Method of making cemented carbide body for tools and wear parts.

The present invention relates to a method of making a cemented carbide body for rock and metal drilling tools and wear parts. The method is particularly useful for preparation of a cemented carbide body which for some reason, e.g. the outer shape, cannot directly be pressed to final form by uniaxial pressing.

Cemented carbide bodies are usually made by powder metallurgical methods: pressing and sintering. The desired form of the sintered body has to be obtained as far as possible before sintering because machining of the sintered body is expensive and in most cases even not profitable. Machining to desired shape is therefore done, if nessesary, in as pressed and/or presintered condition after which the body is finally sintered. Even this is an expensive operation. For said reason the body is generally given such a form that it can be directly pressed by uniaxial pressing. That means, however, great limitations. For example, can be mentioned the necessity of positive clearances in the pressing direction, a critical height to width ratio, no abrupt transitions from small to large diameter etc. It means that the final shape of a cemented carbide body is usually a compromise between what is possible to produce by uniaxial pressing and the really desired one. In certain cases bodies with complicated geometry can be made by use of a collapsible tool in which the die after the pressing is divided in order to expose the compact. Such tools are expensive, however, and sensitive to the high compacting pressures being used in the production of cemented carbide.

The above-mentioned method is suitable to use in the production of bodies in large series e.g. cutting inserts and buttons for rock drilling tools which can carry the costs of producing the necessary pressing tools. For bodies in smaller series such as wear parts one usually starts from a simpler body which then is machined to desired shape. Said machining is expensive with often great material loss because large volumes usually have to be removed. Also in this case the final form is a compromise between desired form and what is possible and reasonable, technically as well as economically.

It has now been surprisingly found that it is possible to produce cemented carbide bodies in a relatively simple way by pressing partial bodies with simple geometry possible to compact directly after which said partial bodies are sintered together to a body with desired, often complex geometry. One example of this technique is SE pat. appl. 8803769-2 which relates to a double-positive cutting insert for chipforming machining. The method can also be used for making other bodies of cemented carbide e.g. rods or blanks for drills and end mills, rock drilling tools and wear parts. The body can also be made of other hard materials e.g. ceramics or carbonitride-based materials so called cermets.

According to the invention there is now available a method of making preferably complex cemented carbide bodies other than inserts for metal cutting by dividing the body in smaller partial bodies which are separately compacted, placed upon each other with the joint lying essentially horizontally and then sintered. At this procedure the bodies are sintered together to a homogenous body and the joint is usually not visible and therefore the strength is fully comparable with the strength of a directly compressed body. It is suitable that the joint if possible is placed so that symmetrical partial bodies are obtained. Furthermore it is suitable that the surfaces which shall be connected are provided with one or more nobs and protrusions or grooves or recesses which fix the relative position of the partial bodies at the sintering and/or that the partial bodies are placed in a suitably shaped fixture. It is naturally desirable that the partial bodies are given their final shape already at the pressing but it is naturally also possible to shape the partial bodies to some extent also after the pressing.

The method according to the invention makes it possible in certain cases to produce cemented carbide bodies simpler and cheaper and with better performance. Examples of cemented carbide bodies according to the invention are shown in Figs. 1-6. It is obvious for a person skilled in the art how the method according to the invention shall be applied also to other embodiments of hard metal bodies.

The method can also be used for making cemented carbide consisting of two or more grades being different with respect to composition and/or grain size e.g. a tough core with a wear resistant cover and vice versa. In the production of such hard metal it is important that the shrinkage is similar in both bodies so that cracking does not occur. This kind of hard metal is particularly suitable to use when brazing parts because a cobalt-rich, tough cemented carbide is easier to braze than a cobalt-poor. This depends upon the difference in thermal expansion coefficient. Steel has high thermal expansion and cemented carbide has low. Cemented carbide with high cobalt content has a higher expansion than cemented carbide with low content of cobalt. Cemented carbide with low content of cobalt is difficult to braze because of increased risks for cracking of the parts due to high brazing stresses and brittle material. In this way an optimal grade for the application can be used without taking any particular consideration to the brazeability.

In a preferred embodiment so called gas pressure sintering of the body is used. It means that the body is first sintered under normal pressure. When closed porosity has been obtained the pressure is increased and final sintering is performed under increased pressure. In this way an increased strength in the body is obtained and the joint will easier sinter to full density.

Example 1

In conventional manufacture of seal rings, A, according to Fig. 1 there are problems in form of cracks at the transition from the larger outer diameter to the smaller one. The reason is the difference in the degree of compaction between the top and bottom parts. At the sintering of the ring great differences in shrinkage will consequently be obtained leading to cracking in the transition zone. Manufacturing of the ring according to the invention, B, was done in the following way: The ring was principally divided in two rings, a and b. The ring a had the dimensions φ_o = 50.4 mm, φ_i = 45.7 mm and h = 7.15 mm and the ring b φ_o = 60.0 mm, φ_i =45.7 mm and h = 4 mm. In order to fix the rings to each other during the sintering process the ring a was provided with totally four protrusions and the ring b with four corresponding grooves. At the sintering the ring a was placed upon the ring b so that the projections and the grooves were fit together and locked the relative position of the rings. The sintering was performed in vacuum at 1450 °C and 2 h sintering time. The material was a corrosion resistant cemented carbide grade having a binder phase of type Ni-Cr-Mo and a hardness of 1520 HV3. Said grade is regarded as difficult to press. In the test 1000 rings were manufactured according to conventional method i.e. with direct-pressing of the whole part. At the same time 1000 rings according to the invention were sintered. The rings were examined with respect to cracks with the following results: <dl id="dl0001"><dt>Conventionally made rings:</dt><dd>738 free of cracks 262 with cracks</dd><dt>Rings according to the inv.:</dt><dd>1000 free of cracks</dd></dl>

Besides, the metallurgical examination of the rings according to the invention showed that the structure was free of defects. Even at high magnification 1500 X no joint could be observed except in connection to the fixing elements.

Example 2

Buttons for raise boring according to Fig. 2 were manufactured, B, (500 pieces) according to the invention, A, (500 pieces) by conventional direct-pressing technique. The cemented carbide had the composition 8 % Co, 92 % WC and a hardness of 1250 HV3. The buttons according to the invention consisted of two separately pressed parts, a and b, according to the figure. At the sintering the chisel part was placed on the cylindrical part. The fixing was done by two protrusions in the chisel part and corresponding grooves in the cylindrical part. An ocular examination gave the following results: <tables id="tabl0001" num="0001"><img file="EP0453428A1_D0001.tif" /></tables>

Because the cracks were small and therefore difficult to detect at an ocular examination it was assumed that several buttons regarded as free of cracks might have had cracks. For that reason twelve buttons per variant were examined metallographically. All buttons according to the invention were free of cracks, however. The joint between the two parts sintered together could not be observed in 1500 X magnification except in connection to the protrusions/grooves. Eight of the conventionally manufactured buttons showed cracks 0.3-0.6 mm deep. Four of these had been detected at the ocular inspection.

Example 3

A cemented carbide body for mineral cutting and road planing according to Fig. 3 with 11 % Co and a grain size of 4 µm (1130 HV3) was directly pressed and sintered according to standard procedure, A. The degree of compaction will be very high at the wall of the die and press-cracks of up to 1 mm could be observed in the collar after the sintering. If the pressing is performed with lower compaction pressure the risks for cracks are decreased but the degree of compaction in the centre of the body will then be so low that an unacceptably high porosity level is obtained.

Instead a cylindrical body was made according to the invention like an ordinary rock tool button according to a in Fig. 3 and an outer ring, b. The button was placed within the ring and the whole was sintered. By choosing the compaction pressure so that the ring shrunk somewhat more than the button during the sintering a body without a visible joint was obtained, B.

Example 4

Bodies according to the preceding example were manufactured by pressing and sintering together a short button, a, and a bottom disk, b, Fig 4. The button had a protrusion in the bottom and the disk had a corresponding groove by which the bodies were fixed relatively to each other during the sintering.

Example 5

In the same way as in Example 4, Fig. 4, a number of bodies were pressed with the difference that the button, a, had the composition 8 % Co and 5 µm grain size (1230 HV3) and the bottom disk, b, 15 % Co and 3.5 µm grain size with the hardness 1050 HV3. The body a was placed upon the body b and the whole was sintered at 1410 °C for 2 h. After the sintering one body was prepared metallographically and a uniform transition between the two cemented carbide grades could be seen in an about 500 µm wide zone. The remaining bodies were brazed in milling tools for comparing tests in middle-hard sandstone with the following results: <tables id="tabl0002" num="0002"><img file="EP0453428A1_D0002.tif" /></tables>

The reason for the improved result of the body according to the invention is the combination of hard and wear-resistant tip on a tougher bottom-part which can better handle the brazing stresses.

Example 6

Chisel inserts for rock drilling tool bits are usually brazed in a milled groove in the bit-end of a drill rod. The inserts consist conventionally of grades with 8-11 % Co and 2.5-5 µm grain size. Chisel inserts were manufactured according to the invention from three together-sintered lamellæ at which the intermediate lamella has a low contentof cobalt while the two surrounding ones have a higher cobalt content.

When drilling in granite-leptite with rock drill BBC-35 and 3 m hole length six rods type H22 were drilled with conventional chisel inserts as well as with chisel inserts according to the invention. The inserts were 10x17 mm. The outer parts had 9.5 % Co and 3.5 µm WC with 1200 HV3 while the intermediate part had 6 % Co and 2.5 µm grain size with 1430 HV3. The conventional insert had 8 % Co and 3.5 µm WC with 1280 HV3. Results: <tables id="tabl0003" num="0003"><img file="EP0453428A1_D0003.tif" /></tables>

Example 7

Blanks for solid cemented carbide drills (diam. 6mm, length 700 mm) with internal coolant channels were manufactured by sintering together three pieces 1,2,3 according to Fig. 6. The individual pieces were tool pressed in an automatic mechanical press. The outer parts contained grooves to form the helicant coolant channels in the final product and means for securing the relative positions of the pieces during sintering.