Well, if the client can't give you any info on what the pipe is and how it was welded the first time then I wouldn't do it.  It's a must that you know what your working with so you know how to attack the situation.  If you have the whole section of pipe out that has the crack in it and they won't give you what you need then just make a new one so you know it gets done right.;-)

If you can get the name of the burner off the name plate you can contact the manufacturer. Most companies have web sites and a contact you can use. Provide them with the model number and the serial number. They may be able to tell you the type of metal used. From that you may deduce the type of filler metal to use.

You can do a quick check to determine if the material is ferromagnetic steel or austenitic stainless steel/nonmagnetic metal by checking it with a magnet. You can easily determine if the ferromagnetic steel is high alloy (chrome or nickle) by subjecting a cleaned area to water to see if it rusts. Do a spark test with a grinder in a darkened room to approximate the carbon content. This will help steer you in the right direction if the manufacturer will not provide some direction for the repair of the burner.

The last alternative is to have a sample analyzed by a laboratory to tell you what the alloy is. Then you will have the actual alloy content and the filler metal can be easily determined.

Best regards - Al