Response of Weld Joints in Stainless Steel 304LN Pipe to Low Temperature Sensitization

Abstract

Heat affected zones adjacent to the welds are generally considered sensitive locations for failure of heat transport system pipelines (made of austenitic stainless steels) in nuclear power industries. Present study includes understanding the sensitization and IGC behavior of nuclear grade stainless steels (AISI 304LN) at low temperatures. Two different solution annealed (pipes) AISI 304LN and weld joints were exposed to 400 and 450°C for 125-8000 hours followed by furnace cooling. Sensitization was quantified in terms of degree of sensitization (DOS) by using double-loop electrochemical potentioldnetic reactivation (DL-EPR) method. The oxalic acid (in 10% oxalic acid solution) etching and copper-copper sulfate-sulfuric acid test as per the ASTM 262 Practice 'A' and 'E' were performed to detect and quantify the susceptibility to intergranular corrosion. Specimens from two different pipes showed variations in susceptibility to sensitization and IGC. Specimens designated, as 304LN-P1 was much less prone to sensitization as compared to 304LN-P2. While HAZ of former showed `step' type structure the other one (304LN-P2) produced mostly 'dual' and `step', in a few cases, during oxalic acid etching. While bending after boiling in copper-copper sulfate- sulfuric acid solution, certain specific time/ temperature combinations produced fissures or even large cracks on the weld pool of both stainless steels.