EP1680250A1

EP1680250A1 - Method for the manufacturing of sintered metal parts having a densified surface

Info

Publication number: EP1680250A1
Application number: EP04775563A
Authority: EP
Inventors: Linnea Forden; Sven Bengtsson
Original assignee: Hoganas AB
Current assignee: Hoganas AB
Priority date: 2003-10-17
Filing date: 2004-10-15
Publication date: 2006-07-19
Also published as: CN1856379A; SE0302763D0; CA2541855A1; KR20060109914A; TW200534942A; RU2311263C1; WO2005037466A1; JP2007508460A

Abstract

The present invention concerns a method for producing powder metal parts having a densified surface. The method comprises the steps: - compacting an iron-based powder by high velocity compacting technique to a density above 7.2 g/cm3 - sintering the parts -subjecting the parts to a surface densifying process.

Description

Method for the manufacturing of sintered metal parts having a densified surface

FIELD OF THE INVENTION

The invention relates to a method for the manufacturing of sintered metal parts. Specifically the invention concerns a method for the manufacturing of sintered metal parts having a densified surface.

BACKGROUND OF THE INVENTION

One area of future growth in the utilization of powder metal parts is in the automotive industry. Of special in- terest within this field is the use of powder metal parts in more demanding applications, such as power transmission applications, for example, gear wheels. Problems with gear wheels formed by the powder metal process are that powder metal gear wheels have reduced bending fa- tigue strength in the tooth root region of the gear wheel, and low contact fatigue strength on the tooth flank compared with gears machined from bar stock or for- gings . These problems may be reduced or even eliminated by plastic deformation of the surface of the tooth root and flank region through a process commonly known as surface densification . Products having a densified surface are described in e.g. the US patents 5 711 187 and 6 171 546.

According to US 5 711 187 a gear wheel is formed from a pressed and sintered powder metal blank, the surface of which is hardened by rolling. It is taught that the sintering is performed at high sintering temperatures up to 1350°C, i.e. at high sintering temperatures. No specific example concerning the exact temperatures used for the sintering are disclosed but normally the term "high sintering temperatures" means that the sintering is performed at about 1250°C. In addition to the high energy consumption the high sintering temperatures will nega- tively affect dimension tolerances of the sintered parts, which may lead to tolerance problems of the rolled part.

Also the US 6 171 546 discloses a method for obtaining a densified surface. According to this patent the surface densification is obtained by rolling or, preferably, by shot peening of a green body of an iron-based powder. From this patent it can be concluded that the most interesting results are obtained if a pre-sintering step is performed before the final densification and sintering operations. According to this patent the sintering can be performed at 1120°C, i.e. at conventional sintering temperatures, but as two sintering steps are recommended the energy consumption will be quite considerable.

A simple and cost effective method requiring minimal dimensional change between the green and sintered compact for the preparation of gear wheels and similar products would thus be attractive and the main object for the pre- sent invention is to provide such a method. Another aspect of the invention is that lower energy consumption and accordingly lower energy costs may be achieved.

SUMMARY OF THE INVENTION In brief it has now been found that by using a method involving the steps of compacting an iron or iron-based powder by high velocity compacting technique to a density above 7.2 g/cm³, low temperature sintering the obtained parts at and subsequently subjecting the parts to a sur- face densifying process such products having a densified surface can be produced. Powder types

Suitable metal powders which can be used as starting materials for the compaction process are powders prepared from metals such as iron. Alloying elements such as car- bon, chromium, manganese, molybdenum, copper, nickel, phosphorous, sulphur etc can be added as particles, pre- alloyed or diffusion alloyed in order to modify the properties of the final sintering product. The iron-based powders can be selected from the group consisting of sub- stantially pure iron powders, pre-alloyed iron-based particles, diffusion alloyed iron-based iron particles and mixture of iron particles or iron-based particles and alloying elements. Most preferable powders are prealloyed iron-based powders due to their high hardenability.

Compaction and sintering

According to the present invention high velocity compaction, HVC, is used in order to obtain the products having the desired high density and narrow dimensional toler- ances . An example of an equipment for HVC-compaction is the computer controlled percussion machine disclosed in US patent 6,207,757 which is referred to above and 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. Preferably a ram speed above 2 m/s is used in order to reach densities above 7.2 g/cm³.

The sintering according to the present invention is performed as low temperature sintering, i.e. below 1200°C, preferably below 1160°C and most preferably between 1120°C and 1160°C. Any conventional sintering furnace may be used and the sintering times may vary between about 15 and 60 minutes. The atmosphere of the sintering furnace may be an endogas atmosphere, a mixture between hydrogen and nitrogen or in vaccuum.

Important features of the inventive method in order to reach the advantages mentioned above are thus that the density of the compacted part is at least about 7.2 g/cm³ and that the sintering can be performed at low temperatures .

Previously known methods of achieving high density of the sintered part are high temperature sintering or double pressing and double sintering.

The possibility of utilizing low sintering temperatures will reduce the energy consumption compared with that required for high temperature sintering. Additionally, the dimensional scatter of the part after low temperature sintering, e.g. within the temperature range of 1120°C to 1160°C, is significantly smaller compared with the dimensional scatter after high temperature sintering. Narrow dimensional tolerances on the sintered blank are essen- tial in order to reach a high quality of the surface densified part.

In comparison with methods involving double pressing and double sintering the method according to the present in- vention has the advantage that one pressing step and one sintering step are eliminated.

Additionally, by using this HVC technique for achieving a high green density it is possible to improve not only the mechanical properties of the final sintered part but also to increase the surface densifying depth. Surface densification

The surface densification may be perfomed by radial or axial rolling, shoot peening, sizing etc. A preferred method is radial rolling as this method provides short cycle times in combination with great densification depth. By the method of the invention a preferred densi- fication depth of up to 1.5 mm, 2 mm and 3 mm or even higher can be obtained on cylindrical parts. For more complex parts such as gears, the achieved densification depth is at least 0.3 mm, preferably at least 0.5 mm. The powder metal parts will obtain better mechanical properties with increasing densifying depth.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 is a diagram showing the relationship between sintered density and the surface densifying depth.

Fig 2a is a photomicrograph of a conventional compacted and surface densified sample.

Fig 2b is a photomicrograph of a high velocity compacted and surface densified sample.

Fig 3 is a diagram showing the standard deviation for the dimensional change between green and sintered components.

The invention is further illustrated by the following non-limiting examples.

Example 1

Cylinders were compacted from a powder metallurgical com- position consisting of the pre-alloyed powder Astaloy Mo with a graphite addition of 0,3 % and amide wax lubricant. For obtaining green densities above 7.2 g/cm³ high velocity compaction were used and for densities up to 7.2 g/cm³ conventional uniaxial compaction were used. The cylinders were sintered at 1120°C, 30 minutes in an atmosphere of 90 % nitrogen and 10 % hydrogen. Surface den- sification were performed by radial rolling and the diameter reduction during rolling was 0.3 mm. Densification depths were evaluated using image analysis, and is defined as the shortest distance from the surface to the point where the density has decreased to 98 % of theo- retical density.

The relationship between sintered density of the blank and the acquired densification depth of the densified part can be seen in figure 1.

Example 2

In figure 2 photomicrographs of compacted, sintered and surface densified cylinders are shown. The cylinder in fig 2a was conventionally compacted to a density of 7.0 g/cm³ and the achieved densified depth is 1 mm, while the cylinder shown in fig 2b was high velocity compacted to a density of 7.5 g/cm³ and the achieved densified depth is 2 mm.

Example 3 High velocity compacted cylinders, sintered at 1120°C for

30 minutes in an atmosphere of 90/10 N₂/H₂ were surface densified by rolling in a two-roll burnishing machine.

The diameter reduction during rolling was 0.3 mm.

Polished cross sections of the rolled cylinders were investigated in light optical microscope and the densification depths were evaluated using image analysis.

The acquired densification depths of the rolled cylinders are shown in table 1. Variant Density Densification # (g/cm³) depth (mm)

A 7.49 2.5

B 7.48 2.5

C 7.42 2.3

D 7.42 2.0

Table 1

Example 4

Gears with the gear data shown in table 2 were compacted and sintered using two different manufacturing routes:

1) High velocity compaction to a green density of 7.2, sintering at 1120°c for 30 minutes

2) Uniaxial compaction to a green density of 7.1, sintering at 1300°c for 120 minutes

No of teeth 18

Module (m_n) 1.5875 mm

Pitch diameter (d) 28.575 mm

Table 2

The gears manufactured by route 1 reached a sintered density of 7.27 and the gears manufactured by route 2 reached a sintered density of 7.36.

The dimensional change from green to sintered state was measured on 20 gears from each manufacturing route. The standard deviation in dimensional change on four different dimensions are presented in figure 3.

Claims

1. Method for producing powder metal parts having a densified surface, comprising the steps of

- compacting an iron or iron-based powder by high velocity compacting technique to a density above 7.2 g/cm³

- sintering the parts

- subjecting the parts to a surface densifying process.

2. Method according to claim 1, wherein sintering is performed at low temperature.

3. Method according to claim 1 or 2, wherein surface den- sifying is performed by rolling.

4. Method according to any one of the claims 1 to 3, wherein the parts are compacted to a density of at least 7.3 g/cm³ preferably at least 7.4 g/cm³.

5. Method according to any one of the claims 1 to 4, wherein the parts are sintered at a temperature below 1160°C.

6. Method according to claim 5, wherein the compacted parts are sintered in a temperature between 1120°C and 1150°C.

7. Method according to any one of the claims 1 to 6, wherein the compacted parts are sintered for a time of 15 to 60 minutes.

8. Method according to any one of the claims 1 to 7 , wherein the compacted parts are sintered in an endogas atmosphere, a mixture between hydrogen and nitrogen or in vaccuum.

9. Method according to any one of the claims 1 to 8, wherein the surface densified parts are densified to a depth of at least 0.3 mm, preferably at least 0.5 mm.

10. Method according to any one of the claims 1 to 9, wherein the produced powder metal parts are gears, bearings, rolls, sprockets etc.