Nigel Willcock, Technical Sevice Engineer at 3M Abrasive Systems Division, discusses the use of Metal Finishing Products to achieve the desired finish for stainless steel
The beauty and advantages of stainless steel are widely appreciated. Resistant to staining and corrosion, stainless steel is not in fact a single metal or even alloy, but rather a family of iron alloys, the properties of which vary according to the balance of elements used. Different types of stainless steel are suited to differing applications. All stainless steel must contain at least 10.5 per cent chromium and 1.2 per cent carbon; however, the exact proportions vary according to type and other elements may be added. The addition of nickel results in austenitic stainless steels, which are the most commonly used.
Stainless steel owes many of its benefits to the thin coating of chromium oxide on its surface, which is passive. This coating protects the bulk metal beneath and even when the surface layer is breached, for example by scratching or removal, it ‘self-heals’ and re-establishes itself to protect the metal once more. The chromium oxide layer is generally between one and ten microns thick, and the smoother the surface, the more compact and durable the passive layer will be. Hence, much stainless steel is finished with a view to maximising the benefits of the passive layer, and is therefore abraded, smoothed and/or polished to a level appropriate for the use that the item will eventually be put to.
Stainless steel is used in a wide range of settings, including professional and domestic kitchens, food and beverage packaging, cookware, cutlery and household goods, architectural, surgical instruments, major appliances and pharmaceutical purposes. For some of these, such as pharmaceutical, food preparation, food storage and surgical applications, the smoothness of the finish given to the stainless steel is important because it minimises contamination and makes the items easier to clean. In other settings, such as household fittings and architectural environments, finishing may be primarily for aesthetic purposes.
The finish of stainless steel also affects its ease of use, for example in food preparation the ability of stainless steel to withstand constant cleaning must be optimised, and this is largely achieved through finishing.
Non-metallic abrasive products
Other factors affect the finishing of stainless steel, too. These include manufacturing processes such as welding and cutting. When stainless steel is welded, the process leaves an oxidised surface in the form of discolouration on the weld that must be cleaned if the weld is not removed. Traditionally, processes such as pickling (a form of cleaning with strong acids) and wire brushing have been used, but these can be hazardous and unpleasant for the people involved, or increase the risk of damaging the passive surface of the stainless steel beyond its ability to heal itself – for example when metal filaments from wire brushes become embedded in the surface of the stainless steel. Scotch-Brite Radial Bristle Brushes by 3M are non-metallic abrasive products that will clean the weld quickly and efficiently without risk of contamination or flying wires leaving a cleaned surface.
When the weld is removed, the aggression and the nature of the product needed to achieve this varies according to context. In some cases, such removal can itself compromise the passivity of the surface layer, because it if done with a traditional abrasive it creates a jagged surface, but the use of Scotch-Brite Metal Finishing Products can overcome this.
Cutting stainless steel, for example to form specific shapes for manufacturing purposes, often causes burring on the underside of the piece. This can be dangerous to those handling and ultimately using the item, so such items must be deburred using abrasive processes. Scotch-Brite Deburring Wheels will remove burrs leaving a rounded edge that is safe to handle.
Recognising the need for versatile, highly effective and user-friendly abrasives with which to finish stainless steel, 3M has developed the Match and Finish system that comprises of an electric in-line sander with a range of coated abrasive and Scotch-Brite Products that help users to achieve the appropriate finish for virtually any final use.
Most stainless steel is given one of the following five finishes:
- 2A, bright annealed – the result of heat treatment, this is a bright, mirror-like finish
- 2B, cold rolled, heat treated, pickled, skin passed – the most common cold treatment, this produces a non-reflective,flat, smooth surface
- 2G (3B in the UK), ground – this has a uni-directional pattern and is not very reflective.2J, brushed or dull polished – this is smoother, with a uni-directional texture, again it is not very reflective
- 2K, satin polish – this gives a smooth, non-reflective finish
Getting the best finish
When finishing stainless steel, attention to detail can make all the difference. Any scratches applied during the finishing process should be parallel, both with the edge of the item and with each other. There should be no start/stop marks that indicate whereabouts on the item the finishing process began and ended, and the scratch pattern should be free of any defects and underlying scratches.
When using the 3M Match and Finish system to optimise straight line patterns, it can be helpful to apply pressure while at the same time making a pulling motion. If the operator stands behind the machine, the turning motion will be limited and so will the curved scratches. To eliminate stop/start marks use a gradual on/off movement, or place a metal part of the same thickness before and after the piece being worked on.
The Match and Finish Tool and appropriate 3M consumables produce superb results in all relevant processes, namely:
- Removal of small welds and/or defects
- Refinement of rotary grind marks and creation of linear marks
- Refinement of deep scratches
- Further refinement (if required by the nature of the finish)
- Blending of scratches (for 2K-type finishes)
Scotch-Brite products offer ease of use and uniform, consistent results with no undercutting. This means less reworking and fewer rejects, and good levels of productivity. The range is wide and offers a large number of options, meaning that users can find the combination of abrasives to achieve precisely the result desired, and achieve those results time after time.
For more information about Scotch-Brite products, visit www.3M.co.uk/abrasives.