Note: Descriptions are shown in the official language in which they were submitted.
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STEEL ALLOY, HOLDERS AND HOLDER DETAILS FOR PLASTIC MOULDING
TOOLS, AND TOUGH HARDENED BLANKS FOR HOLDERS AND HOLDER
DETAILS
TECHNICAL FIELD
The invention relates to a steel alloy and particularly to a steel alloy for
the
manufacturing of holders and holder details for plastic moulding tools. The
invention
also concerns holders and holder details manufactured of the steel, as well as
blanks
made of the steel alloy for the manufacturing of such holders and holder
details.
BACKGROUND OF THE INVENTION
Holders and holder details for plastic moulding tools are employed as clamping
and/or
framing components for the plastic moulding tool in tool sets, in which tool
the plastic
product shall be manufactured through some kind of moulding method. Among
conceivable holder details there can be mentioned bolster plates and other
construction
parts as well as heavy blocks with large recesses which can accommodate and
hold the
actual moulding tool. Said holders and holder details are made of many
different steel
alloys, including martensitic stainless steels. A steel which is manufactured
and
marketed by the applicant under the registered trade name RAMAX S belongs to
that
group and has the following nominal composition in weight-%: 0.33 C, 0.35 Si,
1.35
Mn, 16.6 Cr, 0.55 Ni, 0.12 N, 0.12 S, balance iron and 'impurities from the
manufacturing of the steel. The closest comparable standardized steel is AISI
420F.
Steels of this type have an adequate corrosion resistance, but do not have a
martensitic
micro-structure which is as homogenous that is desirable, but may contain
ferrite and
hard spots, which are due to retained, untempered martensite, which in turn
can be
explained by a certain segregation tendency of the steel. Therefore it exists
a demand of
improvements as far as holder steels are concerned. It is also desirable that
the same
steel, possibly with some modification of the composition, also shall be
useful for the
actual moulding tool.
DISCLOSURE OF THE INVENTION
It is an object of the invention to provide a steel, which after hardening and
tempering
has a more even structure than the above mentioned steel, essentially without
ferrite
and/or spots in the material which have a pronouncedly higher hardness.
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The invention also aims to achieve one or several of the following effects:
- A good machinability.
- An adequate corrosion resistance.
- An adequate hardenability, considering the steel shall be possible to be
used for the
manufacturing of holder blocks made of plates which may have a thickness of up
to
at least 300 mm and in some cases even up to 400 mm thickness.
- An adequate ductility/toughness.
- A hardness of 30 - 42 HRC, preferably 38 - 40 HRC in the tough-hardened
condition.
- A good polishability, at least according to a preferred embodiment, in order
to able
to be used also for moulding tools on which high demands are raised as far as
polishability is concerned.
The above objectives can be achieved if the steel has the chemical composition
which is
stated in the appending patent claims.
As far as the importance of the separate elements and their interaction in the
steel are
concerned, the following may be considered to apply without binding the
claimed patent
protection to any specific theory.
Carbon and nitrogen are elements which have a great importance for the
hardness and
ductility of the steel. Carbon is also an important hardenability promoting
element.
Carbon, however, binds chromium in the form of chromium carbides (M7C3-
carbides)
and may therefore impair the corrosion resistance of the steel. The steel
therefore may
contain max 0.15 % carbon, preferably max 0.13 % carbon (in this text always
weight-
% is referred to if not otherwise is stated). However, carbon also has some
advantageous
effects, such as to exist together with nitrogen as a dissolved element in the
tempered
martensite in order to contribute to the hardness thereof, and also acts as an
austenite
stabilizer and thence counteract ferrite in the structure. The minimum amount
of carbon
in the steel therefore shall be 0.06%, preferably at least 0.07 %.
Nitrogen contributes to the provision of a more even, more homogenous
distribution of
carbides and carbonitrides by affecting the solidification conditions in the
alloy system
such that larger aggregates of carbides are avoided or are reduced during the
solidify-
cation. The proportion of M23 C6-carbides also is reduced in favour of M(C,N),
i.e.
vanadium-carbonitrides, which has a favourable impact on the
ductility/toughness. In
summary, nitrogen contributes to the provision of a more favourable
solidification
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3
process implying smaller carbides and nitrides, which can be broken up during
the
working to a more finely dispersed phase. From these reasons nitrogen shall
exist in an
amount of at least 0.07 %, preferably at least 0.08 %, but not more than 0.22
%,
preferably max 0.15 %, at the same time as the total amount of carbon and
nitrogen
shall satisfy the condition 0.16 < C + N <0.26. Preferably, C + N shall be at
least 0.17
% but suitably max 0.23 %. Nominally, the steel contains 0.20 - 0.22 (C + N).
In the
hardened and tempered steel, nitrogen is substantially dissolved in the
martensite in the
form of nitrogen-martensite in solid solution and thence contributes to the
desired
hardness.
In summary, as far as the content of nitrogen is concerned, it can be stated
that nitrogen
shall exist in the said minimum amount in- order to contribute to the desired
corrosion
resistance by increasing the so called PRE-value of the matrix of the steel,
to exist as a
dissolved element in the tempered martensite which contributes to the hardness
of the
martensite, and to form carbonitrides, M(C, N), to a desired degree together
with
carbon, but not exceed said maximum content, maximizing the content of carbon
+
nitrogen, where carbon is the most important hardness contributor.
Silicon increases the carbon-activity of the steel and thence the tendency to
precipitate
more primary carbides. This is a first reason why it is desirable that the
steel has a low
content of silicon. Further, silicon is a ferrite stabilizing element, which
is a
disadventageous feature of silicon. As the steel also shall contain the
ferrite stabilizing
elements chromium and molybdenum in sufficient amounts to provide desirable
effects
by those elements, at the same time as the steel contains a lower content of
carbon than
is conventional in steels for the application in question, the content of
silicon should be
restricted in order not to cause the steel to contain ferrite in its matrix.
The steel there-
fore must not contain more that 1 % Si, preferably max. 0.7 % Si, suitably
max. 0.5 %
Si, and most conveniently a still lower content of silicon. Generally the rule
shall apply
that the ferrite stabilizing elements shall be adapted to the austenite
stabilizing ones in
order to avoid formation of ferrite in the steel. However, silicon exists as a
residue from
the desoxidation treatment, wherefore the optimum content of silicon lies in
the range
0.05 - 0.5 %, normally in the range 0.1- 0.4 %, and is nominally about 0.2 -
0.3 %.
Manganese is an element which promotes austenite and hardenability, which is a
favourable effect of manganese, and can also be employed for sulphur refining
by
forming harmless manganese sulphides in the steel. Manganese therefore shall
exist in a
minimum amount of 0.1 %, preferably at least 0.3 %. Manganese, however has a
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segregation tendency together with phosphorous which can give rise to
tempering-
embrittlement. Manganese therefore must not exist in an amount exceeding 2 %,
preferably max.1.5 %, suitably max.1.3 %.
Chromium is the main alloying element of the steel and is essentially
responsible for
provision of the stainless character of the steel, which is an important
feature of holders
and holder details for plastic moulding tools, as well as for the plastic
moulding tool
itself, which often is used in damp environments, which may cause less
corrosion
resistant steels to rust.
Chromium also is the most important hardenability promoting element of the
steel.
However, no substantial amounts of chromium are bound in the form of carbides,
because the steel has a comparatively low carbon content, wherefore the steel
can have
a chromium content as low as 12.5 % and nevertheless get a desired corrosion
resistance. Preferably the steel, however, contains at least 13.0 % chromium.
The upper
limit is determined in the first place by the ferrite forming tendency of
chromium. The
steel therefore must not contain more than max. 14.5 % Cr, preferably max.
14.0 % Cr.
Nominally, the steel should contain 13.1 - 13.7 % Cr.
Nickel should exist in the steel in a minimum amount of 0.8 %, preferably at
least 1.0
%, in order to afford the steel a very high hardenability. From cost reasons,
however,
the content should be limited to max. 2.5 %, preferably to max. 2.0 %.
Nominally, the
steel contains 1.4 - 1.8 % or about 1.6 % Ni.
Optionally, the steel of the invention also may contain an active content of
vanadium in
order to bring about a secondary hardening through precipitation of secondary
carbides
in connection with the tempering operation, wherein the tempering resistance
is
increased. Vanadium, when present, also acts as a grain growth inhibitor
through the
precipitation of MC-carbides. If the content of vanadium is too high, however,
there
will be formed large primary MC-carbonitrides during the solidification of the
steel, and
this also occurs if the steel is subjected to ESR-remelting, which primary
carbides will
not be dissolved during the hardening procedure. For the achievement of the
desired
secondary hardening and for the provision of a favourable contribution to the
grain
growth inhibition, but at the same time avoiding formation of large,
undissolvable
primary carbides in the steel, the optional content of vanadium should lie in
the range
0.07 - 0.7 % V. A suitable content is 0.10 - 0.30 % V, nominally about 0.2 %
V.
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Preferably, the steel also contains an active content of molybdenum, e.g. at
least
0.1 %, in order to give a hardenability promoting effect. Molybdenum up to an
amount of at least 1.0% also promotes the corrosion resistance but may have
effect
also if the content is higher. When tempering, molybdenum also contributes to
increasing the tempering resistance of the steel, which is favourable. On the
other
hand, a too high content of molybdenum may give rise to an unfavourable
carbide
structure by causing a tendency to precipitation of grain boundary carbides
and
segregations. Besides, molybdenum is ferrite stabilizing, which is
unfavourable.
The steel therefore shall contain a balanced content of molybdenum in order to
take
advantage of its favourable effects but at the same time avoid those ones
which are
unfavourable. Preferably, the content of molybdenum should not exceed 1.7%. An
optimal content may lie in the range of 0.1 - 0.9%, probably in the range of
0.4 -
0.6% Mo.
Normally, the steel does not contain tungsten in amounts exceeding the
impurity
level, but may possibly be tolerated in amounts up to 1 %.
The steel of the invention shall be possible to be delivered in its tough-
hardened
condition, which makes it possible to manufacture large sized holders and
mould
tools through machining operations. The hardening is carried out through
austenitizing at a temperature of 850 - 1000 C, preferably at 900- 975 C, or
at
about 950 C, followed by cooling in oil or in a polymer bath, by cooling in
gas in a
vacuum furnace, or in air. The high temperature tempering for the achievement
of a
tough hardened material with a hardness of 30 - 42 HRC, preferably 38 - 41 or
about 40 HRC, which is suitable for machining operations, is performed at a
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temperature of 510 - 650 C, preferably at 520 - 540 C, for at least one
hour,
preferably through double tempering; twice for two hours. The steel may, as an
alternative, be low temperature tempered at 200 - 275 C, e.g. at about 250
C, in
order obtain a hardness of 38 - 42 or about 40 HRC.
The steel may, according to a preferred embodiment, also contain an active
content
of sulphur, possibly in combination with calcium and oxygen, in order to
improve the
machinability of the steel in its tough hardened condition. In order to obtain
best
effect in terms of machinability improvement, the steel should contain at
least 0.07%
S if the steel does not also contain an intentionally added amount of calcium
and
oxygen, and at least 0.035%, respectively, if the steel also contains an
active amount
of calcium and oxygen. The maximum sulphur content of the steel is 0.25%, when
the steel is intentionally alloyed with a content of sulphur. A suitable
sulphur content
in this case may be 0.12%. Also a non-sulphurized variant of the steel,
however,
can be conceived, i.e. the steel shall not contain sulphur above impurity
level.
In this case the steel does not contain sulphur above impurity level, and nor
does
that steel contain any active contents of calcium and/or oxygen, i.e. not
contain any
calcium or oxygen above impurity level.
It is thus conceivable that the steel may contain 0.035 - 0.25% S in
combination with
3 - 100 weight-ppm Ca, preferably 5 - 75 ppm Ca, suitably max. 40 ppm Ca, and
10
- 100 ppm 0, wherein said calcium, which may be supplied as silicon-calcium,
CaSi,
in order to globulize existing sulphides to form calcium sulphides,
counteracts that
the sulphides get a non-desired, elongated shape, which might impair the
ductility.
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6A
The steel of the invention can be manufactured conventionally at a production
scale
by manufacturing a metal melt in the normal way, said melt having a chemical
composition according to the invention, and casting the melt into large ingots
or
casting the melt continuously. It is also possible to cast electrodes of the
molten
metal and then remelting the electrodes through Electro-Slag-Remelting (ESR).
It is
also possible to manufacture ingots powder-metallurgically through gas-
atomization
of the melt to produce a powder, which then is compacted through a technique
which
may comprise hot isostatic pressing, so called HIPing, or, as an alternative,
manufacture ingots through sprayforming.
Further characteristics, aspects and features of the steel according to the
invention,
and its usefulness for the manufacturing of holders and moulding tools, will
be
explained more in detail in the following through a description of performed
experiments and achieved results.
BRIEF DESCRIPTION OF DRAWINGS
In the following description of performed experiments and achieved results,
reference will be made to the accompanying drawings, in which
Fig. 1 shows a holder block of a typical design, which can be manufactured of
the steel according to the invention,
Fig. 2A is a chart showing the hardness of a first set of steels, produced in
the
form of so called Q-ingots (50 kg laboratory heats), after hardening but
before tempering, versus the austenitizing temperature at a holding time
of 30 min,
Fig. 2B shows corresponding graphs for another number of tested steels
manufactured as Q-ingots,
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6B
Fig. 3A shows tempering curves for those steels in the first set which have
been
hardened from 1030 C,
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Fig. 3B shows the tempering temperature range 500 - 550 C of the tempering
curves
of Fig. 3A at a larger scale,
Fig. 3C shows tempering curves within the tempering temperature range 500 -
550 C
for those further tested steels, whose hardness versus the austenitizing
temperature was shown in Fig. 2B,
Fig. 4 is a chart which showing hardenability curves for the steels which were
tested
as stated above,
Fig. 5 is a bar chart illustrating results from impact toughness testing of
the above
mentioned steels, and
Fig. 6A and Fig. 6B are bar charts which illustrate the critical current
density, Icr,
measured when corrosion testing samples which had been slowly cooled in a
vacuum furnace at two different cooling rates from the austenitizing
temperature and which thereafter had been high temperature tempered to about
40 HRC.
EXAM NATION OF STEELS MANUFACTURED AT A LABORATORY SCALE
Fig. 1 shows a holder block 1 of a typical design, which shall be possible to
be
manufactured of the steel according to the invention. In the blockl there is a
cavity 2,
which shall accommodate a mould tool, usually a plastic moulding tool. The
block 1 has
considerable dimensions and the cavity 2 is large and deep. Therefore, a
number of
different requirements are raised on the material according to the invention,
i. a. an
adequate hardenability with reference to the considerable thickness of the
block, and a
good ability to be machined by means of cutting tools, such as mill cutters
and borers.
Material
17 Q-ingots (50 kg laboratory heats) with compositions according to Table I
were
manufactured in four rounds. In the first round (Q9043-Q9080), ingots were
manufactured having chemical compositions within a wide range; e.g. variants
having
comparatively high contents of nitrogen were tested. It was revealed that the
alloy
having the most interesting features was Q9068, i.e. with carbon contents
lying in
medium range around 0.10 % and with moderate contents of nitrogen.
In the second round (Q9129-Q9132) one tried to optimize the features that were
obtained by Q9068. The carbon content was slightly varied, vanadium was added
in
order to obtain a finer grain size, and the nickel content was lowered for one
of the
variants.
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In the third round (Q9135-Q9139) variants having increased sulphur contents
were
tested.
In a fourth round only two steels, Q9153 and Q9154, were tested in order to
evaluate
the relations between carbon and nitrogen.
The steels Q9043 and Q9063 are reference materials. Q9043 has a composition
according to SIS2314 and AISI 420, while Q9063 corresponds to W.Nr. 1.2316.
The Q-ingots were forged to the shape of rods of size 60 x 40 mm, whereupon
the
rods were cooled in vermiculite.
Table I - Test materials; chemical composition in weight-%, balance Fe and
unavoidable impurities
Q-ingot C N Si Mn Cr V Ni Mo S
Q9043 0.36 0.026 0.83 0.47 13.9 0.32 0.18 0.12 n.a.
Q9063 0.37 0.12 0.17 0.55 15.7 0.8 1.19 n.a.
Q9064 0.27 0.18 0.14 1.35 16.7 0.3 1.61 0.44 n.a.
Q9065 0.20 0.16 0.185 1.29 15.7 0.15 1.56 0.74 n.a.
Q9067 0.11 0.063 0.18 1.1 12.3 0.73 0.33 n.a.
Q9068 0.11 0.059 0.17 1.06 13.4 0.067 2.1 0.75 n.a.
Q9069 0.075 0.084 0.15 1.01 12.4 0.076 0.75 0.34 n.a.
Q9070 0.076 0.085 0.18 1.14 13.8 0.06 0.74 0.32 n.a.
Q9080 0.15 0.17 0.21 1.26 16 0.12 1.56 0.75 n.a.
Q9129 0.097 0.087 0.16 1.06 12.8 0.2 1.6 0.22 n.a.
Q9131 0.11 0.088 0.15 1.07 12.7 0.19 0.86 0.22 n.a.
Q9132 0.14 0.094 0.14 1.11 12.7 0.19 1.61 0.22 n.a.
Q9135 0.19 0.039 0.12 0.93 13.4 0.27 1.02 0.21 0.07
Q9136 0.07 0.091 0.15 1.17 14.9 0.22 1.04 0.21 0.075
Q9139 0.12 0.092 0.17 1.23 14.2 0.20 1.06 0.22 0.14
Q9153 0.12 0.10 0.14 0.81 12.7 0.20 1.58 0.24 0.0059
Q9154 0.06 0.14 0.17 0.88 12.5 0.21 1.53 0.21 0.0053
n.a. = not analyzed
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Hardness after heat treatment
The hardness versus the austenitizing temperature is shown in Fig. 2A and Fig.
2B. It is
evident from the charts of these drawings that the hardness increases with
increasing
austenitizing temperature for some steels having a higher carbon content, such
as for
Q9043, Q9063, Q9103, Q9104 and Q9135. 1030 C is an austenitizing temperature
which may be appropriate in these cases. For other steels, the hardness
decreases or
remains constant with increasing austenitizing temperature. In that case it
may be more
appropriate to choose 950 C as an austenitizing temperature.
The hardness after tempering of those steels which had been hardened from 1030
C are
shown in Fig. 3A and Fig. 3B, while all the tempering curves for those ones of
the Q-
ingots 9129-9154 which had been hardened from 950 C are shown in the diagram
in
Fig. 3 C. The conclusion can be drawn from the tempering curves that all the
steels can
be tempered down to 40 HRC through tempering in the temperature range 520-600
C.
An appropriate hardness of the steel after tough-hardening is about 40 IHRC.
In Table II
below, the heat treatments are stated which provide the said hardness to the
different
steels.
Table II - Heat treatment for tough-hardening, measured rest austenite,
percent
by volume
Q-ingot Heat treatment Rest austenite
NO (%)
9063 1030 C/30 min + 550 C/2x2 h 0
9064 1030 C/30 min + 550 C/2x2 h 1.3
9065 1030 C/30 min + 550 C/2x2 h 2.3
9067 1030 C/30 min + 5250C/2x2 h 0
9068 1030 C/30 min + 525 C/2x2 h 0
9069 1030 C/30 min + 525 C/2x2 h 0
9070 1030 C/30 min + 525 C/2x2 h 0
9080 1030 C/30 min + 550 C/2x2 h 6.4
9104 1030 C/30 min + 550 C/2x2 h 0
9129* 950 C/30 min + 525 C/2x2 h 0
9131* 950 C/30 min + 525 C/2x2 h+ 535/2h 0
9132* 950 C/30 min + 525 C/2x2 h + 535/2h 0
9135* 950 C/30 min + 525 C/2x2 h 0
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9136* 950 C/30 min + 525 C/2h + 500/2h 0
9139* 950 C/30 min + 525 C/2x2h 0
9153** 950 C/30 min+ 535 C/2x2h 0
9154 950 C/30 min + 540 C/2x2h Not measured
* t8/5=1964s
** t8/5=1885s (t8/5 corresponds to the time for cooling a sample from 800 to
500 C)
Hardenability
5 The hardness after hardening from the austenitizing temperatures which are
given in
Table II, from which temperatures the samples have been cooled at different
rates, is
shown in the hardenability curves of Fig. 4.
Impact toughness tests
10 Impact toughness testing of un-notched test specimens, mean values for four
to six test
rods of each steel, was performed at room temperature. The heat treatments and
cooling
rates, which were employed for the different steels, are given in Table III.
The results
are disclosed by the bar chart in Fig. 5. From this chart it can be recognized
that some
variants, such as Q9067, 9068, 9069, 9129, 9131, 9132 and Q9153 have a very
high
ductility, >350 J, and that the test rods were not ruptured, but also that
some other
steels, including e.g. steel Q9154, have a considerably better ductility than
the reference
steels, Q9063 and 9043, which lie on the 180-200 J level.
Table III
Q-ingot Heat treatment C Cooling rate t8/5 (s)
No
9043 1030/30+560/2h+550/2h 2093
9063 1030/3 0+570/2h+560/2h 2093
9064 950/30+560/2x2h 2093
9065 950/30+550/2x2h 2093
9067 950/30+525/2x2h 2093
9068 950/30+525/2x2h 2093
9069 950/30+525/2x2h 2093
9070 950/30+525/2x2h 2093
9080 950/30+550/2x2h 2093
9129 950/30+525/2x2h 1969
9131 950/30+525/2x2h+535/2h 1969
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9132 950/3 0+525/2x2h+53 5/2h 1969
9135 950/30+525/2x2h 1964
9136 950/30+525/2x2h+500/2h 1964
9139 950/30+525/2x2h 1964
9153 950/30+535/2x2h 1985
9154 950/30+540/2x2h 1863
Corrosion tests
Polarization curves were established in a first test round for the steels
given in Table IV
in terms of critical current density, Icr, for the evaluation of the corrosion
resistance of
the steels. As far as this method of measurement is concerned, the rule is
that the lower
Irc is, the better is the corrosion resistance. The investigations were
performed in two
test series, in which the test specimens were subjected to different cooling
rates. The
heat treatments of the first series are shown in Table IV.
Table 1V - Heat treatment of polarization test specimens. Cooling in vacuum
furnace
Q-ingot Heat treatment T8/5 (s) Hardness
No (ERC)
9063 1030 C/30min + 570 C/2x2h 860 40.8
9064 1030 C/30min + 600 C/2x2h 860 40.5
9065 1030 C/30min + 580 C/2x2h 860 40.0
9067 1030 C/30min + 525 C/2x2h + 535 C/lh 860 38
9068 1030 C/30min + 525 C/2x2h 860 40.1
9069 1030 C/30min + 525 C/2x2h + 535 C/lh 860 40
9070 1030 C/30min + 525 C/2x2h + 535 C/lh 860 39
9080 1030 C/30min + 565 C/2h + 550 C/2h 860 40.6
9129 950 C/30min + 525 C/2h + 535 /2h 876 39.7
9131 950 C/30min + 525 C/2h + 535 C/2h 876 40.2
9132 950 C/30min+ 535 C/2x2h 876 39.7
9153 950 C/30min+535 C/2x2h 957 39.4
The results from this first test round are evident from the bar chart in Fig.
6A. From this
bar chart it is evident that five steels had a better corrosion resistance
than the reference
material, Q9063, namely Q9068, Q9070, Q9129,Q9132 and Q9153.
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Still slower cooling rates t8/5 were employed in a second test round, see
Table V and
Fig. 6B.
Table V - Heat treatment of polarization test specimens. Cooling in vacuum
furnace
Q-ingot Heat treatment T8/5 (s) Hardness
No (HRC)
9063 1030 C/30min+ 570 C/2x2h 1880 38.9
9104 1030 C/30min + 570 C/2x2h 1880 40.1
9129 950 C/30min + 525 C/2x2h 1969 40.6
9131 950 C/30min + 525 C/2x2h + 535 C/2h 1969 39.6
9132 950 C/3Omin + 525 C/2x2h+ 535 C/2h 1969 40.1
9135 950 C/30min + 525 C/2+2h 1964 40.9
9136 950 C/30min + 525 C/2h + 500 C/2h 1964 39.0
9139 950 C/30min + 525 C/2x2h 1964 42.1
9153 950 C/3Omin + 535 C/2x2h 1885 40.3
9154 950 C/30min + 540 C/2x2h 1863 39.0
Fig. 6B illustrates that best corrosion resistances were notified for samples
of Q9063,
9129, 9153 and 9154.
Discussion
In the introductory disclosure of the invention there were listed a number of
purposes of
the invention. Besides a good machinability, the steel shall have a good
ductility, a good
corrosion resistance, and a good hardenability. It can be stated that it is an
aim that the
steel, besides a good machinability, shall have better ductility, corrosion
resistance and
hardenability than steel Q9063. Four steels satisfy those criteria, namely
Q9068, Q9129,
Q9153 and Q9154, which have a rather similar composition; although steel Q9154
has a
higher nitrogen content and a lower content of carbon. On the basis of these
experiences, it can be assumed that an optimal composition could be the
following,
namely 0.10 C, 0.075 N, 0.16 Si, 1.1 Mn, 13.1 Cr, 0.13 V, 1.8 Ni, 0.5 Mo,
balance Fe
and unavoidable impurities. An alternative could be a steel which contains
0.06 C and
0.14 Ni but as for the rest the same composition as the foregoing. Other
alternatives -
suitably conceivable nominal compositions - could be the following ones: 0.12
C, 0.20
Si, 1.30 Mn, 0.10 S, 13.4 Cr, 1.60 Ni, 0.50 Mo, 0.20 V, 0.10 N, balance iron
and
CA 02425893 2003-04-11
WO 02/48418 PCT/SE01/02576
13
unavoidable impurities, and/or 0.14 C, 0.18 Si, 1.30 Mn, 0.10 S, 13.5 Cr, 1.67
Ni, 0.50
Mo, 0.22 V, 0.10 N, balance iron and unavoidable impurities.
MANUFACTURING OF STEEL AT A PRODUCTION SCALE
A 35 tons heat of molten metal was manufactured in an electric arc furnace.
Before
tapping, the melt had the following chemical composition: 0.15 C, 0.18 Si,
0.020 P,
0.08 S, 13.60 Cr, 1.60 Ni, 0.48 Mo, 0.20 V, 0.083 N, balance Fe and
unavoidable
impurities. Of the melt there were manufactured ingots, which were forged to
the shape
of flat rods of varying dimensions. The forging did not cause any problems.
The forged
rods were tough-hardened to a hardness of about 380 BB through austenitizing
at 950
C, holding time 2h, fast quenching in air and tempering at 540 C, 2x2h. The
thus
tough-hardened rods were machined to final gauges.
