WO1996014441A2

WO1996014441A2 - Thermal hardening of hard alloys and implementation in tools

Info

Publication number: WO1996014441A2
Application number: PCT/UA1994/000028
Authority: WO
Inventors: Arkadi A. Mikhailov; Victor I. Trefilov; Sergei A. Firstov
Original assignee: I.N. Frantsevich Institute For Problems Of Materials Science
Priority date: 1994-10-28
Filing date: 1994-10-28
Publication date: 1996-05-17
Also published as: AU3822395A; WO1996014441A3

Abstract

A method for thermally processing a sintered hard alloy to produce a hardened article which comprises heating a sintered hard alloy to a temperature of between about 700 and 850 °C at a first rate which avoids thermal shock; further heating the hard alloy to a maximum temperature of between about 1050 and 1350 °C at a second rate which is faster than the first rate; reducing the temperature of the hard alloy to between about 850 to 950 °C as soon as possible after the maximum temperature is achieved in order to substantially avoid the growth of carbide grains, said reducing being conducted at a third rate; and cooling the heard alloy to ambient temperature at a fourth rate that is slower than the third rate to obtain a hardened article. A further method of making an integral cast body containing a sintered hard alloy article which comprises sintering a hard alloy to form an article; brazing the article to an extension; placing said article and extension into a mold; and casting a body of a castable metal about said article and extension under conditions that provide thermal processing according to the method outlined above to form an integrally cast body about a portion of a hardened hard alloy article.

Description

THERMAL HARDENING OF HARD ALLOYS

AND IMPLEMENTATION IN TOOLS

Field of the Invention The present invention relates to hard metal alloys, and more particularly to heat-treated tungsten carbide hard alloys with or without TiC, TaC, NbC and other carbides and Co at the binder phase. The present invention provides improved durability and utilization properties in products such as cutting tools made of hard alloys by changing the structure of the hard alloys through thermal treatment prior to and during manufacture of the tools.

Background of the Invention Tungsten carbide, because of its high hardness at high temperatures, is used for cutting tools, abrasion- resistant surfaces, and forming tools, mostly in the form of cemented carbides. Tungsten carbide is produced by the reaction of tungsten powder with carbon black at 1500°C. The tungsten carbide is then milled and blended with 3-25% cobalt and pressed and sintered at about 1400°C. Addition of tantalum and titanium carbides improves hardness and wear resistance. Cemented carbides are used for cutting tools, mining and drilling tools, forming and drawing dies, bearings, and numerous other wear-resistant applications.

When a hard alloy pseudobinary system of tungsten carbide/cobalt is heated to temperatures of about 1300°C, up to about 8% by weight of the tungsten carbide will dissolve in the connecting cobalt interlayers. In contrast, only about 1.5% by weight of the tungsten carbide is dissolved in these cobalt layers on achieving equilibrium at temperatures below 500°C.

Because of this, various experts in the field of hard alloys have devoted a great deal of attention to determining the optimum conditions for thermal processing with the objective of improving their mechanical -2 - properties. However, the results obtained have not proven to be totally satisfactory. There is some feeling at the present time that the thermal processing of hard alloys is not worthwhile as a technological approach in commercial production for the purpose of influencing the properties of hard alloys in the desired direction. However, Loshak et al., "Hardening of Alloys", Kiev, Naukova Dumka. 1977, pp. 107-111, suggests a method of thermal hardening that consists of heating hard alloys to a temperature of 1100°C and higher with a subsequent cooling in oil heated to 40°C. This method of thermal treatment increases the value of the bending strength by 12 to 15%. Thus, for example, σ^^ for the hard alloy VK87 is increased from 187 kg/mm² to 208 kg/mm². In Loshak' s opinion, the reason for the hardening is the appearance of compression stress on particles of tungsten carbide due to the difference in the thermal expansion co-efficient of cobalt and tungsten carbide, as well as strengthening of the bond due to a partial dissolution of tungsten carbide in the bond. As can be seen from Auger-spectroscopy studies of the surface breakdown of hard alloys, a typical characteristic of a hard alloy structure is the presence of a thin layer

(1-3 interatomic spacings) of carbon, in the form of graphite, on the tungsten carbide-cobalt boundary. The presence of thin layers of carbon on the WC-Co phase boundary are basically equivalent to the formation of a system of microcracks. However, the presence of substantial thermal compression stresses in carbide granules, due to the difference in the coefficients of thermal expansion of carbide and the connecting phase, overshadows the negative effect of carbon interlayers under certain processing conditions, for instance hardening in oil as taught by Loshak et al . Thus the strengthening characteristics can be increased by 15 to 20%.

The reason for the formation of the carbon layer on the tungsten carbide-cobalt boundary can be understood from an analysis of the equilibrium phase data mentioned above. Specifically, up to 8% os the tungsten carbide can be dissolved in a cobalt-based solid solution at temperatures of up to 1300°C. During this process, the volume fraction of the carbide phase decreases and the fraction of cobalt phase, with the tungsten carbide dissolved in it, increases. The following processes may occur during cooling: separation of tungsten carbide out of the solid solution and its settling on the surface of the particles present, separation of fine carbide particles inside the cobalt phase, incomplete separation of tungsten carbide from the connecting phase and fixation of the oversaturated solid solution and, finally, the formation of carbon segregation on the WC-Co phase boundary.

This last process is the most important one. Of special importance is the fact that the diffusion mobility of carbon and tungsten atoms is different by several orders of magnitude. For this reason, during cooling from high temperatures the tungsten diffusion will initially be sufficient for separating out a solid solution. Subsequently, when a certain temperature is reached, the diffusion of tungsten at a fixed rate of cooling slows down to the extent that the fixation of supersaturated solid solution of tungsten in cobalt occurs,whereas the diffusion of carbon is still substantial.

The more rapid diffusion of carbon from the connecting phase to the WC-Co phase boundary in the temperature range of 500 to 900°C leads to the formation of a carbon interlayer on that phase boundary. The presence of that layer, which can only be determined by local Auger-spectroscopy, has been observed in the work of Ivaschenko et al. as reported in "The Effect of Thermal Processing on the Mechanics of Break-down in Solid Alloy T15K6", Fiziko Khemicheskava Mekhanika Materialov, 1985, No. 6, pp. 34-37. The large carbon interlayers are formed during high rates of cooling (hardening) , while no carbon interlayers are formed at extremely slow rates of cooling (less than 15°C/minute) .

The possible separation of carbon interlayers was not previously recognized. Accordingly, today's conventional hard alloy tips always contain some phase boundaries which contain these interlayers.

Summary of the Invention The present invention provides a method for thermally processing a sintered hard alloy to produce a hardened article which comprises heating a sintered hard alloy to a temperature of between about 700 and 850°C at a first rate which avoids thermal shock; further heating the hard alloy to a maximum temperature of between about 1050 and 1350°C at a second rate which is faster than the first rate; reducing the temperature of the hard alloy to between about 850 to 950°C as soon as possible after the maximum temperature is achieved in order to substantially avoid the growth of carbide grains, said reducing being conducted at a third rate; and cooling the hard alloy to ambient temperature at a fourth rate that is slower than the third rate to obtain a hardened article. The specified heat treatment procedure may be accomplished by stepped heating and stepped cooling while maintaining the specified average rates of heating and cooling in the different temperature ranges.

The present invention further provides a method of making an integral cast body containing a sintered hard alloy article which comprises sintering a hard alloy to form an article; brazing the article to an extension; placing said article and extension into a mold; and casting a body of a castable metal about said article and extension under conditions that provide thermal processing according to the method outlined above to form an integrally cast body about a portion of a hardened hard alloy article.

As a particularly useful embodiment of the present invention, technological methods of tool manufacturing that combine the brazing of hard alloy tips with optimum thermal processing are presented.

This method of thermal processing makes it possible to avoid heating stress, thermal shocks, and to obtain the greatest possible cohesion of carbide in the connecting phase.

Brief Description of the Drawings Figure 1 is a schematic graph depicting temperature change rate ranges and associated time period ranges that may be used in the practice of the present invention.

Figure 2 is a graph depicting actual temperature change rate ranges that were experienced over time in conducting a specific process in accordance with the present invention. Figure 3 shows a schematic cross-section of a mold environment suitable for use in the combined process of forming a body for a cutting tool by casting and thermal hardening of a hard alloy cutting tip.

Description of the Preferred Embodiments One aspect of the present invention relates to a method of optimum thermal processing of products made of hard alloys. This method is distinguished from conventional processes by the fact that (1) the heating of sintered hard alloy products up to 700 to 850°C is carried out fairly slowly in order to prevent thermal shock, preferably at a rate of 10 to 20°C/minute, (2) further heating to 1050 to 1,350°C can be done at a higher rate, up to 600°C a minute, holding at the maximum temperature for a minimal time interval (in order to avoid growth of the carbide grain) , preferably about 1 to 5 minutes, (3) this is followed by an abrupt drop of heating temperature of the product to 850 to 950°C at a rate of about 90 to 130°C per minute, and then (4) further cooling at a low rate, preferably at an average of 1 to 3°C a minute or lower. This cooling stage may be continuous or stepped. However, the average cooling rate here will be significantly slower than the third rate. Subsequent cooling can be accelerated below 500°C, as solubility is practically invariant below that temperature, and additional precipitation of carbon from the solid solution does not occur. Embodiments of these heating rate ranges are depicted graphically in Figure 1. In Figure 1, the vertical axis represents temperatures in °C and the horizontal axis represents time. It should be noted, however, that the horizontal axis is in arbitrary units. The curve can be divided into four stages: i) a rate of from 1 to 25 C° per minute that may last for 3.0 to 480 minutes; ii) a second rate of 6 to 8 C° per second; and iii)a third rate of about 1.5 C° per second that may last between them for 1 to 10 minutes; and iv) a fourth rate of from 1 to 9 C° per minute that may last for 420 to 960 minutes. It should be understood, of course, that Figure 1, as is the case with the remainder of the specific disclosure herein, is illustrative of rather than a limitation upon the present invention. Figure 2 depicts a temperature rate change profile that was actually experienced in molding cast iron in accordance with the teachings of this invention. In Figure 2, the vertical axis represents temperatures in °C and the horizontal axis represents time in minutes. In Figure 2, all of the rates are given in C° per second. The horizontal axis is to scale and that it can be read as representing the continually increasing passage of time. The horizontal scale is discontinuous between 200 and 400 minutes, however. In Figure 2, the first, heating rate ranged from 0.27 to 0.30 C° per second and lasted for just over an hour. The second, heating rate ranged from 6 to 8 C° per second and lasted for about one minute. The third, cooling rate was about 1.5 C° per second and lasted for about four minutes. The fourth, cooling rate slowly declined from the third rapid cooling rate. It was about 0.06 C° per second after two hours and had levelled off to about 0.03 C° per second after five hours.

Heating of the hard alloy product is preferably accomplished in an environment that excludes the possibility of oxidation of the hard alloy, for instance in the presence of ammonia dissociation products as taught in Soviet Inventor's Certificate No. 565,775.

Heating and cooling in accordance with the present invention produce an increase in strength and in the working properties in hard alloys in the WC-Co group and TiC-WC-Co group. Hard alloys of the WC-Co group usually contain from 75% to 97% tungsten carbide with the balance being cobalt. Certain hard alloys of this group can be additionally alloyed with up to 5% by weight of TiC, TaC, NbC, and other carbides. Hard alloys of the TiC-WC-Co group usually contain from 66% to 84% tungsten carbide and from 3% to 30% titanium carbide with the balance being Co.

In summary, analyzing the processes which occur in the hard alloy during thermal processing according to the present invention, the following conclusions-can be drawn: Step (1) ensures a slow heating of the whole volume of the hard alloy product and produces a partial dissolution of the carbide particles in cobalt.

Step (2) provides intensive heating to a temperature of maximum solubility of tungsten carbide in cobalt and ensures dissolution of carbon film.

Step (3) provides rapid cooling that fixates structural transformations.

Step (4) provides slow cooling that ensures complete joint separation of tungsten and carbon from solid solution and relaxes the stresses in the hard alloy.

Observing the above-mentioned conditions and temperatures for heating and cooling produces increased strengthening characteristics in the whole volume of the hard alloy product to be strengthened, which in turn makes for a significant increase in its operating characteristics. Casting The heating and cooling profile according to the present invention is practically impossible to obtain in electrical furnaces. In order to achieve it, intensive forced heating in step (2) as well as forced cooling in step (3) are necessary. Cooling in step (4) can be accomplished in a heating furnace.

Taking into account the complexity of required conditions for thermal furnaces, the heating and cooling profile required by the present invention can be achieved quite successfully using casting technology, where molten metal is used as a heat carrier. It is also possible to use molten glass as a heat carrier.

In each of the mold-related Examples below, it is preferable to use high strength cast iron as the poured metal, except when it is necessary to obtain a thermally hardened hard alloy product without bonding it to the body of the tool, in which case common grey cast iron, which can be used repeatedly, should be used as the heat c^avτiθr The use of steel is possible, but not desirable for the following reasons. Filling with molten steel is conducted at much higher temperatures (about 1600°C) , which produces an increase in the temperature that would be reached in step (2) of up to 1400°C or higher. This leads to the separation of a liquid phase in cobalt and, thus to an uneven subsequent crystallization, which on the whole causes an insignificant increase in the operating properties of the hard alloy. After casting the steel, it must be thermally processed in order to refine its grain structure and to lend higher strength properties, which can have a negative effect on the hard alloy.

Our copending U.S. patent application entitled CUTTING INSERT, filed on even date herewith (Attorney's Docket No. 7989-004) discloses the configuration of a hard alloy cutting insert which can advantageously be thermally hardened in accordance with the present invention and that is currently most preferred. The disclosure of said application is expressly incorporated herein by reference. Example 1 - Metal/Sand Molds

The conditions for thermal processing according to the present invention can be achieved using the method casting into combination metal-sand molds which are heated by natural gas, as shown in Figure 3. In Figure 3, 11 is the sand and 12 is the gas heated areas. Hot molten cast iron flows into the mold filling the hollow area 13. Hollow area 13' has the desired shape of the body of the cutting tool assembly. At the bottom of hollow area 13' is a steel shank 14 into which is placed a hard alloy cutting tool tip 15.

The temperature jump in step (2) occurs at the moment of filling the mold with molten metal. The temperature drop in step (3) is achieved through heat transfer from the poured metal to the metal-sand mold and to the hard alloy product. For step (4) the temperature of the whole system, including mold, article, and poured metal is equalized. Then the whole system cools slowly and evenly to ambient temperature.

The conditions described above make it possible not only to thermally harden hard alloy products but also to combine the forming of the cutting tool bodies during a casting with the heating and thermal hardening of hard alloy cutting tips. To this effect the hard alloy tips are coated with a layer of solder and flux prior to being mounted.

In order to obtain a sufficiently strong hard alloy, the molten metal is maintained at a particular temperature, which depends on the melting temperature of the solder being used and the ratio of the weight of the hard alloy tip to the weight of the metal being poured for the body of the product. The temperature of the liquid metal or glass depends on the melting point of the metal or glass selected as the heat transfer medium, but cannot be below 1050°C or over 1450°C. The preferable temperature range is 1350-1400°C. In combination metal-sand molds, the metal part is basically used as a thermal conductor for transferring heat from the heating gas to the hard alloy tip which is mounted on the metal part of the mold. The gas burning in the closed volume of the heating chamber absorbs oxygen, thus protecting the hard alloy from oxidation. Example 2 - Metal Molds

A similar approach is possible when using metallic molds for forming.

Where electric furnaces are used for heating molds in accordance with step (1) , in order to protect the hard alloy from oxidation, the mold must be filled with protective or protective-restorative gas, for example by using the dissociation products of an ammonia-nitro- hydrogen mixture. Example 3 - Smelting Patterns

In order to achieve the indicated chain of events, the method of casting by smelting patterns can also be used. In that case, step (1) can be extended, a condition which does not have a negative effect on the consequent structural transformations in the hard alloy. Such extension of step (1) is caused by the actual casting technology in melting patterns which requires lengthy heating of the ceramic skin before molds are filled with metal. In that case the protective environment in the mold is created by burning residual wax of the molding mass consisting of CO and C0₂.

The above-mentioned casting methods can be used for the manufacturing of metal cutting tools with increased durability. Example 4 - Sand/Metal Molds

In the case of manufacturing tools designed for operation under especially difficult conditions, such as the breaking of asphalt or granite, in order to produce a highly reliable brazing combined with thermal hardening for the purpose of increasing wear resistance, a combined technology as follows may be employed: a) sintering the hard alloy insert; b) manufacturing the extension piece; c) brazing in a vacuum the hard alloy insert to a steel or cast iron extension piece; d) thermal hardening of the hard alloy insert in the process of casting the tool body; e) mechanical finishing of the product.

The extension piece b) serves to form a reliable connection with the hard alloy insert when brazed in a vacuum and for fastening the pair in the poured metal of the body of the product . Two methods for attaching this pair in the poured metal are possible. In the first method, a mechanical bond of the extension piece with the poured metal is used, i.e., the poured metal thermally hardens the hard alloy insert and forms a mechanical bond with the extension piece. The design of the extension must not allow an axial or radial shift of the brazed pair in the poured metal under the cutting force load. In the second method, an extension piece made of high strength cast iron is used ensuring a diffusion bond in the zone of contact with the poured metal. Example 5 - Properties

As a number of cobalt phase interlayers of various thickness from one nanometer to tens of a fraction of a micron is present in the structure of hard alloys, the WC- Co phase boundary of the thinnest interlayers at medium rates of cooling of 15 to 50°/minute can be free of carbon. However, at cooling rates in excess of 50°/minute, practically 100% of the boundaries contain a carbon film. Thus, cooling after sintering to increase the strengthening and operating characteristics in a temperature range of 900 to 500°C, must be controlled so that the cooling rate does not exceed 30"/minute, and should preferably be in the range of 3 to 10°/minute.

However, even if the hard alloy is produced using optimized technology to prevent carbon film formation, this can still occur during the manufacturing of a tool, e.g. during brazing using high frequency electric heating. High rates of cooling lead to the separation of the carbon film, which will reduce the operating characteristics, in particular the durability, of the cutting tool. Special studies have shown that this method can substantially increase (by several fold) the thermal conductivity as compared with rapidly cooled samples obtained in accordance with the teachings of Loshak et al. The bending strength of hard alloy VK-8 samples increases from 160 to 180 kg/mm² after the usual (uncontrolled) cooling conditions after sintering, thence to 260 ± 30 kg/mm² after thermal processing in accordance with the method described; shock viscosity increases from 0.24 to 0.26 kg/cm² to 0.52 to 0.56 kg/cm². For hard alloy VK-15, the parameters are as follows: σ_n = 200 kg/mm² (after sintering) and σ = 330 +. 30 kg/mm², whereas the shock viscosity P^ increases from 0.3 kg/mm² to 0.9kg/mm².

Although this invention has been described with reference to certain specific embodiments thereof, further embodiments will occur to those skilled in the art based upon the disclosure herein. Applicants propose to be bound, therefore, only by the scope and spirit of the invention as defined in the appended claims.

Claims

THE CLAIMSWhat is claimed is:

1. A method for thermally processing sintered hard alloys, of the WC-Co and TiC-WC-Co systems with additional alloying of up to 10% of other elements to produce a hardened article which comprises heating a sintered hard alloy to a temperature of between about 700 and 850°C at a first rate which avoids thermal shock; further heating the hard alloy to a maximum temperature of between about 1050 and 1350°C at a second rate which is faster than the first rate; reducing the temperature of the hard alloy to between about 850 to 950°C as soon as possible after the maximum temperature is achieved in order to substantially avoid the growth of carbide grains, said reducing being conducted at a third rate; and cooling the hard alloy to ambient temperature at a fourth rate that is slower than the third rate to obtain a hardened article.

2. The method of claim 1 wherein the heated hard alloy is maintained at the maximum temperature for no more than about 30 minutes.

3. The method of claim 2 wherein the heated hard alloy is maintained at the maximum temperature for between about 1 to 5 minutes.

4. The method of claim 1 wherein the first heating rate is less than about 25°C per minute.

5. The method of claim 4 wherein the first heating rate is between about 10 and 20°C per minute.

6. The method of claim 1 wherein the second heating rate is about 20 to 60 times greater than the first heating rate.

7. The method of claim 6 wherein the second heating rate is between about 200 and 600°C per minute.

8. The method of claim 1 wherein the fourth average cooling rate is less than about 10°C per minute.

9. The method of claim 8 wherein the fourth cooling rate is between about 1 and 3°C per minute.

10. The method of claim 1 wherein the third temperature reducing rate is at least about 50°C per minute.

11. The method of claim 10 wherein the third temperature reducing rate is between about 90 and 130°C per minute.

12. The method of claim 1 which further comprises selecting the sintered hard alloy to contain one or more transition metals.

13. The method of claim 1 wherein the sintered hard alloy contains tungsten carbide and cobalt .

14. The method of claim 13 wherein the hardened article contains tungsten carbide and cobalt.

15. A hardened article produced by the method of claim 1.

16. The hardened article of claim 15 in combination with a support structure.

17. The hardened article of claim 16 wherein the support structure is an integral body.

18. A method of making an integral cast body containing a sintered hard alloy article which comprises sintering a hard alloy to form an article; brazing the article to an extension; placing said article and extension into a mold; and casting a body of a castable metal about said article and extension under conditions that provide thermal processing according to the method of claim 1 to form an integrally cast body about a portion of a hardened hard alloy article.

19. The method of claim 18 which further comprises selecting the castable metal to include iron.

20. The method of claim 19 which further comprises selecting the extension to be made of a metal.

21. The article produced by the method of claim 18.

22. A method of thermally processing a sintered hard alloy in accordance with claim 1 and claim 18 by pouring a liquid metal into a container holding the sintered hard alloy article but without bonding the hardened article to the solidified metal.

23. A method of thermally processing a sintered hard alloy in accordance with claim 22, but where the liquid metal is replaced with a molten non-metallic material.