CEMENTED CARBIDE WITH BINDER PHASE ENRICHED SURFACE ZONE

Abstract: The present invention relates to a cemented carbide insert with improved toughness and resistance against plastic deformation containing WC and cubic phases of carbide and/or carbonitride in a binder phase based on Co and/or Ni with a binder phase enriched surface zone. The binder phase content in the insert is 3.5-12 weight-%. In a zone below the binder phase enriched surface zone the binder phase content is 0.85-1 of the content in the inner portion of the insert and the content of cubic phases essentially constant and equal to the content in the inner portion of the insert.

Description: Cemented carbide with binder phase enriched surface zone The present invention relates to coated cemented carbide inserts with a binder phase enriched surface zone and a process for the making of the same. More par¬ ticularly the present invention relates to coated in¬ serts in which the cemented carbide has been modified so that unique technological properties have been obtained at a given chemical composition and grain size regarding the balance between very good toughness behaviour in combination with high resistance against plastic defor¬ mation. 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 has been obtained. Methods or processes to make cemented carbide con¬ taining WC, cubic 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., U.S. Patents 4,277,283 and 4,610,931 nitrogen containing additions are used and sintering ta¬ kes place in vacuum whereas according to U.S. Patent 4,548,786 the nitrogen is added in gas phase. Hereby in both cases, a binder phase enriched surface zone essen- tially depleted of cubic phase is obtained. U.S. Patent 4,830,930 describes a binder phase enrichment obtained through decarburization after the sintering whereby a binder phase enrichment is obtained which also contains cubic phase. In U.S. Patent 4,649,084 nitrogen gas is used in connection with the sintering in order to eliminate a process step and to improve the adhesion of a subse¬ quently deposited oxide coating. From fracture mechanics point of view, an enrichment of binder metal in a surface zone means that the ability • of the cemented carbide to absorb deformation and stop growing cracks increases. In this way, a material is ob¬ tained with improved ability to withstand fracture by allowing greater deformations or by preventing cracks from growing, compared to a material with mainly the same composition but homogeneous microstruc ure. The cutting material, thus, obtains a tougher behaviour. When gradient sintering according to the known technique of vacuum sintering of nitrogen containing ce¬ mented carbide, the nitrogen is usually added by adding of a small amount of nitrogen containing raw materials. Due to the fact that the nitrogen activity in the fur¬ nace atmosphere at the sintering is below the average nitrogen activity in the cubic phase, the nitrogen con¬ taining cubic phase will give off nitrogen through the liquid binder phase to the furnace atmosphere. There is a certain disagreement about the kinetics in this disso¬ lution process. The opinion seems to be that when the nitrogen leaves, this generates conditions for a com¬ plete dissolution of the cubic phase in the surface zone of the material. The process is thought to be controlled by diffusion of nitrogen and by diffusion of the metal components of the cubic phase. The result is that the volume which previously was occupied by the cubic phase after its dissolution is occupied by liquid binder me¬ tal. Through this process a binder phase enriched sur¬ face zone is created after the solidification of the binder phase. The metal components in the dissolved cu- bic phase diffuse inwardly and are precipitated on available undissolved cubic phase present further in the material. The content of these elements therefore increases in a zone inside the binder phase enriched surface zone at the same time as a corresponding decrease in the binder phase content is obtained. A characteristic distribution of Co, Ti and W as a ' function of the distance from the surface of a cemented carbide with binder phase enrichment obtained through the above mentioned process appears, e.g., from fig 1 in U.S. 4,830,930. Outermost, there is a surface zone enriched in binder phase and completely or partly de¬ pleted of cubic phase. Inside this surface zone there is an area with an enrichment of the metallic element (s) present in the cubic phase, in particular Ti, Ta and Nb and where the binder phase content is considerably lower than the average content of binder phase in the interior of the cemented carbide body. The decrease in binder phase content for cemented carbide with about 6 weight-% cobalt and 9 weight-% cubic phase can be up to about 2 weight-%, i.e., a relative decrease of the order of 30 %. Cracks grow easily in this zone, which has a decisive influence on the fracture frequency during machining. It has now turned out that if an essentially vacuum sintered nitrogen containing cemented carbide with a binder phase enriched surface zone is subjected to a nitrogen gas treatment at a temperature where the binder phase is liquid, the toughness behaviour can be increas¬ ed further. This improvement in toughness is obtained simultaneously as the resistance against plastic defor- mation remains essentially unchanged. In this way, an insert can be used in applications which today generally require two or more grades of inserts with homogeneous structure to cover the same application area. Figure 1 shows the distribution of Co and Ti as a function of the distance from the surface of a binder phase enriched cemented carbide according to the inven¬ tion. Figure 2 shows the distribution of Co and Ti as a function of the distance from the surface of a binder phase enriched cemented carbide according to known technique. Figure 3 is a light optical micrograph in 120OX of the surface zone of a cemented carbide according to the invention in which A is surface zone enriched in binder phase and essentially free from cubic phase and B is the upper part of the zone according to the invention. The present invention relates to a process performed after gradient sintering comprising sintering in vacuum or inert atmosphere of a nitrogen containing cemented carbide either as a separate process step or integrated into the gradient sintering process. The process com¬ prises supplying nitrogen gas to the sintering furnace at a pressure of 40-400 mbar, preferably 150-350 mbar, at a temperature between 1280 and 1430°C, preferably between 1320 and 1400°C. A suitable time for the nitro¬ gen gas treatment is 5-100 min, preferably 10-50 min. The nitrogen gas is maintained until a temperature where the binder phase solidifies at about 1275-1300°C. The main part of the effect is, however, achieved even if the binder phase solidifies in vacuum or in inert at¬ mosphere. It is particularly suitable to introduce a holding time for the nitrogen gas treatment of 5-50 min at a temperature of 1350-1380°C and a pressure of 200- 350 mbar for cemented carbides with a content of cubic phase of 6-10 weight-% expressed according to below or at 1280-1320 at a pressure of 50-150 mbar at a content of cubic phase of 8-15 weight-%. The process according to the present invention is particularly intended to be applied to binder phase en- riched cemented carbide made by sintering in vacuum or inert atmosphere at very low pressure of nitrogen of ni¬ trogen containing material. It is effective on cemented carbide containing titanium, tantalum, niobium, tung¬ sten, vanadium and/or molybdenum and a binder phase ba- sed on Co and/or Ni. 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 forming cubic carbides i.e. Ti, Ta, Nb etc is between 6 and 15 weight-%, preferably between 7-10 weight-% at a titanium content of 0.4-10 weight-%, preferably 1-4 weight-% for turning and 2-10 weight-% for milling and when the binder phase content is between 3.5 and 12 weight-% for turning, preferably between 5 and 7.5 weight-% and for milling, preferably between 6 and 12 weight-%. The carbon content can be below carbon saturation up to a content corresponding to maximum C08, preferably C02-C08. With the process according to the present invention a cemented carbide with improved toughness and resis¬ tance against plastic deformation containing WC and cu¬ bic phases of carbonitride and/or carbide, preferably containing Ti in a binder phase based on Co and/or Ni with a, preferably <50 μ thick binder phase enriched surface zone is obtained. Immediately inside the binder phase enriched there is a <300 μm, preferably <200 μm thick zone with a binder phase content of 0.85-1, prefe¬ rably 0.9-1, most preferably 0.92-1 of the content in the inner portion of the cemented carbide and where the content of cubic phase is essentially constant and equal to the content in the inner portion of the cemented car¬ bide. The binder phase enriched zone is essentially free from cubic phase i.e. it contains WC and binder phase except for the very surface where the share of cubic phase is <50 volume-%. The binder phase content in the binder phase enriched zone has within a distance from the surface of 10-30 μ a maximum of >1.1, preferably 1.25-2 of the content in the inner portion of the ce¬ mented carbide. Cemented carbide according to the invention is sui¬ tably coated with in itself known thin wear resistant coating with CVD- or PVD-technique. Preferably a layer of carbide, nitride or carbonitride of, preferably tita¬ nium is applied as the innermost layer. Prior to the coating the cemented carbide is cleaned, e.g., by blas¬ ting so that possible graphite and cubic phase are es¬ sentially removed. The present invention improves the properties of the cemented carbide. When used, no zone is obtained in the material where propagation of cracks is favourable. As a consequence, a cemented carbide is obtained with consi¬ derably tougher behaviour than possible using known technique. By choosing a cemented carbide composition which has great resistance against plastic deformation, it is thus possible with the present invention to obtain the combination of very good toughness behaviour and good resistance to plastic deformation in a way that gi¬ ves a cemented carbide with unique properties . Example 1 From a powder mixture comprising 1.9 weight-% TiC, 1.4 weight-% TiCN, 3.3 weight-% TaC, 2.2 weight-% ΝbC, 6.5 weight-% Co and rest WC with 0.15 weight% over- stoichiometric carbon content turning inserts CΝMG 120408 were pressed. The inserts were sintered with ϊ_2 up to 450°C for dewaxing, further in vacuum to 1350°C and after that with protective gas of Ar for 1 h at 1450°C. This part is completely standard sintering. During the cooling, a treatment according to the in- vention was made as 30 min at 1375°C with an atmosphere of 300 mbar N2 and thereafter continued cooling in N2 down to 1200°C where a gas change to Ar was made. The structure in the surface of the cutting insert consisted then of a 25 μm thick binder phase enriched zone essentially free from cubic phase and below that a zone slightly depleted of binder phase, 0.92-1 of the content in the inner portion of the insert and without essential enrichment of cubic phase, fig 1. On the very surface of the inserts, particles of cu- bic phase were present covering about 40 % together with Co, WC and graphite. The inner portion of the inserts showed C-porosity, C04. After conventional edgerounding and cleaning, part of the cubic phase present on the surface was removed. The cutting inserts were coated by conventional CVD-technique with an 8 μm thick layer con¬ sisting of TiC and TiN. Example 2 (reference example to Example 1) From the same powder as in Example 1 inserts were pressed of the same type. These inserts were sintered according to the standard part of the sintering in Exam¬ ple 1, i.e., with a protective gas of Ar during the hol¬ ding time at 1450°C. The cooling was under a protective gas of Ar. The structure in the surface consisted of a 25 μm thick binder phase enriched zone essentially free from cubic phase. Below that zone, a 100-150 μm thick zone considerably depleted of binder phase, with a minimum of about 70 % of the nominal content in the inner portion of the insert and enriched of cubic phase was found as shown fig 2. The inner portion of the inserts showed C- porosity, C04. This is a typical structure for gradient sintered cemented carbide according to known technique. The inserts were edgerounded and coated according to known technique. Example 3 With the CNMG 120408 inserts from Examples 1 and 2 a test was performed as 'an interrupted turning operation in an ordinary low carbon steel. The following cutting data were used: Speed = 80 m/min Feed = 0.30 mm/rev Cutting depth = 2.0 _____ Thirty edges of each insert were run until fracture. The average life for the inserts according to the inven¬ tion was 4.6 min and for the inserts according to known technique 1.3 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 land width of 0.40 mm was measured for five edges each. Inserts according to the invention obtained an average tool life of 10.9 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 deformation resistance. Example 5 From a powder consisting of, in weight-%, 5.5 TiC, 1.9 TiCN, 5 TaC, 2.5 NbC, 9.5 Co and the rest WC with about 0.05% substoichiometric carbon content milling in¬ serts SPKR 1203 EDR were pressed. The inserts were sin¬ tered according to Example 1 except that the sintering temperature was 1410°C and that the treatment during the cooling was performed with the following parameters: 20 min at 1310°C at an atmosphere of 125 mbar N2. Examination of the structure showed an about 15 μm thick binder phase enriched zone, essentially free from cubic phase, fig 3. Below this surface zone there was a thicker zone insignificantly depleted of binder phase, less than 10% below nominal content. On the surface there were particles of cubic phase covering <10% together with WC and binder phase. The in¬ serts had no C-porosity. After conventional edgerounding and cleaning a con- siderable portion of the cubic phase on the surface was removed particularly in the area close to the edge. The inserts were coated by conventional CVD-technique with an about 6 μm layer of TiC and TiN. Example 6 (reference example to Example 5) From the same powder as in Example 5, blanks were pressed of the same type and inserts were sintered ac¬ cording to the standard part of the sintering in Example 5, i.e., with a protective gas of Ar during the holding time at 1410°C. The cooling was performed under a pro¬ tective gas of Ar. The structure in the surface of the insert consisted of an about 15 μm thick binder phase enriched zone essentially free from cubic phase. Below that there was a zone 100-130 μm thick considerably depleted of binder phase, with a minimum of about 30 % below the nominal content and to the corresponding de¬ gree enriched of cubic phase. The inner portion of the inserts showed no C-porosity. This is a typical struc¬ ture for gradient sintered cemented carbide according to known technique. The inserts were edgerounded and coated according Example 5. Example 7 With the milling inserts from Examples 5 and 6, a milling operation in a quenched and tempered steel SS 2541 was performed as a facemilling over a workpiece 50 mm thick. The milling was performed as one tooth milling with a milling body with a diameter of 125 mm. The il- ling body was positioned such that its centre was above the exit side of the workpiece. The following cutting data were used: Speed = 90 m/min Feed = 0.3 mm/rev Cutting depth = 2 mm The time until insert fracture was obtained was mea¬ sured for 20 edges. The average tool life was 9.3 min for the inserts according to Example 5 and 3.2 min for Example 6. It appears that a clearly improved toughness was obtained for the inserts according to the invention.

Claims: Claims 1. Cemented carbide insert with improved toughness and resistance against plastic deformation containing WC and cubic phases of carbide and/or carbonitride in a binder phase based on Co and/or Ni with a binder phase enriched surface zone c h a r a c t e r i z e d in that the amount of cubic phase expressed as the total content of metallic elements, that forms cubic carbides, is bet¬ ween 6 and 15 weight-%, and that in a zone below the binder phase enriched surface zone the binder phase con¬ tent is 0.85-1 of the content in the inner portion of the insert and the content of cubic phases essentially constant and equal to the content in the inner portion of the insert. 2. Cemented carbide insert according to the previous claim c h a r a c t e r i z e d in that said content of cubic phases in the binder phase enriched zone is essen¬ tially = 0. 3. Cemented carbide insert according to any of the previous claims c h a r a c t e r i z e d in that the surface fraction of cubic phase on the surface of the insert is <50 %. 4. Cemented carbide insert according to any of the previous claims c h a r a c t e r i z e d in that the binder phase content in the binder phase enriched zone has a maximum >1.1 of the binder phase content in the inner portion and the said maximum is at a distance of 10-30 μm from the surface. 5. Cemented carbide insert according to any of the previous claims c h a r a c t e r i z e d in that on said insert is deposited with CVD- or PVD-technique at least one wear resistant coating. 6. Cemented carbide insert according to any of the previous claims c h a r a c t e r i z e d in that inner- most is deposited a coating of carbide, nitride or car¬ bonitride, preferably of titanium. 7. Method of making a binder phase enriched cemented carbide insert by sintering in vacuum of a nitrogen con¬ taining material in in itself known way c h a r a c t e r i z e d in that after the sintering the insert is heat treated in nitrogen at 40-400 mbar at a temperature of 1280-1430°C for a time of 5-100 min.

Inventors: AAKESSON LEIF, GUSTAFSON PER, OESTLUND AAKE, OSCARSSON ULF, AAKESSON, LEIF, GUSTAFSON, PER, OESTLUND, AAKE, OSCARSSON, ULF
Applicants: SANDVIK AB
Application year: 1993
Main classification: C22C29
IPC codes: B22F7, C22C29, C23C14, C23C26