Stainless steel is the most widely used material in various industries, including building, manufacturing, and food processing. It is a popular choice for a range of applications due to its exceptional corrosion resistance, durability, and visual appearance.
The surface roughness of stainless steel is an essential component that influences its usefulness and performance. Mostly the surface roughness is an overlooked feature in the manufacturing process, while more importance is given to the strength and material composition. Increased surface roughness can cause great friction and early failure of the material.
Stainless steel surface roughness refers to imperfections or differences in the texture or finish of its surface, which can affect its appearance, corrosion resistance, and cleanability. The condition of processed surfaces is indicated by surface roughness. Measuring the surface roughness of stainless steel is very important for the final product’s performance.
Understanding stainless steel surface roughness and its impact on performance can assist engineers and designers in material selection and surface finishing decisions.
How to measure the surface roughness of stainless steel
The roughness of a surface is most typically measured with a device that moves a stylus across the surface, amplifies the movement, and records the signal. The result, known as Ra or roughness average, is the arithmetic average of the deviation of the trace below and above the center line.
Ra is traditionally measured in Micrometers. The word CLA is used in ISO standards (Centre Line Average). The values of Ra are high, meaning the material surface is rougher, while the low values show the surface finish.
The roughness of different grades is shown in the surface roughness conversion chart.
| Roughness Grade Number | N 12 | N11 | N10 | N9 | N8 | N7 | N6 | N5 | N4 | N3 | N2 | N1 |
| Ra (Micrometers) Roughness Avg. | 50 | 25 | 12.5 | 6.3 | 3.2 | 1.6 | 0.8 | 0.4 | 0.2 | 0.1 | 0.05 | 0.025 |
| Ra (Micro inches) | 2000 | 1000 | 500 | 250 | 125 | 63 | 32 | 16 | 8 | 4 | 2 | 1 |
| Rt (Micrometers) Total height of roughness profile | 200 | 100 | 50 | 25 | 13 | 8 | 4 | 2 | 1.2 | 0.8 | 0.5 | 0.3 |
| RMS (Micrometers) Root mean square Avg. | 55 | 27.5 | 13.75 | 9.13 | 3.52 | 1.76 | 0.88 | 0.44 | 0.22 | 0.11 | 0.055 | 0.035 |
Effects of surface roughness on stainless steel properties
Surface roughness has various effects on the properties of stainless steel alloys. Some effects can improve the properties of stainless steel, and sometimes the greater surface roughness is unsuitable for applications. Here are some possible effects.
Corrosion resistance
Generally, a smooth surface finish is more successful at resisting corrosion than a rough surface. This is because localized corrosion can result from a rough surface trapping moisture and corrosive substances. A smooth surface, on the other hand, is less likely to catch such chemicals and acts as a barrier against corrosion.
Wear resistance
The wear resistance of stainless steel can be increased by giving the surface a rough texture. This is due to the ability of the surface’s imperfections to capture and hold lubricants, which reduces wear and friction.
Fatigue resistance
The fatigue resistance of stainless steel can be significantly impacted by surface roughness. A smooth surface finish can lower the risk of fatigue failure by lowering stress concentration and the possibility of surface flaws that could serve as micro cracks sites.
Cleanliness
A rough surface is generally not easy to clean as a smooth finish surface. Keeping a material clean can be more difficult if it has rough surfaces that collect dirt, bacteria, and other impurities.
Finally, the unique application and the desired result determine how surface roughness affects stainless steel attributes. In some situations, a rough surface may be favored for greater wear resistance. In contrast, a smooth surface finish may be desired in others for improved corrosion resistance or cleanability.
Measurement units for surface roughness
A common unit mostly used is known as the Ra roughness average for measuring surface roughness. Ra is the average between the peaks and valleys on the surface roughness graph. Some other units are also used in this terminology.
Rz
The Rz is the average of consecutive lowest valleys and greatest peaks. Distances between the lowest valley and the highest peak, the second-highest peak and the second-lowest valley, etc. Usually, the average is determined after performing this for the five largest deviations.
Rp
It is the distance between the mean line and the profile’s tallest peak within the evaluation length.
Rv
It is calculated between the mean line and the profile’s lowest valley within the evaluation length.
Rt
This unit is also known as the total height of the roughness profile. It is the distance between the highest peak and the lowest valley within the evaluation length.
Rmax
The largest continuous variation, calculated during the evaluation length, is between the lowest valley and the highest peak.
RMS
It is the root mean square average of profile height from the mean line within the evaluation length.
Importance of surface roughness measurements
The quality and efficiency of processed surfaces and the finished product’s functionality are significantly affected by the size and layout of features. To ensure that final products perform to high standards, surface roughness measurement is very important. The grit size plays an important role in surface roughness and surface finish. The lower grit number shows a higher surface roughness, while the higher grit number shows a finer surface finish.
The following table shows the comparison between the grit size and surface roughness.
| Grit size | Rq (Micrometers) | Ra (Micrometers) |
| 80 | 2.02 | 1.80 |
| 120 | 1.47 | 1.32 |
| 150 | 1.20 | 1.06 |
| 180 | 0.86 | 0.76 |
| 240 | 0.43 | 0.38 |
| 320 | 0.36 | 0.30 |
| 500 | 0.20 | 0.18 |
| 600 | 0.15 | 0.13 |
How can we reduce the roughness of stainless steel?
The type of product that can be made from stainless steel depends on its surface finish and Ra values. The products with higher levels of purity call for surfaces with finer textures and lower Ra values. Ra can be decreased by gently dissolving the steel’s surface with a chemical and electrical mixture.
This procedure is known as electro-polishing. Only 5 to 10 micrometers of the material is removed, and much of it is the surface’s high peaks. The valleys appear significantly shallower in comparison as a result. Up to 50% of the surface roughness can be removed through this method.
When heavily damaged surfaces result from physical impact, welds, or pitting corrosion, electro-polishing is not the best option. To reduce the Ra values in those circumstances, mechanical polishing techniques like sanding or grinding may be needed.
