Development of high creep resistant MARBN steels for forged components in high efficient power plants

Over decades Böhler Special Steel
has been a full package supplier of customized high
quality
special steels and forgings. Böhler Special Steel
fosters
close relationships to
plant
manufacturers with the aim to provide products ahead of its time.
This paper provides some results
of
our research activities
to raise the operating
temperatures of 9%
Cr steels from 625
°C
(1157
°F)
to 650
°C
(1202
°F).
Increasing the
operating temperature requires adoptions
of our processes
and manufacturing methods to
produce optimized microstructures
which
meet customer specifications and
increase
the
creep
rupture strength
at the same time. The microstructure of two
Boron containing
9% Cr steels
(FB2-2-LN and NPM1-LN) developed within the framework of our COST activities is
compared after heat treatment and discussed after creep rupture.
The results show a dependency
of the creep rupture strength on the stability of
precipitates
and on the recovery of dislocations within the first 10 000 h of creep exposure at 650 °C.
Heat
treatment allows to reduce the total amount of dislocations and to decrease their
recovery during creep. The creep rupture time of both steels was increased by more than 30
%
without negatively affecting the creep rupture strain.