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Materials at High Temperatures (2006), 23(1)
Processing
of high-temperature resistant threedimensional
networks of interconnected metallic fibres by pack
aluminization
E.
Koza and J. Proost*
Division
of Materials and Process Engineering, Universite´ Catholique de Louvain (UCL),
Place Sainte-Barbe 2, B-1348
*E-mail: proost@imap.ucl.ac.be
ABSTRACT
This
paper deals with the use of a pack aluminization process for the transformation
of stainless steel
wires and three-dimensional (3-D) interconnected
fibres into intermetallic compounds. The decomposition
of the pack was first investigated using differential
thermal analysis. For both sample
geometries, wires and interconnected fibres, the
influence of trace amounts of oxygen in the
processing atmosphere on intermetallic formation
is shown. In a normal furnace atmosphere with
Ar-flow,
aluminium deposited from the pack on the sample surface but did not diffuse
inside because
a surface oxidation layer appeared. Upon active
limitation of the oxygen partial pressure, Al was able
to
diffuse into the sample, up to 100 mm after 4 hours at
1000_C for the stainless steel wire, and
resulting in complete transformation of the 3-D
fibre network.
Keywords:
pack aluminization, metallic fibres, three-dimensional network
High
temperature mechanical properties of Inconel
718
pulsed Nd–YAG laser welds
G.D.
Janaki Rama*, A. Venugopal Reddya and
K. Prasad Raob and G. Madhusudhan Reddyc
aRegional Centre for Military
Airworthiness (Materials), Kanchanbagh,
*E-mail:
janakiram_gabbita@yahoo.co.in
bDepartment of Metallurgical
and Materials Engineering, Indian Institute of Technology, Chennai, 600 038
cDefence Metallurgical
Research Laboratory, Kanchanbagh,
ABSTRACT
Bead-on-plate
full-penetration pulsed Nd –YAG laser welds were produced in 2mm thick sheets
of
Inconel
718 and were subjected to aging treatment with and without post-weld solution
treatment at
980_C.
Weld microstructures, high temperature tensile and stress rupture properties
were evaluated.
The
tensile and stress rupture properties of the welds after post-weld aging
treatment were found to be
inferior in relation to the base metal due to the presence
of Nb-rich brittle intermetallic Laves phase in
weld microstructure; however, the weld properties
were found to satisfy the minimum property
requirements generally specified for alloy 718
sheet products (AMS 5596). Post-weld solution
treatment at 980_C was
found to result in considerable Laves dissolution and reduction in Nb
segregation, leading to improvement in weld
properties, although not to the level of base metal.
Keywords:
laser welding, Inconel 718, Laves phase, Nb segregation
Degradation
mechanisms of aluminium diffusion
coatings on 12% chromium steels under elevated
temperature erosion – oxidation conditions
E.
Huttunen-Saarivirtaa*, F.H. Stotta,
V. Rohrb and M. Schu¨ tzeb
aCorrosion and Protection
Centre,
*E-mail:
elina.huttunen-saarivirta@manchester.ac.uk
bKarl-Winnacker-Institut
der DECHEMA e.V., Theodor-Heuss-Allee 25, 60486
ABSTRACT
Aluminium
diffusion coatings offer one possible solution for improving the erosion–corrosion
resistance of candidate steels for future power
plants. However, their successful utilisation at
temperatures higher than currently and under
erosive load of solid particles requires an understanding
of possible degradation phenomena that take place
in both the short and the long term. This paper
reports on the degradation resistance of aluminium
diffusion coatings in the temperature range 550 –
700_C under impacts by silica
sand particles at 30_ and 90_ for
200 h.
For
90_ particle impacts at 550_C
and 600_C and 30_ particle
impacts at 650_C and 700_C,
degradation of the coatings occurs rapidly by
erosion, with oxidation or other diffusional processes in
a minor role. For 30_ particle
impacts at 550_C and 600_C,
the rate of erosion exceeds the rate of
oxidation, with oxidation-affected erosion being
the prevailing erosion – corrosion mode. Chipping
and cutting wear pose the greatest challenge for
long-term performance of the coatings. The coatings
subjected to 90_ particle
impacts at 650_C and 700_C
undergo more rapid oxidation and experience
less erosive wear, with erosion-enhanced oxidation
being the primary erosion – corrosion mechanism.
Here,
oxidation leads to depletion of the near-surface layer in aluminium and void
formation near the
coating surface, with consumption of the aluminium
reservoir and an impairment of the coating
integrity as primary consequences. Phase
transformations and a thermally activated columnar growth
were also observed to occur in the coatings,
causing, for example, formation of new phases with
decreased Al content and variations in the
coating thickness.
Keywords:
aluminium diffusion coating, degradation, erosion – oxidation
Thermal
shock behaviour in ferritic steels:
laboratory tests and comparison with service
conditions
R.P.
Skelton
Consultant,
c/o Science Reviews,
E-mail:
pskelton@scilet.com
ABSTRACT
‘Down’
and ‘up’ thermal shock experiments on cast and forged forms of 9/12Cr
steels and their
weldments at a maximum temperature of 565_C
have demonstrated a marked reluctance to form
traditional craze cracking and propagation into
the body. However, in specimens of ex-service cast
1CrMoV
material tested to the same number of cycles (1000), initiation and growth of
(intergranular)
cracks is much more likely in layers near the inner
surface of the component than in layers deep
within the section. This is due to the effect of prior
thermal cycling of the inner material during start
up and shut down events.
Thermal
shock and isothermal tests at a maximum temperature of 550_C
on 1/2CrMoV/21/4Cr1Mo
weld features have indicated that crack propagation
is generally along the fusion boundary (though for
some cases, in the weld metal itself) and that
cyclic growth rates lie close to an upper bound law valid
for ferritic steels. The results from laboratory
tests are compared with service experience as regards
stresses generated, likely crack path and possibility of
arrest.
Keywords:
downshock, upshock, cylindrical shell, residual stress, temperatureystress
gradient, fusion boundary
cracking, stress intensity factor, transgranularyintergranular
cracking