SEASONING OF CAST IRON

In a recent thread concerning painting cast iron, Andrew Johnston mentioned the seasoning of cast iron. Rather than hijack that thread I have started this one to try and find out a bit more about this subject.

Certainly it used to be fervently believed that seasoning castings made from cast iron was necessary to relieve stresses and avoid distortion after machining. There were many ideas on exactly how to do this, some of doubtful merit including urinating on it.

If seasoning was ever necessary, was this because of the grades of CI available at the time, and possibly their method of production? Therefore not necessary now?

Does Andrew or anyone else ever season CI castings, i.e.is seasoning relevant nowadays? Especially for the size of castings we are likely to encounter in model-making activities.

If it is relevant, what sort of temperature/time cycles are we talking about?

Chas

I was advised to leave new castings outside for 2-3 months. The repeated heating and cooling by the sun would releive the stresses. Others suggested to put them at the base of a large fire pit and allow them to cool slowly in the ashes.

I had plenty of other parts to make and left them to weather outside for about 3 months. Did not have any problems with distortion after machining but don't know if there would have been any if I had machined them sooner.

Paul.

I have seen one Optimum Chinese-made micro/mini lathe whose cast iron bed bent like a banana after a few months, to the tune of 1mm bend in the top bed ways. Most likely but not conclusively a "green" casting.

On the other hand I remember acres of unmachined cast iron heads and blocks deliberately left sitting out in the weather for months behind the Chrysler car factory where I served my time.

The first job I had in 'straya was at a company that made mining machinery. The car park was used to season castings, and there were a lot of them. Apparently the "seasoning time" was related to the length of the grass. It was advisable to be really careful when parking the car to avoid hitting a casting.

cheers

Bill

I would expect that ‘weathering’ castings allowed the surface stresses to be relieved, so that later machining did not need to involve large areas of ‘skin’. I would expect modern-day casting operations involve heat cyling at low temperatures to make things like engine blocks (12” to the foot scale) useable in much shorter timespans. Certainly the engine makers want to work with ‘Just-in-Time’ inventories, these days and I doubt the metal is any different than in the old days - just a rush nowadays, rather than allowing larger time frames for castings to mature. Seems sensible to mature castings at the foundry, under controlled conditions, than at the machining point - plenty of spare heat?

One certainly does not want engine blocks changing shape after the crank bearings have been line-bored!

When I started work in 1963 castings where left out to weather for months and years depending on how many the foundry produced. This kept the foundry busy as they never had to let the furnace die but just kept pouring metal. I was told at the time it was to allow the casting to stress relieve. Fast forward over 50 years and I was asked to machine a batch of castings in my home workshop, I asked the question re how long the castings would be kept for before I received them and the answer was that castings where no longer weathered due to better heat control with the electric furnaces and better quality iron. I did around 90 castings, no chilled skin or blow holes in any of them.

Boxford used to weather castings outdoors for around 6 months. A labourer used to water them daily if it wasn't raining or snowing and, yes, some passing personel did take a leak over them. After 3 months the raw, rusty bed castings were shot blasted and a rough planed top and bottom before being returned outside for another 3 months in the elements. Finally they were shot blasted again, filled and painted before going to the machine shop. The shot blaster was a very violent machine, fully enclosed & used steel slag - it was very effective at cleaning the castings of rust & also probably contributed to stress relief. Good for cleaning up more substantial rusty motorcycle bits as well if you spoke nicely to the operator !

The beds for the later geared head machines that were induction hardened did not get weathered - I did ask why & was told that they were heat treated at the foundry to relieve stresses. They did bend when hardened, though, which some attributed to the lack of weathering. A method was evolved that involved torquing down a clamp in the middle of the bed to pull it hollow before hardening. When the hardening pass was completed, the clamp was removed to allow the middle of the bed to rise before finish grinding the bed formation.

Can't verify the accuracy of the statement, but I was told that the castings for the Swiss SIP & Dixi machines were dropped into lakes to "weather" in the mud for a period of time.

Nigel B.

Many years ago I worked on an "SIP" jig borer that was made in the 1930's. The guy who did the maintenance reckoned the the iron castings were left for 7 years to season before being machined. I initially thought this was a sign of quality. I've no idea how true this is, I'm now sceptical. The castings were, no doubt, of the highest quality, perhaps not needing much seasoning? Surely in the 7 years wait improvements and upgrades to the machine would happen rendering the castings unsuitable? Even back then heat treatments to stress relieve the casting were available.

I would guess a good way to season an iron casting would be to rough machine, take the "skin" off, then heat treat to stress relieve.

John

A spindle moulder which I bought from a "Well known and respected maker" came with a fence that was so distorted as to be useless. I was obliged to have it machined by a local workshop in order to use it. When I remarked on it to the machine man, he just smiled and shook his head and said "They don't bother to season castings anymore, so this is what you get " That was in 1990, so what,s new ?

Even in the late 90's crankcase and bed plate castings at Ruston were 'seasoned' in the manner previously mentioned, left to stand outside initially then part machined and returned outside and then finally brought back in for finishing. Not sure if this appllied to smaller castings like cylinder heads etc.

As regards the aggressive shot blasting would help relive stress, crankshafts are shot peered in the fillet radius of journals to deliberately induce a stress in the surface to better resist fatigue failures initiating at the surface.

Paul.

I don't 'season' my castings as such, partly because they tend to sit around for some time before they get used -and are therefore already pre-aged!

Seriously, most amateurs work with relatively small castings and they may well have been in the suppliers stock for some time, so I don't think it's a big problem for us - different in a factory where turnaround will be quicker and parts larger.

However, I do have a camel-back straight edge casting that I will rough to size and then cycle in the oven for a while before finishing as recommended by the seller. But I won't be doing that for the Hemmingway cast parts I've got in the pipeline. So there may be occasions where it's prudent but generally not essential in my view.

Regards,

IanT

If you do use the oven make sure you have due authority from the domestic authorities.

Edited By Brian Oldford on 27/04/2020 10:12:19

Drummonds of Guildford seasoned their machine castings outside. Not sure of the duration though.

Can't understand the reluctance of some to use a domestic oven for tempering PROVIDING you've thoroughly cleaned the item before sticking it in the oven. Used to save any bits I needed to blue till after the Sunday roast was cooked, frugality, oven up to temp, so dangling bits of metal on wires in there no problem.

Regards Ian.

Hi, the graph below will show that nothing metallurgically alters any iron alloys at temperatures below 500 C with exception to some tempering and trying to stress relieve in a domestic oven will just be a waste of energy.

Regards Nick.

Edited By Nicholas Farr on 27/04/2020 10:38:11

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Still relevant nowadays IMO

I recall seeing piles of ML7s or Drummond M beds in one photo, they just piled them outside in the yard

Nowadays:

My attention was drawn to our banana shaped cable company pavement covers which were mostly laid in the last 20 years. Some are fine, but some of are damn rocky and it's because they are produced and shipped as fast as possible, so the ongoing inherent issues with cast iron are shipped along with them

Part of the problem comes from foundries that kick open the cast just as soon as they can, best left to cool in the sand until easy to handle, earlier ensures you get stresses and chilled areas. Perhaps a good rusting allow the chilled areas to rust away!

I remember reading somewhere that, in a factory, big castings were stress relieved by suspending them and having apprentices hit them with hammers (surely the hammers were small and used in way not to chip the castings); it's on a larger scale what is recommended when stick welding cast iron to avoid cracking the weld.
Massimo

In 2008 I bought a XJ9511 milling machine, which is a clone of the Sieg X2. I think the XJ9511 was made by Real Bull. I bought this machine in the full knowledge that there were likely to be 2 possible problems, one of which happened within a few weeks. (If you remember, in those days there was/are a lot of problems with the plastic gears and the electronics boards on these machines.) What I didn't bargain for was that the table bowed like a banana. At the time I was quite literally a newbie when it came to milling machines - I had never used one, and indeed never seen one on the flesh so to speak, so it was 2011 before I got round to having it reground by Slideway Services, (Brian Caddy?)

On speaking to the proprietor afterwards, he exclaimed that this was one of the worst he had ever come across! So was it lack of ageing? Or something else, although it is difficult to see what else it could be. And before anyone asks, I haven't checked it since.

In fact, as I got to know the machine, I discovered all or most of the problems that afflicted Chinese machinery of that age - casting sand where there should have been none, angle grinder marks on those parts of the slides etc that were normally hidden from sight, etc.

Now, before anyone accuses me of having a rant about Chinese machinery, when I bought I was well aware that there were likely to be problems, but having said that, I learned a lot by rectifying some of them so in that respect I am reasonably happy about having bought the machine in the first place.

Peter G. Shaw