Disc chippers are generally used for producing wood chips to be used for pulp production.

The knife disc of a disc chipper is equipped with knives attached thereto in radial direction, said knives cutting chips from a log against a fixed counter knife. In the sectors between the knives there are wear plates, against which the logs are fed. Continuous abrasion is ex- erted on the wear plates by the logs, and therefore the front edge of the wear plate has been plated with wear-resisting material. Other portions of the wear plates are made of high- tensile material with relatively low strength that can be worked easily. The knives are at- tached against the tapered wear plates by means of knife clamps. Percussion forces are ex- erted on the knives and via knives to the tapered tips of the wear plates by the logs during the chipping, whereby the forces tend to bend the tip portion of the wear plate. Because the surfaces of the traditional wear plates facing the knife and being used about until 1990, are of general construction steel by their hardness (hardness about HB 150), they do not al- ways form an adequately strong support against the knife, and they gradually yield and bend. As a result of the yielding, a small gap arises between the knife and the wear plate, where fibres start to accumulate. The support of the knife is weakened by the transforma- tion of the supporting surface. When the supporting surface of the knife is cambered, the moving knife clamp turns and causes change of the knife position, whereby the clearance between the knives and the counter knife varies between different knives positioned in the knife disc at the same time. Disc chip and its wear plates are known from application WO- Al-96/26817.

A known solution of the above problem is the"cassette system"developed after 1980, comprising elements to be supported against a hardened wear plate for fixing the knives.

The most general cassette system known in the art is described i. e. in publication DE-A1- 33 14 127 (Iggesund Tools). The cassette system comprises a plurality of machined ele- ments, which naturally causes additional investment costs of the chipper. The cassette sys- tem also has typically more stages in changing of the knifes.

In the production, the blanks of the wear plates are pre-worked, after which the front sur- face is plated with wear-resisting material. Generally used for the plating is powder that is sprayed and melted (sintered) on the surface of the plate by means of heat treatment. Tem- perature of the melting furnace is over 1000°C. In connection with the heat treatment, the wear plate blank usually encounters deformations to some extent that cannot be foreseen, whereby the most important dimensions of the wear plate in respect of the operation of the wear plate are machined only after the heat treatment.

In order to prevent permanent deformation of the surface of the wear plate facing the knife, the wear plate should be hardened. A problem with the heat treatment in connection with the hardening of the massive wear plate with a diameter of about 100 mm is changing of the dimensions of the wear plate, and that the fabricating of the wear plate is more difficult after the hardening. In addition, a throughout hardened wear plate is too brittle, and in ca- ses where the chipper is damaged, it can fall to pieces and destroy the whole chipper. In a method of the present invention, the tip of the wear plate stressed by the knife is selectively hardened by heat treatment in connection with sintering, so that its strength and yield limit duplicate or triplicate. Due to the selective hardening, the toughness of the wear plate remains mainly good. One prominent advantage of the invention is that the focused selec- tive hardening is inexpensive, because it is performed in connection with the heat treatment of the sintering.

The present invention and its details will be described in more detail in the following, with reference to the enclosed drawings, wherein Figure 1 shows a knife disc of a disc chipper viewed from the feeding direction of the logs, Figure 2 shows section A-A of figure 1 during chipping, Figure 3 shows attachment of the knife against the wear plate, Figure 4 shows attachment of the knife of prior art with a"cassette system", Figure 5 shows temperature of the wear plate after it has been removed from the sin- tering furnace, Figure 6 shows temperature of the tip portion of the wear plate during wet quenching,

Figure 7 shows selective hardening of the wear plate, and Figure 8 shows rework points of a part of the wear plate, Figure 1 shows a front view of a knife disc 2 of a disc chipper said disc rotating around a horizontal axis 1. Knives 3 attached to the disc extend from the centre of the disc towards the outer edge of the disc. The point lines of the knives 3 are direct and on a same plane with each other, perpendicular to the axis 1 of the disc. Knives 3 are attached to the in- clined wear plate sectors 4 between them (figure 2). During the chipping, the wear plates 4 of the rotating disc 2 rub against the log 6 being supported by the fixed counter knife 5.

Thus, the front surface 7 of the wear plate is exposed to strong abrasion, whereby it is plated with a wear-resisting material 8.

The wear plate 4 is attached to the knife disc with bolts 9, as shown in figure 3. The knife is pressed against the surface 10 of the wear plate facing the wear plate by means of a knife clamp 11 and a bolt 12. In case the knife is pressed too strongly against the surface 10 of the wear plate, the surface 10 can yield. When chipping wood, force F is exerted on the knife 39 as shown in figure 3, that can be divided into components FN and Fp. Fp is exerted on the end support 13 of the knife, said support being formed of a bracket of the wear plate 4. Knife 3 is abutting with the end support 13 via adjustment bolts 14 of its width, whereby force Fp is exerted on a relatively small area. Thus, the end support is exposed to wearing and yielding. The force component FN is exerted on the surface 10 of the wear plate and especially on the tip portion 15 thereof. Also therefore the surface 10 for the part of the tip portion 15 yields in a wear plate of prior art.

As a result of the yielding of the tip portion 15 of the wear plate, a small gap arises be- tween the surface 10 and the knife 3, where cuttings and wood fibres start to accumulate.

When changing the knives, the attachment of the sharp knife it no more as firm. This causes also an allowance between the knife 3 and the clamp in point 23 when the knife bends according to the bending of the tip portion 15 of the wear plate, influenced by the chipping force component FN. Weakening of the firmness of the attachment is especially a problem with narrower knives, whereby the knife comprises the knife 3 and an additional piece 24, the connection 25 being flexible (figure 3).

The knife can move or vibrate, which speeds up the deformation of the wear plate. When the support portion 13 of the wear plate wears and the positions of the surfaces 10 of the

wear plates change, also the positions of the different knifes differ from each other. Then also the knife allowance V differs from one knife to another. As a result, the adjustment of the knife allowance is different and the chip quality declines. Big knife allowance brings about fine grains and long strings from the sapwood, detrimental to the further processing.

Solution shown in figures 1-3 is not suitable to narrow knives.

It is at present usual that the weight of the knives is reduced making them narrower.

Thereby the problems with a narrow knife as shown in figure 3 are emphasized. For solv- ing the problem, many producers of knife systems of chippers have developed systems equipped with turn knives. In the"cassette system", shown in figure 4, the forces are ex- erted on the wear plate or alternatively on the knife clamp 27 on the surface of the knife disc more evenly. The knife cassette comprises hardened elements 28,29 and a bolt 30 locking them together. When positioning the turn-able knife 31 into the cassette, the bolt 30 is loosened so much that the knife 31 fits in between the elements. The bolt 30 is tight- ened, whereby the elements 28, 29 press the turn-able knife 31 in position. When changing the knives in the chipper, the cassette with the knife 31 is positioned between the knife clamp 27 and the knife disc 2'. The knife clamp 27 between the wear plate 4'and the knife is pressed against the cassette by means of a bolt 32. Chipping forces from the knife 31 are exerted on the hardened elements 28,29 having less yielding. Thus, also the force exerted on the knife clamp 27 is more even. The knife clamp is usually medium-hard tempered steel with a hardness of 300 HB. Cassette systems, however, include numerous hardened elements that are destroyed in cases where metallic scrap or stones end up into the chipper.

That causes big maintenance costs.

For the point of view of the use of the chipper and the chip quality, however, it is advanta- geous, that the number of elements is low and the surface rubbing against the log has no jags or cavities. Additionally, narrow knifes are preferably used. These requirements have led to further developments of the present invention. The abovementioned requirements are met by means of a method in accordance with the present invention, wherein the tip 15 of the wear plate and the support 14 are hardened in connection with the sintering heat treat- ment. For that reason they endure the forces from the knife and support the knife better during the chipping. Also the problems with the permanent deformation of the tip of the wear plate are solved. Selective hardening does not prevent machining of the hardened and soft back surface of the wear plate and does not cause embrittlement of the actual con- struction of the wear plate. The wear plate must have adequate toughness to prevent break-

age of the wear plate in case of an accident. Stones or other undesirable material can acci- dentally end up into the chipper.

Temperature of the tip 15 of the wear plate during the hardening process is shown in fig- ures 5 and 6. Horizontal axes of the figures describe time and vertical axes temperature.

Changes of the temperature in the wear plate have been measured in hardening tests.

Straight C of figure 5 describes the change of the surface temperature of the tip 15 after the sintering and immediately after the wear plate is removed from the furnace. The figure shows, that the temperature of the tip 15 decreases in the early stage quickly in the ambient temperature of 20°C of air. Wet quenching has been started after about 1 minute after re- moval from the furnace, whereby the tip portion temperature is still within the austenitic area. The lower limit of the austenitic temperature is, depending on the percent of carbon, 840-870°C. Curve D of figure 6 shows the temperature of the tip portion 15 during quenching. After quenching of about 17 minutes, when the tip portion temperature is be- low 100°C, the quenching was stopped. After that, the inner heat of the basic material of the wear plate increases the tip portion temperature again to more than 200°C, causing ad- ditional annealing of the tip portion 15 of the wear plate. Curve E shows the maximum measured temperature of the medium portion of the wear plate.

The quenching can also be performed with oil, quenching liquid or compressed air. Envi- ronmental aspects and handling problems must in that case be taken into consideration, and the fact that desired quick cooling and hardness is difficult to achieve.

Figure 7 shows water hardening of the wear plate 4, where the tip portion 15 (marked with grate lines) and the support portion 13 are cooled by means of a water jet 16 and 17 di- rected to the surfaces to be in contact with the knife 3 after the heat treatment of the sinter- ing. The present boric steels enable easily the selective hardening, because the boric em- phasizes the hardening by 1000-1500-fold. It is known in the metal technique that the yield limit, in other words the stiffness increases in proportion to the hardness. Tests have shown that by means of selective hardening an about double hardness can be provided to the tip 15 of the wear plate, and thus also increased permanent deformation limit in respect of other portions of the wear plate. As the normal hardness of the wear plate is about HB 150, a hardness of even more than 400 HB can be achieved in the hardened tip portion.

Thus, the ability of the tip of the wear plate and the support portion to encounter forces of the chipping without yielding grows considerably. Preferred composition of the steel to be

used is: C = 0.20-0. 25 %, Si = 0.2 %, Mn= 1. 2 %, Ni = 0.04%, Cr = 0.27 %, Al = 0.03 %, B = 0.002 % and Fe about 98 %.

It has been discovered by hardening tests, that decarburization occurs just quite at the sur- face of the hardened point, resulting that a thin surface layer of 1-3 mm is of softer mate- rial. The harder material is formed under the surface layer. Therefore machining for cor- recting the deformations occurring in the heat treatment for the part of the tip portion 15 and the support portion of the wear plate are easy and the hard surface will be brought out after a'couple of millimetres of material has been removed. Deviations by form are smaller in these parts than in the back surface 18 of the wear plate, and they can be corrected within limits of the softer surface layer remained after hardening. Working after the hard- ening is performed as shown in figure 8 by dashed lines 19,20 and 21. In the front surface 7 of the wear plate a bevel 19 of an angle a can be machined, whereby the wood does not rub the surface 7 for the portion, where no durable coat layer 8 exists. Additionally, it will be made sure that the point 22 of the tip portion 15 sets to the cover of the knife 3. The wear resistance of the point 22 will be improved, when the yielding softer material has been removed. Hardness of the wear plate quite for the part of the sharpest point 22 is im- portant so as to avoid gathering of cuttings and fibres between the knife 3 and the ma- chined surface 20 of the wear plate.

Supporting element for the knife 3 can also be formed of a part 23 separate from the wear plate, as shown in figure 2. In that case, the part 23 is manufactured separately from the wear plate.