Material Defect /Furnace Wall Tube

A furnace wall tube had failed in less than one year of service. It was a SA-210 Grade C carbon steel tube. Six of out 24 tubes with heat flux sensors have failed. Nearby water cannons sometimes malfunction and spray clean tubes; pumps are shut off manually to stop cannon spray. The water cannons and heat flux sensors are part of a complete de-slagging system.

The heat flux sensor is attached to the tube as shown. There was a longitudinal crack (indicated by arrow), about long, on the hot side of the tube near the sensor attachment. The crack was just adjacent to the crown of the tube, at about the 11:00 orientation. The ID was relatively clean, other than some deposit at the crack. The crack ran longitudinally, at the tube rifling. There appear to be multiple cracks at the failure site as shown. A crack indication was observed in the root of the adjacent rifle. Some bulging was observed at the tube ID due to insertion of heat flux sensor. 

 

Heat flux sensor was attached to the tube. Longitudinal cracking is shown by the arrow.

   
   

ID view of cracking after deposit removal.

 

   

There appears to be a couple of cracks; one at the ID (shown by yellow ellipse) and the other in the mid-wall (shown by black ellipse). There was a secondary crack that did not appear to touch the OD or ID, indicated by the red ellipse. The large crack in the middle had intersected the ID initiated crack over a period of time. The tube defect may have been present in the form of lap or inclusion during billet making process. Significant residual stresses were present due to insertion of the heat flux sensor. These residual stresses exacerbated the cracks in the tube.         

Tube defects in the middle and a thermal fatigue crack at the ID surface.

   

Tube manufacturing defect.

   

The cracks initiated at the ID surface were sharp, transgranular and oxide filled, typical for thermal fatigue. The fatigue cracks were initiated due to rapid contraction of the tube when it was blasted with water from a canon. Over a period of time, the thermal fatigue crack at the ID intersected the tube defect and ruptured. The oxide scale in the primary fracture may have been removed as steam escaped.  

 

Crack intersection

   

Thermal fatigue crack

   

The furnace wall tube failed due to a manufacturing defect exacerbated by the residual stresses due to insertion of the heat flux sensor, and thermal fatigue due to the use of water cannons. The tube defects may have been present in the form of laps or inclusions during the billet making process. Thermal fatigue cracks were observed at both ID and OD surfaces. These thermal fatigue cracks were driven by the differential contractions when the water splashed onto the bare tubes. The microstructure was ferrite and pearlite, typical for carbon steel material.