A method of making a cutting insert includes forming a powder mixture containing wc, 2-10 wt % Co, 4-12 wt % cubic carbides, adding N in an amount of 0.9-1.7% of the weight of the cubic carbides, mixing the powder with a pressing agent, milling and spray drying the mixture, compacting and sintering the material at 1300-1500 in an atmosphere of sintering gas 40-60 mbar, applying post-sintering treatment, and applying a coating by CVD or MTCVD.
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1. A method of making a cutting insert comprising a cemented carbide body having a binder phase, with a binder phase enriched surface zone, and a binder phase depleted cutting edge, and a coating, comprising the steps of:
forming a powder mixture containing wc, 2-10 wt. % Co, 4-12 wt. % of cubic carbides of metals from groups 4, 5 or 6 of the periodic table, the binder phase having a CW-ratio of 0.75-0.90;
adding N in an amount of between 0.9 and 1.7% of the weight of the elements from groups 4 and 5;
mixing said powder with a pressing agent;
milling and spray drying the mixture to a powder material;
compacting and sintering the powder material at a temperature of 1300-1500° C., in a controlled atmosphere of sintering gas at 40-60 mbar followed by cooling;
applying post-sintering treatment; and
applying a hard, wear resistant coating by CVD- or MT-CVD-technique.
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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 patients U.S. Pat. No. 5,766,782, U.S. Pat. No. 5,654,035, U.S. Pat. No. 5,674,564, U.S. Pat. No. 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 curbic 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 CVC- or MT-CVD-technique.
A.) Cemented carbide turning inserts of the style C N M G 1 2 0 4 0 8 - P M, D N M G 1 5 0 6 1 2 - P M 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 cement 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 μthick TiN layer on top. The same coating procedures as given in A.) were used.
D.) Commercially available cutting insert in style C N M G 1 2 0 4 0 8 - P M, D N M G 1 5 0 6 1 2 - P M 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 a 4 μm TiN and 6 μm Al2O3. Inserts styles CNMG120408-QM and CNMG120412-MR.
F.) Inserts in styles C N M G 1 2 0 4 0 8 - Q M 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.
Cutting data:
Cutting speed =
140 m/min
Depth of cut =
2.0 mm
Feed =
Starting with 0.12 mm and gradually increased by 0.08
mm/min until breakage of the edge
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).
Cutting data:
Cutting speed =
160
m/min
Feed =
0.7
mm/rev.
Depth of cut =
2
mm
Time in cut =
0.50
min
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.
Cutting data:
Cutting speed:
320
m/min
Feed:
0.3
mm/rev.
Depth of cut:
2
mm
Tool life criteria: Flank wear >0.3 mm
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.
Cutting data:
Cutting speed:
170-180
m/min
Feed:
0.18
mm/rev.
Depth of cut:
3
mm
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.
Cutting data:
Cutting speed:
320
m/min
Feed:
0.3
mm/rev.
Depth of cut:
2
mm
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.
Cutting condition:
Cutting speed:
150
m/min
Feed:
0.3
mm/rev.
Depth of cut:
3
mm
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.
Cutting data:
Cutting speed:
220
m/min
Feed:
0.6
mm/rev.
Depth of cut:
3-5
mm
Total time in cut:
27
min.
The dominating wear mechanism was plastic deformation of the type edge impression causing a flank wear.
Results:
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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