"Pits" in 17-4 SST 3

[Metrics]

After polishing disks of 17-4PH SST to around 0.5 microinch Ra finish, I have enough pits on the surface that it effects the yield of my product.

I hoping somebody could comment on the following:

1) Are the pits likely existing voids in the material?
2) Can I select different grades of this or similar material to reduce these defects? For example 15-5 PH SST
3) Could I be causing the pits during preparation if the parts?

Material: 17-4 PH SST Bar Stock, Cond A, Per AMS5643

Preparation: Turning, Grinding, Diamond Polish, Colloidal Silica Polish

Problem: On a 3" diameter disk, over 100+ noticeable pits with diameters of .0001" or larger. A pit in the wrong place causes a defective part. And at this level of pitting it reduces yield by about 15%.

Please let me know if additional information would be useful.

 

[unclesyd]

At what stage of the process do the pits show up?
Do you think you maybe dragging out some particles when lapping or polishing?

We make spinneretts from 17/4, normally using Carpenter plate material. We grind and lap to your stated requirements or better and the cut or drill the capillaries. After cutting the capillaries we relap to achieve a polished finish. I don't recall seeing or hearing of any pits in the surface of the material. Our process is Blanchard grind the blanks to size with parallel faces, drill and counterbore mounting holes, lap, cut capillaries, relap and polish. Any inclusions in the material would cause problems as one of our requirements is that there be no discernible radius or defects on the capillary edges at 100X.

I think your trouble maybe with using bar stock instead of plate. It has been my experience that bar stock tends to be slightly dirtier than plate. Any time we’ve tried you use bar stock for spinneretts we have had similar problems, though no to the extent you report.

There are some cleaner grades of 17/4 available. The premium for these grade may be worth it to reduce that level of rejects.

 

[EdStainless]

Are you polishing the cross section of the bar? If so you should expect to see some inclusions. Material with heavier amounts of cold reduction will have finer, better dispersed inclusions.
Has this process ever worked for you?
If you are calling 0.0001" a defect then you had better be using a vacuum remelted grade. Inclusions up top about 0.002" are fairly common.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Metrics]

Correct we are polishing the cross section.

The yields have not been better in the past, however they are not unacceptable either.

We've also used plate stock in the past. But the failures were not investigated at the time. They could have easily occured in photolithography or sputtering process that follow polishing.

I think I'll get some costing on the vacuum remelt grades.

Thanks for the insights!

 

[lgearhart]

Metrics, you want 15-5, Type 1, per AMS 5659 rather than the vacuum melted grade of 17-4 per AMS 5622. In almost all respects, 15-5 IS remelted 17-4: the chemistries are almost identical, and the mechanical properties too. Yet 15-5 is a standard product, so the availability and cost should be better than vacuum melted 17-4. You want to note the "Type 1" since 15-5 is available as either Vacuum Arc Remelt (Type I) or ElectroSlag Remelt (Type 2).