The present invention relates to the manufacture of specialised powders of different compositions that can be mixed to make structural and wear components after conventional powder metallurgy processing such as pressing, sintering and secondary operations. Powder metallurgy alloy steel powders have been utilised for the manufacture of wear resistant components in automobiles and for other high duty equipment such as earth moving plant and road construction equipment. The main application has been for the manufacture of valve seats for automotive engines. In this aspect, high speed steel alloy powders are pressed in a die to 6.2 to 7.0 g/cc and are either sintered in a gaseous environment at temperatures generally ranging from 1050- 1300°C where the action of temperature allows the powders to consolidate by known sintering mechanisms, or the compacts are infiltrated by liquid copper, in a process known as sintration, in order to achieve near full density (Metals Handbook 9th Edition, Publ. American Society for Metals, 1984, page 564). The major demand for the powders has been to exhibit high temperature wear resistance and good dimensional stability during processing.

In order to achieve the best properties from such powders a further development has been investigated whereby tool steels including high speed steel alloy powders have been mixed with low alloy powders and, indeed, pure iron with or without further additions. Amongst other objectives the aim of this procedure has been to increase the compressibility of the powder mixes which can contain between 5 and 90% of tool steel powder. In this specification all percentages are by weight. Under specific conditions, dimensional stability can be achieved (Powder Metallurgy, Vol.33, No. 4, page 313) and data has been published by some of the inventors of this application to demonstrate that these powders can achieve excellent wear and rolling contact

SUBSTITUTE SHEET

fatigue properties (Advances in Powder Metallurgy, Vol. 4 Publ. Metal Powders Industries Federation, 1991, page 135). After sintering, the identity of the tool steel powders and that of the pure iron is preserved and there is little diffusion of the alloying elements from one area to another. This means that the non-homogeneous structure, while demonstrating good properties, does not necessarily achieve its ultimate performance since the alloying elements are located in specific regions.

The current art for making powders with large amounts of alloying elements generally consists of melting the alloy composition in a furnace and atomising using water or gas. However, the fact that the powder particles produced are fully alloyed means that the particles are stronger than unalloyed iron and, therefore, cannot be pressed to such high densities.

Another method for making alloys by powder metallurgy is to mix elemental powders in proportions equivalent to the final desired composition. This method has the advantage that the compressibility of the powder mix is not reduced due to solid solution strengthening of the iron by the alloying elements. High pressed densities can therefore be achieved. However, normal sintering conditions are insufficient to produce a fully alloyed material from these powder mixes due to the limited diffusion that can be achieved. This situation can be improved if one or more of the added powders, singly or in combination, produce a liquid phase either as a transient or in a stable condition. This naturally leads to loss of dimensional stability and this may also be the case even if no liquid is present during the sintering reaction.

Another method for making alloys of this type is to blend powders of different chemical compositions (of which one or more may be prealloyed but a significant fraction is a highly compressibile powder such as pure iron). An invention of this type is described in UK Patent No. 2188062. In this

invention it is claimed that the pure iron powder does not melt during sintering but it is necessary that the high speed steel does. Again this leads to problems of the dimensional stability.

SUMMARY OF THE INVENTION

The present invention provides in one form a method of producing a sintered alloy component from powder which comprises forming a green compact from a mixture of a high alloy powder, a low alloy steel or iron powder and free carbon powder and sintering the green compact at a temperature such that no permanent liquid alloy is formed during the sintering process.

In a preferred form the invention provides a method of producing a sintered alloy component from powder, which comprises mixing a high alloy steel powder with a low alloy steel or an iron powder in the presence of a free carbon powder in the range 0.1 to 1.5wt%, optionally with a lubricant, pressing the powder mixture to provide a green compact of density greater than 6.6 gm./cc, (preferably greater than 6.8 gm./cc) and sintering the green compact to consolidate it characterised in that the sintering is at a temperature below the melting point of any component of the powder so that no permanent liquid other than lubricant is formed during the sintering process.

Thus, in contrast to GB-2188062, the present invention involves solid state sintering of all alloy components leading to good dimensional control coupled with high achievable green densities.

The invention extends to components made by the method of the invention.

The present invention evolves from a programme of study into the manufacture of high alloyed steel powders mixed with pure iron and low alloy steels as described above.

A prime consideration is to achieve good compressibility in the high alloy powder and to do this with high fractions of the tool steel addition it is sometimes found necessary to lower the carbon content in the high alloy powder outside normal AISI tool steel specifications. Preferably the carbon content of the high alloy steel powder is between 0.2 and 0.8%. Such normal AISI specifications can be found, for example, in Metals Handbook, 10th Edition, Publ. ASM 1990 or 1991, Vol.l pages 758-759. As such the present invention is a different approach to the method described in the patent GB-2188062.

In order to produce the necessary metallurgical structure this reduction was compensated for by extra carbon powder additions to the mix of high and low alloy powders. These additions were mainly in the form of graphite (Rocol X7119) preferably in the range 0.1 to 1.5% and with advantage in the range 0.5 to 1.2%. Densities in alloy steel mixes of up to 7.4 g/cc were achieved by pressing at 800 MPa. Surprisingly, it was found that the presence of free graphite in specific situations enhanced the solid state diffusion of the alloying elements from the lean carbon alloy steel powders into the iron or lower alloy powders that were mixed with them. Thus, while achieving the expected advantage of improved compressibility, unexpected increases in the distribution of alloying elements in the powder mixtures were also achieved leading to distinct property improvements. The enhanced diffusion available without the need for a liquid phase makes possible the generation of alloyed material with a high degree of homogeneity from a mixture of powders without undue loss of dimensional stability during sintering. US Patent number 4913739 describes the additions of very highly alloyed steel powders to low alloy or pure iron powders as a means for distributing alloying elements within the sintered material. This method relates specifically to the development of compositions based on iron-silicon- manganese-carbon masteralloy powders. This requirement

necessarily limits the compositional specifications of the silicon-manganese-σarbon alloy powders. In the description of that patent it is stated that a temporary liquid phase is present at approximately 1000°C in order to ensure that the elements are distributed uniformly throughout the matrix. In the present invention no liquid phase is however present at normal sintering temperatures. Also in the present invention the carbide formers of interest are those specifically found in tool steels e.g. molybdenum, tungsten, niobium, vanadium, chromium, tantalum, hafnium etc.

In the present invention the homogenisation does not rely on the presence of a liquid phase and is seen to be applicable to a wide range of tool steel and high speed steel powders which are commonly employed in industry.

Thus, the present invention is not restricted in alloy composition and does not depend upon the necessary presence of a temporary or stable liquid phase to achieve enhanced diffusion of the alloying elements.

In order to acquire sufficient hardness, components made from low alloy steel powders require to be austenitised by quenching through a temperature range from about 800 to 900°C through to 200 to 300°C at a high speed in the range of from 800 to 1000°C per minute by, for example, immersion in an oil bath. Considerable distortion can be encountered on low alloy parts when they are treated in this way. Conventionally it has been the practice that a compact made from a mixture of powders containing a significant proportion of low alloy powder or pure iron powder has been subjected to a combination of heat treatments, including the quenching normal for low alloy powder. This has conventionally been done by sintering, then cooling to room temperature at any rate, then reheating to 850 to 900°C to austenitise and then quenching in oil.

We have now found that, surprisingly, controlled cooling of our mixed powder green compacts from the sintering temperature alone can be employed to produce successful

properties. This eliminates the need for a reheating and a second high speed quench with its consequent distortion.

This allows volume production of components such as those used in the valve train of an automobile engine.

Thus in another aspect the invention comprises forming a green compact from a mixture of powders comprising a significant proportion of low alloy or pure iron powder, sintering, and cooling from the sintering temperature, at a controlled rate to produce a component which has significant hardenability requiring no austenitising.

Preferably the present invention provides a method of producing a sintered alloy component from powder which comprises forming a green compact from a mixture of powders comprising a high alloy powder (in a proportion 5 to 90 per cent), a low alloy, iron base or pure iron powder, and free carbon powder up to 1.2 per cent, optionally with a lubricant, sintering the green compact at a temperature such that no permanent liquid alloy phase is formed during the sintering process, and cooling the sintered component, in which the cooling in the range between 900 and 300°C is controlled to between 10 and 400°C per minute, measured as a mean over that cooling range, and in which no following re- austenising heat treatment is applied.

Preferably the cooling from austenising temperature is in the range 900 to 200°C and at rate of 20 to 200°C per minute.

With advantage the high alloy steel is present in the proportion 5 to 50 per cent and is a tool steel modified by reducing its carbon content as discussed above.

A stress relief heat treatment for producing specific surface finishes may be carried out but this is normally in the range 150 to 300°C and does not produce signficant distortion.

In a preferred form of powder particularly for this aspect of the invention the low alloy, iron base powder will range in total alloy content from 0 to 12 per cent

(preferably 0 to 9 per cent) the balance being iron. For example, such a powder is Molybdenum 0.85 per cent - Manganese 0.15 per cent - Carbon 0.003 per cent - Iron balance.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 to 3 show diffusion of carbide forming components across a boundary with and without added carbon, and

Figures 4 and 5 show the structures of components treated according to a preferred heating cycle and containing respectively a powder mixture and a low alloy powder only. The structures are on the same scale as the line marked 100 urn.

DETAILED DESCRIPTION OF THE INVENTION

Alloy steel powders, based on high speed steel, tool steel and high alloy steel compositions can be produced by a number of atomising systems . The specific alloy designations, composition ranges and specifications for these types of alloys are well known by metallurgists and can be found in national standards and reference books.

Specifically the alloys contain carbon at a level between 0.6 and 3wt% which is present to interact with other alloying elements which are strong carbide formers such as vanadium, tungsten, chromium, molybdenum, niobium, tantalum, hafnium etc. in order to form hard carbides within the matrix of the steel after conventional heat treatments. In addition part of the carbon is retained in solution in the matrix to allow the formation of bainite, martensite or tempered martensite in the matrix. The carbon specification for these

alloys depends on the total content of carbide forming elements excluding iron and other elements employed for deoxidation and solid solution strengthening (e.g. silicon and manganese ) .

In this present invention the carbon content of the alloy steels is generally at the minimum specification or less than this in order to achieve good compressibility of the powders. The powder particles are generally less than 250 microns diameter and the powder properties vary depending on the atomising conditions used to produce them.

After production of these alloy powders they are blended with either pure iron or low alloy steel powders e.g. Hoganas NC100.24, Hoganas Distaloy AB , Hoganas 85HP, Mannesmann WPL200, QMP 4601.

Care is necessary to achieve a homogeneous mix of the various powders. The low alloy steels generally contain varying amounts of copper, nickel, manganese and molybdenum whose total content is less than 9wt% . They are characterised by the fact that, with the exception of molybdenum, they do not contain strong carbide forming elements .

The high alloy steel powder can comprise from 5-90% of the mix. For the applications described in the introduction, the high alloy content is more commonly in the range 10-50%. The high alloy steel powders can contain between 0.2% carbon and the maximum described in standard specifications or calculated from the alloy content of carbide forming elements. Preferably the lowest carbon content is chosen (specifically between 0.2 and 0.8% of preferably between 0.2 and 0.4% carbon) in order to achieve high compressibility.

In order to provide sufficient carbon in the mixture to form alloy carbides on subsequent solid state heat treatments, free carbon powder (preferably in the form of graphite) is also mixed with the two powders such that there is sufficient for the formation of carbides by reaction with

the carbide forming elements and to provide sufficient carbon in the matrix of the final material. The amount of carbon required depends on the composition of the high alloy powder, the composition of the low alloy powder and the ratio in which they are mixed. Typically this is greater than 0.5% by weight, preferably in the range 0.5 to 1.2%.

This allows the powder mixtures to be compressed to values in excess of 6.8 g/cc, and specifically to above 7 g/cc, at compacting pressures between 400 and 900 MPa. The final density depends upon the alloy powders, compositions, ratio of different powder types and compacting pressure. This compares with typical green density values of 5.6-6.8 g/cc that can be achieved in high alloy steel powders without dilution and at normal carbon contents.

The powder mix can also contain lubricants that are commonly employed in steel powder metallurgy, such as zinc stearate or proprietary waxes, which can be added at up to 2wt%. The powder mixtures can be placed in compaction dies and pressed at a variety of pressures, commonly between 600 and 900 MPa. On ejection from the die the green compact has sufficient strength to be handled and moved without further support.

Commonly, the mixtures are then heated to a temperature where consolidation takes place by diffusion across powder interfaces in order to form a coherent body. Typically this is undertaken at temperatures in excess of 1000°C and more generally between 1100 and 1200°C. The upper limit of temperature is controlled by the melting point of the final composition of the mixed powders, at which point the dimensional stability of the compacts is lost. Small transitory liquid phases may be present but are not necessary and are not a preferred aspect of the invention.

The addition of a controlled amount of carbon as a specific element to replace the carbon lost during the minimising of the carbon content of the high alloy steel powder results in unexpected increases in the sintering

kinetics of the compact. This is achieved by an increase in the diffusion of the carbide forming elements from the high alloy steel powder to the low alloy or pure iron matrix. While it is not unexpected that carbon diffuses very rapidly and quickly at these sintering temperatures, it is unexpected to see enhanced diffusion of the carbide forming elements. It is this aspect that is the basis of this current invention.

The following examples are given by way of illustration to demonstrate this invention and to demonstrate the general applicability of this invention to a wide range of high alloy steel powders mixed with low alloy steel or pure iron powders containing admixed carbon.

Example 1

In this example, 30wt% of high alloy steel powder HI00 with the following composition:

0.4wt% C, 3wt% W, 2.5wt% Mo, 4wt% Cr, 2wt% V, Balance iron

was mixed with 70wt% of low alloy steel or an iron powder of the following composition:

Carbon additions of up to lwt% and 0.75wt% manganese stearate lubricant were made and samples pressed at pressures of 600 MPa. The green density of such compacts was 7.10 gm/cc at 1.0wt% admixed carbon.

In further examples the above powders were mixed in ratios such that the high alloy steel comprised from 5-60wt%, preferably 10-50wt%, with free carbon comprising 0.1 to 1.5wt% for example 0.4wt% or 0.8wt%.

As a control experiment, similar powder mixes were made where no admixed carbon was added. The sintered compacts were subsequently sintered at 1150°C in a furnace filled with a mixture of hydrogen and nitrogen gas controlled so that no permanent liquid phase formed. The compacts were held either for one or five hours at the temperature of 1150°C which was continuously checked by a thermocouple inserted alongside the specimens of both mixtures. Detailed metallurgical examination of the resulting microstructures indicated a greater than expected degree of diffusion of the carbide forming elements as indicated by the distribution and type of phases present.

A typical linear dimensional change from pressed part to sintered part was measured at 0.2%. This compares with the sintering of high speed steels to full density where a liquid phase is present and a 10% linear dimensional change takes place. (Metal Powder Report Vol.35, No.6, June 1980 page 242. )

It is difficult to demonstrate this quantitatively by mixing the powders randomly so, as a further clarification of this example, diffusion couples have been produced. In this instance, the pure iron and high alloy steel powders have been separately pressed with the appropriate elemental carbon added. Buttons of these powders have been subsequently pressed together so that on one side there is high alloy steel and on the other side pure iron.

These were encased in a steel powder mixture to maintain contact between the two powders and further pressed to form a compact. In some cases no carbon was admixed. This provided a comparison between the presence and absence of admixed carbon.

After sintering in a similar manner to that described

above the diffusion couples were sectioned and analysed using an electron dispersive X-ray system attached to a JEOL 6400 Scanning Electron Microscope. The EDX system was a thin window Tracor analyser and counts were taken at 10 micron intervals either side of the original interface. The number of counts taken was standardised at 100,000 which is necessary to achieve an accuracy of plus or minus 5% of the quoted value. The results for samples sintered for five hours at 1150°C and having HlOO high alloy material on the left side and pure iron on the right side, are shown in figures 1, 2, and 3 where the diffusion profiles respectively for chromium, tungsten and molybdenum into the pure iron are plotted. In figures 1 to 3 the low alloy steel was Mannesmann WPL200 pure iron powder.

In all cases there is a significant increase in the amount of diffusion of the carbide forming elements when the admixed carbon is present. For example, in the case of chromium measured at 30 microns from the original interface into the pure iron side, the chromium content is approximately half of the original value of the high alloy steel compared with an eighth of the original value if no carbon is present. Similar numbers are seen for tungsten and molybdenum as shown in the figures. Indeed, for the case of tungsten there is almost zero presence of the elements 30 microns from the original interface if no carbon is present. Given that the mean iron or low alloy powder particle size in the mixtures is of the order of 50um then it can be expected that a significant amount of homogenisation will take place if carbon is present compared with little diffusion if no carbon is present.

Example 2

Two metallic based powders were mixed together; the first consisted of a 4600 type prealloyed iron powder of composition nickel 1.8 per cent, molybdenum 0.55 per cent,

manganese 0.2 per cent, carbon 0.003 per cent, balance iron, designated QMP 4601 powder. To this was added 20 per cent w/w of modified tool steel powder designated HlOO, of composition chromium 4 per cent, tungsten 3 per cent, molybdenum 2.5 per cent, vanadium 2 per cent, carbon 0.4 per cent, balance iron. To this was admixed 0.5 per cent of synthetic graphite and 0.55 per cent of magnesium stearate.

These constituents were blended in a powder mixer and the blended mixture was then compacted at 6 tonne/sq cm on rectangular section parts of 10.5 x 10.5 x 70mm size. These green compacts were then sintered in a mixed nitrogen- hydrogen atmosphere (50 per cent hydrogen, 50 per cent nitrogen) at a temperature of 1150 C for 30 minutes. The compacts were then cooled at a controlled rate and in the present example the mean cooling rate from 900C to 250C was controlled at approximately 45°C per minute.

In order to relieve the components of any residual stresses formed during cooling a stress relief treatment at 205°C was employed for 60 minutes in air. This was not found to affect significantly the distribution or shape of optically visible alloy carbides formed during the cooling cycle. An example of the final structure is shown in Fig. 4 where the alloy carbides in the area which initially consisted of the modified tool steel composition can be clearly seen. By comparison Fig. 5 shows the structure, on the same scale, of a control sample, treated in the same way, and made from the 4600 type prealloyed powder but without the HlOO.

Surprisingly the mechanical properties that were achieved by controlling the cooling cycle from the sintering temperature were significantly better than would be expected from a simple law of mixtures (e.g. the properties of the low and high alloy powders weighted with respect to their volume percentages). Also such a cooling cycle would not normally be appropriate for the modified tool steel compositions where significant amounts of brittle martensite phase might be

present or retained austenite which would give rise to instability during application.

The properties of the components formed from Blend A (mixed powder) and Blend B (low alloy powder only) are given as follows:

Blend A B(low alloy powder only)

HlOO content % 20 nil

Density after sintering (gm/cc) 7.10 7.15 Apparent hardness after sintering (HRB) 98 87

HlOO microhardness (HV200) 650 Transverse rupture strength (MPa) 1210 1060

Thus the present invention describes a method whereby a significant increase in the degree of diffusion of carbide forming elements from high alloy powders into low alloy or pure iron powders can be achieved compared with mixtures where no free graphite is present or where the total carbon content is fully alloyed in the two powder phases.