Surface Finish of Cold Finished Steel

In previous posts we’ve discussed the features of cold finished steel bars.  Now we’ll get into more details of one important feature, which is the surface finish of cold finished steel.

You can review what a cold finished steel bar is here:  What’s a Cold Finished Steel Bar?

Surface Finish

Probably the easiest of the features to see and verify on a cold finished bar is it’s surface finish.  One can easily recognize the difference in finish when comparing a cold finished bar to a hot rolled bar.

A hot rolled bar comes out of the mill (produced at temperatures of 1600° F to 2200° F) with a rough, abrasive, discolored, and dirty surface.  At these high temperatures, an iron scale forms on the surface of the bars.  You’ll see in the areas where the scale flakes off, that there can also be pits and other marks in the bar.  All in all it’s not very practical in that condition for many applications or end-uses.

Cold finished bars will be free from scale defects, whether it’s from cold drawing, or rough turning (& polishing), CF bars will be suitable for thousands of end use applications.

Examples Showing the Surface Finish of Cold Finished Steel Bars

The examples below show three examples.

1. A hot rolled bar surface.
2. A rough turned bar surface.
3. A turned & polished (turned & machine straightened or burnished) bar surface.

In each photo set, the top image shows the surface zoomed in using a microscope.  The middle image shows the surface profile.  A device called a profilometer measures the microscopic peaks and valleys of the surface.  And the bottom image shows an example of the bar itself.

Hot Rolled Bar

Hot rolled bar will have a rough, abrasive, discolored, and dirty surface.

Typical Ra of 250 to 1,000 microinch.

Photo taken from a hot rolled bar.

• Top – Shows the surface under a microscope.
• Middle – Shows the surface profile using a profilometer.
• Bottom – Shows the image of a hot rolled bar.

Rough Turned Bar

Rough turned bars have a surface that is free from surface defects such as seams, scabs, slivers and decarburization, while holding tighter size tolerances as compared to hot rolled bars.

Typical Ra of 250 microinch.

Photo taken from a rough turned bar

• Top – Shows the surface under a microscope.
• Middle – Shows the surface profile using a profilometer.
• Bottom – Shows the image of a turned & polished (T&P) bar.

Turned & Polished (Burnished / Machine Straightened) Bar

A T&P bar undergoes the rough turning operation, and then an additional burnishing straightening operation to give it a smoother finish and a straighter bar overall.  Shown in the profile image, you can see how the peaks and valleys of the surface are “smashed” flat by way of the rotary straightening operation.  This makes the surface look as though it’s been polished.

Typical Ra below 50 microinch.

Photo taken from a turned & polished bar

• Top – Shows the surface under a microscope.
• Middle – Shows the surface profile using a profilometer.
• Bottom – Shows the image of a turned & polished (T&P) bar.

Northlake Steel Corporation

We employ the latest in polishing and super polishing techniques to supply you with any surface finish needed.  Whether you’re looking for a specific Ra finish, or if you need a super polish with multiple parameters like Rmr, Rk, Rpk, Rvk, Rz, Rmax, or any of the 100+ options.  Northlake will work with you and provide the steel bars to all of your required specifications.  We also have the ability to provide detailed inspection forms showing results of measurements all along the bar, and on multiple planes around the bar.  With the ability to grind & polish to 16″ diameter and 75′ long, we look forward to meeting your challenge!

Contact us online or call 888-278-4682 for more information.