Voids in SB-550 zirc bar

[trottiey]

Machining uncovered some large internal flaws in SB-550 zirc bar, 4.5" diameter round. These flaws are several inches long and almost one-eighth inch in width. Is this normal? To my mind, "bar" implies hot-rolling which would squeeze out voids this size.

The scary part is that I had planned on making this into a pressure vessel without UT. Hollow cylindrical parts up to NPS 4 can be machined from bar with no UT according to UG-14 / code case 2148. Beyond NPS 4, UT is required by code case 2156. So based on that, I had naively waived UT.

I'm eating crow and doing UT now, so the immediate problem is solved. But I'd like to figure out where I took a wrong turn. Did we get bad material, or did I miss a UT requirement in the code, or is this just something that designers are supposed to know?

 

[Duwe6]

Just looked at SB-550: your bar could be a trimmed casting, with little or no hot-work vigorous enough to forge defects shut. It is probable that you have run into a "shrink tree". Are the discontinuities rounded, long and branching?

Other possibility is a true lamination: dross/slag got included inside the bar during pouring & rolling.

 

[trottiey]

It looks like I'm not going to get to see any pictures. But the people who saw them said they were not laminations. So shrinkage does seem like the best explanation. I recognize that SB-550 does not explicitly require hot-work, but I would have thought that this would be required for any boiler code bar.

This seems like a hole in the code. If cast billet is a permitted material, then it seems like volumetric inspection of billets should always be required. In paragraph UG-14, why is the NPS 4 phrase enclosed in square brackets? Does that have something to do with it?