IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
III.C.3. Improvement of Mechanical Properties of
Modified 9Cr-1Mo Steel by Heat Treatment
Modified 9Cr-1Mo steel is
ferritic steel, used extensively in
the standard normalized and
tempered condition (20-30µm
grain size) in fossil power
plants, steam generator of
PFBR, petrochemical industries
and many other heat transport
systems due to its good
mechanical properties up to
823 K and physical properties.
However, the weldments of
these steels are susceptible to
Type IV cracking during service
due to its low creep strength of
the inter-critical heat affected
zone (ICHAZ). Improvement of
Type IV cracking resistance of
these steels by alloying
additions has been suggested
by many authors. In the present
work, it has been shown that
increase in prior austenite grain
size of the modified 9Cr-1Mo
steel by heat treatment
increases the room temperature
and higher temperature yield
strength (Fig. 1) by ~25%
without significantly effecting
the tensile ductility. Increase in
heat treatment temperature
results in increase in grain size
of the material and supersaturated
solid solution of
carbon and alloying elements.
This could be the reason for
increase in strength of the
material in the higher
normalization temperature and
tempering condition. It is
interesting to observe that yield
strength of the weld joints made
with these large prior austenite
grain size materials also
increases (Fig. 2) in same
magnitude at room
temperature and high
temperature.
The increase in yield and
tensile strength of the weld
joints can be correlated with
increase in hardness of the
ICHAZs in the higher grain size
material. It can be mentioned
here that hardness of the
ICHAZ in weld joint, which was
made with 120µm grain size
base metal is increased by 20
VHN 200g . The microstructure
observation shows that
presence of martensite in this
zone could be the reason for
increase in hardness of this
zone. This is further reflected in
the tensile tested weld joints.
The radial profile of tensile
tested specimen shows two
necking (Fig. 3) when the weld
joint was made with 20-30 µm
grain size material, which was
absent when it was made with
120µm grain size material. This
clearly showed that larger grain
sizes can affect the deformation
behavior of cross weld tensile
specimen. The hardness profile
taken from one side of the
tensile tested specimen to the
other side of the specimen
shows that hardness increases
in the broken side of the
specimen when weld joint was
made with 120 µm grain size
material but it decreases when
it was made with 20-30 µm
grain size material (Fig.4),
clearly indicating former
780
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720
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690
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660
650
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630
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1340 1360 1380 1400 1420 1440 1460 1480
Yield Strength, MPa
Yield Strength
UTS
Normalising temperature, K
Fig.1 Change of tensile strength of
base metal with normalizing
temperature in the 1033K/3h
tempered condition (UTS: ultimate
tensile strength)
600
500
400
300
200
100
0
RT
550 0 C
20-30µm
120µm
140µm
Weld Joint
Fig.2 Change of tensile strength of
weld joints with normalizing
temperature in the 1033K/3h
tempered condition
R&D FOR FBRs 47