The present invention relates to coated cemented carbide inserts with a binder phase enriched surface zo¬ ne and a process for the making of the same. More par¬ ticularly, the present invention relates to coated in¬ serts with enhanced properties in applications demanding high edge toughness. Coated cemented carbide inserts with binder phase enriched surface zone are today used to a great extent for machining of steel and stainless materials. Thanks to the binder phase enriched surface zone an extension of the application area for the cutting tool material is obtained.

Methods to make cemented carbide containing WC, cu¬ bic phase (gamma-phase) and binder phase with binder phase enriched surface zones are within the technique referred to as gradient sintering and are known through a number patents and patent applications. According to e.g. US Patents 4,277,283 and 4,610,931 nitrogen contai¬ ning additions are used and sintering takes place in vacuum whereas according to US Patent 4,548,786 the nitrogen is added in gas phase. Hereby in both cases a binder phase enriched surface zone essentially depleted of cubic phase is obtained. US Patent 4,830,930 descri¬ bes a binder phase enrichment obtained through decarbu- rization after the sintering whereby a binder phase en¬ richment is obtained which also contains cubic phase. In US Patent 4,649,084 nitrogen gas is used in con¬ nection with the sintering in order to eliminate a pro¬ cess step and to improve the adhesion of a subsequently deposited oxide coating.

Gradient sintering of cemented carbide inserts ac- cording to known technique results, for essentially

plane surfaces, in a binder phase enriched surface zone essentially free of cubic phase. In edges and corners, however, a complex superposition of this effect is ob¬ tained. The binder phase enriched surface zone is in these parts of an insert generally thinner and the con¬ tent of cubic phase in a corner area is increased rela¬ tive to that of an essentially plane surface with a cor¬ responding decrease in binder phase content, fig 3. In addition, the cubic phase in said area is more coarse grained than in the interior of the insert, fig 1.

However, the edges of a cutting insert has to have a certain radius of the order of 50 - 100 μm or less in order to be useful . The edge radius is generally made after sintering by an edge rounding operation. In this operation the thin outermost binder phase enriched zone is completely removed and the hard, brittle area is ex¬ posed. As a result a hard but brittle edge is obtained. Gradient sintering according to known technique there¬ fore compared to 'straight', not gradient sintered in- serts results in increased risk for problems with britt- leness in the edge particularly in applications deman¬ ding high edge toughness.

This is particularly the case when sintering accor¬ ding to the teachings of e.g. US 4,610,931 but also when using the technique disclosed in Swedish patent applica¬ tion 9200530-5 essentially the same situation occurs.

It has now turned out that if a vacuum sintered nitrogen containing cemented carbide insert with binder phase enriched surface zone is subjected to a nitrogen ' shock' treatment at a temperature where the binder phase is liquid the edge toughness can be increased con¬ siderably. The improvement is obtained at the same time as the resistance against plastic deformation remains essentially constant. The invention is particularly app- licable to grades with relatively high content of cubic

phase.

Figure 1 is a schematic drawing of a cross section of an edge of an insert gradient sintered according to known technique in which the solid dots represent cubic phase and

ER = solid line showing edge rounding after edge rounding treatment

B = binder phase enriched surface zone

C = area enriched in cubic phase and depleted of binder phase. The area used for elemental analysis is indicated by two parallel lines .

Figure 2 is a light optical micrograph in 1000X of a cross section of the edge of a cemented carbide insert according to the invention after edgerounding and coa- ting.

Figure 3 shows the distribution of binder phase (Co) and cubic phase (Ti) as a function of the distance from the corner along a line as indicated in fig 1 essen¬ tially bisecting the edge in a binder phase enriched ce- mented carbide insert according to known technique.

Figure 4 shows the distribution of binder phase (Co) and cubic phase (Ti) as a function of the distance from the corner along a line as indicated in fig 1 essential¬ ly bisecting the edge in a binder phase enriched cement- ed carbide according to the invention.

Figure 5 is a scanning electron microscope image of an edge of a coated insert according to prior art used in a turning operation in stainless austenitic steel.

Figure 6 is a scanning electron microscope image of an edge of a coated insert according to the invention used in a turning operation in stainless austenitic steel.

The present invention relates to a process performed after conventional gradient sintering either as a sepa- rate process step or integrated. The process includes a

nitrogen treatment in two steps. To ensure an abundant nucleation of cubic phase on the insert surface the pro¬ cess is started with a short, <5 min, nucleation treat¬ ment at increased nitrogen pressure, 300-1000 mbar at a temperature between 1280 and 1450°C, preferably 300-600 mbar between 1320 and 1400°C. This treatment is followed by a growth period of the cubic phase at a lower nitro¬ gen pressure optimal for the formation of an even sur¬ face layer of cubic carbide, 50-300 mbar 10-100 min, preferably 100-200 mbar 10-20 min. The nitrogen gas is maintained during cooling to a temperature where the binder phase solidifies at 1265-1300°C.

The process according to the present invention is effective on cemented carbide containing titanium, tan- talum, niobium, tungsten, vanadium and/or molybdenum and a binder phase based on cobalt and/or nickel. An optimal combination of toughness and resistance against plastic deformation is obtained when the amount of cubic phase expressed as the total content of metallic elements for- ming cubic carbides i.e. titanium, tantalum, niobium etc is between 6 and 18 weight-%, preferably between 7-12 weight-% at a titanium content of 0.5-12 weight-%, and when the binder phase content is between 3.5 and 12 weight-% . The carbon content is advantageously below carbon saturation since presence of free carbon can result in precipitations of carbon in the binder phase enriched zone.

With the process according to the invention cemented carbide inserts are obtained with compared to known technique improved edge toughness in combination with a high resistance against plastic deformation. The cement¬ ed carbide contains WC and cubic phases based on carbo- nitride and/or carbide, preferably containing titanium, in a binder phase based on cobalt and/or nickel with a

generally <50 μm thick binder phase enriched surface zo¬ ne essentially free of cubic phase i.e. said surface zo¬ ne contains mainly WC and binder phase. Due to the edge rounding said binder phase enriched zone free of cubic phase is removed in the edge and the cubic phase extends to the rounded surface. The outer surface of the binder phase enriched surface zone is except for an area about <30 μm on each side of the edge, because of the edge rounding, essentially covered by a <5 μm, preferably 0.5-3 μ , thin layer of cubic phase. The binder phase content along a line essentially bisecting the edge in¬ creases towards the edge and with a distance of <200 μm, preferably <100 μm, most preferably <75 μm from the outer rounded edge surface. The average binder phase content in the outermost 25 μm thick surface zone is >1, preferably 1.05-2, most preferably 1.25-1.75, of the binder phase content in the inner of the insert. Fig 2 shows the microstructure of an edge according to the in¬ vention and fig 4 shows the distribution of binder phase and cubic phase.

Cemented carbide inserts according to the invention are after the edge rounding operation suitably coated with in itself known thin wear resistant coatings, e.g. TiC, TiN and AI2O3, with CVD- or PVD-technique. Prefe- rably a layer of carbide, nitride or carbonitride, pre¬ ferably of titanium, is applied as the innermost layer. Inserts according to the invention are particularly suited in applications demanding high edge toughness such as turning and milling of stainless steel, nodular cast iron and low alloyed low carbon steel.

Example 1

From a powder mixture comprising 1.9 weight-% TiC, 1.4 weight-% TiCN, 3.3 weight-% TaC, 2.2 weight-% NbC, 6.5 weight-% cobalt and rest WC with 0.15 weight% over-

stoichio etric carbon content turning inserts C MG 120408 were pressed. The inserts were sintered according to standard practice with H2 up to 450°C for dewaxing and further in vacuum to 1350°C and after that with pro- tective gas of Ar for 1 h at 1450°C.

During the cooling a treatment according to the in¬ vention was made. After cooling to 1380°C and evacuation of the protective Ar gas, 600 mbar N2 was supplied and maintained for 1 min after which the pressure was lower- ed to 150 mbar and kept constant for 20 min. The cooling was continued under the same atmosphere down to 1200°C, where evacuation and refilling of Ar took place.

The structure in the surface of the cutting insert consisted then of a 25 μ thick binder phase enriched zone essentially free from cubic phase. In the area be¬ low the cutting edge a zone had formed where the binder phase content is increased with about 30 % relative com¬ pared to nominal content. This area extended from 20 μ from the surface to 100 μ . In the outermost part of the cutting edge there was an enrichment of coarse cubic phase particles with core-rim structure which essen¬ tially were removed during the subsequent edge rounding treatment. Herewith the binder phase enriched area was exposed.

Example 2 (reference example to example 1)

From the same powder as in example 1 inserts of the same type were pressed and sintered according to the standard part of the sintering in example 1, i.e. with a protective gas of Ar during the holding time at 1450°C. The cooling was under a protective gas of Ar without any heat treatmen .

The structure in the surface consisted as in Example 1 of a 25 μm thick binder phase enriched surface zone essentially free from cubic phase. In the edge area,

however, the binder phase enriched area was missing and instead the corresponding area was depleted of binder phase with about 30% relative to nominal content. The fraction of cubic phase was correspondingly higher. Dur- ing the subsequent edge rounding treatment the binder phase depleted and cubic phase enriched area was expos¬ ed. This is a typical structure for gradient sintered cemented carbide according to known technique.

Example 3

With the C MG 120408 inserts from examples 1 and 2 a test was performed as an interrupted turning operation in a quenched and tempered steel, SS 2244. The following cutting data were used: Speed = 100 m/min Feed = 0.15 mm/rev Cutting depth = 2.0 mm

30 edges of each insert were run until fracture. The average tool life for the inserts according to the in- vention was 7.3 min and for the inserts according to known technique 1.4 min.

Example 4

The inserts from examples 1 and 2 were tested in a continuous turning operation in a quenched and tempered steel with the hardness HB = 280. The following cutting data were used:

Speed = 250 m/min Feed = 0.25 mm/rev Cutting depth = 2.0 mm

The operation led to a plastic deformation of the cutting edge which could be observed as a wear land on the clearance face of the insert. The time to obtain a wear land width of 0.40 mm was measured for five edges each. Inserts according to the invention obtained an

average tool life of 10.0 min and according to known technique an average tool life of 11.2 min.

From the examples 3 and 4 it is evident that inserts according to the invention show a considerably better toughness behaviour than according to known technique without having significantly reduced their plastic de¬ formation resistance.

Example 5 With inserts from examples 1 and 2 a tool life test in austenitic stainless steel (SS2333) was performed. The test consists of repeated facing of a thickwalled tube (external diameter 90 mm and internal diameter 65 mm) . The following data were used. Speed = 150 m/min

Feed = 0.36 mm/rev

Cutting depth = 0-3-0 mm(varying)

The test was run until maximum flank wear = 0.80 mm or until fracture. As an average for five edges the fol- lowing results were obtained.

Prior art = 11 cuts, 5 out of 5 edges fractured.

According to the invention = 51 cuts, 0 of 5 edge fractured.

Example 6

With inserts from examples 1 and 2 a test of the initial wear was performed in austenitic stainless steel (SS2333) . The test consists of facing of a thickwalled tube (external diameter 90 mm and internal diameter 50 mm) . The following data were used. Speed = 140 m/min Feed = 0.36 mm/rev Cutting depth = 0-3-0 m (varying)

The result after one cut is evaluated by studying in a scanning electron microscope the initial wear on the

edge after etching away the adhering work piece materi¬ al. The prior art insert had small chipping damages, fig 5, whereas the inserts according to the invention had no such chippings, fig 6.