Brass Fitting Fails Merc Nitrate Test (Possible SCC) ?

[mscatoloni]

We have machined a hydraulic 90 deg elbow pipe fitting from 360 Brass Alloy (Size -16, 1" OD line) (aprox 600 elbows made). A sample of 13 was subjected to the Mercurous Nitrate Test (ASTM-B154) and 3 of 13 parts were observed to have cracks on the OD radius and wrench pad area. Can anything be done to the "finished part", (which has taper pipe threads), to stress relieve bake the apparent residual stress and cracking that was observed ? ....without compromising the thread geometry from heating ?

A sample of the 360 Alloy Brass forging was Merc Nitrate Tested (successfully)prior to machining it. The elbow was machined on a CNC lathe using standard insert cutting tools (Kenemetal) and water-based coolant was used.
Any idea's to the root cause and possible avoidance of this problem, would be appreciated ???

- Mark S.

 

[kenvlach]

A standard stress relief for this alloy is 1 hour at 475[sup]o[/sup]F. With existing residual stresses sufficient to cause cracking, there could be some warping, but perhaps OK with pipe threads.

I don’t see how the described machining could cause the problem if entirely machining from bar stock. Is there some pipe bending involved? This free-cutting alloy is rated ‘poor’ for cold working by the CDA (Copper Development Assoc.).

 

[kenvlach]

Were the parts all machined from forgings, or just the sample?

Alloy 360 is rated ‘poor’ for hot forming, i.e., it is not suitable for forging.
The recommended brass for forging is 377, next best is 675. Also rated excellent for hot forming are alloys 365-368, 370, 385, and 464-467.

Copper and some silicon bronzes (651 & 655) are rated excellent for both hot and cold forming, but machinability is much lower.

 

[mscatoloni]

Kenvlach,

Thanks for posting.

Actually, the brass alloy was 377 Forgings per QQ-B-626 (not 360 alloy), my mistake. As it turns out, we resolved this problem, unexpectedly. Further examination of the failed samples revealed a surface mark (appearing like a crack), running along both sides of the forging body. We traced this "mark" to our soft-jaws used in the CNC machine for holding the forging body. Once we performed a light polish and grind to remove the marks, re-submitted samples to the merc nitrate test, they all passed. Obviously, we did not have a stress problem (SCC) with this material, only a surface defect caused by our machining method (jaw marks).

Hope this info will help anyone running into this situation.

Thanks again for responding.

-Mark Scatoloni
Faber Enterprises, Inc
Canoga Park, CA
(manufacturer of hydraulic fittings)