LOW TEMPERATURE SENSITIZATION OF AUSTENITIC STAINLESS STEELS

摘要

Austenitic stainless steels are prone to intergranular precipitation and thus sensitization when exposed in the temperature range of 450-900 deg C. However, no sensitization occurs if the temperature of exposure is less than 450 deg C. It has been found that sensitization in austenitic stainless steels can takes place after long exposure at 300 deg C, if such steels were first exposed briefly in the sensitization range prior to low temperature exposure. This kind of thermal cycle is experienced in welded stainless steels used in Boiling Water Reactor (BWR) and termed as low temperature sensitization (LTS). Nitrogen and carbon have significant effect on the sensitization kinetics and it is expected that these two elements will effect LTS behaviour also. In normal sensitization, nitrogen up to 0.16 percent reduces carbide and nitride precipitation. Stainless steels containing more than 0.16 percent nitrogen are however prone to precipitation of chromium rich nitrides. Nitrides, like carbides can also cause intergranular corrosion of austenitic stainless steels. In the present work, an attempt has been made to study effect of low temperature sensitization on isothermally heated 316, 316L and 316LN stainless steels at 500 deg C, 600 deg C and 750 deg C. The level of precipitation has been studied by ASTM A 262 - Practice A and by electrochemical potentiodynamic reactivation (EPR) techniques. From experimental work it is observed that the amount of sensitization in 316 stainless steel is maximum and in 316L stainless steel is the least. 316LN shows more LTS than 316L stainless steel.