Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
IRON-BASED POWDER COMPOSITION
Document Type and Number:
WIPO Patent Application WO/1994/013418
Kind Code:
A1
Abstract:
An iron-based powder for producing high-strength 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 0.20-1.0 % by weight C, and optionally a lubricant and impurities.

Inventors:
LINDBERG CAROLINE (SE)
JOHANSSON BJOERN (SE)
Application Number:
PCT/SE1992/000860
Publication Date:
June 23, 1994
Filing Date:
December 11, 1992
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
HOEGANAES AB (SE)
LINDBERG CAROLINE (SE)
JOHANSSON BJOERN (SE)
International Classes:
C22C33/02; (IPC1-7): B22F1/00; C22C33/02
Domestic Patent References:
WO1992022395A11992-12-23
Foreign References:
EP0200691A11986-11-05
EP0334968A11989-10-04
DE2112944A11971-10-07
DE4031408A11991-04-11
Download PDF:
Claims:
CLAIMS
1. Ironbased powder composition for producing dimen¬ sionally stable highstrength sintered products, c h a r a c t e r i s e d in that the powder of the green product, in addition to iron, essentially consists of 0.5 4.5 % by weight Ni 065 2.25 % by weight Mo 0.20 1.0 % by weight C, less than about 2% by weight, preferably less than about 1% by weight, of impurities and optionally lubricant. 2. Composition according to claim 1, wherein it com¬ prises more than 0.65% by weight C, or from 0.20 to less than 0.35% by weight C.
2. 3 Composition according to claim 1 or 2, wherein Mo, and optionally Ni, is present in solid solution in an ato mised iron powder.
3. 4 Composition according to any of claims 13, where¬ in Ni is present in metallic form.
4. 5 Method of producing a highstrength sintered body without subsequent heat treatment, comprising the steps of al ) preparing an iron powder and diffusionalloying Ni and/or Mo to the iron powder or mixing metal par¬ ticles of Ni and/or Mo to the iron powder, or a2) preparing a melt of iron and molybdenum, water atomising the melt to a powder and diffusionalloying Ni to the resulting powder or mixing metal particles of Ni with the powder, b) adding carbon to the powder obtained, the amount of the included components being so selected that the resulting powder composition is dimensionally stable and, in addition to iron, essentially consists of about 05 4.5 % by weight Ni 0.65 2.25 % by weight Mo 0.20 1.0 % by weight C, less than 2% by weight, preferably less than about 1% by weight, of impurities and optionally lubricant, c) compacting the powder composition, and d) sintering the compacted powder composition. 6. Method according to claim 5, wherein the powder comprises from 0.20 to less than 0.35 or more than 0.65% by weight C.
5. 7 Method according to claims 5 and 6, wherein, for 3 densities above about 7.3 g/cm , the green product is sub jected to presintering and an additional compacting step before the final sintering step.
6. 8 Method according to claim 7, wherein the presin¬ tering step is carried out at a temperature between 700 and 850°C.
7. Method according to claim 5 or 6, wherein the com¬ pacted powder composition is subjected to a final sinter¬ ing at a temperature between about 1070°C and about 1150°C, preferably between 1100°C and 1140°C.
8. Method according to any of claims 57 for produc in9 sintered products having a tensile strength of 5001000 MPa without subsequent heat treatment.
9. Use of an ironpowder composition for the produc¬ tion of dimensionally stable highstrength sintered pro¬ ducts, c h a r a c t e r i s e d in that the powder corti position, in addition to iron essentially consists of 05 4.5 % by weight Ni 065 2.25 % by weight Mo 020 1.0 % by weight C, less than about 2% by weight, preferably less than about 1% by weight, of impurities and optionally lubricant.
10. 12 Use according to claim 9, wherein the powder com¬ position includes from 0.20 to less than 0.35 or more than 0.65% by weight of carbon.
Description:
IRON-BASED POWDER COMPOSITION

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. This, in turn, may give rise to different dimensional changes locally during sintering, thus making the material "warp", which may mean that it will have to be rejected.

The invention is based on the discovery that, during the sintering process, green products made of the powder compositions according to the invention undergo essential¬ ly the same dimensional change in spite of density varia- tions within the green products. Thus, for a density

3 variation of e.g. 0.3 g/cm within the green product, this means that, during the sintering process, the difference in expansion/contraction between parts of the green body

3 having a density of about 6.8 g/cm , and parts having a density of about 7.1 is at most about 0.042%. In view hereof, the compact of the powder composition according to the invention is "dimensionally stable" within the density

3 range 6.7-7.5 g/cm . Naturally, the powder compositions are dimensionally stable also for smaller variations within this range. It should however be noted that the overall dimensional change during the sintering process may vary as in conventional processes, but by using the composition according to the present invention, the pressing tools can be adjusted in size already at the design stage so as to obtain the correct shape after sintering.

The powder compositions according to the invention are especially suited for producing products having com¬ plex or irregular shape, in which density variations occur during the compacting process. Of course, the powders are also suited for producing sintered products in which less or no density variations at all exist within the green product. However, already when sintering green bodies hav-

3 ing a density variation as small as 0.1 g/cm , the advan- tage of using the present compositions is obvious, lead¬ ing to a variation in the dimensional change of at most 0.014%. The advantages of using the powder compositions

according to the invention are of course greater when the

3 density variations are above 0.2 or 0.3 g/cm within the green product.

Another way of expressing the properties of the iron powder according to the invention is that it should satis¬ fy the condition

ΔdL < 0.14

ΔSD ~ wherein ΔdL is the variation in the dimensional change (%) within the sintered product measured from the green to the sintered condition, and

3 ΔSD is the variation in the density (g/cm ) within the sintered product. 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. At

3 a sintered density of e.g. 6.8 g/cm , the sintered pro¬ ducts according to the invention exhibit a tensile strength exceeding 500 MPa, which is considered a high tensile strength.

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 0.20-1.0% by weight of car- bon. In the copending application PCT/SE92/00399, which is hereby incorporated by reference, iron powder compo¬ sitions having a C-content between 0.35 and 0.65% are described and, thus, the present invention particularly relates to compositions outside this range. The invention is also directed to products produced from the stated compositions, and to a method for producing the products on the basis of the compositions. Moreover, the invention relates to the use of the powder compositions for produc¬ ing sintered products. The other features of the inven- tion 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, Mn, 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 prealloyed 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 1-4 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 1120-1140 C for 30 min in endothermic atmosphere at a carbon potential corresponding to the carbon content of the material.

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 also iron-powder compositions having a carbon con-

tent of up to 1% by weight give a low variation in dimen¬ sional 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 similar dimensional

3 5 change at densities above 6.7 g/cm .

Fig. 4 shows that also for densities of up to 7.5 g/ 3 cm , the powder composition according to the invention gives small dimensional changes. Composition C according to the invention includes 0.5% carbon. In this context, o it should be mentioned that densities above about

7.3 g/cm 3 to 7.5 g/cm3 can be obtained by subjecting the compacted green porduct to a presintering step at a tem¬ perature between 700 and 850°C. The presintered product is then compacted once more before the final sintering 5 step.

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 0 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- 5 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, 0 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 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 large 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.