Studies on the Influence of Sodium Hydroxide Concentration on the Stress Corrosion Cracking Behavior of Modified 9Cr-1Mo (P91) Steel Weldment

Influence of sodium hydroxide (NaOH) concentration on the behavior of modified 9Cr-1Mo (P91) steel weldment with respect to stress corrosion cracking (SCC) resistance was studied in this work. Weldment of this steel was prepared using a shielded metal arc welding process using modified 9Cr-1Mo electrode followed by weld heat treatment at 1,033 K/1 h. Stress corrosion cracking experiments were performed at 473 K at a strain rate of 1 × 10−6 s−1 in millipore water (MP) (inert medium) as well as in 1 M, 2 M, 3 M, and 4 M NaOH medium. Ultimate tensile strength (UTS), yield strength (YS), and % total elongation (%TE) determined from stress-strain plots were found to decrease with increasing concentration of NaOH. The SCC susceptibility index (Iscc) evaluated using UTS and %TE was highest for the specimen tested in 4 M NaOH. The number density of cracks determined by optical microscopy increased with the concentration of NaOH. Also, it was higher in number in the base metal than in the weld metal. However, at highest concentration of 4 M NaOH, cracks were observed in the heat affected zone of the weld metal. Fractographic studies by scanning electron microscopic showed mixed mode from intergranular to transgranular cracking and vice versa at all concentrations of NaOH. Failure in the base metal was attributed to coarse precipitates, facilitating easy pitting at the precipitate/matrix interface. From the studies it was inferred that weld metal showed better resistance than base metal to SCC in 1 M to 4 M NaOH concentrations.