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Old 03-14-2007, 03:17 PM   #75
KTM640Dakar OP
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Joined: Nov 2004
Location: Oxbow Lake
Oddometer: 2,120
Originally Posted by Pilbara
I need to weld a bolting mount on the steering head of my KTM950SE. I see they claim the frame is Chrome Moly. I have built a number of frames in the past but bronze welded them as I used thin wall 4130 tube. I have always been a little confused with all the dirt bikes claiming to be CrMo and yet they all appear to be MIG welded. Now when I did my research into 4130 I found it to be an air hardening steel. The electric arc welding processes (and poor gas welding technique) obviously take the material up to the critical hardening temperatures so I would imagine that the HAZ would be very brittle. This is unless they use a tempering process, which I would doubt due to cost. So what is it that the manufacturers are doing to prevent this? My conclusion is that they are simply using thicker walled tube to disipate heat and reduce the HAZ problems. It seems that they could actually make the frames considerably lighter if they used a lower temp welding process or post heat treatment. But I guess it all comes down to cost.

However I digress from my inital enquiry of welding a boss on to the steering head for a bolting bracket.

Like I said any 4130 work I have ever done is with oxy, paying particular attention to not getting the material too hot. (I use Phosphor Bronze BTW)

Question: Can I use the quick and dirty method of MIG welding the brackets on without worrying about any practical metalurgical problems? Of should I use the tried and proven bronze welding.

The rest of the frame is MIGed so I am sure it is going to be OK, but wouldl like an expert opnion please.

Yes you can MIG weld it. Chrome Molly tubing is usually welded with a wire called Super Arc LA-90. It is an ER80S-D2 welding wire. You should use .035 diameter with 90%Argon/10%CO2 shielding gas. The only real issue with 4130 CrMo tubing is that it has a higher carbon content. The trick to successfully welding it is to let it cool slowly so the carbon does not turn to martensite that has a crack sensitive microstructure. Don't quench it with water or it would get brittle. In fact it is always a good idea to weld in a hot garage verses a cold garage. Metal that is cooled quickly will be harder and more brittle then metal that is slowly cooled. Most of the time with mild steel it is not as important, but with steels that have higher carbon levels at about .03% carbon like your 4130 you need to possibly preheat too if it was a very thick section.

Just make sure your parts are at 70 degrees or warmer when you weld. Not 32 degrees F like my garage.
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