WO2002072904A1

WO2002072904A1 - Sintered ferrous materials

Info

Publication number: WO2002072904A1
Application number: PCT/GB2002/000887
Authority: WO
Inventors: Paritosh Maulik
Original assignee: Federal-Mogul Sintered Products Ltd
Priority date: 2001-03-08
Filing date: 2002-02-28
Publication date: 2002-09-19
Also published as: GB0105721D0

Abstract

A sintered ferrous material is described, the material having a matrix, the matrix comprising a microstructure of predominantly tempered martensite resulting from alloying of carbon and a pre-alloyed steel powder, the matrix having therein a uniform distribution of hard-particles and interconnected porosity filled by an infiltrated copper or copper alloy. A method of producing the material is also described.

Description

SINTERED FERROUS MATERIALS

The present invention relates to sintered ferrous materials and to a method for the manufacture thereof.

There is an ever increasing drive to reduce the cost of motor vehicles and of the cost of parts for their manufacture. However, whilst there is an increasing pressure to reduce costs there are also conflicting pressures in that modern internal combustion engines have increased specific power outputs requiring components of increasing quality and performance, capable of withstanding more arduous operating conditions.

Valve seat inserts, for example, used within the cylinder heads of internal combustion engines must be able to withstand high temperatures, erosive and corrosive environments and be wear resistant to the repeated impact loading by their co-operating valves. At one time it was common for such components to be made from cast or sintered steels having a continuous relatively uniform composition of a highly alloyed tool steel type material for example. However, due to the high contents of alloying elements such as chromium, vanadium, tungsten, cobalt, molybdenum, nickel and the like such materials are relatively very expensive. Furthermore, they were very costly to produce since the materials were very hard and difficult to machine.

With the advent of powder metallurgy production techniques, it became possible to produce material microstructures which were not possible by earlier melting and casting methods of production. The highly alloyed monolithic material referred to above gradually gave way to materials formed by processing of powder mixtures comprising, for example, about 50 weight% of a tool steel type powder and about 50 weight% of a pure iron powder with minor additions of graphite to provide a desired carbon level in the resulting steel material. Ferrous materials having two or more separate and distinct ferrous constituents in the matrix became possible due to the ability to form a powder mixture, compact the mixture to form a green compact and then sinter the compacted powder to form a strong component having a desired microstructure. In the example given of a sintered material comprising about 50 weight! of iron powder and about 50 weight% of a pre-alloyed steel powder plus a small carbon powder addition, and exemplified for example in EP-A-0 312 161 of common ownership herewith, the sintered microstructure comprises a reticular structure having regions of iron having a pearlitic and/or ferritic structure and regions of the pre-alloyed steel powder, the regions being intimately mixed and homogeneously distributed throughout the matrix in a reticular structure.

However, a problem with matrices having these two-phase structures including pearlitic and/or ferrite regions is that these regions are inherently less strong and less wear resistant than the hard, wear resistant martensitic regions of the microstructure. This is especially important in modern high performance engines and in diesel truck engines for example in parts such as valve seat inserts. EP—A-0 401 482 describes the manufacture of valve seat inserts having a martensitic matrix and a distribution of added hard particles throughout the matrix. However, the martensitic matrix is formed by separate heating and quenching steps followed by further heat treatment to produce the required tempered martensitic structure. Whilst the possibility of infiltration with copper or a copper alloy is mentioned as an optional step it is not mandatory.

It is always a technical option to increase the alloying additions to give harder, more wear resistant parts but this is not consistent with the need to reduce costs, both intrinsic material costs and processing costs such as machining costs, for example.

According to a first aspect of the present invention there is provided a sintered ferrous material having a matrix, the matrix comprising a microstructure of predominantly tempered martensite resulting from alloying of carbon and a pre-alloyed steel powder, the matrix having therein a uniform distribution of hard-particles and interconnected porosity filled by an infiltrated copper or copper alloy.

The matrix material may preferably be a low-alloy steel having sufficient alloying additions to enable the matrix to transform to martensite either on cooling from sintering or on subsequent thermal treatment which may include cryogenic treatment and tempering. Suitable pre- alloyed powders for the matrix may comprise, for example: Fe-3.5Cr-0.5Mo; Fe-2Ni-0.5Mo-0.3Mn; and, Fe-0.45Mn-lMo- 0.9Ni-0.45Cr . Alloy systems may include those selected from: Fe/0.45-4Ni/0.5Mo/0.2-0.4Mn; Fe/0.5-3.5Mo; and, Fe/2.5-5Cr/0.2-l Mo/0.0-4V, for example.

The pre-alloyed, low-alloy steel powder may preferably be essentially carbon-free. Carbon may be added as a separate addition to an initial powder mixture prior to compaction of green blanks of articles to be sintered, The omission of carbon from the pre-alloyed matrix powder renders the matrix-forming powder softer and hence more compressible than it would otherwise be allowing greater green density on compaction. The added carbon powder, which may be in the form of graphite, also acts as a die and powder lubricant aiding compaction. A suitable level of carbon powder addition may lie in the range from 0.5 to 1.2 weight%. A preferred range may be 0.75 to 1.0 weight%. Below 0.5 weight% the strengthening effect is insufficient whilst above 1.2 weight% the sintered articles become too hard to machine easily.

It is preferred that the total alloying metal content in the matrix low-alloy steel powder material is 7 weight% maximum in the interests of material cost. It is more preferred that the metal alloying content is 5 weight% maximum for the same reason. It has been found that alloyed steels having less than 5 weight% total of additions of chromium, molybdenum and the like which when the matrix material is further alloyed with carbon as a result of diffusion of the carbon powder into the matrix powder, easily form martensite on processing.

The hard particles may be included in the matrix in an amount of from 5 to 50 weight% . A preferred range of hard particles is from 10 to 25%. Below 5% the effect of the hard particles in enhancing wear resistance is insufficient and above 50% the cost rises to unacceptable levels. However, the nature of the hard particles themselves has a strong influence on process economics. For example, where the hard particles comprise a pre- alloyed tool or high-speed steel for example, up to 50 weight% may be economically used. Where the hard particles comprise alloys such as Fe-30Mo-10Cr-2Si, for example, wherein the intrinsic material cost is relatively higher, a maximum of 25 weight% is desirable.

The hard particles may be selected from the group comprising: particles of intermetallic materials or a powder of a pre-alloyed high-speed steel. In the former case such hard particles may be either iron-based or cobalt-based. Examples of these hard particles may be: Fe-30Mo-10Cr-2Si; Co-8Cr-28Mo; Co-17Cr-28Mo, for example, the cobalt-based materials being Stellite (trade name) type materials. A generic composition for hard particles of a high-speed steel material may be: 0.8-1.2 C/0-10 Co/4-10 Cr/0.5-10 Mo/1-3 V/l-14 W/ Bal Fe .

The hard particles should have a minimum hardness of 50 HRC.

The articles are infiltrated with copper or a copper alloy either during or after sintering but prior to any thermal treatment. Depending upon the degree of interconnected porosity, the copper content of the article after infiltration may be in the range from 15 to 25 weight% of the total weight of the article. Infiltration provides a high copper content in the article and consequently produces a higher cooling rate of the article on leaving the sintering zone of the furnace into the cooling zone. The relatively high cooling rate allied to the low-alloy steel composition and carbon promotes the formation of martensite in the article matrix. If the article is not infiltrated, but contains some admixed elemental copper of about 3 to 6 wt% as is known in conventional materials, the cooling rate with a conventional moving belt or walking beam furnace is insufficiently high to cause matensite formation on cooling and a microstructure comprising a mixture of pearlite, bainite, some martensite and some ferrite is formed which has a markedly inferior wear resistance than a substantially fully martensitic matrix. Where the article is infiltrated with copper, the microstructure comprises mainly martensite and perhaps a small amount of bainite and some retained austenite but the formation of pearlite and ferrite is substantially avoided. The retained austenite may easily be converted to martensite by suitable cryogenic thermal treatment.

It is possible to obtain special sintering furnaces which have a rapid cooling facility on exit from the sintering zone so that as the articles exit from the hot zone they are subjected to fast cooling which, if the steel composition is suitable, promotes the formation of martensite. This technique is called "sinter hardening". However, experiments have shown that for articles such as valve seat inserts, for example, the machinability is extremely poor when this technique is used. A subsequent tempering step may be employed to soften the martensite, however, machinability improves only marginally even after tempering.

The presence of copper in the sintered article, e.g. a valve seat insert, increases the thermal conductivity thereof thus, enabling the part to operate at a lower temperature in service which is beneficial for retaining good mechanical properties of the material. Copper also acts as a machining aid. Furthermore, as noted above, the presence of copper in the material enhances the cooling rate of the material following sintering and helps to ensure the formation of martensite in the matrix on cooling. Surprisingly, it has been found that that where a martensitic matrix has been formed as a result of the higher cooling rate afforded by an infiltration step, the machinability of the parts so produced is greatly superior to a martensitic matrix as formed by quenching or other cooling rate enhancing techniques.

Other additions may be made which include solid lubricant materials such as molybdenum or tungsten disulphide, for example, at contents of about 0.75 to 1.25 weight%.

Sulphides like molybdenum and tungsten di-sulphides undergo partial decomposition during sintering. This releases some sulphur and formation of mixed sulphides occurs, which aids machinability. The molybdenum or tungsten thus released enhances the properties of the metallic matrix.

Additions to further improve the machinability of the articles may also be made, a suitable material being manganese sulphide at about 1 weight% addition.

According to a second aspect of the present invention, there is provided a method for the manufacture of a sintered article having a ferrous matrix, the sintered material comprising a matrix of tempered martensite resulting from a pre-alloyed steel powder, the matrix having therein a uniform distribution of hard-particles, the method comprising the steps of: providing a low-alloy iron powder able to form martensite after a sintering step; making a powder mixture of said low-alloy iron powder, a powder comprising hard particles and an addition of carbon powder in the range from 0.5 to 1.2 weight%, optionally up to 1.25 weight% of a solid lubricant material powder and optionally up to 1 weight% of a machinability enhancing powder; pressing said powder mixture to form a green compact; sintering said green compact; infiltrating said article with copper or a copper alloy such that on cooling from said infiltrating step, the matrix transforms to a predominantly martensitic structure.

The martensitic matrix is formed on cooling on exit from the sintering furnace after the infiltration step without the need for a separate austenitising process, ie quenching from elevated temperature. It is preferred that ^' the sintering and infiltration steps are performed simultaneously. Simultaneous sintering and infiltration may be effected by stacking of copper-based ^■ material preforms on the pressed green parts before they are transported through a sintering furnace. In this way a separate process operation is avoided.

The method according to the present invention may also include the step of subjecting said sintered and infiltrated article to post-sintering thermal treatment. Such treatment may comprise tempering and, if necessary, a preceding cryogenic thermal treatment to transform to martensite any retained austenite remaining from the sintering or sintering and infiltration steps.

Sintering temperatures may be in the range from about 1100°C to about 1130°C. A protective gaseous atmosphere is normally used to prevent oxidation and to reduce oxides already present where this is desirable. Such protective gas atmosheres may be based on mixtures of nitrogen and hydrogen for example .

Sintering is frequently effected for articles such as valve seat inserts and valve guides, for example, in furnaces which have continuous moving means, such as a belt or a walking-beam type mechanism, for transporting the articles which are generally supported on trays for example, through the furnace. Generally, a first portion of the furnace raises the temperature of the articles to the sintering temperature; a second portion maintains the articles at the sintering temperature; and, a third portion allows the articles to cool from the sintering temperature to a temperature which will preclude significant oxidation of the articles on exit from the sintering furnace. The articles are generally sintered under a continuous protective gas atmosphere flowing through the furnace substantially at or very slightly above atmospheric pressure and which serves to provide either a neutral or reducing atmosphere and preclude air (oxygen) from entering the furnace. Such furnaces provide convenient high volume production means, are well understood and reliable. It has been found that cooling rates with a conventional mesh belt furnace are less than l°C/sec, typically about 0.5 to 0.6 °C/sec in the temperature range of about 600-800°C for non-infiltrated materials. At these cooling rates the types of ferrous materials contemplated for the matrix of materials according to the present invention will form mainly pearlite with only a small volume fraction of matertensite rather than the desired substantially fully martensitic structure thus, rendering the material less wear-resistant than desired. A cooling rate of 1 to

5°C/sec is required to form matensite depending upon the actual matrix composition, the higher the alloying additions, the lower the cooling rate to form martensite. However, with the process according to the method of the present invention, the infiltration step increases the intrinsic thermal conductivity of the material and hence the cooling rate sufficiently on passing through the furnace cooling zone to form the required martensitic matrix structure.

As noted above the machinability of parts produced by the method of the present invention is surprisingly much better than for parts where the martensitic matrix has been formed by quenching and tempering the martensite or by methods where the cooling rate has been enhanced by methods other than that produced by the intrinsic thermal conductivity of the material per se such as by sinter hardening described above.

The presence of the infiltrated copper material in the interconnected porosity itself further enhances machinability of the sintered parts.

In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying figures, of which:

Figure 1 shows a graph of tool flank wear during machining trials of valve seat inserts made according to the present invention and from a prior art material; and Figure 2 which shows a histogram of valve seat insert wear of materials according to the present invention and prior art materials during an engine test.

Samples of materials according to the present invention were made and having compositions as shown in the tables below relating to Examples 1, 2 and 3. All of the examples were made by forming powder mixtures of the constituent materials plus a fugitive wax pressing lubricant, compacting to form a green compact at pressing pressures of 600 to 800 MPa, simultaneously sintering and infiltrating with copper at a temperature in the range from 1100°C to 1130°C, cryogenically treating at -120°C and tempering at 500 to 600°C.

Example 1

The intermetallic particles were constituted by a water atomised powder having Laves phase (AB₂ type where A and B are both metals) . Particle size was 10 to lOOμm.

Materials according to Example 1 were made in two forms, one with 0.75 wt% carbon and the other with 1.0 wt% carbon. Example 2

Details of the hard particle powder are as for Example 1. In Example 2 two different materials were made, one material having 0.75 wt% carbon and the other having 1.0 wt% carbon. The test data hereinbelow refers to material having a 0.75wt% C addition.

An engine test was conducted on a 2 litre, 16 valve, lOOkW gasoline engine under a high speed cycle at full load with the throttle wide open; test duration 180 hours. Valve seat inserts made from Example 2 with 0.75 wt% carbon were placed in the exhaust positions 1, 3, 5. and 7. Valve seat inserts made from prior art material as described in EP 0 312 016 were placed in exhaust positions 2, 4, 6 and 8. Wear of the valve seat inserts is shown in the histogram of Figure 2. As may be seen from Figure 2, wear of the valve seat inserts is not uniform in each cylinder of the engine. It may be seen that wear of the materials of and made by the method of the present invention is generally less than that of the prior art material. Example 3

The composition of the high speed tool steel powder was: 0.8-1.2C/4-10Cr/5-10Mo/l-6W/l-3V/0-10Co/Bal Fe^'. The high speed tool steel powder possesses about 1.0 wt% carbon and consequently about 0.5 to 0.75 wt% carbon powder is added to raise the total to 1 to 1.25 wt% carbon in the final blend. As with Examples 1 and 2, two materials were made, one having a total carbon content of 1.0 wt% and the other having a total carbon content of 1.25 wt% .

Mechanical property data was obtained on the Examples according to the invention and also on material made according to EP 0 312 161.0.2%. Proof stress figures were obtained and are set out in Table 1 below.

Table 1

As may be seen from Table 1, the effect of the martensitic matrix is considerable in increasing the strength and hence the wear resistance of the material. A direct comparison may be obtained between the materials of Example 3 and the prior art material where, apart from the carbon content, the only substantive difference is the matrix composition and structure.

Example 1 (0.75 and 1.0%C) and Example 2 (0.75%C) contain only 10 wt% of hard particles whereas the prior art materials from EP0312161 contains 50 wt% of hard particles in the form of high speed steel particles yet the properties of the inventive materials are comparable in spite of the greatly reduced quantity of hard phase therein thus, demonstrating the beneficial effect of the stronger, more wear resistant matrix.

Figure 1 shows the results of machining trials of tool flank wear against number of parts machined. Traces 1, 2, 3 and 4 are the results of machining trials on valve seat inserts made from the materials of Example 1 (0.75%C), Example 1 (1%C), Example 2 (0.75%C) and Example 4 (1%C), respectively. Trace 5 relates to prior art material made according to EP 0 312 161 (powder mixture of -50% M3/2 high speed steel powder + -50% unalloyed iron powder + 0.5%C, giving total C of 1%), the material being simultaneously sintered and infiltrated with a copper alloy.

It should be noted that the carbon content referred to in Examples 1 to 3 is the content of the powder blend prior to compaction, sintering and infiltration thus, the carbon content, as a percentage of the total weight after infiltration, when there is an additional 15 to 25wt% of copper present will be somewhat lower.

Figure 1 shows that the machinability (machining speed 70 m/min) of the materials made according to the present invention are at least as good as the prior art material of trace 5. The machinability of Example 3 material (trace 4) which possesses a martensitic matrix and hard particles comprising 49wt% of a high speed tool steel powder was at least as good as the prior art material and significantly better at lower numbers of parts machined whilst possessing greatly superior mechanical properties and strength as may be seen from Table 1. This is a surprising result and indicates that in spite of the better mechanical properties, ease of production is not impaired, and indeed, is superior in some respects. The machinability results demonstrated by traces 1 to 3 show improvements in the region of 50% over the prior art material. Again, this is surprising in view of the fact that all materials have a martensitic matrix and mechanical properties comparable or better than those of the prior art materials.

Claims

1. A sintered ferrous material having a matrix, the matrix comprising a microstructure of predominantly tempered martensite resulting from alloying of carbon and a pre-alloyed steel powder, the matrix having therein a uniform distribution of hard- particles and interconnected porosity filled by an infiltrated copper or copper alloy.

2. A sintered ferrous material according to claim 1 wherein the matrix material is a low-alloy steel having sufficient alloying additions to enable the matrix to transform to martensite either on cooling from sintering or on subsequent thermal treatment which may include cryogenic treatment and tempering.

3. A sintered ferrous material according to either claim 1 or claim 2 wherein the matrix material is selected from the group of pre-alloyed powders comprising: Fe-3.5Cr-0.5Mo; Fe-2Ni-0.5Mo-0.3Mn; and, Fe-0.45Mn-lMo-0.9Ni-0.45Cr; Fe/0.45-4Ni/0.5Mo; Fe/0.5-3.5Mo; and, Fe/2.5-5Cr/0.2-1 Mo/0.0-4V.

4. A sintered ferrous material according to any one preceding claim wherein the pre-alloyed, low-alloy steel powder is substantially carbon-free.

5. A sintered ferrous material according to any one preceding claim wherein carbon is added as a separate addition to an initial powder mixture prior to compaction of green blanks of articles to be sintered.

6. A sintered ferrous material according to claim 5 wherein the carbon addition lies in the range from 0.5 to 1.2 weight% prior to infiltration.

7. A sintered ferrous material according to claim 6 wherein the carbon addition lies in the range from 0.75 to 1.0 weight% prior to infiltration.

8. A sintered ferrous material according to any one preceding claim wherein the total alloying metal content in the iron of the matrix low-alloy steel material is 7 weight% maximum.

9. A sintered ferrous material according claim 8 wherein the metal alloying content is 5 weight% maximum.

10. A sintered ferrous material according to any one preceding claim wherein the content of hard particles included in the matrix lies in the range from 5 to 50 weight% .

11. A sintered ferrous material according to claim 10 wherein the content of hard particles is from 10 to 25%.

12. A sintered ferrous material according to any one preceding claim wherein the hard particles are particles of intermetallic materials.

13. A sintered ferrous material according to claim 12 wherein the intermetallic materials are selected from the group comprising: Fe-30Mo-10Cr-2Si; Co-8Cr- 28Mo; Co-17Cr-28Mo.

14. A sintered ferrous material according to any one preceding claim from 1 to 11 wherein the hard particles are particles of pre-alloyed high speed steel material.

15. A sintered ferrous material according to claim 14 wherein the high speed steel particles have a composition in weight%: 0.8-1.2 C/0-10 Co/4-10 Cr/0.5-10 Mo/1-3 V/l-14 W/ Bal Fe .

16. A sintered ferrous material according to any one preceding claim wherein the hard particles have a minimum hardness of 50 HRC .

17. A sintered ferrous material according to any one preceding claim wherein the infiltrated copper content lies in the range from 15 to 25% of the total weight of the article.

18. A sintered ferrous material according to any one preceding claim further including an addition of solid lubricant material in the range from about 0.75 to 1.25 weight%.

19. A sintered ferrous material according to claim 18 wherein the solid lubricant is molybdenum disulphide or tungsten disulphide.

20. A sintered ferrous material according to any one preceding claim further including an addition of a material to improve the machinability of the ..article.

21. A sintered ferrous material according to claim 20 wherein the machinability improver is manganese sulphide at about 1 weight%

22. A method for the manufacture of a sintered ferrous article, the sintered article comprising a matrix of martensite resulting from a pre-alloyed steel powder, the matrix having therein a uniform distribution of hard-particles, the method comprising the steps of: providing a matrix forming low-alloy steel powder able to form martensite after a sintering and/or infiltration step; making a powder mixture of said low-alloy steel powder, a powder consisting of hard particles and an addition of carbon powder in the range from 0.5 to 1.2 weight%, optionally up to 1.25 weight% of a solid lubricant material powder and optionally up to 1 weight% of a machinability enhancing powder; pressing said powder mixture to form a green compact; sintering and infiltrating said green compact, in a sintering furnace, with copper or a copper alloy during said infiltration step and cooling said infiltrated article at a cooling rate such as to have formed a predominantly martensitic matrix on emergence from a sintering furnace.

23. A method according to claim 22 wherein the sintering temperature lies in the range from about 1100°C to about 1130°C.

24. A method according to either claim 22 or claim 23 wherein a protective gaseous atmosphere is used in the sintering furnace.

25. A method according to any one preceding claim from 22 to 24 wherein a sintering furnace comprises substantially continuous throughput of the articles from entry to exit of the furnace.

26. A method according to claim 25 wherein the type of sintering furnace used is selected from the group comprising: a moving mesh belt furnace; and, a walking beam furnace.

27. A method according to any one of preceding claims 22 to 26 further including the step of subjecting said sintered and infiltrated article to post- infiltration thermal treatment.

28. A method according to claim 27 wherein the post- infiltration thermal treatment comprises cryogenic treatment and tempering.

29. A method according to any one of preceding claims 22 to 28 wherein the cooling rate is at least l°C/sec.

30. A method according to any one of preceding claims 22 to 29 wherein the cooling rate lies in the range from 1 to 5°C/sec.