[ Wiki ]What surface finishes are available/application do i choose on stainless steels?

What surface finishes are available on stainless steels?

There are many different types of surface finish on stainless steel. Some of these originate from the mill but many are applied later during processing, for example polished, brushed, blasted, etched and coloured finishes.

The importance of surface finish in determining the corrosion resistance of the stainless steel surface cannot be overemphasised. A rough surface finish can effectively lower the corrosion resistance to that of a lower grade of stainless steel.

The European standards for stainless steels have attempted to define the most common surface finishes. However, due to the proprietary nature of many suppliers’ finishes, it is unlikely that complete standardisation is possible. This is a summary of the most common types for each product form

Common Surface Finishes for Flat Products from EN 10088-2 (for full list see Specifying finishes for stainless steel flat products (sheet and plate)

Surface Finish Code		Description
Mill finishes
1D		Hot rolled, heat treated, pickled. The most common hot rolled finish. A non reflective, rough surface. Not normally used for decorative applications
2B		Cold rolled, heat treated, pickled, pinch passed. The most common cold rolled mill finish. Dull grey slightly reflective finish. Can be used in this condition or frequently is the starting point for a wide range of polished finishes.
2D		Cold rolled, heat treated, pickled.
2H		Work hardened by rolling to give enhanced strength level. Various ranges of tensile or 0.2% proof strength are given in EN 10088-2 up to 1300 MPa and 1100 MPa respectively dependent on grade
2Q		Cold rolled hardened and tempered. Applies to martensitic steels which respond to this kind of heat treatment.
2R		Cold rolled and bright annealed, still commonly known as BA. A bright reflective finish. Can be used in this condition or as the starting point for polishing or other surface treatment processes e.g. colouring
	In the following codes “1” refers to hot rolled being the starting point and “2” as cold rolled
Special Finishes
1G or 2G		Ground. Relatively coarse surface. Unidirectional. Grade of polishing grit or surface roughness can be specified
1J or 2J		Brushed or dull polished. Smoother than 1G/2G. Grade of polishing grit or surface roughness can be specified
1K or 2K		Satin polish. Similar to 1J/2J but with maximum specified Ra value of 0.5 micron. Usually achieved with SiC polishing belts. Alumina belts are strongly discouraged for this finish as this will have detrimental effect on corrosion resistance. Recommended for external architectural and coastal environments where bright polish (1P/2P) is not acceptable.
1P/2P		Bright polished. Non-directional, reflective. Can specify maximum surface roughness. The best surface for corrosion resistance.
2L		Coloured by chemical process to thicken the passive layer and produce interference colours. A wide range of colours is possible.
1M/2M		Patterned. One surface flat.
1S/2S		Surface coated e.g. with tin = Terne coating
2W		Corrugated. Similar to patterned but both surfaces are affected
Bead blasting		Not in EN 10088-2. Work being undertaken to more accurately define finishes.

How do I choose which stainless steel to use?

Most decisions about which steel to use are based on a combination of the following factors:

1. What is the corrosive environment? – Atmospheric, water, concentration of particular chemicals, chloride content, presence of acid.
2. What is the temperature of operation? – High temperatures usually accelerate corrosion rates and therefore indicate a higher grade. Low temperatures will require a tough austenitic steel.
3. What strength is required? – Higher strength can be obtained from the austenitic, duplex, martensitic and PH steels. Other processes such as welding and forming often influence which of these is most suitable. For example, high strength austenitic steels produced by work hardening would not be suitable where welding was necessary as the process would soften the steel.
4. What welding will be carried out? - Austenitic steels are generally more weldable than the other types. Ferritic steels are weldable in thin sections. Duplex steels require more care than austenitic steels but are now regarded as fully weldable. Martensitic and PH grades are less weldable.
5. What degree of forming is required to make the component? – Austenitic steels are the most formable of all the types being able to undergo a high degree of deep drawing or stretch forming. Generally, ferritic steels are not as formable but can still be capable of producing quite intricate shapes. Duplex, martensitic and PH grades are not particularly formable.
6. What product form is required? – Not all grades are available in all product forms and sizes, for example sheet, bar, tube. In general, the austenitic steels are available in all product forms over a wide range of dimensions. Ferritics are more likely to be in sheet form than bar. For martensitic steels, the reverse is true.
7. What are the customer’s expectations of the performance of the material? – This is an important consideration often missed in the selection process. Particularly, what are the aesthetic requirements as compared to the structural requirements? Design life is sometimes specified but is very difficult to guarantee.
8. There may also be special requirements such as non-magnetic properties to take into account.
9. It must also be borne in mind that steel type alone is not the only factor in material selection. Surface finish is at least as important in many applications, particularly where there is a strong aesthetic component. See Importance of Surface Finish.
10. Availability. There may be a perfectly correct technical choice of material which cannot be implemented because it is not available in the time required.
11. Cost. Sometimes the correct technical option is not finally chosen on cost grounds alone. However, it is important to assess cost on the correct basis. Many stainless steel applications are shown to be advantageous on a life cycle cost basis rather than initial cost. See Life Cycle Costing.

The final choice will almost certainly be in the hands of a specialist but their task can be helped by gathering as much information about the above factors. Missing information is sometimes the difference between a successful and unsuccessful application. See also General principles for selection of stainless steels