Explosive Deslagging - Damage to Stainless Steel 2

[SMF1964]

One of the techniques used to deslag a boiler is through the careful use of explosives to remove the slag from the underlying steel tubes. On occassion, there are consequences of this process in the form of damage to the underlying tubes.

If one looks at a carbon steel (or low alloy) with a ferrite + pearlite structure, a tube that has been damaged during explosive deslagging will exhibit twinning in the ferrite as a result of the explosive impact.

Unfortunately, I have a pair of 304H stainless steel tube that have been 'damaged' by the explosive deslagging process (one more than the other). Any thoughts on microstructural changes in stainless steel as a result of explosive impact?

 

[metengr]

SMF1964;
Regarding explosive cleaning to ferritic materials like C-Mn, C-Mn-Si, C-Mo, Cr-Mo alloys in particular, you can generate ID initiated axial cracks in addition to twinning that results in tube leaks and eventual replacement of steam circuits over time. Let me preface the cleaning method by saying that use of pole charges versus detonation (det) chord seems to be less severe. You did not mention if you used pole charges or det chord.

I have experienced first hand the disastrous effects of explosive cleaning of pendants and horizontal tube assemblies using detonation chord. The sudden concussion or pressure wave subjects the tube material to locally excessive strain rates. Since the alloys mentioned above age with exposure to elevated temperature service, these materials will have reduced ductility and will not be able to accommodate these excessive strain rates from explosive cleaning. We performed extensive laboratory examination to confirm this finding. Bottom line - do NOT use det chord to clean fouled steam circuits.

Now, regarding austenitic stainless steel tube material, this material is indeed more forgiving in comparison to the ferritic boiler tube materials mentioned above. We have not explosively cleaned in areas of the boiler containing stainless steel tube material because typically stainless steel boiler tubing sheds slag – in other words fouling is less prevalent on stainless steel superheater tubing. I would be very concerned if you generated excessive amounts of sigma phase in the 304H material based on long term exposure to elevated temperature service-like in a superheater. I would remove a tube sample that contains remnant explosive damage or was in proximity of the explosive cleaning and make sure you have no twinning or ID initiated cracking due to sigma phase.

I would not want someone else to follow along the same path that I had to walk about 10 years ago when we started burning PRB coals that tend to foul PC boilers. Our upper management got hooked on explosive cleaning and by the time I was involved it was too late.

 

[EdStainless]

If hte tubing has no secondary phases, sigma, chi, or carbides, then you are probably OK. A lot of stainless goes into service with a lot of cold work and it does fine.
ID surface cracking would also be my first concern.

If the structure is clean and there are no cracks I wouldn't worry.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[swall]

Isn't this "twinning" actually adiabatic shear? You see this in the near surface layers of steel that has been heavily shot peened.

 

[SMF1964]

I'm not sure - I don't recognize the term. I'm including a link to a micrograph of a medium carbon steel, shattered in an explosion, that has twinning in the ferrite. If you print the image out at 6.75" wide, the magnification will be 1000X.

http://flypicture.com/bin/?id=rtzzkanR

 

[swall]

SMF1964--your micro is very interesting. Adiabatic shear is defined as "highly localized deformation zones in metals that are observed at very high strain rates, such as those produced by high velocity projectile impacts (330-11,800 ft/s)or explosive rupture". What is different in your case from what I see in shot peening is that you have deformation localized to the ferrite grains whereas shot peening of martensitic steels produces deformation bands that encompass multiple grains. Also, the shot peening induced bands appear to be amorphous, with no apparent metallographic structure.Perhaps "deformation twin" would be a more precise definiation of that what occurs in your case?

 

[SMF1964]

It is a "deformation twin" as opposed to an "annealing twin".

 

[SMF1964]

More information on the stainless steel:

304H, sensitized due to exposure to normal operating temperatures for 16 years. The actual failure was intergranular. The unfailed tube contained subsurface, intergranular fissures in a plane that was parallel to the tube surface and approximately 30 mils (0.030" or 0.75 mm) from the internal surface.

Ok, new question: Is this normal for explosively loaded stainless steel? A function of the sensitization and the loading?

 

[metengr]

I would say that based on your stated conditions for explosive cleaning -yes. If you want to compare just how significant these rapid excessive strain rates are from exposure to explosive cleaning, remove a ring section or strip from the unaffected tube material and perform a flattening or bend test to evaluate the extent of ductility in the 304H sensitized material. If the material can deform with no fissures, the excessive strain rate induced by explosive cleaning is the culprit.

 

[SMF1964]

Yes, a ring section cut from away from the failure and compressed/flattened produced no failure.

 

[metengr]

SMF1964;
Do NOT explosively clean boiler reheater and superheater tube circuits.

 

[EdStainless]

My hunch is that the problem is due to the sensitization. Parts may not fail in low strain rate tests, but I'll bet that you impact values suck.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[metengr]

The H grade austenitic stainless steel boiler tube materials can sensitize from now until the cows come home with no adverse effects on performance under normal boiler operating conditions. I was surprised that someone would even consider explosive cleaning in stainless steel tube elements because normally the austenitic stainless steel tube elements do not develop a tenacious layer of fly ash with PRB fuels.

We just changed out an original ferritic stainless steel reheater with 304H and it is amazing how clean these assemblies are after 2 years of operation on PRB coal.

 

[SMF1964]

We use Central App coal, not PRB, and we've had clinkers in this supercritical unit that were the size of the subcritical sister-unit. And the problem occurred when the unit was cold, so room temperature properties enter into play, although I agree that operating temperatures are a bit more forgiving for "impact" affecting factors like the dreaded sigma-phase, etc.