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Dealing With Hard Spots in Iron Castings

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Dave Otto:
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

Jo:
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

Jasonb:
Thanks Dave, good to know the science behind what's often said to cure hard spots.

steamer:
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'


john mills:
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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