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Journal of Materials Science and Engineering A 7 (9-10) (2017) 271-279 doi: 10.17265/2161-6213/2017.9-10.005
Influence of Composition and Aging Heat Treatment on
the Microstructure and Strength of Innovative-Carbon
Free 10% Cobalt-Maraging Steel Powder Composites
Waleed Elghazaly1, Omyma Elkady1, Sabine Weiss2 and Saied Elghazaly1
1. Central Metallurgical R & D Institute-, P.O. Box 87-Helwan, Cairo, Egypt
2. Head of Material Science & Technology Dept., TU Cottbus-Germany
Abstract: Good combinations between strength and toughness are always the aim of all researchers working in the field of material science. Maraging steel grades (200-300) are one of the well known steel alloys proved to have good strength and toughness and are known as 18% Ni-Co-Mo steel family. Maraging steels production, import, and export by certain countries such as USA is closely monitored by international authorities because it is particularly suited for use in gas centrifuges used for uranium enrichment and in aviation technology. In this research an effort is paid to produce innovative carbon-free maraging steel alloy composites that can compete the well known 18% Ni-8% Co standard (250-300) maraging steel alloy with higher strength and superior toughness. The experimental maraging steel composites having different Ni (18-25%) and Al (0.5-1.5%) together with or without Ti and Mo contents are produced by consolidation from the nano-elemental powders. The mechanism of strengthening in Iron- Nickel- Cobalt-Aluminum composite alloys is studied, however, the changes in microstructures after solution treatment and aging-heat treatment are emphasized using metallurgical microscopy and SEM-TEM aided with EDX analyzing unit. The effect of induced deformation on the properties of the as-sintered samples is also studied. Fracture toughness, impact toughness, hardness, and strength are measured for all alloy composites under investigation and compared with the standard nominal properties for conventional maraging series (250-300).
Key words: High-Strength steels, maraging steels, mechanical properties, microstructure, fracture toughness, precipitation hardening, solution treatment, aging heat treatment.
1. Introduction
The ever increasing demand for superior steel in
aviation and automotive industries requiring high
strength, sufficient ductility, and good weldability
initiated development of maraging and precipitation
hardenable stainless steels [1]. PH (precipitation
hardenable) steels have a unique advantage over
others where they are hardened without quenching and
the absence of distortion and decarburization [2].
These steels rely on the precipitation of intermetallics
compounds and generally contain high levels of cobalt,
Corresponding author: S. El-Ghazaly, Prof., researcher,
research fields: steel metallurgy, steelalloys, superalloys, mechanical failures, microstructures, heat treatment, microstructure-stainless steels, material science, nano and composite materials.
molybdenum and nickel [3]. Elements such as
titanium, vanadium, aluminum and niobium have been
added to enhance the precipitation process thus
increasing the strength. Those steels are generally
hardened by aging at approximately 450-500 °C [4].
Maraging steels based on iron-nickel martensite
constitute a very important family of high-strength
steels, which distinguishes itself by demonstrating an
unparalleled combination of excellent fabricability [5],
high strength and fracture toughness after heat
treatment.
Heat treatment of these steels has now been
perfected to ensure consistently high levels of strength,
ductility, and toughness in a variety of product shapes
and sizes [6]. Cobalt-free variants have been
commercialized as part of efforts to save production
D DAVID PUBLISHING
Influence of Composition and Aging Heat Treatment on the Microstructure and Strength of Innovative-Carbon Free 10% Cobalt-Maraging Steel Powder Composites
272
costs.
Further knowledge has been generated on 18%
nickel maraging steels regarding phases precipitating
during aging, thermal embrittlement, thermal cycling
and austenite reversion/retention and their effect on
mechanical properties [7-9]. The well known alloy
composition of 18% Ni-Co-Mo maraging steels was
designed for maximum strength to toughness ratios due
to their tough Fe-Ni martensite structure which is
hardened at low temperature aging.
It is reported that the primary precipitate responsible
for the strengthening is Ni3Mo, however Cobalt
enhances its precipitation by decreasing the solubility
of Molybdenum in the matrix. Another secondary
hardening by formation of Ni3Al and Ni3Ti
intermetallic precipitates was also reported [10].
In this research some carbon-free maraging steel
alloy composites with different compositions were
produced by sintering them from their powders at
about 1,350-1,450 °C to emphasize the effect of Ni,
Al and Mo contents as well as aging condition on their
microstructures, tensile and fracture toughness.
Microstructure characteristics were investigated on the
light, scanning and transmission electronic
microscope.
2. Experimental Work
2.1 Materials Processing
Maraging steel bars composites 55 10 10 mm
were prepared by consolidation from their powder
constituents by HIP (hot isostatic pressing) at 1,200
MPa. The powders (100-200 microns) were blended
in a tumbling mixer in dry basis for 1.5 h and then
compacted under 1,200 MPa uniaxial pressures in a
special steel die to final product .The bar samples
were then sintered at 1,400 °C for 3 h under vacuum.
The final adjusted compositions of the experimental
steel composites sinter are shown in Table 1.
Some of the sintered bars were forged in the range
1,100 °C-850 °C to about 60% reduction in area to
study the differences in densification, shrinkage,
microstructure and tensile properties as well. Standard
testing specimens were cut and machined from the
forged bars using normal procedures.
2.2 Heat Treatment
Samples of sintered steel rods C-1 to C-4 and others
forged ones were solution treated (900 °C, 120 min,
water cooling). The as-sintered and as-deformed
solution treated samples are then aged at 500 °C for 5
hours and then air cooled to strengthen their matrices
by precipitation hardening mechanism through
forming series of intermetallic phases. Fig. 1
illustrates the procedure of solution treatment and
aging heat treatment of both as-sintered and forged
steel bars.
3. Results and Discussions
3.1 Sintring
Variations in dimensions of the consolidated bars of
produced experimental maraging steel bars were
subjected to changes during processing depending on
the sintering temperature at constant pressure (1,200
MPa) and powders grain size as shown in Fig. 2.
Densities of about 7.8 and 8.3 gm/cm3 were reached
for C-4 maraging steel under such conditions and at
Table 1 Chemical composition of experimental sintered maraging steel forged bars and standard 18%Ni Maraging (250) steel. Element, wt.% Material
Ni Co Mo Ti Al S Cr C
Composite-1 25 10 ---- ---- 1.50 0.003 ---- ----
Composite-2 20 10 4.50 0.55 1.50 0.004 ---- ----
Composite-3 18 10 4.50 0.62 1.50 0.003 ---- ----
Composite-4 20 10 4,50 0.50 0.50 0.005 ---- ----
Standard maraging (250) 18.5 8.4 5.00 0.72 0.12 0.002 ---- 0.03
Influence of Composition and Aging Heat Treatment on the Microstructure and Strength of Innovative-Carbon Free 10% Cobalt-Maraging Steel Powder Composites
273
Fig. 1 Solution and aging heat treatment of the experimental maraging steel alloys.
Fig. 2 Variations in density and shrinkage of C-4 steel after sintering .
sintering temperatures 1,200 °C and 1,300 °C
respectively, while shrinkage of about 22% in such
steel was measured after sintering. The compactness
and density of for example C-4 maraging steel bars,
was upgraded to reach 9.1 gm/cm3 after 85%
reduction in thickness by drop forging at 850 °C.
3.2 Microstructure Variations
Cobalt, Nickel and Molybdenum dissolved in liquid
Iron to form series of solid solutions and intermetallic
compounds as shown in the Fe-Co, Fe-Ni, Fe-Mo and
Fe-Ni-Co equilibrium phase diagrams in Fig. 3 [11].
Nickel forms with Iron series of equilibrium phases
like austenite, martensite, ferritic-austenitic and even
Ni3Fe phases depending on Nickel content, however
Molybdenum prefers to form ferrite (110). The
presence of 10% Cobalt (002) in Iron also enriched
formation of ferrite, however addition of 20% Ni and
Time (Hrs)
0 2 4 6 8 10 12 14 16
Tem
pera
ture
o C
0
200
400
600
800
10002 hrs
5 hrs
SolutionTreatment
Aging
SoftMartensite(28-32 HRc)
Hardening (52 HRc)
Sintering temperature oC
1050 1150 1250 1350
Den
sity
, gm
/cm
3 , x10
30
40
50
60
70
80
90
10
15
20
25
30
Shr
ink
age
%
1100 1200 1300 1400
Density
Shrinkage
Influence of Composition and Aging Heat Treatment on the Microstructure and Strength of Innovative-Carbon Free 10% Cobalt-Maraging Steel Powder Composites
274
Fig. 3 Equilibrium phase diagrams of Fe-Ni , Fe-Co, Fe-Mo and Fe-Ni-Co systems.
5% Mo prefers to form fully austenitic (111)
equilibrium phase even at room temperature.
The microstructures of as-sintered samples
depended to a great extent on the composition of the
experimental steel as projected in Figs. 4A-4C. A
completely fine, interlocked homogeneous
microstructure was obtained at compositions C-2 and
C-4 while the worst microstructure was obtained for
alloy compositions C-1, hence a dendritic and
polygonal microstructures identified the as-sintered
samples. The main differences in microstructures
revealed the absence of segregation at grain
boundaries and the increasing homogeneity in case of
C-4 steel composition. At higher Aluminum content
(1.5%) as in compositions C-1, C-2, C-3 it was
observed the presence of hard clusters of Al2O3
dominated the steel matrix field, while at levels of 0.5%
Al together with 0.5% Ti the matrix was clean from
Alumina particulates. After forging operation, the
structure of as-sintered samples was completely lost,
however, it was observed that re-crystallization of the
slightly banded structure occurred on forging from
high temperatures (1,000-900 °C) as shown in Fig. 4A.
At low forging temperature (800 °C) the aged
microstructures of steel C-4 and C-3 were found to be
the optimal ones hence massive precipitation of
Ni3Mo along with Ni3Ti and Ni3Al took place as in
Fig. 5. Deformation of the sintered steel samples to
60% reduction in thickness altered the microstructure
of all steels to denser, homogenous and grain refined
martensitic-austenite phases which enriched the
formation of intermetallic massive precipitation
during aging process as shown in Fig. 5.
It was observed also that solution treatment of
such steels from 1,100 °C caused coarsening of the
prior austenite and lath martensite phases, however
forging at 800 °C refined the lath martensite
structures.
Fig. 4 Micro
Fig. 5 Appe
The opti
temperature
martensite
crystallograp
differences
density of
interactions
content of n
martensite
mainly she
depending o
Aging of
about 500
precipitating
Ni3Mo
Influence of of Inn
ostructures of
C-4
earance of prec
mal structur
(900 °C) s
(Fe-Ni m
phic structure
as the pres
dislocations
with interm
nickel. Mean
formed by
eared BCT
on the carbon
such carbon
°C introduc
g intermetalli
Ni3A
Compositionnovative-Carb
(A) forged (A), for
cipitations and
res were ob
solution treat
martensite) h
e as body cen
sence of tre
s, fine twin
metallics dep
nwhile, the cr
quenching F
(body cent
content.
-free martens
ed more ha
ic compounds
20,000
Al,Ni3Ti
n and Aging Hbon Free 10%
rged-aged (B)
d lath martensi
btained for
ted. The for
has the s
ntered cubic w
emendously h
ns and com
pending on
rystallograph
Fe-C alloys
ered tetrago
sitic structure
ardening thro
s inside the B
LathRetain
Heat Treatme% Cobalt-Mara
(B)and as-sintere
C-4
ite in forged-ag
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rmed
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T
reve
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reac
h martensite (1ned Austenite
ent on the Micaging Steel P
d aged (C), exp
ged C-4 and M
trix. TEM of
ticulates that
-Fe)3Mo, Ni3
Ti, Al (N) as
nd in any
balt contribu
reasing the
ermetallics wi
ordering in th
The fine har
ealed the tr
formation on
locations an
ctions.
110), (111)
50,000
crostructure owder Comp
perimental C-3
M (250) maragin
f foils showed
had a diffrac
3Ti, Ni3(Ti-A
s projected i
intermetallic
uted to solid
activity of
ith Nickel du
he matrix as w
rd structures
emendous in
the as-sinte
nd sites for
Retained austenite
and Strengthposites
(C) 3 and C-4 steel
M250
ng steels.
d a host of s
ction pattern
Al), Ni3Al an
n Fig. 5. Co
combinatio
d solution ha
f Molybdenu
uring aging an
well.
of steels C
nfluence of
ered samples
r massive
Ni3
h 275
ls, 100.
mall discrete
for (Ni3Mo),
nd dispersion
obalt was not
ns, however
ardening and
um to form
nd increasing
C-3 and C-4
the induced
by creating
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20,000
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e
,
n
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4
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g
c
Influence of Composition and Aging Heat Treatment on the Microstructure and Strength of Innovative-Carbon Free 10% Cobalt-Maraging Steel Powder Composites
276
Two main types of phase transformation in
maraging steels were observed, precipitation and
austenite reversion. Before either of these
transformations happens in these steels (during
heating), there is a third transformation, which is the
martensitic transformation (during cooling).
Precipitation and austenite reversion occur in this
martensite matrix, the former generally desirable as
long as it is not too complete and the latter usually
undesirable. In simple terms, precipitation leads to
hardening and austenite reversion leads to softening.
Although martensitic transformation is a prerequisite
of the functioning of maraging steels, it is easily
achievable and its details do not strongly determine
the final steel properties, at least to a far lesser extent
than precipitation and austenite reversion. Therefore,
the austenite transformation ends at around 720 °C [8].
Complete solution is ensured by heating continuously
to 900 °C and holding at this temperature for 30 min.
During cooling to room temperature, there is drastic
expansion at approximately 135 °C, due to the sudden
start of rapid transformation from austenite to
martensite.
4. Mechanical Properties
4.1 Hardness of Experimental Maraging Steels
Bulk hardness of all the specimens were measured
using IUHTM (identic universal hardness testing
machine), where all the forged hardened samples of
different compositions showed hardness in the range
40-48 HRC as shown in Fig. 6. The maximum
hardness value of about 50 HRC was measured for
solution treated and aged C-4 maraging steel, while
the worst hardness was 40 HRC for C-1 maraging
steel. The hardness of as-sintered aged samples was
found to be in the range 42-43 HRC due to moderate
intermetallic precipitation rate caused by the absence
of activation and nucleation sites created by
deformation. Hardness of composition C-4 is more
than that for standard M (250) produced
conventionally with about 10 HRC due to the more
dens, fine lath martensite and massive precipitation of
intermetallic compounds.
Maraging steel compositions
C-1 C-2 C-3 C-4
Ult
imat
e te
nsile
str
engt
h, M
pa
500
1000
1500
2000
2500
3000
800
65
55
45
40
50
Har
dnes
s, H
Rc
M250
M250
M250
32 HRcAs Sintered
60
U.Tensile Strength
HRc
Fig. 6 Variations of ultimate tensile strength and hardness with composition of experimental maraging steels compared with wrought M (250).
Influence of Composition and Aging Heat Treatment on the Microstructure and Strength of Innovative-Carbon Free 10% Cobalt-Maraging Steel Powder Composites
277
4.2 Tensile Strength of Experimental Maraging Steels
Tensile test coupons of the experimental maraging
steels in their solution treated and aged conditions
were applied to standard tensile testing using
mini-sample sizes. The results of testing steels C-1 to
C-4 were compared with those for the standard known
M (250) maraging steel as shown in Fig. 6. Tensile
values in the range 800-2,750 MPa were detected for
steels C-1 to C-4 comparing with about 1,800-2,100
MPa for M (250). The tensile strength of as-sintered
aged test samples showed only moderate values
between 890-1,000 MPa due to the depletion of
precipitates inside the matrix and in some cases due to
the bad effect of Al2O3 non-metallic inclusions at high
aluminum levels. The variations in tensile strength for
the maraging samples depend on the cleanness of the
matrix and on the ordering inside the matrix itself.
The counteraction of using aluminum as intermetallic
former with nickel and its amount forming Al2O3
inclusions must be adjusted.
4.3 Fracture Toughness Measurements
The maraging steel alloys under investigation have
superior ductility despite their high volume fraction of
intermetallic hard precipitations like Ni3Mo, Ni3Ti and
Ni3TiAl. Therefore from fracture mechanics point of
view the most straight forward parameter to
characterize fracture toughness is the critical stress
intensity factor (K) or dynamic fracture parameter
(Kid). A fracture toughness Kic (Pa.
measurements was made at room temperature using
the well known fracture standard test, meanwhile
Charpy toughness (CVN) test was also used .The
obtained results are projected in Fig. 7. Best values of
fracture toughness 72, 76, and 80 MPa.√ were
measured for steels C-2, C-3 and C-4 respectively,
while the fracture toughness values for standard M
(250-300) are 67-71 MPa. √ . In other toughness
measures, values of about 35-42 J were obtained
for C-2, C-3 and C-4 experimental steels while
only 28-32 J were reported for M (250) maraging steel
Maraging steel compositions
0 C-1 C-2 C-3 C-4
Fra
ctur
e to
ughn
ess
(Kic
), M
pa.m
-1/2
45
50
55
60
65
70
75
80
85
M250
45
35
25
20
30
40
55
50
60
CV
N, J
M (250, 300)
M (250, 300)
As Sintered
Fracture Toughness
CVN, J
Fig. 7 Fracture toughness values for experimental steels compared with M (250).
278
Fig. 8 Fract
category. T
from lack
measured in
superior tou
steels, and t
for the as-sin
of deforme
contained ab
some appare
8B.
5. Conclus
(1) Fine
obtained at
deformation
worst mic
composition
to the prese
the absence
(2) Tensil
maximum h
measured fo
C-4 after sol
(3) Fractu
was measur
toughness ap
appearance
facets.
Influence of of Inn
(A
ture appearanc
The as-sinter
of toughnes
n average for
ughness was
the worst on
ntered sampl
d aged stee
bout 85-90%
ent precipitat
sions
e and inter
steel compos
n and then sol
rostructure
ns C-1, C-2 in
ence of Al2O
of Mo and Ti
le values of
hardness valu
or deformed
lution treatme
ure toughnes
red for comp
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of fracture sh
Compositionnovative-Carb
A)
ce for experim
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ss and only
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measured f
ne was measu
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el C-4, the
% ductile face
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was obtain
n as sintered
O3 nonmetalli
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about 2,500-
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experimenta
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ss of about
positions C-
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howed high c
n and Aging Hbon Free 10%
mental steel C-4
samples suff
y 20-27 J w
In all cases,
for C-3 and
ured for C-1
of the hard ma
fracture sur
ets, Fig. 8A w
as shown in
crostructure
nd C-4 after 6
d, aged, while
ned for a
or deformed
ic inclusions
ell.
-2,750 MPa,
47-50 HRC w
al steels C-3
g.
75-80 MP
3 and C-4 w
the same time
content of du
DuctPrecipi
Heat Treatme% Cobalt-Mara
4.
fered
were
, the
C-4
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atrix
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Fig.
was
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e the
alloy
due
and
and
were
and
√
with
e the
uctile
(4
as-s
due
prec
mar
Re
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
tile Fracture Fitates Ni3Mo, N
ent on the Micaging Steel P
4) In all c
sintered, aged
e to the absen
cipitation o
rtensite matri
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