GB2270928A - A method of producing sintered alloy steel materials with a high degree of chemical homogenisation by mixing high alloy powders,low alloy powders,and carbon. - Google Patents

Abstract

The method 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 1.5 wt%, optionally with a lubricant, pressing the powder mixture to provide a green compact of density greater than 6.8 gm./cc, and sintering the green compact to consolidate it at a temperature below the melting point of any component of the powder so that no liquid other than lubricant is formed during the sintering process. The process produces metal alloy components, for example wear resistant components for use in the automobile industry, which have high density and high dimensional stability.

Description

A METHOD OF PRODUCING SINTERED ALLOY STEEL MATERIALS WITH A HIGH DEGREE OF CHEMICAL HOMOGENISATION BY MIXING HIGH ALLOY POWDERS, LOW ALLOY POWDERS, AND CARBON BACKGROUND TO THE INVENTION 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 mixtures of 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 105013000C 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. 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.

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 fatigue properties (Advances in Powder Metallurgy, Vol. d 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 nonhomogeneous 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 the 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 strengthening of the iron by the alloying elements. High pressed densities, and potentially good mechanical properties, 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. 218 806 2. 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 stability of dimensional control.

SUMMARY OF THE INVENTION The present invention provides 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 transient liquid alloy is formed during the sintering process.

Thus, in contrast to UK Patent No. 218 806 2, 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 highly 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. Such normal AISI specifications can be found, for example, in Metals Handbook, 10th Edition, Publ. ASM 1990, Vol.1 pages 758-759. As such the present invention is a different approach to the method described in the above patent.

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) and 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.

Other inventions (USA Patent number 4913739,1990) have described 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. In the description of the invention it is stated that a temporary liquid phase is present at approximately 10000C 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.

This requirement necessarily limits the compositional specifications of the silicon-manganese-carbon alloy powders.

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 to 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. This general applicability of the results is described by reference to a specific example that is not intended to show that the invention is restrictive.

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 etc.

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 5-908 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 0.4% 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, carbon (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.

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 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 10000C but more generally between 1100 and 12000C. 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 apsect 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.

Examples In this example, 30wt% of high alloy steel powder H100 with the following composition: 0.4wt% C, 3wt% W, 2.5wt% Mo, wtZ Cr, 2wt% V, Balance iron was mixed with 70wit% of low alloy steel or an iron powder of the following composition: Carbon 0.02% Silicon 0.05% Manganese 0.20% Phosphorus 0.02% Sulphur 0.015% Iron balance 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 gmtcc at l.Owt% admixed carbon.

In further examples the above powders were mixed in ratios such that the high alloy steel comprised from 5-60wt%, preferably 10-50wti, 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 11500C in a furnace filled with a mixture of hydrogen and nitrogen gas. The compacts were held either for one or five hours at the temperature of 11500C which was continuously checked by a thermocouple inserted alongisde the specimens of both mixtures. Detailed matallurgical 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 Miscroscope. 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 were 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 1150C and having H100 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 ppwder particle size in the mixtures is of the order of 5 Oum 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.

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.

Claims (14)

1. 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 transient liquid alloy is formed during the sintering process.

2. A method of producing a sintered metal 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 l.Swt%, optionally with a lubricant, pressing the powder mixture to provide a green compact of density greater than 6.8 gm./cc, and sintering the green compact to consolidate it at a temperature below the melting point of any component of the powder so that no liquid other than lubricant is formed during the sintering process.

3. A method according to claim 1 or claim 2 in which the high alloy steel powder has a carbon content less than the normal specification.

4. A method according to any of claims 1 to 3 in which the high alloy steel is a high speed steel, die steel or tool steel excepting that the carbon content is less than 0.8wt%.

5. A method according to any of claims 1 to 4 in which the carbon content of the high alloy steel is less than that necessary for the stochiometric balance when combined with all the available carbide formers other than iron.

6. A method according to any of claims 1 to 5 in which the percentage of high alloy steel powder is between 5 and 60wt%.

7. A method according to any of claims 1 to 6 in which the quantity of admixed carbon is between 0.5 and 1.2wt%.

8. A method according to claim 7 in which the amount of carbon admixed is between 0.5 and 1.Owt%.

9. A method according to any of claims 1 to 8 in which the high alloy steel powder is (H100) and contains carbon at substantially 0.4wt%.

10. A method according to claim 9 where the proportion of H100 is between 10 and 50wt%.

11. A method according to claim 9 or claim 10 in which the sintering temperature is between 1100 and 12200C.

12. A method according to any of claims 1 to 8 in which the carbon addition is substantially 0.8wt%.

13. A method of producing a sintered metal component substantially as described herein.

14. A sintered alloy component formed by the method of any of claims 1 to 13.