Author Topic: Cast iron wheels : To anneal or not to anneal, that is the question?!  (Read 215 times)

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Offline John Candy

I had wheels specially cast by Mark Wood for my GWR 33XX class "Bulldog" 4-4-0.

Mike Danby arranged for them to be turned by Walsall and today reported that they intend annealing them. This surprised me, since I have always understood that a tungsten carbide cutter would remove the "crust" left by the casting process.

I immediately contacted Mark Wood who sent me a very full and helpful reply. He drew attention to the fact that his wheels are investment/loss wax castings and not from sand moulds, so don't have the hard "crust" associated with that method. He is not aware of anyone having annealed his wheels and has never had to do so himself.

I was concerned that annealing may weaken the crystalline nature of the iron but Mark said that he was not aware of sand cast wheels having been weakened by the process.

John.

My fellow Members, ask not what your Society can do for you, ask what you can do for your Society.

Offline Peaky 556

Re: Cast iron wheels : To anneal or not to anneal, that is the question?!
« Reply #1 on: August 28, 2018, 02:53:55 PM »
This seems to be a very specialised subject, and I am no expert.  I suspect I am not the only one to have consulted Wikipaedia, and the discussion there raises more questions than answers.  In essence the process of tempering cast iron is to increase its malleability (ability to plastically deform), but the process is guided by the original composition of the CI, and may involve heating to around 950 to 1000C for lengthy periods (several days) before being allowed to air cool.
My feeling would be to support Mark Wood in not undertaking any tempering, based upon his experience, and that of hundreds of users of his wheels, and supported by the potential cost of doing so!
Sorry that’s all I can offer John.
Regards, Tim

Offline IanT

Re: Cast iron wheels : To anneal or not to anneal, that is the question?!
« Reply #2 on: August 28, 2018, 05:13:24 PM »
I can't remember all the specifics now but I have certainly 'annealed' castings in the past (not that much recently though). The reason has always been 'cold' spots (caused by the iron cooling unevenly or 'chilling' ) - which will quickly blunt a carbon or HSS tool. Normally you can hear them rub as you are turning the casting (sounds a bit like an interrupted cut) and they show up as 'bright' spots if you examine the surface carefully.

A tipped tool might help (they certainly do when getting under sand crust) but if you are already near to dimension on a casting - then annealing is worth trying. In my case, this has involved burying the part in a hot (wood) fire, stoking the fire up and then letting everything cool overnight in the ashes (the casting will still be too hot to handle the next day even if the fire has completely died down).
   
Generally, I've only done this after discovering cold spot problems - although I have 'pre-annealed' several large (tool) castings recently because they looked discoloured/burnt in some areas - suggesting something might have gone a bit amiss. As an aside- some 'scrap' steel and any small welded bits I make also get the overnight treatment (de-stressing) if possible before machining them.

Having said all this - I wouldn't expect Mark Woods wheel casings to have cold/chilled spots, mainly because all his castings that I have seen have not only had very fine definition (due to the lost wax process) but also a very even (clean) colour - indicating (I think) good process & quality control. They are also far from cheap - so if I did find a chilled area, I'd just return it and ask for a replacement.

So yes, annealing can be useful (certainly was required at one time - as casting quality varied a lot) but no - I wouldn't anneal Mark Wood castings as I very much doubt they require it. Puzzling, as I would also consider WI as being perfectly competent in most things 'engineering' - so maybe there is some reason/logic here that I'm not aware of (maybe simply Belt & Braces?). Perhaps worth asking them?

Regards,


IanT


Nothing's ever Easy - At least the first time around.

Offline John Candy

Re: Cast iron wheels : To anneal or not to anneal, that is the question?!
« Reply #3 on: August 28, 2018, 07:22:21 PM »
Thanks Ian and Tim.

The response from Walsall (as quoted to me by Mike Danby) goes like this.

Quote
WMI insists on annealing MW castings as they have had previously broken some and “it costs a bleeding fortune “.
Their words.

I guess WMI must do what they think is best!

Regards,
John.

My fellow Members, ask not what your Society can do for you, ask what you can do for your Society.

Offline IanT

Re: Cast iron wheels : To anneal or not to anneal, that is the question?!
« Reply #4 on: August 28, 2018, 08:53:49 PM »
I've never turned any of Mark Woods wheel castings John but others have told me they are very nice to work with. However, since you are paying WMI to do the job, as long as you are happy with the end product, then I guess it doesn't matter too much how they do it.

I thought I had a photo of some wheel cold spots somewhere but if I do - I can't find it.

Just out of interest however, I dug out these two castings from my stock box to show how they can vary. They are both from the same source but purchased at least five or six years apart - and I suspect they are from different foundries. The one on the left was from the first batch ordered (I used some of them for another engine). The one on the right is from the castings I [eventually] ordered to replace the ones already used - so I had a complete set for the original engine (just in case they became unavailable later).

I'd be fairly sure the pattern used was the same for both castings but quite apart from the colour - I hope you can also see other differences in the surface finish (lumps in centre) and poor spoke definition of the left hand one when compared to the right hand one.

I can't remember now whether I had problems with the first batch or not but when I do come to use them, I will probably give the left-hand ones the 'heat' treatment just in case...

By the way - this issue of 'chilling' has nothing to do with getting under surface 'crust' (a problem encountered with sand castings) - so although a lost-wax casting will obviously have no embedded sand in its surface - they could still suffer from cold/chill spots I guess.





Regards,

IanT
Nothing's ever Easy - At least the first time around.

Offline John Candy

Re: Cast iron wheels : To anneal or not to anneal, that is the question?!
« Reply #5 on: August 28, 2018, 09:52:32 PM »
Thanks Ian,
The differences between the two examples can be clearly seen.
Mark Wood said in his reply,
Quote
the castings cooling rate is slowed by the ceramic outer case so that they do not chill
.
Since WMI have elected to anneal them, if anything goes wrong then the ball will be in their court.
Regards,
John.

My fellow Members, ask not what your Society can do for you, ask what you can do for your Society.

Offline Nick

Re: Cast iron wheels : To anneal or not to anneal, that is the question?!
« Reply #6 on: August 30, 2018, 09:09:23 AM »
Having turned wheel castings from Mark Wood and WMI, I think I can usefully comment. Mark Wood's castings are lost wax, don't have a hard shell, and certainly don't need annealling. I have not found any hard spots nor would expect to. I prefer a carbide tipped tool because the wear rate is lower than for a HSS tool, but the latter is usable. I do use Mark's own form tool, which is HSS, for the final profiling of the tread.

WMI's castings do have a hard shell and in my experience, may have hard spots. Annealling is a good idea, as is a carbide tipped tool to get through the shell is pretty much essential.

Nick

Offline John Candy

Re: Cast iron wheels : To anneal or not to anneal, that is the question?!
« Reply #7 on: August 30, 2018, 10:21:34 AM »
Thanks Nick,

That confirms what others have told me, including Mark Wood.

Regards,
John.
My fellow Members, ask not what your Society can do for you, ask what you can do for your Society.

Offline IanT

Re: Cast iron wheels : To anneal or not to anneal, that is the question?!
« Reply #8 on: August 30, 2018, 10:31:02 AM »
Seems like a general consensus on MW castings and the need (or not) for annealing.

For anyone considering turning some sand cast wheels - I do agree with Nick that a tipped tool will greatly help to take the surface crust off them and I do use one to do so. However, I still use a [sharp] HSS rounded nose tool for finish turning the rest of the work as they give (in my opinon) a better finish and also help in creating the final tread/flange form.

As I think has been discussed here before, I have never used a form tool to finish G3 wheels, as I really don't see the need for them (and it also costs £40 plus postage). A rounded nose tool and a few file strokes (usual "care is required" disclaimer) is all that is required in G3 in my experience.
   
I described my approach to wheel turning in the G3S Newsletter a few years back (about 2015 I think) but if anyone would like the text, just PM me. There is no 'absolutely' right or wrong way of doing some of these things and every machinist has their own personal preferences - and provided your way works (and suits you) then that's probably all that matters.
 
Regards,

IanT
Nothing's ever Easy - At least the first time around.