CEMENTED CARBIDE WEAR PART AND LAPPING METHOD

Abstract: The present invention relates to a cemented carbide wear part with a wear surface with improved friction and wear properties. This has been obtained by providing the wear surface with a surface layer with a thickness of 0.5 mu m to 25 mu m, preferably 1 to 10 mu m with an average WC grain size of less than 500 nm. The surface layer is obtained by lapping the wear surface with an abrasive disc for at least 10 minutes at 1000 to 3000 rpm at a pressure of 0.1 to 0.5 MPa preferably without any grinding media.

Description: Cemented carbide wear part and lapping method. The present invention relates to cemented carbide wear parts particularly useful in demanding wear parts applications used during sliding at high pressure and high sliding speed including seal rings with improved friction and wear properties. Cemented carbides, particularly WC-Co, WC-TiC-Co, C-TaC-Co, C-TiC-TaC-Co materials, find many applications in the field of wear parts. This is linked to their outstanding mechanical properties, e.g. a unique combination of hardness and toughness resulting in high wear resistance, low friction and high thermal conduc- tivity. Cemented carbides with a very fine grain size, often referred to as nanocrystalline structure, i.e. with carbide grains in the order of nanometers in size, show better mechanical properties than conventional, coarse grained materials. Although nanocrystalline cemented carbides have been manufactured, problems in their production make them expensive. There are however several examples of applications where nanocrystalline materials are very interesting candidates, such as metal cutting inserts, wood cutting teeth, etc. An industrially important example is cemented carbides used in seal rings for pumps working under heavy- duty conditions, such as drainage pumps and seawater pumps. The seal ring package, Figure 1, is often left on the shelf for a prolonged period of time before use, resulting in a joining process caused by the surrounding air, with severe seal/pump damages during the starting up moment as a result. Also the running-in period of the sealring package or the bearing package could give prob- lems in applications with high demands on the friction and the wear properties. An early fracture of the cemented carbide in the seal surface can totally destroy the seal surface. In some applications it is therefore necessary to use a combination of two types of materials in the seal rings, e.g. hard sintered SiC against cemented carbide. Such material combinations put high demands on the surface hardness and the friction properties of the softer cemented carbide. An interesting alternative to the manufacture of entirely nanocrystalline parts would be the creation of nanocrystalline surface layers on parts with a bulk of conventional coarse grained cemented carbides. In acidic environments the corrosion resistance of Co is low. By replacing Co with another more corrosion resistant metal, e.g. Cr, Ni or Mo or combinations thereof, the corrosion resistance of the material is increased. An alternative to the manufacture of parts with the entire material based on corrosion resistant metal would be the creation of a surface layer consisting of WC and a corrosion resistant metal mixed with Co. Figure 1 shows a seal ring package . Figure 2 is a SEM-micrograph in 4000x of a surface layer according to the invention Figure 3 shows an XRD diffraction pattern from a seal surface according to the invention. According to the invention seal rings with improved properties have been obtained by treatment of the seal surfaces with a lapping process. Surprisingly it was found that during the surface treatment the outer surface portion of the seal ring surface transforms to a hard and wear resistant layer of a very fine grained structure with excellent friction properties. According to the present invention there is pro- vided a seal ring with a surface layer on the seal sur- face with a thickness of 0.5 μm to 25 μm, preferably 1 to 10 μ . The seal ring consists of WC and 4 to 15%, preferably 5 to 12 weight-% Co, Ni and/or Fe . Additions of a few percent of Cr and/or Mo are present in the binder phase at least in the surface layer. The WC grain size is 2 to 6 μ . Up to 5% of cubic carbides such as TiC, TaC, NbC can be present in the cemented carbide. Alternatively, the seal ring is made of binderless cemented carbides containing <1 wt-% Co and/or Ni and/or Fe. In one embodiment the outermost part of the layer consists essentially of very fine grained WC less than 500 nm preferably less than 200 nm and most preferably about 100 nm in average size and 1 to 25 vol-% prefera- bly 5 to 12 vol-% of about 10 nm of oxidized crystallites such as CaWC-4, W0 3 or C0WO4. In a second embodiment the layer also contains hydroxides of the binder phase metals. In a third embodiment there are no oxides or hy- droxides at all. In all three embodiments the binder phase content in the layer is essentially the same or somewhat higher as in the interior portion of the seal ring. According to the method of the present invention the cemented carbide seal rings are lapped on the seal surfaces with abrasive discs such as pure sintered alumina or SiC or diamond for at least 10 minutes at 1000 to 3000 rpm at a pressure of 0.1 to 0.5 MPa preferably without any grinding media. Preferably a lapping liquid of 10 to 25% of CaOH at a pH level of 8 to 9 is used. Also B, S and/or metals such as Cr and Mo can be present as suitable soluble salts . Alternatively, the method may be carried out with an addition of corrosion resistant metal (s) at the sur- face, e.g. by using known deposition methods such as PVD or CVD or by covering the surface with small amounts of metal powder prior to the lapping treatment to get the fine grained surface layer. The process may also be carried out in gases not resulting in any oxidation of the material such as Ar or N 2 . In nitrogen only small amounts of nitrides are formed . The treatment can be performed by other methods such as brushing with grinding media of diamond, alumina or SiC. Alternatively cemented carbide rings can be used instead of the ceramic discs mentioned above. The invention has been described with reference to seal ring but it is obvious that it can be applied also to other wear parts applications including high speed sliding and high surface pressure with high demands on the sliding properties, such as bearings, e.g. bearings in dPE (Low density poly ethylene) reactors. Example 1 Cemented carbide seal rings with OD 55 mm and composition of 6 weight-% Co and WC with a grain size of about 5 μm were lapped with pure alumina ceramic disc as sliding counterpart for 30 minutes in air without addi- tion of any grinding media. The surface pressure during the lapping procedure was about 0.2 MPa and rotating speed of 2000 rpm. After the treatment the rings had a completely covering layer of thickness 3 μm consisting of nanocrystalline WC and a mixture of Co and Al 2 0 3 . The amount of alumina in the surface composite was minor. Small amounts of W0 3 and C0WO4 were also present according to XRD analyses in the surface layer. Example 2 Two self-mated cemented carbide seal rings with OD of 55 mm with a composition of 6 weight-% Co and WC with a grain size of about 5 μm were rotated relative to each other, one in a stationary position, with surface pressures of about 0.2 MPa. The rings were rotated for 15 minutes, rotating speed of 2000 rpm without any addition of grinding media. After the treatment the rings had a completely covering layer of thickness 4 μm consisting of nanocrystalline WC and Co, Figure 2. Small amounts of W0 3 and C0WO 4 were also present in the surface layer . Example 3 Two self-mated flat cemented carbide rings with OD of 55 mm and a composition of 6 weight-% Co and WC with a grain size of about 5 μm with a dull surface smoothness, were rotated relative to each other, one in a stationary position, with surface pressures of about 0.2 MPa. The test was performed with a chamber filled with nitrogen gas surrounding the rotating rings . The rings were rotated for 15 minutes with a rotating speed of 2000 rpm without any addition of grinding media. After the treatment the rings had a completely covering layer of thickness 3 μm consisting of nanocrystalline WC and Co. No oxides could be detected in the layer and only small amounts of nitrides. Example 4 Two self-mated flat cemented carbide rings with OD of 55 mm and a composition of 11 weight-% Co and WC with a grain size of about 5 μm, were rotated relative to each other, one in a stationary position, with surface pressures of about 0.2 MPa. Before testing the stationary ring was coated with a 100 nm PVD surface layer of Cr. The rings were rotated for 15 minutes with a rotating speed of 2000 rpm without any addition of grinding media. After the treatment the rings had a completely covering layer of thickness 2 μm consisting of nanocrystalline WC, Cr and Co. Small amounts of W0 3/ Cr-oxides and mixed oxides of W-Co-Cr were also present in the surface layer. Example 5 Two self-mated flat cemented carbide rings with OD 55 mm and a cemented carbide composition of 6 weight-% Ni and WC with a grain size of about 5 μm, were rotated relative to each other, one in a stationary position, with surface pressures of about 0.2 MPa. Before testing the stationary ring was covered with about 1 mg of Cr powder. The rings were rotated for 15 minutes with a ro- tating speed of 2000 rpm without any addition of grinding media. After the treatment the rings had a completely covering layer of thickness 3 μm consisting of nanocrystalline WC, Cr and Ni . Small amounts of W0 3 , Cr-oxides and oxide mixtures of W-Ni-Cr were also found by XRD analyses in the surface layer. Example 6 Two self-mated cemented carbide seal with OD 55 mm a cemented carbide composition of 6 weight-% Ni and WC with a grain size of about 5 μm, were rotated relative to each other, one in a stationary position, with surface pressures of about 0.2 MPa. The rings were rotated for 15 minutes with a rotating speed of 2000 rpm without any addition of grinding media. After the treatment the rings had a completely covering layer of thickness 1-2 μm consisting of nanocrystalline WC and Ni . Small amounts of W0 3 were also present in the surface layer. Example 7 The initial friction coefficient for two self-mated cemented carbide rings according to prior art was 0.4 at a pressure of 300 kPa. Two seal rings according to exam- pie 2 showed an initial friction coefficient of 0.17 in the same test. Example 8 Two self-mated cemented carbide rings according to prior art showed a regularly spaced crack pattern after sliding in water lubricated contact for 700 hours at 3 kPa. Two self-mated cemented carbides rings according to example 6 did not show any cracks after running for 700 hours at the same conditions. Example 9 Two self-mated flat rings of "Binderless" WC-MeC grade with OD of 55 mm and a composition of, in addition to WC, 3 weight-% TiC, 1.5 weight-% TaC, 0.5 weight-% NbC and 0.5 weight-% Co and a WC grain size of about 3 μm, were rotated relative to each other, one in a stationary position, with surface pressures of about 0.2 MPa. During testing the chamber of the rings was filled with a CaOH solution with pH=8. The rings were rotated for 15 minutes with a rotating speed of 2000 rpm without any addition of grinding media. After the treatment the rings had a completely covering layer of thickness 2 μm consisting of nanocrystalline WC, TiC, CaW0 4 and W0 3 according to XRD analysis, figure 3. Example 10 A comparison with seal rings according to prior art was performed in a qualification procedure. Test conditions were as follows: Medium on the pressure side: Water Medium at the Atm. side: Dry Rotating speed: 3000 rpm Seal ring pressure (MPa) : 0.4 Temp. (°C) : 40 Time (h) : 800 The seal surface on the rings according to prior art had smooth shiny surface. The rings according to the invention (example 6) had a flat and dull seal surface. The rings according to the invention performed very well with a light running during the whole test period. No early fractures were observed. The rings according to prior art were stopped after 12 hours because of high friction and hazardous running conditions. Early fractures with cracks and chipping of the cemented carbide from the surface occurred.

Claims: Claims 1. Cemented carbide wear part with a wear surface based on WC and a binder phase of Co, Ni and or Fe c h a r a c t e r i s e d in that said wear surface com- prises a surface layer with a thickness of 0.5 μm to 25 μm, preferably 1 to 10 μm in which the average grain size is less than 500 nm. 2. Cemented carbide wear part according to claim 1 c h a r a c t e r i s e d in that said surface layer further contains 1 to 25 vol-% preferably 5 to 12 vol-% of about 10 nm of oxidized crystallites such as CaW0 4 , W0 3 or CoW0 4 . 3. Cemented carbide wear part according to any of the preceding claims c h a r a c t e r i s e d in that said surface layer further contains hydroxides of the binder phase metals . 4. Cemented carbide wear part according to any of the preceding claims c h a r a c t e r i s e d in consisting of WC and 4 to 15%, preferably 5 to 12 weight-% Co and/or Ni and/or Fe . 5. Cemented carbide wear part according to any of claims 1-3 c h a r a c t e r i s e d in consisting of WC and <1 weight-% binder phase. 6. Method of making a cemented carbide wear part with a wear surface c h a r a c t e r i s e d in lapping the wear surfaces with an abrasive disc for at least 10 minutes at 1000 to 3000 rpm at a pressure of 0.1 to 0.5 MPa preferably without any grinding media. 7. Method according to claim 6 c h a r a c t e r i s e d in using lapping liquid of 10 to 25% of CaOH at a pH level of 8 to 9. 8. Method according to any of claims 6 or 7 c h a r a c t e r i s e d in using a lapping liquid containing also B, S and/or metals such as Cr and Mo pres- ent as suitable soluble salt. 9. Method according to any of claims 6-8 c h a r a c t e r i s e d in applying corrosion resistant metal (s) at the surface, e.g. by using known deposition methods such as PVD or CVD or by covering the surface with small amounts of metal powder prior to the lapping treatment . 10. Method according to any of claims 6-9 c h a r a c t e r i s e d in using as abrasive disc a cemented carbide ring.

Inventors: AXEN NIKLAS, EDERYD STEFAN, ENGQVIST HAAKAN, AXEN, NIKLAS, EDERYD, STEFAN, ENGQVIST, HAAKAN
Applicants: SANDVIK AB
Application year: 2000
Main classification: C23C30
IPC codes: B24B37, B24B57, C22C29, C23C30, F16C33, F16J15