Machining of stainless steel
Only very few uses stainless steel without machining it. The steel can be cut, bent, welded, polished, or in another way be exposed to mechanical processing. Unfortunately, this has consequences for the corrosion resistance of the steel. Stainless steel is only conditionally stainless, and the corrosion resistance is dependent on how the steel is treated. As a starting point, the steel is “perfect” from the supplier. The moment it leaves the steelworks it possesses its maximum corrosion resistance and the vast majority of mechanical processes that the steel can be exposed to lowers its corrosion resistance. All processing and handling of stainless steel should, therefore, be performed in a way which neutralises these weakening factors as much as possible. If this is not possible, the processes should be followed by an appropriate chemical treatment.
Welding of stainless steel
Regarding corrosion, one of the most serious “actions” is welding. Besides introducing a new phase (the welding metal), the steel is also exposed to powerful heat stress, which involves at least three potential dangers: sensitization, tempering, and inner tensile stress.
Those corrosion risks directly connected to the welding metal itself are often attempted to be minimised by the use of an “over-alloyed” filler material. It is more difficult, though, to ensure that there are no submerged cracks in the system. Those can occur in the form of pits, shrinkage holes, lack of fusion, burn-throughs, etc. and the corrosion risk is primarily crevice corrosion.
A good rule of thumb is that there is a possibility of crevice corrosion at a temperature 20-25°C lower than the critical pitting temperature (CPT) – the temperature above which there is a possibility of pitting corrosion. The solution is either to completely hinder submerged cracks (= intensified control) or to choose a better steel which has a higher Pitting Resistance Equivalent (PREN) and thereby a greater encapsulated safety (e.g. 4404 instead of 4301).
Drawn cut through a welding seam. A: Basis steel; B: Welding seam; C: Natural oxide film; D: Tempering; E: de-chromed layers (just below the tempering); F: Heat Affected Zone (HAZ); G: Pits, shrinkage holes, lack of fusion, etc.
Heating of the steel to temperatures between 500 and 850°C (an inevitable adverse effect of e.g. the welding process) entails a risk of the formation of harmful chromium carbides (= sensitization). This does not occur in the actual welding seam but in a heat stressed zone nearby (“Heat Affected Zone” = HAZ) and the problem is of greatest risk at welding in heavy material thicknesses. In practice, this is most effectively opposed by using low-carbon steel (e.g. 4306, 4307, or 4404) or titanium stabilised steel grades (4541 or 4571).
A related phenomenon is the formation of harmful intermetallic phases (e.g. “sigma” (Cr-Fe) or “ksi” (Cr-Mo)), which is something that is especially possible to encounter in welding of highly alloyed “super duplex” steel grades (e.g. 4410, duplex 2507, Zeron 100) or the highly alloyed ferritic steel grades (e.g. 4509, 4526, and 4521).
Just as harmful, at least, is the blueish or yellowish tempering that is formed on the surface of the steel beside the welding seam. These occurrences of tempering are strongly thickened oxides of chromium and iron and are caused by a warm oxidation of the actual stainless steel surface. In practice, such oxidation entails a critical weakening of the steel in regard to corrosion. If you want your steel to live up to its full potential, you need to secure that all welding is conducted under completely deoxidised conditions which requires the use of an extreme amount of shielding gas (see FORCE’s “Referenceatlas”).
A more economic and often quicker alternative is to tolerate a certain degree of blue colouring and subsequently remove the tempering again either by use of pickling or by use of a combination of polishing and chemical finishing treatment (pickling or passivation). Glassblowing is not a suitable approach since both the tempering and the de-chromed layer will be forced into the surface instead of being removed. This is handled by a pickling before the glassblowing.
Finally, any welding process, like any other form of mechanical processing, results in the formation of inner tensile stress and thereby an increased risk of stress corrosion. Nothing can be done to avoid this besides facing the problem already during the design phase and pick a steel which is predominantly resistant to stress corrosion at the planned operating conditions. It is not advisable to fight against stress corrosion by assuming that the end product has no tensile stress at all…
Welding methods and heating of pipes
HF – High-frequency welding
The HF-method is used in connection with the production of pipes for constructional purposes as well as the production of exhaust systems for cars. Within these scopes of application, the HF-method is favoured because of its cost-effective productivity benefits. On the other hand, the small welding seam that is accomplished with HF (see photo 3 – x50) is not always optimal in regard to the ability to perform, to resist pressure and resist corrosion due to the lack of fusing the strip edges together and the oxide formation on the welding edges.
Bright annealing
Bright annealing is performed in an oven filled with hydrogen (H2) at temperatures between 1040° C and 1100° C and is followed by a quick cooling. The hydrogen is NOT an oxidising agent. Therefore, no surface oxidisation is formed, and pickling is no longer required after the bright annealing.
The predominant advantage of this solution is – besides a blank and even surface that eases further finishing of the pipes – the enhanced corrosion resistance of the material.
A preparation like that, which is done in the final step of the production process, ensures a complete solution to possible carbides formed at the grain border by which an austenitic matrix is achieved without any defects. This renders it possible to avoid the harmful phenomenon of intergranular corrosion (the presence of sulphur and chlorine during welding due to high temperatures).
The austenitic structure, which is achieved through off-line bright annealing, is homogeneous with regular grain size (the dimension varies from 6 to 8 ASTM); as a consequence of this the tensile properties – especially prolongation – with increased plasticity and a decrease of residual stress.
This material property is extremely valued by every ultimate consumer who performs further processing of pipes such as bending and moulding.
Pipes, non-annealed – pickled
Welded pipes can be delivered with a non-annealed finish. This product undergoes the same production process except for the heat treatment. Instead, the pipes are sent to a chemical pickling treatment. The pickling bath consists of sulphuric acid and fluorine acid.
This process can – both on the outward and inward surface as well as the points of the material – eliminate any sign of ferrous-contamination as well as potential oxides which can appear on the metal surface as a result of mechanical finishing (contact rollers, sand belts, cutting equipment) and welding.
Brushed pipes
Brushed pipes are available on the market. Only the outward surface is brushed to avoid the chemical treatment in connection with pickling.
These products, however, have a lower corrosion resistance compared to pickled pipes if they are exposed to attacks in identical environments. This is both caused by the deposits on the metal surface, which have been contaminated during the production process and by the fact that the surface is rough, which easily can contain oxides and traces of ferrous contamination. The sand belts themselves can leave material that can be a cause of corrosion.
Brushed pipes require, due to their finish, a more frequent periodic maintenance compared to pickled pipes.
It should be emphasised that this exclusively refers to outward brushing. Therefore, it cannot remove potential contamination on the inward surface and the points of the material that are cut off by the use of a cutter constructed of steel-based materials.
The Eddy Current test
The welded pipes that are delivered with a TIG and Laser welded finish from Damstahl undergoes – after they have been calibrated – an Eddy Current test. Such a non-destruction test is conducted by forming a magnetic field around the pipe and trace any interruptions caused by leaks and holes.
Cutting, sawing, and other cutting methods
The most dangerous processes are generally exothermic since there is a risk of tempering as in the case of welding, which has to be removed mechanically/chemically. A “hot classic” is angle grinders that, besides resulting in very rough tempered surfaces, also have a tendency to shoot the warm particles left and right to other surfaces than those being processed. These particles can be permanently burned into the steel surface and cause both crevices and tempering – an extremely unfortunate combination that can result in vastly reduced corrosion resistance. The way to handle this is to remove all particles carefully by the use of a screwdriver or chisel and then carry out a pickling.
Meanwhile, even the cold cutting processes can disturb the corrosion resistance of the steel since you expose the centre of the steel that, ceteris paribus, contains more impurities than the surface. This effect stems from the solidification of “slabs” weighing tons. The solidification naturally occurs from the outside towards the inside and in this process, impurities are pushed in front of the solidified metal and ultimately end in the centre of the steel. Even a step-by-step rolling from e.g. 300 to under 1 mm does not change the fact that impurities are concentrated towards the centre of the steel.
The centre of a sheet is by that means less resistant to corrosion compared to the surface – a phenomenon that is connected to the actual production of the steel at the steelworks, and, as above, the problem can be minimised by carrying out a concluding pickling.
Brushing, blasting, polishing, and other machining processing of stainless steel
Any mechanical treatment of stainless steel affects the surface roughness and thereby the corrosion resistance of the steel. As a general rule, the corrosion resistance decreases with increasing surface roughness, and a very rough surface (say, sandblasted) performs markedly worse in a corrosion testing than the normal, smooth 2b.
The reason for this is double: At first, a rough surface is much better than a smooth one at “collecting” dirt and corrosive salts, thus forming “local elements”. Secondly, a rough grinding will tend to expose a larger concentration of impurities from the steel itself. Such impurities, in particular sulfides, may act as points of attack for pitting corrosion, and thereby lower the corrosion resistance
Two stainless steel plates, both EN 1.4301 (AISI 304). The left one has been grinded while the right one has been electro polished. It is not hard to imagine which surface is the most effective collecting salts and moisture. The white line in the bottom of each photo is 100 µm. Both photos courtesy of Technical University of Denmark (DTU).
In addition, a rough grinding will tend to increase the level of tensile stress in the surface of the steel, increasing the risk of stress corrosion cracking. In contrast, a fine blasting (shot peening or glass blasting – not sand blasting) may increase the level of compressive stress and thus increase the resistance against SCC.
From a corrosion point of view, it is normally an advantage not to perform any kind of mechanical surface treatment at all! The smooth and pickled 2B surface of the cold-rolled sheets possesses its maximum corrosion resistance and no matter how much we grind, it just gets worse. As above, a proper chemical surface treatment will reduce the damage of the steel.
Handling and transport of stainless steel
A particular risk in almost any handling of stainless steel is iron stains; a problem which is especially seen if, for instance, your bending tools, fork-lift truck or truck has been used to handle black steel. Besides an ugly appearance, iron stains lower the actual corrosion resistance of the stainless steel as the corrosion of the iron stains can continue into the actual stainless steel and result in corrosion there.
A grim example of iron stains in the form of a particle that, at a rolling mill, has been forced into the stainless steel. The particle must have been quite solid as it has been pushed deep into the stainless steel and even through pickling will remove all black steel/rust, the treatment will leave a small hole.
Iron stains can be removed chemically but it is just as effective is to preclude the problem. It is especially important only to use tools that are exclusively used for stainless steel, which includes everything from bending tools to the forks of one’s truck.
Even in the case of a total separation of tools, metal dust is a “classic”. Metal dust can be horrifyingly mobile, and the preclusion of iron stains can become a sort of a Sisyphean task. Ideally, black steel and stainless steel should be prepared on two different addresses, but there is often turned a blind eye to this requirement. In that case, there is no way around chemical finishing treatment.
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