Stainless Steel - General Information - Surface Finishes & Polishing
Surface Finishes & Polishing
Surface finish is an important element in any specification of stainless steel regardless of the intended use. For those applications where appearance is important, finish is a design element and must be specified. In non-decorative applications the surface finish may have implications for friction, wear, maintenance or corrosion resistance and must, therefore, also be carefully chosen and clearly specified.
Types of Finish Stainless steel is available in a wide variety of standard and special finishes. The majority of finishes can be divided into three categories: ~Mill finishes ~ Mechanically polished finishes ~ Special finishes In each case the finishes are described under either their appropriate Standard or the name by which they are commonly known. In some instances a finish may commonly be known by several names, leading to some misunderstandings.
Finishes and Design
There are a wide range of decorative finishes available; therefore, it is important to pay close attention to the selection of the most appropriate finish for the application required. For highly visible applications the appearance of stainless steel is a critical design element and a misunderstanding of the wrong finish can alter the desired effect. In commercial and hygienic applications, such as restaurants and hospitals, properly finished stainless steel is easier to keep clean. In consumer products, such as catering equipment, the lustre from a well polished sheet of stainless steel has strong sales appeal.
In addition to the visual appearance of polished stainless steel there are a number of functional considerations. In sanitary applications correctly polished stainless steel not only looks good but it helps to reduce the risk of bacteria being retained by the material.
In aggressive environments, such as in the nuclear or offshore industries, a correctly polished stainless steel surface has a better resistance to corrosion than a surface that is roughly or badly polished. A smooth surface is less susceptible to an accumulation of deposits and stainless, which often become focal points for localised corrosion. All stainless steel finishes perform better when cleaned and maintained and details of correct cleaning procedures may be found in our publication, “The Cleaning and Maintenance of Stainless Steel”.
Finishes and Fabrication
Some fabrication operations, such as grinding prior to painting or gluing, may require a rough surface finish but, generally speaking, a smooth, well finished sheet requires less physical effort than a coarse, rough one when it comes to blending. Certain finishes are more difficult to recreate by hand than others, causing fabrication difficulties; this is especially true of the “special” finishes which cannot be easily replicated in a fabrications workshop.
The surface of stainless steel is actually and extremely thin but stable and passive Chromium rich oxide film, on which Stainless Steel relies for its excellent corrosion resistance. The surface finish on Stainless Steel should therefore be developed and maintained to ensure this vital property, and also for the secondary reason of the pleasing aesthetic appearance of stainless steel. Standard Mill Finished - Stainless Steel Flat Rolled Products
The Standard Mill Surface Finished are laid down in Specifications BS 1449, Part 4, and the Committee of Stainless Steel Producers, American Iron & Steel Institute.
The finished are designated by a system of numbers, and these are broadly described hereunder relative to the finishing operations employed. It should be remembered that different grades of Stainless Steel can result in a variation of visual appearance for the same finishing operation. The thickness can also have an effect, generally the thinner the material the smoother the surface finish.
The thicker gauge sizes of Stainless Steel are hot rolled. This is done at high temperatures and will always result in a scaled surface. Stainless Steel Flat Product is supplied in the annealed ie fully softened condition.
This is also a high temperature operation and unless carried out in a very closely controlled inert atmosphere, will result in oxidation (scaling) of the surface.
The scale is usually removed by a pickling process, that is the removal of the scale by use of suitable acids, and the passivated by the use of Nitric Acid.
No. 0 Finish HRA
Also referred to as Hot Rolled Annealed (HRA). The plate is hot rolled to required thickness, and then annealed. No pickling or passivation operations are effected, resulting in a scaled black finish.
This does not develop the fully corrosion resistant film on the Stainless Steel, and except for certain high temperature heat resisting applications, this finish is unsuitable for general end uses.
No 1 Finish Plate is hot rolled, annealed, pickled and passivated. This results in a dull, slightly rough surface; quite suitable for industrial applications which generally involve the range of plate thicknesses. Grinding marks may be visible in isolated areas.
Some of the thinner thicknesses within the plate range are Cold Rolled; but Sheet, Coil and Strip gauge are produced by Cold Rolling, ie rolled without and heating of the material. Cold rolling hardens the material, and the thinner sizes may have to be subjected to an intermediate annealing and pickle, or bright annealed, during the reduction of thickness to final gauge.
The starting material for Cold Rolling always has a No. 1 finish. Cold Rolled material is supplied with the following standard mill finishes.
No 2D Finish A no. 1 Finish after being Cold Rolled, Annealed, Pickled and Passivated. This results in a uniform dull matt finish, superior to a No. 1 Finish.
Suitable for industrial application, and eminently suitable for severe deep drawing as the dull surface, (which may be polished after fabrication) retains the lubricant during the drawing operation.
No 2B Finish A 2D Finish is given a subsequent light skin pass cold rolling operation between polished rolls.
This is the most common finish produced and called for on sheet material. It is brighter than 2D and is semi-reflective. It is commonly used for most deep drawing operations, and is more easily polished to the final finished required than is a 2D finish.
No 2 BA Finish This is more commonly referred to as a BRIGHT ANNEALED (BA) FINISH. Material with a No. 1 finish is Cold Rolled using highly polished rolls in contact with the steel surface. This smooths and brightens the surface.
The smoothness and reflectiveness of the surface improves as the material is rolled to thinner and thinner sizes. Any annealing which needs to be done in order to effect the required reduction in gauge, and the final anneal, is effected in a very closely controlled inert atmosphere. No oxidation or scaling of the surface therefore occurs and there is no need for additional pickling and passivating.
The final surface developed can have "MIRROR" type finish similar in appearance to the highly polished No. 7 and No. 8 Finishes.
Note Much of the 2B Finish sheet imported is not a true 2B Finish. Mills which operate bright annealing facilities will often carry out all the annealing operations of Cold Rolled material in such facilities. This leads to a superior "2B" finish as no oxidation or scaling takes place during the annealing operation, even though the actual rolling may be effected on polished rolls as for normal 2B Finish, but not highly polished as would be needed to produce a BA finish.
The following finishes are all mechanically produced polished finishes. As well as being standard mill finishes, they are also applied to stainless steel articles and components to meet the required aesthetic criteria. It should be appreciated that factors such as hand polishing vs. mechanical polishing; polishing a flat product as against a component of complex shape; thickness and composition of material can affect the visual appearance of the final surface.
No. 3 Finish This is a ground unidirectional uniform finish obtained with 80 - 100 grit abrasive.
It is a good intermediate or starting surface finish for use in such instances where the surface will require further polishing operations to a finer finish after subsequent fabrication or forming.
No. 4 Finish This is a ground unidirectional finish obtained with 150 grit abrasive. It is not highly reflective, but is a good general purpose finish on components which will suffer from fairly rough handling in service (eg restaurant equipment).
No. 6 Finish These finishes are produced using rotating cloth mops (Tampico fibre, muslin or linen) which are loaded with abrasive paste.
The finish depends on how fine and abrasive is used, the uniformity and finish of the original surface.
The finish has a non-directional texture of varying reflectiveness. "Satin Blend" is an example of such a finish.
No. 7 Finish This is a buffed finish having a high degree of reflectiveness.
It is produced by progressively using finer and finer abrasives and finishing with Buffing compounds. Some fine scratches (grit lines) may remain from the original starting surface.
No. 8 Finish This is produced in an equivalent manner to a No. 7 Finish, the final operations being done with extremely fine buffing compounds.
The final surface is blemish free with a high degree of image clarity, and is the true mirror finish.
Note The finer polished finishes (No. 4, No. 6, No. 7 and No. 8) are generally only produced one side of the sheet, the reverse side being either a 2B or No. 3 Finish.
Polishing Precision Stainless Steel Tubing
Procedure: Electropolishing and Mechanical polishing .
Definition: Stainless steel pipe polishing process is actually the inside or outside surface of the pipe polishing process, using polishing equipment and the friction of pipes surface, to meet the roughness requirement.
Type: Stainless steel tube of polishing with internal surface polishing and external polishing; Dry and Wet
Grade: Polishing grade : the outer surface of # 400 , # 600, brushed , Silk hair
Difference between Wet bright and Dry Bright: Grinding resistance: light resistance and abrasive grinding machinery, the rigidity of the workpiece support system , but also with grinding in the grinding temperature, vibration , or a relationship , but also affect the life of grinding tools , grinding the surface of the characters .
Grinding temperature: grinding thermal deformation temperature would reduce dimensional accuracy , surface grinding process also affected the metamorphic layer
Electropolishing features: Electro polishing is the use of stainless steel in the electrolyte solution in the selectivity of the anode to achieve the purpose of polishing and cleaning the surface of a surface treatment .
Advantages: 1. Greatly improved corrosion resistance . Since electropolishing selective dissolution of the elements , making a solid surface of chromium-rich layer of dense solid transparent membrane , and formation of electric surface , eliminating and reducing micro-cell corrosion .
2. Micro- electrolytic polishead surface is more smooth than the mechanical polishing , reflecting a higher rate . This makes the device non-stick wall , not hanging material , easy to clean surface about a reduction of approximately one wire , to GMP and FDA regulatory requirements .
3. Electrolytic polishing from the workpiece size and shape restrictions. Mechanical polishing on the appropriate implementation of the workpiece can be polished , for example, thin tube wall , elbow , bolts , nuts and containers inside and outside the wall .
Electropolishing superior than mechanical polishing :
A. a surface potential conditions; B. difficult to capture pollutants; C. surface can form a more solid salt corrosion resistance of the transparent membrane; D. does not produce Beillby layer .
Electrochemical polishing characteristics:
Electrochemical polishing is the use of electrochemical anodic dissolution of metal grinding polishing principle . The electrochemical pre- polishing and fine polishing machine together organically , playing two types of electrochemical polishing expertise and institutions . It is not material hardness and toughness of restrictions , complex shapes can be polished workpiece . The method and electrolytic grinding similar .
Related References: 1. ISO 1302 DIN 4768 Comparison of Surface Roughness Values 2. Electrical Discharge Machining EDM Roughness Comparator 3. Polishing Polished Seamless Stainless Steel Pipe Tube 4. Stainless Steel Tubes Surface Roughness Review 5. Stainless Steel Bright Annealing/Annealed Tubes 6. China USA Surface Roughness Standard Comparison 7. Surface Roughness Conversion Chart of Stainless Steel Tube 8. Stainless Steel Tube Surface Finish Types 9. Cleaning of Stainless Steel Tubes 10. Post weld cleaning and finishing of stainless steels 11. Care and maintenance of stainless steel 12. British and American standards for tolerances, surface finish and testing of stainless steel 13. Surface Finish Degree Comparison Table between Rz Ra RMS
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Stainless Steel Pipes Specifications
ASTM A312: Seamless and straight-seam welded austenitic pipe intended for high temperature and general corrosive service. Filler metal not permitted during welding.
ASTM A358: Electric fusion welded austenitic pipe for corrosive and/or high temperature service. Typically only pipe up to 8 inch is produced to this specification. Addition of filler metal is permitted during welding.
ASTM A790: Seamless and straight-seam welded ferritic/austenitic (duplex) pipe intended for general corrosive service, with a particular emphasis on resistance to stress corrosion cracking.
ASTM A409: Straight-seam or spiral-seam electric fusion welded large diameter austenitic light-wall pipe in sizes 14” to 30” with walls Sch5S and Sch 10S for corrosive and/or high
ASTM A376: Seamless austenitic pipe for high temperature applications.
ASTM A813: Single-seam, single- or double- welded austenitic pipe for high temperature and general corrosive applications.
ASTM A814: Cold-worked welded austenitic pipe for high temperature and general corrosive service.
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