Compared to other materials, the use of stainless steel as a construction material has a number of important advantages: high mechanical strength, relatively good machinability (weldable and deformable), good cleanability, high corrosion resistance and an inert surface that does not allow any substances to get into the end product. These properties make stainless steel very suitable for use in the food, pharmaceutical and chemical industries.  

Many visible products and constructions in public (outdoor) areas are also made of stainless steel, because of its high aesthetic value, vandalism resistance and the many possible treatments to achieve a shiny and smooth result. In addition, stainless steel is virtually maintenance-free.

The oxide film protects stainless steel against corrosion and is able to regenerate itself if there is sufficient oxygen.

To be able to speak of stainless steel, the steel must contain a minimum of 11% chrome and a maximum of 1.2% carbon. The higher the carbon content, the harder the steel. The higher the chromium content, the higher the corrosion resistance. When chromium comes into contact with oxygen, it forms an invisible layer of chromium oxide, the oxide skin. This layer protects the underlying metal against further corrosion. If the oxide skin is damaged, a new protective oxide skin is always formed when there is sufficient oxygen.  

Separate processing and storage of stainless steel and steel

The oxide skin can be damaged by machining, falling or daily use. In addition, the oxide skin can also be affected by, for example, iron particles or other base metals that come into contact with the stainless steel. We call this contamination corrosion. The iron particles come, for example, from steel tools or are released during the machining of steel. Corrosion then occurs when the stainless steel becomes damp. Condensation can be enough to cause rust. The moisture dissolves part of the free iron and the acidic environment will further accelerate rusting. Even more serious is the fact that the oxygen supply to the stainless steel is thus cut off, which impedes the self-healing capacity of the protective oxide layer and the corrosion process is not stopped. To avoid contamination with base steel particles, it is important to process and store carbon steel and stainless steel in separate rooms.  

Stainless steel grinding

To preserve the unique stainless properties of this metal, the choice of abrasives is also essential. Choose abrasives that are especially suitable for working stainless steel. These are free of iron, chlorine and sulphurous fillers and thus cannot contaminate the oxide skin. Grinding and cutting discs are often marked INOX.  

These discs are also designed to introduce minimal heat and thus avoid discolouration of the stainless steel as much as possible.  

For larger machines such as press brakes, the blades should be taped so that there is no contact between the steel of the blades and the stainless steel. Another solution is to leave present laser foil on the stainless steel product for protection.

Furthermore, the smoother the surface is, the more difficult it is for foreign particles to adhere and cause damage.  

Stainless steel cleaning and processing

Surface condition

How well stainless steel can be cleaned depends very much on the surface condition, which in turn is influenced by all the treatments the metal has undergone before reaching its final shape. Strict hygiene requirements are imposed to prevent undesired growth of micro-organisms. This growth mainly takes place in micro cracks and other damages that are often not even visible to the naked eye.  

Also, adhesion of dirt must be prevented. The surface roughness is important here. This is expressed in µm or micrometers, for example Ra, Rz or Rt. The most commonly used value is the Ra value. This roughness is the arithmetic mean of the deviations in the roughness profile compared to the centre line. For the food industry the Ra value is often 0.8µm and for the pharmaceutical industry 0.4µm or lower.  

Grinding stainless steel to roughness.

Grinding to Ra value is a precision and time-consuming job that requires professional skill. Step one is to remove the mill scale with an aggressive abrasive. Depending on the desired roughness, we then sand with grit 60, 120, 240 and, if necessary, 400. Another option is to switch to a Scotch Brite Surface Conditioning Disc after grit 120. Another excellent product for grinding to roughness is Trizact. Due to its unique construction, this product gives a consistent finish throughout its life.  

Scotch Brite is the product of choice for a perfect finish to your stainless steel.

In 1958, over sixty years ago now, 3M introduced its first Scotch-Brite™ cleaning pad for industrial use. Since then the range has continually expanded. The success story began with the development of 3M's non-woven technology in the late 1940s. Some time later, in 1958, the Scotch-Brite scouring fleece was born. This fleece combines synthetic fibres, resin and abrasive grains to create a three-dimensional structure. In this way, new abrasives are exposed to the work surface each time and a uniform, reproducible surface is created. In industrial metalworking, this technology is used today in Scotch-Brite sanding discs and belts, wheels as well as the well-known rollers and hand pads.

General tips for fine-tuning the desired finish with Scotch Brite:

1. Play with the pressure.
   With the same product you can create a different finish by playing with the amount of pressure. Less pressure gives a finer finish through thinner scratches. While more pressure creates deeper, coarser scratches.
2. Watch your speed.
   Some go for slow but steady, others opt for big steps fast home. But did you know that the speed/speed can make a big difference in the final finish? If you want a coarser finish, make long strokes with your machine at low speed. For a finer finish, increase the speed.
3. Choose the right sanding mineral.
   Many Scotch Brite products feature an aluminium oxide grit. But we also find the CubitronII grit in several Scotch Brite products. These minerals are very suitable for universal applications. However, if you want more shine on your end product, consider a silicon carbide grit. This is sharper and more pointed, providing a finer finish with more gloss.
4. Choose the right density.
   A consistent finish is not always easy. To achieve this, even on curves, recesses, contours, etc. the density of the disc can make a difference. Changing to a lower density results in a more flexible product, better for working in corners and on contours. Higher densities result in stiffer discs, allowing for more aggressive work. Choose the right density depending on the application and desired finish.

Cleaning stainless steel of weld discolouration and light deburring with 3M Bristle Discs

Disfiguring weld discolouration is caused by oxidation of the metal surface. This can be (partially) prevented by using an inert gas such as argon during the welding process. If discolouration does occur, it can easily be removed with the Bristle Disc. A good alternative to steel brushes and, thanks to their plastic bristles, also suitable for use on stainless steel. The cast wires are fitted with the ceramic Cubitron grain, which guarantees fast work. Only slight pressure is required and the flexible filaments mean that complex shapes and contours can also be worked on and cleaned well with the Bristle Disc.  

Bristle Discs are also aggressive enough for light deburring without affecting the parent material.  

Inside corner weld machining with 3M Rapid Cut and EXL discs

Even a perfectly laid weld has irregularities or is slightly porous. Dirt can accumulate there. A smooth finish and seamless connection of all parts is therefore essential. By performing this operation with a Rapid cut disc and then using an EXL disc 2A Medium, you are done in two steps. Your weld is perfectly finished and dirt has no chance.

Cleaning stainless steel of rust, coatings, mill scale and other contaminants with 3M Clean & Strip

The Clean & Strip disc has long been known as a disc for removing paint or rust. Not so long ago, this disc has undergone a true metamorphosis and has been greatly improved. The renewed Clean & Strip XT-Pro and XT-Pro Extra Cut is more aggressive than ever. The XT-Pro has a silicon carbide grain and is by far the best disc on the market for removing mill scale, rust and paint. The green variant has an aluminium oxide grain for cleaning, blending and preparing surfaces. It also removes thick layers of paint with ease. Both discs work quickly, do not clog up and do not damage the material to be processed.  

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Stainless steel brushing and directional grinding with Scotch Brite belts and wheels

With Scotch Brite belts and rollers you can achieve a beautiful visual finish. By brushing, a polished structure is matted. At the same time, with Scotch Brite wheels you bring the roughness down considerably.  

Rough grinding is a trade. We are happy to advise you on how to achieve your desired finish in the fewest possible steps. Ask for a demo without any obligations. Call 033-4558788 for an appointment with one of our technical specialists.  

For more information and videos on stainless steel processing and various applications, please visit our website at www.metaaltechniek.nl.  

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