Author Topic: Dealing With Hard Spots in Iron Castings  (Read 478 times)

Offline Dave Otto

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Dealing With Hard Spots in Iron Castings
« on: November 07, 2021, 05:38:12 PM »
First I want to say that I'm no expert in the field of metallurgy, but I do work with one at my day job who is. A while back I offered to help my buddy to try and soften up a cylinder casting for his Forest engine that had areas that would just laugh at a carbide end mill. I asked my colleague at work about how to deal with this and below is what she gave me. Her comment was to stay below 925 degrees C and soak for one hour at temperature.

We did some polishing and etching on one of the bearing caps that under high magnification showed excessive carbides in the mix. After annealing the test piece was polished and etched again and showed that the excessive carbides had been removed, and the part was easily machined. So the cylinder casting was wrapped in SS foil and went through the same treatment, afterwords it machined and drilled easily.

Copy of text supplied to me by my colleague;

If the microstructure of gray iron contains massive carbide particles, higher annealing temperatures are necessary. Graphitizing annealing may have the purpose simply of converting massive carbide into pearlite and graphite, although in some applications it may be desired to carry out a ferritizing annealing treatment to provide maximum machinability.

Production of free carbide that must later be removed by annealing is almost always an accident resulting from inadequate inoculation or the presence of excess carbide formers, which inhibit normal graphitization; thus the annealing process is not considered to be part of the normal production cycle.

To break down massive carbide with reasonable speed temperature of at least 870 C (1600 F) are required. With each additional 55 C (100 F) increment in holding temperature the rate of carbide decomposition doubles; consequently, it is general practice employ holding temperatures above 900 to 955 C (1650 to 1750 F). However, at 925 C (1700 F) and above, the phosphide eutectic present in irons containing 0.10% P or more may melt.

The holding time at temperature may vary from a few minutes to several hours. The Chill (white iron) in some high-silicon, high-carbon irons can be eliminated in as little as 15 min at 940 C (1720 F). In all applications, unless a controlled-atmosphere furnace is used, the time at temperature should be as short as possible, because at these high temperatures gray iron is susceptible to scaling if moisture is present in the furnace atmosphere.

The cooling rate depends on the final use of the iron. If the principal object is of the heat treatment in to break down carbides, and it is desired to retain maximum strength and wear resistance, the casting should be air cooled form the annealing temperature to about 540 C (1000 F), to promote formation of a pearlitic structure. If maximum machinability is the object the casting should be furnace cooled to 540 C (1000 F) and special care should be exerted to ensure slow cooling through the transformation range. In both instances, cooling from 540 C (1000 F) to about 290 C (550 F) at not more than 110 C/h (200 F/h) is recommended to minimize residual stresses.


Hopefully this may remove some confusion about dealing with hard castings.
Dave


Offline Jo

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Re: Dealing With Hard Spots in Iron Castings
« Reply #1 on: November 07, 2021, 05:51:24 PM »
Thanks Dave, If you don't have a kiln at home:

Your standard home open wood fire burns from around 600 degrees (red flame) to 1000 degrees (Orange flame).  (White flames are around 1300 degrees)

If you have an enclosed log burner you get a hotter burn (brighter intense orange flame) and they burn at around 1000 degrees. We use our log burner to normalise castings  ;)

Jo
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Offline Jasonb

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Re: Dealing With Hard Spots in Iron Castings
« Reply #2 on: November 07, 2021, 06:24:18 PM »
Thanks Dave, good to know the science behind what's often said to cure hard spots.

Online steamer

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Re: Dealing With Hard Spots in Iron Castings
« Reply #3 on: November 07, 2021, 07:16:40 PM »
I would add the following

If you ask for the right material, you will get a better product

If you ask for Meehanite GC40....Stress releived to 220 BHN +/- 20%    You should get some really nice iron...that machines like butter!

If you ask for "Gray iron"   you're going to get everything the foundry can sweep up off the floor...and you won't be happy

While at Heald, I was brought over to see a 52" work table being machine on a Bullard lathe

The point of my supervisor bringing me over was an object lesson.    The part had been specified as "Gray Iron".....and neatly profiled in the edge of this casting was the distinct outline of a railroad spike....hard as glass...it was breaking every tool used at the time.

I was told...Dont ever do that!!!...and to specify the 40,000 psi tensile iron above as the Meehanite process was by far the best available at the time.

Just sayin'


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Offline john mills

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Re: Dealing With Hard Spots in Iron Castings
« Reply #4 on: November 07, 2021, 07:37:21 PM »
i would think it is up to the foundry to have a good furnace man that willuse a good recipe to give a suitable grade of iron for the job
it sould not be a haphazard  lot of what ever.
    John

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Re: Dealing With Hard Spots in Iron Castings
« Reply #5 on: November 07, 2021, 07:38:40 PM »
i would think it is up to the foundry to have a good furnace man that willuse a good recipe to give a suitable grade of iron for the job
it sould not be a haphazard  lot of what ever.
    John

Good luck with that
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Offline Jasonb

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Re: Dealing With Hard Spots in Iron Castings
« Reply #6 on: November 07, 2021, 08:07:28 PM »
No doubt there will be a cost implication for such requests

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Re: Dealing With Hard Spots in Iron Castings
« Reply #7 on: November 07, 2021, 08:16:30 PM »
Pick your poison.....I suspect if you ask for r the good stuff, and can be a little patient...you can get what you want without too much extra cost when they run a bigger job.

Dave
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Offline Jasonb

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Re: Dealing With Hard Spots in Iron Castings
« Reply #8 on: November 07, 2021, 08:51:54 PM »
Am I right in thinking that only licenced foundries can put the Meehanite name onto their iron? if so that would rather limit the choice of foundry and they may not want out little orders

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Re: Dealing With Hard Spots in Iron Castings
« Reply #9 on: November 07, 2021, 08:54:59 PM »
Check your foundry.   My foundry followed the process, and I got good results.   
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Offline fidlstyks

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Re: Dealing With Hard Spots in Iron Castings
« Reply #10 on: November 11, 2021, 02:11:42 PM »
My family started having castings made for hit and miss engines back in the early 70s. I can still recall Progressive Foundry at Perry Iowa USA, after going in the front door and up some 3 steps to a little room off to the side, we met the owner. Now one needs an appointment and they have about 10 minutes for me. But always very glad to help some one playing I call it with iron.
   Now it is a huge factory foundry that only uses one sided match plates where they make two halves and glue them together. They charge $250 per hour plus costs. And they charge to change the plates. Casting cast iron 3 days per week and ductile 2 days. They town has boomed now with hispanic workers who also work a the huge meat processibg plant. They cast Ford drivetrain parts for one thing.
   I still go there and mooch materials. Once, while there a guy walked up and showed a casting to the metal mixer man, it was broke in half, the main way to look at the grain.  It was silver white about 1\8" all around. The guy just said, "some one did something wrong". Looking at their scrap pile they had precharged loads ready to melt. They use a huge electric furnace which means an open pot maybe 10 feet across and 15' deep?
   But anyway, the scrap charges included new cast iron ingots, scrap castiron, and shreaded auto steel, which now means everything imaginable. I scrap some, and have noted they drag the more expensive 3' prepared steel and 5 foot number 1 over to the shreader and push it through, mixing all of it together, after hours so no one knows they lay 30 cents in the dollar for what they sell for all one price.
   The melting pot has an up and down meter input that had 2 prongs a set distance apart. Through the electric current a screen off to the side had graffs which stated all elements in the mix. Pretty smart! In the old days they cut 1" cubes, then placed them on the surface grinder and flattened them perfectly.
   They then placed a single drop of Nitric acid on the iron. One could smell that outside. Nasty stuff. They then held the piece up to a poster that resembled a map of the universe with set cubes all over it. Each picture-cube had either little red dots, half moons or other shapes. With a symbol chart in the corner, one would determine what the iron held. Silicon, Manganese and other elements.
    When casting all molds are like in groups, so they can dump any bad iron used molds, the first thing poured are little molds with wedges cast in them. They put them first, which casts a knife blade wedge and then they break it. If it is silver for more than maybe 3/32" they dump the whole pot into a pig mold. So they have pretty good quality control.
   I have some other interesting comments, will post later.