Note: Descriptions are shown in the official language in which they were submitted.
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INERT MATERIAL WITH INCREASED HARDNESS FOR THERMALLY
STRESSED PARTS
BACKGROUND OF THE INVENTION
1. Field of the Invention
[00021 The present invention relates to a material with high inertness, in
particular high
oxidation stability, and increased hardness for the manufacture of thermally
resistant parts
and tools.
2. Discussion of Background Information
[0003] According to DIN (German Industrial Standard) 50900, a reaction of a
metallic material
with its surroundings which causes a measurable change in the material, is
defined as
corrosion. Corrosion can occur with and without mechanical stress of the part,
as well as
after various kinds of chemical attack and at different temperatures.
[0004] A surface attack of objects is most often caused by an electrochemical
corrosion
in the presence of an ion conducting phase or by chemical corrosion and hot
corrosion at
elevated temperatures. A corrosion attack can also occur in molten media at
elevated
temperature, e.g., in liquid glasses, with a change in the surface of a metal
part in contact
therewith.
[0005] In modem technology, parts and tool parts are mostly exposed to a
plurality of
different stresses at the same time, of which in particular the thermal and
mechanical stresses
can act in an alternating or increasing manner. Accordingly, multiply
intensified corrosion
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conditions exist which are possibly reinforced by a deformation of the zone
close to the
surface of the part.
100061 Corrosion- and heat-resistant steels and alloys should have a cubic
face-centered
atomic lattice structure or an austenitic microstructure, respectively, also
for a thermal
resistance at temperatures above 600 C. In terms of'alloy technology, this
means that for
increased strength and hardness at high temperatures, such materials have
higher nickel
and/or cobalt contents or are formed as nickel-based or cobalt-based alloys.
However, for
reasons of chemical corrosion they must have a chromium content of higher than
13 % by
weight.
(00071 Although a material with a high nickel concentration invariably shows
an
increased mechanical strength and high material hardness, respectively, which
improves the
performance characteristics of parts and tool parts at high temperature, for
economical
reasons there is a desire to reduce the nickel content to below 36 % by weight
and to increase
the chromium content of the alloy to above 16 % by weight in order to increase
corrosion
resistance.
100081 Although due to a high chromium concentration, an austenitic iron-based
material
having a nickel content of less than 36 % by weight can withstand, if
necessary, in
combination with further elements inhibiting corrosion, a corrosion attack at
high
temperatures, e.g., at 600 C and above, for a required minimum period, the
material exhibits
a low hardness and strength and a limited behavior under long period
stressing. Despite
these disadvantages, alloys, e.g., according to DIN material Nos. 1.2780 and
1.2782 and
1.2786, are used as tools for glass processing for reasons of cost
effectiveness and for
manufacturing reasons.
[00091 It would be desirable to have available a material of the type
mentioned at the
outset which has a hardness of higher than 230 HB and shows a high creep
resistance and an
improved fatigue strength behavior and corrosion resistance even at
temperatures above
600 C.
[00101 It would also be desirable to provide a process for the economical
production of
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a material for parts and tools which have improved performance characteristics
as well as
high hardness and increased corrosion resistance.
SUMMARY OF THE INVENTION
100111 The present invention provides a material for the manufacture of parts
and tools
for use at elevated temperature. The material comprises an alloy having a
composition of, in
% by weight:
Carbon (C) 0.01 to 0.25
Silicon (Si) 0.35 to 2.5
Manganese (Mn) 0.4 to 4.3
Chromium (Cr) 16.0 to 28.0
Nickel (Ni) 15.0 to 36.0
Nitrogen (N) 0.01 to 0.29
provided that the nickel content of the alloy is equal to or higher than the
value formed by
the chromium content plus 1.5 silicon minus 0.12 manganese minus 18 nitrogen
minus 30
carbon minus the numerical value of 6:
Ni z Cr + 1.5 xSi-0.12xMn- 18xN-30xC-6
The balance of the alloy is iron (Fe) and accompanying elements and
impurities. The material
shows a hardness, provided by cold forming, of at least 230 HB.
[00121 It is noted that unless indicated otherwise, all weight percentages
given in the
present specification and the appended claims are based on the weight of the
total
composition. Moreover, all numerical values herein are approximate values.
100131 According to one aspect of the material, the hardness of the material
is higher than
250 HB, e.g., at least 300 HB.
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100141 According to another aspect of the material, the nickel content of the
alloy is
higher by not more than 4.8 % than the value obtained according to the above
equation.
100151 In another aspect, the alloy comprises one or more (e.g., all) of the
above elements
in the following concentrations, in % by weight: C 0.02 to 0.20; Si 0.50 to
2.48; Mn 0.62 to
4.05;Cr20.1 to27.6;Ni 16.1 to 27.3; and N 0.014 to 0.23.
100161 According to yet another aspect, the alloy comprises one or more (e.g.,
all) of the
above elements in the following concentrations, in % by weight: C 0.04 to
0.15; Si 1.22 to
2.36; Mn 1.00 to 3.95; Cr 23.9 to 26.5; Ni 17.9 to 25.45; and N 0.018 to 0.20.
100171 According to a still further aspect, the alloy comprises one or more
(e.g., all)
accompanying elements in the following concentrations, in % by weight:
Molybdenum (Mo)
less than 1.0; Vanadium (V) up to 0.5; Tungsten (W) up to 0.5; Copper (Cu) up
to 0.5;
Cobalt (Co) up to 6.5; Titanium (Ti) up to 0.5; Aluminum (Al) up to 1.5;
Niobium (Nb) up
to 0.5; Oxygen (0) up to 0.05; Phosphorus (P) up to 0.03; and Sulfur (S) up to
0.03. In this
regard it is noted that the expressions "up to" and "less than" include the
absence of the
corresponding element; i.e., 0 % by weight.
[0018] The present invention also provides a process for producing a material
for parts
and tools for use at a temperature of up to 750 C. The process comprises the
provision of an
initial product from an alloy having a composition of, in % by weight, as
indicated above and
the subsequent cold forming of the initial product to a hardness of at least
230 HB.
[0019] In one aspect of the process, the initial product is formed by a
process comprising
hot forming and, subsequently, subjecting it to solution annealing or cooling
down from the
forming temperature, e.g., by forced cooling.
[0020] According to another aspect of the process, the cold forming is carried
out over
the whole circumference, and radially perpendicular to the longitudinal axis
of the initial
product.
[0021] According to yet another aspect, the degree of cold forming is such
that the
hardness of the material is higher than 250 HB, e.g., at least 300 HB.
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[0022] In a still further aspect of the process, the degree of cold forming is
at least 6
for example, at least 12 %.
100231 Also provided by the present invention is a hot working tool which
comprises a
cold formed material of an alloy having a composition of, in % by weight, as
indicated
above.
100241 According to one aspect of the tool, the cold formed material has a
hardness of at
least 230 HB, e.g., higher than 250 HB.
100251 In another aspect, the hot working tool of can be used at a working
temperature
of higher than 555 C, e.g., at a working temperature of higher than 602 C.
For example,
the working temperature may be up to 750 C.
[0026] The present invention additionally comprises a mold for machine pressed
glass.
The mold is made, at least in part, from the cold formed material as indicated
above. Also
provided is a tool in the glass industry which comprises this nlaterial, as
well as a process for
the manufacture of a part or tool for use at elevated temperature. The process
comprises
providing the above cold formed material and making it into said part or tool.
[0027] The advantages obtained according to the invention lie, in particular,
in the
synergy of corrosion chemical resistance of the selected alloy and the
properties of the
material that can be achieved with this chemical composition by means of cold
forming. The
cold forming or forming below the recrystallization temperature of the cubic
face-centered
austenite results in a strengthening of the material by a blocking of
dislocations in the crystal
lattice. An increase in hardness and an increase in the strength of the
material according to
the invention associated therewith is retained even at use temperatures of
above 600 C,
which is surprising to those of skill in the art. Recovery processes that
would be expected
to occur in the distorted lattice, such as, e.g., a thermally activated cross
slip and a
recombination of dislocations, cannot be observed within the usual time
periods. In other
words, contrary to the expectation of those skilled in the art, the hot
strength that is increased
by means of a cold forming of the material composed according to the invention
is retained,
even at high use temperatures of the part, because a high creep resistance of
the steel
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improves the fatigue strength thereof. Particularly with increasing thermal
stress, as is the
case with a metal mold for the manufacture of utility glasses, strong
temperature fluctuations
and, thus, local changes in the volume of the material occur on the working
surface thereof.
It has been found that due to the hardness and hot strength of the material
which are
increased according to the invention, the local deformation or the deformation
close to the
surface, respectively, of the material, e.g., of a glass mold, occurs in the
elastic area thereof,
and that a fatigue crack formation, which occurs already with slight plastic
defoi-rnations and
can lead to the breakdown of the mold, is counteracted thereby.
[00281 In order to ensure an improved property profile of the material, it is
important for
it to remain in the stable austenitic region even during a cold forming, and
to not exhibit any
zones with strain-induced martensite. This is achieved according to the
invention by the
stated nickel and chromium concentration ranges and by the concentration range
of nickel
as a function of chromium, silicon, manganese, nitrogen and carbon. As has
been shown,
higher nickel contents may impair the fatigue strength behavior. However, with
low nickel
concentrations, there may be a sudden decrease in the austenite stability and
the hot strength
of the material. Essentially the same applies to the elements carbon and
nitrogen, where in
particular nitrogen increases the fatigue strength of the material.
100291 The performance characteristics of parts and tools according to the
invention can
be improved if the material has contents of one or more alloying elements, in
% by weight,
of
C = 0.02 to 0.20, preferably 0.04 to 0.15
Si = 0.50 to 2.48, preferably 1.22 to 2.36
Mn = 0.62 to 4.05, preferably 1.00 to 3.95
Cr = 20.1 to 27.6, preferably 23.9 to 26.5
Ni = 16.1 to 27.3, preferably 17.9 to 25.45
N = 0.014 to 0.23, preferably 0.018 to 0.20.
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[0030) It should be noted here that, as is known per se, starting at a content
of 0.52% by
weight, cobalt can improve the hot strength of the material also in the alloy
according to the
invention.
100311 Although the elements molybdenum, vanadium, tungsten, titanium and
niobium
increase the creep resistance of the material at high temperatures, and copper
and aluminum
represent classic hardening elements, in the material according to the
invention these steel
impurities should have a reliable concentration, because, as has been found,
higher contents
thereof reduce corrosion resistance, in particular during temporary contact
with pasty glass,
and impair glass transparency because of an occurring surface roughness of the
mold. The
reason for this has not yet been adequately established, but the acceptor
atoms Na+, K+, Ca2+,
B3+, A13+ and Si4+ are among the hard Lewis acids, whereby a hot corrosion
stress occurs in
the mold after each glass forming.
[00321 Naturally, impurities can impair the material properties, so that for
the
accompanying elements and/or impurity elements the alloy according to the
invention should
show concentrations, in % by weight, of
Molybdenum (Mo) less than 1.0
Vanadium (V) up to 0.5
Tungsten (W) up to 0.5
Copper (Cu) up to 0.5
Cobalt (Co) up to 6.5
Titanium (Ti) up to 0.5
Aluminum (Al) up to 1.5
Niobium (Ni) up to 0.5
Oxygen (0) a max. of 0.05
Phosphorus (P) a max. of 0.03
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Sulfur (S) a max. of 0.03.
100331 In the process according to the present invention for producing a
material for parts
and tools with high inertness, in particular high oxidation stability, and
increased hardness
uilder thermal stresses at a temperature of up to 750 C, an initial product is
made from an
alloy with a composition, in % by weight, of essentially
Carbon (C) 0.01 to 0.25
Silicon (Si) 0.35 to 2.5
Manganese (Mn) 0.4 to 4.3
Chromium (Cr) 16.0 to 28.0
Nickel (Ni) 15.0 to 36.0
Nitrogen (N) 0.01 to 0.29
provided that the nickel content of the alloy is equal to or higher,
optionally by a maximum
of 4.8% by weight, than the value formed by the chromium content plus 1.5
silicon minus
0.12 manganese minus 18 nitrogen minus 30 carbon minus the numerical value of
6:
Ni _ Cr + 1.5 x Si - 0. 12 x Mn - 18 x N - 30 x C -- 6
the balance being iron (Fe) and accompanying elements and impurities. This
initial product
is subsequently further processed by cold forming to produce a material with a
hardness of
higher than 230 HB.
[0034] By means of a cold forming of the alloy according to the invention, the
elasticity
limit of the material can be increased to a tension level that is not reached
even close to the
working surface of the part or too] through a change in volume caused by
altemrnating thermal
stress. Accordingly, even in the area of the grain boundaries no zones occur
that are
plastically deformed during the temperature change, whereby a crack formation
due to
fatigue can be avoided. An attack at grain boundaries by chemical or hot
corrosion can thus
be largely avoided, so that, such as, e.g., with a glass mold, a high working
surface or surface
quality is retained over a long period even under high stress and with large
production
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quantities. In comparison, conventional glass molds often show erosion of
material at the
grain boundaries of the microstructure after a short time in use, with
erosions spaced in the
range of a few um. Thus, unevenness in the wave length range of visible light
is imparted
to the formed glass, which can result in reflection interferences and milky
glass effects.
100351 The corrosion resistance and hot strength can be further increased and
a fatigue
crack formation can effectively be suppressed if in the process according to
the invention,
a material having a hardness of higher than 250 HB, in particular 300 HB and
higher, is
formed by cold forming.
100361 When an initial product with a composition according to the invention
is formed
by means of hot forming, is subjected to a solution annealing or cooled down,
optionally in
a forced manner, from the forming temperature, and cold formed, a material
with a
particularly homogeneous microstructure and improved corrosion resistance can
be
produced.
[00371 In particular for largely axially symmetrically shaped tools, such as
bottleneck
ingot molds and the like, it can be advantageous if the cold forming of the
material is carried
out over the whole circumference radially perpendicular to the longitudinal
axis of the initial
product.
100381 For an increased quality of the product, the alloy of the invention
advantageously
contains one or more alloying elements in the following concentrations in % by
weight
C = 0.02 to 0.20, preferably 0.04 to 0.15
Si = 0.05 to 2.48, preferably 1.22 to 2.36
Mn = 0.62 to 4.05, preferably 1.00 to 3.95
Cr = 20.1 to 27.6, preferably 23.9 to 26.5
Ni = 16.1 to 27.3, preferably 17.9 to 25.45
N = 0.014 to 0.23, preferably 0.018 to 0.2.
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[0039] Furthermore, the concentrations of the individual alloying elements, in
% by
weight, in the iron-based alloy may be as follows:
Carbon (C) up to 0.25
Silicon (Si) up to 2.5
Manganese (Mn) up to 4.3
Chromium (Cr) 16.0 to 28.0
Nickel (Ni) 15.0 to 36.0
Nitrogen (N) 0.01 to 0.29
provided that the nickel content of the alloy is equal to or higher,
optionally by a maximum
of 4.8 % by weight, than the value formed by the chromium content plus 1.5
silicon minus
0.12 manganese minus 18 nitrogen minus 30 carbon minus the numerical value of
6:
Ni>_ Cr + 1.5 x Si - 0.12 x Mn - 18 x N - 30 x C - 6
the balance being iron (Fe) and accompanying elements and impurities. The
alloy is
strengthened to a material hardness of' at least 230 HB, preferably higher
than 250 HB, by
cold forming the initial product made thereof, to result in a material for hot
working tools
with a working temperature of higher than 555 C, preferably higher than 602
C, in particular
up to 750 C.
[0040) The use of the above-mentioned iron-based alloy as a tool material in
the glass
industry, in particular as a mold material for machine pressed glasses, is
particularly
advantageous with regard to product quality and cost-effective production.
100411 The material according to the invention will be described in more
detail on the
basis of comparative test results.
BRIEF DESCRIPTION OF THE DRAWINGS
100421 The present invention is further described in the detailed description
which
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follows, in reference to the noted plurality of drawings by way of non-
limiting examples of
exemplary embodiments of the present invention, in which like reference
numerals represent
similar parts throughout the several views of the drawings, and wherein:
Fig. 1 shows the strength as a function of the degree of the cold forming of a
material
according to the invention at 604 C;
Fig. 2 shows the hardness curve at room temperature after a long-term thermal
stress at
600 C.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
j00431 The particulars shown herein are by way of example and for purposes of
illustrative discussion of the embodiments of the present invention only and
are presented
in the cause of providing what is believed to be the most useful and readily
understood
description of the principles and conceptual aspects of the present invention.
In this regard,
no attempt is made to show structural details of the present invention in more
detail than is
necessary for the fundamental understanding of the present invention, the
description taken
with the drawings making apparent to those skilled in the art how the several
forms of the
present invention may be embodied in practice.
(0044) Fig. 1 shows the strength of the material according to the invention at
a test
temperature of 604 C as a function of the extent of the cold forming. The
test material was
forged at a temperature of 1010 C and cooled in a forced manner from the
forming heat and
subjected to a solution annealing at 1060 C. A cold forming was carried out
on parts of the
material with a forming degree of 21 %, 35 %, 47 % and 55 %, respectively.
Specimens for
tensile tests were subsequently made from these materials. The strength tests,
namely the
determination of the 0.2 % yield point and the tensile strength, were
conducted at a
temperature of 604 C, keeping the specimens at this temperature for 20
minutes. For
comparison, standard material was solution annealed at 1060 C, and samples
made
therefrom were likewise tested at 604 C. The bar chart in Fig. I clearly
shows an increase
in the strength values of the material as a function of the forming degree,
wherein (not shown
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in the diagram) a large increase in strength is already present at a degree of
cold forming of
more than 6 %, in particular more than 12 %.
100451 Fig. 2 shows the fatigue strength of the material according to the
invention at a
temperature of 600 C, determined by a hardness test of the specimens in a
cold state,
compared with DIN materials Nos. 1.2083 and 1.4028.
(0046] The material according to the invention was smelted with a composition,
in % by
weight, of C = 0.08, Si = 1.7, Mn = 1.15, P= 0.01, S = 0.002, Cr = 24.8, Ni =
19.8, N= 0.02,
Mo=0.26,V=0.09,W=0.11,Cu=0.12,Co=0.4,Ti=0.01,A1=0.02,Nb=0.001,0=
0.0029, and cast into a test block. This test block was hot formed to produce
test material.
A solution annealing was carried out on the test material at 1060 C with a
subsequent
quenching in water. Thereafter specimens with the designation H 5 (unformed)
and
specimens with the designation H 525 (with a degree of cold forming of 35 %)
were
subjected to a long-term annealing at 600 C. The comparison materials Nos.
1.2083 and
1.4028 were hardened in oil from 1020 C, tempered at 630 C and likewise
subjected to the
long-term annealing. After 45, 90, 140 and 180 hours, the test material was
removed from
the oven, allowed to cool, and the hardness of the material was determined.
Thereafter, the
samples were reinserted (with a temperature cycle stress). The comparison
material H 5
showed an expected hardness behavior, whereas the material H 525 according to
the
invention, cold-formed at 35 %, exhibited an increased hardness of 315 HB and
a high
fatigue strength. At 600 C, no reduction of hardness and no creeping of the
material could
be detected even under alternating thermal stress. In contrast, a clear
decrease in hardness
was detected in the martensitic standard steels as a function of the annealing
time of the
samples.
(0047] It is noted that the foregoing examples have been provided merely for
the purpose
of explanation and are in no way to be construed as limiting of the present
invention. While
the present invention has been described with reference to an exemplary
embodiment, it is
understood that the words which have been used herein are words of description
and
illustration, rather than words of limitation. Changes may be made, within the
purview of
the appended claims, as presently stated and as amended, without departing
from the scope
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and spirit of the present invention in its aspects. Although the present
invention has been
described herein with reference to particular means, materials and
embodiments, the present
invention is not intended to be limited to the particulars disclosed herein;
rather, the present
invention extends to all functionally equivalent structures, methods and uses,
such as are
within the scope of the appended claims.
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