As a general rule, Ra – typically referred to as ‘average roughness’ – of the surface of EPS-processed strip steel will be higher than that of the acid pickled steel it replaces. Upon hearing this, some may incorrectly conclude that the higher Ra is a drawback to EPS. After all, isn’t a smoother surface a better surface? Not necessarily. In fact, numerous studies have shown the ‘rougher’ EPS surface is actually better for important properties of coating adhesion and corrosion resistance. This article explains why and also gives suggestions for how to ‘smooth’ the EPS surface (lower the Ra) when the end user believes – rightly or wrongly – that a lower Ra is needed for their application.
|As a measure of surface texture or topography, Ra is a fairly crude tool. It measures only the average absolute value of deviations (peaks and troughs) of the surface profile from the arithmetic mean. For example, the surface profiles shown at the right, while having very different surface textures, all have the same Ra. →|
The typical surface of EPS processed steel is characterized as having a ‘micro jagged’ surface with tight, sharp peaks and valleys as shown in the profilometer trace below. ↓ Note how uniform the profile is.
Compare this EPS profile to the one below ↓ from a sample of the same substrate material that was acid pickled:
This acid pickled profile is much more ‘wavy’. Instead of sharp peaks and valleys it has rolling ‘plateaus and canyons.’ But guess what? The acid pickled profile has a lower Ra than the EPS profile. Clearly, Ra doesn’t tell the whole story. An important characteristic of EPS processed steel is the consistency of the profile, and consistency matters when it comes to appearance of a painted surface.
The clearest evidence of this was a study in which the surface of EPS and acid-pickled steel strips were analyzed side-by-side using optical profilometery. The study first measured the surface texture of bare samples, then repeated the measurements after chromating (paint pretreatment) and then again after e-coat painting of the samples. The acid pickled sample had a lower Ra than the EPS sample to begin with, and the Ra of both EPS and acid pickled samples became lower with each successive layer of coating applied.
|However, the surface ‘waviness’ of these samples told the real story. The average waviness, Wa, of the chromated and painted EPS sample was roughly half that of the acid pickled sample, as shown in scan traces on the right. → The result was the consistency of the EPS surface yielded a more uniform, higher quality finished paint appearance. Get the full study details.|
This ‘rougher’ but more uniform EPS surface also offers paint adhesion and corrosion resistance benefits over an acid pickled surface. This was proven in the recent tests that lead to General Motors and Chrysler approvals of EPS (click for details), as well as many other studies that are documented in the EPS END USE AND APPLICATION TEST RESULTS report..
Despite the acknowledged benefits of the more uniform, higher Ra EPS surface, a few users who are buying EPS processed material have asked for EPS product with a lower Ra value. While the standard EPS Ra ranges from 2.45 to 3.50 μm (96 to 140 μin), these users are typically asking for EPS with an Ra < 2.5 μm (100 μin). In some cases, they simply want to match an existing spec for acid pickled material and are unaware of the benefits of the higher Ra EPS surface. In other cases, they’re very aware of the benefits, but don’t want to have to ‘sell’ their customer on relaxing a longstanding spec that calls for Ra < 2.5 μm.
An EPS Ra < 2.5 μm is actually pretty easy to accomplish: The EPS Producer reduces the rpm (revolutions per minute) of the Slurry Turbines to ‘soften’ the impact of the abrasive grit on the strip surface. Unfortunately, this slows EPS line speed – about 30% – which penalizes economics. To avoid this penalty, we have evaluated other ways to lower the Ra of EPS processed steel and offer the following for consideration:
1. Skin Pass:
Last month’s EPS Blog entry explained how a European galvanizer wanted to ‘smooth’ the EPS Ra prior to running galvanizing trials. The idea was to galvanize EPS with an Ra more like what they expect from acid pickled strip. They ran the EPS strips through a skin pass mill, applying different roll forces on different strips. All of the EPS strips began with a thickness of 1.99 mm (0.078″) and Ra of 2.45 μm (96 μin). The table below shows the resulting Ra values as a function of rolling force. We hope it offers guidance for others wishing to use this method for ‘smoothing’ their EPS.
|Strip #||Skin Pass Rolling Force, kN (lbs.)||Resulting Ra|
|1||NA – original EPS strip||2.45 μm (96 μin)|
|2||160 kN (36,000 lbs.)||1.88 μm (74 μin)|
|3||265 kN (59,600 lbs.)||1.09 μm (43 μin)|
|4||340 kN (76,400 lbs.)||1.04 μm (41 μin)|
2. Leveling and Brushing:
Here at TMW, we did some Ra ‘smoothing’ tests using EPS coils with an Ra of 3.0 μm (118 μin). We first ran some of this material through a conventional roller leveler, which reduced Ra to 2.25 μm (89 μin). Next, we ran the leveled strip through a set of United Rotary Brush Corporation Silicon Carbide 240 Grit brushes. This further dropped Ra to 2 μm (79 μin).
We then tested some of the the EPS material with various brush types (no roller leveling). We found that running the material once through a single set of Osborn Silicon Carbide 240 Grit brushes managed to reduce Ra to 2.25 μm (89 μin).
We recognize that results will vary based on thickness and grade of steel being processed, but we feel that, in general, the approach of roller leveling and/or brushing EPS processed material presents a viable, low cost way to ‘knock down’ Ra without sacrificing the uniformity of the EPS surface and without incurring undue economic penalty by slowing EPS processing speed.