Windy, I remember reading about that process on here. **LINK**

Try doing a search. John

Hi Windy,

Its been mentioned in ME. As I recall you soak it at red heat for a good while as if you were normalising it and add a thou or two per inch, then re-lap it.

Obviously you can only do this once.

Neil

I found this – makles it nice and simple, eh?

"

Irreversible Carbon Diffusion in Growth of Cast Iron

1984 MRS Fall Meeting.

K. Nagaoka^a1 and M. Souma^a1

^a1 Faculty of Eng., Hokkaido University, Kita 13 Nishi 8, Kitaku, Sapporo, 060, Japan

ABSTRACT

In gray cast iron carbon diffuses irreversibly and the result is a growth remaining castoff voids and graphite redistribution. The origin of primary growth is in the mechanism of carbon diffusion which is controlled by a ratchet at graphite where carbon is carried by vacancy flow. Growth of flake cast iron in oxygen reduced air at low oxygen content is accounted for by self carburizing due to a reaction of penetrated oxygen and graphite flakes. For the irreversible carbon migration by gaseous carrier another ratchet is suggested. Cracking theory for primary growth is replaced by castoff void theory, and oxidation theory is modified."

Hi Neil ,

As I'm sure you know already – there are almost as many Great Experts in the academic world as there are in model engineering – a knowledge based filter is nescessary when reading some of their astonishing works !

What is in that paper is not wrong but it can be explained much more simply and has in any case been known about by engineers and metallurgists for many years – possibly centuries .

Several metal alloys grow a little with time when heated above a critical temperature but beware they are also degrading – heat grey cast iron for long enough and you'll get a crumbly sponge .

There are several ways to increase the size of a cast iron component by a very small amount by more controlled and less damaging processes .

(1) Using variations of case hardening without the final hardening bit – for instance heating in a bath of finely ground cast iron with an added activator and leaving to cool naturally .

(2) Using deliberately initiated creep – contrive a moderately heat resistant jig which acts so as to force the sections of the casting in the desired direction . For a piston this would be some variant of circular wedges in/on a mandrel fitted inside piston and moved progressively by a screw or pressing arrangemnt as heating progresses .

Methods of adding larger amounts of metal involve weld deposition and Chrome plating .

Very large pistons have sometimes been given emergency repairs by sleeving .

NB: Method (2) above works well on some aerospace materials including Titanium and was once used quite extensively for recovering the proper shape of distorted aeroengine components and putting deliberate distortions into things for experimental purposes .

Personally for a tiny piston on a model engine I'd just make a new one – advanced repair methods are only of use on higher value components or where a new one cannot easily be obtained .

Regards ,

Michael Williams .