Effect of Low Temperature Sensitization on IGSCC Behavior of Type 304L and Type 304LN SS in Oxygenated High Temperature Water

Abstract

Shirish Chandrakant Bali and V. S. Raja, IIT Bombay, INDIA; V. Kain, BARC, INDIA

Austenitic stainless steels are widely employed as structural materials in nuclear power plants. Boiling water reactors (BWRs) operate at high temperature (~ 288oC) and high pressure conditions (~ 7 MPa). Stress corrosion cracking (SCC) of weldments is one of the major issues concerning these materials at these operating conditions. Sensitization is the basic reason for intergranular stress corrosion cracking and it occurs due to the formation of chromium carbides (Cr23C6) at grain boundaries of weldments.

Hence, low carbon stainless steels are employed to prevent sensitization and IGSCC. Though low carbon stainless steels such as type 304L SS does not sensitize just after welding, prolonged exposure to BWR conditions are found to promote low temperature sensitization (LTS). N addition to 304L SS is expected to not only enhance to alloy strength, but also sensitization resistance. Hence study of LTS is very useful

The motive of this study is to examine the IGSCC behavior of low temperature sensitized type 304L and type 304LN SS in oxygenated water at 288°C and compared to that of high temperature sensitized materials. Experiments were performed by slow strain rate test (SSRT) in aerated pure water at 288°C with the strain rate of 5.68 x 10-7s-1. Small fractions of surfaces with intergranular cracks were observed on fracture surface in oxygenated high temperature water environments in the sensitized condition. The LTS treated samples of both the grades of SS did not show intergranular facets indicating resistance to IGSCC.