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The Curious Case of the Cracked Injector

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Nigel Bennett22/01/2022 12:29:06
456 forum posts
20 photos

Here was a curious (not to say irritating) thing; I made a batch of five injector bodies. I happened to have some nickel brass in stock, so I used that. Some folk call it nickel silver, others German silver, but the "official" term seems to be nickel brass. Anyway, I used it because it's a very similar colour to silver solder, so it might not look so very obvious a fabrication.

All went well; I made them all with the same bar, used the same brazing fixture and the same stick of solder for all five. Four were absolutely fine, but one has cracked to a quite amazing degree:





What on earth could have happened? I can only imagine it's a material fault in the bar; but I hope it's not something that will suddenly appear in the others. I have heard about brass self-destructing due to high inbuilt stresses (stress-corrosion cracking) after extruding - particularly with thin sections, but not 3/8" round bar, surely?

Next batch will probably be in brass...

Nigel Bennett22/01/2022 12:36:12
456 forum posts
20 photos

Try and enlarge them a bit...





Seems a bit clearer!

Speedy Builder522/01/2022 12:43:17
2590 forum posts
207 photos

Did you quench in cold water (wintertime) to clean off the flux? Subsequent ones may not have cracked as the water had been warmed up by that time ??

noel shelley22/01/2022 13:14:32
1278 forum posts
21 photos

Hum ! Brass can be strange, all cracks seem to be on the main component part, there may have been a flaw in manufacturing. Pessure test the good ones to 2x and see what happens ? Speedy may have it ? Noel.

SillyOldDuffer22/01/2022 13:17:52
8469 forum posts
1885 photos

Amazing! New one on me, but I did find this on the web:

Nickel materials or parts stressed or cold worked may be subject to stress cracking when coming in contact with molten filler metal.

To avoid such cracking, stress relieving should be performed either before or during Brazing-nickel.

I guess some slight difference in the process took one example over the edge. Perhaps the faulty one was made from the bar end, or was over or under heated compared with the others. I notice Nickel-brass is intended to be wrought (rolled or stamped), rather than machined. Possibly machining increased the injector's internal stresses and then the extra stress caused by brazing heat broke it.

Stress relieving is probably just gently heating the metal up to low red heat, holding for a few minutes (increasing in proportion to weight), and then quenched.


Nigel Bennett22/01/2022 14:13:29
456 forum posts
20 photos

Ha! The mystery is solved! I've just picked up a "good" injector body and done a careful length check on it to be sure of its actual dimension before I started on the cones. It was miles out! Whaaat??? Check the others... They had all shrunk in length by about 0,2mm - and I'd made them all to within 0,01mm. What must have happened is that the body had expanded during heating, but my brazing fixture is made from a big block of hollow square cast iron. The body had nowhere to go and so it got squashed as it expanded and softened.

Thinking about it I've often seen dents in brass parts of a fabrication when I've tightened up a G clamp or a fixture screw a bit too tightly.

I've just taken a length of nickel brass bar now, turned it accurately to length and heated it up to dull red just by itself. No change in length once it had cooled down, and no cracking.

Then I put it in the fixture, heated it up to dull red again, and surprise, surprise, it's 0,2mm down on length and has cracked.

My scrap bin now has five too-short nickel brass injector bodies and I'm changing the way that the bodies are held to ensure they can expand unrestrained when heated. Back to the lathe now to make some more - and in nickel brass because I still have a fair length of the stuff.

SillyOldDuffer22/01/2022 14:36:54
8469 forum posts
1885 photos
Posted by Nigel Bennett on 22/01/2022 14:13:29:

Ha! The mystery is solved! ...

Thanks for explaining, I'd never have thought of that.


Martin Connelly22/01/2022 15:56:55
2123 forum posts
222 photos

We used to do a part at work that had two autogenous welds in Ø32 x 1.2mm wall 321 stainless steel. The central part had to be made 2mm longer than nominal to allow for the weld shrinkage that occurred due to the two welds. We regularly had to re-melt welds to pull parts back to square after welding. Elbows in pipes were the biggest cause of this as they had a thicker inside wall due to being bent when made. The forces involved with trying to prevent the movement were very large. I am not surprised they were high enough to cause cracking in a relatively brittle material. We were always having a go at the people who put out drawings with tight tolerances on welded parts, pointing out that if they wanted such exact sizes then the part needed machining after welding and stress relieving. I have seen a 35mm thick plate bend after welding was carried out on one side only.

Martin C

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