EVALUATION OF SENSITIZATION AND STRESS CORROSION CRACKING BEHAVIOR OF 304H STAINLESS STEEL

Abstract

The present work is concerned with investigation of the sensitization and stress corrosion cracking (SCC) of austenitic stainless steel grade 304H in time - temperature conditions, which could be experienced during tat>rication or operation at normal, transient or even abnormal operating conditions of power boiler, and other applications, and trial to conclude the optimum conditions to eliminate these risks in various phases. Potentiodynamic polarization test in 0.5 M NaCl was carried out. Electrochemical Potentiokinetic Reactivation (EPR) test with its two techniques single and double loop was used to evaluate the degrees of sensitization of the studied alloy at various temperatures and time durations. These techniques were used firstly in the current study to quantifY the degree of sensitization of alloy 304H. However, they are originally specified in ASTM G 108 for alloys 304 and 304L. The ASTM standard committee does not restrict the use of these techniques to the two grades only, but allow studying the validity of them to other stainless steels. In the current study, the applicability of these techniques in evaluation the degree of sensitization of alloy 304H was assured. Also the time temperature " itization (TTS) diagram was also firstly plotted for this value of carbon content.

Boiling magnesium chloride (MgCb) test was used to evaluate the susceptibility to stress corrosion cracking of sensitized 304H SS at two levels of stress. A correlation between degree of sensitization and stress corrosion cracking was established.

It was concluded that the strength properties were significantly reduced as tbe agmg temperatures and duration increase, which was attributed to grain growth and chromium carbides precipitations. Specimens sensitized at 550, 600, 650, 700, and 750 °C showed significant increase in degree of sensitization (DOS) with duration. This may be explained by tbe fact that the tendency to carbide precipitation shows considerable boundary-to-boundary variation. This is reversely related to grain boundary energy. Subsequently the degree of sensitization of the specimen increases with increasing the length of attacked grain boundary. Meanwhile, the specimens that sensitized at 800 °C attain the highest degree of sensitization at I h aging duration, and then this value starts to decrease with sensitizing duration. This could be explained by the so-called healing effect occurred at high temperatures due to chromium diffusion back into the depleted zone from the matrix.