Why 6010 root 7018 fill? 4

[SKIAK]

I do structural but I have become the "go to guy" for a contractor and he was asking me about boiler pipe welding. He told me, and I've found thorugh some internet searches, that the pipe welds usually use 6010 rod for a root pass and 7018 (or others) for subsequent fill and cover passes. Why is that? It turns out the welders used 6010 for all passes and they were asking me if it was OK. I couldn't seem to figure it out so I told the guy to talk to whomever or whatever authority was requiring it (mechanical engineers? ASME?), but now I'm curious and nobody around me seems to know.

 

[metengr]

Typically, 6010 weld rod provides good penetration for the weld root region when making open root butt welds using the SMAW process for low pressure applications. We, as well as many others in the Power Generation business do not permit the SMAW process for making weld roots for open root butt welds for boiler tubing, or as some call it "high pressure" butt weld applications. For this, we require a GTAW root with SMAW fill. The reason is that pulling a root using the SMAW process requires a significant amount of welding skill to achieve a quality that will pass an RT examination. Since we x-ray these high pressure butt welds welds, the chances for re-work using SMAW versus GTAW is high.

ASME B&PV Code and most piping codes do not get into process details like the above.

 

[stanweld]

E6010 is a quick freeze electrode that is partially shielded by CO2 gas developed from the cellulosic coating making it ideal for all position root pass welds. Because of the moisture content in the coating, it generates hydrogen and welds thereof are more prone to hydrogen cracking when coolng rates are high. As a result, many Engineers/Owners restrict its use to the root pass or root and hot pass with fill passes to be made using low hydrogen electrodes, especially for boiler and power piping applications.

 

[stanweld]

In addition to comments by metengr, it is almost universally prohibited for use in pipe systems generating and tranporting steam to a steam turbine (slag deposited on root ID surface with potential damage to turbine blades).

 

[btrueblood]

Nice answers, stan and met.

 

[SKIAK]

Yes, nice answers. Thank you both.

 

[Ron]

In addition to the excellent comments by metengr and stanweld, it is unlikely that a welder using 6010 electrodes and SMAW could pass an RT examination of the weld in a boiler tube. As metengr noted, the use of 6010 requires a great deal of skill. It penetrating capability also creates a large arc blow, prone to slag inclusions, thus making RT pass results more difficult.