NO159773B - CARBID MATERIALS AND PROCEDURES FOR THE MANUFACTURE AND USE OF THIS. - Google Patents

Abstract

A wear-resistant carbide material consists of 30-80. % by weight of a hard carbide material with a particle size not finer than 400 mesh, for example 2.5-400 mesh and 20-70% by weight of a steel-based matrix material, in which the carbide material is baked in and bonded to the matrix. Such a carbide material is prepared by mixing 20-70% by weight of a steel-forming powder with 30-80% by weight of hard carbide particles having a particle size in the range of 2.5-400 mesh, after which the mixture is introduced into a form of desired configuration and isostatically pressed by a pressure above 700 kp / cm, for example 2450 kp / cm, after which the compacted preform is sintered at a temperature below the melting point of the steel, and then isostatically pressed at a pressure above 700 kp / cm at a temperature in the region of 870 ° C and the melting point of the sub-steel. In certain cases, the hard carbide particles may be precoated with a metal coating, such as copper, before being mixed with the steel-forming powder. a steel-based or steel-forming powder mixture which is given a Danish shape, after which the composite body consisting of the powder and the carbide material is subjected to that described above ne treatment.

Description

The present invention relates to a composite material

of the kind specified in claim 1's preamble, a production thereof, as specified in claims - 5 - 7, as well as use of the composite material in wear-resistant tools, as specified in claim 8.

Since 1940, wear-resistant parts for tools and equipment have been manufactured from cemented carbide alloys consisting of a finely divided hard carbide phase based on metals selected from groups IVB, VB and VIB of the periodic table, cemented by means of cobalt and/or nickel sintering.

Such cemented carbide alloys exhibit microstructures

which is characterized by hard carbide grains with a size generally in the range 1 - 15 <y>m.

The use of iron or steel as binder material has proven to be difficult because the finely divided state and high specific surface area of the dispersed hard phase promotes the formation of relatively brittle binary boundary alloys of tungsten and iron with carbon, which reduces a proportion of the free binder volume and makes the sintered body more or less brittle, depending on the care taken with regard to composition and sintering conditions, and the addition of free carbon to achieve a satisfactory affinity between iron and carbon.

Unlike cobalt and nickel, iron forms a stable carbide Fe^C and has a greater tendency to form brittle, binary carbides than cobalt and nickel binder materials. Carbon transfer from the hard carbide phase or phase to iron is promoted by the presence of the liquid or plastic state of the iron or steel binder during liquid state sintering carried out at a temperature near or above the melting range of the binder. In recent times, useful wear parts have been produced by casting a liquid steel or iron melt into a ready-made layer of relatively coarse-grained sintered cemented carbide with a particle size in the range of 3 - 4.8 mm.

The present invention differs from the molten steel casting method in US patents 4,024,902 and 4,140,170

and the molten cast iron method in accordance with US patent 4,119,459 with two main features: (1) a powdered compact mixture of steel or iron and graphite containing dispersed powder of sintered cemented carbide or a number of pieces of dimensioned sintered cemented carbide or, mainly, unground macrocrystalline carbide crystals is sintered at a temperature below the melting temperature of steel or cast iron, and (2) instead of using matrix alloy melting temperatures to achieve densification of the alloy, high compaction unit pressures are used, both before and after sintering, thereby avoiding degradation of the surfaces of the dispersed phase of hard particles by cleavage , melting or carbon diffusion reactions. The composite material is thus characterized by what is stated in the characterizing part of claim 1.

Casting methods also lack the well-known economic advantages characteristic of powder metallurgical methods, especially when a number of different wear parts or smaller or thin sections are to be produced. Also due to the relatively high processing temperatures and liquid state, excessive amounts of unwanted binary carbides will be formed despite the use of relatively coarse carbide particles with a small surface area.

Since both the conventional powder metallurgical method of pressing and sintering finely ground steel cemented carbide powders and methods including casting of liquid steel or liquid cast iron in powdery cemented carbide arranged in molds lead to the above-mentioned problems, it is a main purpose of the present invention to develop a method with which steel-cemented hard carbide alloys can be produced essentially free of binary boundary alloys of iron and tungsten with carbon, and in which the dispersed hard carbide phase is free from boundary area cleavage phenomena, melting or thermal cracking, and is firmly bonded to a steel matrix which in the substantially free of macro-porosity.

It is also a purpose of the invention to produce an object in which a hard carbide material is dispersed which is firmly and adhesively bound in a metal matrix by means of powder metallurgical techniques including compaction and

high temperature and high pressure diffusion bonding.

It is a further purpose of the invention to produce tools with hard carbide grinding or cutting parts baked in and bonded to a consolidated steel powder matrix or object in accordance with present invention. It is a further object of the invention to produce parts which are essentially non-machinable and with sufficient impact resistance strength to make them suitable as safety plates and as parts for padlocks.

The present method includes the mixing of sintered, cemented tungsten carbide particles or primary unground macrocrystalline (i.e. larger than 0.03 mm) tungsten carbide crystals with a matrix of iron and graphite powders or steel powders with a subsequent cold isostatic pressing of the mixture in a shape to the desired configuration with a subsequent sintering in the solid state at a relatively low temperature, in particular at a temperature below the melting temperature of the steel, in particular at a temperature in the range 1030 - 1235°C, after which the sintered body is pressed isostatically at a temperature well below the melting range of the steel to obtain a final densification. A diffusion bond is formed between the hard carbide particles and the surrounding steel powder which holds the wear-resistant hard carbide particles in place. The method is characterized by what is stated in claim 5's characterizing part.

A critical factor in the present invention is the high pressure densification, both cold and hot in order to avoid production temperatures that cause the steel binder phase to flow and thus promote the previously mentioned undesirable reactions between the steel binder and the hard dispersed phase. The technique is enhanced in this respect by the use of a hard dispersed particle or particles with a low specific surface area. The method provides a significant advantage with regard to manufacturing possibilities for steel carbide slite parts of relatively small size or in the form of thin sections or with intricate patterns, compared to the methods indicated in the previously mentioned US patents i.h. t. in which molten steel or molten cast iron is poured into a mold containing particles of cemented carbide. Furthermore, both chemical control of and compositional flexibility for the matrix alloy is significantly better compared to the melt-metal casting methods. Avoidance of the high manufacturing temperatures necessary to melt and cast steel or cast iron provides better economy for the moulds, which can be reused and for the matrix metals which are not exposed to casting losses and recycling costs. The present method is suitable for the production of parts that can resist high grinding/wearing stresses as well as impact impacts. The process is ideally suited for the production of wear-resistant parts and cutting tools for equipment in agriculture, road construction and maintenance, crushing, fine distribution, excavation etc. As the wear resistance strength of the products manufactured in accordance with present method is so high that they practically cannot be machined, they are also ideally suited for use as security plates in safes. The abrasion resistance in combination with the impact strength of these compounds also makes them suitable for use in padlocks.

The present invention will be apparent from the following description with reference to the attached drawings in which: Fig. 1 is a photomicrograph at 1500 x magnification showing a cemented carbide particle with a cobalt anchor baked into and bound to a consolidated steel powder matrix. Fig. 2 shows a cross-sectional perspective of a wear plate with cemented carbide parts baked into and bonded to a consolidated steel powder matrix, and Fig. 3 is a cross-section of a cutting tool with a cemented carbide button baked into and bonded to a consolidated steel powder matrix.

Pre-alloyed steel matrix powder or a mixture of iron powder and graphite powder comprising 20-70% by weight of the finished mixture is mixed with 30-80% by weight of hard carbide particles of W, Ti, Ta, Nb or Zr, V, Hf, Mo, B,

Si Cr or mixtures thereof, either in the form of sintered cemented carbide particles or as primary, uncemented, unsintered, unground carbide crystals. About. 3% naphtha or other liquid hydrocarbon is added to the powder mixture during mixing to prevent segregation of the heavier carbide particles during mixing and filling in the mold, especially when the dispersed hard phase consists of hard carbide particles coarser than 250 pm.

In the event that the dispersed phase of hard particles is finer than 250 µm, paraffin wax or other solid lubricant such as zinc stearate can be used because the possibilities for particle segregation during mixing are then reduced.

The matrix powder containing the dispersed hard carbide phase is then packed into a preform mold made of polyurethane or other elastomeric plastic. The steel powder with other chemical compositions (such as carbon, alloy or stainless steel powders) or elemental powders such as iron, copper or nickel can also be packed in the same form with the main mixture consisting of the steel powder-carbide mixture, on which anywhere adjacent and in contact with a body containing the hard, dispersed carbide phase, or around such a body or enclosing a dimensioned, sintered cemented carbide part.

The packed form with a suitable suitable cover is sealed and introduced into a rubber bag or balloon which is then evacuated, sealed and pressed isostatically, preferably by

2 2

2;450 kp/cm , but not at a pressure below 7 00 kp/cm .

The compacted powder preform is then removed from the mold and heated under vacuum or in a protective or reducing gas atmosphere, for example hydrogen, to a temperature below the melting temperature of the steel matrix, preferably in the range 1030 - 1150°C for 20-90 min.

An alternative preforming method consists in packing the composite mixture containing preferably a liquid hydrocarbon, for example naphtha, preferably 7% by weight methylcellulose which respectively press lubricant and binder in an unsintered state, in a steel preform mould. The non-sintered preform is then air-dried at room temperature in the mold and then removed from it and placed in a rubber sack which is evacuated and sealed, and is then ready for cold isostatic compaction as previously described. The compacted blanks sintered in the solid state retain some porosity, but shrinkage during sintering does not exceed 1%. However, it has been found that densification is achieved by high-pressure isostatic compaction followed by sintering, as previously described, which is sufficient to eliminate any communicating pore network and that the sintered bodies can therefore be subjected to a final densification by known non-isostatic pressure (HIP) methods. .

Hot isostatic pressing is carried out in an inert atmosphere, preferably at 870 - 1260°C or at any temperature below the melting temperature of the steel for 20-90 min. at a pressure of at least 700 kp/cm 2 and more preferably at a pressure

2

of approx. 1050 kp/cm for 60 min. It is important that an alloy layer is formed at the interface between the cemented carbide particles and steel matrix<n.> This interface alloy

bond that is first formed during sintering and later strengthened during the hot isostatic pressing consists of a thin boundary area, for example a 0.75% carbon steel matrix and dispersed cobalt cemented carbide particles in the size range 3-4.8 mm. The thickness of the bonding layer is typically less than 40 µm and not greater than 50 µm. Under these conditions, the interface bonding alloy mainly consists of

cobalt and iron. The bond formation becomes particularly important when the hard dispersed phase consists of relatively coarse particles because these can be pulled out if they are not securely anchored in the matrix alloy.

Cemented tungsten carbide particles constituting the dispersed hard phase are generally selected within the sieve range of 8.0-0.15 mm, preferred ranges being 1.63-0.84 mm, 3.36-1.68 mm and 4.76-3.36 mm. Specially selected screening areas can be produced by known methods such as crushing and sorting of sintered, cemented carbide tool components, and which alloys usually consist of cobalt or nickel-cemented tungsten carbide (WC) as the main component, sometimes also containing TiC, TaC or NbC or combinations of these hard, carbides.

A further useful feature of the present method

is to apply a coating of an alloy or metal, preferably Corson bronze or nickel, to the surface of a dimensioned sintered cemented tungsten carbide insert of selected shape and size or a number of such inserts, which are then baked into a steel or iron graphite matrix powder in selected positions in the preform before the filled form is compacted isostatically. The Corsonbronze coating used can be one of the two nominal compositions shown in Table I.

After cold isostatic compaction and during subsequent sintering and hot isostatic pressing of the carbide steel press piece, the coating on the cemented carbide body will autogenously form a diffusion bond that increases the bond strength between the dimensioned, cemented carbide bodies and is held in the matrix. In this way, a cemented carbide body or a number of such with a special shape and size can replace the hard carbide phase of a powdery nature and thereby form a wear-resistant body or tool for cutting or drilling metal or stone. It will be understood that in relatively low processing temperatures used in the present method in cases where a steel matrix-powder mixture is used which does not bind well to the particles of the dispersed hard carbide phase will lead to insufficient bond strength at the matrix-carbide particle interface.

In such cases, when it is known, for example, that the steel alloy powder used is less sinterable at the relatively low "solid state" sintering temperature prescribed in invention, it has been found advantageous to pre-coat the hard carbide particles with, for example, nickel or copper, by known processes such as non-galvanic metal coating or vacuum vapor phase coating. Thus, nickel coating was placed on the dispersed hard carbide fraction, before mixing, it has been found to improve the bond strength of the carbide particles. Such a pre-coating of the hard carbide particles will also be advantageous when stainless steel powder is used.

A further useful feature of the described method is the incorporation of a dispersed hard carbide phase in the compacted steel or iron-graphite powder blank, which phase consists of unground macrocrystalline carbide crystals in the sieve fraction between 0.25 mm and 0.03 mm and in the preferred sight ranges, namely 0.25 - 0.15 mm, 0.18 - 0.07 mm or 0.09. -

0.04 mm, instead of and to replace the particles of cemented carbide. The procedure in accordance with the invention for the composition and shaping of macrostructured cemented carbide mixtures is the one previously described.

The relatively low processing temperature used leads to a macrostructure that is essentially free of brittle double carbides of iron and tungsten (eta phase) and free of porosity.

The tendency for liquid-phase sintered, microstructured, cemented tungsten carbide alloys in which, for example, a steel binder is used instead of the usual cobalt binder, is well known to develop brittle eta-type phases. It is believed that when liquid phase sintering is avoided and consequently also the carbon transfer promoted by liquid phase sintering, as well as the particularly low specific surface area of unground, macrocrystalline carbide particles that make up the dispersed hard phase are necessary for the formation of the two-phase steel carbide macrostructure obtained by the present approach. It should be understood that the liquid phase sintering referred to here means sintering at a temperature where the steel binder is at least partially liquid. The ban on liquid phase sintering in accordance with the present invention therefore does not apply to any low-melting metals or alloys (for example copper or Corson bronze) which may be added as powder or coated to promote bonding or densification and which intentionally become liquid during the shaking or during the hot isostatic pressing.

The use of unground, macrocrystalline hard carbide crystals as a dispersed hard phase constitutes a preferred embodiment of the present method as an effective way of maintaining a hard phase exhibiting a low specific surface area. However, it will be understood that essentially binder-free, hard aggregates of finer or ground hard carbide crystals can also be used.

An important feature of the aforementioned macrostructured bodies is the avoidance of ball milling or size reduction of the matrix carbide powder mixtures or each of these two materials separately before the cold isostatic compaction, sintering and HIP. The known practice, which is generally accepted as necessary for commercial, cemented carbide structures, leads to an enhanced reaction between carbides and jerri-based matrix powders with the consequent formation of brittle double carbides• By avoiding the powder being ground, costs are also reduced.

In the present method, any of the microcrystalline carbides can be used, either in combination or as solid solutions of these, especially WC, TiC,

TaC or NbC, all of which exhibit low specific surface area and angular block shapes that are typical of these coarse-crystalline mono- and binary carbides. It is known that primary macrocrystalline carbide materials can be finely ground together with cobalt or nickel to give cemented carbide microstructures by liquid phase sintering in the temperature range 1315 - 1538°C

in which the resulting dispersed hard carbide phase is typically within the size range of 1-10 µm. In contrast to this, it is obtained in accordance with present method dispersed, simple macrocrystalline carbide grains in a size range that lies within the coarse extremes of 2 50 µm -

40 µm.

The invention shall be further explained by the following examples:

EXAMPLES

Abrasion-resistant cutting edges were produced for rotary sugar cane crushers. A smooth mixture consisting of approx. 55% by weight 3.2-4.8 mm cobalt cemented tungsten carbide granules, approx. 44.67 wt.% 0.15 mm sieve atomized iron powder and 0.33 wt.% <^0.04 mm sieve graphite powder were prepared. During mixing, 5% by weight of naphtha was added to minimize segregation of the heavier, cemented carbide particles. The moist mixture was manually compacted into the cavity of an elastomeric polyurethane mold of the desired shape and dimensioned to account for the cold isostatic powder compaction plus 1% sintering shrinkage. After cold isostatic compaction at 2,450 kp/cm 2 , the preform was removed from the mold and vacuum sintered at . 1093°C for 60 min., after which the sintered body was isostatically pressed at 1232°C for 60 min. by a

2

pressure of 1,050 kp/cm under helium.

Metallographic examination showed a matrix structure consisting essentially of pearlite and some ferrite which is typical of conventional slow cooled 0.75% carbon steel with low porosity. The cemented carbide matrix interfaces were filled with bands with a width of approx. 5 um of an alloy believed to consist mainly of iron and cobalt. The cemented carbide particles appear to be unaffected by thermal degradation, and there was no indication of dissolution, melting or splitting of the dispersed carbide phase at or near the internal interfaces. These boundaries were sharp except for the aforementioned iron-cobalt alloy diffusion zone. No potentially harmful concentrations of etaphase were observed. Specimens were manually bent over a mandrel by hammering at room temperature and found to exhibit high impact strength and were substantially free of brittleness cracks.

Fig. 1 is a photomicrograph of a typical area of compositions prepared in accordance with example 1, except that the sintering was carried out at 1150°C. A cobalt-cemented tungsten granule 40 is shown metallurgically bonded to pure carbon steel which essentially exhibits pearlite structure 50 at a diffusion zone 45 containing iron and cobalt. The diffusion zone 45 is approx. 3 µm thick.

Example 2

A 50 mm x 50 mm x 9.5 mm thick wear-resistant plate was prepared consisting of 60 wt% unground 0.25 - 0.15 mm sieve macrocrystalline WC cemented at 40 wt% 0.75% carbon steel containing 2 wt% Cu. A uniform dry-blended mixture of 0.25 - 0.15 mm sieve macrocrystalline WC crystals, ^0.04 mm sieve graphite powder, <C 0.15 mm sieve iron powder and <0.04 mm sieve copper powder was dry-blended, unground, into a uniform mixture and then moistened with a mixture of liquid naphtha and methyl cellulose to resp. 7% by weight and 0.5% by weight of the dry mixture and then packed in a steel preform into a solid, (green) plate form with the dimensions approx. 102%

of the desired final dimension.

After air drying in the mold at room temperature, the blank was removed from the mold, placed in a rubber bag and further processed by cold isostatic compaction, sintered and HIP as described in example 1. Metallographic examination showed a macrostructure of macrocrystalline WC evenly dispersed in the steel matrix. A 5 µm thick bonding layer of unknown composition was observed at the WC steel boundary rafts. These interfaces were free of binary carbide phases and cracks.

Example 3

A composite wear-resistant steel body in the form of

a 38 mm cube enclosing a dimensioned plate of sintered cemented 5 wt% cobalt-tungsten carbide was prepared and intentionally the dimensioned plate of sintered cemented carbide was in the "green" powder before isocompaction so that its outer surface was in flush with the outer surface of the steel cube. A dry unground mixture consisting of 97.25 wt.% <[o, 15 mm sieve atomized iron powder, 2 wt.% <^0.04 mm sieve Cu powder: and 0.75 wt.% graphite was prepared and then mixed with naphtha and methyl cellulose to a length of respectively 5% by weight and 0.3% by weight of the dry mixture. This was then packed into an elastomeric mold after which a 25.4 x 25.4 x 6.35 mm thick plate of sintered cemented carbide was pressed into the mold of the iron powder mixture so that the outer surfaces were congruent.

After sealing the mold, it was placed in a rubber bag, evacuated, sealed and then isostatically compacted, removed from the mold, sintered and hot-compacted isostatically, as indicated in Example 1. Metallographic examination showed that the placed sintered carbide plate was bonded by means of a 5 pm boundary bonding phase to the surrounding steel matrix on its three sides, so that the entire structure appeared solid and free of cracks.

Fig. 2 shows a wear plate 30 produced as described in this example, except that instead of one cemented carbide insert 30, three were baked into the plate 20, so that a surface 45 of each of the inserts 30 is essentially flush with the work plate 40 of the tool 20. It will be noted that the interface bond 35 is substantially smooth and continuous and forms a tough and adhesive bond between the cemented carbide and the consolidated carbon steel and copper matrix' 45.

For certain utility applications, depending on a corrosive nature

of the environment in which the wear plate is to be used, stainless steel or steel alloy powders can be used instead of the iron, carbon and copper powders that were used in this example.

Fig. 3 shows a cross-section of another embodiment of a tool in accordance with the invention. This tool 1 can be produced in essentially the same way as described in example 3 except that the cemented carbide insert 5 has its working end 2 protruding from and past the steel body 10 of the tool 1. As can be seen from the figure, the insert 5 is anchored to the steel body 10 by a diffusion zone 15 which is formed by interdiffusion of cobalt from the insert 5 and iron from the steel body 10 during the high temperature and high pressure sintering operations.

Claims (8)

1. Composite material comprising 30 - 80% by weight of a carbide material, such as tungsten carbide, titanium carbide, tantalum carbide, niobium carbide, zirconium carbide, vanadium carbide, hafnium carbide, chromium carbide, boron carbide, silicon carbide or a mixture thereof, as well as their solid solutions and cemented mixtures, and 20 - 70 weight of a matrix material comprising steel, steel and iron, steel and copper, as well as steel and nickel, the carbide material being baked in and bonded to the matrix, characterized in that the grain size of the carbide material is 0.04 - 0.08 mm, and that the interface between the carbide material and the matrix is not thicker than 50 µm, preferably 0-40 µm and is essentially free of eta carbides, and that the carbide material is optionally provided with a metal coating, which forms a strong and adhesive bond between the carbide material and the matrix, in that the carbide material and the matrix material are evenly mixed without significant grain size reduction and where the interface is formed by sintering the homo -gene mixture at a temperature below that at which the steel is at least partially liquid. 2. Composite material according to claim 1, in which the carbide material is a cemented mixture with cobalt as binder and characterized in that the interface comprises iron and cobalt, and has a thickness of 0 - 40 µm. 3. Composite material according to claim 1 or 2, characterized in that the carbide material includes tungsten carbide. 4. Composite material according to claim 1, characterized in that the steel is alloy steel or stainless steel. 5. Process for the production of a composite material according to claims 1-4, by mixing steel-formed powders and hard carbide particles to give a homogeneous mixture which is cold-pressed to give a compacted preform which is then sintered at high pressure and elevated temperature, characterized by that carbide particles with a particle size in the range of 0.04 - 0.08 mm are used and that the sintering treatment takes place at a temperature below that at which the steel is at least partially liquid. 6. Method according to claim 5, characterized in that the hard carbide particles are pre-coated with a metal coating before the mixture with the steel-formed powders. 7. Method according to claim 5 or 6, characterized in that densification of the compacted preform via diffusion bonding takes place by sintering the compacted preform at a temperature below 1375°C and below the melting temperature of the steel, in order to minimize connecting porosity in the preform, and then press the preform isostatically at a pressure above 700 kg/crn^ at a temperature in the range between 870°C and the melting temperature of the steel. 8. Use of the composite material according to claims 1-4 in wear-resistant tools.