Effect of chromium content on stress corrosion cracking susceptibility of shielded metal arc weld metals for 600 type alloy in high-temperature pressurised pure water

Abstract

The stress corrosion cracking (SCC) susceptibility of SMAW metals for Inconel alloy 600 to which Cr was added to 14.8–21.4 mass% has been investigated on the basis of a creviced bent beam test in pressurized hot water (corresponding to the service condition of boiling water reactor nuclear power plant), since the TIG weld metal of alloy 82 involving 18–22 mass% Cr possesses much better resistance to SCC than the SMAW metal of alloy 182 (Cr content = 3–17 mass%). When their Cr contents were increased to the same level as those of the alloy 82, the weld metals of alloy 182 sustained only slight SCCs in the as-welded state, and no crack was detected after the post-weld heat treatment stress relief+low-temperature ageing (SR+LTA) of stress relief annealing at 893 K followed by ageing at 673 K. These results suggest that the higher Cr content of the alloy 82 is responsible for its higher resistance to SCC than that of the alloy 182. The Cr carbides precipitated at the grain boundary during the welding, and the SR+LTA treatment was also changed from M7C3 type to M23C6 type with the increase in the Cr content. Although the Cr content at the grain boundary in weld metal containing 14.8 mass% Cr subjected to the SR+LTA treatment was 3 mass%, the Cr content of weld metal containing 18.5 mass% Cr was not less than 10 mass%. The addition of the Cr to the alloy 182 increased the Cr content in the grain boundary region, suggesting that the inter-granular SCC can be suppressed when the Cr content at the grain boundary is not less than 10 mass%. In addition to the carbide, Ni16(Mn, Cr)6Si7 (G phase) was precipitated at the grain boundary in the alloy 182 containing 18.5 mass% Cr when the SR+LTA treatment was applied. transmission electron microscope-EDS analyses suggested that the G phase was enriched in P, and therefore could decrease the P content in the grain boundary region. Probably, the decreased P content at the grain boundary due to the precipitation of G phase contributed to the enhancement of the SCC resistance of the Cr-added alloy 182 by the SR+LTA treatment.

Keywords:

• stress corrosion cracking
• alloy 182
• alloy 82
• chromium content
• creviced bent beam test
• chromium depletion layer
• phosphorus content
• G phase