The service-exposed (similar to 60 000 h/873 K) Alloy 625 ammonia cracker t
ubes showed higher strength and lower ductility compared to the virgin mate
rial in the solution annealed state. Precipitation of intermetallic gamma "
and Ni-2(Cr,Mo) phases and the inter and intragranular carbides were found
to be responsible for higher strength of the service-exposed alloy. Subjec
ting the service-exposed alloy to thermal aging treatments subsequently at
923 It and 1123 It (above the service temperature of the exposed alloy) led
to the dissolution of the intermetallic phases that in turn increased the
ductility of the alloy. Post-service aging of the alloy at 923 K for short
durations resulted in the dissolution of the Ni-2(Cr,Mo)phase. The dissolut
ion of the Ni-2(Cr,Mo)-phase exhibited significant influence upon yield str
ength OCS) but negligible effect on ductility. Prolonged aging of the alloy
for 500 h at 923 K resulted in the precipitation of intermetallic F-phase.
Post-service aging of the alloy at 1123 K promoted the dissolution of both
Ni-2(Cr,Mo) and gamma " formed during service. Longer duration aging at th
e same temperature led to the precipitation of the delta -phase with an ass
ociated increase in strength and loss in ductility. Re-solution annealing o
f the service-exposed alloy at 1423 K caused the dissolution of the strengt
hening phases. When the re-solution annealed alloy was subjected to prolong
ed exposure at 923 K, the yield stress was found to increase rapidly with a
ging time with attendent loss in ductility due to the precipitation of gamm
a ". (C) 2001 Elsevier Science B.V. All rights reserved.