FIELD OF THE INVENTION
[0001] The present invention concerns an iron-based powder, in particular a stainless steel
powder, which is useful for powder injection molding; a composition for powder injection
molding; a method of making sintered components from the powder composition; and sintered
components made from the powder composition. Using the powder composition it may be
possible to obtain sintered parts with densities above 96% of the theoretical density,
thus resulting in excellent mechanical properties.
BACKGROUND OF THE INVENTION
[0002] Powder injection molding, also called metal injection molding (MIM) is an interesting
technique for producing high density sintered components of complex shapes. In general,
fine carbonyl iron powders are used in this process. Other types of powders used are
gas-atomized or water-atomized of very fine particle size, the cost of which is relatively
high. In order to improve the competitiveness of the MIM process it is desirable to
reduce the cost of the powder used. One way of achieving this is by utilizing coarser
powders. However, coarse powders have a lower surface energy than fine powders and
are thus much less active during sintering. Another issue is that the use of coarse
and irregular powders leads a lower packing density and thus the maximal powder content
of the feedstock is limited. Lower powder content results in a higher shrinkage during
sintering and may lead to,
inter alia, high dimensional scatter between components produced in a production run.
[0004] WO2012089807 discloses the use of a coarse powder which achieves a theoretical density of more
than 95%. There is still a need for technology which can achieve even higher density.
[0005] Normally, the solid loading (
i.e. the portion of iron- based powder) of an iron-based MIM feedstock (
i.e. the iron- based powder mixed with organic binder ready to be injected) is about 50%
by volume, which means that, in order to reach high density after sintering (above
93% of theoretical density), the green component must shrink almost by 50% by volume.
This is in contrast to PM components produced through uniaxial compaction which already
in green state obtain relatively high density. Therefore, fine powders having high
sintering activity are normally used in MIM. By elevating the sintering temperature
coarser powders may be used. This, however, results in grain coarsening which in turn
gives mechanical properties which are not optimal.
[0006] It has unexpectedly been found that a coarse metal powder, wherein the metal powder
has a certain composition, can be used in a feedstock for powder injection molding
in order to obtain components with a sintered density of at least 96% of the theoretical
density.
SUMMARY
[0007] An object of invention is to provide a metal injection molding feedstock composition
comprising said relatively coarse stainless steel powder composition.
[0008] Another object of the invention is to provide a method for producing injection molded
sintered components from the feedstock composition, said components having a density
of at least 96% of the theoretical density.
[0009] Still another object of the present invention is to provide a sintered component
produced according to the MIM process having a density of 96% and above, of theoretical
density and a tensile strength above 800MPa as sintered, without hardening.
[0010] At least one of these objects is accomplished by:
A metal injection molding feedstock comprising
- a) an iron based powder, having an median particle size of 25-45 µm, and 99% of the
particles less than 120 µm, wherein the iron-based powder comprises, by weight percent;
15-17%Cr; 3-5% Ni; 3-5%,Cu; 0.15-0.45% Nb; <1.0% Mn; <1.0% Si; less than 0.08% C,
balanced with Fe; and
- b) 30-65% by volume of the feedstock of a binder.
[0011] The particle size is determined by laser diffraction using a Sympatec Helos instrument.
The median particle size as defined above means that 50% of the particles in the powder
is larger than this value. This value is normally termed the "X50" value.
[0012] A method for producing a sintered component comprising the steps of:
- a) preparing a metal injection molding feedstock as suggested above;
- b) molding the feedstock into an unsintered blank;
- c) removing the organic binder;
- d) sintering the obtained blank in a reducing atmosphere at a temperature between
1 200-1 400° C.
- e) cooling the sintered component, and;
- f) optionally subjecting the component to post sintering treatment such as precipitation
hardening, case hardening, nitriding, carburizing, nitrocarburizing, carbonitriding,
induction hardening, surface rolling and/or shot peening.
[0013] A sintered component made from the feedstock composition, the component having a
density of at least 96% of theoretical density, and a tensile strength above 800MPa.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The stainless steel powder composition includes at least one iron based powder and/or
pure iron powder. The iron based powder and/or pure iron powder can be produced by
water or gas atomization of an iron melt and optionally alloying elements. The atomized
powder can further be subjected to a reduction annealing process, and optionally be
furthered alloyed by using a diffusion alloying process. Alternatively, iron powder
may be produced by reduction of iron- oxides.
[0015] The particle size of the iron- or iron- based powder composition is such that the
median particle size is 25-45 µm, preferably 25-35µm. Further, X
99 shall be at most 120µm, preferably at most 100 µm. (X
99 means that 99% of the particles have a particle size less than X
99)
[0016] Copper, Cu will enhance the strength and hardness through solid solution hardening.
Cu, will also facilitate the formation of sintering necks during sintering as copper
melts before the sintering temperature is reached providing so called liquid phase
sintering. The powder may optionally be admixed with Cu, preferably in the form of
Cu-powder in an amount of 0-5-wt%, or 3-5-wt%.
[0017] Other substances such as hard phase materials and machinability enhancing agents,
such as MnS, MoS
2, CaF
2, different kinds of minerals etc. may optionally be added to the iron based powder
composition.
[0018] The feedstock composition may be prepared by mixing the iron based powder composition
described above and a binder.
[0019] The binder in the form of at least one organic binder may be present in the feedstock
composition in a concentration of 30-65% by volume, preferably 35-60% by volume, more
preferably 40-55% by volume. When using the term binder in the present description
also other organic substances that are commonly in MIM-feedstocks are included, such
as e.g. releasing agents, lubricants, wetting agents, rheology modifiers, dispersant
agents. Examples of suitable organic binders are waxes, polyolefins, such as polyethylenes
and polypropylenes, polystyrenes, polyvinyl chloride, polyethylene carbonate, polyethylene
glycol, stearic acids and polyoxymethylen.
[0020] The feedstock composition is molded into a blank. The obtained blank is then heat
treated, or treated in a solvent or by other means to remove one part of the binder
as is known in the art, and then further subjected to sintering in a reducing atmosphere
in vacuum or in reduced pressure, at a temperature of about 1200-1400°C.
[0021] The sintered component may be subjected to a heat treatment process in order to obtain
a desired microstructure, e.g. by heat treatment and by controlled cooling rate. The
hardening process may include known processes such as precipitation hardening, quench
and temper, case hardening, nitriding, carburizing, nitrocarburizing, carbonitriding,
induction hardening and the like. Alternatively, a sinter-hardening process at high
cooling rate may be utilized.
[0022] Other types of post sintering treatments may be utilized, such as surface rolling
or shot peening which introduce compressive residual stresses enhancing the fatigue
life.
[0023] Sintered components according to the invention reach a sintered density of at least
96% of the theoretical density, and tensile strength above 800 MPa.
EXAMPLE 1
[0024] Iron based powder compositions according to Table 1 were prepared.
Table 1
Element |
A |
B |
D |
E |
C (comparative) |
Cr |
16.5 |
16.5 |
17 |
16.5 |
16.1 |
Ni |
4.09 |
4.3 |
4.3 |
4.09 |
13.3 |
Cu |
4 |
4.04 |
3.96 |
4 |
|
Nb |
0.37 |
0.37 |
0.47 |
0.37 |
|
Mn |
0.1 |
0.1 |
0.04 |
0.1 |
0.096 |
Si |
0.68 |
0.53 |
0.95 |
0.68 |
0.881 |
Mo |
|
|
|
|
2.12 |
C |
0.016 |
0.079 |
0.011 |
0.016 |
0.022 |
O |
0.351 |
0.433 |
0.146 |
0.351 |
0.236 |
N |
0.04 |
0.025 |
0.021 |
0.04 |
0.044 |
S |
0.007 |
0.006 |
0.003 |
0.007 |
0.009 |
Fe |
Bal |
Bal |
Bal |
Bal |
Bal |
X10 |
10.9 |
14.2 |
14.4 |
21.4 |
12.2 |
X50 |
24.4 |
32.6 |
31.0 |
35.0 |
26.4 |
X90 |
46.7 |
57.0 |
52.1 |
56.7 |
46.9 |
x99 |
72.2 |
79.8 |
86.8 |
104.0 |
66.9 |
Example 2
[0025] The compositions were compacted to a density about 4.5g/cm
3 (58% of theoretical density) into cylinders with a diameter 25mm and a height of
8mm and thereafter A, C and E were sintered at 1350°C in an atmosphere of 100%H
2 by volume, during 1200 minutes. Sample C was sintered at 1380°C, during 120 minutes,
100% H
2. Sintered density was measured using the water displacement method as described in
standard SS-EN ISO 3369:2010.
Table 2 shows the test results.
|
A |
C (comparative) |
E |
SD |
7.63 |
6.65 |
7.37 |
% of theoretical density |
98.2 |
83.4 |
95.0 |
Example 3
[0026] A feedstock containing the metal powder composition A, B, and D, respectively, were
prepared and compared with a feedstock made from composition C, by mixing the powder
compositions with an organic binder. The binder was composed of 47.5 % polyethylene,
47.5% paraffin wax and 5% stearic acid. All percentage in weight percentage. The organic
binder and the powder compositions were mixed in a ratio of the metal powder:binder
of 53:47 by volume.
[0027] The feedstock were injection molded into standard MIM tensile bars according to ISO-
SS EN ISO 2740 The samples were then debound in hexane for 4 hours at 60°C to remove
the paraffin wax, followed by sintering at 1350°C in an atmosphere of 100% hydrogen
for 120 minutes.
[0028] The Sintered Density were measured using the water displacement method. Tensile test
was tested according to SS EN ISO 2740. Results are shown in table 3. Standard values
were taken from ISO22068 and shows values for the standard alloys 17-4PH and 316L
in the sintered state. The mechanical properties are presented as % of standard value
in order to be able to compare two different alloys.
Table 3
|
A |
B |
D |
C |
|
Absolute value |
% of standard value |
Absolute value |
% of standard value |
Absolute value |
% of standard value |
Absolute value |
% of standard value |
Sintered Density (g/cm3) |
7,68 |
|
7,68 |
|
7,69 |
|
7,38 |
|
Hardness (HRC) |
27.8 |
103 |
26.4 |
98 |
29,8 |
110 |
58.2 |
49 |
Tensile strength (MPa) |
1129 |
141 |
1124 |
141 |
1086 |
135 |
286.5 |
64 |
Yield strength 0.2% (MPa) |
897 |
138 |
877 |
135 |
860 |
132 |
130.9 |
94 |
Elongation (%) |
2.9 |
97 |
2.84 |
95 |
1,4 |
47 |
22.81 |
57 |
1. Feedstock for metal injection molding, comprising;
a) an iron based powder, having an median particle size of 25-45 µm, and 99% of the
particles less than 120 µm, wherein the iron-based powder comprises, by weight percent;
15-17%Cr; 3-5% Ni; 3-5%,Cu; 0.15-0.45% Nb; <1.0% Mn; <1.0% Si; less than 0.08% C,
balanced with Fe; and
b) 30-65% by volume of the feedstock of a binder.
2. Feedstock according to claim 1 wherein the binder is in the form of an at least one
organic binder.
3. Feedstock according to claim 2 wherein the at least one organic binder is chosen from
the group of waxes, polyolefines, such as polyethylenes and polypropylenes, polystyrenes,
polyvinyl chloride, polyethylene carbonate, polyethylene glycol, stearic acids and
polyoxymethylen.
4. Use of a feedstock according to any of claims 1-3 for metal injection molding.
5. A method comprising the steps of:
a) preparing a metal injection molding feedstock according to any of claims 1-3,
b) molding the feedstock into an unsintered blank,
c) removing the organic binder
d) sintering the obtained blank in a reducing atmosphere at a temperature between
1 200-1 400°C
e) cooling the sintered component, and
f) optionally subjecting the component to post sintering treatment such as precipitation
hardening, case hardening, nitriding, carburizing, nitrocarburizing, carbonitriding,
induction hardening, surface rolling and/or shot peening.
1. Feedstock zum Metallpulverspritzgießen, welcher Folgendes umfasst:
a) ein Pulver auf Eisenbasis, das eine mittlere Partikelgröße von 25 bis 45 µm und
99 % der Partikel kleiner als 120 µm aufweist, wobei das Pulver auf Eisenbasis in
Gewichtsprozent umfasst:
15 bis 17 % Cr; 3 bis 5 % Ni; 3 bis 5 % Cu; 0,15 bis 0,45 % Nb; < 1,0 % Mn; < 1,0
% Si; weniger als 0,08 % C, der Rest bis auf 100 % ist Fe; und
b) 30 bis 65 Vol.-% des Feedstocks eines Binders.
2. Feedstock nach Anspruch 1, wobei der Binder in Form mindestens eines organischen Binders
vorliegt.
3. Feedstock nach Anspruch 2, wobei der mindestens eine organische Binder aus der Gruppe
von Wachsen, Polyolefinen, wie Polyethylenen und Polypropylenen, Polystyrolen, Polyvinylchlorid,
Polyethylencarbonat, Polyethylenglycol, Stearinsäuren und Polyoxymethylen ausgewählt
ist.
4. Verwendung eines Feedstocks nach einem der Ansprüche 1 bis 3 zum Metallpulverspritzgießen.
5. Verfahren, welches die Schritte umfasst:
a) Herstellen eines Metallpulverspritzguss-Feedstocks nach einem der Ansprüche 1 bis
3,
b) Formen des Feedstocks zu einem ungesinterten Rohling,
c) Entfernen des organischen Binders,
d) Sintern des erhaltenen Rohlings in einer reduzierenden Atmosphäre bei einer Temperatur
zwischen 1200 und 1400°C,
e) Abkühlen des Sinterformteils und
f) gegebenenfalls Unterziehen des Formteils einer Behandlung nach dem Sintern, wie
z.B. Ausscheidungshärten, Einsatzhärten, Nitrieren, Aufkohlen, Nitrocarburieren, Carbonitrieren,
Induktionshärten, Oberflächenwalzen und/oder Kugelstrahlen.
1. Charge d'alimentation pour le moulage par injection de poudre métallique, comprenant
:
a) une poudre à base de fer présentant une valeur médiane de la taille des particules
de 25 à 45 µm, 99 % des particules étant d'une taille inférieure à 120 µm, la poudre
à base de fer comprenant, en pourcentage en poids :
15 à 17 % de Cr ; 3 à 5 % de Ni ; 3 à 5 % de Cu ; 0,15 à 0,45 % de Nb ; < 1 % de Mn
; < 1 % de Si ; moins de 0,08 % de C, le reste étant du Fe ; et
b) un liant à une teneur de 30 à 65 % en volume de la charge d'alimentation.
2. Charge d'alimentation selon la revendication 1, dans laquelle le liant se présente
sous la forme d'au moins un liant organique.
3. Charge d'alimentation selon la revendication 2, dans laquelle l'au moins un liant
organique est choisi dans le groupe constitué des cires, des polyoléfines, telles
que les polyéthylènes et les polypropylènes, des polystyrènes, du polychlorure de
vinyle, du carbonate de polyéthylène, du polyéthylèneglycol, des acides stéariques
et du polyoxyméthylène.
4. Utilisation d'une charge d'alimentation selon l'une quelconque des revendications
1 à 3 pour le moulage par injection de poudre métallique.
5. Procédé comprenant les étapes consistant à :
a) préparer une charge d'alimentation de moulage par injection de poudre métallique
selon l'une quelconque des revendications 1 à 3,
b) mouler la charge d'alimentation afin d'obtenir une ébauche non frittée,
c) éliminer le liant organique,
d) fritter l'ébauche obtenue dans une atmosphère réductrice à une température comprise
entre 1 200 et 1 400 °C,
e) refroidir le composant fritté, et
f) soumettre éventuellement le composant à un traitement après frittage tel qu'un
durcissement par précipitation, un durcissement superficiel, une nitruration, une
cémentation, une nitrocarburation, une carbonitruration, une trempe par induction,
un laminage superficiel et/ou un grenaillage de précontrainte.