By the term "overheating", I meant getting the tubes too hot. From a distortion point of view, the more heat put into a section of wall tubes, the greater the distortion to the tube section. From my experience, the tubes tend to expand outwards with excessive heat input from cladding, be it from either cladding for corrosion resistance, or for cladding to increase minimiun wall thickness. During the cladding on this job, for min. wall thickness, I carried a variety of temp sticks, ranging from 125 degrees F to 500 degrees F. The tubes never got above 250 degres F in the center section(an area roughly 28" X 48", all totaled more than 1000sq.in., and did I mention it's a last-minute finding in a shutdown situation, and the initial response was to put 4 welders on the cladding?), well below the interpass temperature for carbon steel. But if you were to look along a side profile view of the roof tubes, you will still see a distortion, as my partner didn't feel the same way as I did about giving the plant a nice job, it being overhead and all?!?!
Niekie2,
You speak of the quality of the welds. I tend to agree. How much porosity from bad starts will result in a failure? I attempted to show my partner how little time needed to be spent grinding, if one were to identify the defective areas, and once located, just go ahead and grind them out. This was to no avail. You can possibly imagine how much I wanted to do him some bodily harm when I became aware of the arc strikes and resultant porosity he left on my overlay!
And lets speak about bead profiles for a moment. What would you call a good multipass bead profile in a case such as this( this case being such that the cladding was performed longitudinally)? How do deep interbead valleys perform in this situation? My guess is not very well. I would think it reduces the overall thickness of the cladding, and potentially creates a notch stress riser( is this correct?).
What did you mean by "overheating"? And are you talking of the same distortion as I am?
I personally thought that if the tube was too hot to put your bare hand on, then it was too hot to weld on. I might add that the minimum wall thickness determined by UT examination was .080", so "burn through" was also an issue,(and thank GOD it didn't turn into an issue!) and high travel speeds were a necessity. I've used this philosophy on several occasions, and it has repeatedly proven itself to me as an excellent distortion control technique. If I were to clad a panel with no mind for distortion issues, you might not even see some of the tubes from a side view, for with the heat came the need to grow. But where to grow? I wonder if we aren't inadvertently changing some of the flow dynamics of the gas flow by the distortion of the tubes, be they wall or roof tubes, and thereby changing the wear patterns.
And by the nature of this distortion, I was wondering if this had an effect on expected tube lifetime.
Also, I was wondering if someone out there had a bad experience with an overlay job, that I could use as an example the next time someone told me " it doesn't matter".
After my all too brief discussions with the plant's chief metallurgist, I must say I feel I'm on the right track. If I need correction on some point, by all means gentlemen, please do so.
Dale Simonds