INOX – Steel processing guide
8- SURFACE FINISHINGS
The considerable diversification of surface finish semi-manufactured or manufactured goods with steel plays a decisive role.
In fact, this not only responds to an aesthetic reason, but optimizes the behavior of the material with respect to certain corrosive environments or enhances the characteristics of this material: think, to cite an example, to the relationship that exists between the surface and hygiene of sanitary stainless steels.
These considerations are generally valid for each type of material, in particular for autopassive materials, such as stainless steels.
It’s necessary to distinguish two types of finishes: those obtained by rolling and those by abrasion.
FINISHES BY ROLLING
These surface finishes are obtained on the steel product (tape, sheet, tube, etc.), directly from the steel by successive cycles at the mill.
When the generic user requires, at the steel mill or at retailers, a laminated material, to define the desired degree of surface finish, he must refer to conventional abbreviations.
Such designations vary according to the different entities of national unification: Table 6 shows comparisons between abbreviations relating to the main current regulations (in the table, reference was made, for completeness, also to other types of steel products, such as long products and forgings).
In any case, it is important to remember that the rules define the various technological stages of finish obtainable on stainless steel sheets, but do not specifically refer to correlations with the roughness of the individual finishes.
Referring to the legislation (“Finished stainless steel flat products resistant to corrosion and heat. Sheets, thin sheets, tapes and long tapes”), so you can distinguish the following degrees of finishes:
- 0 FINISH – Provides for the hot rolling of the tape or sheet of stainless steel and the reactive heat treatment. The pickling it’s not provided.
- 1 FINISH – is the relative degree to plates and hot rolled strip. It is obtained by hot rolling followed by an annealing heat treatment, recrystallization or solubilization, depending on whether, respectively, of martensitic steels, ferritic or austenitic.
Both the scale formed by the hot rolling, both the one formed due to the subsequent heat treatment, must be eliminated by a suitable pickling. The appearance of this degree of finish is silvery gray, dull and poor surface smoothness.
- 2D FINISH – The letter D means “dull” that is opaque. This degree is achieved by cold rolling of sheet and strip already hot rolled, annealed and pickled. The cold rolling involves, obviously, a hardening of the stainless steel which will be the higher, the greater will be the reduction made to undergo the sheet or tape and/or in function of the type of steel.
It follows that, downstream of this cold working, must re-homogenize and anneal the material, after having suitably eliminated the cold rolling oil. This new annealing still produce a flake and you will need a pickling.
The surface which is obtained is not yet reflective, but certainly more polished finish of 1, thanks to the cold rolling suffered; Furthermore, the sheet thickness or the tape is much more constant and with tolerances which may also be very restricted.
- 2B FINISH – The letter B stands for “bright” that is brilliant. It gets you on plates or 2D finish tapes using a cold lamination film-mill “skinpass” equipped with two shiny cylinders.
This compact machining the surface, but does not cause appreciable reductions of the thickness and, consequently, the work hardening is almost nil.
The 2B finish has a brilliant silvery-gray look and it is certainly the most widely used for sheets and ribbons. It also allows a more rapid polishing and with best results of the 2D finish.
- BA FINISH – This designation is the word “bright annealed,” that is “bright annealing”.
It is a finish rolling and cold-rolled strip, designed to annealing heat treatment, recrystallization or solubilization (depending on whether it is martensitic, ferritic or austenitic), in an inert atmosphere after rolling and the subsequent degreasing.
This heat treatment in total absence of oxygen, causes the stainless steel bait from the oven in the same conditions in which there is joined, without any surface alteration.
Preventing the formation of surface oxides, it is missing the decromizzazione then highlighted by pickling. It is therefore lacking any possible alteration on the most important alloying elements: chromium, whose surface concentration must therefore equal to that inside of the thickness.
Next to “standard” finishing the of steel mill products (especially sheets and strips), there are also “non-standard” finishing, that aren’t unified, that steel mills can provide depending on the market needs. These finishes, always for lamination, are obtained with special cylinders that allow to realize, through the incision, different aspects on the sheet, like sandblasting, brushing, embossing, etc.
Table 6 – Designation and matching of different finishes by lamination according to UNI, AFNOR, AISI, ASTM, BSI e DIN.
FINISHES BY ABRASION
This kind of finish is made on special stainless steels, using abrasive of suitable grain size.
The succession of these operations must be such as to provide the use of abrasive with more and more fineness for finishes ranging from those most rough (lower reflectivity) and those most smooth (higher reflectivity).
Table 7 shows descriptions of standard finishes by abrasion, with their correspondences with UNI, AFNOR, AISI, ASTM and DIN.
It is important to note that the grit of abrasive must always be uniform (to avoid scratches) and that the abrasive must be absolutely free from any contaminant, expecially iron, to avoid spotting and real phenomena of corrosion of the stainless steel.
Table 7 – Designation and matching of different finishes by abrasion according to UNI, AFNOR, AISI, ASTM, BSI e DIN.
It’s important to recall the concept of “roughness” as defined by the legislation.
The roughness of a profile, for a generic material, depends on the surface irregularities, namely the deviations of the actual surface respect to the nominal (ideal geometric surface area). Such irregularities have nature of various kinds, essentially due to the action of the machine tool or to the action of the same tools that have worked the metal.
The surface therefore presents a series of “peaks” and “valleys”, as can be seen in the following figure.
In this scheme, are given some well-defined parameters:
- detected profile: line that identifies a good approximation of the actual area; it is usually detected by instruments;
- laverage line of the profile: line that divides the detected profile in such a way that the sum of the areas above is equal to the sum of the areas below the same;;
- base length: length of the detected profile, choosen for the assessment of the roughness.
In the figure there are two cartesian axes: the X axis coincides with the center line while, on the ordinate, the different values of Y represent the deviation of each point from the mean line of the detected profile.
The Ra roughness, therefore, is defined as the sum of the ordinates with respect to the average line, computed in absolute value, ie:
where n is the number of ordinates.
The parameter Ra, whose unit of measurement is the micron (μm), is the most widely used and recognized by all world standards for surface roughness.
However, there are several other parameters to make this measurement as Rmax defined, in microns, as the distance of the highest point and the lowest point of the profile detected; or Rtm defined as the mean value of the detected roughness Rmax of 5 of adjacent base lengths.
In addition to these, there are also other important parameters to completely identify a surface profile: these do not refer to the “vertical” variations of the detected profile than the average line (as seen to define Ra, Rmax, Rtm), but to the “horizontal phenomena“, that is, relative to the existing spacing between the various peaks (λa, λq, Δa, Aq).
From what illustrated above, it can be noted that it is not possible to express a close relationship between roughness and type of the surface treatment of a specific metal material, given that this ratio depends on many factors.
Then, it is important to note that the foregoing as “degree of finish” for stainless steels, refers to considerations made in legislation. It’s therefore intuitive that these directions are approximate and may happen to find on the market two sheets or two tapes with surface finishes designated with the same letters (for example: 4), but with slightly dissimilar aspects, due to different roughness values obtained: although if they are included in the “range” in the legislation.
If, for example, will lead to roughness tests on three different samples, in the direction perpendicular to the rolling, the surface finish identified with the same symbol, you can see three different types of roughness, and this confirms that it is not possible to find an exact correspondence between the roughness and the type of finish.
If you want to accomplish a particular surface appearance, especially in finishes by abrasion, it is advisable to refer, in addition to the legislation, to sample sets, of which it is known the cycle of the processing.
Of course, there are other types of surface treatments that can be performed on stainless steel, depending on the performance and aesthetic needs requests to the various products.
We quote below some of the most used technologies:
This technique is interesting for objects with a particular shape, where the mechanical finishing is uncomfortable, because of the difficult accessibility of some parts, or where there is a high “empty/full” ratio, as, for example, in grids.
Big advantages it also presents for those elements of considerable “responsibility”, both aesthetically and functional (high-value tableware, surgical instruments, electronic components, etc.).
The articles are immersed in a bath of electrolyte solution where the article to be polished constitutes the anode of a cell for electrolysis, while the cathode is usually made from a copper plate or stainless steel AISI 304 or, sometimes, lead.
It is used only for parts with a small size, produced in large series.
In vessels (tumblers) are housed objects to be finished together with appropriate amounts of abrasive.
Usually, the ratio between the weights of the articles to be finished and of the abrasive ranges from 1:3 to 1:5; the filling ratio of the tumbler with the set of abrasive and objects is determined so as to not occupy more than 50÷60% of the entire volume.
The mixture is loaded dry or with liquids such as water, soap and water, etc. The movement of the tumbler is rotated around its axis, usually combined with appropriate vibrational stresses.
Abrasives (in natural or synthetic suitable form) must be intended only for stainless steel work: we must therefore ensure that they do not contaminate, with less noble material, the material being processed (typically carbon steel).
The stainless steel coloring can be achieved by immersion in an aqueous acid solution.
With the same concentration of acid in the bath, the variation of the parameters “temperature” and “soak time” allows you to create different colors, ranging from bronze to blue, to red, to green, with several intermediate shades.
The technology of realization involves three essential steps: surface preparation, coloring and fixing. This process results in a surface conversion phenomenon, since on the metal takes place the formation of subsequent oxide layers.
Thus, the different color appears for “interference”. It is caused by the existing phase difference between the reflected light beams from the different surfaces of formed oxide in the treatment and those reflected instead by the underlying metal surface.
PAINTING AND GLAZING
Although these technologies are not very suitable on stainless steels, they can be used with good aesthetic and functional success, following a careful surface preparation with a thorough degreasing and with mechanical activation of the surface itself.
The most widely used paints are those based on epoxy resins that have a good anchorage.
You should follow the directions of the manufacturers of paint or varnish and predict, when required, the use of primer before performing the actual varnishing.
As for the enamelling, enamels are used dry or in aqueous suspension (100 parts of enamel and 40 parts of water) with heating in the oven (neutral atmosphere or oxidized) at 800÷820 °C for about 3 minutes.