|
|
|
|
 |
Letter 34414 Bad finish (pitting or orange peel) on 316L SS tubing after electropolishing [Quebec]+++++ We are an OEM of equipment for the Pharmaceutical industry. Our components are made of welded 316-316L SS sheet metal, bar or tubing parts which are polished to 20µin as RA and then electropolished. Components made of sheet metal are always goog looking (< 20µin)after electropolishing. However, some parts made of tubing or bar loose their finish (> 20µin). Finish as some light pitting or orange peel. We made some test and our conclusions lead us toward a material defect: we tested many sample of different heat number and, everytime the result was the same. One MTR was always good and the other one bad. Is this possible? Can the material be the cause of this bad finish? We use ASTM A249-A269-A270. We also subcontract our electropolishing. On the other hand can the electropolishing process in cause? Eric Potvin
+++++ This is a common problem electropolishing 316. T316 bar stock has sulfur added to improve machinability, and these sulfur inclusions are preferentially dissolved during electropolish, leaving a surface with many small pits. Sheet stock does not have sulfur added, and does not exhibit the problem. You can confirm this by carefully looking at the parts machined from bar stock. You will find the pitting is much more obvious on the cut ends, ie perpendicular to the direction of rolling. Using the minimum acceptable time for electropolish will minimize the problem; changing to T304 will eliminate it.
+++++ T304 is not an option for us. Our customers request 316/316L. Does the added sulfur shows on the MTR? We just received a sample (see picture)were the good part had a sulfur content of 0.003% and the bad part a content of 0.013%. We also had occurrence of the problem with sulfur content of 0.007%.
Eric Potvin
First of two simultaneous responses -- +++++ They are normal residual sulphur levels. The range 0.002 to 0.02 percent is typical. That is, sulphur was not added during manufacture to confer free-machining properties. Free-machining sulphur levels are typically 0.2-0.3 percent. So the cause of your problem would seem to be somewhere else. Jeffrey Holmes says that T304 (I assume from the context that the "T" designation denotes the sulphurised free-machining version) is OK. The significant difference between 316 and 304 is that 316 contains 2 to 2.5 percent molybdenum but 304 contains negligible molybdenum (usually less than 0.1 percent). Perhaps the problem is related to the homogeneity of the molybdenum distribution, which would depend on the amount of deformation (reduction in cross-sectional area) of the bar during its rolling and the original size of the bar/billet/ingot that was the rolling stock. Of course you have no control over such aspects and indeed it's likely that such information could not readily be obtained. So maybe you need to stick with a supplier whose product electropolishes OK and accept that the reason for the difference isn't known. I'm a metallurgist primarily concerned with bulk properties of steels rather than surface finishing aspects - I cannot conceive that normal residual sulphur levels could segregate to give the problem you experience.
Second of two simultaneous responses -- +++++ Check with your supplier and see what they can offer as far as low sulfur. I'm not certain what is available. There have been reports that extended precleaning (an hour or longer) in hot alkaline cleaner helps to mimize this problem. We have not been able to see that this helps, but others have reported some success with extended precleaning. It's worth a try. Other than that, and using the minimum electropolish time, I have no other suggestions. This is really a material problem, not an electropolishing problem.
I need to find out why stainless steel parts are pitting and orange peeling in an electropolish process. Sometimes the defects are localized.
Dear Reader: please choose what you want to do--
 |
|
Save
This Page (why?) - Home - ©1995-2008 finishing.com
