A Surface Optimized For Appearance and Performance
WHY THE EPS SURFACE IS BETTER
As it removes scale, EPS creates a new surface characterized by 'micro
jagged' peaks and valleys. It's more uniform than a typical acid-pickled surface, as shown in the measured surface profiles shown below:
These two profiles have similar Ra values, but notice how the acid-pickled
surface exhibits larger deviations from the average (red line) and a very irregular surface. The left of the profile is very smooth, which is inhibits
paint adherence, whereas the right
of the profile is more irregular. These
two different topographies on the same surface means the material has a
high Rz value, which makes
it more difficult to get a consistent
paint finish.
The EPS surface doesn't have extreme peaks and valleys. It has a uniform
(low Rz value) surface profile, which produces a smooth paint finish. It's
also more 'jagged' which better holds coatings for superior adhesion.
This was made very clear in a side-by-side study of EPS and acid- pickled steel using non-contact optical profilometry. Scans of the EPS and bare acid-
pickled surface showed EPS had a higher Ra, but more uniform profile. After chromate pretreatment and e-coat were applied to both materials, the painted EPS surface actually had a lower Ra and less "waviness" than the acid pickled surface, as can be seen in the topographical scans below. READ THE FULL TEST REPORT
With EPS, you can specify a desired surface finish, in terms of Ra, for your processed material. The EPS process offers two distinct ranges of surface Ra:
R80: 80 - 110 microinches
(2.0 - 2.75 microns)
R110: 110 - 140 microinches
(2.75 - 3.5 microns)
Different roughness levels present a way for you (or your customers) to choose a surface that's more suitable for your needs. The R110 finish is typically selected to allow the EPS line to run at speeds that optimize economics. But since EPS' surface is very uniform, even at this higher Ra level, you do NOT sacrifice the smooth appearance of the finished painted surface.
Other applications may benefit from a lower Ra surface that is still characterized by a very uniform topography. The R80 EPS surface would be preferable for these.
With EPS, you no longer have to accept whatever finish was on the steel when acid pickling removed the scale. You can now actually specify the surface finish best suited to your needs.
Mitigating Common Surface Defects
As EPS processing removes scale, it also 'conditions' the surface of the strip, mitigating common flaws and defects. This results in steel with more a uniform, higher quality appearance.
Defects which are actual cavities in the surface (pitting) are not removed, but become 'smoothed' so as to be less severe and less susceptible to trapping oil and contaminants. Flaws which reside on the surface, like silicon streaks or pickle stains are completely gone after EPS processing.
The gallery below shows side-by-side photos of common surface defects. The left side shows a section of a strip that was not EPS processed (or prior to processing), while the right side shows a section of that same strip, originally having the same surface defect, after it underwent EPS processing.
EPS benefits are also available to material that has already been acid pickled, but became rusted or exhibits high levels of surface flaws that makes it 'secondary' material. EPS processing can restore it to 'primary' status and, in the case of processing on an EPS Coil Line, improve the material's shape by tension leveling.
SEEING IS BELIEVING
Side-by-side photographs show how EPS processing mitigates common mill defects. Click on the name of each defect below:
Pitted Surface
Rusted Surface
Roll Marks
Silicon Streaks
Pickling Stains
Wood Grain
A coil with repeated roll marks was the source of samples that were acid pickled or EPS processed. Acid pickling did not visibly effect the roll 'scrape' mark but EPS processing left the mark barely visible, as it blends into the very uniform surface.
This sample is a single piece from a coil with severe silicon streaks, a problem of excess silicon in the steel depositing on the surface. The left half of the sample was masked off, then the sample was EPS processed. Removing the masking, it was apparent that the streaking was removed by EPS.
A manufacturer has coils of stainless steel EPS'd so as to 'roughen' the surface, which has demonstrated improved paint adherence and performance. A side benefit is the 'wood grain' pattern on the surface of the strip, shown in the 'Before' portion is eliminated by the EPS processing.
Coupons taken from a coil of hot roll all featured severe pitting. Some were acid pickled and some were EPS'd. The EPS process served to smooth out the contours of any cavities that remained on the steel (see right side above), something acid pickling cannot do.
Coupons from a hot roll coil with significant surface rust were acid pickled or EPS processed. Acid pickling removed the rust, but left a surface covered with pitting and cavities. EPS achieved significant 'smoothing' of these defects, as verified by the Ra of the samples: EPS: 80.4 Ra . . . HRPO: 123 Ra.
A coil arrived with bad pickling stains – probably a result of halting the acid pickling line as it was being processed. A closeup of the surface is shown on the left. The balance of the coil was EPS processed, which completely removed the pickling stains, as shown on the right.
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