Note: Descriptions are shown in the official language in which they were submitted.
l *****
In applications for tool s~eels, such as in the
manufacture o~ Llot working rolls and tooling used in rolling hot
metal, the tooling is subjected to condition~ of extreme wear as a
result of contact with the workpiece, thermal shoc~, as a result
5 of being subjected to high temperatures when in contact with the
hot workpiece and then rapid cooling when ou~ of contact with the
wor~piece, and high compressive stresses, as a result of the roll
separating ~orces encountered during rolling. In view of these
service conditions it is desirable that tool steels from which
10 wor~rolls and other similar hot work tooling are made be
charac~erized by good wear resistance, toughness, strength and
resistance to thermal fatigue and shock. In tool steels o~ this
type it is known to provide vanadium and sufficient carbon to
combine therewith to produce vanadium carbides, which impart wear
15 resistance to the alloy. For purposes of strength it is typical
to provide excess carbon over that necessary to combine with
vanadium so tilat there is carbon in the alloy matrix to contribute
significantly to the strength. It is generally believed bv those
skilled in the art that carbon may be stoichiometrically balanced
20 with vanadium to produce vanadium carbides by having 0. 270 carbon
for each 1% vanadium present-. Although alloys of this ~ype
provide good strength and wear resistance they have been deficient
in ~ertain applications such as for the manufacture of hot work
rolls and tooling in that they tend to crack due to thermal fatigu~
25 and shock when subjected to drastic temperature cyclPs during use.
It is accordingly a primary object of the present invention
to provide a powder metallurgy tool steel article having in
combination wear resistance, toughness, strength and resistance to
thermal fatigue and shock, and thus particularly adapted for hot
30 working applications.
~.
In accordance with the invention the composition limits
for the alloy would be as follows, in weight percen~;
Manganese 0.2 to 1.5
Silicon 2 max.
Chromium 1.5 to 6
Molybdenum 0.50 to 6
Sulfur 0.30 max.
Vanadium 7 to iO
Carbon .25 min., .40 max. + .16 x %
v nadium
Tungsten, Columbium, Up to 5
Cobalt
Iron Balance and incidental elements
and impurities characteristic of
steelmaking practice
The alloy would be processed by powder metallurgy
techniques in the conventional manner and would be characterized
by a fully martensitic structure with essentially no carbon in
the steel matrix in excess of carbon necessary to combine with
vanadium present to form vanadium carbides and to insure a fully
mar~ensitic structure. After quenching from austenitizing
temperature tne hardness may be at least 50 Rc for hot working
applications; lower hardnesses may be provided for cold wor~
tooling requirements. At the relatively high vanadium carbide
content of tne alloy powder metallurgy processing is required to
ensure a fine, even carbide distrlbution necessary for toughness
and grindability.
The term "powder metallurgy article" as used herein is
used to designate a compacted prealloyed particle charge that has
been formed by a combination of heat and pressure to a coheren~
mass having a density in final form, in excess of 99% of
theoretical density; this includes intermediate products, such as
~ 3~
billets, blooms, rod and bar and the like, as well as final
products, sucn as tool steel articles including rollq, punches,
dies, wear plates, slitter knives, shear blades and the like~
which ar~icles may be fabricated from intermediate product forms
from the initial prealloy particle charge. The particle charge
may be produced by conventional gas atomization.
Tne term '~C-type vanadium carbides" as used herein
refers to the carbide characterized by the face-centered cubic
crystal structure with M representing the carbide-forming element
essentially vanadium; this also includes M4C3-type vanadium
car~ides and includes the partial replacement of vanadium by other
carbide forming elements such as iron, molybdenum, chromium and of
carbon by nitrogen to encompass what are termed carbonitrides.
Although the powder me~allurgy article of this invention is defined
herein as containing all MC-type and M4C3 vanadium-carbides, it is
understood that other types of carbides, such as M6C, M2C, and
M23C6 carbldes, may also be present in minor amounts, but are not
significant from the standpoint of achieving the objects of the
invention specifically from the standpoint of wear resistance.
To determine the optimum composition for the alloy of
this invention, experimental compositions were prepared by powder
metallurgical technology and microstructural studies were
conducted on heat treated specimens to determine the compositional
balance with respect to van~dium and carbon which is requir.ed to
develop a fully martensitic structure. A summary of the relation
between the microstructural observations and compositions is
shown in lable 1.
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~9~
1 TABLE I
hemical Composition (Wt. ~O)
Steel C Mn Si _r Mo V Heat Treated Microstructure
Alloy 1 2.46 .50 0.95 5.00 1.33 9O75 Fully Martensitic
~lloy 2 2.00 .67 lo 39 4.82 1.36 10.23 " "
Alloy 3 1.92 .49 1.14 4.89 1.34 9.78 " "
Alloy 4 1 r 80 .49 1,14 4.89 1.34 9O78 Minor amounts of ferrite
otherwise martensite
Alloy 5 1.60 .49 1.14 4.89 1.34 9.78 Notable amounts of ferrite
remainder martensite
CPM 9V* 1.78 .49 0.81 5.33 1.20 8.80 Fully Martensite
*invention alloy
~ study of these results shows that the 1.78 carbon-
8.80 vanadium alloy is the leanest composition which develops
the fully martensitic structure desired in our invention. At
least about 0.25% carbon is required in the matrix to develop a
fully martensitic structure with the remainder present in the
form of MC or M4C3 carbides and also that a matrix carbon con-
tent of over about 0.40% may be detrimental to toughness. To
further assess the effect of the same compositional variables
on a key property for the alloy of this invention, C-notch impact
tests were conducted on specimens heat treated to the HRC ~8 to
50 hardness range. The results in Table II show that a signi-
ficant toughness advantage was presented by the 1.78 carbon -
8.80 vanadium alloy (CPM 9V) of the invention. Specifically,
the 1.78 carbon - 8.80 vanadium alloy in accordance with
the invention exhibited a C-notch impact strength value
(ft-lbs) of 74 at an HRC of 49.5 which demonstrates a drastic
improvement in toughness at a hardness level comparable to the
ha~dnesses of the conventional alloys set forth in Table II.
~ 3
TABLE II
CHARPY C-NOTCH IMPACT TOUGHNESS VALUES
Hardness C-Notch Impact
C~ V HRC Strength (ft-lbs)
2.46 9.75 48 29
2.00 10.23 48 23.5
1.92 9.78 49.5 29.5
1.80 9.78 48.5 26
. 1.7& ~.80 49.5 74
The results of the Charpy C-notch impact tests are
shown in Tabie III for the CPM 9V alloy of the invention a~
various heat treatments and hardnesses.
TABLE lII
CHARPY C-NOTCH ~PACT TEST RESULTS FOR CPM 9V
Impact
Heat Treatment ~.RC Temp.(F) Strength (ft-lbs)
52050F~10 min, AC, 1025F/2+2 hr. 56 RT 26
~2050F/30 min, AC, 1050F/2+2 hr. 53 RT 54
2050F/30 min, AC, 1125F/2+2 hr. 49 RT 53
1950F/1 hr,AC, 1100F/2+2 hr. 49.5 RT 74
" 300 81
10 ~ " " 500 94
" " 800 82
1950FIl hr, AC, 1140F/2+2 hr. .46.5 RT 61
" " 300 78
- " " 500 ~4
- " " 800 ~0
1850F/1 hr, AC, 1090F/2+2 hr. 4~.0 RT 75
" " 300 87
" " 500 103
" " 800 1~0
201850F/1 hr, AC, 1125~ 2+2 hr. 44.0 RT 70
300 7 G
" " 500 84
" " . 800 78
For comparison purposes Table IV shows the C-notch
impact results, as well as hardness (Rockwell C scale), for a
conventional powder metallurgy produced tool steel with a nominal
composition, in weight percent, carbon 2.4, manganese .45, silicon
.89, chromium 5.25, vanadium 9.85, molybdenum 1.25 and balance
iron. ~ne distinguishing featurP between this composition and the
30 above-reported CPM 9V composition is that with this latter
conventional composition there is excess carbon present in the
_7_ .
~ 3~
matrix wnlch is intended for strengthening. As may be seen from
the comparison of the toughness values for the ma~erial in
accordance witn the invention presented in Table III at similar
heat treated conditions the material of the invention exhibits far
superior Charpy C-notch impact test values than the conventional
material, the ~est results of which are presented on Table IV
along witn comparison values for CPM 9~. At the relatively lower
vanadium content of the invention alloy compared to the vanadium
content of the conventional alloy, it has been determined, in
accordance with the invention, that if carbon is present in an
amount equaling 0.2% carbon for each 1% vanadium this can result
in carbon being present in the matrix in an amount in excess of
that necessary to ensure a fully martensi~ic structure and thus
toughness is impaired. Hence, in accordance with the invention
lS carbon = .25% minimum, .40% maximum ~ .16 x percent vanadium.
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As may be seen from Table V, which gives the hardness
values for the material in accordance with the invention its
hardness is comparable to that of the conventional hot work tool
materials after elevated temperature exposures slightly above the
expected ~aximum temperature range of application for ~he steel
article o~ this invention.
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1 As may be seen from the data presented in Tables I to V,
by controlling carbon at a level expressed by the formula
C = .25 min., .40 max. + .16 x ~V
one is able to achieve a significant improvement with respect to
toughness, as demonstrated by the Charpy C-notch impact test
results for the material of the invention without sacrificing
the required strength and hardness. In addition, by the presence
of vanadium and sufficient carbon to combine there~ith to pro-
duce vanadium carbides the material has excellent wear resistance.
Table VI compares, after heat treatment, the wear re-
sistance of the CPM 9V material of the invention which conven-
tional high alloy hot work tool steels of conventional cast and
wrou~ht production. As may be seen from Table VI the CPM 9V
material of the invention shows drastically improved wear re-
sistance over the AISI H13, AISI Hl9 and AISI H21 steels even
in instances wherein the hardenss of the CPM 9V material is sig-
nificantly lower than that of the conventional steels.
TABLE VI
WEAR RESISTANCE OF HOT WORK STEELS
Hardness Wear Resistance
Grade Heat Treatment (HRC) (xlol psi)
-
CPM 9V 2150F/1 hr, AC, 1025F/2-~2 hr 56 71
CPM gV 2050F/1 hr, AC, 1050F/2+2 hr 53 61
CPM 9V 1850F/1 hr, AC, 1095F/2+2 hr 48 22
CPM 9V 1850F/1 hr, AC, 1125F/2+2 hr 45 21
AISI H13 1850F/1 hr, AC, 1050F/2+2 hr 52 3.6
AISI Hl9 2150F/1 hr, AC, 1025F/2+2 hr 56 3.7
AISI H21 2150F/1 hr, AC, 1025F/2+2 hr 56.5 2.1
For evaluation of wear resistance, the cross-cylinder
wear test was used. In this test, a cylindrical specimen (5/8 in.
diameter) of the respective cold-work or warm-work tool material
- ]2 -
and a cylindrical specimen (1/2 in. diameter) of tungsten carbide
(with 6% cobalt binder) are positioned perpendicularly to one
another. A fifteen-pound load is applied through weight on a leve~
arm. Then the tungsten carbide cylinder specimen is rotated at a
speed of 667 revolutions per minute. No lubrication is applied.
As the test progresses, a wear spot develops on the specimen of the
tool material. From time to time, the extent of wear is determined
by measuring the depth of the wear spot on the specimen and
converting it into wear volume by aid of a relationship
specifi~ally derived for this purpose. The wear resistance, or the
reciprocal of the wear rate, is then computed according to the
following formula:
Wear resistance = - 1 = L~s = L~ d~N
wear rate ~ v ~ v
where
v = the wear volume, (in.3)
L = the applied load, (lb.)
s = the sliding distance, (in.)
d = the diameter of the tungsten carbide cylinder, (in.)
and
-1~ = the number of revolutions made by the tungsten
carbide cylinder, (rpm)
This test has provided excellent correlations with wear
situations encountered in prac~ice.
The thermal fatigue properties of the steel of the
invention when compared with conventional powder metallurgy produced
cold wor~ tool steels and conventional cast and wrought steels of
this type are shown in Table VII; in this Table, the steel of the
invention, CPM 9V, is compared with a conventional powder
30 metallurgy produced tool steel containing 2.46% carbon and 9.75%
-13-
vanadium and a conventional cast and wrought steel of this type,
which is identified as AISI H13.
TABLE VII
RESIST~CE T0 THE~*~L FATIGUE
. . _ .
Thermal Fatigue
Grade Cycles ~o Failure
AISI H13 27,000
.46C - 9.75V 4,500
CPl~i 9V Greater than 60,000
- As may be seen from Table VII the thermal fatigue
resistance of the CPM 9V material of the invention is drastically
greater than that of both of the other conventional steels tested,
including the 2.46 carbon - 9.75 vanadium material which is a
powder metallurgy produced steel designed for cold or warm work
Itooling.
'ihe thermal fatigue test involves the use of an
electrically heated lead pot, a hot water quenching bath and a
solenoid valve operated, pneumatic-operated mechanical transfer
for transferring the specimens between the lead pot and the bath.
Specimens are transferred into the lead bath for a 4-second
heating period: They are then transferred quickly to a position
abovP the water bath wherein they are quenched for 2 s~conds at a
water bath temprature of 180F. This cycle is repeated at a
rate of 3 cycles per minute. Each specimen during each cycle is
dried above the lead pot for a period of 5 seconds. Including
transfer time each cycle takes approximately 20 seconds. ~uring
each cycle dif-ferential heating occurs in the rim and hub of each
specimen and nence from the thermal expansion, the rim periphery
is mechanically strained to set up compressive stresses in this
region. ~pon quenching the reverse of the phenomenon takes place.
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During this por~ion of the cycle, the hub opposes the thermal
con~raction of the rim causing residual (peripheral) tensile
stresses to be set up. Typically, fatigue is demonstrated by the
beginning of cracks in the rim periphery of ~he samples which
propagate toward the hub with the rate of cracking being
determined by the thermal fatigue resistance of the steel being
tested.
With reference to the tou~hness and wear resistance
advantages offered by CPM 9V over adominant cold work ~ool steel
grade namely AISI D2, Table VIII shows the Charpy C-notch impact
and the wear resistance comparisons of these steels.
! TABLE VIII
CHARPY C-NOTCH IMPACT STRENGTH OF WEAR RESISTANCE C02~PARISONS
` OF CPM 9V AND AISI D2
L5 Hardness C-Notch Impact Wear Resistance
Steel HRC Strength (ft-lb) (xlol0 psi)
CPM 9V 53 54 61
CPM 9V 49 53 30
AISI D2 59 21 4
~O AISI D2 56 19
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