Correcting chilled castings

I have recently had problems with cast iron castings which have very hard areas around the edges, not the skin formed when sand cast. I believe they are caused by the casting cooling too fast in those areas or the casting being poured too slowly. Does anyone know how this can be corrected? I am thinking heat up the casting to above critical temp and soak at this temp for some time followed by slow cooling to allow the casting to anneal. Am I on the right track?

Hi Martin, can you not return the casting to your supplier for a replacement? Most suppliers seem happy to replace a defective or unmachineable casting.

Regards, (another) Martin.

Yes I can and have done in the past, but turnaround time is weeks or sometimes months. I am trying to speed things up.

hi martin,

if you have a solid fuel stove or open fire leave them in the banked up fire overnight. or use a tungsten carbide tool (not insert) and very slow speed and keep regrinding the tool on a green wheel.

cheers,

julian

Edited By julian atkins on 11/09/2013 23:45:21

For small protrusions on castings I have got away with 20mins with a propane torch keeping the offending bit of the casting red hot. For whole castings as julian says leave in a fire or woodburner overnight and leave in the ashes to cool.

If its just a flat surface you need to machine then carbide will get through quite a bit of hard stuff but if you then need to drilla nd tap that area you will have problems if its left hard. The slightly darker area on this castings corner which you can see as a slightly darker/shinier area was chilled but the indexable cutter got through it OK,

I have just finished a cored cast iron liner for the ETW Centaur Gas Engine, the outer edges of the bore were rock hard and not doing the tool any good, I heated up the whole casting with the propane torch and kept it at a dull red heat for around 10/15 minutes.

What I thought was a scrap casting is now a finished liner.

I had big problems with the body of a Potts Spindle, overnight in the coal fire made no difference.

A solid carbide tool cut it, but the surface finish was poor. I think the problem was it had a thick layer of pure white cast iron (cementite) which takes more than just annealing to soften it.

Surprisingly (Stuart have a reputation for excellent castings) the corners of the valve face of my Stuart 10V's cylinder were chilled, but nowhere near as severely.

Other castings I have had have been fine.

Neil

Martin,

I've just dug out my "Metallurgy for Engineers" by Rollason. He has some words on rate of cooling which, if too quick, produces cementite, or white iron. This can be a problem with castings having considerable variation in section - the thin bits cool much quicker than the thicker bits giving a tendency for cementite to form. As one of the other respondents has observed, cementite is very hard and only carbide tipped tools have any chance of cutting through it.

In the old days, iron castings were left in the yard "to weather" before they were machined. This, I believe, allowed the hard surface layer to rust away giving hardened carbon steel or , later, high speed steel half a chance of getting under the "skin" and into the grey iron.

If you can get a copy of Rollason from the library, look up the blackheart process on page271. This gives the correct times and temperatures to develop a machinable structure from a white iron.

Jack Meatcher

The other problem is that once poured the foundry sometimes kicks open the sand before it has all cooled down far enough and so it chills locally where exposed, because it saves time. A long hot soak usually sorts them and an indexable insert can get under it, but don't pussyfoot about, if you can hold the casting firmly then a strong heavy cut is called for.

Thats provided you don't need to drill and tap the offending chilly bit, like I did a couple of weeks ago.

The darker areas are the hard parts after the surface was faced with an indexacle 40mm endmill

Bit of heat

Threaded 3/8BSP for fuel cap

~In another life I have had to deal with cast steel castings of a primitive nature which I had to machine on a large slotting machine. These castings were riddled with hard "nuts" and caused me no end of problems. Using a carbide tipped tool was out of the question as the backward stroke of the slotter did not "relieve" the tool as it would on a shaper with its clapper box and consequently the carbide tips simply fractured away - they don't like being dragged.

The solution was to use HSS bits and when I hit a "nut" I would ride over it (damaging the edge of course) resharpen and refit the cutter then attack the "nut" with a die grinder to just below the level of the finishing cut before resuming the finishing cut.

This means of course that the finished surface will have a number of craters in it but that mattered not in this case as the finished surface was just a bed for flat hardened die inserts to rest on.

Edited By Rik Shaw on 24/10/2013 19:32:22

Hi folks, I have just restarted work on my build of the James Coombes...set up in the mill one of the two (very fragile) top slide assemblies clocked it in relative to cutting tool needed 45 thou'' off each side of the slot ...I had already done the file test ''not happy'' soon as cutter started to bite i knew I was going to have a problem I managed to ease the cutter down both sides only to discover I needed another 20 thou '' off each side...

I do not want to try annealing it as it is quite a fragile casting and very well may warp...Thinking of setting up the Dremel on the mill which will give me the speed I need for grinding ...Any thoughts would be appreciated

John.

Hi All

When I bought all my castings from Norman Spink I found half of them had hard spots, he said just grind it off but I pointed out that if I did and then broke a tap in it he would not change them. I took mine back and he changed them after moaning ! the only thing about heating cast iron is as it cools it can crack badly with stress. My advice would be to change them.

Regards

Stan