Title of Invention	A PROCESS FOR THE MANUFACTURE OF A FERROUS-BASED SINTERED ARTICLE
Abstract	A process for the manufacture of a ferrous-based sintered article containing copper in the range from 12 to 26 weight% is described, the process including the steps of: making a powder mixture having a desired composition, at least a proportion of a total content of iron and copper being provided by an iron powder having copper indivisibly associated therewith; compacting said powder mixture to form a green compact of an article to be produced and sintering said green compact.

Title of Invention

A PROCESS FOR THE MANUFACTURE OF A FERROUS-BASED SINTERED ARTICLE

Abstract

A process for the manufacture of a ferrous-based sintered article containing copper in the range from 12 to 26 weight% is described, the process including the steps of: making a powder mixture having a desired composition, at least a proportion of a total content of iron and copper being provided by an iron powder having copper indivisibly associated therewith; compacting said powder mixture to form a green compact of an article to be produced and sintering said green compact.

Full Text	A PROCESS FOR THE MANUFACTURE OF A The present invention relates to a process for the manufacture of a ferrous-based sintered article made therefrom and to a method for their manufacture particularly, ferrous materials containing copper. The powder metallurgy route enables the design of metallic materials which it is not possible to make by- conventional casting and ingot working processes. It is known to infiltrate sintered ferrous powder metallurgical products with metals having lower melting points such as lead and copper, for example. Lead is used to improve machinability of sintered ferrous materials whilst copper also has this effect but also has other desirable properties which it confers on the sintered material. Lead is nowadays avoided due to its harmful environmental properties. Copper improves machinability and also improves the thermal conductivity of the sintered article. Copper infiltrated products are used extensively in the automotive industry for applications such as valve seat inserts in the cylinder heads of internal combustion engines, for example. Such products have to perform under very arduous conditions including repeated impact loading, marginal lubrication, elevated service temperatures and hot corrosive gases. Properties to withstand these conditions are achieved by the suitable design of the ferrous matrix system. Such ferrous matrices are often highly alloyed which adversely affects machinability. Machinability is important to an engine builder in a production context as it affects productivity. Copper infiltration provides improved machinability whilst the copper, itself provides improved thermal conductivity which has the effect of lowering operating service temperatures which helps to retain mechanical properties. The infiltration process is effected by stacking a copper alloy compact in contact with the ferrous component and passing the stacked assembly of the two items through a sintering furnace at a sintering temperature in the region of about 1100°C under an inert or reducing gaseous atmosphere thus effecting sintering and infiltration simultaneously. During this sintering process the copper alloy compact melts and the molten alloy infiltrates and :¦ fills the pores of the ferrous component by capillary action. Only interconnected pores can be filled in this manner, isolated or otherwise unconnected porosity cannot be so filled. The composition of the copper alloy compact is so chosen that it is compatible with the ferrous material and undesirable reactions or erosion thereof is avoided as far as possible. The weight of the copper alloy compact is chosen so as to be able to fill the majority of the pores, however, as noted above there is inevitably some residual porosity. In a variation of the above process, the copper alloy compact is stacked with a pre-sintered ferrous component and the two passed through a sintering furnace to effect infiltration. The infiltration process is an expensive process owing to the extra process steps involved. The process requires the additional steps of: making a separate copper alloy powder mixture; pressing suitable compacts of the correct weight from the powder mixture; stacking the compacts with the ferrous articles themselves prior to passing through the sintering furnace; and, barrelling the sintered and infiltrated articles after cooling to remove the powdery deposit which inevitably forms on the articles during the sintering process. In conventional copper infiltrated ferrous products, the level of copper content generally lies in the range from. 15 to 25 weight%. In non-infiltrated products it is common to add up to about 5 weight% of copper powder in the pre-compacted powder mixture. Such relatively small additions of copper to non-infiltrated ferrous materials assist the sintering process due to the liquid copper phase being present. People have tried to add levels of copper achieved in the infiltration process by means of additions of the appropriate amount of elemental copper in the initial powder mixtures prior to compaction and sintering. However, due to differences in, for example, powder particle size, powder density and powder particle morphology, segregation of the copper tends to occur during handling of the powder mixtures. Such powder segregation causes unacceptable variations in the resulting products. Where only small amounts of elemental copper powder is present such as the case of up to about 5 weight% noted above, segregation still occurs but the effect in the resulting products is minimised and does not cause a serious problem. At one time components such as valve seat inserts for engines having the most arduous service environment were made entirely from highly alloyed steels such as M3/2 class steels for example. Such steels contain relatively high quantities of . chromium, tungsten, - molybdenum, vanadium and the like. Whilst components made from such materials have excellent performance and long service lives, they are inherently expensive to make and process. They are expensive to make firstly because of the high intrinsic material cost and secondly expensive to process because of the difficulty in machining components having high contents of hard carbide in the microstructure thereof. In the never ending quest to lower costs, much work has been carried out to reduce material cost by adding relatively high proportions of substantially pure iron powder to the powder mixes and consequently reducing processing costs by making the resulting sintered materials easier to machine by reducing the amount of hard phases and adding phases which assist machinability such as copper or chip-breaking phases. A disadvantage in terms of performance and longevity of life of these newer materials such as may be exemplified in GB-A-2 188 _Q62 for example is the retention in the cores of the iron grains, formed by the sintering together of the original compacted iron powder particles in the powder mixture, of soft ferrite phase which can deleteriously affect the wear and strength, properties thereof. Such materials may initially comprise mixtures of about 50% of the highly alloyed M3/2 material, for example, and about 50% of pure iron powder and minor additions of carbon, die lubricating waxes and the like. Even when fully sintered the iron grains have ferrite cores with only some diffusion of chromium, from the M3/2 regions, into the surface regions of the iron grains, where martensite may be formed, after sintering. This structure still applies even when the material is infiltrated or when up to about 5 weight% of elemental copper has been added to the powder mixture. It is an object of the present invention to provide a process for making ferrous material articles having a high copper content commensurate with that of infiltrated material but without the disadvantage of the additional process steps required in the prior art processes. Other advantages will become apparent from the description of the invention below. According to a first aspect of the present invention, there is provided a process for the manufacture of a ferrous-based sintered article containing copper in the range from 12 to 26 weight%, the process including the steps of: making a powder mixture having a desired composition, at least a proportion of a total content of iron and copper being provided by an iron powder having copper indivisibly associated therewith; compacting said powder mixture to form a green compact of an article to be produced and sintering said green compact. The copper content is primarily intended to enhance the thermal conductivity of articles produced, however, other important benefits are also provided to articles made by the method of the present invention. Below 12 weight% copper the required enhancement in thermal conductivity is not achieved whilst above 26 weight% "bleeding" of molten copper from the material during sintering is a problem. Preferably, the copper content may lie in the range from 15 to 20 weight%. In the process according to the present invention, the iron powder indivisibly associated with copper is effectively a pre-alloyed powder in that the individual powder particles comprise both iron and copper and consequently significant segregation between the iron and copper is not possible. The iron and copper powder particles may be selected from two basic types of powder stock: a pre-alloyed iron-copper powder; or, a diffusion bonded iron-copper powder. The pre-alloyed iron-copper powder may be produced by known techniques of melting the constituent materials together and then atomising the molten melt by water or gas, for example, to produce the required pre-alloyed powder. The diffusion bonded iron- copper material is produced by making a mixture of elemental iron and copper powders, for example, and passing the mixture, uncompacted, through a furnace such that diffusion between the particles occurs so as to bond them together. The "cake" so formed is given a light crushing operation to break it up into particles comprising both iron and copper adhered to each other. Such a process causes diffusion of some copper into the outer regions of each iron particle. The method of the present invention obviates several of the process steps of prior art processes in that a separate copper alloy powder mixture and consequent compacts do not need to be made, they do not need to be stacked with the ferrous material compacts and the final sintered articles do not need to be treated to remove the adherent deposit thereon as with prior art infiltration processes. A particular advantage conferred by the method of the present invention relates to the processing of those ferrous materials which comprise mixtures of an alloyed steel powder and a low-alloy iron or substantially pure iron powder. It is known to use such mixtures with additions of carbon powder, for example, and to process them by compaction, sintering and post-sintering thermal treatment into articles such as valve seat inserts for internal combustion engines, for example. Such prior art materials may or may not be infiltrated with a copper alloy by one of the conventional processes described above. Such materials are exemplified by those materials and production processes described in GB-A-2 188 062 and EP-A-0 312 161 for example. These materials may comprise a proportion, e.g. about 50 weight% of a highly alloyed steel powder with about 50 weight% of a substantially pure iron powder. The alloyed steel powder usually contains chromium which under the prevailing sintering conditions of about 1100°C is one of the most mobile element atoms after carbon, in terms of rate of diffusion, of those alloying elements which promote the formation of martensite on cooling of the article following sintering. Carbon atoms are the most mobile, moving into the interstices of the iron atoms in the crystal structure. However, since chromium is of a similar atomic size and weight to iron it substitutes for iron and consequently has a similar mobility to iron under the prevailing sintering conditions. The presence of chromium promotes the formation of martensite in those regions of the sintered material into which it diffuses, the martensite being formed on cooling of the material at the end of the sintering cycle. Sintering is frequently effected for such articles in furnaces which have continuous moving means, such as a belt or a walking-beam type mechanism, for transporting the articles, 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 which serves to provide either a neutral or reducing atmosphere and preclude air (oxygen) from entering the furnace. The atmosphere is at substantially atmospheric pressure with only a slight positive pressure within the furnace to prevent air from entering therein. Where the sintered material contains a significant quantity of iron powder in the original mix it is frequently found that the iron grains resulting from the sintering of the compacted iron powder particles possess a microstructure ranging from ferrite to pearlite and mixtures of the two phases, depending upon the carbon content, in the core of the iron-rich non-tool steel regions. The outer region of the iron grains generally comprises martensite resulting from chromium which has diffused in during the sintering operation but the core remains essentially as ferrite or pearlite or a mixture of ferrite and pearlite depending upon the added carbon level. In the as-sintered condition, the iron-rich non-tool steel phase or grain structure consists of mainly pearlite, though there may be some ferrite, at the centre and the outer regions of the grains are a mixture of martensite/bainite. If there is any retained austenite in the sintered article it is generally transformed by cryogenic treatment after sintering. During a tempering operation usually carried out after cryogenic treatment, partial decomposition of the pearlite phase occurs leading to the formation of ferrite areas within the iron-rich grains or phase. This can result in the material having inferior wear properties due to the presence of ferrite and also lower strength due to the ferrite. The post-sintering thermal treatments comprising cryogenic treatment to transform any remaining y-phase (austenite) to martensite followed by tempering treatments are to reduce the degree of hardness and brittleness of the martensite phase rather than to effect decomposition of the pearlite which is an undesirable side effect of the tempering process. Since the tempering treatment is carried out at a temperature in excess of the expected service temperature, size stability of the article in its service environment (e.g. a valve seat insert in the combustion chamber of an internal combustion engine) is ensured. However, such treatments do not affect the presence (other than to be responsible for generating at least a proportion of the ferrite) of the ferrite phase or its inherently poor wear and mechanical properties. It has been found that with the method of the present invention that there appears to be a synergistic effect of the copper (either from the diffusion-bonded form or in the pre-alloyed form with the iron) and chromium together in promoting the diffusion of copper and chromium towards the centre of the iron grains and, instead of the core of the iron grains remaining as ferrite or pearlite or a mixture of these, the core of the iron grains is found to transform to martensite during normal furnace cooling. Sintered ferrous materials made according to the process of the present invention using either pre-alloyed iron-copper or diffusion bonded iron-copper powders reveal the presence of martensite in the cores of the iron-rich grains due to the diffusion of chromium or other martensite promoting elements into the iron grains. The martensite is formed during the cooling of austenite and any retained austenite is transformed by cryogenic treatment following sintering. During the cooling process from the sintering temperature some of the austenite can also transform to bainite. The martensite may then be tempered to form a structure of tempered martensite which is readily machinable. However, it is important to note that the previously soft ferritic/pearlitic cores of the iron grains now comprise material which is harder, stronger and more wear resistant due to the process of the present invention. It is believed that the processing used to form the pre- alloyed and diffusion bonded iron-copper material causes at least some diffusion of the copper phase into the iron constituent and the presence of the copper assists in the diffusion of chromium and other martensite promoting elements into the cores of the iron grains formed on sintering thus, enabling martensite to be formed. Tests making materials according to the method of the present invention and making substantially identical materials by prior art infiltration processes, but using substantially identical processing parameters of pressing pressure and sintering temperature for example, have shown the beneficial effects of using an iron-copper pre- alloy or diffusion bonded powder as described hereinabove. Materials of largely identical composition except for the copper content were made by 1) the method of the present invention; 2) by the route of simultaneous sintering and infiltration; and, 3) by adding 13 weight% elemental copper powder to the initial powder mixture and sintering (i.e. without infiltration and without the addition of pre-alloyed iron-copper powder). Materials made by conventional infiltration techniques under the same processing conditions do not show the beneficial effect of martensite formation in the iron grain core. Analysis by scanning electron microscope has shown the presence of chromium in the particle core in materials made by the method of the present invention. It is to be emphasised that the processing conditions used in the comparative tests are the same processing conditions used for production of commercial prior art materials and thus represent the current optimum processing conditions taking all factors into account. Materials made according to the method of the present invention may also receive post-sintering thermal treatments such as cryogenic treatment at -120°C or below to convert any residual austenite phase to martensite, followed by tempering to make the martensite softer, more dimensionally stable and make it amenable to machining. Thus, according to a feature of one embodiment of the present invention, the powder mixture contains a powder component comprising a relatively un~alloyed iron powder and a powder component comprising a steel powder containing at least some chromium or other martensite promoting element as an alloying element in addition to the pre-alloyed or diffusion bonded iron-copper powder. Alternatively or additionally the powder mixture may contain addition(s) of elemental martensite promoting material such as molybdenum and/or nickel for example. Examples utilising M3/2 high speed steel powders are described herein, however, any other suitable tool steel or high speed steel, for example, chromium-containing steel powder may be employed depending upon the application in which the article produced therefrom is to be used. An example of an. alternative steel material is so-called 316 steel which is a stainless steel comprising in weight%: 17 Cr/ 2 Mo/ 13 Ni/ Bal Fe and which is substantially carbon free. Thus, it appears that the manner in which copper . is introduced into the sintered ferrous material, i.e. by being associated with the iron where there has been prior treatment causing reaction therebetween, has an unexpected and synergistic effect in aiding diffusion of chromium or other martensite promoting elements through the iron matrix to assist in the transformation to martensite on cooling after sintering or by transformation of retained austenite by cryogenic treatment. The composition of the iron-copper pre-alloyed or diffusion-bonded material may be any desired, e.g. iron- 2 0 copper. Powder mixtures may be made up having powder components comprising: iron; iron-copper; pre-alloyed steel powder; and, carbon powder, for example. The amount of iron-copper pre-alloy powder will depend upon the final required copper content in the article and on the initial composition of the iron-copper pre-alloy powder. The use of iron-copper pre-alloyed and/or diffusion bonded material in a powder mixture together with an addition on elemental copper powder is not precluded and in some circumstances may be beneficial. The use of both pre-alloyed and diffusion bonded iron-copper powder may also be employed in a powder mixture. The pre-alloyed iron-copper material appears to be somewhat more effective in promoting the formation of martensite in iron grains than does diffusion bonded iron-copper material. Therefore, the use of the pre- alloyed material is preferred, however, it is pointed out that the diffusion bonded material produces martensite after sintering and subsequent processing whereas prior art infiltrated materials do not produce any martensite in the iron grain cores, the cores comprising only mixtures of pearlite and ferrite. According to a second aspect of the present invention, there is provided a sintered article produced by the first aspect of the present invention. 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 drawings, of which: Figure 1 shows a histogram showing wear of valve seat inserts in an engine test on material made according to . the present invention; and Figure 2 which shows a graph of tool wear vs number of parts machined for materials made according to the present invention and prior art material. Example 1 was a material prepared by the method of the present invention where all of the iron and a proportion of the copper were added as pre-alloyed iron-20 copper powder. The pre-alloy powder contributes about 9.5 weight% of copper to the final material. A further 6 weight% of elemental copper powder was added to the initial powder mixture to bring the total copper up to 15 weight%. The steel pre-alloy powder was a water atomised M3/2 powder having a nominal composition of: 1 C; 4 Cr; 5 Mo; 3 V; 5 W. Since only 6 weight% of elemental copper powder was added, segregation was minimised. Example la is powder mixture wherein all of the iron powder content is provided as pure iron powder and the copper content as 13 weight% of elemental copper powder. Whilst such material would not normally be made with such a high content of elemental copper powder for the reasons discussed hereinbefore, the material was made to Valve Seat Insert Material- Example 1 Ferrous powder mixtures of a typical composition used in the production of valve seat inserts for internal combustion engines were prepared by various routes. The compositions of the powder mixtures in terms of the actual constituent component powders used to make them were as set out below in Table 1: determine the effect of the copper content on the diffusion characteristics of the chromium into the iron constituent. Example 1b was made by the prior art process according to GB-A-2 188 062 wherein the copper is supplied via a simultaneous sintering and infiltration step. All of the powders were blended according to established principles in a Y-cone mixer. Compaction pressure was in the range 650-800MPa in each case followed by sintering at around 1100°C in a conveyor furnace, all Examples being sintered under the same conditions. Following sintering all Examples were cryogenically treated at 12C°C to transform any remaining austenite (y-phase) in the structure and then tempered at 600°C for 2 hours to soften the martensite, make it more dimensionally stable and enhance machinability qualities. Table 2 below gives the actual compositions in terms of the constituent elements, the density of the sintered material and its final hardness following cryogenic and tempering post-sintering treatment. The microstructure of samples made according to Example 1 showed a tempered martensite structure even in the cores of the iron grains. The martensite was formed on cooling from the sintering temperature. Cryogenic treatment was used to transform any retained austenite in the M3/2 phase of the material to martensite. The change from austenite to martensite is not easily seen under the microscope, the change being evidenced by increased hardness on the change from austenite to martensite. Samples from Example la showed a microstructure comprising some martensite formed on cooling from the sintering temperature and retained austenite. Following cryogenic treatment, the retained austenite transformed to martensite in the M3/2 regions and the iron grains comprised mainly pearlite (a phase comprising a lamellar structure of ferrite and cementite) and some ferrite. The pearlite was formed by virtue of the carbon powder added as graphite, however, owing to the absence of chromium in the iron grain cores, no martensite was formed. On tempering, extensive decomposition of pearlite took place and the volume fraction of ferrite increased compared with that of the as-sintered state. Thus, the wear resistance of Example la material is inferior and the mechanical properties, as evidenced by the hardness figures, are also inferior. Samples from Example 1b demonstrated almost identical structure and properties as did Example la. This material was made according to the known process of GB-A-2 188 062^, The hardness of Example lb was slightly higher than Example 1, this being attributed to the higher density of the material following infiltration. However, the material of Example lb showed extensive quantities of inherently weaker ferrite areas after tempering and not the desirable tempered martensite structure shown by Example 1 made according to the process of the present invention. Figure 1 shows a histogram of valve seat insert wear of valve seat inserts, made from the material of Example 1, in the exhaust positions of a 1.81, 4-cylinder, 16-valve engine which was run for 180 hours at 6000 rev/min on unleaded gasoline, the engine having Stellite (trade name) faced valves. The success criteria for this test is that valve seat insert wear must not exceed 100mm. As may be seen from Fig.l the maximum wear was at valve seat position 4 at 60mm, all other inserts having wear of about 30pm or less. Thus, it is clear from Examples 1, la and 1b that the only substantive difference in the manufacture thereof was the manner in which the copper was introduced into the sintered material. It is believed that the improved structure and properties are directly attributable to the use of the iron-copper pre-alloyed materials wherein at least a proportion of the copper is indivisibly associated with the iron and stem from the enhanced diffusion promoted by this pre-alloyed material. Example 2 A powder mixture comprising 45 wt% M3/2 tool steel powder/ 0.55C/ 1 MoS2 / 6 Cu/ 47.45 FeCu20 (diffusion bonded powder)/ 0.75 lubricating wax was made. This mixture was compacted into green compacts at 770MPa to a green density 7.1 Mgm-3 and sintered at about 1100°C under a continuous flowing nitrogen-hydrogen gas atmosphere in a conveyor furnace. The sintered articles were cryogenically treated at -120°C or below to convert retained austenite to martensite and finally tempered at 600°C. Density of the sintered material was 7.0 Mgm3. The hardness of the as sintered material was 61HRA; that of the cryogenically treated material 65HRA; and that of the cryogenically treated and tempered material 62-65 HRA. The microstructure of the Example 2 material (made with diffusion-bonded iron-copper powder) after tempering , (following sintering and cryogenic treatment) showed some small occasional areas of ferrite in the iron-rich non- tool steel phase. However, this iron-rich phase comprised essentially pearlite rather than the extensive regions of ferrite typified by the prior art material made using the infiltration technique. Example 3 A mixture comprising in weight%: 75% pre-alloyed Fe-Cu20 powder/ 23% 316 stainless steel powder/ 0.75% MoS2 powder/ 1% carbon powder was prepared; this material being coded Nl. The composition of the 316 stainless steel was 17 Cr/ 2 Mo/ 13 Ni/ bal Fe. A comparative example coded N was made from the following mixture in weight%: 70.9% unalloyed iron powder/ 27% 316 stainless steel powder/ 0.9% MoS2 powder/ 1.2% carbon powder. Both materials were compacted at 770MPa. However, material Nl was sintered only (as there was about 15 wt% Cu provided by the Fe-Cu pre-alloy) and material N was simultaneously sintered and infiltrated according to the known prior art process. The final theoretical overall composition of both materials Nl and N in weight% was: 1 C/3.9 Cr/15 Cu/0.9 Mo/3 Ni/S 0.3/bal Fe. The sintering/infiltration steps were carried out at about 1100°C under a flowing nitrogen/hydrogen atmosphere. Both materials following sintering were cryogenically treated and tempered. The Nl material showed a microstructure having no ferrite, even in the cores of the grains which were predominantly iron. The structure of this material showed essentially a tempered martensite structure. The N material on the other hand showed extensive ferrite in the iron grains with a pearlitic structure in the transition zones between prior iron particles and 316 stainless steel particles even though this material had slightly higher carbon at 1.2%. Thus, again the influence of the copper being indivisibly associated with the iron is shown in the resulting structure after processing. Example 4 Further mixtures denoted as material FMCA and FMCD were made according to the present invention. The blend compositions of these materials in terms of the constituents in the powder mixtures are given below in Table 3. The materials were compacted at 770 MPa and sintered at about 1100°C under a continuous gaseous atmosphere as with previous examples. The resulting densities and hardnesses of the sintered materials are given below in Table 3 FMCA 1 FMCD Fe-20 Cu (pre-alloyed) 75 75 C 1.35 1.35 Mo 0.5 ______________________ MoS2________________________1_________________ Unalloyed Fe_____________23.15 22.65 Lubricating wax__________ 0.75 0.75______________ Table 4. For these samples no post-sintering heat treatment was carried out. In the FMCA material made according to the present invention pre-alloyed Fe-Cu powder and 0.5% elemental Mo powder were used in the initial powder mixture. The FMCA material showed extensive Mo-rich zones and martensitic and bainitic areas associated with these zones. The FMCA material also showed grain boundary carbides. The microstructure of the FMCA material was somewhat" similar to a comparative material, coded FMC (unalloyed iron powder, 1.35% C, 0.5% Mo), wherein the copper content was provided by a simultaneous sintering and infiltration process according to the prior art. Apart from the infiltration step, the sintering conditions were the same as those for the FMCA and FMCD materials. In the FMC material grain boundary carbide was present, the matrix was pearlite and the Mo-rich zones associated with the Mo particles were present but very small compared with the FMCA material. During sintering, the MoS2 in the FMCD material undergoes partial decomposition and donates free Mo to the structure which potentially is able to generate a localised martensitic/bainitic structure associated with the Mo-rich zones. Some of the sulphur from decomposed MoS2 reacts with iron and copper to form metallic sulphides which are beneficial for improving machinability. In the FMCD material no carbide networks could be seen and the matrix was pearlitic. Figure 2 shows a graph of tool wear vs number of parts machined for FMC, FMCA and FMCD materials. The Figure confirms that the materials using pre-alloyed Fe-Cu powders which give rise to extensive martensitic/bainitic areas do not have their machinability impaired in spite of the stronger, more wear resistant material structures so formed. Indeed, the machinability of the both the FMCA and FMCD materials is superior to the FMC material made by a prior art process. WE CLAIM : 1. A process for the manufacture of a ferrous-based sintered article containing copper in the range from 12 to 26 weight%, the process involving the steps of: making a powder mixture of the desired composition, said mixture comprising: a pre-alloyed steel powder, a powder in which iron and copper are indivisibly associated with each other, being selected from the group consisting of a diffusion bonded iron-copper powder and pre-alloyed iron-copper powder, and a material for promoting the formation of martensite, being selected from the group consisting of carbon, chromium, molybdenum, nickel and molybdenum disulphide, said material being either added to the mixture as a powder or already present in the pre-alloyed steel powder, the process also involving compacting said powder mixture to form a green compact of the article, and sintering said green compact to produce the article which has a ferrous matrix with a martensitic structure. 2. A process as claimed in claim 1 wherein the copper content lies in the range from 15 to 20 weight%. 3. A process as claimed in claim 1 wherein the steel powder is a high-speed steel powder. 4. A process as claimed in claim 3 wherein the steel powder is an M3/2 steel powder. 5. A process as claimed in claim 1 wherein the steel powder is a stainless steel powder. 6. A process as claimed in claim 5 wherein the stainless steel powder is 316 steel. 7. A process as claimed in claim 1 wherein the powder mixture contains carbon powder. 8. A process as claimed in claim wherein the iron-copper powder has a composition in weight% of 20 copper and the balance iron. 9. A process as claimed in claim 1 wherein the powder mixture also contains elemental copper powder. 10. A process as claimed in claim 1 wherein said material for promoting the formation of martensite is alloyed with the pre-alloyed steel powder. 11. A process as claimed in claim 1 wherein said material for promoting the formation of martensite is included in the powder mixture as a powder. A process for the manufacture of a ferrous-based sintered article containing copper in the range from 12 to 26 weight% is described, the process including the steps of: making a powder mixture having a desired composition, at least a proportion of a total content of iron and copper being provided by an iron powder having copper indivisibly associated therewith; compacting said powder mixture to form a green compact of an article to be produced and sintering said green compact.

Full Text

A PROCESS FOR THE MANUFACTURE OF A
The present invention relates to a process for the
manufacture of a ferrous-based sintered article made
therefrom and to a method for their manufacture
particularly, ferrous materials containing copper.
The powder metallurgy route enables the design of
metallic materials which it is not possible to make by-
conventional casting and ingot working processes. It is
known to infiltrate sintered ferrous powder metallurgical
products with metals having lower melting points such as
lead and copper, for example. Lead is used to improve
machinability of sintered ferrous materials whilst copper
also has this effect but also has other desirable
properties which it confers on the sintered material.
Lead is nowadays avoided due to its harmful environmental
properties. Copper improves machinability and also
improves the thermal conductivity of the sintered
article.
Copper infiltrated products are used extensively in the
automotive industry for applications such as valve seat
inserts in the cylinder heads of internal combustion
engines, for example. Such products have to perform under
very arduous conditions including repeated impact
loading, marginal lubrication, elevated service
temperatures and hot corrosive gases. Properties to
withstand these conditions are achieved by the suitable
design of the ferrous matrix system. Such ferrous
matrices are often highly alloyed which adversely affects
machinability. Machinability is important to an engine
builder in a production context as it affects
productivity. Copper infiltration provides improved
machinability whilst the copper, itself provides improved
thermal conductivity which has the effect of lowering
operating service temperatures which helps to retain
mechanical properties.
The infiltration process is effected by stacking a copper
alloy compact in contact with the ferrous component and
passing the stacked assembly of the two items through a
sintering furnace at a sintering temperature in the
region of about 1100°C under an inert or reducing gaseous
atmosphere thus effecting sintering and infiltration
simultaneously. During this sintering process the copper
alloy compact melts and the molten alloy infiltrates and :¦
fills the pores of the ferrous component by capillary
action. Only interconnected pores can be filled in this
manner, isolated or otherwise unconnected porosity cannot
be so filled. The composition of the copper alloy compact
is so chosen that it is compatible with the ferrous
material and undesirable reactions or erosion thereof is
avoided as far as possible. The weight of the copper
alloy compact is chosen so as to be able to fill the
majority of the pores, however, as noted above there is
inevitably some residual porosity.
In a variation of the above process, the copper alloy
compact is stacked with a pre-sintered ferrous component
and the two passed through a sintering furnace to effect
infiltration.
The infiltration process is an expensive process owing to
the extra process steps involved. The process requires
the additional steps of: making a separate copper alloy
powder mixture; pressing suitable compacts of the correct
weight from the powder mixture; stacking the compacts
with the ferrous articles themselves prior to passing
through the sintering furnace; and, barrelling the
sintered and infiltrated articles after cooling to remove
the powdery deposit which inevitably forms on the
articles during the sintering process.
In conventional copper infiltrated ferrous products, the
level of copper content generally lies in the range from.
15 to 25 weight%. In non-infiltrated products it is
common to add up to about 5 weight% of copper powder in
the pre-compacted powder mixture. Such relatively small
additions of copper to non-infiltrated ferrous materials
assist the sintering process due to the liquid copper
phase being present.
People have tried to add levels of copper achieved in the
infiltration process by means of additions of the
appropriate amount of elemental copper in the initial
powder mixtures prior to compaction and sintering.
However, due to differences in, for example, powder
particle size, powder density and powder particle
morphology, segregation of the copper tends to occur
during handling of the powder mixtures. Such powder
segregation causes unacceptable variations in the
resulting products. Where only small amounts of elemental
copper powder is present such as the case of up to about
5 weight% noted above, segregation still occurs but the
effect in the resulting products is minimised and does
not cause a serious problem.
At one time components such as valve seat inserts for
engines having the most arduous service environment were
made entirely from highly alloyed steels such as M3/2
class steels for example. Such steels contain relatively
high quantities of . chromium, tungsten, - molybdenum,
vanadium and the like. Whilst components made from such
materials have excellent performance and long service
lives, they are inherently expensive to make and process.
They are expensive to make firstly because of the high
intrinsic material cost and secondly expensive to process
because of the difficulty in machining components having
high contents of hard carbide in the microstructure
thereof. In the never ending quest to lower costs, much
work has been carried out to reduce material cost by
adding relatively high proportions of substantially pure
iron powder to the powder mixes and consequently reducing
processing costs by making the resulting sintered
materials easier to machine by reducing the amount of
hard phases and adding phases which assist machinability
such as copper or chip-breaking phases.
A disadvantage in terms of performance and longevity of
life of these newer materials such as may be exemplified
in GB-A-2 188 _Q62 for example is the retention in the
cores of the iron grains, formed by the sintering
together of the original compacted iron powder particles
in the powder mixture, of soft ferrite phase which can
deleteriously affect the wear and strength, properties
thereof. Such materials may initially comprise mixtures
of about 50% of the highly alloyed M3/2 material, for
example, and about 50% of pure iron powder and minor
additions of carbon, die lubricating waxes and the like.
Even when fully sintered the iron grains have ferrite
cores with only some diffusion of chromium, from the M3/2
regions, into the surface regions of the iron grains,
where martensite may be formed, after sintering. This
structure still applies even when the material is
infiltrated or when up to about 5 weight% of elemental
copper has been added to the powder mixture.
It is an object of the present invention to provide a
process for making ferrous material articles having a
high copper content commensurate with that of infiltrated
material but without the disadvantage of the additional
process steps required in the prior art processes.
Other advantages will become apparent from the
description of the invention below.
According to a first aspect of the present invention,
there is provided a process for the manufacture of a
ferrous-based sintered article containing copper in the
range from 12 to 26 weight%, the process including the
steps of: making a powder mixture having a desired
composition, at least a proportion of a total content of
iron and copper being provided by an iron powder having
copper indivisibly associated therewith; compacting said
powder mixture to form a green compact of an article to
be produced and sintering said green compact.
The copper content is primarily intended to enhance the
thermal conductivity of articles produced, however, other
important benefits are also provided to articles made by
the method of the present invention. Below 12 weight%
copper the required enhancement in thermal conductivity
is not achieved whilst above 26 weight% "bleeding" of
molten copper from the material during sintering is a
problem.
Preferably, the copper content may lie in the range from
15 to 20 weight%.
In the process according to the present invention, the
iron powder indivisibly associated with copper is
effectively a pre-alloyed powder in that the individual
powder particles comprise both iron and copper and
consequently significant segregation between the iron and
copper is not possible. The iron and copper powder
particles may be selected from two basic types of powder
stock: a pre-alloyed iron-copper powder; or, a diffusion
bonded iron-copper powder. The pre-alloyed iron-copper
powder may be produced by known techniques of melting the
constituent materials together and then atomising the
molten melt by water or gas, for example, to produce the
required pre-alloyed powder. The diffusion bonded iron-
copper material is produced by making a mixture of
elemental iron and copper powders, for example, and
passing the mixture, uncompacted, through a furnace such
that diffusion between the particles occurs so as to bond
them together. The "cake" so formed is given a light
crushing operation to break it up into particles
comprising both iron and copper adhered to each other.
Such a process causes diffusion of some copper into the
outer regions of each iron particle.
The method of the present invention obviates several of
the process steps of prior art processes in that a
separate copper alloy powder mixture and consequent
compacts do not need to be made, they do not need to be
stacked with the ferrous material compacts and the final
sintered articles do not need to be treated to remove the
adherent deposit thereon as with prior art infiltration
processes.
A particular advantage conferred by the method of the
present invention relates to the processing of those
ferrous materials which comprise mixtures of an alloyed
steel powder and a low-alloy iron or substantially pure
iron powder. It is known to use such mixtures with
additions of carbon powder, for example, and to process
them by compaction, sintering and post-sintering thermal
treatment into articles such as valve seat inserts for
internal combustion engines, for example. Such prior art
materials may or may not be infiltrated with a copper
alloy by one of the conventional processes described
above. Such materials are exemplified by those materials
and production processes described in GB-A-2 188 062 and
EP-A-0 312 161 for example. These materials may comprise
a proportion, e.g. about 50 weight% of a highly alloyed
steel powder with about 50 weight% of a substantially
pure iron powder. The alloyed steel powder usually
contains chromium which under the prevailing sintering
conditions of about 1100°C is one of the most mobile
element atoms after carbon, in terms of rate of
diffusion, of those alloying elements which promote the
formation of martensite on cooling of the article
following sintering. Carbon atoms are the most mobile,
moving into the interstices of the iron atoms in the
crystal structure. However, since chromium is of a
similar atomic size and weight to iron it substitutes for
iron and consequently has a similar mobility to iron
under the prevailing sintering conditions. The presence
of chromium promotes the formation of martensite in those
regions of the sintered material into which it diffuses,
the martensite being formed on cooling of the material at
the end of the sintering cycle. Sintering is frequently
effected for such articles in furnaces which have
continuous moving means, such as a belt or a walking-beam
type mechanism, for transporting the articles, 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 which serves to
provide either a neutral or reducing atmosphere and
preclude air (oxygen) from entering the furnace. The
atmosphere is at substantially atmospheric pressure with
only a slight positive pressure within the furnace to
prevent air from entering therein. Where the sintered
material contains a significant quantity of iron powder
in the original mix it is frequently found that the iron
grains resulting from the sintering of the compacted iron
powder particles possess a microstructure ranging from
ferrite to pearlite and mixtures of the two phases,
depending upon the carbon content, in the core of the
iron-rich non-tool steel regions. The outer region of the
iron grains generally comprises martensite resulting from
chromium which has diffused in during the sintering
operation but the core remains essentially as ferrite or
pearlite or a mixture of ferrite and pearlite depending
upon the added carbon level. In the as-sintered
condition, the iron-rich non-tool steel phase or grain
structure consists of mainly pearlite, though there may
be some ferrite, at the centre and the outer regions of
the grains are a mixture of martensite/bainite. If there
is any retained austenite in the sintered article it is
generally transformed by cryogenic treatment after
sintering. During a tempering operation usually carried
out after cryogenic treatment, partial decomposition of
the pearlite phase occurs leading to the formation of
ferrite areas within the iron-rich grains or phase. This
can result in the material having inferior wear
properties due to the presence of ferrite and also lower
strength due to the ferrite. The post-sintering thermal
treatments comprising cryogenic treatment to transform
any remaining y-phase (austenite) to martensite followed
by tempering treatments are to reduce the degree of
hardness and brittleness of the martensite phase rather
than to effect decomposition of the pearlite which is an
undesirable side effect of the tempering process. Since
the tempering treatment is carried out at a temperature
in excess of the expected service temperature, size
stability of the article in its service environment (e.g.
a valve seat insert in the combustion chamber of an
internal combustion engine) is ensured. However, such
treatments do not affect the presence (other than to be
responsible for generating at least a proportion of the
ferrite) of the ferrite phase or its inherently poor wear
and mechanical properties.
It has been found that with the method of the present
invention that there appears to be a synergistic effect
of the copper (either from the diffusion-bonded form or
in the pre-alloyed form with the iron) and chromium
together in promoting the diffusion of copper and
chromium towards the centre of the iron grains and,
instead of the core of the iron grains remaining as
ferrite or pearlite or a mixture of these, the core of
the iron grains is found to transform to martensite
during normal furnace cooling. Sintered ferrous materials
made according to the process of the present invention
using either pre-alloyed iron-copper or diffusion bonded
iron-copper powders reveal the presence of martensite in
the cores of the iron-rich grains due to the diffusion of
chromium or other martensite promoting elements into the
iron grains. The martensite is formed during the cooling
of austenite and any retained austenite is transformed by
cryogenic treatment following sintering. During the
cooling process from the sintering temperature some of
the austenite can also transform to bainite. The
martensite may then be tempered to form a structure of
tempered martensite which is readily machinable. However,
it is important to note that the previously soft
ferritic/pearlitic cores of the iron grains now comprise
material which is harder, stronger and more wear
resistant due to the process of the present invention. It
is believed that the processing used to form the pre-
alloyed and diffusion bonded iron-copper material causes
at least some diffusion of the copper phase into the iron
constituent and the presence of the copper assists in the
diffusion of chromium and other martensite promoting
elements into the cores of the iron grains formed on
sintering thus, enabling martensite to be formed.
Tests making materials according to the method of the
present invention and making substantially identical
materials by prior art infiltration processes, but using
substantially identical processing parameters of pressing
pressure and sintering temperature for example, have
shown the beneficial effects of using an iron-copper pre-
alloy or diffusion bonded powder as described
hereinabove. Materials of largely identical composition
except for the copper content were made by 1) the method
of the present invention; 2) by the route of simultaneous
sintering and infiltration; and, 3) by adding 13 weight%
elemental copper powder to the initial powder mixture and
sintering (i.e. without infiltration and without the
addition of pre-alloyed iron-copper powder).
Materials made by conventional infiltration techniques
under the same processing conditions do not show the
beneficial effect of martensite formation in the iron
grain core. Analysis by scanning electron microscope has
shown the presence of chromium in the particle core in
materials made by the method of the present invention. It
is to be emphasised that the processing conditions used
in the comparative tests are the same processing
conditions used for production of commercial prior art
materials and thus represent the current optimum
processing conditions taking all factors into account.
Materials made according to the method of the present
invention may also receive post-sintering thermal
treatments such as cryogenic treatment at -120°C or below
to convert any residual austenite phase to martensite,
followed by tempering to make the martensite softer, more
dimensionally stable and make it amenable to machining.
Thus, according to a feature of one embodiment of the
present invention, the powder mixture contains a powder
component comprising a relatively un~alloyed iron powder
and a powder component comprising a steel powder
containing at least some chromium or other martensite
promoting element as an alloying element in addition to
the pre-alloyed or diffusion bonded iron-copper powder.
Alternatively or additionally the powder mixture may
contain addition(s) of elemental martensite promoting
material such as molybdenum and/or nickel for example.
Examples utilising M3/2 high speed steel powders are
described herein, however, any other suitable tool steel
or high speed steel, for example, chromium-containing
steel powder may be employed depending upon the
application in which the article produced therefrom is to
be used.
An example of an. alternative steel material is so-called
316 steel which is a stainless steel comprising in
weight%: 17 Cr/ 2 Mo/ 13 Ni/ Bal Fe and which is
substantially carbon free.
Thus, it appears that the manner in which copper . is
introduced into the sintered ferrous material, i.e. by
being associated with the iron where there has been prior
treatment causing reaction therebetween, has an
unexpected and synergistic effect in aiding diffusion of
chromium or other martensite promoting elements through
the iron matrix to assist in the transformation to
martensite on cooling after sintering or by
transformation of retained austenite by cryogenic
treatment.
The composition of the iron-copper pre-alloyed or
diffusion-bonded material may be any desired, e.g. iron-
2 0 copper. Powder mixtures may be made up having powder
components comprising: iron; iron-copper; pre-alloyed
steel powder; and, carbon powder, for example. The amount
of iron-copper pre-alloy powder will depend upon the
final required copper content in the article and on the
initial composition of the iron-copper pre-alloy powder.
The use of iron-copper pre-alloyed and/or diffusion
bonded material in a powder mixture together with an
addition on elemental copper powder is not precluded and
in some circumstances may be beneficial. The use of both
pre-alloyed and diffusion bonded iron-copper powder may
also be employed in a powder mixture.
The pre-alloyed iron-copper material appears to be
somewhat more effective in promoting the formation of
martensite in iron grains than does diffusion bonded
iron-copper material. Therefore, the use of the pre-
alloyed material is preferred, however, it is pointed out
that the diffusion bonded material produces martensite
after sintering and subsequent processing whereas prior
art infiltrated materials do not produce any martensite
in the iron grain cores, the cores comprising only
mixtures of pearlite and ferrite.
According to a second aspect of the present invention,
there is provided a sintered article produced by the
first aspect of the present invention.
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
drawings, of which:
Figure 1 shows a histogram showing wear of valve seat
inserts in an engine test on material made according to .
the present invention; and
Figure 2 which shows a graph of tool wear vs number of
parts machined for materials made according to the
present invention and prior art material.
Example 1 was a material prepared by the method of the
present invention where all of the iron and a proportion
of the copper were added as pre-alloyed iron-20 copper
powder. The pre-alloy powder contributes about 9.5
weight% of copper to the final material. A further 6
weight% of elemental copper powder was added to the
initial powder mixture to bring the total copper up to 15
weight%. The steel pre-alloy powder was a water atomised
M3/2 powder having a nominal composition of: 1 C; 4 Cr; 5
Mo; 3 V; 5 W. Since only 6 weight% of elemental copper
powder was added, segregation was minimised.
Example la is powder mixture wherein all of the iron
powder content is provided as pure iron powder and the
copper content as 13 weight% of elemental copper powder.
Whilst such material would not normally be made with such
a high content of elemental copper powder for the reasons
discussed hereinbefore, the material was made to
Valve Seat Insert Material- Example 1
Ferrous powder mixtures of a typical composition used in
the production of valve seat inserts for internal
combustion engines were prepared by various routes. The
compositions of the powder mixtures in terms of the
actual constituent component powders used to make them
were as set out below in Table 1:
determine the effect of the copper content on the
diffusion characteristics of the chromium into the iron
constituent.
Example 1b was made by the prior art process according to
GB-A-2 188 062 wherein the copper is supplied via a

simultaneous sintering and infiltration step.
All of the powders were blended according to established
principles in a Y-cone mixer. Compaction pressure was in
the range 650-800MPa in each case followed by sintering
at around 1100°C in a conveyor furnace, all Examples
being sintered under the same conditions. Following
sintering all Examples were cryogenically treated at
12C°C to transform any remaining austenite (y-phase) in
the structure and then tempered at 600°C for 2 hours to
soften the martensite, make it more dimensionally stable
and enhance machinability qualities.
Table 2 below gives the actual compositions in terms of
the constituent elements, the density of the sintered
material and its final hardness following cryogenic and
tempering post-sintering treatment.
The microstructure of samples made according to Example 1
showed a tempered martensite structure even in the cores
of the iron grains. The martensite was formed on cooling
from the sintering temperature. Cryogenic treatment was
used to transform any retained austenite in the M3/2
phase of the material to martensite. The change from
austenite to martensite is not easily seen under the
microscope, the change being evidenced by increased
hardness on the change from austenite to martensite.
Samples from Example la showed a microstructure
comprising some martensite formed on cooling from the
sintering temperature and retained austenite. Following
cryogenic treatment, the retained austenite transformed
to martensite in the M3/2 regions and the iron grains
comprised mainly pearlite (a phase comprising a lamellar
structure of ferrite and cementite) and some ferrite. The
pearlite was formed by virtue of the carbon powder added
as graphite, however, owing to the absence of chromium in
the iron grain cores, no martensite was formed. On
tempering, extensive decomposition of pearlite took place
and the volume fraction of ferrite increased compared
with that of the as-sintered state. Thus, the wear
resistance of Example la material is inferior and the
mechanical properties, as evidenced by the hardness
figures, are also inferior.
Samples from Example 1b demonstrated almost identical
structure and properties as did Example la. This material
was made according to the known process of GB-A-2 188
062^, The hardness of Example lb was slightly higher than
Example 1, this being attributed to the higher density of
the material following infiltration. However, the
material of Example lb showed extensive quantities of
inherently weaker ferrite areas after tempering and not
the desirable tempered martensite structure shown by
Example 1 made according to the process of the present
invention.
Figure 1 shows a histogram of valve seat insert wear of
valve seat inserts, made from the material of Example 1,
in the exhaust positions of a 1.81, 4-cylinder, 16-valve
engine which was run for 180 hours at 6000 rev/min on
unleaded gasoline, the engine having Stellite (trade
name) faced valves. The success criteria for this test is
that valve seat insert wear must not exceed 100mm. As may
be seen from Fig.l the maximum wear was at valve seat
position 4 at 60mm, all other inserts having wear of
about 30pm or less.
Thus, it is clear from Examples 1, la and 1b that the
only substantive difference in the manufacture thereof
was the manner in which the copper was introduced into
the sintered material. It is believed that the improved
structure and properties are directly attributable to the
use of the iron-copper pre-alloyed materials wherein at
least a proportion of the copper is indivisibly
associated with the iron and stem from the enhanced
diffusion promoted by this pre-alloyed material.
Example 2
A powder mixture comprising 45 wt% M3/2 tool steel
powder/ 0.55C/ 1 MoS2 / 6 Cu/ 47.45 FeCu20 (diffusion
bonded powder)/ 0.75 lubricating wax was made. This
mixture was compacted into green compacts at 770MPa to a
green density 7.1 Mgm-3 and sintered at about 1100°C under
a continuous flowing nitrogen-hydrogen gas atmosphere in
a conveyor furnace. The sintered articles were
cryogenically treated at -120°C or below to convert
retained austenite to martensite and finally tempered at
600°C. Density of the sintered material was 7.0 Mgm3. The
hardness of the as sintered material was 61HRA; that of
the cryogenically treated material 65HRA; and that of the
cryogenically treated and tempered material 62-65 HRA.
The microstructure of the Example 2 material (made with
diffusion-bonded iron-copper powder) after tempering ,
(following sintering and cryogenic treatment) showed some
small occasional areas of ferrite in the iron-rich non-
tool steel phase. However, this iron-rich phase comprised
essentially pearlite rather than the extensive regions of
ferrite typified by the prior art material made using the
infiltration technique.
Example 3
A mixture comprising in weight%: 75% pre-alloyed Fe-Cu20
powder/ 23% 316 stainless steel powder/ 0.75% MoS2
powder/ 1% carbon powder was prepared; this material
being coded Nl. The composition of the 316 stainless
steel was 17 Cr/ 2 Mo/ 13 Ni/ bal Fe. A comparative
example coded N was made from the following mixture in
weight%: 70.9% unalloyed iron powder/ 27% 316 stainless
steel powder/ 0.9% MoS2 powder/ 1.2% carbon powder. Both
materials were compacted at 770MPa. However, material Nl
was sintered only (as there was about 15 wt% Cu provided
by the Fe-Cu pre-alloy) and material N was simultaneously
sintered and infiltrated according to the known prior art
process. The final theoretical overall composition of
both materials Nl and N in weight% was: 1 C/3.9 Cr/15
Cu/0.9 Mo/3 Ni/S 0.3/bal Fe. The sintering/infiltration
steps were carried out at about 1100°C under a flowing
nitrogen/hydrogen atmosphere. Both materials following
sintering were cryogenically treated and tempered.
The Nl material showed a microstructure having no
ferrite, even in the cores of the grains which were
predominantly iron. The structure of this material showed
essentially a tempered martensite structure. The N
material on the other hand showed extensive ferrite in
the iron grains with a pearlitic structure in the
transition zones between prior iron particles and 316
stainless steel particles even though this material had
slightly higher carbon at 1.2%. Thus, again the influence
of the copper being indivisibly associated with the iron
is shown in the resulting structure after processing.
Example 4
Further mixtures denoted as material FMCA and FMCD were
made according to the present invention. The blend
compositions of these materials in terms of the
constituents in the powder mixtures are given below in
Table 3.
The materials were compacted at 770 MPa and sintered at
about 1100°C under a continuous gaseous atmosphere as
with previous examples. The resulting densities and
hardnesses of the sintered materials are given below in
Table 3
FMCA 1 FMCD
Fe-20 Cu (pre-alloyed) 75 75
C 1.35 1.35
Mo 0.5 ______________________
MoS2________________________1_________________
Unalloyed Fe_____________23.15 22.65
Lubricating wax__________ 0.75 0.75______________
Table 4. For these samples no post-sintering heat
treatment was carried out.
In the FMCA material made according to the present
invention pre-alloyed Fe-Cu powder and 0.5% elemental Mo
powder were used in the initial powder mixture. The FMCA
material showed extensive Mo-rich zones and martensitic
and bainitic areas associated with these zones. The FMCA
material also showed grain boundary carbides. The
microstructure of the FMCA material was somewhat" similar
to a comparative material, coded FMC (unalloyed iron
powder, 1.35% C, 0.5% Mo), wherein the copper content was
provided by a simultaneous sintering and infiltration
process according to the prior art. Apart from the
infiltration step, the sintering conditions were the same
as those for the FMCA and FMCD materials. In the FMC
material grain boundary carbide was present, the matrix
was pearlite and the Mo-rich zones associated with the Mo
particles were present but very small compared with the
FMCA material.
During sintering, the MoS2 in the FMCD material undergoes
partial decomposition and donates free Mo to the
structure which potentially is able to generate a
localised martensitic/bainitic structure associated with
the Mo-rich zones. Some of the sulphur from decomposed
MoS2 reacts with iron and copper to form metallic
sulphides which are beneficial for improving
machinability. In the FMCD material no carbide networks
could be seen and the matrix was pearlitic.
Figure 2 shows a graph of tool wear vs number of parts
machined for FMC, FMCA and FMCD materials. The Figure
confirms that the materials using pre-alloyed Fe-Cu
powders which give rise to extensive martensitic/bainitic
areas do not have their machinability impaired in spite
of the stronger, more wear resistant material structures
so formed. Indeed, the machinability of the both the FMCA
and FMCD materials is superior to the FMC material made
by a prior art process.
WE CLAIM :
1. A process for the manufacture of a ferrous-based sintered article containing
copper in the range from 12 to 26 weight%, the process involving the steps of:
making a powder mixture of the desired composition, said mixture comprising:
a pre-alloyed steel powder,
a powder in which iron and copper are indivisibly associated with each other,
being selected from the group consisting of a diffusion bonded iron-copper powder
and pre-alloyed iron-copper powder, and
a material for promoting the formation of martensite, being selected from the
group consisting of carbon, chromium, molybdenum, nickel and molybdenum
disulphide, said material being either added to the mixture as a powder or already
present in the pre-alloyed steel powder,
the process also involving compacting said powder mixture to form a green
compact of the article, and sintering said green compact to produce the article which
has a ferrous matrix with a martensitic structure.
2. A process as claimed in claim 1 wherein the copper content lies in the range
from 15 to 20 weight%.
3. A process as claimed in claim 1 wherein the steel powder is a high-speed
steel powder.
4. A process as claimed in claim 3 wherein the steel powder is an M3/2 steel
powder.
5. A process as claimed in claim 1 wherein the steel powder is a stainless steel
powder.
6. A process as claimed in claim 5 wherein the stainless steel powder is 316
steel.
7. A process as claimed in claim 1 wherein the powder mixture contains carbon
powder.
8. A process as claimed in claim wherein the iron-copper powder has a
composition in weight% of 20 copper and the balance iron.
9. A process as claimed in claim 1 wherein the powder mixture also contains
elemental copper powder.
10. A process as claimed in claim 1 wherein said material for promoting the
formation of martensite is alloyed with the pre-alloyed steel powder.
11. A process as claimed in claim 1 wherein said material for promoting the
formation of martensite is included in the powder mixture as a powder.
A process for the manufacture of a ferrous-based sintered article containing
copper in the range from 12 to 26 weight% is described, the process including the
steps of: making a powder mixture having a desired composition, at least a
proportion of a total content of iron and copper being provided by an iron powder
having copper indivisibly associated therewith; compacting said powder mixture to
form a green compact of an article to be produced and sintering said green compact.

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Patent Number

214071

Indian Patent Application Number

00870/KOLNP/2003

PG Journal Number

05/2008

Publication Date

01-Feb-2008

Grant Date

30-Jan-2008

Date of Filing

07-Jul-2003

Name of Patentee

FEDERAL-MOGUL SINTERED PRODUCTS LTD.

Applicant Address

HOLBROOK LANE, COVENTRY, WEST MIDLANDS, CV6 4BG

Inventors:

#	Inventor's Name	Inventor's Address
1	MAULIK PARITOSH	FEDERAL-MOGUL SINTERED PRODUCTS LTD., HOLBROOK LANE, COVENTRY, WEST MIDLANDS, CV6 4BG

PCT International Classification Number

C 22 C 33/02

PCT International Application Number

PCT/BG02/00176

PCT International Filing date

2002-01-17

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	0101770.6	2001-01-24	U.K.