Multiple grade cemented carbide inserts for metal working and method of making the same The present invention relates to multiple grade, composite cemented carbide bodies and a method of making such bodies. The said bodies comprise cemented carbide grades with individu- ally different compositions and/or microstruc- tures and, therefore, correspondingly different properties at different locations in the same body. Such bodies are herein referred to as com- pound bodies. They are especially aimed at acting as insert in a drill, soldered or by other means attached to a shaft or used as a separate insert in drilling, milling or turning.

In tools where the demands on different parts thereof are varying it is proposed to use com- pound technique. In drill bits for rock drilling the demands differ between the surface (wear re- sistance) and the inner part (toughness) as dis- cussed in US 5,541,006, in which is emphasised on the use of two grades in a rock-drilling bit. The grades are both straight grades with tungsten carbide and Co. Much attention is given the abil- ity to control. Co migration, which is in this case preferred to result in an abrupt borderline.

This problem is also solved with the technique known as Dual-Phase or DP-technique, US 4,743,515. Tools as wear parts, rolling rings and slitter/trimming knifes can be manufactured with a method described in US 5,543,235, including re- moving a partitioning means.

The use of two active grades in the same in- sert is presented in US 3,482,295. The wear re- sistant grade formed as a top layer on an insert

is just around 0,2 mm thick and seems more like an attempt to solve a problem later on solved by the PVD and CVD techniques.

Patents dealing with cemented carbide drills containing cubic carbides are US 6,086,980 and US 4,971,485. The former deals with cylindrical solid tools that are not manufactured by ordinary tool pressing. Also the latter describes a cylin- drical tool where the WC-Co grade is used in the shaft to avoid damage due to vibrations in the machine and the shaft is soldered to the cutting part of the tool.

Two or more grades in the same insert is also described in AT 269598 where a method is pre- sented with a number of press stages and using frames of rubber or other elastic materials to form the cavities needed for filling the differ- ent powders. AT 269598 thus discloses inserts consisting of two or more cemented carbide grades made by (pre) compacting a blank of one grade pro- vided with groove (s), recess (es) and/or depres- sion (s). These are filled with cemented carbide powder of the other grade and subsequently com- pacted to a green body, which is finally sin- tered.

DE 19634314 discloses a compound component consisting of at least two constituent parts with different material compositions. At least one of such parts-which are joined into a single com- ponent by a concluding sinter process-consists of a hard alloy or a cermet. The joining surface between its constituent parts is an uneven sur- face.

However, the choice of grades, final compac- tion pressure and sintering conditions have to be performed with great care in order to avoid cracks developing in the transition region bet- ween the two grades. One reason hereto is that it is generally not possible to obtain the optimum compaction pressure to both grades to obtain the same shrinkage. Generally the one grade shrinks more than the other leading to a distorted body after sintering, see Fig. 1 which shows a cross section of an RNGN insert and the same part of the insert from above. That is why AT 269598 dis- closes a heat treatment after sintering to de- crease the stresses at the boundaries. Even if no cracks develop the body needs excessive grinding in order to be useful as a cutting tool.

It is therefore an object of the present in- vention to provide a method of making cemented carbide inserts containing two different cemented carbide grades, which are less sensitive to de- veloping cracks in the transition zone between the grades.

It is a further object of the present inven- tion to provide a cemented carbide insert con- sisting of two different grades which needs less grinding after sintering.

Fig. 1 shows a compound insert according to prior art.

Figs. 2 a-e illustrates the method of the present invention.

Figs. 3 a-d shows cross sections and view from above of RNGN inserts according to the in- vention.

Fig. 4 shows a light optical micrograph at about 1000X of the uneven boundary between the two grades.

It has now surprisingly been found that the above mentioned problems can be overcome by using a specially designed press tool for making com- pound cemented carbide inserts. The method is il- lustrated i Figs. 2 a-e. In Fig. 2a powder PI is filled from a filling shoe, F, into the main cav- ity, A, of the die and powder P2 into an addi- tional cavity, B. In Fig. 2b the filling shoe has been withdrawn and the lower punch, C, lowered to a position where the powder P2 can be introduced on top of the powder PI as shown in Figs. 2c and 2d. The resulting insert after compaction and sintering is shown in Figs. 3a and b. Alterna- tively, the level is chosen somewhat higher so that the powder P2 is pushing powder PI away dur- ing introduction and thereby forming a portion deeper on the rake face. The resulting compacted and sintered insert is shown in Figs. 3c and 3d.

The ratio 11/12 in Fig. 2e shall not exceed 1/2.

The multi axial filling procedure allows the two powders to be compacted simultaneously and a compact with more optimal press density is ob- tained. The sintered body will need very little grinding.

The invention also relates to a cemented car- bide insert of a first grade in which at least one cutting point consists of a cemented carbide of a second grade with different composition and/or grain size. Preferably, the first grade is a WC-Co-grade and the second grade a WC-Co-gamma phase grade. The boundary between the first and the second grade after sintering is uneven with no cracks, see Fig. 4. The shape of the bodies of the second grade will always be different within an insert and between inserts.