Metal expansion

Hello, I am trying to get my head around what happens to, say a cast Iron slab with a hole in the centre, when it heats up. Does the hole increase or decrease in size as the temperature increases? Logic probably says that it increases but if the metal surrounding the hole increases in size in all dimensions, perhaps the hole would decrease in size? Can anyone tell me what happens in reality? Many thanks, David

I remember asking this exact question to my metalwork teacher at school. He told me off for being a smart arse, most likely because he couldn't ask the question. There is a lot of info on this subject out on the internet, the TLDR answer is the hole does get bigger.

By observation holes increase in size as metal heats up and shrink when it cools down. The effect has long been used to fit iron tyres on to wooden wheels, and steel tyres on to cast-iron railway wheels.

Dimly remembered explanation: Heating causes metal to expand evenly but the overall distance moved is bigger in the longer dimension. As the forces pushing the hole open due to its circumference are greater than the forces across the diameter tending to close it, the hole's diameter increases.

Fun question based on a fact I came across by accident. No prizes, but can anyone name the three metal elements that expand on cooling?

Dave

Edited By SillyOldDuffer on 04/10/2019 12:19:50

There are doubtless videos, showing the ‘ball and ring’ thermal expansion school experiments, on the net?

As SOD points out, shrink fitting of parts could not happen if the hole ‘shrunk’!

Invar, a metal alloy, has a nearly zero coefficient of expansion.

Imagine a block with a hole drilled through. Now image a plug of the same material that exactly fills the hole. Ignoring friction etc, the plug can obviously be removed and inserted at will. Now, with the plug in position, heat the block and plug up together. Clearly the plug still fits the hole, so the hole expands the same as the plug.

SOD

One of them is antimony used in metal type metal to make sure letters are crisp

SOD

I have an invar pendulum rod that expands with cold and contracts with warming up

Roy

ps I have been told that it should have been heat treated before I used it

Thanks Chaps, appreciate your responses.

David

Correct! There are a few alloys that expand on cooling, but I was thinking of pure elements. It would make a good pub quiz question I think! I came across the fact in a recent secondhand bookstore find: Hampel, Clifford (Ed.), Rare Metals Handbook, Reinhold Publishing Corporation New York, 1956

'Bismuth is one of the few metals (Antimony and Gallium are the others) which increases in volume on solidification., this expansion being 3.32 per cent.'

Quoting the handbook I see I've been careless again: the metals expand on solidification, which isn't necessarily the same as expanding on cooling.

Dave

Same as water, which expands when it freezes. In its solid or liquid state it expands on heating, it's just the transition that is odd. Lucky though, otherwise ponds etc would freeze from the bottom up - but unlucky too 'cos Titanic hit a floating iceberg.

Cast iron.

JA

Hi, if you think about it, being that most things expand equally in all directions, unless otherwise restrained, think of the metal bar in a vice being heated up and then just drops out when it cools without opening the vice, then take the amount that the metal would expand towards the hole compared to the circumference of the hole, then it stands to reason that the inside of the hole will expand a greater distance and will have to become bigger. Of course, this is assuming that the whole piece is heated uniformly, because if it was only heated around the hole, it would be restrained by the cold metal surrounding it and the expansion would be greater sideways and the hole may even stay more or less the same size.

As for water does it really expand when it freezes? water becomes ice when it freezes, so maybe it can be considered no longer water. Ice of course is less dense because it expands and is lighter than water for the same volume, hence it floats on water. Change of state and all that.

Regards Nick.

Just because it has different names doesn't mean its different substances. Ice, water, and steam are all names for the substance also known as H²O. Water starts to expand when it gets below 3.98°C and reaches its minimum density at 0°C.

The alloy invar still expands when heated, but at a lower rate than other metals.

old mart

my pendulum rod doesn't, see post above

Roy

Roy

Is your pendulum a grid iron type? If correctly proportioned, a grid iron pendulum should remain the same length irrespective of temperature.

Which is why Harrison used brass and steel (possibly iron ) elements in his grid iron pendulum

On Google, Colunbia metals state that invar has a near zero rate of thermal expansion.

So a straight pendulum made of invar will have coefficient of thermal expansion that is low.but not zero or negative.

Howard

Howard

It is a straight rod and contracts in warm weather and expands in winter ie the clock gains in summer and loses in winter . The pendulum bob is mild steel and is supported dead centre. As I said before I was told that it should have had heat treatment before I used it, and I believe that this would have been two days in a butchers fridge then left at room temperature for two days then repeated. This was from an RHS member who contacted me after I contacted them for advice

Roy

Roy is correct that Invar needs to e in the right state of heat treatment to have near zero expansion. Chuck it away and replace it with carbon fibre, readily available as bar or (more convenient) tube I've got a bit in the rafters for my clock, which currently has a mild steel pendulum. It gains in winter and loses in summer, over a year it's pretty good!

We learn something every day!

Howard