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Some imagery shown of the CyberTruck, shows some shiny smooth surfaces.
Let's see what the final product surface finish will show.
2B, 2D and BA Cold Rolled Finishes What are Cold Rolled Finishes?
Cold rolled finishes apply to flat products such as sheet or coil, with thickness less than about 5mm and usually less than 3mm. They are firstly hot rolled into a strip (or cast into a slab which is hot rolled into a strip) and then cold rolled. Cold rolling reduces the thickness by at least 50%. This smooths the surface, refines the grain structure and causes differences in the mechanical properties along and across the rolling direction. In the case of austenitic and duplex alloys, the process hardens and strengthens the stainless steel. Finally, the steel is softened by annealing in a furnace? Each surface finish may undergo additional processes to improve the surface further. The smoother the surface finish is, the higher resistance to corrosion it will be.
These mill produced finishes must be handled carefully as surface damage such as scratches, grinding marks or spatter cannot be matched by polishing with abrasives or etching with chemicals. Of the 3 finishes, BA is most nearly able to be matched by a mirror polish.
Standards
The most common definitions of these surface finishes are provided by ASTM A480 and EN 10088. In both cases it is the cold rolled manufacturing method that is specified rather than specific, measurable characteristics about the surface. We have used ASTM A480 as an example:
ASTM A480:
No. 2D - A smooth, non-reflective cold-rolled annealed and pickled or descaled finish. This non-directional finish is favourable for the retention of lubricants in deep drawing applications.
No. 2B - A smooth, moderately reflective cold-rolled annealed and pickled or descaled finish typically produced by imparting a final light cold-rolled pass using [large diameter] polished rolls. This general-purpose finish is more readily polished than No 1 or 2D finishes. Product with 2B finish is normally supplied in the annealed plus lightly cold-rolled condition unless a tensile-rolled [harder and stronger] product is specified.
Bright Annealed [BA] Finish- A smooth, bright, reflective finish typically produced by cold rolling followed by annealing in a protective atmosphere so as to prevent oxidation and scaling during annealing.
2B
2B is the most widely used stainless steel surface finish. It is especially common in industrial, chemical and food processing applications such as process vessels and tanks. It is also used in some architectural applications that will not be closely examined for uniformity of finish such as downpipes and gutters.
When specifying this finish think about these attributes:
2B is the most economical finish
It is highly corrosion resistant because it has been chemically pickled and is smooth
Over broad areas and between batches etc., it is not uniform and may not match in appearance It has been produced in the mill and can't be matched after fabrication
It if often protected by plastic films until final cleanup and commissioning
2B, 2D and BA Cold Rolled Finishes
2D
2D is used around the world in applications where its low reflectivity is important. The largest application is in roofing materials. The surface is rougher than 2B and retains lubricants better making it appealing for deep drawing. 2D surfaces are not designed for appearance so the limitations on matching of weld and other surface damage is not as critical. Railcars are a typical example where thicker sheet and ongoing abrasive damage make the rougher 2D a suitable finish.
When specifying this finish think about these attributes:
2D is not commonly available in Australia
It is highly corrosion resistant because it has been chemically pickled and is relatively smooth Over broad areas and between batches etc., it is not uniform and may not match
It has been produced in the mill and can't be matched after fabrication
Bright Annealed (BA)
The classic use of a BA finish is domestic: the interior of a dishwasher or clothes washing machine. In the clothes washer, it also provides a smooth, non- abrasive surface for the clothes to slide around. The mirror like surface is also used in road mirrors where a precise image is not required.
When specifying this finish think about these attributes:
BA is common in some grades and thicknesses, but not all
It is highly corrosion resistant because it is very smooth
Different batches may not match
It is has been produced in the mill and can't be exactly matched after fabrication although a mechanical mirror polish can be close.
Post Production Processing to Improve Corrosion Resistance
All the cold rolled processes include a pickling stage. Pickling is the removal of high temperature scale and the adjacent low chromium layer of metal from the surface of stainless steel by chemical means. Pickling will also remove manganese sulphide inclusions and any other contamination on the surface. Pickling results in a very clean, highly corrosion resistant surface, but will slightly roughen the surface.
___________________________________________________________
This article is the final in a series on common finishes. Previous articles in this series dealt with the workhorse No. 4 (2J/2K) (AS36, Winter 2006) and the mill finishes 2R (BA), 2B and 2D (AS38, Summer 2006). This article looks at mirror, profiled and coloured surfaces.
Mirror polished surfaces, as the name implies, have a bright reflective surface which give a mirror like image. They are the most labour intensive mechanically finished surface with an obvious impact on cost and time of production. As flat product, mirror finishes are produced by post mill abrasive polishing of either hot rolled or cold rolled sheet or strip.
Although mirror finishes are only defined in standards for flat products, it is also possible to generate a mirror polish on surfaces where there is sufficient access for the grinders and polishing mops. The exception to this is materials with large or variable microstructures such as some castings or heavily cold worked items or welds. In these cases, it may not be possible to obtain a mirror surface with a clear image or a uniform lustre.
Mirror polished surfaces are produced by grinding with
successively finer abrasives where the next grit size is not selected
until all the scratches produced in the previous stage have been
removed. The surface is then buffed with “mops” (which may be
soft or hard) and sticks compounded with binders and rouge of
whose particle size depends on the required finish. The buffing
stage does not remove much material and, if there are scratches
from earlier grinding stages, they will show up as rounded furrows. Contamination with a larger grit particle will give isolated, but very unsightly scratches.
A mirror finish is the only one that will produce a clear image of its surroundings. Finishes such as the cold rolled BA or an
electropolished 2B or even a No 4 finish with a very low Ra that is then electropolished, will be brilliant and reflective but they will not form a sharp reflected image.
Specifications
Both ASTM A480 and EN10088-2 include mirror finish specifications called No 8 and 2P respectively. Mirror finish is described as a non-directional finish which is reflective and has good image clarity. The surface will be essentially free from grit lines due to the initial grinding stages but there will be visual differences between surfaces produced by different suppliers to these specifications.
For mirror finishes, requiring that the surface has a specific surface roughness (Ra) is not very useful. Gloss measurements are a useful technique on flat surfaces
especially if both diffuse and specular reflection are measured. Not surprisingly, diffuse reflectance is always higher. If a higher level of confidence is required
for critical applications, then agreement on comparison with a finished sample in agreed conditions is recommended. It is the most reliable method of detecting
random scratches.
Applications and Limitations
Mirror finish is most widely used for plates in presses, on the interior surface of moulds and also for small mirrors, reflectors and architectural panels.
The two primary limitations are:
• the most commonly used grades (304 and 316) are quite soft so that any cleaning process must avoid scratching the surface with residual dust or applied cleaning compounds, and
• large flat areas of mirror polish throw scratches, grease or dirt markings into sharp contrast.
The first limitation is overcome by careful cleaning protocols using adequate water rinsing. The effect of random marks and scratches is reduced if the surface is corrugated at the macro level described below when using surfaces that are texturised (single-sided deformation) or embossed/rigidised (through-thickness deformation). However, while mirror sheet may be profiled, it is more common to apply profiling treatments to a cold-rolled sheet.
Profiled or Patterned Surfaces
EN10088-2 uses categories 1M and 2M for sheet material that has been patterned on one side only. The 1M group applies to hot rolled base materials while the much more common 2M applies to cold rolled base materials, usually with a 2B, 2D or 2R (BA) finish. In corrosive environments, it is essential to orient the pattern to allow free drainage.
Through Thickness Deformation
EN10088-2 lists 2W for a cold-rolled sheet that has been cold rolled into a through-thickness corrugated pattern. The cold work strengthens the sheet and may permit the use of the thinner sheet. The rigidity also helps control oil-canning, i.e. local buckling caused by thermal expansion during fixing or temperature changes during the year or day. The surfaces are often described as embossed or rigidised.
Coloured Stainless Steel
Stainless steel can be painted provided that the surface is scrupulously clean and has sufficient profile to mechanically anchor the paint. Using a stainless steel base metal offers the advantage that, even if the coating is damaged, any rusting is superficial and will not lead to long term structural failure as has occurred when protective coatings fail on steel or aluminium structures.
However, painted organic coatings are a clumsy means of colouring stainless steel. If it is simply to be blackened, then the molten sodium dichromate process or a hot sulphuric acid treatment or even an electrodeposited and baked organic coating could offer greater durability.
Other colours are offered by an electrochemical deposition process that deposits and hardens an enhanced oxide film on the stainless steel although usually only on 304. The colour depends on the thickness of the oxide and progresses from blue to gold to mauve and green with time of exposure.
The colour is similar to the interference colours in an oil slick or soap bubble with interference between the light reflected from the top of the oxide and base metal. In theory the colouring could be reproduced on any simple shape but in practice, it is only available on sheets. The coated sheets may be deep drawn, formed, bent and fixed mechanically but welding destroys the coloured film. While the oxide film is harder and more resistant to scratching than the basic 304 substrate, it is still susceptible to mechanical damage and so is not suitable in areas subject to heavy traffic. Corrosion tests on blue coatings on 304 showed slightly improved resistance to atmospheric and acidic chloride exposure over the resistance of 304.
Let's see what the final product surface finish will show.
2B, 2D and BA Cold Rolled Finishes What are Cold Rolled Finishes?
Cold rolled finishes apply to flat products such as sheet or coil, with thickness less than about 5mm and usually less than 3mm. They are firstly hot rolled into a strip (or cast into a slab which is hot rolled into a strip) and then cold rolled. Cold rolling reduces the thickness by at least 50%. This smooths the surface, refines the grain structure and causes differences in the mechanical properties along and across the rolling direction. In the case of austenitic and duplex alloys, the process hardens and strengthens the stainless steel. Finally, the steel is softened by annealing in a furnace? Each surface finish may undergo additional processes to improve the surface further. The smoother the surface finish is, the higher resistance to corrosion it will be.
These mill produced finishes must be handled carefully as surface damage such as scratches, grinding marks or spatter cannot be matched by polishing with abrasives or etching with chemicals. Of the 3 finishes, BA is most nearly able to be matched by a mirror polish.
Standards
The most common definitions of these surface finishes are provided by ASTM A480 and EN 10088. In both cases it is the cold rolled manufacturing method that is specified rather than specific, measurable characteristics about the surface. We have used ASTM A480 as an example:
ASTM A480:
No. 2D - A smooth, non-reflective cold-rolled annealed and pickled or descaled finish. This non-directional finish is favourable for the retention of lubricants in deep drawing applications.
No. 2B - A smooth, moderately reflective cold-rolled annealed and pickled or descaled finish typically produced by imparting a final light cold-rolled pass using [large diameter] polished rolls. This general-purpose finish is more readily polished than No 1 or 2D finishes. Product with 2B finish is normally supplied in the annealed plus lightly cold-rolled condition unless a tensile-rolled [harder and stronger] product is specified.
Bright Annealed [BA] Finish- A smooth, bright, reflective finish typically produced by cold rolling followed by annealing in a protective atmosphere so as to prevent oxidation and scaling during annealing.
2B
2B is the most widely used stainless steel surface finish. It is especially common in industrial, chemical and food processing applications such as process vessels and tanks. It is also used in some architectural applications that will not be closely examined for uniformity of finish such as downpipes and gutters.
When specifying this finish think about these attributes:
2B is the most economical finish
It is highly corrosion resistant because it has been chemically pickled and is smooth
Over broad areas and between batches etc., it is not uniform and may not match in appearance It has been produced in the mill and can't be matched after fabrication
It if often protected by plastic films until final cleanup and commissioning
2B, 2D and BA Cold Rolled Finishes
2D
2D is used around the world in applications where its low reflectivity is important. The largest application is in roofing materials. The surface is rougher than 2B and retains lubricants better making it appealing for deep drawing. 2D surfaces are not designed for appearance so the limitations on matching of weld and other surface damage is not as critical. Railcars are a typical example where thicker sheet and ongoing abrasive damage make the rougher 2D a suitable finish.
When specifying this finish think about these attributes:
2D is not commonly available in Australia
It is highly corrosion resistant because it has been chemically pickled and is relatively smooth Over broad areas and between batches etc., it is not uniform and may not match
It has been produced in the mill and can't be matched after fabrication
Bright Annealed (BA)
The classic use of a BA finish is domestic: the interior of a dishwasher or clothes washing machine. In the clothes washer, it also provides a smooth, non- abrasive surface for the clothes to slide around. The mirror like surface is also used in road mirrors where a precise image is not required.
When specifying this finish think about these attributes:
BA is common in some grades and thicknesses, but not all
It is highly corrosion resistant because it is very smooth
Different batches may not match
It is has been produced in the mill and can't be exactly matched after fabrication although a mechanical mirror polish can be close.
Post Production Processing to Improve Corrosion Resistance
All the cold rolled processes include a pickling stage. Pickling is the removal of high temperature scale and the adjacent low chromium layer of metal from the surface of stainless steel by chemical means. Pickling will also remove manganese sulphide inclusions and any other contamination on the surface. Pickling results in a very clean, highly corrosion resistant surface, but will slightly roughen the surface.
___________________________________________________________
This article is the final in a series on common finishes. Previous articles in this series dealt with the workhorse No. 4 (2J/2K) (AS36, Winter 2006) and the mill finishes 2R (BA), 2B and 2D (AS38, Summer 2006). This article looks at mirror, profiled and coloured surfaces.
Mirror polished surfaces, as the name implies, have a bright reflective surface which give a mirror like image. They are the most labour intensive mechanically finished surface with an obvious impact on cost and time of production. As flat product, mirror finishes are produced by post mill abrasive polishing of either hot rolled or cold rolled sheet or strip.
Although mirror finishes are only defined in standards for flat products, it is also possible to generate a mirror polish on surfaces where there is sufficient access for the grinders and polishing mops. The exception to this is materials with large or variable microstructures such as some castings or heavily cold worked items or welds. In these cases, it may not be possible to obtain a mirror surface with a clear image or a uniform lustre.
Mirror polished surfaces are produced by grinding with
successively finer abrasives where the next grit size is not selected
until all the scratches produced in the previous stage have been
removed. The surface is then buffed with “mops” (which may be
soft or hard) and sticks compounded with binders and rouge of
whose particle size depends on the required finish. The buffing
stage does not remove much material and, if there are scratches
from earlier grinding stages, they will show up as rounded furrows. Contamination with a larger grit particle will give isolated, but very unsightly scratches.
A mirror finish is the only one that will produce a clear image of its surroundings. Finishes such as the cold rolled BA or an
electropolished 2B or even a No 4 finish with a very low Ra that is then electropolished, will be brilliant and reflective but they will not form a sharp reflected image.
Specifications
Both ASTM A480 and EN10088-2 include mirror finish specifications called No 8 and 2P respectively. Mirror finish is described as a non-directional finish which is reflective and has good image clarity. The surface will be essentially free from grit lines due to the initial grinding stages but there will be visual differences between surfaces produced by different suppliers to these specifications.
For mirror finishes, requiring that the surface has a specific surface roughness (Ra) is not very useful. Gloss measurements are a useful technique on flat surfaces
especially if both diffuse and specular reflection are measured. Not surprisingly, diffuse reflectance is always higher. If a higher level of confidence is required
for critical applications, then agreement on comparison with a finished sample in agreed conditions is recommended. It is the most reliable method of detecting
random scratches.
Applications and Limitations
Mirror finish is most widely used for plates in presses, on the interior surface of moulds and also for small mirrors, reflectors and architectural panels.
The two primary limitations are:
• the most commonly used grades (304 and 316) are quite soft so that any cleaning process must avoid scratching the surface with residual dust or applied cleaning compounds, and
• large flat areas of mirror polish throw scratches, grease or dirt markings into sharp contrast.
The first limitation is overcome by careful cleaning protocols using adequate water rinsing. The effect of random marks and scratches is reduced if the surface is corrugated at the macro level described below when using surfaces that are texturised (single-sided deformation) or embossed/rigidised (through-thickness deformation). However, while mirror sheet may be profiled, it is more common to apply profiling treatments to a cold-rolled sheet.
Profiled or Patterned Surfaces
EN10088-2 uses categories 1M and 2M for sheet material that has been patterned on one side only. The 1M group applies to hot rolled base materials while the much more common 2M applies to cold rolled base materials, usually with a 2B, 2D or 2R (BA) finish. In corrosive environments, it is essential to orient the pattern to allow free drainage.
Through Thickness Deformation
EN10088-2 lists 2W for a cold-rolled sheet that has been cold rolled into a through-thickness corrugated pattern. The cold work strengthens the sheet and may permit the use of the thinner sheet. The rigidity also helps control oil-canning, i.e. local buckling caused by thermal expansion during fixing or temperature changes during the year or day. The surfaces are often described as embossed or rigidised.
Coloured Stainless Steel
Stainless steel can be painted provided that the surface is scrupulously clean and has sufficient profile to mechanically anchor the paint. Using a stainless steel base metal offers the advantage that, even if the coating is damaged, any rusting is superficial and will not lead to long term structural failure as has occurred when protective coatings fail on steel or aluminium structures.
However, painted organic coatings are a clumsy means of colouring stainless steel. If it is simply to be blackened, then the molten sodium dichromate process or a hot sulphuric acid treatment or even an electrodeposited and baked organic coating could offer greater durability.
Other colours are offered by an electrochemical deposition process that deposits and hardens an enhanced oxide film on the stainless steel although usually only on 304. The colour depends on the thickness of the oxide and progresses from blue to gold to mauve and green with time of exposure.
The colour is similar to the interference colours in an oil slick or soap bubble with interference between the light reflected from the top of the oxide and base metal. In theory the colouring could be reproduced on any simple shape but in practice, it is only available on sheets. The coated sheets may be deep drawn, formed, bent and fixed mechanically but welding destroys the coloured film. While the oxide film is harder and more resistant to scratching than the basic 304 substrate, it is still susceptible to mechanical damage and so is not suitable in areas subject to heavy traffic. Corrosion tests on blue coatings on 304 showed slightly improved resistance to atmospheric and acidic chloride exposure over the resistance of 304.
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