Qualify Brazing Procedure Based On Groove Welds? 2

[tc7]

We have a customer who specifically asked for a repair using a TIG arc braze method on carbon steel. The scope of the repair is to buildup a thin (< 0.125") layer on one face of a CrMo (4130 normalized) machined housing then finish machine the surfaced area to final dimension. Either Alum bronze or silicon bronze filler will be used. In use, the new surface will be in compression and will experience very low static operating loads. Presumably, the point is to buildout the surface without need for a high temp post weld stress relief and risk distortions to critical dims which exist elsewhere in the equipment. Temper beading was discussed and dismissed since the intended additional surface layer is so thin.

Since all we are really doing is a surfacing operation, I feel inclined to NOT qualify an arc brazing procedure IAW ASME Sect IX QB100 which does not address surfacing, but rather to to treat this as a "buildup" and perform groove weld tests and qualify IAW QW-202.3

Am I correct in this thinking? Opinions please and thankyou.

 

[tc7]

I should add that our customer's product does not fall in ASME jurisdiction but has chosen to apply qualifications IAW either ASME or AWS.

 

[metengr]

tc7:
Since this is depositing braze metal on a steel substrate (similar to a weld overlay except with braze metal), I would perform a mock-up by following a brazing procedure. Once the mock-up is completed, I would cross section the mock-up for metallographic examination to evaluate the bond integrity of the braze metal and surrounding 4130 BM.

 

[tc7]

The more I thought about this overnight, the less I understand about what it is we are being asked to do. This isn't a fusion process (or is it??), so the base metal won't melt. But the temperature still must be hot enough to melt the braze material (1880 deg F) and allow dissolving into the steel substrate to occur. Will this not potentially lead to HAZ issues that must be dealt with? How about if we perform a long low temperature stress relief? say 900 deg F for a couple of hours (based on a weld overlay thickness of only ~1/8")?

Also would a preheat of about 350-400 deg F be advisable?

 

[metengr]

Tc7;
Preheat should be used just like fusion welding. Brazing results in a chemical bond and heat is used so the bond integrity and steel BM should be evaluated using a mock-up.

 

[GRoberts]

If you treat this like an overlay qualification (and do the bend tests), it should give you a reasonable approximation of the ductility of the 4130 HAZ after brazing. I agree with metengr that preheat should be required as you will be heating the 4130 above the lower-transformation temperature, so it will re-form martensite if cooled quickly. You are correct in that the 4130 should not be melted, but a metallurgical bond will be formed between it and the braze metal.