GB2451898A

GB2451898A - Sintered valve seat

Info

Publication number: GB2451898A
Application number: GB0716103A
Authority: GB
Inventors: Paritosh Maulik; Iain Robert Whitaker
Original assignee: Federal Mogul Sintered Products Ltd
Current assignee: Federal Mogul Coventry Ltd
Priority date: 2007-08-17
Filing date: 2007-08-17
Publication date: 2009-02-18
Also published as: WO2009024809A1; GB0716103D0

Abstract

A valve seat made by compacting and sintering a water atomised vacuum annealed high alloy powder which comprises (by weight): 15-30% Cr, 0.5-5% Mo, 1-2% C, optionally one or more other alloying elements, with the balance being Fe. (Claim 15 species an alloy which comprises 0.5-5 % of at least one of Mo, V, W and Nb). The valve seat comprises at least 10 % of this powder and it may be completely formed therefrom (allowing up to 2 % for additives). Other powders which may be mixed with the above powder are: iron powder, low alloy steel powder, machinability aids, solid lubricants, sintering enhancers, and/or hard particles to a level of 30 % or less of the total powder mixture. Processing can involve cryogenic treatement followed by tempering to form a martensitic matrix with carbides distributed therein.

Description

A Valve Seat Insert This invention relates to a valve seat insert, and more specifically to a specific powder metallurgy (PM) composition for valve seat inserts, and further specifically to the use of a pie-alloyed powder, optionally combined with, among other things, iron powder in the manufacture of valve seat inserts for combustion engines.

BACKGROUND

PM is widely practised, and almost universal in the manufacture of valve seats. A significant body of prior art already exists in this field, and a description of some of the more relevant art is provided below.

Sintered valve seat production using different types of pre-alloyed powder has been considered, and US5462573 to Baker and Kettle, and assigned to applicant herefor, claims a valve seat insert consisting of a ferrous-based material having a matrix comprising a pressed and sintered powder, the metallurgical structure of the matrix comprising tempered martensite containing spheroidal alloy carbides, bainite and ferrite, the powder having been pressed to greater than 80% of the theoretical density from a mixture including two different ferrous based powders. This mixture includes between 40-70% of a pie-alloyed powder having a composition in wt%, apart from impurities, within the ranges: C 0.45-1.05; W 2.7-6.2; Mo 2.8-6.2: V 2.8-3.2; Cr 3.8-4.5; Fe balance, and between 60-30% of an iron powder having carbon powder therein, optionally together with 4-6% Cu, optionally up to 5% of one or more metallic sulphides, and optionally up to 1 % Suphur, the final composition being such that the total carbon content of the sintered material lies in the range 0.8-1.5 wt%.

The use of M312 high speed toot steel (which is employed in the above example) has been widely practised, and it is well known that compaction of a mixture of M3/2 tool steel powder and iron powder, or indeed M3/2 powder alone, followed by a standard sintering process gives rise to an entirely satisfactory valve seat. For example, GB2210894 to Fellgett and Lane discloses sintered ferrous materials 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 2.5-5/Others 3 max/Fe balance, such being an example of a diluted form of M3/2 steel. Also, up to 1 % S may be present, with or without 4-6% Copper.

The pnmary disadvantage of the use of M3/2 high speed steel powder is its relative cost, certainly as regards elemental Iron powder. In an effort to reduce the cost of valve seat production, a number of different alternative compositions have been proposed, in particular compositions which include carbides, or carbide forming elements in conjunction with elemental C. Examples include GB2087436 to Cadle and Smith which discloses a sintered alloy formed by compacting and sintering a mixture of a powder containing 0.1-0.9% carbon, 8-18% chromium and, optionally, 0-1% manganese, 0-1% molybdenum, 0-1% silicon, 0- 0 1% phosphorus, 0-0.1% sulphur, 0-2.5% nickel, balance iron and 1% to 10%, by weight of a powder lubricant which remains solid at the sintering temperature of the mixture. The lubricant is MoS2 or CuS2, such being typical. A further example is found in EP480495 to Purnell and MauUk which discloses sintered materials and a method for their manufacture, such as piston rings and valve seat inserts wherein the sintered material comprises a porous matrix with a composition lying in the range expressed in wt% of 8 to 12 chromium, 0.5 to 3 molybdenum, up to 1.5 vanadium, 0.2 to 1.5 carbon, other impurities 2 max., up to 1 manganese sulphide, optionally up to 5 molybdenum disuiphide, balance iron, the matrix having a uniform dispersion of submicroscopic particles of molybdenum rich carbides which render the material resistant to thermal softening. In this example, the hardenability of the material is enhanced by pre-alloying rather than elemental addition of Mo and/or V. Elemental Mo addition, according to GB2087436, only undergoes partial dissolution and its effects on the hardening process are somewhat limited.

A further example is found in EP946775 to Maulik which discloses a powder mixture suitable for use in a compaction and sintering process may comprise three constituents.

The first constituent is substantially carbon-free and is formed by one or more alloyed powders and has a composition lying in the range expressed in % by weight: 8 to 12 chromium, 0.5 to 3 molybdenum, optionally up to a total of 10 of at least one metal selected from vanadium, cobalt and tungsten, a maximum of 2 of other materials including impurities, and the balance iron. The second constituent is formed by sufficient carbon powder to bring the total carbon content of the mixture to 1.5 to 3.0% by weight.

The third constituent is formed by sufficient ferro-phosphorus powder to bring the total phosphorus content of the mixture to 0.1 to 0. 5% by weight. This high density Fe-Cr alloy (around 7.4Mg m) is achieved without Copper infiltration. The microstructure consists of relatively coarse carbides in a tempered martensitic matrix, and this gives rise to material having high rolling contact fatigue resistance.

A yet further example is found in US6 123748 to Perrin and Whitaker which teaches an iron-based powder which is a mixture comprising a major proportion of a first alloy powder, a minor proportion of a second alloy powder and a proportion of a solid lubricant. The first alloy powder consists of, in weight percentages, 14 to 30 chromium, 1 to 5 molybdenum, 0 to 5 vanadium, 0 to 6 tungsten, the total of molybdenum, vanadium and tungsten being at least 3, a total of 0 to 5 of other strong carbide forming elements, 0 to 1.5 silicon, carbon with a minimum level sufficient to form carbides with substantially all of the molybdenum, vanadium, tungsten, and any other strong carbide forming elements present, and a balance which is iron and incidental impurities. The second alloy powder is an austenitic stainless steel. Coarse carbides are present in this material, which does not significantly respond to heat treatment, and is primarily suitable for low load, high temperature applications.

A further example includes US6679932 which discloses a ferrous sintered valve seat material made of mixed powders comprising a sinter-hardenable phase and a finely dispersed carbide phase. The powder mixture comprises a sinter-hardening prealloyed powder forming 75 to 90 wt. % of the mixture and a tool steel (M312 type or S6-5-2 type) powder with finely dispersed carbides forming 5 to 25% of the mixture. Machinability additives of MnS, CaF2 or MoS2 types are added in an amount of 1 to 5 wt. %. Improved thermal conductivity is obtained by infiltrating the compact with Cu or Cu alloy up to 25 wt.%.

GB2298869 discloses articles produced by a PM process involving forming of a shape by conventional single press compaction followed by sintering without the application of external pressure from a high Cr-Fe base alloy powder produced by rapid atomisation followed by an annealing treatment. The powder consists essentially of, in weight percent, chromium 14 to 30, molybdenum 1 to 5, vanadium 0 to 5, tungsten 0 to 6, silicon 0 to 1.5, carbon minimum as specified below to one fifth chromium content minus 2, other strong carbide forming elements (eg Nb, Ta, Ti) totalling together 0 to 5, the total of Mo, V and W being at least 3, the balance being iron including incidental impurities; the alloy powder (including any addition of free graphite powder mixed therewith before sintering) having a sufficient carbon content to form carbides with all the Mo, V, W and other strong carbide forming elements present; the articles consisting of a distribution of carbides embedded in a substantially ferritic matrix containing at least 12% by weight of chromium in solution, and which articles do not require further heat treatment As can be appreciated from the above, the concepts of water atomisation, annealing, pre-alloying, and infiltration with Copper or Copper alloy are known, as is the concept of including carbide forming elements together with elemental Carbon so that carbides are formed in the matrix of the final sintered product to provide wear resistance. However, the inclusion of a higher wt.% of carbide-forming elements like Mo, V, W, and/or high grade tool steel such as M3/2 necessarily renders the resulting products expensive.

It is therefore an object of this invention to provide a valve seat made from a new PM composition which significantly reduces costs as compared to a valve seat made by traditional methods incorporating M3/2 tool steel and optionally Iron or pre-alloyed Iron powder, and which, post compaction and sintering, possess a similar microstructure to the standard M3/2 + Fe blends in which the microstructure consists of a distribution of high carbide-containing areas and areas relatively free of carbides.

BRIEF SUMMARY OF THE DISCLOSURE

According to the present invention there is provided a valve seat made by compacting a powder mixture and sintering the green compact formed thereby, said powder mixture comprising 10 wt% or more of a water atomised vacuum annealed high alloy powder A, the remainder of the mixture consisting of at least one of: -Iron powder or a low alloy steel powder -one or more conventional machinability aids, solid lubricants, sintering enhancers, and -hard particles to a level of 3Owt.% or less of the total powder mixture, Wherein said powder A comprises 15-30 wt. % Cr, 0.5-5wt.% Mo, and 1-2wt.% C with the balance being Fe, one or more optional other alloying elements, and incidental impurities.

Preferably, powder A further comprises up 3wt% W, up to 3 wt% V, and up to 2wt.% Si.

By low-alloy steel" is meant less than 5wt.% of alloying elements in combination with Fe and incidental impurities. In this invention, powder A may be termed a high alloy powder, having alloying elements (particularly Cr) present in amount greater than 5wt.%.

Typical machinability aids include Suiphides such as MnS or MoS2, typical sintering enhancers include phosphides or phosphorous alloys of iron and/or copper.

Typical hard-phase particles include Ferro-Molybdenum or Ferro-Chrome hard particles, and others as are disclosed in GBO6 15929.7 in the name of applicant herefor. This application discloses a hard-phase consisting of at least 3Owt.% Fe, with at least some of each of the following elements, the weight% being chosen from the following ranges such that together with the wt.% Fe, the total is 100%: 1-3% C; 20-35% Cr; 2-22% Co; 2-15% Ni; 8-25% W Preferably, the hard phase composition also includes one or more of the following elements in greater than trace amounts, but not totalling any more than Swt.% of all such elements:V, Ni, Ti, Cu.

In a preferred embodiment, powder A comprises 20-25wt.% Cr, 1-2wt.% C, 1-2wt.% Mo, 0.5-3wt.% V, 0-lwt.% Si and 1.5-3.5wt.% W with the balance being Fe excepting incidental impurities.

Most preferably, powder A comprises 22wt.% Cr, 1.74wt.% C, 1.5wt.% Mo, 0.85wt.% V and 2.2wt.% W with the balance being Fe excepting incidental impurities.

Most preferably, the microstructure of the compacted, sintered and heat treated material of the present invention exhibits carbides distributed in a tempered martensitic matrix.

In a preferred embodiment, the powder mixture which is compacted and sinterea is subsequently subjected to a heat treatment and a machining step.

Preferably, the powder mixture comprises 25wt.% or more of powder A with the balance being Fe excepting incidental impurities and allowing for 2wt.% or less of the conventional additives mentioned above.

Preferably, the powder mixture comprises 5Owt.% or more of powder A with the balance being Fe excepting incidental impurities and allowing for 2wt.% or less of the conventional additives mentioned above.

Most preferably the powder mixture comprises 75wt.% or more of powder A with the balance being Fe excepting incidental impurities and allowing for 2wt.% or less of the conventional additives mentioned above.

In a particular embodiment, the powder mixture consists entirely of powder A excepting incidental impurities and allowing for 2wt% or less of conventional additives.

By using the powder mixture of the invention, regardless of whether Fe or low-alloy powder is used to form a powder mixture with A, the compaction and sintering processes do not serve to cause the particles within the green compact to undergo complete dissolution into each other. Limited diffusion occurs at the particle boundaries, but the particles generally become densified and are still generally separately identifiable when the resulting components are microscopically examined. Thus while there may be a physical change in the particle shape, voids in the resulting physical structure may still exist. This also occurs in the more conventional, prior art valve seat powder mixture of M3/2 steel and Fe.

In a most preferred embodiment, a Cu infiltration process may occur simultaneously with the sintering process to enhance the thermal conductivity, machinability and also strength of the resulting article.

Most preferably, a post-sintering cryogenic process is conducted by immersing the component in a bath containing liquid Nitrogen such that the component temperature is reduced rapidly to preferably at least -78°C, and more preferably -120°C or below, and then a tempering process is conducted for 1-2 hours at between 550-600°C. In an alternative embodiment, a bath containing solid CO2 is used, the meiting point of which is -78°C, but it is generally more preferable that a bath containing liquid nitrogen is used, the boiling point of which is -196°C, as a lower ambient temperature can be obtained therein, and furthermore the rate of cooling down to a temperature of -120°C is also improved. In any event, the low temperature and rate of cooling thereto have to be sufficient so that all austenite in the matrix transforms to martensite.

In a further preferred embodiment, a dual layer valve seat component is provided wherein the component comprises a first layer being compacted from a powder mixture as provided above, and a second layer compacted from a powder mixture of significantly lower quality or grade in terms of alloying elements and composition, said first and second layers being superposed immediately prior to sintering and then sintered together to provide a unitary component comprising two broadly discrete layers, excepting for elemental or alloy diffusion during the sintering process from either layer into the other near the boundary zone.

In a yet further aspect, the invention provides a valve seat made by compacting a powder mixture and sintering the green compact formed thereby, said powder mixture comprising at least 10 wt.% or more of a water atomised vacuum annealed high alloy powder A, the remainder of the mixture consisting of at least one of: -Iron powder or a low alloy steel powder -one or more conventional machinability aids, so!id lubricants, sintering enhancers, and -a hard phase component to a level of 3Owt.% or less of the total powder mixture, wherein said powder A comprises 18-25 wt. % Cr, 0.5-5wt.% of at least one of Mo, V, W and Nb and 0.5-2wt.% C (or more preferably 0.7-2 % C or most preferably 1-2 % C), and optionally up to 2wt % Si, with the balance being Fe, one or more optional other alloying elements, and incidental impurities, and wherein said powder A has a matrix comprising of less than 10 % by weight of Cr, and wherein powder A comprises large chromium carbides.

A preferred valve seat includes a powder A comprising 19-23wt.% Cr, 1-2wt.% C, 1- 2wt.% Mo, 0.5-1.5 wt.% V, 0.2-lwt.% Si and 1.5-3.5wt.% W with the balance being Fe excepting incidental impurities.

A method of making a valve seat is also provided and comprises the steps of: making a powder mixture comprising at least 10 wt.% or more of a water atomised vacuum annealed high alloy powder A, the remainder of the mixture consisting of at least one of: -Iron powder or a low alloy steel powder -one or more conventional machinability aids, solid lubricants, sintering enhancers, and/or hard particles to a level of 2wt. % or less of the total powder mixture, Wherein said powder A comprises 15- 25 wt. % Cr, 0.5-5wt.% of each of at least one of Mo, V, W and Nb and 0.5-2wt.% C (or more preferably 0.7-2 % C, or most preferably 1-2 % C) with the balance being Fe, one or more optional other alloying elements, and incidental impurities wherein said powder A has a matrix comprising of less than 10 % by weight of Cr, and wherein powder A comprises large chromium carbides compacting said powder mixture sintering said powder mixture subjecting the resulting sintered valve seat to a cryogenic treatment followed by a heat or tempering treatment.

it is to be understood that there is significant overlap between this latter aspect of the invention and former aspects, and that it is within the contemplation of the applicant that such aspects may be combined to provide an overarching aspect of the invention covering both specific aspects.

In a basic equivalence test conducted by the applicant, it has been shown that a compacted, sintered powder mixture comprising -5Owt.% Fe with -5Owt.% powder A and -0.5wt.% C, when powder A is of the composition: 1.5wt.% C, 22wt.% Cr, lwt.% Mo, lwt.% V, 1.5wt.% W, with the balance Fe (excepting incidental impurities) gives rise to a valve seat component of comparable wear characteristics with a more conventionally product component such as that which might be produced according to the method of US5462573 to Baker and Kettle in which -5Owt.% of a pre-alloyed (low-alloy) steel powder of the M3 type (e.g. M3/2) is mixed with -5Owt.% Fe powder and -0.5wt.% C powder, compacted and sintered. importantly however, the cost of producing the component constituted by the powders of the present invention is up to 17% less expensive in terms of raw ingredients, compaction and sintering (and final machining) processes being generally identical The reader will immediately appreciate that this represents a significant reduction in the ult:mate component cost without any appreciable reduction in component quality in terms of wear resistance, and additionally there is a perceptible increase in corrosion resistance on account of the fact that the overall chemical composition of the resulting component contains a significant amount of Cr. Such enhanced corrosion resistance is becoming a necessity with the introduction of more advanced engine fuels such as bio-diesel and ethanol because.

-the fuels may be prone to moisture retention, and -combustion can result in the formation of acids in the exhaust gas.

It is considered by the applicant that the presence of the large carbide particles in powder A render it equally as wear resistant as a conventional low-allow or tool steel powder without having any deleterious effect on structural integrity of the resulting component.

In terms of comparative technical details, it should be noted that high speed steel-type alloys containing high percentages of Mo, V, W, form M6C and MC carbides where M is typically a complex metal alloy (W-Mo-Fe-Cr-V). These carbides are stable at high temperatures. Cr rich carbides form in Fe -Cr -C material. These carbides may not be very stable at elevated temperatures, but for wear resistance applications at moderate temperatures, these carbides can provide adequate performance. Since Cr is comparatively inexpensive compared to Mo, V and W, the former is the preferable alloying element.

For high load bearing wear resistance applications it is highly preferred that the microstructure of the material of the present invention exhibits carbides distributed in a tempered martensitic matrix. In order to render Fe -Cr -C alloys amenable to martensitic transformation and therefore responsive to heat treatment, it is essential to judiciously maintain a ratio of alloying elements. This has been achieved by maintaining Cr -C ratio in the present powder. The alloy (Fe -Cr -C -others) described in GB2298869 is ferritic in microstructure and does not undergo martensitic transformation.

Additionally the high concentration of alloying elements in the present powder results in a microstructure with high volume fraction of carbides, and these are coarser compared to those observed in the material made with M3/2 types of powder. A high volume fraction of carbides also improves wear resistance of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1, 2, 3, show comparative graphical results for physical properties of test samples described below; Figure 4 shows a graphical plot of the results of standard flank wear tests on the various compositions of the prior art and the present invention as detailed below.

Figure 5, 6 show electron micrographs of a 49 75/49.75 wl.% M3/2+Fe, with 0.5% C prior art material specimen, and a specimen having a composition according to the present invention, namely 49.75/49.75% Al (see below)+Fe with 0.5%C, and both having been prepared according to the examples below; Figure 7 shows an electron micrograph of a specimen composed of 100% M3/2 high speed tool steel prepared as by the example below (i e. a prior art material) and infiltrated with Copper during sintering; again indications are provided of chemical compositions of different areas within the micrograph; (it is to be mentioned that the compositions shown in this Figure may not correspond to theoretical values); Figures 8, 9 show optical micrographs of two additional specimens, both having been infiltrated with copper, the first specimen including 49.75% M3/2, 49.75% Fe and 0.5% C and prepared according to US5462573 above, and the second specimen including 49.75% of powder Al referred to below with 49.75% Fe, and 0.5%C.

DETAILED DESCRIPTION

Example 1

In a first embodiment, a water atomised, vacuum annealed powder having a composition of 1.74wt.%C, 22wt.%Cr, 1.5wt.%Mo, 0.85wt %V, 2.2lwt.%W with the balance being Fe excepting incidental impurities (hereinafter referred to as powder Al) was compacted to approximately 800MPa as is conventional in PM processes. The subsequent sintering and copper infiltration occurs at around 1100-1120°C in an inert (N2 -H2) atmosphere for about 30mm. in an industrial mesh belt furnace, The sintered components were then allowed to cool in ambient manner during exrt from the furnace at approx. 10-i 5°C min1 from the sintering temperature. This cooling rate is not high enough to cause martensitic transformation in the sintered article, which is therefore ideally cryogenically treated by placing cages or trays of the sintered articles in a chamber containing liquid N2 such that the ambient temperature in the chamber is -120°C or below. The cryogenic treatment is followed by a heat treatment at around 600°C for about 1-2 hours. The sintered articles were machined to suitable test pieces for the determination of physical properties.

Machinability performance of the present invention was compared with that of the prior art material

Example 2

In a second embodiment, a powder Al was mixed with Fe powder in equal amounts of 49.75wt.%, the remainder being 0.5wt.% C excepting incidental impurities, said mixture being subsequently compacted, sintered and copper infiltrated, cryogenically and heat treated as described above, and machined into test pieces.

The table provided below summarises the blends examined:

Bnds Examplel Example2 Prior art Prior art

Sample 1 Sample2 Powder Al 100 49.75 M3/2 49.75 100 Un-alloyed Fe 49.75 49.75 Graphite (C 0.5 0.5 I addition) _______________ ___________ _____________ Fugitive 0.75 f 0.75 0.75 0.75 [jbricant __________________________ Sintered blanks were produced from example 2 and prior art blends. These were machined to valve set inserts and were tested to simulate valve/valve seat wear in a motorised valve seat insert test set up In this set up a gas flame maintained the valve seat insert temperature around 250 °C and the rotation of the motor operating at around 3000 rpm provided the valve lift which was around 6 mm. The test ran for 5 hr and following the test the valve seat wear was measured.

The following tables provide details of the physical properties of the test articles and

prior art articles:

0.2% Proof stress, MPa Test temperature, Present powder, M3/2, 100% rM3/2+ Fe, 50/50+ 100% 0.5C 919 170 1090 300 658 1240 860 456 1080 770 Young's modulus, GPa � Fe, 50/50 + 161 170 165 300 658 1240 860 500 146 155 150 Hot hardness,HR3ON Test Present M3/2, 100% Present M3/2+ Fe, temperature, powder, 100% powder + Fe, 50/50 + 0.5 C 50/50 + 0.5 C 64 67 64 63 300 63 68 64 63 500 59 65 60 59 These results are all plotted graphically as shown ri Figures 1, 2,3 Thermal properties Thermal conductivity, Wm1 K1 M3/2, 100% Present alloy, 100% 20-300°C 40 37 -500°C 41 40 Thermal expansion conductivity, *106K1 M3/2, 100% Present alloy, 100% 20-300°C 13 13 [20-500°C j 14 13.6 As can be seen from the above, the thermal properties of the prior art and new sintered materials are comparable, which is desirable.

Referring to Figure 4 there is shown the results of a standard flank wear test on components made as described above and having the following compositions 1) 50% M3/2+ 50% Fe + 0.5 C; Cu infiltrated; (prior art) 2) Present powder Al; Cu infiltrated; present invention

3) M3/2, Cu infiltrated; (prior art)

4) 50% Present powder Al + 50% Fe + 0.5 C; Cu infiltrated; present invention Again, the results for the new composition are comparable to that of the prior art, which is desirable.

Referring to Figures 5, 6, 8, 9, the similarities of the microstructures of the prior art materials and those of the novel materials according to the present invention can easily be seen, and the implication from these micrographs that the overall physical properties, temperature and wear resistance characteristics will be similar is borne out by the technical data provided above and as shown in Figures 1-4.

The table below shows the results from valve -valve seat wear test set up. The results from the prior art and present invention samples compare well.

Valve -valve seat wear test set up; valve seat face were coated with hard phase 50% M3/2 + 50% Al � 50% M3/2; Cu Al; Cu 50% Fe + 0.5 Fe + 0.5 C; Cu infiltrated; prior infiltrated, C; Cu infiltrated; art present infiltrated; prior present invention ______________ art invention _______________ Valve seat 29 19 35 10 wear ________________ Valve wear 2 2 2 13 Referring to Figures 5, 6, 8, 9, the similarities of the microstructures of the prior art materials and those of the novel materials according to the present invention can easily be seen, and the implication from these micrographs that the overall physical properties, temperature and wear resistance characteristics will be similar is borne out by the technical data provided above and as shown in Figures 1-4.

Claims

1. A valve seat made by compacting a powder mixture and sintering the green compact formed thereby, said powder mixture comprising at least 10 wt.% or more of a water atomised vacuum annealed high alloy powder A, the remainder of the mixture consisting of at least one of: -Iron powder or a low alloy steel powder -one or more conventional machinability aids, solid lubricants, sinteririg enhancers, and -a hard phase component to a level of 3Owt.% or less of the total powder mixture, Wherein said powder A comprises 15-30 wt. % Cr, 0.5-5wt.% Mo, and 1-2wt.% C with the balance being Fe, one or more optional other alloying elements, and incidental impurities

2. A valve seat according to claim 1 wherein powder A further comprises up to 3.5wt% W, up to 3 wt% V, and up to 2wt.% Si.

3 A valve seat according to claim 2 wherein powder A comprises 20-25wt.% Cr, 1- 2wt.% C, 1-2wt.% Mo, 0.5-3wt.% V, 0-lwt.% Si and 1.5-3.5wt.% W with the balance being Fe excepting incidental impurities.

4. A valve seat according to claim 3 wherein powder A has a composition of 22wt.% Cr, 1.74wt.% C, 1.5wt.% Mo, 0B5wt.% V and 2.2wt.% W with the balance being Fe excepting incidental impurities.

5. A valve seat according to any preceding claim, which having been subjected to a cryogenic treatment, followed by a tempering or heat treatment process after sintering, has a microstructure which exhibits carbides distributed in a tempered rnartensitic matrix.

6. A valve seat according to any preceding claim wherein the powder mixture comprises 25wt.% or more of powder A with the balance being Fe excepting incidental impurities and allowing for 2wt.% or less of the conventional additives.

7. A valve seat according to any of claims 1-5 wherein the powder mixture comprises 5Owt % or more of powder A with the balance being Fe excepting incidental impurities and allowing for 2wt.% or less of the conventional additives.

8. A valve seat according to any of claims 1-5 wherein the powder mixture comprises 75wt.% or more of powder A with the balance being Fe excepting incidental impurities and allowing for 2wt % or less of the conventional additives.

9. A valve seat according to any of claims 1-5 wherein the powder mixture consists entirely of powder A excepting incidental impurities and allowing for 2wt.% or less of conventional additives.

10. A method of making a valve seat comprising the steps of: making a powder mixture comprising at least 10 wt.% or more of a water atomised vacuum annealed high alloy powder A, the remainder of the mixture consisting of at least one of: -Iron powder or a low alloy steel powder -one or more conventional machinability aids, solid lubricants, sintering enhancers, and/or hard particles to a level of 2wt.% or less of the total powder mixture, said powder A comprising 15-30 wI. % Cr, 0.5-5wt.% Mo, and 1-2wt.% C with the balance being Fe, one or more optional other alloying elements, and incidental impurities, compacting said powder mixture sintering said powder mixture subjecting the resulting sintered valve seat to a cryogenic treatment followed by a heat or tempering treatment.

11. A method according to claim 10 wherein the method includes the further step of infiltrating the valve seat as it is sintered with Copper or an alloy thereof.

12. A method according to either of claims 10 or 11 wherein the post-sintenng cryogenic process is conducted by immersing the valve seat in a chamber containing liquid Nitrogen at -120°C or below, and wherein the subsequent tempering process is conducted for 1-2 hours at between 550-600°C.

13. A method according to any of claims 10-12 wherein the resulting microstructure of the post-heat treated material exhibits carbides distributed in a tempered martensitic matrix.

14. A method according to any of claims 10-13 wherein the composition of powder A and its wt.% in the overall powder mixture is determined according to any of claims 2-4 and 6-9.

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15. A valve seat mace by the method claimed in any preceding claim wherein the resulting microstructure of the post-heat treated material exhibits carbides distributed in a tempered martensitic matrix.

15. A valve seat made by compacting a powder mixture and sintering the green compact formed thereby, said powder mixture comprising at least 10 wt.% or more of a water atomised vacuum annealed high alloy powder A, the remainder of the mixture consisting of at least one of: -Iron powder or a low alloy steel powder -one or more conventional machinability aids, solid lubricants, sintering enhancers, and -a hard phase component to a level of 3Owt % or less of the total powder mixture, wherein said powder A comprises 18-25 wt. % Cr, 0.5-5wt.% of at least one of Mo, V, W and Nb and 0.5-2wt.% C, and optionally up to 2wt.% Si, with the balance being Fe, one or more optional other alloying elements, and incidental impurities, and wherein said powder A has a matrix comprising of less than 10 % by weight of Cr, and wherein powder A comprises large chromium carbides.

16 A valve seat according to claim 15 wherein powder A comprises 19-23wt.% Cr, 1-2wt.% C, 1-2wt.% Mo, 0.5-1.5 wt.% V, 0.2-lwt.% Si and 1.5-3.5wt.% W with the balance being Fe excepting incidental impurities.

17. A valve seat according to claims 15 or 16 wherein powder A comprises 0.7-2 % C. 18 A valve seat according to claims 15 or 16 wherein powder A comprises 1-2 % C. 19. A method of making a valve seat comprising the steps of: making a powder mixture comprising at least 10 wt.% or more of a water atomised vacuum annealed high alloy powder A, the remainder of the mixture consisting of at least one of: -Iron powder or a low alloy steel powder -one or more conventional machinability aids, solid lubricants, sintering enhancers, and/or hard particles to a level of 2wt. % or less of the total powder mixture, Wherein said powder A comprises 15- 25 wt. % Cr, 0.5-5wt.% of each of at least one of Mo, V, W and Nb and O.5-2wt,% C with the balance being Fe, one or more optional other alloying elements, and incidental impurities wherein said powder A has a matrix comprising of less than 10 % by weight of Cr, and wherein powder A comprises large chromium carbides.

compacting said powder mixture sintering said powder mixture subjecting the resulting sintered valve seat to a cryogenic treatment followed by a heat or tempering treatment 20. A method according to claim 19 wherein powder A comprises 0.7-2% C. 21 A method according to claim 19 wherein powder A comprises 1-2% C. mendrnents To The Claims Have Been Filed As Follows 1. A method of making a valve seat compristng the steps of: a) making a powder mixture comprising at least 10 wt.% of a water atomised vacuum annealed high alloy powder A, the remainder of the mixture consisting of: -optionally iron powder or a low alloy steel powder -one or more conventional machinability aids, solid lubricants, sintering enhancers -hard particles up to a level of 3Owt% of the total powder mixture, said powder A comprising 15 -30 wt.% Cr, 0.5-5wt.% of at least one of Mo, V, W and Nb, and 0.5 -2wt.% C with the balance being Fe, one or more optional other alloying elements, and incidental impurities, and wherein said powder A comprises large chromium carbides in a Cr containing matrix; b) compacting said powder mixture to form a green compact valve seat; c) sintering said green compact and infiltrating the same with copper or an alloy thereof; and d) subjecting the resulting sintered valve seat to a cryogenic treatment followed by a heat or tempering treatment such that iron in the alloy matrix undergoes martensitic transformation.

2. A method according to claim 1 wherein the post-sintering cryogenic process is conducted by immersing the valve seat in a chamber containing liquid nitrogen to cool to -120°C or below, and wherein the subsequent tempering process is conducted for 1-2 hours at between 550-600°C.

3. A method according to claim 1 or 2 wherein said powder A has a matrix comprising of less than 1 Owt% of Cr.

4. A method according to claims 1, 2 or 3, wherein said powder A comprises 18-25 wt%Cr.

5. A method according to any preceding claim wherein powder A comprises 0.7-2% C. 6. A method according to any preceding claim wherein powder A comprises 1-2 %

C

7. A method according to any preceding claim wherein powder A further comprises up to 2wt% Si.

8. A method according to any preceding claim wherein powder A comprises 20- 25wt% Cr, 1-2wt% C, 1-2wt% Mo, O.5-3wt% V, 0-lwt% Si and 1.5-3.5wt% W with the balance being Fe excepting incidental impurities.

9. A method according to any preceding claim wherein powder A has a composition of 22wt% Cr, 1.74wt% C, 1.5wt% Mo, 0.85wt% V and 2.2wt% W with the balance being Fe excepting incidental impurities.

10. A method according to any of claims 1 to 6 wherein powder A comprises 19- 23wt% Cr, 1-2wt% C, 1-2wt% Mo, 0.5-1.5wt% V, 0.2-lwt% Si and 1.5-3.5wt% W with the balance being Fe excepting incidental impurities.

11. A method according to any preceding claim wherein the powder mixture comprises at least 25wt% of powder A with the balance being Fe powder or low alloy steel powder excepting incidental impurities and allowing for up to 2wt% of the conventional additives as well as hard particles up to 3Owt%.

12. A method according to any of claims 1 to 10 wherein the powder mixture comprises at least 5Owt% of powder A with the balance being Fe powder or low alloy steel powder excepting incidental impurities and allowing for up to 2wt% of the conventional additives as well as hard particles up to 3Owt%.

13. A method according to any of claims 1 to 10 wherein the powder mixture comprises at least 75wt% of powder A with the balance being Fe powder or low alloy steel powder excepting incidental impurities and allowing for up to 2wt% of the conventional additives as well as hard particles up to 3Owt%.

14. A method according to any of claims 1 to 10 wherein the powder mixture consists of powder A excepting incidental impurities and allowing for up to 2wt% of conventional additives as well as hard particles up to 3Owt%.