View Full Version : Micro Cracking
08-17-2001, 03:11 AM
I was making a jack shaft so I grabbed some 3/4" 1018... while turning down the end to .6259 for the bearings I encountered micro cracking at this dimension. Thinking it could be from when the piece was sheared I cut it back and turned it again... again the cracks appeared. So I just started to turn the entire piece and low and behold is had these cracks intermittently along it's length some 2.5" long others .5". One thing I did notice the chip change from running coils to short curls and then mini chips. I guess from the crack to brittle center. any body run into this? And what would cause this?
03-06-2006, 11:50 AM
Ok anybody care to hazard a guess as to what would cause this condition to occur. I'm just glad I was not turning threads for a critical application cause I'd of never seen um' and they would of been at the root of the threads. I'll try and post a picture of these I'm not as good at micro photography as Evan... but I'll give it my best try.
03-06-2006, 12:06 PM
"micro cracking" Radial in an axial plane? Radial in a eadial plane? "Laminated?"
In the meantime reject the material and the parent bad it came from for use in anything critical and important. I'd paint the bar in red and sliver stripes to denote its reject tlassification and use the stuff for drifts, punches, pins, and temporary welded items in jigs etc.
03-06-2006, 12:16 PM
Sounds like junk steel that was full of inclusions when it was drawn. If so, the inclusions got stretched out into long stringers, which now appear as longitudinal cracks.
LES A W HARRIS
03-06-2006, 01:43 PM
Tinkerer, transfered from the first posting.
<http://bbs.homeshopmachinist.net//icons/icon1.gif> posted 08-18-2001 04:13 AM
Yes it's quite common, that's why aerospace materials specifications were written to require the material to meet cleanliness to a given requirement, and of course costs more but if cracks show up, the material can be returned for replacement. Plain old commercial quality 1018 cold roll etc, no gaurantee. If Your material was purchased to ASTM A1018 or UNS G10180 it should be returnable.
Causes, various, most cold finished bars are produced by cold drawing, oversize hot rolled bars are drawn through dies to the required size, improper cleaning will cause inclutions, if the material has been drawn down a long way will cause longitudinal cracks as You decribe.
[This message has been edited by LES A W HARRIS (edited 03-06-2006).]