CN108367356B

CN108367356B - Iron-based powder for powder injection molding

Info

Publication number: CN108367356B
Application number: CN201680072717.4A
Authority: CN
Inventors: A·拉松
Original assignee: Hoganas AB
Current assignee: Hoganas AB
Priority date: 2015-10-15
Filing date: 2016-10-05
Publication date: 2020-10-27
Anticipated expiration: 2036-10-05
Also published as: TW201728769A; EP3362210B1; WO2017063923A1; EP3156155A1; TWI714649B; ES2808207T3; EP3362210A1; CN108367356A; DK3362210T3

Abstract

The invention relates to a feedstock for metal injection moulding comprising a coarse stainless steel powder having a median particle size of 20-60 μm and 99% of the particles being smaller than 120 μm and a binder, wherein the iron-based powder comprises, in 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.

Description

Iron-based powder for powder injection molding

Technical Field

The present invention relates to iron-based powders, particularly stainless steel powders, useful for powder injection molding; a composition for powder injection molding; a method for producing a sintered part from the powder composition; and a sintered part made from the powder composition. Using this powder composition it is possible to obtain sintered parts having a density above 96% of the theoretical density, thus resulting in excellent mechanical properties.

Background

Powder injection molding, also known as Metal Injection Molding (MIM), is a useful technique for manufacturing high density sintered parts of complex shapes. Generally, fine carbonyl iron powder is used in this process. Other types of powders used are gas-or water-atomized with very fine particle sizes, which are relatively costly. To improve the competitiveness of MIM processes, it is desirable to reduce the cost of the powders used. One way to achieve this is to use coarser powders. However, coarser powders have lower surface energy than fine powders and are therefore much less active during sintering. Another problem is that the use of coarse and irregular powders results in a lower bulk density, thus limiting the maximum powder content of the feedstock. Lower powder contents lead to higher shrinkage during sintering and may in particular lead to high dimensional dispersion between the parts produced in the production flow.

WO2012089807 discloses the use of a coarse powder that achieves a theoretical density of more than 95%. There is still a need for techniques that can achieve higher densities.

Typically, the solid loading (i.e. the iron-based powder fraction) of the iron-based MIM raw material (i.e. the iron-based powder mixed with the organic binder to be injected) is about 50 vol%, which means that in order to reach a high density after sintering (93% of the theoretical density) the green part has to shrink by almost 50 vol%. This is in contrast to PM parts made by uniaxial compaction, which have achieved relatively high densities in the green state. Fine powders with high sintering activity are therefore commonly used in MIM. By increasing the sintering temperature, coarser powders may be used. But this causes grain coarsening, which in turn leads to non-optimal mechanical properties.

It has been surprisingly found that metal powder powders whose metal powder has a specific composition can be used in the raw material for powder injection molding to obtain parts having a sintered density of at least 96% of the theoretical density.

SUMMARY

It is an object of the present invention to provide a relatively coarse stainless steel powder composition with a low amount of alloying elements suitable for metal injection molding.

It is another object of the present invention to provide a metal injection molding feedstock composition comprising the relatively coarse stainless steel powder composition.

It is another object of the invention to provide a method for producing an injection molded sintered part from said raw material composition, said part having a density of at least 96% of the theoretical density.

It is a further object of the invention to provide sintered components made according to the MIM process that have a density of 96% and higher of the theoretical density and a tensile strength higher than 800MPa when sintered without hardening.

At least one of these objectives is achieved as follows:

an iron-based powder composition for metal injection moulding having a median particle size of 20-60 μm, preferably 20-45 μm, most preferably 25-45 μm or more preferably 25-35 μm. Particle size was determined by laser diffraction using a Sympatec Helos instrument. Median particle size, as defined above, means that 50% of the particles in the powder are greater than this value. This value is commonly referred to as the "X50" value.

A metal injection moulding feedstock composition comprising an atomised iron-based powder composition having a median particle size of 20-60 μm, preferably 20-45 μm, most preferably 25-45 μm or more preferably 25-35 μm and an organic binder.

A method of manufacturing a sintered component comprising the steps of:

a) preparing a metal injection molding raw material as set forth 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 of 1200-1400 ℃;

e) cooling the sintered part, and;

f) the component is optionally subjected to a post-sintering treatment such as precipitation hardening, case hardening, nitriding, carburizing, nitrocarburizing (carbonitriding), carbonitriding (carbonitriding), induction hardening, surface rolling, and/or shot peening.

A sintered part made from the feedstock composition, the part having a density of at least 96% of theoretical density and a tensile strength of greater than 800 MPa.

Detailed Description

The stainless steel powder composition comprises at least one iron-based powder and/or a pure iron powder. The iron-based powder and/or the pure iron powder may be produced by water atomization or gas atomization of an iron melt and optionally alloying elements. The atomized powder may be further subjected to a reduction annealing process and optionally further alloyed using a diffusion alloying process. Alternatively, the iron powder may be produced by reduction of iron oxide.

The particle size of the iron or iron-based powder composition is such that: the median particle size is from 20 to 60 μm, preferably from 20 to 45 μm, most preferably from 25 to 45 μm, more preferably from 25 to 35 μm. Further, X₉₉Preferably it should be at most 120 μm, preferably at most 100 μm (X)₉₉Means that 99% of the particles have a size less than X₉₉Particle size of (d).

Copper Cu will enhance strength and hardness through solution hardening. Cu also facilitates the formation of sintering necks during sintering, since copper melts before the sintering temperature is reached to provide a so-called liquid phase sintering. The powder may optionally be mixed with Cu, preferably in the form of a Cu powder, in an amount of 0-5 wt% or 3-5 wt%.

Other substances, such as hard phase materials and machinability enhancing agents, such as MnS, MoS, may optionally be added₂、CaF₂Different kinds of minerals etc. are added to the iron-based powder composition.

The raw material composition may be prepared by mixing the above-described iron-based powder composition and a binder.

The at least one binder in the form of an organic binder may be present in the raw material composition in a concentration of 30-65 vol%, preferably 35-60 vol%, more preferably 40-55 vol%. When the term binder is used in this specification, other organic substances which are customary in MIM raw materials are also included, such as mold release agents, lubricants, wetting agents, rheology modifiers, dispersants. Examples of suitable organic binders are waxes, polyolefins, such as polyethylene and polypropylene, polystyrene, polyvinyl chloride, polyethylene carbonate, polyethylene glycol, stearic acid and polyoxymethylene.

The feedstock composition is molded into a preform. The resulting green body is then heat treated, either in a solvent or by other means to remove a portion of the binder as is known in the art, and then further sintered in a reducing atmosphere in vacuo or reduced pressure at a temperature of about 1200-1400 ℃.

The sintered component may be subjected to a heat treatment process, for example by heat treatment and by controlled cooling rates, to obtain the desired microstructure. The hardening process may include known processes such as precipitation hardening, quenching and tempering, case hardening, nitriding, carburizing, nitrocarburizing, carbonitriding, induction quenching, and the like. Alternatively, a sinter hardening process at high cooling rates may be employed.

Other types of post-sintering treatments may be used, such as surface rolling or shot peening, which introduces compressive residual stresses to enhance fatigue life.

The sintered component according to the invention achieves a sintered density of at least 96% of the theoretical density and a tensile strength of more than 800 MPa.

Example 1

An iron-based powder composition according to table 1 was prepared.

TABLE 1

Element(s)	A	B	D	E	C (contrast)
						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

The composition was compacted to a density of about 4.5g/cm3 (58% of theoretical density) to make a cylinder having a diameter of 25mm and a height of 8mm, after which A, C and E were at 100% by volume H₂In an atmosphere of 1350 c for 1200 minutes. Sample C was sintered at 1380 ℃ for 120 minutes, 100% H₂. The 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 (contrast)	E
				SD	7.63	6.65	7.37
% of theoretical density%	98.2	83.4	95.0

Example 3

By mixing the powder compositions with an organic binder, raw materials containing metal powder compositions A, B and D, respectively, were prepared and compared to the raw material made from composition C. The adhesive consisted of 47.5% polyethylene, 47.5% paraffin wax and 5% stearic acid. All percentages are by weight. The organic binder and powder composition were mixed at a metal powder to binder volume ratio of 53: 47.

The feedstock was injection molded into a standard MIM tensile bar according to ISO-SS EN ISO 2740. The samples were then debinded in hexane at 60 ℃ for 4 hours to remove paraffin followed by sintering at 1350 ℃ for 120 minutes in a 100% hydrogen atmosphere.

Sintered density was measured using a water displacement method. Tensile testing was performed according to SS EN ISO 2740. The results are shown in table 3. The standard values are taken from ISO22068 and show the values of standard alloy 17-4PH and 316L in the sintered state. The mechanical properties are presented as% of the standard value to enable comparison of two different alloys.

TABLE 3

Claims

1. A feedstock for metal injection molding comprising:

a) an iron-based powder having a median particle size of 20-60 μm and 99% of the particles being smaller than 120 μm, wherein the iron-based powder comprises, in 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; and

b) a binder present in the feedstock at a concentration of 30-65% by volume.

2. Use of a feedstock according to claim 1 for metal injection moulding.

3. Use according to claim 2, comprising the steps of:

a) preparing a metal injection molding material according to claim 1,

b) the raw material is molded into an unsintered blank,

c) the organic binder is removed and the organic binder is removed,

d) sintering the obtained blank in a reducing atmosphere at a temperature of 1200-1400 ℃ to obtain a sintered part,

e) cooling the sintered part, and

f) optionally, the part is post-sintered.

4. Use according to claim 3, wherein the post-sintering treatment is precipitation hardening, case hardening, nitriding, carburizing, nitrocarburizing, carbonitriding, induction quenching, surface rolling and/or shot peening.

5. Sintered component made according to claim 3 or 4, having a density of at least 96% of the theoretical density.

6. Sintered component according to claim 5, having a tensile strength higher than 800 MPa.