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 P91 steel was prepared by shielded metal arc welding (SMAW) process using modified 9Cr- 1Mo electrode followed by weld heat treatment at 1033 K/1h. SCC experiments were carried out at 473 K at a strain rate (SR) of 1 × 10-6 s-1 in Millipore water (MP) (inert medium) as well as in 1, 2, 3 and 4 M sodium hydroxide (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. SCC susceptibility index (Iscc) evaluated using UTS and % TE was highest for specimen tested in 4M NaOH. Number density of cracks determined by optical microscopy increased with the concentration of NaOH. Also, number of cracks was higher in the base metal than in the weld metal. At highest concentration of 4M NaOH, cracks were also observed in the heat affected zone (HAZ) of the weldmetal. Fractographic studies by Scanning electron microscopic (SEM) 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 1-4 M NaOH concentrations.

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