GB2210894A - Sintered materials - Google Patents

Abstract

Sintered ferrous materials have a composition expressed in wt% lying within the ranges: C 0.4-1.5/W 2-4/Mo 1.5-4/V 2-4/Cr 2.5-5/Others 3 max/Fe balance. Up to 1% S may be present, with or without 4-6% Copper.

Description

SINTERED MATERIALS The present invention relates to materials for engineering components and particularly to components produced by a powder metallurgy route for use in, for example, internal combustion engines.

Valve seat inserts produced by a powder metallurgy route from powdered tool steel alloys are known.

One known alloy has the composition range in weight per cent C 0.6-1.5/W 4-6/Mo 4-6/V 2-3/Cr 2.5-4/Cu 15-25/Others 2 max/Fe balance, the copper. being added by infiltration of a copper alloy during sintering of the pressed powder. Such highly alloyed materials have desirable properties such as, for example, retention of hardness and strength at high temperatures. These properties are especially important for exhaust valve seat inserts in internal combustion engines as they are associated with wear-resistance. Such alloys also have the strength necessary to withstand the forces imposed upon them during fitting of the insert into a cylinder head.

These alloys are also hard and, therefore, powders made from them do not have a very high compressibility at the levels of compacting pressure normally employed.

Additional disadvantages of such alloys and the procedures required to manufacture components from them relate to cost; they employ relatively high levels of costly alloying elements and the process step of infiltration with a copper-based alloy is also expensive.

Tungsten, chromium, vanadium and molybdenum are employed variously to improve temper-restistance, hot hardness, hot strength and wear-resistance.

An example of a production route for a prealloyed tool steel powder might typically be compacting to about 75% of full theoretical density then simultaneously sintering and infiltrating with a copper alloy under a protective atmosphere.

It is an object of the present invention to provide an alloy composition for the manufacture of engineering components such as, for example, valve seat inserts and valve guides and which possesses the necessary properties for arduous implications.

It is a further object of the invention to provide an alloy composition which is both intrinsically cheaper than some known alloys and from which it is cheaper to produce components due to simplified manufacturing procedures. It is yet another object of the invention to provide a material having higher compressibility than some known powders in order that higher pressing densities may be attained to remove the need for infiltration during processing.

It is, however, pointed out that the alloys of the present invention may be infiltrated with copper or a copper-based alloy and for many applications have advantageous properties when so infiltrated.

According to the present invention an alloy for the manufacture of an engineering component by a powder metallurgy route comprises the following composition, expressed in weight per cent: C 0.41.5/W 2-4/Mo 1.5-4/V 2-4/Cr 2.5-5/Others 3 max/Fe balance.

Preferably the carbon content may be in the range 0.6 to 1.1%. The carbon may not all be combined in the prealloyed powder. The powder blend prior to compacting may, for example, contain about 0.5% carbon in prealloyed form and about 0.3t carbon in the form of graphite.

A preferred range of composition expressed in weight % comprises: C 0.6-1.1/W 2.4-3.6/Mo 2.0-3.6/V 2.4-3.6/Cr 3.3-4.7/Others 3 max/Fe balance.

An even more preferred range of composition expressed in weight % comprises: C 0.6-1.1/W 2.63.3/Mo 2.7-3.2/V 2.7-3.6/Cr 3.4-4.5/Others 3 max/Fe balance.

The material may optionally contain up to 1.0 sulphur as an aid to machinability. Sulphur may, for example, be added as elemental sulphur or prealloyed into the ferrous base powder.

The material may further comprise additions of up to 5 of metallic sulphides which may include, for example, molybdenum disulphide or manganese sulphide.

Such additions may be made for their beneficial effect on wear-resistance, solid lubrication and machinability. Additions may be made at the powder blending stage but, however, the resulting sintered material will comprise a complex sulphide structure owing to diffusion effects between constituents during wintering.

The alloy powder may be produced by any known method such as, for example, gas or water atomisation.

Due to the lower levels of carbon and other alloying additions, especially where some of the carbon is added as graphite, powders of the present invention possess superior compressibility than other known alloys and thus may be compacted to higher initial densities and obviate the need for infiltration.

It is intended that the alloys of the present invention may be compacted to green densities in excess of 80% of theoretical density and preferably in excess of 85go.

It will be appreciated, as noted above, that the present alloys may be infiltrated if desired and that there is no technical reason why this could not be done. Infiltration may be effected with any known alloy used for this purpose. Infiltration may be successfully accomplished at compacted densities of substantially greater than 85% of theoretical although this is conditional on the presence of interconnected porosity. Much lower densities of material may be infiltrated.

Where the material is manufactured in the uninfiltrated condition an addition of 4 to 6 weight per cent of copper powder may optionally be added to the initial powder mixture to function as a sintering aid.

Articles produced in the material of the present invention may be thermally processed after sintering or sintering and infiltration. Such thermal treatment may comprise a cryogenic treatment such as, for example, in liquid nitrogen followed by a heat treatment in the range 5750C to 7100 C.

In order that the present invention may be more fully understood some examples will now be described by way of illustration only.

An alloy powder according to the present invention and having a composition in wt of C 0.51-W 2.93 Mo 2.96/V 2.86/Cr 3.95/S 0.33/Fe balance was produced by water atomisation. The powder was processed according to known methods and a size fraction of minus 80 B.S. Mesh was blended with an additional 0.28 wt% of graphite powder and 1% of a wax to assist compaction. Samples for testing were produced to a theoretical density of 87% by pressing at a pressure of 1035 MN/m . The samples were sintered at a temperature of 11000C in a nitrogen based protective atmosphere, followed by thermal processing as described above.

Tests were carried out to measure the Young's Modulus, 0.2t compressive proof stress and hardness at 25 0C, 3000C and 5000C. These tests were also conducted on three other known materials. The compositions of the tested materials are shown in Table 1 below.

TABLE 1 MATERIAL COMPOSITION (wt.%) ELEMENT 1 2 3 4 Fe Bal. Bal. Bal. Bal.

Cr 3.95 3.02 4.1 29.0 Mo 2.96 4.51 7.2 V 2.86 2.56 1.84 0.62 W 2.93 6.66 6.73 15.0 Co - - - 10.0 Cu - 19.4 Ni - - - 39.0 C 0.78 0.83 1.44 2.35 S 0.33 - - Si - - 0.81 Material 1 is an alloy according to the present invention.

Material 2 is a known alloy produced by powder metallurgy methods.

Material 3 is a known alloy produced by casting.

Material 4 is a known alloy produced by casting.

The results of the mechanical tests are shown below in Table 2.

TABLE 2

MATERIAL PROPERTY TEMP. C 1 2 3 4 Youngs 20 136 181 234 235 Modulus 300 , 128 ' 169 217 225 (GPa) 500 117 147 197 210 0.2% Com- 20 1557 1372 1088 1205 press vie Proof 300 1343 1173 919 1176 Stress I (MPa) 500 1118 : 985 ' 788 1081 Hardness 20 2 66.7 62.8 64.6 68.1 Hr30N 300 63.3 56.7 57.1 64 500 58.9 50.0 51.0 63 Apart from the Young's Moduli the other mechanical properties, however, are in most cases higher than the other materials with the exception of the hardness values for Material 4 which are slightly, though not significantly higher. Material 4, as may be seen from Table 1, is a very expensive alloy in intrinsic metal value having high levels of nickel, cobalt, tungsten and chromium. Materials 2 and 3 also have relatively high levels of molybdenum and tungsten.

Further materials according to the present invention have been prepared having the compositions as given in Table 3 below for Materials 5 to 9, the production parameters for these materials also being given in tabulated form.

TABLE 3

EXAMPLE 5 6 7 8 9 %C 0,82 0,60 0,82 0,51 0,51 %W 3,10 3,24 3,10 2,93 2,93 %MO 2,85 3,12 2,85 2,96 2.96 %V 3,03 3,60 3.03 2,86 2,86 %Cr 4,12 3,47 4.12 3,95 3,95 %S 0,33 0,33 % Graphite 0,41 0,28 0,28 %S 0,43 % MoS2 3,50 % MnS 0,35 B. S. Mesh Size -80 -100 -80 -80 -80 (Prealloy) Compacting 925 925 1000 1050 1000 Pressure (MPa) Green Density 85 86 85 87 87 (% theoretical) Sintering 1100 1120 1150 1100 1120 Temperature ( C) Sintering Nitrogen Dissociated Nitrogen Nitrogen Nitrogen Atmosphere Base Ammonia Base Base Base Material 5 is similar to Material 1 but all the carbon is in the form of combined carbon in a prealloyed powder. Material 6 has a slightly different composition to Material 1 but has been produced using different processing parameters. Material 7 is similar to Material 5 but has a sulphur addition blended with the prealloyed powder.Materials 8 and 9 utilise the powder blend of Material 1 with additions of molybdenum disulphide and manganese sulphide respectively.

Material 10 of composition: C 0.91/W 3.58/Mo 2.93/V 2.88/Cr 3.61/S 0.26/Mn 0.30/Ni 0.19/Co 0.36/Cu 0.25/Fe balance was prepared and manufactured into exhaust valve seat inserts for a 1.8 litre, 4 cylinder engine. The engine was run for 60 hours at 6000 rev./min. At the end of the test the wear on both the valve seat inserts and the valves was measured.

The valve seat inserts were produced to a mean density of 7gm/cc. and a hardness of 68 HRA. The valve material was non-stellited. Mean wear for the inserts was slightly under 20 micrometres and mean wear of valves was 7 micrometres. A comparative engine test using the original parts fitted to the production engine gave mean insert wear of 31 micrometres and valve wear of 7 micrometres. The acceptance limit of the engine manufacturer is that insert wear in this test should not exceed 100 micrometres.

Claims (18)

1. A sintered ferrous material having a composition, expressed in wt% lying within the ranges: C 0.41.5/W 2-4/Mo 1.5-4/V 2-4/Cr 2.5-5/Others 3 max/Fe balance.

2. A sintered material according to Claim 1 having a composition expressed in wt% lying within the ranges: C 0.6-1.1/W 2.4-3.6/Mo 2.0-3.6/V 2.4-3.6/Cr 3.3-4.7/ Others 3 max/Fe balance.

3. A sintered material according to either Claim 1 or Claim 2 having a composition expressed in wt% lying within the ranges: C 0.6-1.1/W 2.6-3.3/Mo 2.7-3.2/V 2.7-3.6/Cr 3.4-4.5/Others 3 max/Fe balance.

4. A sintered material according to any one preceding claim further including up to 1't sulphur.

5. A sintered material according to any one preceding claim further including up to 5 wt't of one or more metallic sulphides.

6. A sintered material according to any one preceding claim further including from 4 to 6 wt% of copper.

7. A sintered material according to any one preceding claim having a density in excess of 80% of the full density.

8. A sintered material according to Claim 7 having a density in excess of 85% of full density.

9. A method of making a sintered ferrous material, the method comprising the steps of preparing a powder having a composition, expressed in wt, lying within the ranges: C 0.4-1.5/W 2-4/Mo 1.5-4/V 2-4/Cr 2.55/Others 3 max/Fe balance, pressing a green body from, the powder and then sintering the green body.

10. A method according to Claim 9 and comprising the step of adding up to 1 wt% of sulphur.

11. A method according to either Claim 9 or Claim 10 and further comprising the step of adding up to 5 wt% of one or more metallic sulphide.

12. A method according to any one of Claims 9 to 11 and further comprising the step of adding a part of the carbon as carbon powder.

13. A method according to any one of Claims 9 to 12 and further comprising the step of adding between 4 and 6 wt% of copper powder.

14. A method according to any one of Claims 9 to 12 and further comprising the step of infiltrating the material with a copper-based alloy.

15. A method according to any one of Claims 9 to 14 and further comprising the step of cryogenically treating the sintered material.

16. A method according to Claim 15 wherein the cryogenically treated material is subsequently heat-treated at a temperature in the range from 5750 C to 710 0C.

17. A sintered material substantially as hereinbefore described with reference to any one of examples 1 to 10 of the accompanying specification.

18. A method of making a sintered ferrous material substantially as hereinbefore described with reference to any one of examples 1 to 10 of the accompanying specification.