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17-4 Hot Roll

We make a structural bracket out of 17-4 Hot Roll Bar stock that is heat treated to the H900 condition. For reference, the dimensions of the stock is 1.75 wide X .25 thick and the preform has a length of about 8 inches. After coming back from the heat treater, the parts are bead blasted, industrially cleaned by a commercial plater in a HCl solution and passivated in 25% nitric acid solution. The problem is that evan after all this, there are these straight black lines in the surface layer of the material that run lengthwise on the top and bottom surface. The width of the lines can be up to 1" and they won't come out no matter what we throw at it. I suspect that there is significant porosity of the surface layer and the hot roll process introduces all kinds of stuff into it. We are going to try some other steps to get this out, but if anybody has any suggestions please pass them on.

Mark Sorensen
casters - Redwood CIty, California, USA
2006

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First of two simultaneous responses --

I didn't know designation 17-4 was used for hot rolled stock. I have always related this Spec to cast materials. For rolled stuff I've always seen SAE 300 series designations such as 304, 310 or 316. Your mention of porosity also points to an inferior quality. Are you sure your material is rolled? You should confirm if it is porosity there's no way a conventional cleaning or passivation will eliminate it. On the contrary.

Guillermo Marrufo
Monterrey, NL, Mexico
2006

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Second of two simultaneous responses -- 2006

Maybe carbide formed from a graphite-containing hot rolling lubricant?
"Other steps to get this out" should include cleaning prior to heat treatment.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

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PH 17-4 is a High Iron and chrome with little nickel stainless steel and you can get it almost any type of condition, this material can be hardened.

the veins could also be mill lines

Chris Snyder
plater - Charlotte, North Carolina
2006