Aluminum Preheat How hot?????

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Hay Folks got a quick question bout preheating an Aluminum wheel. It is cracked on the inside of the rim not in a high stress area and a small bout 1 inch crack, does go trough the bead. Fixed it at the track the other day but really didn't have the right tools. Had a little tig and noway to grind a V grove so I just stop drilled it and filled the crack. Well it held but it is re cracking now kinda thought it would so I have the wheel back at the shop and going to do it right. I normally use around 400F on Aluminum that I am not sure what alloy. What do y'all think?? it is not a forged wheel and it did weld ok the first time. Going to leave the bead on the outside and not grind it down.....ThanksBrandonLincoln Tombstone 225 A/C Steel StickerOld Victor O2/Acetylene setupMiller Syncrowave 250 Tig/PC-300 PulserMiller Millermatic 212 Mig HTP Invertatig 201 Giant Teck D50 Plasma cutterLots of HF grinders
Reply:depending on the wheel and where it was made, they are typically cast and pretty soft.getting all of the crack out and adding enough filler during welding is very important.You will get a good bit of shrinkage in the weld, use plenty of filler and peen the weld as soon as it starts to cool. 400 is fine, you really just want to overcome the heatsinking effect.if the wheels are of chinese vintage ( most are since @ 1998 ) all bets are off.insert thoughtful quote from someone else2000 Thermal Arc 300GTSW 3.5 hours1946 Monarch 20 x 54 Lathe1998 Supermax 10x54 Mill2004 Haco Atlantic 1/2" Capacity Lasernot mine but i get to play with it
Reply:@ brandon i trick i use when preheating thick or cast alu is to blacken it with an acetylene torch (gas only), then i open up the oxygen and burn off the soot. once the soot's all g0ne, i keep heating it a touch more then go at her. you'll know if it's hot enough as soon as you try to puddle. or check this http://www.scribd.com/doc/3515813/We...-eBook-7-Pages hope it helps youLast edited by Unklmo; 06-24-2009 at 01:46 PM.
Reply:was taught that lil trick by an old naval pipefitter  soot up, then heat, when soot gone its readya good way to  help in bending alum, but there is  a fine line between toooo hot  an not hot enough
Reply:I have also used the acetylene soot method on thin aluminum sheet as a guide to anneal, not to preheat. I read some posts of people using soot method for preheat guide and was curious if it works. I used it on some .050 and it got over 650 and was fully annealed (which is what I have done in past to soften aluminum for hanmmer forming). I then sooted some 1/2 inch plate and it burned off in low to mid 200 degree range. Tempilsticks are accurate and not expensive, IR guns have come down in price and have a multitude of uses. Invest in the IR gun.I weld quite a few rims without preheat. I chop saw through crack and start weld on outside. Weld to half way point, stop, sart welding from end and meet previous weld in middle.The only reason I would use preheat is if I did not have enough amps available. To get puddle started on cold wheel I use 250 amps, and have to back peddle as rim heats up. A preheat of 350 will reduce amps needed to start puddle by about 50 percent.My 2 cents.Peter