Iron inclusions in 17-4 stainless barstock

[Vonmagnuson]

We have a simple part of 17-4 stainless steel HR bar stock. It's 3" x 3" x 1/2" thick. There is a 3/4-10 tapped hole centered in it. Generally it's made from 3" x 1/2" barstock, sawn and machined. The part is then welded to a structure. The structure is exposed to cleaning agents in the food and pharma industries.
Until a few weeks ago we have never encountered any contamination that was not within our control. The last two heats of bars stock from different locations have actually begun to "rust" in the field.
We looked at our internal stock and did some testing (before welding). We found that after exposure to salt water, the part began to "rust" in a matter of minutes. It does not "rust" on any of the machined surfaces. It seems to have iron included in the rolled surfaces only.
After we found this, we took the parts we had in stock and tryed both chemical passivation and electropolishing them to remove the inclusions. Nothing has worked.
The funny thing is that there seems to be so much iron in the surface that it should inherently change the chemistry of the heat, if the bar was tested on it's own.

My question is: is there a specification somewhere that tells my vendor the amount of allowable contaminant in the surface?
I have searched ASTM a480,a484,a564 for reference to this and cannot find it. I don't have a262 but I believe it's our vendors responsibility to supply "clean" steel and do the testing. I don't want to change steel unless I have to do so, because what we have works well and there have been no incidents like this in 8 years of history.
I'll listen to any suggestions at this point.
Thanks

 

[rorschach]

IMHO, parts machined from rolled stock should always be machined from bar stock a bit larger than the part and a minimum of a 1/16" should be removed from the surface to remove laps and seams which can be crack and corrosion initation points. Most metallurgists routinely make these recommendations but a lot of people don't heed this advice. These laps and seams can harbor contaminates such as you've found. Additionally a lot of stainless steel is now being made in facilities that were originally carbon steel mills and they don't have the AOD or VIM/VAR equipment that more industrialized nations have. Not all 17-4PH is equal in quality. Some facilities do a better job. if you buy only from major suppliers (Sandvik, Thyssen-Krupp, Carpenter, Allvac and others)you'll generally have better and more consistant quality material.

 

[unclesyd]

We have used 17/4 PH SS from it's inception in all forms and have never had any problem as you describe with contamination.

Have you been able to ascertain whether the iron is free iron or an oxide?

Another possibility is that the material may have been solution annealed in a scaley heat treating basket. I have seen a little of this on some stock.

What were the specifications on the purchase order?
What does the supplier say?

 

[rorschach]

Were the parts Passivated after machining? you may be seeing contamination from tooling or from handling with carbon steel fixtures.

 

[Vonmagnuson]

Don't know if it's free iron or an oxide. I don't have any method to verify either.
The material has been solution annealed. So far the supplier has said nothing.
The passivation can be ruled out as the source. We did salt water tests before and after passivation with the same results. Also, there is no "rust" in the areas of the parts where they have been sawn or machined. The only places this oxide is showing up is on the surfaces that have been rolled. We have never seen anything like this before either, at least not to this degree.
We are going to blanchard off .002" at a time and see how deep this stuff is.
Thanks for all the input.
Bill

 

[EdStainless]

Maybe you should try pickling some and see if that removes it. My guess in rolled in debris. It may be some 400 series stainless or some oxide.
You have mill finish bar, most mills would not consider this a service surface.

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