Cobalt and the Toughness of Steel

Abstract

In developing compositions for new ultra-high strength steels it is important to understand the effects of alloying additions on the microstructure, strength and fracture resistance. Cobalt has been widely used in the development of high toughness ultra-high strength steels and the effects of cobalt on the strength of steels has been studied extensively but there is remarkably little known about how cobalt influences toughness. In this article are reviewed the effects of cobalt on the toughness of steel. The literature suggests that cobalt in solid solution will act to raise the ductile-to-brittle transition temperature but that it can act to increase the upper-shelf toughness of steel.

Full text

Cobalt and the Toughness of Steel
Warren M. Garrison, Jr.
Department of Materials Science and Engineering
Carnegie Mellon University, Pittsburgh PA 15213, USA
E-mail: wmg@andrew.cmu.edu
Key words: Steel, cobalt, fracture, toughness, DBTT
Abstract. In developing compositions for new ultra-high strength steels it is important to
understand the effects of alloying additions on the microstructure, strength and fracture
resistance. Cobalt has been widely used in the development of high toughness ultra-high
strength steels and the effects of cobalt on the strength of steels has been studied extensively but
there is remarkably little known about how cobalt influences toughness. In this article are
reviewed the effects of cobalt on the toughness of steel. The literature suggests that cobalt in
solid solution will act to raise the ductile-to-brittle transition temperature but that it can act to
increase the upper-shelf toughness of steel.
Introduction
The purpose of this paper is to discuss the effects of cobalt on the toughness of ultra-high
strength steels. Cobalt is an addition to two types of ultra-high strength steels associated with
high room temperature fracture toughness. One type are the conventional maraging steels,
which contain about eight wt. % cobalt. Their compositions are given in Table 1 and their
mechanical properties are given in Table 2 [1,2]. These steels are strengthened, in part, by the
precipitation of particles of Ni3Mo on tempering. Cobalt additions enhance strengthening due to
Ni3Mo precipitation [3]. The second are high toughness secondary hardening steels such as
HY180, AF1410 and AerMet® 100. The compositions of these steels are given in Table 3 and
their mechanical properties are given in Table 4 [4-6]. The mechanical properties given in Table
4 are all from heats which have been modified with rare earth additions. These steels contain 8
to 14 wt. % cobalt. Cobalt enhances strengthening by the precipitation of M2C on tempering,
where M is Cr and Mo [4]. These two types of ultra-high strength steels have high room
temperature fracture toughness but there are other steels also containing large amounts of
cobalt, which can have very low room temperature toughness, as will be discussed.
In this paper, unless otherwise stated, the measure of fracture resistance used to determine
the ductile-to-brittle transition temperature (DBTT) is the Charpy impact test. Here the DBTT is
the test temperature at which the toughness or ductility is the average of the toughness or
ductility measured at low temperatures where the fracture is cleavage and the toughness or
ductility measured at high temperatures where the fracture is ductile.
Effect of alloying additions and fine-scale microstructure on the DBTT
Alloying additions and the fine-scale microstructure of a steel can influence the DBTT. One of
the earliest papers on such effects is that of Hodge and Manning, who examined the effects of
nickel and grain size on the DBTT of ferritic steels [7]. They showed refining the grain size

lowered the DBTT and increasing the nickel from 0.03 wt. % to 3.6 wt. % lowered the DBTT
by about 33°C at all grain sizes. Rinebolt and Harris examined the effects of alloying additions
Table 1 Nominal compositions in wt. % of the conventional maraging steels
Alloy Ni Co Mo Al Ti
C200 18 8.5 3.0 0.1 0.2
C250 18 8.0 4.8 0.1 0.4
C300 18.5 9.0 4.8 0.1 0.65
Table 2 Typical mechanical properties of the conventional maraging steels
Alloy Y.S U.T.S. R.A. CV KIC
(MPa) (MPa) (%) (J) (MPa√m)
C200 1425 1500 50 50 150
C250 1740 1800 48 35 130
C300 1930 2000 40 21 115
Table 3 Nominal compositions in wt. % of HY180, AF1410 and AerMet® 100
Alloy C Ni Co Cr Mo
HY180 0.10 10 8 2 1
AF1410 0.16 10 14 2 1
AerMet 100 0.23 11.1 13.4 3.1 1.2
Table 4 Typical mechanical properties of HY180, AF1410 and AerMet® 100
Alloy Y.S U.T.S. R.A. CV KIC
(MPa) (MPa) (%) (J) (MPa√m)
HY180 1225 1350 79 200 300
AF1410 1500 1660 69 85 190
AerMet 100 1725 1965 65 40 125
on the DBTT of ferritic-pearlitic steels[8]. Their results indicate chromium, copper,
molybdenum and silicon raise the DBTT while manganese and nickel lowered the DBTT. In
the discussion to this paper they presented data showing that increasing the cobalt content from
0.04 wt. % to 1.55 wt. % lowered the DBTT by about 10°C. Jolley investigated the effects of
nickel additions of 3.28 wt. % and manganese additions of 1.0 and 1.8 wt. % on the DBTT of
ferritic steels[9]. At the low carbon level of 0.002 wt. % he found the manganese additions
slightly increased the DBTT but the nickel addition lowered the DBTT by about 70°C. He
concluded nickel lowered the DBTT through its effect on the slip behavior of the ferrite. Thus
nickel in solid solution lowers the DBTT but manganese in solid solution has no effect on the
DBTT. Floreen et al. examined the effect of nickel additions of 1.6 and 3.6 wt. % on the DBTT
of a ferritic steel containing about 0.01 wt. % carbon and 0.3 wt. % nickel using titanium to
getter the carbon as particles of Ti(C,N)[10]. The addition of 1.6 and 3.3 wt. % nickel lowered
the DBTT by about 20°C and 50°C, respectively. They concluded nickel lowered the DBTT
through the effect of nickel in solid solution on the slip characteristics of ferrite. Floreen found

increasing the nickel content of low carbon martensitic steel containing 9.3 wt. % chromium,
from 5.6 to 11.3 wt. % significantly lowered the DBTT[11]. Thus, it appears that nickel in solid
solution will lower the DBTT of ferritic and martensitic steels. Floreen and Hayden examined
the effects of iridium, rhodium, ruthenium and platinum on the DBTT of low carbon alloys and
found these elements also significantly lowered the DBTT[12].
Stoloff et al. considered the effects of 1, 5 and 10 wt. % cobalt on the DBTT of low carbon
ferrite, where the DBTT was determined using tensile ductility[13]. The additions of 1, 5 and 10
wt. % cobalt increased the DBTT by about 20°C, 55°C and 110°C, respectively. They
concluded that cobalt raised the DBTT because cobalt tends to restrict cell formation and cross
slip in a manner similar to that obtained by lowering the test temperature.
Squires et al.[14] and Squires and Wilson[15] discuss the effect of cobalt on the DBTT.
They considered two alloys. The first alloy had a composition in wt. % of
0.008C/5.85Ni/5.3Mn/2.45Mo. The second alloy was obtained by adding 8.8 wt. % cobalt to
this first alloy. Both alloys were dislocated martensite with very similar, and low, hardness. The
cobalt addition increased the DBTT by about 160°C. They concluded the increase in the DBTT
was due to the effect of cobalt on slip behavior as did Stoloff et al. [13]. Squires and Wilson
suggested the high room temperature toughness of the conventional maraging steels is probably
due to the high nickel content of these alloys compensating for the adverse effect of the cobalt
on the DBTT[15].
While nickel and cobalt in solid solution can influence the DBTT through their effects on
slip behavior, the DBTT of martensitic steels is also influenced by the fine-scale microstructure
of the steel. Considering lath martensite, microstructural features which could influence the
DBTT would include the packet size, the amount and mechanical stability of retained austenite,
the formation of interlath carbides during the decomposition of the retained austenite on
tempering, the introduction of reverted austenite on tempering and the nature of the precipitates
formed during tempering.
Refining the prior austenite grain size would lower the DBTT because refining prior
austenite grain size refines the packet size and the packet size is the effective grain size of lath
martensite[16]. In the as-quenched condition a martensitic steel will normally contain a few
volume % of retained austenite and this austenite has low mechanical stability. It is likely that
this mechanically unstable austenite adversely affects the DBTT. If the steel is then tempered at
a low temperature, on the order of 200°C, the mechanical stability of the retained austenite is
increased. It is suggested that this mechanically stable retained austenite acts to lower the
DBTT. Tempering steels containing carbon at higher temperatures can result in the
decomposition of the retained austenite leading to the precipitation of interlath carbides and a
reduction in the mechanical stability of the remaining retained austenite. It is felt that both the
interlath carbides and the remaining mechanically unstable retained austenite will act to raise
the DBTT. An important method of lowering the DBTT is the introduction of reverted austenite
during tempering. The low DBTT of the 9 nickel steels developed for cryogenic service is due
to both their high nickel content and the introduction of reverted or precipitated austenite on
tempering[17,18]. Morris and his students have discussed why reverted austenite lowers the
DBTT[19,20].
High cobalt ultra-high strength steels of low toughness
Considered first are ultra-high strength steels which contain large amounts of cobalt and which
do not contain large amounts of nickel. Such steels can have very low toughness at room
temperature. One such a steel is AFC77 developed in the early 1960’s which has a composition,

in wt. %, of 0.15C/14.5Cr/13.5Co/5Mo/0.5V. This steel is austenitized, quenched and then
tempered at temperatures on the order of 500°C. This steel is strengthened by the precipitation
of alloy carbides and of an intermetallic called R-phase. The cobalt in the steel enhances the
secondary hardening and is required for the formation of the R-phase. When this steel is
tempered in the range of 425°C to 650°C its room temperature fracture toughness varies from
about 40 MPa√m to less than 30 MPa√m[21]. This suggests the fracture mode for these
fracture toughness tests is not ductile and that if a DBTT were to be measured for this steel
using Charpy impact specimens its DBTT would be well above room temperature.
Holloway and Hopkins [22] considered the effects of cobalt additions of 1, 2, 5, 7 and 10 wt.
% on the strength and toughness of a base secondary hardening steel of a composition in wt. %
of 0.25C/3.3Ni/3.3Mo/0.56Cr/0.69Mn/0.25Si. They determined the strength and fracture
toughness of these alloys for the as-quenched condition and after tempering at 200°C, 400°C,
500°C, 550°C and 600°C. Considering the tempering temperature of 550°C, they found that the
strength increased as the cobalt was increased but the fracture toughness decreased rapidly with
increasing cobalt. When the alloy contained no cobalt the fracture toughness was about 61
MPa√m but increasing the cobalt to 10 wt. % decreased the fracture toughness to 20 MPa√m.
Again these results suggest that at high cobalt levels the DBTT as measured by Charpy impact
energies would be well above room temperature.
A third example of the detrimental effect of cobalt on toughness comes from a study of the
effects of 2 wt. % silicon, 2 wt. % aluminum, 4 wt. % nickel, 4 wt. % cobalt and 8 wt. % cobalt
on the tempering response and Charpy impact energy of a base secondary hardening steel of a
composition in wt. % of 0.38C/4.5Cr/2.0Mo/0.5W0.5V[23]. Cobalt additions result in a
substantial decrease in toughness compared to the other alloying additions. After tempering at
550°C the Base+4Ni and Base+4Co steels have almost the same hardness (RC≈54) but the
Charpy impact energy of the Base+4Ni steel was 32.3 J while that of the Base+4Co steel was
13.6 J. Also, the additions of 2 wt. % silicon, 2 wt. % aluminum and 8 wt. % cobalt enhance
the secondary hardening to the same degree and result in a hardness after tempering at 550°C of
about 57 RC. After tempering at 550°C the Charpy impact energy of the Base+8Co steel is 6.1 J,
much less than the Charpy impact energies of the Base+2Si and Base+2Al steels, which were
14.9 and 16.3 J, respectively. These results suggest cobalt additions result in higher ductile-to-
brittle transition temperatures than the other steels when steels of similar hardness are
compared.
These examples are consistent with the hypothesis that large cobalt additions in the absence
of large additions of nickel will result in low toughness at room temperature because cobalt
increases the DBTT. However, the situation is more complicated. The three examples all
consider toughness when the steels are tempered at high temperatures, on the order of 550°C.
All of these three steels contain carbon and large amounts of chromium so the retained austenite
in these steels will start decomposing on tempering at about 500°C[24]. Thus on tempering
these steels at 550°C they will contain interlath carbides and mechanically unstable retained
austenite and both of these features are believed to promote cleavage fracture and a higher
DBTT. If these steels had contained mechanically stable retained austenite and no interlath
carbides the high cobalt contents should still favor a higher DBTT but the mechanically stable
retained austenite might be sufficient to lower the DBTT below room temperature so that the
room temperature toughness levels would be relatively high and the room temperature
toughness levels would give no indication of an embrittling effect of cobalt additions. Thus one
could reduce the increase in the DBTT due to cobalt additions by ensuring the microstructure
contained a few volume % of mechanically stable retained austenite.

This suggestion is consistent with the behaviors of AFC77 and the steels considered by
Holloway and Hopkins when the toughness on tempering at 200°C is considered. After
tempering at 200°C the strength of AFC77 is less than after tempering at 550°C, but the fracture
toughness is quite high, about 95 MPa√m[21]. After tempering at 200°C the compositions
considered by Holloway and Hopkins all have reasonably high toughness. On tempering at
200°C the ultimate tensile strength increases with increasing cobalt content and the fracture
toughness also increases with increasing cobalt content[22]. The fracture toughness of the alloy
containing no cobalt is about 69 MPa√m and this increases to about 78 MPa√m when the cobalt
is increased to 10 wt. %. The results of Holloway and Hopkins suggest that if there is
mechanically stable retained austenite in the microstructure then large cobalt additions will not
necessarily result in a high DBTT, or at least a DBTT at or above room temperature. These
results also suggest cobalt additions might increase the fracture toughness when fracture is
ductile.
The following is another example of how introducing mechanically stable austenite can
reduce the undesirable effect of cobalt on the DBTT[25]. In this study of precipitation
strengthened martensitic stainless steels two sets of alloys were considered. The first set of
alloys was made to investigate the effect of cobalt on strength and cobalt additions of 9, 12, 15,
18 and 21 wt. % were made to a base steel containing, in wt. %, 0.005C/12Cr/5Mo/1.5Ni. The
yield strength which could be achieved increased rapidly with increasing cobalt content. The
yield and ultimate tensile strengths achieved for the alloy containing 18 wt. % cobalt after
tempering at 550°C were 1592 MPa and 1718 MPa, respectively. The Charpy impact energies
of these alloys were very low for all tempering temperatures for which there was substantial
precipitation strengthening and the fracture mode was a very brittle cleavage fracture. The
Charpy impact energy of this alloy after tempering at 550°C was 8 J. These alloys contained
no retained austenite or reverted austenite. The second part of this study was to develop similar
alloys of improved toughness. The approach taken was to alter the composition so that the
martensite start temperatures of these new alloys would be lower than for the alloys used in the
study of the effect of cobalt on strength. This was done to ensure these new alloys would
contain a few volume % retained austenite. One of these alloys had a composition in wt. % of
0.002C/14Cr/5Mo/1.5Ni/19.5Co. After tempering at 550°C this alloy had yield and ultimate
tensile strengths of 1666 MPa and 1866 MPa, respectively, and the Charpy impact energy was
32.9 J. This alloy contained 2.7 vol. % retained austenite and 3.5 vol. % reverted austenite after
tempering at 550°C. The improved toughness was attributed to the alloy containing a
reasonable amount of austenite, but the relative importance of the retained and reverted
austenite is not clear.
It is suggested the high cobalt contents of the maraging steels and the secondary hardening
steels of the AF1410 type act to raise the DBTT and that the high nickel contents of these alloys
act to counteract this effect of cobalt. It is interesting to note that despite their high nickel
contents the ductile-to-brittle transition temperatures as measured by Charpy impact energies
are about 50°C and 100°C for the C250 and C300 alloys, respectively[2]. While nickel in solid
solution can modify slip behavior in a way which compensates for the effect of cobalt on slip
which promotes cleavage fracture, high nickel also promotes the formation of reverted austenite
which should also help lower the DBTT. Both the maraging steels and the secondary hardening
steels AF1410 and AerMet® 100 contain reverted austenite after their usual heat treatments.
AerMet® 100 contains about 4 vol. % reverted austenite of an interlath morphology after its
usual heat treatment[6,26,27].

Effect of cobalt on upper shelf toughness
The results of Holloway and Hopkins suggest cobalt additions can increase the upper shelf
toughness of martensitic steels. More recent studies suggest cobalt additions can increase the
upper-shelf fracture toughness of ferritic and ferritic-pearlitic steels.
Srinivas et al. [28] considered the effects of additions of 0.5 and 3.5 wt. % silicon, 0.5 and 5
wt. % molybdenum, 0.5 and 5 wt. % nickel and of 0.5 and 5 wt. % cobalt on the room
temperature tensile properties, Charpy impact energy and fracture toughness as measured by JIC
of Armco iron. When the mechanical properties are compared at very similar grain sizes they
found that the addition of 0.5 wt. % cobalt lowered the yield strength by about 70 MPa, had
little effect on the ultimate tensile strength, reduced the room temperature Charpy impact energy
from 260 J to 210 J but increased JIC from 140 kJ/m2 to 162 kJ/m2. An addition of 5 wt. %
cobalt lowered the yield strength by about 50 MPa, had little effect on the ultimate tensile
strength, increased the room temperature Charpy impact energy from 260 J to 270 J and
increased JIC from 140 kJ/m2 to 187 kJ/m2. The fracture modes reported for the JIC tests for the
Armco iron and the Armco iron modified with cobalt additions were ductile. Thus the work
indicates that the cobalt additions can increase the upper-shelf toughness of ferritic iron. The
authors suggested that cobalt additions increased JIC because they increased the work hardening
exponent.
Srinivas et al. [29] examined the effects of additions of 5 wt. % cobalt and of 5 wt. % nickel
to 0.2 wt. % carbon steel on the room temperature tensile properties, Charpy impact energy and
fracture toughness as measured by JIC. The microstructures examined were ferritic-pearlitic and
the ferrite grain sizes and the volume fractions of pearlite were nearly the same for all three
alloys. The cobalt addition had no effect on yield strength but increased the ultimate tensile
strength by about 10 %. The nickel addition increased the yield strength by about 20 % and the
ultimate tensile strength by about 27 %. The room temperature Charpy impact energies of the
base steel and the steel containing cobalt were 16 J and 15 J, respectively while that of the
nickel modified steel was 120 J. JIC was 130 kJ/m2, 232 kJ/m2 and 153 kJ/m2 for the base steel
and the cobalt modified steel and the nickel modified steel, respectively. These results suggest
that nickel lowers the DBTT but that nickel increases the fracture toughness modestly while
cobalt increases the fracture toughness by almost 80 %. Thus these results also indicate that
cobalt can increase the upper-shelf toughness of steels.
Conclusions
The literature strongly suggests that cobalt in solid solution will act to raise the DBTT. This
adverse effect of cobalt can be minimized in several ways. First, one can add nickel to the steel,
as nickel in solid solution will act to lower the DBTT. Second, one can balance the composition
in such a way that the alloy contains mechanically stable retained austenite or the alloy contains
reverted austenite after heat treatment. It is suggested that the conventional maraging steels and
the secondary hardening steels such as AF1410 and AerMet 100 have high toughness at room
temperature because they contain large amounts of nickel and the nickel in solid solution will
compensate for the tendency of cobalt to raise the DBTT and the nickel will, as well, result in a
small volume % of reverted austenite after heat treatment. In addition, the literature suggests
that as long as the fracture is by micro-void coalescence cobalt additions can act to increase the
fracture toughness.

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