The specification section on surface condition usually begins with a statement about undesirable surface imperfections. In many cases, this statement reads: "The surface must be free of scratches. It shall have no stress raisers, pits or seams." This is a generalized statement which, if taken literally, constitutes overspecification. Strip with "no" surface imperfections can be produced only at great expense under laboratory-quality conditions.
A more realistic and economical approach is to consider the effect of surface imperfections on the finished part. This effect will be determined by the configuration, direction, depth and severity of the imperfections, and by the deflections, loads and stresses imposed on the part.
Usually, surface imperfections are not detrimental if they are light and longitudinal, and if stress concentrations are negligible. Thus, minimal stress raisers can often be tolerated.
Conversely, relative freedom from stress raisers is critical in circular parts that deflect in all directions under load. Otherwise, application of load and resultant deflection might cause fatigue problems.
Type and depth of imperfection must be considered. A sharp V-notch in some parts can cause early fatigue failure in high loading situations. In the same circumstances, a shallow U-shaped notch might prove much less sensitive.
Two types of flaws - seams and laminations - are usually detrimental to a part. They are the most common cause of surface-related fatigue failure. Seams, slivers and laminations result from steelmaking processes. A seam can be created, for example, if foreign particles are trapped in the steel ingot during pouring, or if the strip is scratched in hot rolling.
Specifying Surface Roughness
Surface roughness in high carbon strip is commonly measured and specified in terms of RA (arithmetical average roughness) value. For noncritical applications, users frequently do not designate an RA value. In these cases, cold-rolled and hardened strip will have a surface roughness ranging from 6-15 RA (the exact figure depends on strip processing operations).
Sometimes it is desirable to specify an RA value. For example, manufacturers who require a mirror bright finish on their parts might request strip with an RA of 3-6. To convert RA to RMS, multiply RA x 1.1 = RMS.
Measuring Surface Profile
To measure surface roughness, a profilmeter is used. The profilometer's sensitive stylus runs across the width of the strip sample, recording surface variations in microns. An arithmetic average of these variations is displayed.
The profilometer can provide an average value and an RA value, based on customer need for surface texture requirements. We can also measure scratch depth with the Rmd value.