Re: [CAD_CAM_EDM_DRO] cast iron

I set up heat treat furnaces for foundries. The heat profile for cast iron can differ greatly depending on the type of cast iron,
its thickness and geometry. The important factors are the ramp up, soak and ramp down. Usually the temperature can ramp up rapidly
to 600F. The ramp up is then tightly controlled at not more than a 200 degree F rise per hour until the soak temperature is reached
. For some castings this could be 1050F or as high as 1650F, This temperature is held for a soak cycle of two hours per inch of
casting thickness. The casting is then slowly cooled in the ramp down cycle at a rate of not less than 200 degrees F per hour, to
600F. Often a fully loaded HT furnace will naturally ramp down if it is shut down and bottled up at the end of the soak cycle.

Aging castings is no where as effective as heat treating. Some castings have an extremely tough skin that can take an edge right off
a cutting bit. Aging won't help, but heat treating can make it cut like butter.

Hope this helps.

Jay Hayes

Smoke wrote:

> Well, I've seen lots of answers posted about this one. Wonder how many of
> those answers are based on REAL experience?
>
> At any rate, having worked in the auto industry for more than 20 years, I
> can tell you how they do it and the reasons for it.
>
> Castings develop stress due to various sections cooling at different rates.
> This is caused by varying thicknesses throughout the castings. This process
> will take place on both investment cast and sand cast parts. This WILL
> happen even if the casting is left to cool completely in the sand bed.
> Since the entire sand bed is of a much lower temperature than the molten
> metal, it is impossible for the casting to cool uniformly as the metal
> solidifies..hence the stresses. Even cast flat plates will develop some
> stresses.
>
> The auto industry stress relieves the castings by bring the temperature of
> the castings up to about 1800 degrees (this temperature will vary based on
> the alloy being cast) and HELD there for an hour or more (depends on casting
> size and complexity). The furnace is turned off and the castings are
> allowed to slow cool IN THE FURNACE (doors are all left shut).
>
> If your getting small casting made, this same procedure can be done at your
> local ceramics shop (if you can talk them into using the kiln for this
> purpose.
>
> Or, if you do your own castings at home via investment castings, you could
> use your wax burnout oven to do this.
>
> With this process, you won't have to wait months for the casting to age
> before machining. This same techniques is also used for weldments.
>
> Smoke
>
> >I've got some questions about some castings I'll be having made soon.
> There
> >was recently some mention of aging the cast iron before machining, and I'm
> >wondering, how long one should let it age? Is it a thermal issue? Also,
> >how common and important is stress-relieving for parts that will only be
> >lightly machined? Several of the foundries I've spoken with said that
> >stress-relieving was not a "must", but mostly for troublesome parts.
> >
> >Thanks in advance,
> >
> >Carlos Guillermo
> >
>
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