US20030033903A1

US20030033903A1 - High density stainless steel product and method for the preparation thereof

Info

Publication number: US20030033903A1
Application number: US09/963,651
Authority: US
Inventors: Anders Bergkvist; Sven Allroth; Paul Skoglund
Original assignee: Hoganas AB
Current assignee: Hoganas AB
Priority date: 2001-06-13
Filing date: 2001-09-27
Publication date: 2003-02-20
Also published as: KR100923604B1; CA2446225A1; CN1512926A; WO2002100581A1; JP2008248389A; ES2274040T3; EP1395383A1; MXPA03011533A; TW570850B; DE60216756T2; BR0210346B1; KR20040003062A; DE60216756D1; EP1395383B1; US20040062674A1; BR0210346A; JP2004528482A; CN1330444C; CA2446225C; US7311875B2

Abstract

The invention concerns a method of preparing compacts having a sintered density of above 7.3 g/cm^3.This method comprises the steps of subjecting an annealed, water-atomised, essentially carbon free stainless steel powder, which in addition to iron, comprises at least 10% by weight of chromium, not more than 0.4%, preferably not more than 0.3% by weight of oxygen, not more than 0.05%, preferably not more than 0.02% and most preferably not more than 0.015% of carbon, not more than 0.5% by weight of Si and not more than 0.5% of impurities, to HVC compaction with an uniaxial pressure movement with a ram speed of at least 2 m/s, and sintering the green body.

Description

FIELD OF THE INVENTION

This invention relates to the general field of powder metallurgy. Particularly the invention is concerned with high-density stainless steel products and a compacting and sintering operation for achieving such products.

BACKGROUND OF THE INVENTION

Currently used methods for preparing high density products, such as flanges, of stainless steel powders involve compacting the stainless steel powders to densities of between about 6.4 and 6.8 g/cm ³at compaction pressures of 600-800 MPa. The obtained green body is then sintered at high temperatures, i.e. temperatures between about 1250° C. and 1400° C. for 30 to 120 minutes in order to get densities of about 7.25 g/cm³. The requirement for the long sintering times at the comparatively high temperatures is of course a problem considering the high energy costs. The necessity for special, high temperature furnaces is another problem.

OBJECTS OF THE INVENTION

An object of the invention is to provide a solution to these problems and provide a method for the preparation of high-density products, particularly products having a sintered density above 7.25, preferably above 7.30 and most preferably above 7.35 g/cm ³.

A second object is to provide a compaction method adapted to industrial use for mass production of such high-density products.

A third object is to provide a process for the sintering of such compacted products requiring less energy.

A fourth object is to provide a process for sintering the stainless steel compacts to densities above about 7.25 g/cm ³which can be performed in conventional furnaces without need for special high temperature equipment.

A fifth object is to provide a process for the manufacturing of large sintered stainless steel PM products, such as flanges, having a relatively simple geometry.

SUMMARY OF THE INVENTION

In brief the method of preparing such high density products comprises the steps of

subjecting an annealed, water-atomised stainless steel to HVC compaction with a uniaxial pressure movement at an impact ram speed above 2 m/s;

and sintering the green body.

DETAILED DESCRIPTION OF THE INVENTION

The annealed stainless steel powder is preferably an essentially carbon free stainless steel powder which in addition to iron comprises at least 10% by weight of chromium, not more than 0.4%, preferably not more than 0.3% by weight of oxygen, not more than 0.05%, preferably not more than 0.02% and most preferably not more than 0.015% of carbon, at most 0.5% by weight of Si and not more than 0.5% of impurities. Such powders and the preparation thereof are described in the international patent publication WO 98/58093 that is hereby incorporated by reference.

Specifically the annealed powder could include, by percent of weight, 10-30% of chromium, 0-5% of molybdenum, 0-15% of nickel, 0-0.5% of silicon, 0-1.5% of manganese, 0-2% of niobium, 0-2% of titanium, 0-2% of vanadium and at most 0.3% of inevitable impurities and most preferably 10-20% of chromium, 0-3% of molybdenum, 0.1-0.3% of silicon, 0.1-0.4% of manganese, 0-2% of niobium, 0-0.5% of titanium, 0-0.5% of vanadium, 0-8% of tungsten and essentially no nickel or alternatively 7-10% of nickel.

A preferred annealed powder which may be used comprises, by percent of weight, 10-20% of chromium, 0-3% of molybdenum, 0.1-0.3% of silicon, 0.1-0.4% of manganese, 0-2% of niobium, 0-0.5% of titanium, 0-0.5% of vanadium, 0-8% of tungsten and essentially no nickel or, alternatively, 7-10% by weight of nickel, the balance being iron.

In order to obtain the products having the desired high density according to the present invention the compacting method is important. Normally used compaction equipment does not work quite satisfactorily, as the strain on the equipment will be too great. It has now been found that the high densities required may be obtained by the use of the computer controlled percussion machine disclosed in the U.S. Pat. No. 6,202,757 which is which is hereby incorporated by reference. Particularly, the impact ram of such a percussion machine may be used for impacting the upper punch of a die including the powder in a cavity having a shape corresponding to the desired shape of the final compacted component. When supplemented with a system for holding a die, e.g. a conventionally used die, and a unit for powder filling (which may also be of conventional type) this percussion machine permits an industrially useful method for production of high-density compacts. An especially important advantage is that, in contrast to previously proposed methods, this arrangement driven by hydraulics permits mass production (continuous production) of such high density components.

In the U.S. Pat. No. 6,202,757 it is stated that the use of the percussion machine involves “adiabatic” moulding. As it is not fully clarified if the compaction is adiabatic in a strictly scientific meaning and we have used the term high velocity compaction (HVC) for this type of compaction wherein the density of the compacted product is controlled by the impact energy transferred to the powder.

According to the present invention the ram speed should be above 2 m/s. The ram speed is a manner of providing energy to the powder through the punch of the die. No straight equivalence exists between compaction pressure in a conventional press and the ram speed. The compaction which is obtained with this computer controlled HVC depends, in addition to the impact ram speed, i.a. on the amount of powder to be compacted, the weight of the impact body, the number of impacts or strokes, the impact length and the final geometry of the component. Furthermore, large amounts of powder require more impacts than small amounts of powder. Thus the optimal conditions for the HVC compaction i.e. the amount of kinetic energy which should be transferred to the powder, may be decided by experiments performed by the man skilled in the art. Contrary to the teaching in the U.S. Pat. No. 6,202,757 there is, however, no need to use a specific impact sequence involving a light stroke, a high energy stroke and a medium-high energy stroke for the compaction of the powder. According to the present invention the strokes (if more than one stroke is needed) may be essential identical and provide the same energy to the powder.

Experiments with existing equipment has permitted ram speeds up to 30 m/s and, as is illustrated by the examples, high green densities are obtained with ram speeds about 10 m/s. The method according to the invention is however not restricted to these ram speeds but it is believed that ram speeds up to 100 or even up to 200 or 250 m/s may be used. Ram speeds below about 2 m/s does, however, not give the pronounced effect of densification. It is preferred that the ram speed above 3 m/s. Most preferably the ram speed is above 5 m/s.

The compaction may be performed with a lubricated die. It is also possible to include a suitable lubricant in the powder to be compacted. Alternatively, a combination thereof may be used. Alternatively the particles may be provided with a coating. This coating or film is achieved by mixing the powder composition, which includes the free or loose, non agglomerated powder particles with the lubricant, subjecting the mixture to an elevated temperature for melting the lubricant and subsequently cooling the obtained mixture during mixing for solidifying the lubricant and providing the powder particles or aggregates thereof with a lubricant film or coating. The lubricant can be selected among conventionally used lubricants such as metal soaps, waxes and thermoplastic materials, such as polyamides, polyimides, polyolefins, polyesters, polyalkoxides, polyalcohols. Specific examples of lubricants are zinc stearate, H-wax® and Kenolube®. The amount of lubricant may vary up to 1% by weight of the powder composition.

Furthermore the compaction may be performed at ambient or at elevated temperature e.g. between 90 and 180° C. In the latter case a pre-heated powder composition is subjected to compaction in a pre-heated die and the lubricant may be selected from lubricants specifically developed to this end. Examples of such warm compaction lubricants are disclosed in e.g. the U.S. Pat. No. 5,154,881 and 5,744,433.

Generally the subsequent sintering may be performed at a temperature between about 1120 and 1250° C. for a period between about 30 and 120 minutes. According to a preferred embodiment the sintering is performed in a belt furnace at temperatures below 1180° C., preferably below 1150° C. and most preferably below 1160° C. The invention is however not restricted to sintering at such low temperatures and by sintering at higher temperatures, such as up to 1400° C. even higher densities may be obtained. It is also preferred that the sintering atmosphere includes hydrogen. Preferably a hydrogen/nitrogen atmosphere should be used.

A particular advantage of the invention is that the compacts having near theoretical density may be sintered at low temperatures, such as 1120-1150° C., in conventional furnaces, such as belt furnaces. This is in contrast to conventional compaction methods where it is not possible to obtain such high green densities and where a high sintered density is obtained by high temperature sintering, which causes shrinkage of the compacts. By using the HVC compaction method with no or a very small amount of lubricant included in the powder composition to be compacted, the green density will be essentially identical with the sintered density. This in turn means that very good tolerances are obtained.

The method according to the invention permits the manufacture of green and sintered compacts having high density, such as densities above 96 or even above 98% of the theoretical density. For stainless steel powders having the usual chemical compositions this corresponds to densities above 7.25, 7.30 and even 7.35 g/cm ³

The invention as described in the present specification and the appended claims is believed to be of especial importance for large sintered stainless steel PM compacts having a comparatively simple geometry, such as flanges. Other products which may be of interest are gas-tight oxygen probes. The invention is, however, not limited to such products.

The invention is further illustrated by the following example:

EXAMPLE 1

Two powders, A and B respectively, having the compositions given in the following table were subjected to HVC compaction using a compaction machine Model HYP 35-4 from Hydropulsor AB, Sweden. [0025]

TABLE 1

Material C Ni Fe Mn Cr Si O N

A 0.01 0.1 Base 0.1 11.4 0.9 0.19 0.036

B 0.005 0.05 Base 0.1 11.2 0.13 0.20 0.001
A is a standard stainless powder [0026]
B is an annealed stainless powder which should be used according to the invention [0027]

The die was lubricated with zinc stearate dissolved in acetone. After drying 70 g of the powder was poured into the die. As can be seen from the following tables 2-5 the method of according to the present invention makes it possible to obtain stainless steel products having both green and sintered densities above 7.3 g/cm ³. These tables also shows the impact of the stroke length on the density. The stroke lengths, which varied between 10 and 70 mm, correspond to ram speeds between about 3 and about 8 m/s.

	TABLE 2


	Powder	Theor. density 7.75 g/cm³

A	Stroke	Green Density
Sample	length (mm)	(g/cm³)	Rel dens (%)

1	10	5.50	71.0
2	20	6.06	78.1
3	30	6.41	82.7
4	40	6.67	86.0
5	50	6.91	89.2
6	60	7.12	91.8
7	65	7.15	92.2
8	70	7.21	93.0

	TABLE 3


	Powder	Theor. density 7.78 g/cm³

B	Stroke	Green Density
Sample	length (mm)	(g/cm³)	Rel dens (%)

1	10	5.86	75.3
2	20	6.44	82.8
3	30	6.81	87.6
4	40	7.10	91.3
5	50	7.27	93.4
6	55	7.35	94.5
7	60	7.41	95.3
8	65	7.41	95.2

Sintered properties of Powder A are disclosed in the following table 3.

TABLE 3


	Temp		Sintered density	Rel. Dens.
Sample	(° C.)	time (min)	(g/cm³)	(%)

1	1250	30	5.56	71.7
2	1150	30	6.04	77.9
3	1250	30	6.52	84.2
4	1150	30	6.66	86.0
5	1250	30	7.02	90.6
6	1150	30	7.10	91.6
7	1250	30	7.22	93.2
8	1150	30	7.19	92.8

Sintered properties of Powder B are disclosed in the following table 4.

TABLE 4


	Temp		Sintered density	Rel. Dens.
Sample	(° C.)	time (min)	(g/cm³)	(%)

1	1250	30	5.93	76.3
2	1150	30	6.42	82.5
3	1250	30	6.87	88.3
4	1150	30	7.06	90.7
5	1250	30	7.33	94.2
6	1150	30	7.32	94.1
7	1250	30	7.45	95.8
8	1150	30	7.39	95.0

Claims

1. Method of preparing compacts having a high density comprising the steps of

subjecting an annealed, water-atomised, essentially carbon free stainless steel powder, which in addition to iron, comprises at least 10% by weight of chromium, not more than 0.4%, preferably not more than 0.3% by weight of oxygen, not more than 0.05%, preferably not more than 0.02% and most preferably not more than 0.015% of carbon, at most 0.5% by weight of Si and not more than 0.5% of impurities to HVC compaction with a uniaxial pressure movement with an impact ram speed above 2 m/s; and

sintering the green body.

2. Method according to claim 1 wherein the annealed powder comprises, by percent of weight

10-30% of chromium

0-5% of molybdenum

0-15% of nickel

0-0.5% of silicon

0-1.5% of manganese

0-2% of niobium

0-2% of titanium

0-2% of vanadium

0-8% of tungsten

and at most 0.3% of inevitable impurities, the balance being iron.

3. Method according to any one of the claims 1-2 wherein the annealed powder comprises, by percent of weight

10-20% of chromium

0-3% of molybdenum

0.1-0.45% of silicon

0.1-0.4% of manganese

0-2% of niobium

0-0.5% of titanium

0-0.5% of vanadium

0-8% of tungsten

and essentially no nickel the balance being iron.

4. Method according to any one of the claims 1-2 wherein the annealed powder comprises comprising, by percent of weight,

10-20% of chromium

0-3% of molybdenum

0.1-0.3% of silicon

0.1-0.4% of manganese

0-2% of niobium

0-0.5% of titanium

0-0.5% of vanadium

0-8% of tungsten and 7-10% of nickel the balance being iron.

5. Method according to any one of the claims 1-4 characterised in that the compaction is performed at a ram speed above 3, preferably above 5 m/s.

6. Method according to any one of the claims 1-5 characterised in that the compaction is performed as warm compaction.

7. Method according to any one of the preceding claims for the preparation of compacts having a density above about 96%, preferably above 98% of the theoretical density.

8. Method according to any one of the claims 1 to 7 wherein the sintering is performed at a temperature between about 1120 and 1250° C. for a period between about 30 and 120 minutes.

9. Method according to any one of the claims 1 to 8 characterised in that the sintering is performed in a belt furnace at temperatures below 1250° C., preferably, below 1200° C. and most preferably below 1160° C.

10. Method according to any one of the claims 1 to 10 wherein the sintering atmosphere includes hydrogen.

11. Compacted and sintered stainless steel products, such as flanges, having a sintered density of above 7.25 g/cm³, preferably above 7.30 g/cm³and comprising at least 10% by weight of chromium, not more than 0.05%, preferably not more than 0.02% and most preferably not more than 0.015% of carbon, and not more than 0.5% by weight of silicon.

12. Compacted and sintered bodies, according to claim 11 having a composition, by percent of weight, of

10-30% of chromium

0-5% of molybdenum

0-15% of nickel

0-0.5% of silicon

0-1.5% of manganese

0-2% of niobium

0-2% of titanium

0-2% of vanadium

0-8% of tungsten

the balance being iron and at most 0.3% of inevitable impurities

13. Compacted and sintered bodies according to claim 12 including

10-20% of chromium

0-3% of molybdenum

0.1-0.3% of silicon

0.1-0.4% of manganese

0-2% of niobium

0-0.5% of titanium

0-0.5% of vanadium

0-8% of tungsten

and essentially no nickel, the balance being iron and at most 0.3% of inevitable impurities.

14. Compacted and sintered bodies according to claim 12 including

10-20% of chromium

0-3% of molybdenum

0.1-0.3% of silicon

0.1-0.4% of manganese

0-2% of niobium

0-0.5% of titanium

0-0.5% of vanadium

0-8% of tungsten

and 7-10% of nickel, the balance being iron and at most 0.3% of inevitable impurities