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ASTM G36 - STRESS CORROSION CRACKING in a BOILING MAGNESIUM CHLORIDE SOLUTION

Presented in this report are the results of stress corrosion cracking testing per ASTM G36 (Standard Practice for Performing Stress-Corrosion Cracking Tests in a Boiling Magnesium Chloride Solution) of welded bare and finned tubes fabricated from steel.  

 

TUBE SAMPLES 

Two (2) 1-foot long pieces of 0.750 O.D. tube of each of the bare and finned tube were supplied for the testing.  Accompanying paperwork states that both types of tube were fabricated from the same heat lot of material. 

 

TEST SPECIMENS 

One (1) 4 long full tube section test specimen was prepared from each type of tube.  No preparation other than deburring and degreasing was performed on the test specimens prior to testing. 

 

TEST PROCEDURE 

Test procedures conformed to the referenced ASTM test method.  As specified, the concentration of the boiling magnesium chloride solution was adjusted to achieve a boiling point of 155 1C.  The test specimens were immersed in the boiling solution and supported using the suggested ladder-back cradles.  Regular examination periods for test specimen cracking were scheduled. 

 

RESULTS 

Circumferential cracking of the bare tube test specimen was apparent within the first 8 hours of testing even while the test specimen was on test.  Both test specimens were taken off test at 8 hours for examination at low magnification.  Cracking of both test specimens was observed, and the tests were terminated.  Numerous circumferential cracks with occasional connecting longitudinal cracks were observed on the base metal of the bare test specimen; however, cracking in the weld/HAZ was mostly longitudinally-oriented.  On the finned test specimen, cracks extended down the fins from the outer edge to the tube wall in a direction roughly normal to the tube wall; at the tube wall the cracks extended in a short arc both longitudinally and circumferentially, the arcs stopping before the next fin was encountered.  The weld and HAZ of the finned tube appeared to have more cracks than the base metal, and the cracks appear to nearly connect from fin to fin.  Photographs of typical appearance of the cracks on both tubes are shown in Figures 1 and 2. 

 

METALLOGRAPHY 

Crack characteristics were investigated by preparing metallographic sections of the test specimens.  It was decided that cracks in the bare tube could be best examined using a longitudinal cross-section, while the cracks in the finned tube could be best examined using a transverse cross-section.  Microphotographs of the deepest cracks found on the mounted sections are shown in Figures 3 and 4.  The microphotographs show that the deepest crack on the bare tube extends through 80% of the tube wall, while the deepest cracks on the finned tube extend through 12% of the tube wall.  Etching both sections did not reveal any additional crack depth as sometimes occurs in metallographic studies of stress-corrosion cracking.

 

Figure 1. Circumferential cracks with connecting longitudinal crack in base metal of bare tube. (25x original magnification)

Figure 2. Longitudinal crack in weld/HAZ of bare tube.(12.5x original magnification)

   

Figure 3. Deepest crack found on longitudinal cross-section of bare tube. (62.5x original magnification)

 

Figure 4. Deepest cracks found on transverse cross-section of finned tube.  Black lines indicate the approximate location of the outer surface of the tube. (62.5x original magnification)

 

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