Welding cst iron to steel

My neighbor's fireplace has an opening to the outdoors for cleaning out ashes. It has a cast iron frame, but the door is missing. They have asked
me to weld it shut, since they don't use it, but my only portable welder is a 120 v flux core "MIG." What will happen if I try to weld cast iron to steel with this? All I need is enough strength to keep a 4"x4" cover from falling off. Strength and safety are not involved.
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My guess is that the cast iron is dirty and rusty. Might be easier to secure the cover on with bolts. If the idea is to have it air tight, how about using bolts to secure the cover on, but use cement or epoxy to make it air tight.
Dan
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^^^^^^^^^^^^^^^^^^^ It's rusty, but not really dirty. I think I could hit it with my HF grinder to get some clean metal. ^^^^^^^^^^^^^^^^^^^ Might be easier to secure the cover on with bolts. If the idea is to have it air tight, ^^^^^^^^^^^^^^^^^^^ that's the idea ^^^^^^^^^^^^^^^^^^^ how about using bolts to secure the cover on, but use cement or epoxy to make it air tight. ^^^^^^^^^^^^^^^^^^^ That approach would certainly work. I also considered setting half a brick in the opening with mortar. But if welding can be expected to hold somewhat, I could be in and out of there without drilling and tapping, or mixing anything. Thanks for helping me think about this.
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On Wed, 16 Dec 2009 18:04:37 -0800, snipped-for-privacy@krl.org wrote:

If you're going to use epoxy, how about just glue the cover in with epoxy and call it done?
Use JB Weld.
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wrote:

Can it take the heat?
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Bob La Londe wrote:

Cleanout doors at the bottom of a chimney under a fireplace never see any heat.
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"Bob La Londe" wrote: Can it take the heat? ^^^^^^^^^^^^^^^^^^^^ No heat to worry about. The opening is on the outside bottom of the ash pit, a few feet from the actual "firebox."
Progress report: I have placed half a brick in the opening, which blocks 80% of the air flow. I'm going to cruise around until I eventually find someone doing concrete work, and bum a coffee can full of mix to finish the job. Funny thing--I tried to buy a small amunt of mix from Home Depot, and, yes, they have it, but it costs as much as three regular bags. (Maybe I could flag down a Ready-mix truck <G>.)
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On Fri, 18 Dec 2009 09:10:50 -0800, "Leo Lichtman"

I doubt you will find a ready mix truck with mortar mix on it.
Thank You, Randy
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You can usually get a small tack weld to hold for this situation. Use a high heat to get through the rust, fire it up long enough to get a small puddle formed, quit. Do that 4 times and quit. In cold weather the whole assembly never gets above 70 degrees.
Leo Lichtman wrote:

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You can insert a piece of bar that catches on the inside of the frame that has a hole or two drilled and tapped for a stove bolt in your lid, a bit like a toggle bolt. When you tighten the bolts, it all becomes one. The work can all be done indoors and the installation is easy with a battery drill. You could use a piece of EPDM between the plate and frame to be air tight, etc.
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