CEMENTED CARBIDE BODY WITH INCREASED WEAR RESISTANCE

Abstract: A cemented carbide button, based on WC and cobalt, for rock drilling comprises a core and a surface zone. The core contains eta-phase and the surface zone has an inner part with a higher content of cobalt than the nominal and an outer part with a lower content of cobalt than the nominal. The cemented carbide is characterized by a narrow grain size distribution of the hard constituent, i.e. WC, in the surface zone part with a high cobalt content and in the core. The button has improved resistance against plastic deformation. The method of preparing the cemented carbide includes pressing and sintering a powder which has not been milled in the conventional way; instead the binder has been uniformly distributed by coating the hard constituent particles with the binder phase.

Description: Cemented carbide bodv with increased wear resistance The present invention relates to cemented carbide bodies useful in tools for rock drilling, mineral cut- ting, oil drilling and in tools for concrete and asphalt milling. In US 4,743,515 cemented carbide buttons are dis¬ closed having a core with finely and evenly distributed η-phase embedded in the normal α + β - phase structure, and a surrounding surface zone with only α + β - phase, (α = tungsten carbide, β = binder-phase, e.g., cobalt and η = MgC, M12C and other carbides, e.g., C03 3C) . An additional condition is that in the inner part of the surface zone situated close to the core, the cobalt con- tent is higher than the nominal content of cobalt and that the cobalt content in the outermost part of the surface zone is lower than the nominal and increases in the direction towards the core up to a maximum, usually at the η-phase core. US 5,286,549 discloses an improvement of the above- mentioned US patent according to which the cobalt con¬ tent is essentially constant in the outer surface zone resulting in further increased wear properties. According to US 5,413,869 it has been found that further improvement are obtained in certain rock drill¬ ing applications if the core containing η-phase is ex¬ posed on the top surface. Cemented carbide bodies according to the mentioned patents are manufactured according to powder metallurgi- cal methods: milling, pressing and sintering. The mil¬ ling operation is an intensive mechanical milling in mills of different sizes and with the aid of milling bodies. The milling time is on the order of several hours up to days. Such processing is believed to be ne- cessary in order to obtain a uniform distribution of the binder phase in the milled mixture, but it results in a wide WC grain size distribution. In US 5,505,902 and US 5,529,804 methods of making cemented carbide are disclosed according to which the milling is essentially excluded. Instead in order to ob¬ tain a uniform distribution of the binder phase in the powder mixture the hard constituent grains are precoated with the binder phase, the mixture is further mixed with pressing agent, pressed and sintered. In the first men- tioned patent the coating is made by a SOL-GEL method and in the second a polyol is used. An important restriction of the above mentioned prior art patents is the toughness properties of the co¬ balt-rich zone. During the heat treatment process after the sintering the η-phase is transformed to WC-Co re¬ sulting in a structure with both fine and coarse WC grains. Fine WC-grain size in a cobalt rich matrix gives low resistance against plastic deformation in all appli¬ cations where high forces and high temperatures are pre- sent such as in rock and coal cutting and hot forming. In these types of applications, there is substantial risk for damage of the whole tool caused by plastic de¬ formation. Another disadvantage of the prior art structure is the presence of both fine and coarse WC grains in the cobalt rich zone and the η-phase core, leading to low resistance against crack propagation. It has now surprisingly turned out that it is pos¬ sible to control the manufacturing process in such a way that both fine and abnormally coarse grains can be avoided in both the cobalt rich zone and the η-phase containing core. Fig 1 shows in 1200x magnification the microstruc¬ ture of the cobalt rich zone according to prior art. Fig 2 shows in 1200x magnification the microstruc¬ ture of the η-phase core according to prior art. Fig 3 shows in 1200x magnification the microstruc¬ ture of the cobalt rich zone according to the invention. Fig 4 shows in 1200x magnification the microstruc¬ ture of the η-phase core according to the invention. According to the present invention a powder is used which has not been milled mechanically in the conven¬ tional way. Surprisingly, it has been found that the formation of fine and abnormally coarse grains when the η-phase is dissolved can be avoided in this way. Rock bit buttons according to the invention have a core containing at least 2 % by volume, preferably at least 5 % by volume, of η-phase but at the most 60 % by volume, preferably at the most 35 % by volume. The η- phase shall be fine-grained with a grain size of 0.5 - 10 μm, preferably 1 - 5 μm, and be evenly distributed in the matrix of the normal WC-Co-structure. The width of the η-phase core shall be 10 - 95 %, preferably 25 - 75 % of the cross section of the cemented carbide body. The binder phase content in the zone free of η-phase increases in the direction towards the η-phase core up to a maximum usually at the η-phase core of at least 1.2 times, preferably at least 1.4 times, compared to the binder phase content in the centre of the η-phase core. The WC grain size distribution is characterized in being relatively narrow. That is, at least about 90 % of the WC grains are within 0.4-2.5 times the mean WC grain size. Preferably, the number of WC grains smaller than 0.4X of the mean-grain size is less than 5% in number and the number of WC grains larger than 2.5X the mean grain size is less than 5% of the total number of grains. The cobalt-portion in the η-phase can completely or partly be replaced by at least one of the metals iron or nickel i.e. the η-phase itself can contain one or more of the iron group metals in combination. Up to 15 % by weight of tungsten in the α-phase can be replaced by one or more of the metallic carbide for- mers Ti, Zr, Hf, V, Nb, Ta, Cr and Mo. According to the method of the present invention a cemented carbide body is manufactured by powder metal¬ lurgical methods such as mixing, pressing and sintering whereby a powder with substoichiometric content of car¬ bon is sintered to an η-phase containing body which af¬ ter the sintering is given a partially carburizing heat treatment whereby an η-phase containing core surrounded by an η-phase free surface zone is obtained. By starting from a powder in which the WC-grains are previously coated with binder phase, preferably using the above mentioned SOL-GEL-technique, the conventional milling can be replaced by mixing with pressing agent and possibly additional WC- or Co-powder in order to obtain the desired composition. Example 1 In a coal mine in South Africa a test with point at¬ tack cutting tools was run as follows : Seam: Grit coal, top part of seam containing coarse grained sandstone lenses. Sandstone floor Machine: Voest Alpine AM. Cutting speed: 2 m/s Penetration rate: 80 mm/revolution Cemented carbide grade: Variant A: Buttons made from conventionally milled WC-Co-powder according to US 4,743,515. The WC grain size distribution in the Co-rich zone was 15 % less than 0.4 times the mean grain size, 15 % greater than 2.5 times the mean grain size and a WC mean grain size of 3.5 μm. Variant B: Buttons made in the same way but from WC- Co-powder which was produced from powder which was made by coating the WC-grains with the cobalt by the SOL-GEL method, disclosed in above mentioned US 5,505,902. The WC grain size distribution in the Co-rich zone was 5 % less than 0.4 times the mean grain size, 5 % greater than 2.5 times the mean grain size and a WC mean grain size of 3.5 μm. The cobalt content was 10 weight % . All buttons were sintered and heat treated in order to get the outer zone with low cobalt content, the co¬ balt rich zone and the η-phase containing zone. Results Variant A: worn out after three shifts and 3,5 tons/tool Variant B: worn out after nine shifts and 11,3 tons/tool The main reason for the poor performance of variant A was plastic deformation of the cobalt-rich zone due to high temperature in the cutting edge because of high cutting forces when cutting in sandstone of the bottom of face. Example 2 Rock: Quartzite, heavily abrasive Machine: Tamrock Super Drilling, Datamaxi Drilling data: Impact pressure: 200 Bar Feeding pressure: 140 Bar Rotation: 130 rpm Water pressure: 15 bar Drill bits: 45 mm button bits with five peripheral buttons φ =11 mm ballistic top Hole depth: 5 m Variant 1 : Cemented carbide according to the inven¬ tion with 6 weight-% Co. The WC grain size distribution in the Co-rich zone was 4 % less than 0.4 times the mean grain size, 5 % greater than 2.5 times the mean grain size and a WC mean grain size of 2.5 μm. Variant 2 : Same as variant 1 but made according to US 4,743,515. The WC grain size distribution in the Co- rich zone was 20% less than 0.4 times the mean grain size, 10% greater than 2.5 times the mean grain size and a WC mean grain size of 2.5 μm. Variant 3: Same as variant 1 but with no η-phase core and even cobalt-distribution. In this rock there is obtained in addition to heavy- wear also crack formation in the wear surface. The final damage of the bits is often button damages. Result Drilled length, m Variant 1 415 Variant 2 330 Variant 3 290 Variant 3 obtained early damages due to crack forma¬ tion in the wear surface. Variant 2 also obtained cracks but they were stopped partly in the cobalt-rich zone. Variant 1 obtained less cracks in the wear surface because of the narrow grain size distribution in which the finest WC grain size fraction is lacking. The cracks stopped in the cobalt-rich zone. Example 3 Production drilling in iron ore, magnetite. Rock: Magnetite, forming snake skin. Machine: Tamrock SOLO 1000 with HL1500 hammer Buttonbits: φ=115 mm Hole depth: 15 - 30 m upwards, one ring about 350 - 400 m. Drilling data: Impact pressure: 170 bar Feeding pressure: 120 bar Water pressure: 6 bar Rotation: About 70 rpm Variant 1: WC and 6 weight-% Co according to the present invention. The WC grain size distribution in the Co-rich zone was 2 % less than 0.4 times the mean grain size, about 5 % greater than 2.5 times the mean grain size and a WC mean grain size of 5 μm. Variant 2 : Same as variant 1 but made according to US 4,743,515. The WC grain size distribution in the Co- rich zone was 20 % less than 0.4 times the mean grain size, about 10 % greater than 2.5 times the mean grain size and a WC mean grain size of 5 μm. Variant 3 : Same as variant 1 but with no η-phase core and even cobalt-distribution. Drilling without grinding of the buttons. Result Variant 1: One ring, 350 m, could be drilled. No button damages. Snake skin on the wear surfaces which, however, did not cause button damages. The bits could be reground and used to drill another ring of holes. Variant 2: Snake skin formation causing button dam¬ ages. The bit could not be used after 200 m. Variant 3: As variant 2 with life 195 m. Example 4 Test in a copper mine. Rock: Biotite gneiss, mica schist Machine: Bucyrus Erie with feedforce 400 kN. Bits: Roller bits φ=311 mm CS1 with test buttons in row 1 in all cones. Variant 1: Bit with buttons according to the present invention. Cemented carbide with 6 weight-% nominal co¬ balt content. The WC grain size distribution in the Co- rich zone was about 3 % less than 0.4 times the mean grain size, about 5 % greater than 2.5 times the mean grain size and a WC mean grain size of 5 μm. Variant 2: Bit with buttons with composition and grain size as Variant 1 but made according to prior art US 4,743,515. The WC grain size distribution in the Co- rich zone was about 20 % less than 0.4 times the mean grain size, about 10 % greater than 2.5 times the mean grain size and a WC mean grain size of 5 μm. Variant 3 : Bit with buttons with no η-phase core and even cobalt-distribution and 9.5 weight-% Co and 3.5 μm WC grain size. Result Variant drilled length, m 1 2314 2 1410 3 1708 Variant 1 had worn out buttons and bearing failure as final damage. Variant 2 had button damages on row 1 as final damage. Variant 3 had worn out buttons and low drilling rate as final life length determining factor.

Claims: Claims 1. Cemented carbide body preferably for use in rock drilling and mineral cutting, comprising a cemented car¬ bide core and a surface zone surrounding the core where- by both the surface zone and the core contains WC, in which up to 15 % by weight of W can be replaced by one or more of Ti, Zr, Hf, V, Nb, Ta, Cr and Mo, and 3-25 % by weight of binder phase based on cobalt, iron and/or nickel, the surface zone having an outer part with a binder phase content which is lower than the nominal and an inner part having a binder phase content which is higher than the nominal, at which the average binder phase content in said outer part is 0.2-0.8 of the nominal and the binder phase content in said inner part reaches a highest value of at least 1.2 of the nominal binder phase content, and the core in addition contains 2-60 % by volume of η-phase with a grain size of 0.5-10 μm, while the surface zone is free of η-phase, the width of the core being 10-95 % of the cross section of the body, c h a r a c t e r i z e d in that at least 90 % of the WC grains in the cobalt rich zone and the η-phase core is between 0.4 and 2.5 times the mean WC grain size. 2. Cemented carbide button according to claim 1 c h a r a c t e r i z e d in that max 5% of the total number of WC-grains is smaller than 0.4X the mean grain size and that max 5% of the total number of WC-grains is coarser than 2.5X the mean grain size. 3. Method of manufacturing a cemented carbide button for rock drilling by powder metallurgical methods such as milling, pressing and sintering whereby a powder with substoichiometric content of carbon is sintered to an η- phase containing body which after the sintering is given a partially carburizing heat treatment whereby an η- phase containing core surrounded by an η-phase free sur- face zone is obtained c h a r a c t e r i s e d in using a powder mixture in which the WC-grains are coated with binder phase whereby the conventional milling is replaced by mixing with pressing agent and possibly additional WC- or Co-powder in order to obtain the desired composition.

Inventors: FISCHER UDO, HARTZELL TORBJOERN, WALDENSTROEM MATS, FISCHER, UDO, HARTZELL, TORBJOERN, WALDENSTROEM, MATS
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 1996
Main classification: C22C29
IPC codes: B22F1, C22C29, C23C30, E21B10