ABSTRACT
A computational approach based on thermodynamic and kinetic calculations (using commercial software) to assess the sensitization propensities of titanium and niobium stabilized stainless steels (UNS S32100 and UNS S34700, respectively) was investigated. The thermodynamic part consisted of calculating the carbon and nitrogen solubilities in austenite in the stabilized steels and comparing them to the corresponding solubilities in the unstabilized grades to determine the effectiveness of the stabilizing elements in decreasing carbon supersaturation at the service temperature. The kinetic part of the analysis consisted of determining the precipitation rate of carbides and carbo-nitrides during extended service. In addition, detailed microstructural studies were performed on selected steels exposed to service to validate some of the computational results. The study revealed the critical role of nitrogen in these steels with respect to sensitization and how its role differs between the two steels. Quantitative insights from the study provide guidance to fine-tune existing ASTM composition specifications and defining of optimum steel chemistry window for improved resistance to sensitization. With future calibration studies with field samples, the calculations may also be effectively used to predict the sensitization potential of components in service based on the alloy chemistry and heat treatments prior to service.
INTRODUCTION
It is well known that titanium (UNS S32100) (1) or niobium (UNS S34700) is added to ”stabilize” stainless steels through the formation of titanium or niobium carbides with the intent to decrease the carbon content in the austenite and prevent the formation of grain boundary chromium carbides during service.1,2 Previous workers did extensive experimental work to determine the carbon solubilities in austenites and demonstrated the effectiveness of Ti/C ratio in controlling the carbon solubility.3-5 Based on past studies and industry experience, ASTM(2) has established specifications for the use of these steels.6 However, there are some inconsistencies in the chemistry specifications. For example, ASTM A213 and A240 specifications which cover super heater / heat exchanger tubes and plate products, respectively, specify a ratio Ti/(C+N) > 5 for UNS S32100 steel while nitrogen is not included in the specification of UNS S34700 steel (Nb/C>10). Since both titanium and niobium are known to form carbo-nitrides (e.g. Ti(CN), Nb(CN)) the rationale for this difference is not clear. It is, however, recognized that these ratios are relevant only when these steels are used at elevated temperatures when precipitation of carbides can occur during service causing sensitization and localized corrosion. Therefore, API 571(3) which addresses Polythionic Acid Stress Corrosion Cracking (PASCC) damage mechanism which is caused by sensitization, also recommends an additional “stabilizing” heat treatment at 900 0C to maximize the effectiveness of the titanium or niobium additions.7
