The two February 1997 patch repairs on tank car ACAX 80010 were inspected and tested in accordance with these new procedures, and the repair patch that failed was completely removed and replaced, and the replacement patch was heat-treated. The National Transportation Safety Board determines that the probable cause of the failure of tank car ACAX 80010 was inadequate heat treatment to reduce the hardness of the weld material used in the repair of the tank to a level that would retard or prevent hydrogen-assisted cracking and inadequate testing to determine whether the weld material hardness exceeded established limits. In 1990, head shields were added to the tank car, and it was converted to a specification 112S400W car. 2 A hydrogen blister in steel is a bulge caused by molecular hydrogen trapped at an internal flaw within the steel. The Rockwell C hardness scale is used for rating the hardness of steels.
Higher values indicate higher hardness.
4 A river pattern is a series of radiating lines on the fracture surface. The lines indicate the direction of crack propagation. 6 A material safety data sheet is developed by the producer of a chemical product and contains general information about it, including its chemical and physical properties, the health and environmental hazards it poses, and guidelines for responding to product releases. 7 Schuyler, Roy L., III, Ph.D., Hydrogen Blistering of Steel in Anhydrous Hydrofluoric Acid , National Association of Corrosion Engineers, Houston, Texas, March 1979. 10 Twenty-six tank cars made of TC 128 steel, including ACAX 80010, were in Canada at this time.