Digester agitator blade.
Alloy 31 (high nickel (30.0 – 32.0 percent), chromium (26 – 28 percent), molybdenum (60 – 70 percent) alloy steel)
The blade is used to mix a solution of mined slurry and sulfuric acid in a 100,000-liter digester tank.
The blades had fractured and cracked due to sensitization of the heat-affected zone of the weld. The sensitization of the heat-affected zone of the Alloy 31 blades resulted in severe, localized metal loss along the heat-affected zone of the welds (Figure A and B).
Sensitization is defined as the precipitation of carbides and intermetallic phases, usually at grain boundaries, on exposure to high temperatures, leaving the near-grain boundary region depleted of corrosion resistant elements and therefore susceptible to preferential attack by a corroding medium. In the situation of the corroded agitator blades, the high temperature of the welding would have caused the precipitation of carbides (most likely chromium carbides).
The precipitation of the carbides caused the heat-affected zones of the blades to become susceptible to corrosion in the corrosive environment of the digester. The localized metal loss along the heat-affected zone of the weld(s) weakened the load carrying cross-section of the blades resulting in the observed fracture and cracking. It is important to note that this grade of Alloy 31 did not contain any carbide stabilizing elements, such as niobium or titanium. The lack of carbide stabilizers makes the blades perfect candidates for sensitization and preferential corrosion of the heat-affected zone. Consequently, the Alloy 31 blades should not have been welded.