Non-Destructive Testing on the Lime Hydrater Plates to Test for SCC
450 plates in a lime hydrator were reportedly cracking in service. Hardness and chemical analysis results indicated that the plates met the applicable specification requirements for composition and mechanical properties. Magnetic particle evaluations indicated that no surface cracking resulted from forming or welding operations.
As an actual crack defect was unavailable for analysis, the exact cause of the cracking remained undetermined. Stress corrosion cracking (SCC) was suspected.
Two likely possibilities for the observed cracking are: 1) rapid cooling of the welded paddles while the remaining material was at maximum temperature, or 2) thermal fatigue due to temperature differences between the paddles and the underlying Hardox plate.
Fluorescent magnetic particle testing was performed on the formed and welded plates, with no surface crack
indications found. Rockwell C Hardness (HRC) of the plate was 46 HRC, indicating mechanical properties were within specification.
The plate defect fracture surfaces were not available for examination. As all cracking requires an applied tensile load, a possible cause of the observed cracking is a rapid cooling of the welded flights (paddles) while the remaining material was at maximum temperature. While this grade of steel is susceptible to stress corrosion cracking (SCC) in caustic environments, it is unlikely given the single intense crack front with reported operating temperatures below the threshold temperature of 210 F for SCC.
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– Tom O’Donnell, PE
– Introduction to Stress Corrosion Cracking
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– Publications – Heat Exchangers, Pressure Vessels, Welds and Other Applications, Fatigue, Elevated Temperature