The present invention concerns compacted bodies and more particularly compacted and optionally presintered boαies, which are prepared from metal powders and wnich have a densified surface. Materials used for components subjected to a bending stress e.g. gear wheels are subjected to local stress concentrations, and it is preferred that these materials have superior properties at the local stress maximum regions. An example of such a material is disclosed in EP 552 272 which concerns sintered powder metal blanks having densified surface regions. According to this publication the densified regions are obtained by rolling.

It is also known that the surfaces of sintereα pow- der metallurgical parts can be densified by using shot peening. The purpose of shot peenmg the surfaces of these sintered parts is to induce compressive stress in the surfaces, which in turn results in sintered parts having improved fatigue strength, surface hardness etc. It has now been found that important advantages can be obtained if the densification of the surface is per¬ formed before the sintering of the compacted parts. The most interesting results have been obtained when the com¬ pacted parts are subjected to the densification process after a presmtering step. Accordingly, the present in¬ vention concerns a process for preparing compacted and preferably presintered bodies having a densified surface as well as the bodies obtained by this process.

By performing the densification of metal powder bodies in green and optionally presintered condition, a larger degree of deformation can be obtained than _n the case where sintered bodies are densifieα. When the σreen and optionally presintered parts are subsequently sin¬ tered, the orevious pores are sintered together ana a layer with full or almost full density is created. In this context the term "full or almost full αensitv" is

intended to mean that a densification in the range of 90 - 100 per cent of full density is established.

By using the process according to the present inven¬ tion not only the densification or deformation depth will be improved. Also the energy requirement will be conside¬ rably lower than when the densification process is car¬ ried out after the sintering step m accordance with known methods. After sintering the bodies prepared according to the present invention can be treated with secondary operations as usual.

Suitable metal powders which can be used as starting materials for the compacting process are powders prepared from metals such as iron and nickel . In the case of iron- based powders, alloying elements, such as carbon, chro- mium, manganese, molybdenum, copper, nickel, phosphorus, sulphur, etc. can be added in order to modify the proper¬ ties of the final sintered products. The iron-based pow¬ ders can be selected from the group consisting of sub¬ stantially pure iron particles, pre-alloyed iron-based particles, diffusion-alloyed iron-based particles and mixtures of iron particles and alloying elements.

In order to obtain sufficient bending strength for the suosequent densification process the starting metal powder is uniaxially compacted at a pressure between 200 and 1200, preferably between 400 and 900 MPa. The compac¬ tion is preferably carried out in a lubricated die. Other types of compaction are warm and cold compaction of metal powders mixed with lubricants, such as stearates, waxes, metal soaps, polymers, etc. According to a preferred embodiment of the invention the compacted body is also presintered at a temperature above 500°C, preferably between 650 and 1000°C before the densification operation.

The green and optionally presintered bodies sub- jected to the densification process according to the pre¬ sent invention should be compacted and optionally pre¬ sintered to a minimum bending strength of at least 15

MPa, preferably at least 20 MPa, and most preferably at least 25 MPa.

The densification process according to the invention is preferably carried out by shot peening although other densification processes such as different types of roll¬ ing are not excluded. In shot peening, rounded or essen¬ tially spherical particles (termed "shot") made from cast or wrought steel and stainless steel, as well as from ceramic or glass beads, are propelled against a workpiece with sufficient energy and for a sufficient time to cover the surface with overlapping cold worked dimples (see e.g. the article by J. Mogul et al "Process controls the key to reliability of shot peening", Process Controls & Instrumentation, November 1995) . The shot peening time according to the present in¬ vention normally exceeds 0.5 seconds and is preferably between 1 and 5 seconds and the Almen intensity is nor¬ mally in the range 0.05 - 0.5. The deformation depth depends on the final use of the product and should exceed 0.1 mm, preferably 0.2 mm and most preferably the depth should exceed 0.3 mm.

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

The starting metal powder was Distaloy DC-1, which is an iron-based powder containing 21 nickel and 1.5% mo¬ lybdenum available from Hoganas AB, Sweden.

This powder was warm compacted at 700 MPa to a density of 7.4 g/cm ³ having a bending strength of 25 MPa. The compacted bodies were divided into the following three groups:

Group 1 The bodies were left green, I e not subjected to any additional treatment. Group 2 The bodies were presintered at 750°C for 20 minutes in protective atmosphere.

Group 3 The bodies were sintered at 1120°C for 15 minutes in endogas.

Group 1

The green bodies were shot peened. At too high in¬ tensities, i.e. Almen intensities (cf the Mogul article referred to above) above 0.14 for 3 seconds, the partic¬ les were torn loose and the surface was destroyed. It turned out that the Almen intensities should be below about 0.14 and the exposure time should be less than 2 seconds. This was true for both green bodies which ιad been warm compacted and for bodies which were produced in a lubricated die. As can be seen in Fig. 1, the densifi- cation was somewhat better in the bodies obtained wnen the compaction was performed in a lubricated die.

Group 2

The presmtering of the green bodies was done _n or¬ der to remove lubricant that could create porosity, to remove deformation hardening and to improve the strength of the material. It was essential that the graphite difu- sion was limited in order to avoid solution hardening effects in the iron powder particles. After the presm¬ tering, the strength of the material had improved significantly and much higher Almen intensities couid be used, especially for the bodies manufactured in lubricated dies. Almen intensities up to 0.3 could De used without problems, I.e. no particles were torn loose from the surface, and deformation depths of 300 μm were achieved. For the warm compacted bodies the erosion started at intensities of 0.14. Due to the removal of lubricant and deformation hardening, the deformation depth had increased significantly in comparison with the green bodies of group 1.

Group 3

Only warm pressed materials were tested as no sig¬ nificant pore structure difference from various compac¬ ting methods is considered to remain after a full sinte¬ ring operation. The sintered body had their full strength, and therefore very high Almen intensities, up to 0.3, could be used. The effect of the shot peening operation is, however, much less in comparison with the bodies which were shot peened in green or presintered condition according to the present invention. It can be seen that only one third of the deformation depth was achieved at the same intensity due to the high hardness of the presintered body.

The experiments are listed in the following table.