[rtv]

i have a cast iron piece with 2 hairline cracks. I have read about a hundred different thoughts on how to fix it. tig stick gas braze hot cold etc. etc. I have never welded cast iron before, I'd love to do it myself (w/tig) but i don't want to destroy the part either. what chance do I have as a first timer w/ cast? here are a couple pix

[weldprof]

The first thing we need to know is what is your welding experience and process ability? It could be Oxy-fuel brazed or GTAW with silicon bronze rod. First the crack should be dye penetration tested to see where the crack ends and starts. Sometimes the human eye cannot see the extent of the cracking. I have seen cracks that looked like they ended in a single line only to "spiderweb" all over the place. Welding cast-iron requires preheat, controlled inter-pass temperature and post heat treatment. I have had best luck with silicon bronze rod and GTAW for the process. Let's discuss your skill level and experience and we will continue with the repair.

weldprof

[rtv]

Well...as to my skill level, i'm sure in comparison to others, it is low. Ive been teaching myself TIG via the web and books.
and i think i do a pretty ok job on AL, Steel is a piece of cake in comparison so I haven't had any problems there.
In the past i have used MIG and some stick (not a lot). Anyway.. I guess I'd say I know enough to get into trouble

[rtv]

oh..in case it matters..my welder is a miller dynasty 200dx

[weldprof]

Excellent machine. Set it up as you were doing steel. Tungsten same, polarity same, so forth and so on. Once you have determined the length of the cracks and their severity then drill holes at the end of the cracks to stop the propagation of the cracks. If the holes are not beginning from an edge then drill at both ends. You will need a oxy-fuel torch with heating tip or propane torch to pre-heat the material. A digital readout infrared thermometer will work great to monitor the preheat and inter-pass temp plus the post heat stress relief. Can get at Home Depot or Harbor Freight or online. Preheat material to 450 F and then proceed to weld with silicon bronze rod. You can practice the sil/bronze on steel first to get a feel as to how it runs. It will be much like brazing with a TIG torch. But much more control of the puddle. Maintain your inter-pass temp of the surrounding area outside the heat affected zone of about 750 to 800 F for the duration of the welding. If it gets a little hotter no worry it is the cooling rate that is critical. Once the weld is complete you will want to cover your piece with some sort of thermal blanket used for welding. Check with your local welding vendor. By wrapping the piece in the blanket you can control it's cooling rate. It should take about an hour to two to drop to room temp. Check it periodically without completely unwrapping the piece. If it is dropping too fast, apply a little heat to stabilize the temperature. The longer it takes to cool the better your chances of success.

Success with cast iron is relative. Expect some failures. Even the best welders have difficulty with Cast. Good luck.
John

[rtv]

Thanks John! What do you think for the prep of the crack? should i bevel it out or just go right over it? i read somewhere that sand is a good insulator for slow cooling... i was thinking about using that as it would be able to be welded while buried in the sand (mostly)

[Helix]

Vee out the groove (both sides if you can) wherever you weld it and don't forget the small hole at the end of the crack.
A gas grill will work to preheat and control post heat if you have one.
I'd clean the paint back also to minimize contamination in the HAZ (heat affected zone)

[rtv]

yes.. the gas grill was what i was thinking. as for the small hole..1/8" 1/4" ?

[Helix]

3/32 or 1/8"
Drill (rpm and feed) slower than steel with cast iron and that small of drill dia. or you may break the drill off.

[rtv]

with this Silicon bronze rod.. am i using it like any other filler or is this suppose to be like brazing? (as in not melting the cast iron)

[ajlskater1]

silicon bronze is basically like brazing with a tig with a tig torch. It works really well with using a lay wire technique vs. dipping. Also works good for thin sheet meal for like outside corner joints. I use a lot of it a my work for like 24 ga and 22 ga steel.

[rtv]

ok...thanks..but now i wonder how much i should "open" the crack. if this stuff melts low (relatively) it might run out like solder if there is a serious groove..will it wick into the crack on the root side if i leave it closed at the bottom? as in: V the crack but not all the way through

[ajlskater1]

It will not flow exactly like brazing rod. Its kinda hard to explain but you should defienetely bevel out the crack, I would bevel both side of the crack leaving a small gap in between the bevels.

[ajlskater1]

silicon bronze does have a tendenancy to leave more bleed through than steel rod because it flows so much easier. My suggestion would be to bevel both sides and not leave a big gap in the middle of the two bevels. And if you are really worried about bleed through, put a peice of copper on the backside and weld the top side first, then weld up the bottom side.

[rtv]

got it! once again..just to be sure...I am NOT welding...I'm using the rod as "solder", a eutectic sort of thing. i am not melting the cast iron ..correct?

[ajlskater1]

yep. The whole reason for the silcon bronze is to limit heat input for anything you are welding with it. Thats why it also works really well for really thin steel. And with casting it helps cause you do not heat up the material and cause internal stress and it helps it weld alittle cleaner cause it gives you a better chnace at not heating up the sand pockets, which can be a real headache if you get a really dirty casting.

[rtv]

alright..then that leads me to believe that i should not open up the crack very much...maybe just an 8th?

[ajlskater1]

ya I would say that should be about right and then have about a heavy 1/16 gap in the middle of the two bevels.