More specifically, the invention concerns agglomerated iron-based powders for the preparation of wear resistant materials, which combine low friction, good wear resis- tance and self lubrication and which advantageously can be used in e.g. heavy-duty diesel engines. When using the agglomerated powders these materials can be prepared by conventional technique from inexpensive raw materialsin existing plants.

From theoretical and practical considerations it has been found that the starting materials for such wear re- sistant material could be selected from the following materials.

a) 63-95 % by weight of a fine base powder consist- ing essentially of iron and having a particle size essentially less than 75 m b) 5-20 % by weight of a lubricating phase having a particle size essentially less than 120 Fm and preferably less than 60 m c) 0-15 % by weight of a hard phase material having a particle size essentially less than 10 Fm, and d) 0-7 % of additives including binding agent(s), solvent(s)and optionally lubricant(s) The base powder could be selected from the group consisting of prealloyed powders, partially prealloyed powders or pure iron powders. Examples of prealloyed pow- ders are e.g. Astaloy Mo and the partially prealloyed powders can be e.g. Distaloy SE and Distaloy AE. Pure iron powders which advantageously could be used are e.g.

ASC 100.29, NC 100.24, SC 100.26 and AHC 100.29. All powders are available from Höganäs AB, Sweden.

The lubricating phase according to the invention is present also after the sintering process and is a solid inorganic material. Examples of such materials are metal sulphides, metal chlorides and metal fluorides. A preferred material is MnS. The lubricating phase could also be MnX available from Nbgands AB, Sweden. If more than 20 % is added the strength will be adversely affected and if less than 5 W is added the friction will be too high. According to a preferred embodiment the amount of the lubricating phase is 5 - 15 % by weight.

The hard phase powder is selected from carbides, such as NbC, TiC, VC, TaC. If the amount of the hard phase powder is more than 15 % the compressibility will be too low. According to a preferred embodiment the amount of hard phase powder is not more than 10 %. In practice the amount of the hard phase powder is chosen in view of the desired wear resistance.

The different additives could be selected from the group consisting of Fe3P, graphite and/or various conven- tional lubricants, such as waxes, stearates and polymers.

Unexpected problems were encountered when these pow- der mixtures should be used on an industrial scale, as it turned out that the powders had essentially no flow and good flow is a necessary prerequisite for industrial production. Other disadvantages involved too much segregation and dusting during handling.

According to the invention these problems have been obviated by using a process including the following steps: 1) Mixing the dry ingredients according to points a)-d) above in a mixing chamber.

2) Evacuating the mixing chamber to less than 25, preferably less than 15 mbar.

3) Filling the mixing chamber with an inert gas to slight underpressure to about 950, preferably about 900 mbar.

4) Mixing the ingredients with less than 1 % by weight, based on the whole mixture, of a binding agent and adding a solvent.

5) Drying the obtained powder.

An important feature of the granulation process is the low amount of binding agent, which is beneficial to the subsequent sintering process and, consequently, to the final product. The binding agent could be any conventioanl binding agent used within the P/M field.

More specifically, the binding agent could be selected from the group consisting of polyesters and polyalcohols.

Cellulose acetate butyrate is a presently preferred binding agent.

The solvent depends on the binding agent and is selected from the group consisting of water, alcohols and ketones. A preferred solvent is acetone.

The agglomerated powder, which has a particle size essentially between about 75 and 150 pm, can be uniaxially compacted to a green body having a density exceeding 85 and preferably exceeding 90 percent of the theoretical density.

In order to prepare the final wear resistant mate- rial, the agglomerated powder is compacted at a pressure between about 400 and 800 MPa and subsequently sintered at e.g. 12500C for 45 minutes in 95/5 N2/H2 Sizing is per- formed at eg 800 MPa, carburizing at 8600C for eg 30 minutes in about 0.9 WOC and tempering is carried out at a temperature of about 180"C for about 60 minutes.

The properties of a compacted and sintered product obtained from an agglomerated powder according to the in- vention were superior to the properties of a correspon-

ding material which was obtained with a non-agglomerated powder.

The invention is illustrated by the following non- limiting examples.

Group Material 1 Astaloy Mo* <75 Fm + MnS(5%, 15%)+ MnS (20 Fm, <60 Fm) 2 Cold PMo* + MnS (5%, 15%) 3 M3/2** + MnS(5%, 15%) + 7,74% NbC/5% TiC * Available from Höganäs AB, Sweden ** Standard quality of high-speed steel available from Coldstream A.S., Belgium.

Granulation A powder mix of 20 kg is prepared and put in a Y-cone mixer. The acetone and the binder (cellulose acetate butyrate) are added to the mix according to the schedule stated below.

0.15% binder (group 1 and 2 materials) 0.3% binder (group 3 materials) 4.0% acetone (group 1 and 2 materials) 6.0% acetone (group 3 materials) Process schedule: 1. Mixing of dry powder 2. Evacuation of mixer 3. Fill mixer with N2 4. Start the intensifier, add the solvent with the in- tensifier running. Continuously adjust the pres sure so that slight under-pressure is kept 5. Let the intensifier run until the mixture is homogeneous.

6. Dry/evacuate the powder until the pressure is about 2-10 mbar 7 Run the mixer 2-10 more minutes 8. Fill the mixer with N2 to atmospheric pressure 9. Empty the mixer The group 3 materials needed extra binder and solvent for the granulation to be sufficient.

Materials Group 1 Two parameters and two levels are tested with one addi- tional mid-point. The first parameter is the amount of MnS added, the low level is 5% MnS and the high is 15% MnS. The second parameter is the type of MnS. The first type of MnS is the normal MnS which is added to PM mixes as machining aid and the second type of MnS is a course MnS with a particle size essentially between 60 Fm and 120 Fm using a Tyler mesh standard sieve. The mid-point is 10% MnS, that is a mix of 50% normal MnS that has an average particle size essentially less than 60 Fm and 50% of material that has a particle size essentially between 60 Rm and 120 Zm. As no hard phase is added, the amount of binder can be kept low and the compressibility is not much reduced.

Material Composition ST-1 95% Base material + 5% MnS less than 60pm + 0.4% H- wax ST-2 95% Base material + 5% MnS 60-120pm + 0.4% H-wax ST-3 85% Base material t 15% MnS less than 60 m + 0.4% H- wax ST-4 85% Base material + 15% MnS 60-120pm+ 0.4% H-wax ST-5 90% Base material + 10% MnS mix Base material 97.6% Astaloy Mo <75 + 0.4% graphite MnS mix 50% MnS having an average particle size essentially less than 60 Fm and 50% MnS having a particle size essentially between 60 and 120 Mm Granulation aid 0.15% binder Material AD Flow GD P Mn Mo Cu g/cm3 sec/50g g/cm3 ST-1 3,39 25,77 6,66 0,21 3,0 1,4 1,6 ST-2 3,42 26,97 6,64 0,20 3,2 1,3 1,6 ST-3 3,02 31,98 6,13 0,17 8,8 1,1 1,4 ST-4 3,08 29,88 6,08 0,18 8,8 1,1 1,4 ST-5 3,10 29,90 6,40 0,20 5,7 1,1 1,4 Group 2 A sintered component based on Cold PMo contains a lot of carbides after sintering. Addition of hard phase requires an increased sintering temperature and is not good for the mechanical properties of the material.

As in the previous group when no hard phase is added, the amount of binder can be kept low and the compressibility is not much reduced.

Material Composition A-l 100% Cold PMo A-2 95% Cold PMo + 5% MnS mix + 0,4% H-wax A-3 90% Cold PMo + 10% MnS mix + 0,4% H-wax A-4 85% Cold PMo + 15% MnS mix + 0,4% H-wax A-5 85% Cold PMo + 15% MnS mix A-6 90% Astaloy Mo<75 Fm + 10% MnS mix Cold PMo = 95% prealloyed, water atomized with 10% molybdenum, to which are added 1.15% graphite and 3.85% Fe3P MnS mix = 50% MnS having a particle size essentially less than 60 Fm and 50% MnS having a particle size essentially between 60 and 120 Wm Granulation aid 0.15% binder Material AD flow GD P Mn Mo Cu g/cm3 sec/50g g/cm3 A-1 3,32 24,06 6,45 0,46 2,8 10 A-2 3,49 23,33 6,32 0,42 3,0 9,6 A-3 3,29 25,17 6,10 0,40 5,8 9,1 A-4 3,17 26,18 5,91 0,43 9,4 8,3 A-5 3,11 25,46 5,88 0,45 9,4 8,3 A-6 3,20 29,95 6,44 - 5,8 1,2 1,6 Group 3 The third group of materials is high-speed steel mixes.

The carbides are useful in order to improve the wear re- sistance. The hard phase together with the M3/2 that has poor compressibility gives the materials with the lowest compressibility.

Material Composition BF-1 86,76% M3/2 + 5% MnS* + 7,74% NbC + 0,5% H-wax BF-2 76,76% M3/2 + 15% MnS* + 7,74% NbC + 0,5% H-wax BF-3 89,5% M3/2 + 5% MnS* + 5% TiC + 0,5% H-wax BF-4 79,5% M3/2 + 15% MnS* + 5% TiC + 0,5% H-wax *Powder having a particle size essentially less than 60m Granulation aid 0.3% binder Material AD flow GD P Mn Mo Cu g/cm3 sec/5 g/cm3 Og BF-1 2,62 36,23 6,07 BF-2 2,74 36,62 5,85 BF-3 2,62 36,23 5,88 BF-4 2,72 37,00 5,71 The above tables disclose that a flow between 25 and 40 sek/50 g can be obtained by using the agglomeration process according to the present invention. No flow could be obtained for the untreated non-agglomerated powders.