Strange 304L HX Tube Construction and Failure
Strange 304L HX Tube Construction and Failure
(OP)
I attach a few photos of a failed refinery reboiler tube that has me puzzled.
Description: ¾"Ø x 16GA (0.062"), arc welded 304L. Chemical analysis of base metal is in spec, although P is surprisingly high at 0.034%.
Service: Tubeside steam, shell side cat cracker feed (heavy hydrocarbons). Chloride ions reported <10 ppm.
My first thought on seeing the double track on the OD was 'selective corrosion of HAZ', but the microscopy showed different.
- base metal looks heavily sensitized; possible sigma phase. Despite carbon of only 0.02%.
- failure was by a single pit originating on the STEAM side (NOTE: and adjacent tube failed by an apparently entirely different mechanism: 50% lack of fusion in the factory long seam followed by high cycle fatigue).
- the groove on the OD looks less like corrosion pitting and more like a dent.
- aggressive corrosion from the steam side but only at a single pit
Any thoughts?
Description: ¾"Ø x 16GA (0.062"), arc welded 304L. Chemical analysis of base metal is in spec, although P is surprisingly high at 0.034%.
Service: Tubeside steam, shell side cat cracker feed (heavy hydrocarbons). Chloride ions reported <10 ppm.
My first thought on seeing the double track on the OD was 'selective corrosion of HAZ', but the microscopy showed different.
- base metal looks heavily sensitized; possible sigma phase. Despite carbon of only 0.02%.
- failure was by a single pit originating on the STEAM side (NOTE: and adjacent tube failed by an apparently entirely different mechanism: 50% lack of fusion in the factory long seam followed by high cycle fatigue).
- the groove on the OD looks less like corrosion pitting and more like a dent.
- aggressive corrosion from the steam side but only at a single pit
Any thoughts?
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"





RE: Strange 304L HX Tube Construction and Failure
This really looks like defects associated with the seam weld and possibly in the skelp. I couldn't tell in the first photograph if the OD crevices are actually dents because I was looking for locally deformed grains along the surface.
RE: Strange 304L HX Tube Construction and Failure
I've attached a second micrograph done with a different etchant; I cranked up the brightness to show things better. I don't see evidence of either corrosion or mechanical damage, it looks more like the weld just solidified that way. I will go back to the lab for clarification.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Strange 304L HX Tube Construction and Failure
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Strange 304L HX Tube Construction and Failure
Poor quality HX tubing.
RE: Strange 304L HX Tube Construction and Failure
The lines do look like forming tooling marks, even if they were just grooves they could trap impurities and be a site for initiation of corrosion.
This look very much under annealed. I suspect a very rapid inline anneal that is just enough to say they annealed it, they didn't reach solution conditions at all. What is the elongation in the MTR?
Yes it looks sensitized, did they report passing a A262?
I wonder what the residual delta ferrite in the weld is?
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P.E. Metallurgy, Plymouth Tube
RE: Strange 304L HX Tube Construction and Failure
metengr, the bundle was installed 2003, replacing carbon steel. There is no MTR or other data about this, only that the client's standard requires seamless HX tube and that these were installed in error.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Strange 304L HX Tube Construction and Failure
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Strange 304L HX Tube Construction and Failure
RE: Strange 304L HX Tube Construction and Failure
I believe an intelligent welding system would have detected problems by the effect on the arc.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Strange 304L HX Tube Construction and Failure
The root issue could have been a non-metallic inclusion (polite for trash).
With that narrow of a weld it doesn't surprise me that you found lack of penetration in places.
And it doesn't look like the weld was cold worked as required by A249. It looks like it was just sized from the OD.
It might be good to close the loop on this to pull a few tensiles and check for residual ferrite. Let me know if you want a hand.
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P.E. Metallurgy, Plymouth Tube
RE: Strange 304L HX Tube Construction and Failure
I can positively locate the weld, but everywhere I tested the result was in the range 0.1~0.2FN.
The tube geometry is not ideal, but for low ferrite numbers the curvature correction is very low.
I would think that if there was a normal amount of ferrite in the weld the scope would have found it.
EdS, when you say anneal, do you mean solution anneal? Not sure how that could be accomplished effectively onstream.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Strange 304L HX Tube Construction and Failure
I would like to know the manufacturing process for welded SS tube in detail, hopefully in flowchart form. I am especially interested in the annealing cycle.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Strange 304L HX Tube Construction and Failure
The inline anneals work very well for ferritic alloys (439 and superferritics) since these alloys don't contain Ni and diffusion rates are very high.
Material that is specified as furnace anneal or welded and drawn would all be annealed in an off line furnace where hold times are >3 min. Here you will dissolve any preexisting carbides and secondary phases, and you will get some diffusion to break up dendrites (a little) and get some grains forming in the welds.
If the tube is cold drawn then the combined effects of the mechanical deformation and re-anneal will nearly eliminate any weld metal structures.
The specific steps can have some variation. On the weld mill you form, weld, bead condition (roll or forge), and size. If you inline anneal it goes before sizing. Other operations such as straightening and NDT can be done inline or offline.
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P.E. Metallurgy, Plymouth Tube
RE: Strange 304L HX Tube Construction and Failure
That still leaves the severe sensitization; what could have gone wrong to cause that? Very slow cooling of some tubes in a furnace batch?
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Strange 304L HX Tube Construction and Failure
The ferrite could be very low because of low FN raw material combined with very fast weld speeds.
With a weld that is only 0.020" wide this may be laser welded
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P.E. Metallurgy, Plymouth Tube
RE: Strange 304L HX Tube Construction and Failure
Nickel is at 9.2% which is generous, and N is 0.064%, so FN potential in the weld would indeed be low.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Strange 304L HX Tube Construction and Failure
What country was this steel made in? You never see that chem here.
The FN would be low, if it is less than about 3 you get virtually zero retained ferrite in the welds.
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P.E. Metallurgy, Plymouth Tube