SINTERED PRODUCTS HAVING IMPROVED DENSITY

Abstract: The present invention concerns low alloy PM materials which after single pressing and sintering utilising traditional powder metallurgy processes and equipment combine high mechanical strength and high density with maintained precision of tolerance. As base material is used an iron powder having at least one alloying element diffusion-bonded to the outer surfaces of unalloyed iron particles.

Description: SINTERED PRODUCTS HAVING IMPROVED DENSITY Field of the invention The present invention concerns sintered powder- metallurgically produced sintered products having im¬ proved density. More specifically the invention concerns products prepared from iron or steel powders having the alloying elements diffusion-bonded to the iron or steel particles. These products are prepared by a warmcompac- tion process using high temperature lubricants. Background of the invention The market for sintered structural components could be markedly extended if enhanced mechanical properties could be obtained with maintained precision of tolerance control. Higher densities than the range currently pro- duced (6.5-7.2 g/cc) would be required for more highly stressed applications in automotive engineering as den¬ sity affects the mechanical properties and fatigue strength in particular. A number of options are available to increase sin- tered density. Methods such as double pressing/double sintering, copper infiltration and powder forging pro¬ vide higher densities than traditional single press and sintered operations but their use is constrained by cost and geometry considerations. In order to retain competi- tive viability a satisfactory process route is required which will not add any extra processing step which will increase the processing cost. Summary of the invention The present invention concerns low alloy PM materi¬ als which after single warm pressing and sintering uti¬ lising traditional powder metallurgy processes and equipment combine high mechanical strength and high den¬ sity with maintained precision of tolerance. These mate- CONFIRMATION COPY rials are obtained according to a process including the following steps: mixing an iron powder, a high temperature lubricant and optionally an organic binder; heating the mixture, preferably to a temperature of at least 120°C; transferring the heat powder composition to a die which is preheated to a temperature of preferably at least 120°C; and compacting the composition, at an ele- vated temperature of preferably at least 120°C. sintering the compacted body at a temperature of at least 1120°C; whereby the iron powder is a diffusion-bonded powder having one or more alloying elements diffused into the outer surfaces of unalloyed iron particles, and whereby the lubricant is added in one step. The US patent 5 154 881 discloses metal composi¬ tions subjected to a warm compacting process. Compacted and sintered products prepared according to this patent exhibit improved densities and other strength properties and the advantages are obtained when the increased pressing temperatures are applied to iron based powders, which could be pure iron powders, mixtures of iron pow¬ ders with alloying elements or pre-alloyed iron powders. The US patent 5 256 185 discloses a method for the preparation of a powder metallurgical product, wherein an iron powder, a lubricant and a binder are mixed, the mixture is compacted at an elevated temperature and the compacted body is subsequently sintered. More specifically, this patent concerns a method of lubricant addition, which enables suitable adjustment of the apparent density, either upwards or downwards, as decided, without significant effect on the flow rate. This is accomplished by the following steps: (a) provid- ing a dry admixture of an iron based powder, at least one alloying powder and a first amount of organic lubri- cant, (b) providing a liquid mixture of an organic bind¬ ing agent dissolved or dispersed in a solvent, (c) wet¬ ting the dry admixture with the liquid mixture, (d) re¬ moving the solvent thereby forming a dry powder composi- tion and (e) admixing a second amount of organic lubri¬ cant with the dry powder composition to form a metallur¬ gical powder composition. Contrary to the process disclosed in this US patent the present invention includes only one lubricant addi- tion step. Furthermore, the present invention does not concern a method of manipulating the apparent density and thus the present invention concerns a quite diffe¬ rent problem than the problem which is solved by the in¬ vention disclosed in the US patent. Quite unexpectedly, it has now been found that in¬ creased densities and conc-equently improved mechanical properties can be obtained if the iron powder is a dif¬ fusion-bonded powder having one or more alloying ele¬ ments diffused into the outer surfaces of unalloyed iron particles. Examples of such diffusion-bonded powders are Distaloy AB, Distaloy AE, Distaloy SA and Distaloy SE, all available from Hδganas AB, Sweden. These powders are all distinguished by a low carbon content, i e a carbon content below 0.01 % by weight. The invention is further defined by the accompany¬ ing claims. The high temperature lubricant is generally one which can withstand a compaction temperature up to about 370°C. Examples of such lubricants are molybdenum sul- phide, boric acid and those polyamide lubricants which are disclosed in the US patent 5 154 881 which is hereby incorporated by reference. Particularly preferred are the commercially available lubricants ADAVA AX 450 or PROMOLD 450, polyamide sold by Morton International of Cincinnati, Ohio, which according to the US patent is an ethylene bis-stearamide having an initial melting point between about 200°C and 300°C. Other lubricants which could be used are oligomers of "polyamide" type as de¬ scribed in our copending Swedish patent application 9401922-1 filed June 2, 1995 which is hereby incorpo- rated by reference. These lubricants may be used in com¬ bination with minor amounts, e.g. from 0.05 to 0.15% by weight, of conventional lubricants for cold-compaction, e.g. metal stearates, such as zinc stearate. The total amount of lubricant is 0.1-2, preferably 0.2-1% by weight of the composition. The binding agent is preferably a cellulose ester such as those manufactured by Eastman Chemical products designated as CA, CAB and CAP resins. If present, the binding agent is used in an amount of 0.01-0.40% by weight of the composition. The invention is further illustrated by the follow¬ ing examples. Materials tested The following two mixes were prepared using the special developed lubricant/binder system for warmcom- paction: 1) Distaloy AE + 0.6% graphite + 0.6% lubricant/binder 2) Distaloy DC*-l+0.6% graphite + 0.6% lubricant/binder *pre-alloyed iron-powder for comparison The lubricant used was Promold 450 and the binder was a cellulose ester. The weight ratio lubricant/binder was 3:1. The same mixes were also prepared using 0.8% zinc- stearate as lubricant for conventional processing at room temperature. Processing conditions Tensile test and TRS specimens were compacted at pressure of 400, 600 and 800 MPa on a mechanical DORST- press, using both the warmcompaction process as well as the conventional single pressing/single sintering (1P1S) . The compacted specimens were sintered at 1120°C for 30 minutes in endothermic atmosphere with controlled carbon potential. Green and sintered properties were analysed accord¬ ing to standard test methods, i.e. the green density according to ISO No 3927-1977 the green strength according to ISO No 3995-1977 the sintered density according to ISO No 3369-1975 and the tensile strength according to ISO No 2740-1973 The powder properties are summarised in the follow¬ ing table. Warm compacting Cold compacting Compacting pressure 400 600 800 400 600 800 GD[g/cc] Distaloy AE 7,05 7,30 7,39 6,80 7,10 7,25 Distaloy DC 6,91 7,20 7,35 6,72 7,03 7,22 GS[MPA] Distaloy AE 18 27 28 8 12 13,5 Distaloy DC 12 18 18 7 11 12 SG[g/cc] Distaloy AE 7,05 7,25 7,40 6,83 7,08 7,25 Distaloy DC 6,91 7,21 7,37 6,74 7,04 7,23 TS[MPa] Distaloy AE 720 780 830 815 720 780 Distaloy DC 645 720 780 575 665 725 GD = green density GS = green strength SD = sintered density TS = tensile strength The above table discloses that the use of iron pow¬ ders having alloying elements diffusion-bonded to the outer surfaces of unalloyed iron particles results in products having properties superior than what can be ob¬ tained if pre alloyed iron powders are used. This supe¬ riority is especially marked when a warm compacting process is used.

Claims: CLAIMS 1. Process for the preparation of a powder-metal¬ lurgical product having increased density including the following steps: mixing an iron powder, a high temperature lubricant and optionally an organic binder; heating the mixture to a temperature above ambient tem¬ perature; transferring the heat powder composition to a preheated die; compacting the composition at an elevated tempera¬ ture, and sintering the compacted body at a temperature of at least 1120°C; whereby the iron powder is a diffusion-bonded powder having one or more alloying elements diffused into the outer surfaces of unalloyed iron particles, and whereby the lubricant is added in one step. 2. Process according to claim 1, wherein the com¬ pacting is carried out in a single step and at a pres- sure between about 400 and 800 MPa. 3. Process according to claim 1 or 2, wherein the sintering is carried out in a single step. 4. Process according to any of the previous claims, wherein both the compacting and the sintering are car- ried out as single steps. 5. Process according to claim 4, wherein the alloy¬ ing elements are selected from Ni, Cu, Mo, Cr, Mn and Si. 6. Process according to any of the claims 1-5, wherein the powder composition and the die are preheated to at least 120°C 7. Process according to claim 6 wherein the compo¬ sition is compacted at a temperature of at least 120°C 8. Process according to any of the preceding claims wherein the lubricant is selected from the group con¬ sisting of polyamides or oligomer compounds of amide type, optionally in combination with metal soaps and/or waxes. 9. Product prepared according to any of the preced¬ ing claims having a sintered density between about 7.1 and 7.5 g/cc. 10. Product according to claim 9 including 0-6% Ni, 0-5% Cu, 0-6% Mo, 0-4% Cr, 0-4% Mn and 0-3% Si. 11. A green body prepared from an iron powder hav¬ ing at least one alloying element diffusion-bonded to the outer surfaces of unalloyed iron particles whereby the green body has a green density between about 7.0 and 7.5 g/cc when compacted at a pressure between about 400 and 800 MPa and at a temperature above 120°C, preferably above 150°C. 10. Use of an iron powder having at least one al¬ loying element diffusion-bonded to the outer surfaces of unalloyed iron particles for the preparation of single compacted and single sintered products having a sintered density between about 7.1 and 7.5 g/cc.

Inventors: ENGSTROEM ULF, JOHANSSON BJOERN, ENGSTROEM, ULF, JOHANSSON, BJOERN
Applicants: ENGSTROEM ULF, HOEGANAES AB, JOHANSSON BJOERN, ENGSTROEM, ULF, JOHANSSON, BJOERN
Application year: 1995
Main classification: B22F1
IPC codes: B22F1, B22F3, C22C33