blow-hole at the end of a run

[valvieboy]

more advice from you guys in the know please.
i am trying to butt weld(TIG) some .9mm thick mild steel cones that are rolled to make some bike exhausts,but i am on the verge of giving up.because of my lack of experiance i have to go around the circumference in about 4 or 5 runs,but where i end every run i'm getting a nasty blow hole.the welder is a very basic sureweld unit with no slope controls,but one knob with a clock and gas bottle symbol,which i have altered from one end of the spectrum to the other with no effect.other than the holes, the beads dont seam to bad,but i'm tearing my hair out with the hole problem.

 

[the snooper]

are the cones air tight if so thats your problem the air inside heats up and causes pressure which will blow out the molten metal on the last bit

 

[hotrodder]

Avoiding craters with a basic scratch set takes some practice (like striking an arc), especially thin material.

When you get to the end of a run don't stop and then break the arc, keep the arc moving as you pull away. Once the arc is broken move the torch back over the weld. For the final tie in keep going round the pipe moving faster (to lower the heat input) and then snap it away

Been a while since i've used a basic scratch set (been spoilt!), someone else may be able to describe it better/have a different method

 

[valvieboy]

not airtight

are the cones air tight if so thats your problem the air inside heats up and causes pressure which will blow out the molten metal on the last bit

thanks for the reply but i get the problem on the 1st run and i doubt if they are butted up so as to be air tight,but.... it must be a problem with air.

 

[pedrobedro]

It's the heated metal solidifying and contracting isn't it ? You probably need to pause and add a bit more filler or backtrack up the run adding a bit more again.

 

[Dave Richards]

difficult one this, with no slope out try putting a small spot tack using .6 wire then run up to it. 4 or 5 runs is not bad on small diameter tubes, are you using filler wire on the joints or going autogenous ( melting the two parts together).