The present invention relates to a coated cemented carbide insert for turning of steel, like low alloyed steels, carbon steels and tough hardened steels at high cutting speeds. The cemented carbide consists of WC, 2-10 wt. % Co and 4-12 wt. % of cubic carbides of metals from groups 4, 5 or 6 of the periodic table, preferably Ti, Ta and Nb. The Co-binder phase is highly alloyed with W with a CW-ratio of 0.75-0.90. The insert has a binder phase enriched and essentially cubic carbide free surface zone A of a thickness of <20 μm and along a line C essentially bisecting the edge, in the direction from the edge to the centre of the insert, a binder phase content increases essentially monotonously until it reaches the bulk composition. The binder phase content at the edge is 0.65-0.75 times the binder phase content by volume of the bulk and the depth of the binder phase depletion is 100-300 μm, preferably 150-250 μm. The insert is coated with 3-12 μm columnar TiCN-layer followed by a 2-12 μm thick Al2O3-layer.
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1. A cutting tool insert for machining steel, comprising a cemented carbide body and a coating, wherein:
the cemented carbide body comprises WC, 2-10 wt. % Co, 4-12 wt. % of cubic carbides of metals from groups 4, 5 or 6 of the periodic table, and N in an amount of between 0.9 and 1.7% of the weight of the elements from groups 4 and 5;
the cemented carbide body comprises a Co-binder phase which is highly alloyed with W, and has a CW-ratio of 0.75-0.90;
the cemented carbide body has a surface zone with a thickness of <20 μm, which is binder phase enriched and essentially cubic carbide free;
the cemented carbide body has a cutting edge which has a binder phase content of 0.65-0.75 by volume of the bulk binder phase content, and the binder phase content increases at a constant rate along a line which bisects said cutting edge until it reaches the bulk binder phase content at a distance between 100 and 300 μm from the cutting edge; and
the coating comprises a 3-12 μm columnar TiCN layer followed by a 2-12 μm Al2O3 layer.
2. The cutting tool insert of
3. The cutting tool insert of
4. The cutting tool insert of
5. The cutting tool insert of
6. The cutting tool insert of clam 1, wherein the surface zone of the cemented carbide body is 5-15 μm thick.
7. The cutting tool insert of
10. The cutting tool insert of
0. 11. The cutting tool insert of
0. 12. The cutting tool insert of
0. 13. The cutting tool insert of
0. 14. The cutting tool insert of
0. 15. The cutting tool insert of
0. 16. The cutting tool insert of
0. 17. The cutting tool insert of
0. 18. The cutting tool insert of
0. 19. The cutting tool insert of
0. 20. The cutting tool insert of
0. 21. The cutting tool insert of
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Inserts according to the invention are further provided with a coating consisting of essentially 3-12 μm columnar TiCN-layer followed by a 2-12 μm thick Al2O3-layer deposited, for example according to any of the patents U.S. Pat. Nos. 5,766,782, 5,654,035, 5,674,564, 5,702,808 preferably with an α-Al2O3-layer, possibly with an outermost 0.5-4 μm TiN-layer.
The present invention is applicable to cemented carbides with a composition of 2-10, preferably 4-7, weight percent of binder phase consisting of Co, and 4-12, preferably 7-10, weight percent cubic carbides of the metals from groups 4, 5 or 6 of the periodic table, preferably >1 wt. % of each Ti, Ta and Nb and a balance WC. The WC preferably has an average grain size of 1.0 to 4.0 μm, more preferably 2.0 to 3.0 μm. The cemented carbide body may contain small amounts, <1 volume %, of η-phase (M6C).
By applying layers with different thicknesses on the cemented carbide body according to the invention, the property of the coated insert can be optimised to suit specific cutting conditions. In one embodiment, a cemented carbide insert produced according to the invention is provided with a coating of: 6 μm TiCN, 8 μm Al2O3 and 2 μm TiN. This coated insert is particularly suited for cutting operation with high demand regarding crater wear. In another embodiment, a cemented carbide insert produced according to invention is provided with a coating of: 8 μm TiCN, 4 μm Al2O3 and 2 μm TiN. This coating is particularly suited for cutting operations with high demands on flank wear resistance.
The invention also relates to a method of making cutting inserts comprising a cemented carbide substrate consisting of a binder phase of Co, WC and a cubic carbonitride phase with a binder phase enriched surface zone essentially free of cubic phase and a coating. The powder mixture consists 2-10, preferably 4-7, weight percent of binder phase consisting of Co, and 4-12, preferably 7-10, weight percent cubic carbides of the metals from groups 4, 5 or 6 of the periodic table, preferably >1 wt. % of each Ti, Ta and Nb and a balance WC, preferably with an average grain size of 1.0-4.0 μm, more preferably 2.0-3.0 μm. Well-controlled amounts of nitrogen are added either through the powder as carbonitrides and/or added during the sintering process via the sintering gas atmosphere. The amount of added nitrogen will determine the rate of dissolution of the cubic phases during the sintering process and hence determine the overall distribution of the elements in the cemented carbide after solidification. The optimum amount of nitrogen to be added depends on the composition of the cemented carbide and in particular on the amount of cubic phases and varies between 0.9 and 1.7%, preferably about 1.1-1.4%, of the weight of the elements from groups 4 and 5 of the periodic table. The exact conditions depend to a certain extent on the design of the sintering equipment being used. It is within the purview of the skilled artisan to determine whether the requisite surface zones A and B of cemented carbide have been obtained and to modify the nitrogen addition and the sintering process in accordance with the present specification in order to obtain the desired result.
The raw materials are mixed with pressing agent and possibly W such that the desired CW-ratio of the binder phase is obtained and the mixture is milled and spray dried to obtain a powder material with the desired properties. Next, the powder material is compacted and sintered. Sintering is performed at a temperature of 1300-1500° C., in a controlled atmosphere of between 40 and 60 mbar, preferably about 50 mbar, followed by cooling. After conventional post sintering treatments including edge rounding a hard, wear resistant coating, such as defined above, is applied by CVD- or MT-CVD-technique.
A.) Cemented carbide turning inserts of the style CNMG120408-PM, DNMG150612-PM and CNMG160616-PR, with the composition 5.5 wt % Co, 3.5 wt. % TaC, 2.3 wt. % NbC, 2.1 wt. % TiC and 0.4 wt. % TiN and balance WC with an average grain size of 2.5 μm were produced according to the invention. The nitrogen was added to the carbide powder as TiCN. Sintering was done at 1450° C. in a controlled atmosphere consisting of Ar, CO and some N2 at a total pressure of about 50 mbar.
Metallographic investigation showed that the produced cemented carbide inserts had a cubic-carbide-free zone A with a thickness of 10 μm. Image analysis technique was used to determine the phase composition at zone B and the area along line C (FIG. 1). The measurements were done on polished cross sections of the inserts over an area of approx. 40×40 μm gradually moving along the line C. The phase composition was determined as volume fractions. The analysis showed that the cobalt content in zone B was 0.7 times the bulk cobalt content and the cubic carbide content 1.3 times the bulk gamma phase content. The measurements of the bulk content were also done by image analysis technique. The Co-content was gradually increasing and the cubic carbide content gradually decreasing along line C in the direction from the edge to the centre of the insert.
Magnetic saturation values were recorded and used for calculating CW-values. An average CW-value of 0.84 was obtained.
B.) Inserts from A were first coated with a thin layer <1 μm of TiN followed by 6 μm thick layer of TiCN with columnar grains by using MT-CVD-techniques (process temperature 850° C. and CH3CN as the carbon/nitrogen source). In a subsequent process step during the same coating cycle, an 8 μm thick α-Al2O3 layer was deposited according to patent U.S. Pat. No. 5,654,035. On top of the α-Al2O3 layer a 1.5 μm TiN layer was deposited.
C.) Inserts from A were first coated by a thin layer <1 μm of TiN followed by a 9 μm thick TiCN-layer and a 5 μm thick α-Al2O3 layer and a 2 μm thick TiN layer on top. The same coating procedures as given in A.) were used.
D.) Commercially available cutting insert in style CNMG120408-PM, DNMG150612-PM and CNMG160616-PR, with the composition given below were used as references in the cutting tests:
E.) Inserts with the same cemented carbide composition as in D were coated with 4 μm TiN and 6 μm Al2O3. Inserts styles CNMG120408-QM and CNMG120412-MR.
F.) Inserts in style CNMG120408-QM and CNMG120412-MR with the composition: 4.7 wt. % Co, 3.1 wt. % TaC, 2.0 wt. % NbC, 3.4 wt. %, TiC 0.2 wt. % N and rest WC with a grain size of 2.5 μm were produced. The inserts were sintered according to the method described in patent U.S. Pat. No. 5,484,468, i.e., a method that gives cobalt enrichment in zone B. The sintered carbide inserts had a 25 μm thick gradient zone essentially free from cubic carbide. The inserts were coated with the same coating as in E.
Inserts from B and C of Example 1 were tested and compared with inserts from D with respect to toughness in a longitudinal turning operation with interrupted cuts.
Results: | mean feed at breakage | |
Inserts B | 0.23 mm/rev | |
Inserts C | 0.23 mm/rev | |
Inserts D | 0.18 mm/rev | |
Inserts from B, C and D of Example 1 were tested with respect to resistance to plastic deformation in longitudinal turning of alloyed steel (AISI 4340).
The plastic deformation was measured as the edge depression at the nose of the inserts.
Results: | Edge depression, μm | |
Insert B | 43 | |
Insert C | 44 | |
Insert D | 75 | |
Examples 2 and 3 show that the inserts B and C according to the invention exhibit much better plastic deformation resistance in combination with somewhat better toughness behaviour in comparison to the inserts D according to prior art.
Inserts from E and F of Example 1 were tested with respect to flank wear resistance in longitudinal turning of ball bearing steel SKF25B.
Results: | Tool life | |
Insert E | 8 min | |
Insert F | 6 min | |
Variant F exhibited micro plastic deformation resulting in more rapid development of the flank wear.
Inserts from E and F of Example 1 in inserts style CNMG120412-MR were tested at an end-user in machining of a steel casting component.
The component had the shape of a ring. The inserts machined two components each and the total time in cut was 13.2 min.
After the test the flank wear of the inserts were measured.
Results: | Flank wear | |
Insert E | 0.32 mm | |
Insert F | 0.60 mm | |
Example 4 and 5 illustrate the detrimental effect of cobalt enrichment in the edge area B typical for inserts produced by prior art gradient sintering technique as described in e.g. U.S. Pat. No. 5,484,468.
Inserts from B and D from Example 1 were tested under the same condition as in Example 4. Inserts style CNMG120408-PM
Results: | Tool life | |
Insert B | 8 min | |
Insert D | 8 min | |
Inserts from B and D of Example 1 were tested at an end user in the machining of cardan shafts in tough hardened steel. Insert style DNMG150612-PM.
The inserts machined 50 component each. Afterwards the flank wear of the inserts was measured.
Results: | Flank wear | |
Insert B | 0.15 mm | |
Insert D | 0.30 mm | |
Examples 6 and 7 illustrate that inserts with an optimised edge zone composition according to the invention do not suffer from micro plastic deformation and hence no rapid flank wear as prior art gradient sintered insert F does (see examples 4 and 5).
In a test performed at an end-user inserts from B, C and D in Example 1 in style CNMG160616-PR were run in a longitudinal turning operation in machining of crankshaft in low alloyed steel.
The inserts were allowed to machine 90 crankshafts and the flank wear was measured and compared.
The dominating wear mechanism was plastic deformation of the type edge impression causing a flank wear.
Flank wear | ||
Insert B | 0.2 mm | |
Insert C | 0.2 mm | |
Insert D | 0.6 mm | |
The example illustrates the superior resistance to plastic deformation of the inserts B and C produced according to the invention compared to prior art inserts D.
Lenander, Anders, Lindholm, Mikael, Palmqvist, Lisa, Ljungberg, Bjorn, Thysell, Michael
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