[Ed Caffrey]

Brian,

It's very likely that it's a flaw in the material. I had this happen enough times within the past couple of years to almost make me quit using 5160. I'm not 100% sure, but my belief is that these flaws come during the rolling process at the mill, when the rollers are allowed to either run out of cooling fluid, or the rollers are not kept clean, and the steel basically gets an inclusion in it. Then, becasue I believe it happens early in the rolling process, the successive trips through the rollers cover it up.
It always seems to become visible for me at about a 220 or 400 grit belt. Sometimes, which is the most frustrating, is that it doesn't appear until I'm hand finishing. The more I try to get it out, the larger it becomes! At that point about all you can do is start over.
It's just one more signal that in an attempt to make more money, the steel companies are slacking on quality control. Of course this type of defect doesn't seem to effect 5160 that is being used as springs (it's primary use), and we are such a small percentage of 5160 buyers that the steels companies could care less if we have complaints.