Silver steel increase in diameter on hardening.

Does anyone have experience of and/or an explanation for silver steel "growing" by up to 1/2 a thou' when it has been hardened and tempered to say straw?

Yes, it changes its structure when you heat treat it and it grows a tiny bit.

regards David

Hi Alan, this is a well known phenomenon but I can't remember why it happens?

Tony

I believe the grain structure changes, the material alters completely.

You heat it to red, you leave to stand a few minutes to refine it and reheat it to red and then quench it.

regards David

David is right. It's abit like putting snooker balls in a box. If they are packed as hexagons they take up less room as if they are packed on a square grid. With iron atoms it's more complicated, but the same principle. I expect wikipedia has a full explanation.

Neil

Evidently, there are many instances when materials behave `badly’.

Heading slightly off topic, this reminds me of the phase change in PTFE (Teflon), which only `surfaced’ for me when I was making small valves for biological research. Refer to my album - `A Plastic Valve'

The body of the valve was PMM (Perspex) and the spools were PTFE. The success of the valve came from its leak resisting performance. A primary requirement to achieve this was selective assembly between the bore of the PMM body and the PTFE spool. Even minuscule differences in their respective diameters could cause the spool to be too tight or conversely, they would leak.

It needs to be noted here that PTFE in its natural state (ie. no filler), has a low creep resistance so that a marginally oversize spool will become `squashed’ over time.

As with most materials, a change of temperature causes the volume of PTFE to increase or decrease correspondingly. However, at 19 degrees C, there is a phase change, causing the volume of this material to increase sharply. Expansion then continues at a rate similar to that below 19C.

I had developed the valve for use at around 38C although one particular series of experiments required that the valve as supplied, had to perform at temperatures in the range of 5C to 15C.

That was fine except that they couldn’t be stored above 19C while assembled due to their low creep resistance. I had to sell them with the spools removed, and with instructions to store them refrigerated after assembly. Prolonged storage above 19C would ruin the requisite degree of fit.

Alan, hope you don't mind me asking a question in your thread. It's somewhat related.

Is it possible to increase the diameter/overall size of a lathe shaft by heating? I have one that's made of mild steel and has understandably worn over time running in oilite bearings.

Thanks,
Ed.

I think you are out of luck on that one.

Metal spraying might be the way to build up the shaft, then it would have to be turned/ground back to size.

Or get it centreless ground to remove any scores or taper and then make new, unndersize, bronze bearing bushes.

Russell

Ok, thank you gentlemen.

Ed.

If you normalise the component by heating to a cherry red and leave to cool, preferably in a container filled with lime before hardening, should help to stop any movement, however small, caused by the heat treatment. This is especially true if the component has been subjected to several machining operations

Thanks for the replies. There has been an ongoing saga each Saturday night drive to York from Ripon to attend York and District SMEE. It has even involved the wager of a whole fiver between two Scotsmen. Now you know how serious the discussions have been.

Regards,

Alan.