Safe specificiation of polished finishes

Safe specification of polished finishes on Stainless Steel Sheet and Plate.

Peter Davies; Professional Polishing Services Ltd.

Almost every industry has a regime for technical appraisal and measurement of its product or process to ensure repeatability and suitability for purpose.

In terms of grade specification, Stainless Steel is covered by British and European standards so that users can ascertain the grade which will perform best in given circumstances. This may be in terms of corrosion resistance, formability, welding, tensile strength etc., etc. Producing Mills and the advisory service at the British Stainless Steel Association all provide help and advice on the selection of material.

In contrast, there is little help from standards when it comes to specifying surface finishes. BS EN 10088-2 fails to cover the subject definitively as there is only one finish which has any form of surface measurement noted and all the others have just vague descriptions.

Background.

Back in the 1950s/60s mechanically polishing Stainless Steel sheet was mainly the province of the producing Mills and apart from offering final finishes to the market was also a way of reclaiming poor quality product. All sheets went through several passes, on several grit belts, culminating in final finishing with a silicon carbide abrasive. The final finish was visually quite bright and very smooth so presented few problems with regard to surface staining or corrosion.

During the 1970s/80s the quality of Mill surface finishes improved with continuous production of coil. This enabled polishing routes to be shortened from several stages to one using pinch feed machines such as the Grindingmaster. A number of Stockholders moved into added value processing but few had the expertise to develop a range of finishes nor recognised the dangers inherent in producing finishes suitable for onerous service conditions. Cost driven, the whole industry in the UK moved towards the use of Aluminium Oxide abrasives as these offered longer life and consistent, if inferior, finishes. By sheer force of numbers the ‘Alox’ finish became the norm from Stockholders as this is by far the easiest finish to produce. It was left to independent Polishing Sub Contractors to develop finishes using Silicon Carbide media that produced clean cut finishes with a very low Ra [surface roughness] reading.

From the late 1980s to date the Aluminium Oxide finish, usually 240 grit, has been the norm and is generally what will be supplied by many Stockholder processors if a customer specifies a ‘Dull Polish’. This finish is acceptable in many applications but can have serious shortfalls if specified for external use.

The other consideration, of course, is the fact that ‘Dull Polished’ sheets from

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a number of different supply routes may be visually very different. This can be a major problem where sheets from different sources may be used adjacent to one another resulting in a ‘Battenberg Cake’ effect.

Silicon Carbide finishes are produced by some, but not all, Stockholder processors and it is essential that serious consideration be given to the service conditions before accepting a ‘Dull Polish’ without checking as to the type of abrasive used.

Selecting and specifying a finish.

So given the background described above it is apparent that gradually there was a shift from Silicon Carbide to Aluminium Oxide materials some years ago and it is only relatively recently that there has been a swing back to the use of Silicon Carbide media.

Why is this important you may enquire? Well essentially, whilst attractive, easy to produce and perfectly acceptable for a wide variety of [mainly indoor] applications sheet polished using Aluminium Oxide belts is likely to fail in external applications such as Architectural, Street Furniture, Signage etc.

Essentially, Alox belts ‘rip’ through the surface of Stainless Steel leaving a coarse surface which can often show readings up to 2.0microns Ra. This leaves minute crevices which are perfect for harbouring airborne particles.

Such particles will usually have some Ferrous content which when trapped in the crevices will produce staining and sometimes severe corrosion. Coastal locations and the added presence of salt laden atmosphere will speed up the corrosion process.

In contrast, a Silicon Carbide belt will cut cleanly through the surface. It breaks down the media fragments cleanly rather than having the ‘blunting’ effect of Aluminium Oxide abrasive. The resultant effect, using like for like grit belts, is a smoother slightly brighter finish with a surface roughness reading of 0.5Ra or better. This is the one finish, as mentioned earlier, that is specified in BS EN 10088-2 under the designation 2K for Cold Rolled sheet or 1K for Hot Rolled material.

Seeking specialist advice.

Professional Polishing Services Ltd, [PPS] have tried long and hard to educate stockholders and end users alike as to the dangers inherent in the specification of polished finishes. As stated earlier, polishing is an area where there is no definitive specification. A ‘Dull Polish’ may be anything from a 120 grit to a 240 grit with surface readings anywhere between 2.0 Ra and 0.6 Ra. All such material would comply with a 2J finish in BS EN 10088-2 even though very different in appearance and it would be difficult to justify a rejection.

The old adage ‘prevention is better than cure’ is very true so PPS always suggest that where the service conditions are known they are involved to assist with the selection process. Obviously stockholders have their standard ‘stocked’ finish but any reputable company will discuss an application with

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their sub-contract polishing company to ensure that the correct finish is specified.

Many end users who have an appreciation of the pitfalls inherent in specifying a generic finish to a stockholder have developed their ideal finish in conjunction with Professional Polishing Services. This is then specified by the client to his supplier and enables orders to be placed with the knowledge that even when material comes from several sources the finish will match batch to batch, sheet to sheet. It may seem an easy option to use a generic description but this can be a very dangerous route to take as the examples have shown.

There are other important advantages to using one sub contract polisher and these are that the specifier has the potential price advantage of buying material from the cheapest source, splitting orders where stock is not all available from one source and obtaining consistent finish on material of varying quality.

Other product forms.

Whilst this article has focussed on volume sheet polishing the exact same principle applies to any other material form. There are situations where major projects have been jeopardised by ignorance when specifying a finish resulting in massive rework at extra cost. There are no standards at all for polished finishes on tube, bar, blanks, components etc. It is true that there is probably more likelihood of potential problems as there are literally hundreds of small polishing companies who have the capability and the skill to polish Stainless Steel but not necessarily the technical appreciation of potential problems. It is not unheard of for these type of companies to polish Mild Steel and Stainless Steel in the same workshop and even on the same machine with the likelihood of serious cross contamination.

Conclusion.

It is worth talking to Professional Polishing Services if you have any doubts as to the type of finish you require. Remember they have spent over twenty five years developing their high quality finishes. They are market leaders and specialists in their field and can not only give advice on specification of standard finishes but where necessary can work with the specifier to develop bespoke finishes. As they do not sell Stainless Steel they are able to concentrate on their core business which is to give the customer the finish they need rather than sell a standard finish which may, or may not, be what is right for the application.

It is obvious that in the current climate stocks, at Stockholders and end users alike, have been reduced and any way of improving consistency of standards and reduction in process lead times is to be welcomed.

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Stainless Steel polishing is not restricted to the ‘Dull /Satin’ finishes which have been discussed but can encompass a range of alternative aesthetic finishes such as Bright / Mirror and Ultrabright, Satin-web [produced with a Scotchbrite type’ roll, Pro-Circ circle finish, Pro–Wave and Pro-Stripe which when combined can offer an imaginative alternative to a conventional polished finish.

The Company.

Based in Smethwick, West Midlands the Company has a product range second to none. From polishing of smaller profiled items through Long Products to large plate via Dull / Satin finishes, Mirror polishes and Circle / Patterned finishes on sheet there is a very strong probability that Professional Polishing Services can help deliver the finish you need.

With nearly thirty years of service to the Stainless Steel Industry Professional Polishing are the market leaders in mechanical polishing. The Company ethos is one of flexibility. Whether developing bespoke finishes, achieving tight delivery schedules or providing a shoulder to cry on when you’ve got it wrong, PPS will almost certainly be able to help you.

Please feel free to contact the Sales team who will be delighted to help you with any requirements or technical questions you may have.

Tel; 0121 555 6569 Fax; 0121 555 6613 e-mail admin@professionalpolishing.co.uk

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