IRON-BASED POWDER COMPOSITION HAVING GOOD DIMENSIONAL STABILITY AFTER SINTERING

Abstract: An iron-based powder for producing highly resistant components with a small local variation in dimensional change, by powder compacting and sintering. The powder contains, in addition to Fe, 0.5-4.5 % by weight Ni, 0.65-2.25 % by weight Mo and 0.35-0.65 % by weight C, and optionally a lubricant and impurities. The maximum variation in dimensional change is 0.07 % for a minimum density of 6.7 g/cm<3>.

Description: IRON-BASED POWDER COMPOSITION HAVING GOOD DIMENSIONAL STABILITY AFTER SINTERING The present invention relates to an iron-based powder which after powder compacting and sintering gives dimen- sionally stable products, i.e. products inherently exhi¬ biting similar dimensional changes, also in the event of local density variations. A major advantage of powder-metallurgical processes over conventional techniques is that components of varying complexity can be sintered into final shape immediately after powder compacting, and they therefore require but a relatively limited aftertreatment as compared with e.g. a conventional steel blank. Also in the development of new powder-metallurgical materials, it is an aim to ensure that the dimensional change is small during sintering, since it has been found difficult in practice to maintain the dimensional stability if the dimensional change is considerable. This is especially important in the case of high-strength materials which are difficult to adjust to correct measurement after sintering. Therefore, it is vital that the dimensional change is minimal and as inde¬ pendent as possible of variations in the process para¬ meters sintering time, sintering temperature, carbon con- tent and distribution of alloying substances. In the deve¬ lopment of high-strength diffusion-alloyed materials during the 1970s, the primary objective precisely was to make the dimensional change as independent as possible of these process variables. By the diffusion-alloying technique, the alloying substances Ni, Cu and Mo have become uniformly distributed in the material and the contents of these substances can be so selected that variations in the other process para¬ meters time, temperature and C-content have but a small effect on the dimensional change. On the other hand, it has been found that the dimensional change is not constant for different density levels in these materials. In the compaction of powder mixtures, the density may in fact vary considerably within the compacted component and in particular if the geometrical shape is complex. For exam- 3 pie, density differences about 0.4 g/cm are not at all unusual in practice. This, in turn, may give rise to dif¬ ferent dimensional changes locally during sintering, thus making the material "warp", which may mean that it will have to be rejected. One object of the present invention is to provide a dimensionally stable sintered product. The expression "dimensionally stable" as used in this context means that the product undergoes a similar dimensional change despite inherent density differences. Thus, it is possible accord¬ ing to the invention to produce a product which, although exhibiting inherent density differences, has a variation in the dimensional change of at most about 0.07%, pre¬ ferably at most about 0.05% at a minimum density of 3 about 6.7 g/cm , especially in the density range of 3 6.8-7.2 g/cm . The dimensional change during the sintering process need however not be zero, since the pressing tools can be adjusted in size already at the design stage so as to obtain the correct shape after sintering. Another object of the invention is to produce an iron-powder-based material which after compacting and sin- tering yields a dimensionally stable product having high strength. For instance, it is possible with the iron- powder-based material according to the invention to pro¬ duce sintered products having a tensile strength above about 450 MPa, especially between 500 and 1000 MPa, and preferably between 550 and 950 MPa, without the sintered product being subjected to subsequent heat treatment. Yet another object of the invention is to produce a powder which by a simple and inexpensive low-temperature sintering process yields a product having the properties specified above. The invention embraces also such powders as after compacting and sintering exhibit not only good dimen¬ sional stability and high strength but also high fatigue strength. In these powders, the nickel content is compa- ratively high and preferably is in the range of 2-4.5% by weight. According to the invention, these objects can be achieved by a powder composition which, in addition to iron, includes 0.5-4.5% by weight of nickel, 0.65-2.25% by weight of molybdenum, and 0.35-0.65% by weight of car¬ bon. The invention is also directed to products produced from the stated compositions, and to a method for produc¬ ing the products on the basis of the compositions. More¬ over, the invention relates to the use of the powder com- positions for producing sintered products. The other fea¬ tures of the invention are recited in the accompanying claims. Compositions containing the components Fe, Ni and Mo in approximately the same contents as in the present invention are previously known from EP 0,334,968. These known compositions are intended for use in the making of products which after sintering and heat treatment (quench¬ ing and tempering) are distinguished by a very high strength and high hardness. However, the EP publication does not contain any information or indication whatever of any particular advantages of these powder compositions when it comes to producing dimensionally stable and high- strength products obtained by simple sintering without any subsequent heat treatment. Since it is well-known that the dimensional accuracy is impaired in heat treatment, it is not possible by using the method disclosed in EP 0,334,968 to achieve the object: of the present invention. DOS 2,112,944 also discloses powder compositions including Ni and Mo in such amounts as to place the pre- sent powder compositions within the ranges here suggested. However, the compositions of DOS 2,112,944 also include Mn as a compulsory component, whereas any Mn present in the powder composition according to the invention is an unde¬ sirable impurity. Consequently, it is preferred according to the present invention that the content of Mn is at a minimum and less than 0.3% by weight, preferably less than 0.1% by weight. The DOS publication further mentions Ni, y.n r Mo and Fe as completely prealloyed powders. Reference is also made to DE 1,207,634, in which Ni and/or Mo and/or Mn is/are added to an iron base powder, either as pure substances, or as master alloys (which means that at least two of the included alloying substances form a chemically homogeneous powder) or as ferro-alloy powder (chemically homogeneous material in which iron is included, but with essentially higher alloying contents as compared with the material of the invention). These variants of powder mix- tures are not comprised by the present invention. Nor do these publications teach or suggest anything whatever about the advantages that can be gained with the inven¬ tion. The powder compositions according to the invention have proved well suited for use in so-called low-tempera¬ ture sintering, which means sintering at temperatures below about 1150°C. Such sintering may advantageously be performed in belt furnaces. Sintering in such furnaces usually takes place at temperatures of about 1120°C-1140°C for at most 1 hour, generally between 20 and 40 min. Before the powder compositions are passed into the sinter¬ ing furnace, they are first admixed with a lubricant and thereafter moulded in a pressing tool under high pressure. For highly resistant products, the compacting pressure is in practice about 600 MPa. For the powder compositions according to the inven¬ tion, preference is given to such powders in which the nickel content varies between 1.0 and 3.0% by weight and the molybdenum content varies between 0.8 and 2.0% by weight. The best results have hitherto been achieved with compositions in which the content of Ni > the content of Mo, and particularly preferred are compositions containing 1.5% by weight of molybdenum and about 2% by weight of nickel. For products requiring higher fatigue strength, the amount of nickel should be higher, preferably between 2 and 4% by weight. In addition to the indicated substances, the powder compositions may contain impurities, the content of which should be as low as possible. Examples of impurities in the compositions according to the invention are copper, tungsten and phosphorous, which interfere with the dimen- sional stability. Other impurities that may also have an adverse effect on the sintered product because of oxida¬ tion are chromium, manganese, silicon and aluminium. The total content of impurities should be maintained below 2% by weight, preferably below 1% by weight. In addition, the powder composition of the invention may optionally contain a lubricant of the type which is known to those skilled in the art. In a particularly preferred embodiment, Mo is present in solid solution in a water-atomised iron-based powder. This embodiment provides a powder which imparts to the sintered components a more homogeneous structure on micro level as compared with powders in which Mo is not prealloye'd to the iron. At the same time, the sintered density is affected only insignificantly when Mo is pre- alloyed to the iron. If, on the other hand, Ni is present in solid solution in the iron-based powder, the compress¬ ibility of the material is impaired, as is also the sin¬ tered density (the Example below shows, for instance, how material B in Table 2 will have a very low density after sintering at the compacting pressures used as compared with the other materials. This material includes about 2% Ni and 0.5% Mo as prealloyed elements in the iron-based powder while material A, which also is completely pre¬ alloyed but with about 1.5% Mo, will have a much higher density after sintering under the same process conditions as for material B). Therefore, Ni preferably is in metal¬ lic form, it being diffusion-alloyed with the iron-based powder prealloyed by means of Mo. Ni may also in this case be mixed with the prealloyed powder. The alloying content ranges are selected under the consideration that the material of the invention should satisfy at least three of the conditions stated above, viz., within the limits specified, provide a dimensionally stable sintered product despite varying density levels within the product, provide an iron-powder-based material which after compacting and sintering yields a dimensional- ly stable product having high strength, and provide a powder which by simple and inexpensive low-temperature sintering without subsequent heat treatment can yield a product having the properties specified above. The accompanying Figs 1-3 show how the dimensional change varies at different density levels during sinter¬ ing, and how the tensile strength is affected by the sin¬ tered density at different contents of alloying substances Ni, Mo and C. These Figures show compacted and sintered powder mixtures where Mo (if present) has been prealloyed in an atomised iron-based powder having a particle size substantially below 200 μm, while Ni (if present) having a particle size substantially below 15 μm has thereafter been diffusion-alloyed to the iron-based powder. C in the form of graphite having a particle size substantially below 15 μm has thereafter been added to the powder. The powder mixtures have then sintered in a belt furnace at 1120°C for 30 min in endothermic atmosphere at a carbon potential corresponding to the carbon content of the mate¬ rial. Fig. la shows how the tensile strength is improved at increasing density and Ni-content, while Fig. lb shows how the dimensional change is similar at different den¬ sity levels for the material of the invention. A too high or a too low Ni-content, i.e., falling outside the stated limits of the inventive material, results in too large variations in dimensional change at different density levels. Fig. 2a illustrates how an increased carbon con- tent improves the tensile strength, while Fig. 2b shows that too high a carbon content results in too large a variation in dimensional change at different density levels. Figs 3a and b show that a certain Mo-content is required to meet the requirements as to strength and 3 similar dimensional change at densities above 6.7 g/cm . The invention will be illustrated by the Example below. This Example is intended merely to illustrate an embodiment of the invention in a non-restrictive manner. Example Two different powders (A, B) were prepared by water- atomising an iron melt alloyed both with Mo and with Mo and Ni. The oxygen content was reduced by annealing the atomised powders in reducing atmosphere. In addition, Ni was diffusion-annealed in reducing atmosphere in two con¬ tents to the iron-based powder which was prealloyed with Mo (C, D). A non-alloyed iron powder was also prepared by water-atomisation and annealed to reduce the oxygen con¬ tent. The resulting powder was thereafter diffusion- annealed with different amounts of Mo, Ni and.Cu (E, F, G, H). The chemical composition of the different powders appears from Table 1 below. Powder Chemical composition (%) Ni Mo Cu Fe A - 1.51 - balance B 1.92 0.48 - balance C* 1.98 1.52 - balance D* 2.97 1.50 - balance E* 2.01 1.48 - balance F 3.92 0.54 1.47 balance G 3.99 0.53 - balance H 1.72 0.53 1.47 balance * powder according to the present invention. Table 1. Chemical composition of the powder materials tested. The different powders having a particle size sub¬ stantially below 200 μm were admixed with 0.5% graphite having a particle size substantially below 15 μm and 0.6% Kenolube as lubricant. After mixing, tensile testpieces were compacted at 400, 600 and 800 MPa. Sintering was performed at 1120°C for 30 min in reducing atmosphere (endogas) at a carbon potential of 0.5%. Methane was added to control the carbon content. After sintering, the tensile strength and the dimensional change were measured for the different materials at varying densities. The result appears from Table 2 below. Material Tensile Sintered Dimensional strength density change (MPa) (g/cm" 3 ) (%) A 400 6.67 -0.03 540 7.05 -0.01 602 7.22 -0.01 B 346 6.55 -0.37 458 6.98 -0.33 528 7.19 -0.32 C* 597 6.75 -0.38 727 7.10 -0.36 785 7.27 -0.37 D* 640 6.79 -0.53 796 7.13 -0.50 877 7.30 -0.49 E* 591 6.75 -0.21 696 7.08 -0.19 774 7.24 -0.18 F 699 6.80 -0.37 855 7.11 -0.26 895 7.25 -0.24 G 578 6.84 -0.27 694 7.14 -0.22 757 7.32 -0.18 H 519 6.81 -0.18 620 7.11 -0.12 655 7.30 -0.09 * Material according to the present invention. Table 2. Tensile strength and dimensional change at vary¬ ing densities. Materials A, B, F and H are previously known, and as appears from the Table, material F gives high strength, but a relatively low variation in dimensional change at different densities. Material G has been produced in the same way, but without addition of Cu. The strength value has therefore dropped, but still is quite acceptable. On the other hand, the variation in dimensional change still 3 is too high in the density range exceeding 6.7 g/cm . By lowering the Ni-content in material F from about 4% by weight to about 1.75% by weight (= material H), the variation in dimensional change at different densities decreases, but still is too high. The prealloyed materials A and B exhibit a small variation in dimensional change at different densities, but the strength values are too low. However, it has been found that the combination of a higher Mo-content than in material B, with an Ni-addition gives a material having high strength and a small variation in dimensional change at different densities. As appears from Table 2, the properties become similar in materials C and E, whether Mo is prealloyed (i.e. is added before atomisation) or it is diffusion-alloyed. The only difference is the level of dimensional change, which does not conflict with the invention. Adding more Ni (material D) gives improved strength, but a slightly higher variation in dimensional change than for materials C and E. The variation in dimensional change at different densities however is in compliance with the requirements of the invention.

Claims: CLAIMS 1. Iron-based powder composition for producing a dimensionally stable sintered body having a density of 3 at least about 6.7 g/cm , which composition, in addition to iron, essentially consists of about 0.5 - 4.5% by weight Ni 0.65 - 2.25% by weight Mo 0.35 - 0.65% by weight C less than about 2% by weight, preferably less than about 1% by weight, of impurities, and optionally a lubricant. 2. Composition as claimed in claim 1, wherein it essentially consists of about 0.5 - 3.5% by weight Ni 0.65 - 2.25% by weight Mo. 0.35 - 0.65% by weight C. 3. Composition as claimed in claim 1 or 2, wherein it includes about 1.0 - 3.0% by weight Ni 0.8 - 2.0% by weight Mo. 4. Composition as claimed in claim 1, wherein it includes about 2 - 4.5% by weight Ni. 5. Composition for producing a dimensionally stable sintered body which, in addition to iron, essentially con¬ sists of about 0.5 - 4.5% by weight Ni 0.65 - 2.25% by weight Mo 0.35 - 0.65% by weight C less than about 2% by weight, preferably less than about 1% by weight, of impurities, and optionally a lubricant, wherein Mo is present in solid solution in an atomised iron-based powder. 6. Composition as claimed in any one of claims 1-3, wherein Ni is present in metallic form. 7. Composition as claimed in any one of claims 1-3, wherein Ni is diffusion-alloyed to an iron-based powder. 8. Method for producing a dimensionally stable sin- 3 tered body having a density of at least about 6.7 g/cm , comprising the steps of a) preparing an iron powder, b) diffusion-alloying Ni and/or Mo to the iron powder, or mixing metal particles of Ni and/or Mo to the iron powder, c) adding carbon to the powder obtained, the amounts of the included components-being so selected that the resulting powder composition, in addition to iron, essentially consists of about 0.5 - 4.5% by weight Ni 0.65 - 2.25% by weight Mo 0.35 - 0.65% by weight C less than about 2% by weight, preferably less than about 1% by weight, of impurities, and optionally a lubricant d) compacting the powder composition, and e) sintering the compacted powder composition. 9. Method as claimed in claim 8, wherein the powder composition essentially consists of about 0.5 - 3.5% by weight Ni 0.65 - 2.25% by weight Mo 0.35 - 0.65% by weight C. 10. Method for producing a dimensionally stable Sin¬ 's tered body having a density of at least about 6.7 g/cm , comprising the steps of: a) preparing a melt of iron and molybdenum, b) water-atomising the melt to a powder, c) diffusion-alloying Ni to the resulting powder, or mix- ing metal particles of Ni with the powder, d) adding carbon to the powder obtained, the amounts of the included components being so selected that the resulting powder compositic , in addition to iron, essentially consists of about 0.5 - 4.5% by weight Ni 0.65 - 2.25% by weight Mo 0.35 - 0.65% by weight C less than about 2% by weight, preferably less than about 1% by weight, of impurities, and optionally a lubricant, e) compacting the powder composition f) sintering the compacted powder composition. 11. Method as claimed in claim 10, wherein the compo¬ sition essentially consists of about 0.5 - 3.5% by weight Ni 0.65 - 2.25% by weight Mo 0.35 - 0.65% by weight C 12. Method for producing a dimensionally stable sin¬ tered body as claimed in claim 10 or 11, wherein the com¬ pacted powder composition is subjected to sintering at a temperature between about 1070°C and about 1150°C, prefer- ably between 1100°C and 1140°C. 13. Use of an iron-based powder composition which, in addition to iron, essentially consists of about 0.5 - 4.5% by weight Ni 0.65 - 2.25% by weight Mo 0.35 - 0.65% by weight C less than about 2% by weight, preferably less than about 1% by weight, of impurities, and optionally a lubricant for producing a dimensionally stable sintered body having 3 a density of at least about 6.7 g/cm . 14. Use as claimed in claim 13, wherein the powder composition consists of about 0.5 - 3.5% by weight Ni 0.65 - 2.25% by weight Mo 0.35 - 0.65% by weight C. 15. Use as claimed in claim 13, wherein the powder composition includes about 1.0 - 3.0% by weight Ni 0.8 - 2.0% by weight Mo 16. Use as claimed in claim 13, wherein the powder composition includes about 2.0 - 4.0% by weight Ni 0.8 - 2.0% by weight Mo 17. Dimensionally stable, sintered body having a den- 3 sity of at least about 6.7 g/cm based on a powder compo¬ sition which, in addition to iron, essentially consists of about 0.5 - 4.5% by weight Ni 0.65 - 2.25% by weight Mo 0.35 - 0.65% by weight C less than about 2% by weight, preferable less than about 1% by weight, of impurities.

Inventors: JOHANSSON BJOERN, LINDBERG CAROLINE, JOHANSSON, BJOERN, LINDBERG, CAROLINE
Applicants: HOEGANAES AB
Application year: 1992
Main classification: C22C33
IPC codes: B22F1, C22C33, C22C38