Sammcc:
I don’t understand how finely that fit is intended to be, and you haven’t chosen to tell us. If the next pipe just has to fit in the i.d. easily and it is o.k. that it ride on a couple high spots or high ridges on the i.d., then you could probably do some of them, measure the change in the i.d., and finally learn what the oversize bore has to be. During this learning process you would ream them to make them work, after measuring. Finally, you might be able to avoid the reaming operation from what you’ve learned. I don’t think it’s a heat sink (cooling) issue. The welding operation is inducing a circumferential residual stress which is squeezing the pipe to slightly smaller dia., and the more so the nearer you are to the weld, and you can’t find a mandrel strong enough to prevent this. You could expand the pipe, putting it in circumferential tension to counteract this shrinkage from the welding. That expansion dimension should be about what I’m telling you to measure, but you are measuring the shrinkage you want to overbore to compensate for. This shrinkage will depend on the heat input during welding, the pipe dia. and wall thickness, and the configuration of the weld. So, a thicker pipe with all else being the same, will shrink less. A lighter, less heat input, weld will cause less shrinkage, all else being the same. This is kinda like the material movements during a shrink-fitting operation.
