Refo
rme
r fu
rnace heate
r tubes made of cast austenitic alloy steels se
rve in the tempe
ratu
re
range of 1073–1173 K and p
ressu
re a
round 3–4 MPa. Though the design life is about 100,000 h (
rc="http://www.sciencedi
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r" bo
rde
r="0">11.4 yea
rs), p
rematu
re failu
res within 3–8 yea
rs a
re mo
re common, mainly caused by p
rematu
re c
reep induced by ove
rheating. The cast tube mate
rial unde
rgoes mic
rost
ructu
ral changes with se
rvice exposu
re and as a consequence, deg
radation in st
rength. Mic
rost
ructu
ral changes, tensile st
rength and accele
rated st
ress
ruptu
re st
rength of 96,000, 105,120 and 131,400 h se
rvice exposed tubes of 24%Ni24%C
r1.5%Nb alloy (equivalent to IN-519) se
rved in a
refo
rme
r fu
rnace we
re analysed. P
resence of coa
rsened ca
rbides at the inte
rdend
ritic bounda
ries, absence of significant amount of seconda
ry ca
rbides in the int
ra-dend
ritic
regions togethe
r with sigma (
σ) phase at both inte
r and int
ra-dend
ritic
regions and isolated c
reep cavities indicated seve
rity of se
rvice induced ageing and damage p
rocess accumulation. Howeve
r, the st
ress
ruptu
re life obtained th
rough La
rson–Mille
r pa
ramet
ric method was good fo
r fu
rthe
r life extension. Attempts to co
rrelate the mic
rost
ructu
ral changes to
remaining c
reep life assessment (RLA) p
roved to be difficult as e
rro
rs we
re mainly a
rising f
rom the fact that tempe
ratu
re expe
rienced by the individual tubes in a fu
rnace diffe
rs and fo
r a single tube th
rough out its ve
rtical length it va
ries. Non-dest
ructive ult
rasonic scan method employed too could not estimate the c
reep damage level accu
rately and the
results va
ry with dest
ructive test
results. Lack of data on both mic
rost
ructu
ral changes and mechanical st
rength of the se
rvice exposed tube mate
rials makes RLA mo
re difficult.