T23 Boiler Tube with Longitudinal Crack & Hidden Gems 2

[Guest102023]

The micrograph is in the HAZ very near unaffected base metal (~235 HV). We are seeing a number of these TiN (or TiCN) particles throughout the tube wall, even embedded in the 2 mil thick oxide covering an old fracture surface. The tube has a short (1/4") vertical tear just above the header weld, and it has been there for some time gauging by the oxide thickness. The tube is in one of the outer rows and is bent in the region just above the header. The crack is exactly on the intrados side.

Q1: Are the particles abnormal?
Q2: What could cause a vertical tear? My only guess is that the problem started at the factory - perhaps a mistake with the bending tooling?

I have a suspicion the two conditions are related ... chemical analysis has been ordered. For those who appreciate the difference, this tube did not originate from either V&M or Sumitomo.

 

[Guest102023]

Metengr,

By 'incorrect' do you mean secondary hardening can be an issue at lower temperatures also, or that my reheater tubes' SRC did not originate with the original PWHT? These tubes have been in two-shift service only about three years.

Actual operating temp. is not more than 1025°F, well within what is permitted by Code. The literature I have read indicates that secondary hardening is much more of a concern above 600°C (1112°F).

 

[metengr]

Depending on the level of residual cold work, this can be lower temperature.

 

[Guest102023]

That would make sense from physical metallurgy principles of course.

It looks like a perfect storm around the header joints for tubes made from inferior steel, not heat treated after bending then not properly PWHT ... it would require a very long post to document everything I have found wrong in a very small chunk of tube. The composition is the only thing that is right.

The weld joint, made from conventional B3 filler, has 295 HV1 hardness, when WPS data indicate something closer to 240 HV1. That to me can only indicate inadequate PWHT and/or welding preheat.