inclusions v. stringers v. segregation??? 1

[gearHTr]

we make some pump gears from aircraft quality 52100 steel(not vac melted). Outside NDT house rejected 15 of 58 pcs for inclusions over spec. limit of .125"
When we mag. particle inspected in-house, the indications are long thin lines running length of part, almost perfectly straight to axis of shaft. Stock was 1.25" dia, most inications are on a .500 shaft

Our Level III thinks these lines are from the structure of the alloy, kind of a segregation, and not inclusions.

I am looking for any input as to what we are seeing and alternate methods of inspecting to verify.

Thanks

 

[metengr]

gearHTr;

Outside NDT house rejected 15 of 58 pcs for inclusions over spec. limit of .125"

Did the contractor NDT firm also use WFMT (wet fluorescent magnetic particle) or other method of NDT? Was an ultrasonic examination performed to evaluate forging cleanliness?


Over the years, I have seen where surface conditions do produce reportable indications by WFMT (faint lines that are like seams). After careful local polishing, these indications disappeared and after confirmation by ultrasonic examination, the material was accepted for use.

 

[TVP]

Manganese sulfide inclusions can become quite elongated, which is why they are frequently called stringers. What you are trying to determine is whether or not the indications are elongated inclusions or carbide streaks (due to segregation). The best method is to cross-section the item(s), polish, etch and observe the microstructure using a microscope. The German standard SEP 1520 is the best reference fon this subject, and even has a series of images for rating the carbide structure in terms of size, network, and degree of streakiness. It is available as a dual-language English-German document:

http://www.stahleisen.de/html/home....sh&PHPSESSID=01bb6ec4ac0242608e1181bfb9e93866

 

[gearHTr]

metengr - NDT house did use WFMT per ASTM E1444, but these were made from CF bar, not forgings. The thing that drives me nuts about some vendors, is that they write up the reject and move on, no further exam, no call to discuss, nothing

TVP- what is a carbide stringer?

Thank you both for great info.

 

[TVP]

The SAE steel grade 52100 nominally contains 1% carbon. This is a large amount for steel, and it means that there will be a large amount of it tied up in the form of carbide particles (iron carbide is called cementite, but since this grade also contains ~ 1% Cr, many of the carbides will be alloy carbides of the form Cr₂₃C₆). Since there are so many carbides in this grade they can end up segregating into streaks that align during thermomechanical processing. The following link contains a number of microstructural images that include carbide streaks (click next page for more examples):

http://www.metallograf.de/start-eng.htm?werkstoffkartei-eng/3505/3505-4.htm

 

[gearHTr]

Thanks again. Those images look similar to what we are seeing. I will section a part to confirm.