngineerOnLine Ltd. with the knowledge of BSSA.

 


For your convenience, the following is adapted and reproduced from Wikipedia as at December 2nd 2005, complete with active hyperlinks.

STAINLESS STEEL CORROSION

Intergranular Corrosion

Some compositions of stainless steel are prone to intergranular corrosion. When heated to around 700 C, chromium carbide forms at the intergranular boundaries, depleting the grain edges of chromium, impairing their corrosion resistance. Steel in such condition is called sensitised. Steels with carbon content 0.06% undergo sensitisation in about 2 minutes, while steels with carbon content under 0.02% are not sensitive to it.
There is a possibility to reclaim sensitised steel, by heating it to above 1000 C and then quenching it in water. This process dissolves the carbide particles and keeps them in solution.
It is also possible to stabilize the steel to avoid this effect and make it welding-friendly. Addition of titanium, niobium and/or tantalum serves this purpose; titanium carbide, niobium carbide and tantalum carbide form preferentially to chromium carbide, protecting the grains from chromium depletion. Use of extra-low carbon steels is another method. Light-gauge steel also does not tend to display this behaviour, as the cooling after welding is too fast to cause effective carbide formation. 
 

Pitting Corrosion

When subjected to high concentration of chloride ions (eg. sea water) and moderately high temperatures, localized severe corrosion known as pitting corrosion can occur. Acidic environment worsens the problem. This can cause localized perforation of the parts. Composition rich in chromium and particularly molybdenum and nitrogen display higher resistance to this mode of corrosion. Solution of ferric chloride is used for laboratory testing of pitting corrosion. Download more info.  

Crevice Corrosion

In the presence of reducing acids or exposition to reducing atmosphere, the passivation layer protecting steel from corrosion can break down. This wear can also depend on the mechanical construction of the parts, eg. under gaskets, in sharp corners, or in incomplete welds. Such crevices may promote corrosion, if their size allows penetration of the corroding agent but not its free movement. The mechanism of crevice corrosion is similar to pitting corrosion, though it happens at lower temperatures. Download more info.  

Stress Corrosion Cracking

Stress corrosion cracking is a rapid and severe form of stainless steel corrosion. It forms when the material is subjected to tensile stress and some kinds of corrosive environments, especially chloride-rich environments (sea water) at higher temperatures. The stresses can result of the service loads, or can be caused by the type of assembly or residual stresses from fabrication (eg. cold working); the residual stresses can be relieved by annealing. This limits the usefulness of stainless steel for containing water with higher than few ppm content of chlorides at temperatures above 50 C.
Stress corrosion cracking applies only to austenitic stainless steels and depends on the nickel content.   

Sulphide Stress Cracking

Sulphide stress cracking is an important failure mode in the oil industry, where the steel comes into contact with liquids or gases with considerable hydrogen sulfide content eg. sour gas. It is influenced by the tensile stress and is worsened in the presence of chloride ions. Very high levels of hydrogen sulfide apparently inhibit the corrosion. Rising temperature increases the influence of chloride ions, but decreases the effect of sulfide, due to its increased mobility through the lattice; the worst temperature range for sulphide stress cracking is between 60 and100 C.   

Galvanic Corrosion

Galvanic corrosion occurs when a galvanic cell is formed between two dissimilar metals. The resulting electrochemical potential then leads to formation of an electric current that leads to electrolytic dissolving of the less noble material. This effect can be prevented by electrical insulation of the materials, eg. by using rubber or plastic sleeves or washers, keeping the parts dry so there is no electrolyte to form the cell, or keeping the size of the less-noble material significantly larger than the more noble ones (eg. stainless-steel bolts in an aluminium block won't cause corrosion, but aluminium rivets on stainless steel sheet would rapidly corrode.   

 

Further recommended reading regarding the maintenance of stainless steel (click to visit) Courtesy of Atlas Steels

Carbon steel is a very common contaminant here, coming from nearby grinding of carbon steel or use of tools contaminated with carbon steel particles. The particle forms a galvanic cell, and quickly corrodes away, but may leave a pit in the stainless steel from which pitting corrosion may rapidly progress. Some workshops therefore have separate areas and separate sets of tools for handling carbon steel and stainless steel, and care has to be exercised to prevent direct contact between stainless steel parts and carbon steel storage racks.

Particles of carbon steel can be removed from a contaminated part by passivation with dilute nitric acid, or by pickling with a mixture of hydrofluoric acid and nitric acid. However if the stainless has been mirror polished it will need re-polishing after using those methods.

Phosphoric acid at 35% dilution which is commercialy available by various names like Naval Jelly, RustX or Rust Kill etc is very effective for removing the staining caused by carbon steel contamination. The following is extracted from metalwebnews.com :-

"Another technique for removing rust is etching with Phosphoric Acid. Phosphoric Acid has a unique property of dissolving iron oxide (rust) quickly while etching iron very slowly. This means that you can leave metal in Phosphoric Acid for much longer than necessary with very little damage. The acid will attack bare metal slowly however and will start the process of hydrogen embrittlement, so use the minimum etch time that removes all rust.

Another unique advantage of Phosphoric Acid is that it leaves a fine coating of iron phosphate behind. Iron phosphate prevents rust. However, the iron phosphate coating is not very thick or durable. Some additional protection is still required.

Phosphoric Acid etch will leave a hard, bright metal finish. This is because it will etch the surface slightly, exposing new, bare metal. Often this is desirable. It leaves an attractive surface and a surface ready to paint. A common product which contains Phosphoric Acid is Naval Jelly. The soft drink Coca-Cola also contains Phosphoric Acid, so Coke will etch rust. But Coke also contains carbonic acid and other nasty things. You're going to drink that stuff?" 

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