Expansion of brass component after heating to 800 deg C

[iFixCameras]

Hello and thanks for letting me join this forum. I have a question about changes in dimensional characteristics of machined brass components after heating, in this case after a repair using silver solder.

The component in question is a tubular machined component made out of brass (no idea what alloy but this is about 50 years old at least). A localized repair was necessary and in the end it was decided that a silver solder was required to do the job. The dimensions are approx 30mm ID and 35mm OD. The external face has a helical thread - part of a lens. Care was taken to heat as evenly as possible and the part was left to cool to room temperature.

I didn't take any 'before' measurements unfortunately, as I didn't expect there to be a problem but it appears that the part now is slightly oversized. The threads are not even trying to 'bite'. I made a mandrel to mount the component onto, to. It is a tight fit and it doesn't look as if there is any distortion to the circular shape. I did quite a bit of searching and have found a lot about grain sizes etc from a metallurgy point of view but nothing about before and after dimensional changes to components after heating. Has anyone any idea of the likely percentage or otherwise increase in size of the component - or even to confirm my suspicion that the part is now somewhat bigger than it was before I did the repair? I'm hoping I can save this component rather than have to consider making one from scratch - a bit out of my comfort zone - never cut helical threads! Thanks.

 

[EdStainless]

It could either be from stress relief or simply the silver filling a gap and making it larger.
Though 800C is very hot for a brass and you could have formed some secondary phases that happen to take up more room than the original material.

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P.E. Metallurgy

 

[Compositepro]

Brass threads are soft and easily deformed. Your problem can be fixed by hand "chasing" the threads with a triangular jeweler's file or sharp single point tool. You will probably only need to work on the start of both the male and female threads to get them to engage. If they are still too tight, then continue to chase until the threads fit. The process involves scraping the thread grooves to remove any burrs or high spots caused by deformation of the brass. The scraping tool will naturally follow the thread groove. You do not need to use a lathe. The most likely cause of your problem is an initial unsuccessful attempt to engage the threads, which caused some damage. Additional attempts will just cause more damage. After chasing, make sure the threads are completely clean of any particles before attempting to engage them.

 

[TugboatEng]

That's such a small piece of stock I can't comprehend why you wouldn't start from scratch. Threading such a thin section seems problematic to me. I would much rather finish the bore after threading the OD.

 

[GarryB1957]

Hi IFixCameras,

Before coming over to the dark side of corrosion and cathodic protection engineering, I was a toolmaker, so I am very well versed with threads and how to repair them. I can't really give you much insight into the cause of not being able refit the threaded components, as this could be as suggested about contamination or distortion due to the heat. However, before any attempt is made to repair a thread it is important to identify what thread it is. There are many threads that all look similar. Most people think of thread forms be a 60 degree incuded angle, this is not true. It would be of benefit to know where the component was manufactured, if in USA it is almost certain to be 60 degree thread, the same can be said for europe as most metric threads are 60 degree form. There are however, other thread forms that are 47.5 degree and 55 degree. If it is one of these different form using a jewelers triangular file will cause more harm and maybe even destroy part of the thread. Also to repair a damaged thread a thread chaser (thread file) should be used.

As your componet is a multi start thread, just guessing from the picture something in the region of 24 thread the effective pitch for a thread file for repeir purposes can be calculated from the distance a single thread travels per revolution divide by the number to threads start on the circumfrence.