Clean and preheat area to be welded, use 1/8" or 5/32" 7018ac rod at around 160-180 amp, Cat used to recommend 6011 but I wouldn't use it for temporary strength

-for an amateur/ beginner level welder i would use 7018 rod with a DC welder, reverse polarity (personally dislike AC=horrific)
- i find 1/8" 7018 the easiest/ most common rod to burn and apply.
- probably start in the 125 to 140 amp range(depends on the unit)....and as your skill improves you may turn it up a bit
-cleanliness helps immensely, no oil, grease, paint, rust, scale, etc...

you can also do it with standard 0.9mm(30thou) mig wire as it is a lower hydrogen content anyway. yes you can get true hydrogen controlled wire but not worth it for a temporary job.
and take it slow do a pass and move to the next tooth and work your way around you dont want to fully weld one tooth at a time can create over heating and forming cracks and yes preheat the sprocket teeth. its better to work on say three teeth at a time this helps spread the heat.

save gas and time buy a proper rose bud heating tip oxy-lpg is better than oxy acetlyene for heating. i see people using a cutting attachment to heat stuff, wrong way to go use a proper heating tip you use less gas get a more even heat and it heats quicker. heating tips for heating cutting tips for cutting welding tips for welding.

a gentle post heat wouldnt go astray either to help normalise the steel
as for amperage and polarity use the electrode manufacturers recommendation for that electrode starting in the middle of that range. and yes a dc machine is always better than ac and the inverter units available now for the price why wouldn't you buy one. just watch your duty cycle(how long your passes are) some machines have a very short duty cycle and can overheat easily. particularly the cheaper models.

If this is just to move the machine and not work, I wouldn't go to much trouble, just get some mild steel !/2" rod and cut to width of sprocket and run a low hydrogen bead each side. I built up sprockets with 3/4" and run low hydrogen weld to fill in and build up the sprocket root a bit, they lasted for 500 hours or more, never pre heated or anything fancy.

Regards, Wombat

Hi, Neil.
If my guess is right, what Wombat has suggested will work as he is suggesting building up the teeth a quick, easy and cheap way. I have never done it but I have heard of it being done to get a job finished when sprocket teeth and pins and bushings are worn to about 200%(???????????????????) - - - like one time when I had to reverse a Cat 943 track loader onto a rear loading low bed ' coz it just plain would not drive forward on one track, the sprocket teeth were so worn.

I suspect that the rebar would be better than the mild steel round bar too as I think it is higher quality steel. If you put pretty heavy tacks on all 4 corners of each piece of bar on about 3 teeth atta time and then go back and full weld each of them in turn, the heavy tacks will help to pre-heat the tooth and the bar a little. You can even keep on adding tacked on pieces of bar ahead of where you are full welding - weld one, tack one - to help with the pre-heating/warming.

But do it BEFORE your 'radical' winter cold. LOL.

Just my 0.02.

yes wombat's idea could work well and be less work you used to be able to get barstock to do the same thing to grousers and get a second life from them. a bit of medium tensile bar would be the best. and with round you have a nice natural vee to fill with weld.

I have seen many sprockets built up using round bar and low hydrogen rods and no preheat what so ever. Not once have I seen a crack from this process.
Now if you take into consideration Niels statement that he only wants to make the machine mobile enough to "move itself around" then a piece of 1/2" round bar welded to the tip with a run each side would be more than strong enough to make the machine mobile.