A 6% Mo superaustenitic stainless alloy (UNS N08367) was exposed at temperatures in the 1000 to 1900°F (538 to 1038°C) range for times up to 10,000 hours. The effect of these exposures on mechanical properties and corrosion resistance has been examined.
INTRODIJCTION
Six percent molybdenum alloys have row been used in a wide variety of chemical process applications where the combination of chromium, molybdenum and nitrogen has provided these alloys with excellent corrosion resisting properties, particular against high chloride environments. Unfortunately, the high molybdenum content (6% minimum) of these alloys renders them susceptible to precipitation of intermetalic second phases (typically sigma Fe-Cr-Mo) if exposed for extended times within the 650 to 1000°C (1200 to 1830°F) temperature range. This instability (with regard to precipitation of intermetallic phases) is reduced by the addition of nitrogen, which helps to reduce the temperature range in which intermetallic phase precipitation occurs and to extend the times required for observable precipitation.2 In this study, UNS N08367 was chosen as a representative 6% Mo superaustenitic alloy. UNS N08367 alloy is produced under the tradename AL-6XN® alloy.
UNS N08367 alloy is currently restricted by the ASME Boiler and Pressure Vessel Code to 800°F (427°C) maximum use temper ature.4-5 This restriction is typical for alloys for which creep and rupture data have not been supplied. At times, it has been requested that the ASME limit be raised to 1000F (538°C) or higher. Creep and stress rupture resistance must be demonstrated for ASME approval to be granted for high temperature (greater than 800°F) use. The known tendency for 6% Mo alloys to suffer loss of corrosion resistance (typically more rapid) z nd embrittlement (typically slower) during exposures at high temperature led to the Iiecision to investigate these effects before investigating the creep and stress rupture properties of N08367 alloy. At the same time, it became desirable to investigate the effects of short-term heat treatments carried out below the --1950°F (1065°C) sigma S01VUS[21. In support of these goals, samples of N08367 alloy were aged at various temperatures between 1000 and 1900F (538 and 1038°C). The resultant materials were mechanically testec, their microstructure described, and their performance in ferric chloride was measure 3. The ferric chloride test is an aggressive laboratory test solution often used as an accelerated test for resistance to natural seawater or other oxidizing, high-chloride environments. Although the work described here is specific to the N08367 alloy, the phenomena described are believed to be generally applicable to all the 6% Mo stainless alloys.
In a previous study, it was demonstrated that the sigma solvus for N08367 alloy is below 1950°F (1065°C). Thus, at the inception of this study, it was anticipated that N08367 alloy would be resistant to embrittlement at some temperatures between 800 and 1950°F (427 and 1065°C). It was also anticipated that, at the edges of the embrittlement temperature range, embrittlement would be slow. Since it is impractical to expose specimens for the actual design lifetime of plant equipment, which may extend to 40 years, this investigation was started with the idea of extrapolating shorter-term exposures at higher temperatures to longer times at lower temperatures. The initial conditions investigated were for material aged at l500°F (815°C), since this was believed to approximate the temperature of most rapid intermetallic phase precipitation. Subsequent times and temperatures were selected based upon the results of preceding tests. Because the higher temperatures (1800 and 1900°F ? 982 and 1
