WO2016186528A1

WO2016186528A1 - Method for manufacturing knives for forage harvesters, in particular shear bars

Info

Publication number: WO2016186528A1
Application number: PCT/PL2016/000053
Authority: WO
Inventors: Dominik JANICKI
Original assignee: Janicki Dominik
Priority date: 2015-05-15
Filing date: 2016-05-13
Publication date: 2016-11-24
Also published as: PL412359A1; PL233914B1

Abstract

In a method for manufacturing knives for forage harvesters the edges of the extended steel part (1) are machined by performing insert seats (1a) therein for reinforcement. Around these insert seats (1 a) the edges of the extended steel part (1 ) are reinforced. For the fabrication of the extended steel part (1) heat-treatable steel is selected, while as a reinforcement carbide inserts (2) are applied, then in the austenitizing temperature of steel, from which the extended steel part (1) is made, carbide inserts (2) are combined with the material of the extended steel part (i) by means of brazing, using composite filler materials, preferably brass-based, wherein during brazing, a clearance (3) is maintained with the range from 0.2 mm to 0.8 mm, whereupon the whole is cooled at a rate ensuring the martensitic or bainitic transformation in the extended steel part (1). To fabricate the extended steel part (1) steel is selected preferably with carbon content, preferably from 0.22% to 0.33%. Used as an insert (2) is sintered tungsten-cobalt carbide WC-Co of mining grade with cobalt content by weight from 10% to 30% and grain size of carbides WC below 35μm.

Description

Method for manufacturing knives for forage harvesters, in particular shear bars

The invention relates to the manner of manufacturing knives for forage harvesters, especially shear bars intended primarily for use in forage harvesters, used for harvesting maize, grass, alfalfa and other green plants.

A shear bar, which is also called counter knife or fixed knife, comprises an elongated steel part made of carbon steel, which is sometimes referred to as base material. Shear bars have reinforced cutting edges. Their resistance has a major influence on the quality of the machine operation. The higher the edge resistance, the better the chopping quality and shorter the machine downtimes. For the cutting quality it is crucial to maintain the smallest possible gap between cutting and shear bar. Hence the desire to fabricate the edge of the shear bar from materials that ensure as lowest possible and even wear. At present, mainly used in forage harvesters are shear bars made of carbon steels, with cutting edges ciadded using different welding techniques. The most commonly used methods of reinforcing cutting edges of the shear bar are the following: flame spraying, plasma cladding, laser cladding. Less frequent is the technology of covering shear bars with wear resistant mats of the Brazecoat^® type. These mats are applied using infiltration brazing in a vacuum or hydrogen atmosphere. And this method is disclosed in the description of the patent for an invention US3,743,556. Thus brazed layers contain tungsten carbides distributed in the metallic matrix of the nickel- chromium-silicon-boron alloy.

Cladding techniques produce layers containing tungsten carbides in a metallic matrix of nickel-chromium-silicon-boron alloy comprising by weight 50-60% of tungsten carbide. The applied particle size of carbides is usually 40-150pm. Assumed adhesion of the weld to the substrate in the existing solutions is above 100 Pa. For applications exposed to the impact of stones, cladding of cutting edges of the shear bar is effected with alloys with a composition similar to high speed steels. This technique is most commonly used for cutting grass and alfalfa.

In the existing solutions we encounter uneven wear of the cutting edges. The applied technology and materials no longer allow to achieve better performance characteristics. Wear of the cutting edge also occurs by cracking and chipping of the cladded layers. Another very important aspect is the self-sharpening of the cutting edge of a shear bar. Previous experience shows that it is essential for the process of cutting the plant material. Self-sharpening occurs due to excessive wear of the base material as compared to the cladded layer. However, too large a difference between abrasion of the cutting edge and the base material is also undesirable, since strongly grooved shear bar causes incorrect flow of the green mass. The hardness of the surface layer of the steel base part in the existing solutions is usually 25HRC. Therefore, for more reinforced edges of shear bars apply additional hardening of the surface layer is applied.

Description of the patent application for an invention No. US 3,635,271 discloses a solution for shear bar of the forage harvester wherein the edges are reinforced by overlaying, using flame spraying, a thin layer , since the thickness is from 0.004 inch to 0.012 inch, of highly wear-resistant material containing tungsten carbides. This description also shows the effect of self-sharpening associated with a different wear resistance of a hard edge and a soft base material.

Description of the invention of the European Patent No. EP 0 878 23B1 discloses a solution for a shear bar for forage harvester wherein edges are reinforced by overlaying, through cladding, a thin film (strip- shaped) with a wear-resistant material containing tungsten carbides in a matrix of two or more elements e.g. nickel, silicon, boron and carbon. The weld deposit of this embodiment has a thickness from 2 mm to 3.5 mm, and is placed on the cutting edge of the extended steel part of the shear bar to give a self-sharpening effect. Description of the invention to the US Patent No. 1156,637,687 shows an embodiment of a shear bar for a forage harvester, wherein on the upper part, by brazing in vacuum or hydrogen atmosphere, is applied a highly wear resistant mat, such as Brazecoat^®, manufactured by Innobraze GmbH. Under the highly wear resistant mat, along the edges of the shear bar there is a layer of steel hardened to 40-70 HRC, made by induction hardening or cladding. This layer has a hardness intermediate between the relatively soft base material and a very hard layer of the wear-resistant mat. This solution ignores the self-sharpening effect of cutting edges of the counter knife.

The purpose of this invention is to produce a shear bar with a higher wear resistance while maintaining the self-sharpening effect and even wear process of the cutting edge.

The essence of the invention being a method for manufacturing knives for forage harvesters, wherein the edges of the extended steel part are machined by performing insert seats therein for reinforcement, while around these insert seats the edges of the extended steel part are reinforced and subsequently, the whole is subjected to grinding until a size falling within the intended tolerances is reached, characterized in that for the fabrication of the extended steel part heat-treatable steel is selected, while as a reinforcement sintered carbide inserts are applied, then in the austenitizing temperature of steel, from which the extended steel part is made, carbide inserts are combined with the material of the extended steel part by means of brazing, using composite filler materials, preferably brass-based, wherein during brazing, a clearance is maintained with the preferably range from 0.2 mm to 0.8 mm, whereupon the whole is cooled at a rate ensuring the martensitic or bainitic transformation in the extended steel part. According to another preferred feature of the invention to fabricate the extended steel part steel is selected preferably with carbon content, preferably from 0.22% to 0.33%. According to a further preferred feature of the invention, used as an insert is sintered tungsten-cobalt carbide WC-Co of mining grade with cobalt content by weight from 10% to 30% and grain size of carbides below 35μηπ. According to a further preferred feature of the invention, the dimensions of the insert seat for reinforcement of the edge of the extended steel part, measured in a cross- section of the edge of the extended steel part, correspond to the dimensions of the cross-section of the carbide insert used. According to a further preferred feature of the invention, the insert seats in the edge of the extended steei part are performed by means of machining. According to a further preferred feature of the invention, segments with cross-sections from 5 mm x 2.5 mm to 15 mm x 3.5 mm are used as carbide inserts. According to a further preferred feature of the invention, the clearance is maintained by means of reinforcement in the form of a steel mesh or dispersed fibers or distancers made of sheet with a thickness from 0.15 mm to 0.75 mm. According to another preferred feature of the invention, as a brazing operation a brass-based composite filler material is used with the brazing temperature ensuring austenitic transformation of the steel part. According to a further preferred feature of the invention, the brazing operation is performed simultaneously on all inserts with cross heating of the entire extended steel part to the austenitizing temperature.

Brazing of sintered carbide to the edge of the shear bar has encountered a number of difficult to overcome obstacles of technical nature, such as deformations of the base material as a result of heat from the brazing process and no adequate strength of the brazing joint. This prevented edge enhancement using sintered carbides, in order to eliminate this difficulty, distancers were used in the clearance in the form of a steel mesh of higher melting temperature than the brazing temperature, which elements evenly distributed in the filler material influence the quality of the brazing joint. As a result of these procedures, a composite filler material of appropriate quality was obtained. To homogenize the structure of the base part and thus reduce the occurring stresses, brazing was performed by heating the material of the extended steel part in the entire volume. Thereby, the structure of the steel part was homogenized and the stresses were reduced. Thanks to elastification (recrystallization) of the base part, it became possible to straighten and make final adjustments to the shape of the shear bar. Giving the final shape of the shear bar and cooling is provided by the quenching press. Actions taken gave an unexpected effect of being able to obtain a uniform hard structure of the base material by quenching. For this purpose, sintered carbides were selected with adequate shrinkage-resistant chemical composition and structure, and heat-treatable steel containing boron, giving a high hardness and abrasion resistance, and the possibility to use service without tempering. In this manner a shear bar was obtained whose extended steel part is characterized by a uniform martensitic structure, brazing joints are of excellent quality and provide adequate resistance, while carbide inserts have no defects. Selected grade of carbides and steel ensures superior product quality without incurring high manufacturing costs. Brazed segments made of sintered carbides, as compared to cladded layers, are characterized by high dimensional accuracy. Due to the straightness of the shear bar after brazing and quenching, and because of the edge already shaped before the brazing, savings are achieved on the grinding operation. The shear bar is made more precisely and does not require large expenditures for grinding.

The invention is explained in more details using exemplary embodiments illustrated in the drawings, wherein Fig. 1 is a schematic longitudinal section view of a shear bar, Fig. 2 shows a schematic cross- sectional view, and Fig. 3 is a detail of a schematic enlarged cross- sectionai view.

Example 1

In the embodiment of knives for forage harvesters according to one of the many possible embodiments, from heat-treatable steel of grade SB27 12CB, the extended steel part A is machined, wherein insert seats la are made through miliing to strengthen the cutting edge. Placed as reinforcement are inserts 2 made of sintered metal powder WC-Co of mining grades with cobalt content, by weight, of 15% and WC grain size of 6pm, in the form of segments with cross sections of 3 mm x 10 mm. Inserts 2 are protected against accidental movement relative to the insert seat la, and then, the inserts 2 are bonded with the material of the extended steel part 1 by means of induction brazing, using brass-based composite filler material. The brazing operation is performed simultaneously on all inserts 2 with the cross-heating of the entire extended steel part i to the austenitizing temperature. The composite filler material contains spacer elements of higher melting point than the brazing temperature. Thus the filler material consists of two basic components: the matrix 6, which provides satisfactory plasticity and reinforcement that ensures an adequate size of the clearance and its adequate mechanical characteristics. During brazing a clearance 3 is obtained with the thickness of from 0.5 mm to 0.55 mm by means of reinforcement 4 in the form of a steel mesh with a thickness of 0.5 mm. Then the whole is subjected to straightening and cooling in the quenching press at a rate ensuring the attainment of a martensitic or bainttic transformation in the extended steel part i. After quenching, shear bar are subjected to grinding until a size falling within the intended tolerances is reached. The insert seats a on the edge of the extended steel part i are made by milling, wherein the dimensions of the insert seat la, in order to reinforce the edge of the extended steel part I measured in the cross-section of the edge of the extended steel part i correspond to the cross-sectional dimensions of the applied insert 2. The type of steel intended for the extended steel part % and cooling with the rate of the martensitic transformation ensure steel hardness of in the cross-section of the extended steel part i at the level from 49HRC to 50HRC and the possibility to use it under service conditions without tempering. By using the quenching press the hardening process takes place directly from the brazing temperature and after obtaining in this manner high-hardness of the extended steel part I there is no need for additional hardening of the surface layer of the shear bar. On the other hand, the application of the composite filler material provides adequate width of the clearance 3, which allows to compensate quenching deformations and shrinkage stresses resulting from the differences in thermal expansion (linear) of sintered carbides and steel of the extended steel part I. Thus executed filler materia! provides excellent performance. In this way and thanks to a correct choice of the sintered carbide grade, cooling with the rate of the martensitic transformation does not damage the inserts 2 already brazed. Sintered insert 2 is formed under pressure and sintered in special furnaces usually with a high isostatic pressure densification function. The main advantage of sintered tungsten carbide is its mechanical strength, including resistance to cracking and chipping as well as its hardness and abrasion resistance. This is achieved by advanced technology of sintering production. Carbides of selected grades have a strength suitable for a sudden drop of temperature which does not cause any cracks. The abrasion resistance of sintered carbides is many times greater than that of the cladded layers. The steel part with a hardness of 50 HRC exhibits a better wear resistance than previously obtained hardness of 25HRC, therefore there is no need for additional covering of the upper surface of the extended steel part JLof the shear bars. Since the sintered carbide has many times greater resistance to erosive wear than steel hardened to 50HRC, behind the cutting edge of the extended steel part 1, due to the flow of the cut elements, a small cavity 5 is formed, which causes self-sharpening of the cutting edge. The applied technique of induction brazing with a composite filler material allows to obtain the brazing joint strength above 300 MPa.

Example 2

In the embodiment of knives for forage harvesters according to one of the many possible embodiments, from heat-treatable steel of grade 33MnCrB5-2 the extended steel part I is formed, wherein insert seats 1a are made through milling to strengthen the cutting edge. Placed as reinforcement are inserts 2 made of sintered metal powder WC-Co with cobalt content, by weight, of 20% and grain size of approximately 12pm, in the form of segments with cross sections of 4 mm x 12 mm. Inserts 2 are protected against accidental movement relative to the insert seat la, and whereupon at the austenitizing temperature of steel used for the extended steel part 1 the inserts 2 are bonded with the material of the extended steel part 1 by means of induction brazing, using composite filler material on brass matrix. The brazing operation is performed simultaneously on all inserts 2 with the cross-heating of the entire extended steel part 1 to the austenitizing temperature. The composite filler materials contain spacer elements of significantly higher melting point than the brazing temperature. Thus the filler material consists of two basic components: the matrix 6, which provides satisfactory plasticity and strengthening of the reinforcement 4 that ensures an adequate size of the clearance and its adequate resistance. During brazing a clearance 3 is obtained with the thickness of from 0.55 mm to 0.6 mm by means of reinforcement 4 in the form of a spacer made of perforated metal sheet with a thickness of 0.5 mm. Then the whole is straightened and cooled in the quenching press at a rate ensuring the attainment of a martensitic transformation in the extended steel part 1. After quenching, the whole is subjected to grinding until a size falling within the intended tolerances is reached. The insert seats la on the edge of the extended steel part 1 are made by milling, wherein the dimensions of the insert seat la, in order to reinforce the edge of the extended steel part I measured in the cross-section of the extended steel part ± correspond to the cross-sectional dimensions of the applied insert 2. The type of steel intended for the extended steel part i and cooling with the rate of the martensitic transformation ensure steel hardness of in the cross-section of the extended steel part 1 at the level from 48HRC to 50HRC and the possibility to use it under service conditions without tempering. By using the quenching process directly from the brazing temperature and after obtaining in this manner high- hardness of the extended steel part I there is no need for additional hardening of the surface layer of the shear bar. On the other hand, adequate size of the clearance 3 allows to compensate quenching deformations and shrinkage stresses resulting from the differences in thermal expansion of sintered carbides and steel of the extended steel part 1. In this way and thanks to a correct choice of the sintered carbide grade, cooling with the rate of the martensitic transformation does not damage the inserts Sintered insert 2 is formed under pressure and then sintered in a vacuum. The main advantage of sintered tungsten carbide is its mechanical strength, including resistance to cracking and chipping as well as its hardness and abrasion resistance. This is achieved by advanced technology of sintering production. Carbides of selected grades have a strength suitable for a sudden drop in temperature which does not cause any cracks. The abrasion resistance of such sintered carbides is many times greater than that of the cladded layers likewise the resistance of the steel with a hardness of 50HRC as compared to steel with a hardness of 25HRC, therefore there is no need for additional covering of the upper surface of the extended steel part 1 of the shear bars. Since the sintered carbide has many times greater resistance to erosive wear than steel hardened to 50HRC, behind the cutting edge of the extended steel part I, due to the flow of the cut elements, a small cavity 5 is formed, which causes self-sharpening of the cutting edge. The applied technique of induction brazing with a composite filler material allows to obtain the joint strength above 300 MPa.

Claims

1. A method for manufacturing knives for forage harvesters, wherein the edges of the extended steel part are machined by performing insert seats therein for reinforcement, while around these insert seats the edges of the extended steel part are reinforced and subsequently, the whole is subjected to grinding until a size falling within the intended tolerances is reached, characterized in that for the fabrication of the extended steel part (1) heat-treatable steel is selected, while as a reinforcement carbide inserts (2) are applied, then in the austenitiztng temperature of steel, from which the extended steel part (i) is made, carbide inserts (2) are combined with the material of the extended steel part (1) by means of brazing, using composite filler materials, preferably brass-based, wherein during brazing, a clearance (3) is maintained with the range from 0.2 mm to 0.8 mm, whereupon the whole is cooled at a rate ensuring the martensitic or bainitic transformation in the extended steel part (1).

2. Method for manufacturing knives for forage harvesters, according to claim 1 , characterized in that to fabricate the extended steel part (1) steel is selected preferably with carbon content, preferably from 0.22% to 0.33%.

3. Method for manufacturing knives for forage harvesters, according to claim 1 , characterized in that that used as an insert (2) is sintered tungsten-cobalt carbide WC-Co of mining grade with cobalt content by weight from 0% to 30% and grain size of carbides WC below 35pm.

4. Method for manufacturing knives for forage harvesters, according to claim 1 , characterized in that the dimensions of the insert seat (la) for reinforcement of the edge of the extended steel part (i), measured in a cross-section of the edge of the extended steel part (I), correspond to the dimensions of the cross-section of the sintered carbide insert (2) used.

5. Method for manufacturing knives for forage harvesters, according to claim 1 , characterized in that the insert seats (1a) on the extended steel part (1) are performed by means of machining.

6. Method for manufacturing knives for forage harvesters, according to claim 1 , characterized in that segments with cross-sections from 5 mm x 2.5 mm to 5 mm x 3.5 mm are used as carbide inserts (2).

7. Method for manufacturing knives for forage harvesters, according to claim 1 , characterized in that the clearance (3) is maintained by means of reinforcement (4) in the form of a steel mesh or dispersed fibers or distancers made of sheet with a thickness from 0.15 mm to 0.75 mm.

8. Method for manufacturing knives for forage harvesters, according to claim 1 , characterized in that as a brazing operation a brass-based composite filler material is used with the brazing temperature ensuring austenitic transformation of the steel part.

9. Method for manufacturing knives for forage harvesters, according to claim 1 , characterized in that the brazing operation is performed simultaneously on all inserts (2) with the cross-heating of the entire extended steel part (I) to the austenitizing temperature.