Background of the invention Fatigue performance of sintered steels are influ- enced by several factors which are interacting. The den- sity was early established as one of the most influential factors together with the microstructure and alloy ele- ment content but also homogeneity, pore size and pore shape are known to influence the dynamic properties. This makes fatigue performance one of the most complex proper- ties of PM materials.

Obiects of the invention An object of the present invention is to improve the dynamic properties of sintered steels, specifically sin- tered steels having a density between 6.8 and 7.6 g/cm3.

Another object of the invention is to eliminate the influence of the particle size of the lubricant on the dynamic properties, especially the fatigue strength of the sintered parts.

A third object is to provide a method of improving the fatigue strength by selecting the particle size of the lubricant in view of the particle size of the iron powder.

Summary of the invention According to the invention it has now been found that, even if the amount of the very largest particles of

a lubricant constitutes a negligible or almost negligible fraction of the lubricant particle size distribution as well as of the amount of the lubricant, this fraction has an unexpectedly large detrimental effect on the pore size and accordingly on the dynamic properties.

Similarly it has been found that the very largest particles of the iron powder, i. e. the maximum size of the iron powder has an unexpectedly large detrimental effect on the dynamic properties. Thus in order to get improved dynamic properties the maximum size of the lubricant particles as well as the maximum size of the iron powder should be reduced. For presently commercially used ferrous based press powders this means that the maximum particle size of the lubricant should be less than about 60 Um as measured by laser diffraction measurement.

In order to achieve the best dynamic properties for a given iron based powder (at a given density) a relationship between the maximum size of the particles of the lubricant and the maximum size of the particles of the iron based powder has also been established. The term "maximum size"as used in this context is defined in the formula below.

In accordance with the invention it has been found that the particle size of the lubricant in a composition including the lubricant and an iron based powder for powder metallurgical preparation of compacted and s-intered products should be selected so that the largest pores of the compacted and sintered product prepared from this composition should be equal to or less than the largest pores obtained in a compacted and sintered product prepared from the same composition without the lubricant, which in practice means that the compaction is performed in a lubricated die.

Empirically we have found the following relationship between the largest lubricant particles and the largest

iron powder particles in order to avoid the influence of the lubricant on the size of the largest pores.

LUbmax< 0.31 x Femax-26, wherein Lub ax is the lubricant particle size in p. m whereas 99.99 % of the lubricant is finer.

Femax is the iron particle size in m whereas 99.99 % of the iron powder is finer (this could also be expressed as Lubmax is the size of the largest one hundredth of a percent fraction of lubricant particles in pm, Femax is the size of the largest one hundredth of a per- cent fraction of the particles of the iron based composi- tion in hum). This means that the maximum particle size of the lubricant as defined above should be less than about 0.3 of the maximum size of the iron or iron-based particles.

Detailed description of the invention The iron based powder according to the invention may be an alloyed iron based powder, such as a prealloyed iron powder or an iron powder having the alloying ele- ments diffusion-bonded to the iron particles. The iron based powder may also be a mixture of an essentially pure iron powder and the alloying elements.

The alloying elements which can be used in the com- positions according to the present invention may be one or more elements selected from the group consisting of Ni, Cu, Cr, Mo, Mn, P, Si, V and W. The particle sizes including the maximum particle sizes of the alloying elements are smaller than those of the iron or iron-based powder. The various amounts of the different alloying elements are between 0 and 10, preferably between 1 and 6 % by weight of Ni, between 0 and 8, preferably between 1 and 5 % by weight of Cu, between 0 and 25, preferably

between 0 and 12 % by weight of Cr, between 0 and 5, preferably between 0 and 4 % by weight of Mo, between 0 and 1, preferably between 0 and 0.6 % by weight of P, between 0 and 5, preferably between 0 and 2 % by weight of Si, between 0 and 3, preferably between 0 and 1 % by weight of V and between 0 and 10, preferably between 0 and 4 % by weight of W.

The iron based powder may be an atomised powder, such as a wateratomised powder, or a sponge iron powder.

The particle size of the iron based powder is selected depending on the final use of the sintered product and, according to the present invention is has been found that also the maximum particle size of the iron based powder has an unexpectedly large detrimental effect on the dynamic properties of the sintered product.

The type of lubricant is not critical and the lubri- cant may be selected from a wide variety of solid lubri- cants. Specific examples of suitable lubricants are con- ventionally used lubricant such as KenolubeX, Metalub, (both available from Hoganas AB Sweden) H-WachsX (available from Clariant), and zinc stearate (available from Megret). The amount of the lubricant may vary between 0.1 and 2, preferably between 0.2 and 1.2.

Furthermore the vaporising temperature of the lubricant should be below the sintering temperature of the compacted part. Presently used lubricant which may be used according to the present invention have vaporising temperatures less than about 800°C.

The amount of graphite varies between 0 and 1.5, preferably between 0.2 and 1% by weight of the composi- tion. Also, the maximum particle size of the graphite powder should be equal to or smaller than the maximum particle size of the lubricant.

In addition to the iron based powder, optional alloying elements, graphite and lubricant (s) the composi- tions according to the invention may also include op- tional conventionally additives, such as MnS, MnxTM.

The improved dynamic properties which can be ob- tained according to the present invention are especially interesting in sintered products having densities between 6.8 and 7.6 g/cm3, especially between 7,0 and 7,4/cm3.

Examples of preferred iron based powders plus pre- ferred amounts of graphite follows below: Iron + 4% Ni + 1.5% Cu + 0.5% Mo where the alloying elements are diffusion bonded to the iron particle mixed with 0.4 to 1% graphite.

Iron + 1.75% Ni + 1.5% Cu + 0.5% Mo where the alloy- ing elements are diffusion bonded to the iron particle mixed with 0.4 to 1% graphite.

Iron + 5% Ni + 2% Cu + 1% Mo where the alloying ele- ments are diffusion bonded to the iron particle mixed with 0.4 to 1% graphite.

Iron prealloyed with 1.5% Mo and 2% Ni diffusion bonded to the iron/Mo particle which are mixed with 0.4 to 1% graphite.

Iron prealloyed with 1.5% Mo and 2% Cu diffusion bonded to the iron/Mo particle which are mixed with 0.4 to 1% graphite.

Iron prealloyed with 1.5% Mo and 2% Cu and 4% Ni diffusion bonded to the iron/Mo particle which are mixed with 0.4 to 1% graphite.

Iron prealloyed with 1.5 or 0.85% Mo and mixed with 0.4 to 1% graphite.

Iron prealloyed with 3% Cr and 0.5% Mo and mixed with 0.2 to 0.7% graphite.

These iron based powders all contains powders with a particle size below 212 m sieved.

According to one especially preferred embodiment of the invention the maximum particle size of the iron based powder should be less than about 220 Hm (which is obtained for e. g. Astaloy Mo-106 J. m, through sieve analysis), and for this powder the maximum particle size of the lubricant should be less than 60 pm as measured by laser diffraction measurement.

The compacting and sintering steps for the prepara- tion of the final products, which-are distinguished by essentially the same or better dynamic properties as ob- tained for the same composition but without lubricant are performed under conventional conditions, i. e. the compac- tion is carried out at pressures between 400 and 1200 MPa and the sintering is performed at temperatures between 1100 and 1350°C.

The invention is further illustrated by the follow- ing non limiting examples.

Example 1 Five mixes with the same nominal composition were prepared from Distaloy AE which is a pure iron powder which has 4% Ni, 1.5% Cu and 0.5% Mo diffusion annealed to it and which has a main particle size range between 20 and 180 jum. The mixes mainly consisted of Distaloy AE + 0.3% C (UF-4) + 0.8 % Metalub Distaloy AE + 0.3% C (UF-4) + 0.8% zinc stearate Distaloy AE + 0.3% C (UF-4) + 0.8% Hoechst wachs Distaloy AE + 0.3% C (UF-4) + 0.8% Kenolube Distaloy AE + 0.3% C (UF-4) (reference, lubricated die) The following maximum particles sizes of the lubri- cants were measured by the laser diffraction measurement technique: Type of lubri-Maximum particle cant size, m Metalube 147 zinc stearate 73 Hoechst wachs 51 Kenolubes 73 From these mixes 5 TS bars were compacted to a den- sity of 7.10 g/cm3. For the mix without lubricant the

tool surface was lubricated with zinc stearate dispersed in acetone. All bars were sintered at 1120°C for 30 min- utes in endothermic atmosphere with a carbon potential corresponding to 0.3% carbon content. After sintering the density, carbon content and pore size distribution were evaluated. Also the particle size distribution of the different lubricants was measured using a Sympatec Helos laser diffraction particle size analysing equipment. The lubricants were dispersed in air for the particle mea- surement.

The bars manufactured from the different mixes de- fined above had a very even carbon content and density after sintering. Metallografic samples were prepared and the pore size distribution was measured on a surface of 25 mm2 for every material.

The relationship between the different lubricants are identical for the pore size distribution and the par- ticle size distribution, which indicates that the size of the largest lubricant particles governs the size of the largest pores at least for the lubricants containing par- ticles larger than approximately 60 pm. The pore size distribution for the lubricated die however shows that the reduction of the internal friction with addition of lubricants decreases the size of the intermediate poro- sity. In the case with the lubricant with the smallest coarse fraction/maximum particle size, lubricant C, the lubricant does not at all contribute to the amount of coarse pores. As can be seen from the above experiment lubricants with particles larger than 60 m creates coarse pores in a component made of Distaloy AE + 0.5% C at 7.1 g/cm3. A decrease of the fraction of coarse pores will increase the dynamical properties.

These results demonstrate the possibility to produce a material with finer porosity at a given density by using a lubricant, the maximum particle size of which is determined in view of the maximum particle size of the

iron based powder in accordance with the present inven- tion.

Example 2 The following example illustrates the effect on the fatigue strength of eliminating the largest particles of the lubricant as well as the largest particles of the iron based powder.

The following mixes were used Astaloy Mo + 0.3% C (UF-4) + 0.8% of Hoechst Wachs Astaloy Mo (-106 pm) + 0.3% C (UF-4) + 0.8% of Hoechst Wachs.

Astaloy Mo is a prealloyed material with 1.5% Mo (available from Hoganas AB, Sweden) which has an approxi- mate particle size range distribution of 20-180 pm. The sieved finer grade powder Astaloy Mo-106 pm was used to demonstrate the effect of eliminating the largest par- ticles of the iron based powder. The maximum particle size of Astaloy Mo as measured by laser diffraction measurement (Sympatec Helos laser) and the maximum particle size of Astaloy Mo-106ym were 363 and 214 Hm, respectively.

From all materials 20 fatigue test bars and 7 ten- sile strength bars were pressed to 7.1 g/cm3 and sintered at 1120°C for 30 minutes in endothermic atmosphere with a controlled carbon potential. The bars were then evaluated with respect to static properties and fatigue strength according to the staircase method described in Sonsino C. M."Method to determine relevant material properties for the fatigue design of powder metallurgy parts", Pow- der Metallurgy International 1984 vol. 16 p. 34-36. The pore size distribution was evaluated according to the method described in Example 1.

The results obtained demonstrate that the products prepared from the finer base powder Astaloy Mo (-106 pm) and finer lubricant powder Hoechst Wachs have less large pores and an increase of the fatigue strength of about 15% is obtained with the decreasing fraction of coarse pores. For the tensile strength there is a small increase of approximately 5% with decreasing fraction of coarse pores.