Note: Descriptions are shown in the official language in which they were submitted.
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CORROSION AND WEAR RESISTANT COLD WORK TOOL STEEL
TECHNICAL FIELD
The invention relates to corrosion and wear resistant cold work tool steel and
a method
of making the cold work steel and use of the cold work tool steel.
BACKGROUND OF THE INVENTION
Nitrogen alloyed martensitic tool steels have recently been introduced on the
market and
attained a considerable interest, because they combine a high wear resistance
with an
excellent corrosion resistance. These steels have a wide rang of applications
such as for
moulding of aggressive plastics, for knives and other components in food
processing
and for reducing corrosion induced contamination in the medical industry.
The steels are generally produced by powder metallurgy. The basic steel
composition is
firstly atomized and subsequently subjected to a nitrogenation treatment in
order to
introduce the desired amount of nitrogen into the powder. Thereafter the
powder is
filled into a capsule and subjected to hot isostatic pressing (HIP) in order
to produce an
isotropic steel.
The amount of carbon is generally reduced to a very low level as compared to
conventional tool steels. By substituting most of the carbon with nitrogen it
is possible
to substitute the chromium rich carbides of the type M7C3 and M23C6 with very
stable
hard particles of the type MN-nitrides.
Two important effects are achieved. Firstly, the relative soft and anisotropic
phase of
M7C3-carbide (z1700HV) is replaced by the very hard and stable phase of small
and
evenly distributed hard phase of the type MN (z2800HV). Thereby, the wear
resistance
is improved at the same volume fraction of hard phase. Secondly, the amount of
Cr, Mo
and N in solid solution at the hardening temperature is very much increased,
because
less chromium is bound in the hard phase and because the carbides of the type
11/123C6
and M7C3 do not have any solubility for nitrogen. Thereby, more chromium is
left in
solid solution and the thin passive chromium rich surface film is
strengthened, which
leads to an increased resistance to general corrosion and pitting corrosion.
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Hence, in order to obtain good corrosion properties the carbon content has
been limited
to less than 0.3 %C, preferably less than 0.1 %C in DE 42 31 695 Al and to <
0.12 % C
in WO 2005/054531 Al.
DISCLOSURE OF THE INVENTION
The general object of the present invention is to provide a powder metallurgy
(PM)
produced nitrogen alloyed cold work tools steel alloy having improved
properties, in
particular a good corrosion resistance in combination with a high hardness.
A particular object is to provide a nitrogen alloyed martensitic cold work
tools steel
alloy having improved corrosion resistance at a fixed chromium content.
A further object is to provide a method of producing said material.
The foregoing objects, as well as additional advantages are achieved to a
significant
measure by providing a cold work tool steel having a composition as set out in
the
present application.
DETAILED DESCRIPTION
The importance of the separate elements and their interaction with each other
as well as
the limitations of the chemical ingredients of the alloy
are briefly explained in
the following. All percentages for the chemical composition of the steel are
given in
weight % (wt. %) throughout the description.
Carbon (0.3 ¨ 0.8 %)
is to be present in a minimum content of 0.3%, preferably at least 0.35%. At
high
carbon contents carbides of the type M23C6 and M7C3 will be formed in the
steel. The
carbon content shall therefore not exceed 0.8%. The upper limit for carbon may
be set
to 0.7% or 0.6%. Preferably, the carbon content is limited to 0.5%. Preferred
ranges are
0.32 - 0.48%, 0.35 - 0.45%, 0.37 - 0.44% and 0.38 - 0.42%. In any case, the
amount of
carbon should be controlled such that the amount of carbides of the type M23C6
and
M7C3 in the steel is limited to 10 vol. %, preferably the steel is free from
said carbides.
011% %Oat&
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Nitrogen (1.0 ¨2.2%)
Contrary to carbon, nitrogen cannot be included in M7C3. The nitrogen content
should
therefore be much higher than the carbon content in order to avoid the
precipitation of
M7C3-carbides. In order to get the desired type and amount of hard phases the
nitrogen
content is balanced against the contents of the strong carbide formers, in
particular
vanadium. The nitrogen content is limited to 1.0¨ 2.2%, preferably 1.1 ¨ 1.8%
or 1.3 ¨
1.7%.
(C+N) (1.3 ¨ 2.2%)
The total amount of carbon and nitrogen is an essential feature of the present
invention.
The combined amount of (C + N) should be in the range of 1.3 ¨ 2.2%,
preferably 1.7 ¨
2.1% or 1.8- 2.0%.
C/N (0.17 - 0.50)
A proper balance of carbon and nitrogen is an essential feature of the present
invention.
By controlling the carbon and nitrogen contents the type and amounts of the
hard phases
can be controlled. In particular, the amount of the hexagonal phase M2X will
be reduced
after hardening. The C/N ratio should therefore be 0.17 ¨ 0.50. The lower
ratio may be
0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 024 or 0.25. The upper ratio may be 0.5,
0.48, 0.46,
0.45, 0.44, 0.42, 0.40, 0.38, 0.36 or 0.34. The upper limit may be freely
combined with
the lower limit. Preferred ranges are 0.20 ¨ 0.46 and 0. 22 ¨ 0.45.
Chromium (13 ¨ 30%)
When it is present in a dissolved amount of at least 11%, chromium results in
the
formation of a passive film on the steel surface. Chromium shall be present in
the steel
in an amount between 13 and 30 % in order to give the steel a good
hardenability and
oxidation and corrosion resistance. Preferably, Cr is present in an amount of
more than
16% in order to safeguard a good pitting corrosion resistance. The lower limit
is set in
accordance to the intended application and may be 17%, 18%, 19%, 20%, 21% or
22%.
However, Cr is a strong ferrite former and in order to avoid ferrite after
hardening the
amount need to be controlled. For practical reasons the upper limit may be
reduced to
26%, 24% or even 22%. Preferred ranges include 16 - 26%, 18 ¨ 24%, 19 ¨ 21%,
20 -
22% and 21 ¨ 23%.
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Molybdenum (0.5 ¨ 3.0%)
Mo is known to have a very favourable effect on the hardenability. It is also
known to
improve the pitting corrosion resistance. The minimum content is 0.5%, and may
be set
to 0.6%, 0.7%, 0.8% or 1.0%. Molybdenum is a strong carbide forming element
and
also a strong ferrite former. The maximum content of molybdenum is therefore
3.0%.
Preferably Mo is limited to 2.0 %, 1.7% or even 1.5%.
Tungsten (< 1%)
In principle, molybdenum may be replaced by twice as much tungsten. However,
tungsten is expensive and it also complicates the handling of scrap metal. The
maximum amount is therefore limited to 1%, preferably 0.2% and most preferably
no
additions are made.
.. Vanadium (2.0 ¨ 5.0%)
Vanadium forms evenly distributed primary precipitated nitrocarbides of the
type
M(N,C) in the matrix of the steel. In the present steels M is mainly vanadium
but
significant amounts of Cr and Mo may be present. Vanadium shall therefore be
present
in an amount of 2-5. The upper limit may be set to 4.8%, 4.6%, 4.4%, 4.2% or
4.0%.
The lower limit may be 2.2%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8% and 2.9%. The
upper and lower limits may be freely combined within the limits set out in the
present
application. Preferred ranges includes 2 ¨ 4%.
Niobium (< 2.0%)
Niobium is similar to vanadium in that it forms nitrocarbides of the type
M(N,C) and
may in principle be used to replace vanadium but that requires the double
amount of
niobium as compared to vanadium. Hence, the maximum addition of Nb is 2.0%.
The
combined amount of (V + Nb/2) should be 2.0 ¨ 5.0%. However, Nb results in a
more
angular shape of the M(N,C). The preferred maximum amount is therefore 0.5%.
Preferably, no niobium is added.
Silicon (< 1.0%)
Silicon is used for deoxidation. Si is present in the steel in a dissolved
form. Si is a
strong ferrite former and should therefore be limited to < 1.0%.
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Manganese (0.2 ¨ 2.0%)
Manganese contributes to improving the hardenability of the steel and together
with
sulphur manganese contributes to improving the machinability by forming
manganese
5 sulphides. Manganese shall therefore be present in a minimum content of
0.2%,
preferably at least 0.3%. At higher sulphur contents, manganese prevents red
brittleness
in the steel. The steel shall contain max.2.0%, preferably max. 1.0 %Mn.
Preferred
ranges are 0.2 ¨ 0.5%, 0.2 ¨ 0.4%, 0.3 ¨ 0.5% and 0.3 - 0.4%.
Nickel ( < 5.0%)
Nickel is optional and may be present in an amount of up to 5%. It gives the
steel a
good hardenability and toughness. Because of the expense, the nickel content
of the
steel should be limited as far as possible. Accordingly, the Ni content is
limited to 1%,
preferably 0.25%.
Copper 3.0%)
Cu is an optional element, which may contribute to increasing the hardness and
the
corrosion resistance of the steel. If used, a preferred range is 0.02 ¨ 2% and
a most
preferred range is 0.04 - 1.6%. However, it is not possible to extract copper
from the
steel once it has been added. This drastically makes the scrap handling more
difficult.
For this reason, copper is normally not deliberately added.
Cobalt (.< 10.0%)
Co is an optional element. It contributes to increase the hardness of the
martensite. The
maximum amount is 10 % and, if added, an effective amount is about 4 to 6%.
However, for practical reasons such as scrap handling there is no deliberate
addition of
Co. A preferred maximum content is 0.2%.
Sulphur 0.5%)
S contributes to improving the machinability of the steel. At higher sulphur
contents
there is a risk for red brittleness. Moreover, a high sulphur content may have
a negative
effect on the fatigue properties of the steel. The steel shall therefore
contain < 0.5 %,
preferably < 0.035%.
Be, Bi, Se, Mg and REM (Rare Earth Metals)
'I'hese elements may be added to the steel in the amounts in order to
further
improve the machinability, hot workability and/or weldability.
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Boron 0.01%)
B may be used in order to further increase the hardness of the steel. The
amount is limited to
0.01%, preferably <0.004%.
Zr, Al and Ta
These elements are carbide formers and may be present in the alloy in the
ranges for
altering the composition of the hard phases. However, normally none of these
elements are
added.
Hard phases
The total content of the hard phases MX, M2X, M23C6 and M2C3 shall not exceed
50 vol. %,
wherein M is one or more of the above specified metals, in particular V. Mo
and/or Cr and X is
C, N and/or B and wherein the contents of said hard phases fulfil the
following requirements (in
vol. %):
MX 3-25 preferably 5-20
M2X < 10 preferably < 5
M23C6 M7C3 < 10 preferably < 5
More preferably the content of MX is 5 -15 vol. %, the content of M2X is <
vol. 3% and the
content of M23C6 M7C3 is < 3 vol. %. Most preferably the steel is free from
the component
M7C3.
PRE
The pitting resistance equivalent (PRE) is often used to quantify pitting
corrosion resistance of
stainless steels. A higher value indicates a higher resistance to pitting
corrosion. For high
nitrogen martensitic stainless steels the following expression may be used
PRE= %Cr +3.3 %Mo + 30 %NI
wherein %Cr, %Mo and %N are the calculated equilibrium contents dissolved in
the matrix at
the austenitising temperature (TA), wherein the chromium content dissolved in
the austenite is
at least 13 %.. The dissolved contents can be calculated with Thermo-Cale" for
the actual
austenitising temperature (TA) and/or measured in the steel after quenching.
?tRitARJA WWN/PAW1-(14--11
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The austenitising temperature (TA) is in the range of 950 - 1200 C, typically
1080-
1150 C.
It follows from the above reasoning that the austenite composition at
austenizing
temperature may have a considerable effect on the pitting corrosion resistance
of the
steel. The lower limit for the calculated PRE-value may be 25, 26, 27,28, 29,
30, 31,
32 or 31
High nitrogen stainless steels are based on a replacement of carbon with
nitrogen. By
substituting most of the carbon with nitrogen it is possible to substitute the
chromium
rich carbides of the type M7C3 and M23C6 with very stable hard particles of
the type
MN-nitrides. The amount of Cr, Mo and N in solid solution at the hardening
temperature is therefore very much increased, because less chromium is bound
in the
hard phase and because the carbides of the type M23C6 and M7C3 do not have any
solubility for nitrogen. Thereby, more chromium is left in solid solution and
the thin
passive chromium rich surface film is strengthened, which leads to an
increased
resistance to general corrosion and pitting corrosion. Accordingly, it is to
be expected
that the pitting corrosion resistance would decrease if carbon replaces part
of the
nitrogen. High nitrogen stainless steels known in the art therefore have a low
carbon
content
However, the present inventors have surprisingly found that it is possible to
increase the
corrosion resistance by increasing carbon content to above 0.3% as will be
discussed in
relation to the examples.
Steel production
The tool steel having the chemical composition can be produced by
conventional gas atomizing followed by nitrogenation of the powder before HIP-
ing.
The nitrogen content in the steel after gas atomizing is normally less than
0.2%. The
remaining nitrogen is thus added during the nitrogenation treatment of the
powder.
After consolidation the steel may be used in the as HIP-ed form or formed into
a desired
shape. Normally the steel is subjected to hardening and tempering before being
used.
Austenitising may be perfiprmed by annealing at an austenitising temperature
(TA) in
the range of 950 - 1200 'V, typically 1080- 1150 C. A typical treatment is
annealing
at 1080 C for 30 minutes. The steel may be hardened by quenching in a vacuum
furnace by deep cooling in liquid nitrogen, and then tempered at 200 C for 2
times at
2 hours (2x2h).
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EXAMPLE 1
In this example a steel according to the invention is compared to a steel
having lower
carbon content and a different balance between carbon and nitrogen. Both
steels were
produced by powder metallurgy.
The basic steel compositions were melted and subjected to gas atomization.
Subsequently the obtained powders were subjected to a nitrogenation treatment
in order
to introduce the desired amount of nitrogen into the powders. The nitrogen
content was
increased from about 0.1% to the respective content.
Thereafter the nitrogenated powders were transformed to isotropic solid steel
bodies by
conventional hot isostatic pressing (HIP) at 1100 C for 2 hours. The applied
pressure
was 100 MPa.
The steels thus obtained had the following compositions (in wt. %):
Inventive steel Comparative steel
C 0.35 0.18
1.5 1.9
(C+N) 1.85 2.08
C/N 0.23 0.09
Si 0.3 0.3
Mn 0.3 0.3
Cr 18.2 19.8
Mo 1.04 2.5
V 3.47 2.75
balance iron and impurities.
The steels were austenitised at 1080 C for 30 minutes and hardened by
quenching by
deep cooling in liquid nitrogen in a vacuum furnace followed by tempering at
200 C
for 2 times at 2 hours (2x2h). The inventive steel had a hardness of 60 HRC
and the
comparative steel a hardness of 58 HRC.
The alloy microstructure consisted of tempered martensite and hard phases. Two
distinct hard phases were identified in the microstructure of both steels: MX
and M2X.
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In the comparative steel the hexagonal MA was the majority phase and the face
centred cubic MX-phase was the minority phase. However, in the inventive steel
MX
was the majority phase and MA was the minority phase.
The materials susceptibility for pitting corrosion was experimentally examined
by
anodic polarisation sweep. An electrochemical cell with a saturated Ag/AgC1
reference
electrode and a carbon mesh counter electrode, were used for cyclic
polarization
measurements. The 500 mesh grounded sample was first open circuit potential
(OCP)
recorded with a 0.1 M NaCl solution to ensure a stable potential was reached.
Next, the
cyclic polarization measurements were performed with a scan rate of 10 mV/min.
Start
potential was -0.2 V vs. OCP, and the final potential was set to the OCP. By
choosing a
setting in the software, the upward potential scan was automatically reversed
when the
anodic current density reached 0.1 mA/cm2.
Fig. 1 discloses a schematic anodic polarization curve and the information
that can be
obtained from the curve. The forward scan gives information about the
initiation of
pitting and the reverse scan provides information about the alloys
repassivation
behavior. Eb is the value of the potential for pitting breakdown above which
new pits
will initiate and existing pits will propagate. As the potential is decreased
on the reverse
scan, there is a decrease in current density. The alloy is repassivated where
the reverse
scan crosses the forward scan. Ep is the repassivation potential, or
protection potential
i.e. the potential below which no pitting occur. The difference between Eb and
Ep is
related to the susceptibility to pitting and crevice corrosion. The greater
the difference
the greater the susceptibility.
Steel Eb (V) Ep (V)
Inventive 0.38 0.07
Comparative 0.30 -0.10
Table 1. Result of the anodic polarisation.
Table 1 discloses that the inventive steel with the increased carbon content
has the less
tendency to suffer localised corrosion and also that the inventive steel also
repassivate
more easily than the comparative steel. Accordingly, the inventive steel is
much less
sensitive to pitting and crevice corrosion.
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These results were totally unexpected because the inventive steel had lower
contents of
Cr, Mo and N than the comparative steel. The reasons therefore are presently
not fully
understood. However, the present inventors suspected that the differences may
be
related to the type and amount of hard phases remaining in the steel after
austenizing
5 and quenching.
EXAMPLE 2
The influence of the relative amounts of carbon and nitrogen on the formation
of the
different hard phases in the steel was calculated in Thermo-Calera for a steel
having
10 variable C and N contents and the following basic composition in weight
%: Cr: 19.8,
Mo: 2.5, V: 2.75; Si: 0.3, Mn: 0.3, Fe balance.
C/N MX M2X M23C6 Cr Mo N PRE
Comp. 0.1 2.05 0.05 4.2 12.7 0 13 2.5
0.23 28.2
Comp. 0.2 1.9 0.11 4.0 11.3 0 14
2.6 0.24 29.7
Inv. 0.3 1.75 0.17 3.9 9.8 0 15 2.6
0.26 31.4
Inv. 0.4 1.6 0.25 3.9 8.0 0.6 16 2.6 0.27
32.7
Inv. 0.5 1.45 0.34 4.2 6.0 2.6 16 2.4 0.27 32.0
Inv. 0.6 1.3 0.46 4.6 3.7 4.6 16 2.3 0.26 31.4
Comp. 0.7 1.15 0.60 5.0 1.5 6.5 16.5
2.2 0.26 31.4
Table 2. Results of Example 2 at 1080 C. Elemental concentrations in wt. %.
Hard
phases in vol. %. Cr, Mo and N denotes the calculated dissolved contents of
the
elements in the matrix at 1080 C. PRE is calculated from the dissolved
contents.
Fig. 2 discloses the amount of hard phases as a function of the ratio C/N and
it can be
seen that amount ofM2X decreases rapidly with increasing ratio C/N. However,
M23C5
starts to form already at a C/N ratio of about 0.25.
Fig. 3 discloses calculated PRE-values as a function of the ratio C/N and it
can be seen
that the highest values are obtained for the steels according to the
invention.
EXAMPLE 3
The influence of the relative amounts of carbon and nitrogen on the formation
of the
different hard phases in the steel was calculated in Thermo-CalcTra for a
steel having
variable C and N contents and the following basic composition in weight %: Cr:
18.2,
Mo: 1.04, V: 3.47; Si: 0.3, Mn: 0.3, Fe balance.
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C/N MX M2X M23C6 Cr Mo N PRE
Comp. 0.1 2.05 0.05 7.0 7.4 0
14.0 1.15 0.23 24.7
Comp. 0.2 1.9 0.11 6.8 6.1 0 14.5
1.16 0.24 25.5
Inv. 0.3 1.75 0.17 6.7 4.7 0 15.5 1.16 0.26
27.1
Inv. 0.4 1.6 0.25 6.6 3.1 0 16.5 1.16 0.27
28.4
Inv. 0.5 1.45 0.34 6.8 1.2 1.6 16.8 1.1 0.27
28.5
Inv. 0.6 1.3 0.46 6.8 0 3.5 16.8 1.0 0.25
27.6
Comp. 0.7 1.15 0.60 6.3 0 5.2
16.4 0.9 0.21 25.7
Table 3. Results of Example 3 at 1080 C. Elemental concentrations in wt. %.
Hard phases in
vol. %. Cr, Mo and N denotes the calculated dissolved contents of the elements
in the matrix at
1080 C. PRE is calculated from the dissolved contents.
Fig. 4 discloses the amount of hard phases as a function of the ratio C/N and
it can be seen that
amount of M2X decreases very rapidly with increasing ratio C/N. It can also be
seen that M23C6
starts to form at a C/N ratio of about 0.3.
Fig. 5 discloses calculated PRE-values as a function of the ratio C/N and
again it can be seen
that the highest values are obtained for the steels according to the
invention.
These results verify that a proper balance of carbon and nitrogen is an
essential features of the
present invention. A carefully controlled increase of the carbon content can
be made without
obtaining problems with carbides of the type M23C6 and M7C3 in the steel.
These results also
reveals that if the carbon and nitrogen contents are controlled as defined in
the present
application., then the amount of the hexagonal phase M2X will be reduced after
hardening. This
phase is mainly referred to as Cr2N but it may also include a substantial
amount of Mo. The
reduction of the amount of M2X is a result of dissolution during the
austenizing. Although a
part of these elements under certain circumstances may be found in the
increased fraction of
MX (Fig. 2) it would appear that the dissolution of M2X results in increased
amounts of Cr, Mo
and N dissolved in the matrix with a corresponding increase of the PRE-number
until a certain
limit. Thereafter the PRE-value will decrease as a result of the formation of
M23C6, because
said phase is rich in Cr and Mo.
Ftnyom W-%iNtVi
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Another mechanism, which may contribute to the improved corrosion resistance
disclosed in Table 1 and Fig. 1, may be that the boundary regions surrounding
the hard
phase M2X may be depleted in Cr and Mo due to the formation of Cr and Mo rich
M2X.
Another possibility mechanism that may influence the corrosion resistance is
that the
increased carbon content in the hard phase MX may result in a lower solubility
of Cr in
this phase. This would result in a reduced volume fraction of MX and more
chromium is
retained in solid solution, which helps to improve the corrosion resistance.
Accordingly, the present invention provides a to provide a powder metallurgy
(PM)
produced nitrogen alloyed cold work tools steel having an improved corrosion
resistance in combination with a high hardness.
INDUSTRIAL APPLICABILITY
The cold work tool steel of the present invention is particular useful
in applications requiring good wear resistance in combination with a high
resistance to
pitting corrosion.
Date Recue/Date Received 2021-11-11
