Patent Application
20140000467
The invention relates to a blade shaped tool belonging to that type of tools which comprises flat, strip shaped doctor and coater blades, which have a body of steel and at least one end portion, which is the working part of the tool during use thereof.
Blade shaped tools of many kinds are employed for various industrial applications. Doctor blades of the typed mentioned in the preamble are used in printing offices and printeries for scraping excess ink from an engraved printing or ductor roller. In the paper industry, coater blades, which also are made of strip steel of the above mentioned kind, are employed for scraping excess coating material from a running paper web in order to leave just an even layer of coating material of desired thickness on the web. Further, also creping doctor blades for creping a paper web should be mentioned in this connection. I all these cases, high quality requirements are raised on the employed blade shaped tools, which preferably are made of strip steel. They need to have, for their respective fields of use, an adequate thickness, width, length and shape, flexibility, hardness, and resistance to wear. Particularly, the combination of all these requirements have been difficult to satisfy.
The importance of wear resistance is elucidated in SE 519 466. This patent document suggests coating the doctor or coating blade electrolytically with a nickel layer containing wear resistant particles in order to improve the wear resistance of the blade. However, an electrolytically deposited nickel layer does not have a sufficient basic hardness in order to function satisfactorily for retaining the wear resistant particles. This is particularly true when the ink compositions which are used in those printing offices, or the coating compositions which are used in those paper industries, where the doctor blades or coater blades, respectively, are used, contain wearing particles of aluminium oxide, Al2O3, which they normally do. The Al2O3-particles, which are extremely hard, namely “hollow out” the nickel layer and “carry away” the wear resistant particles from the nickel layer. This is at least a plausible reason why the working part of the tool blade is worn out too soon.
It has also been suggested, US 2005100673, to use a bar made of an austenitic stainless steel as a doctor element, which due to its shape has a sufficient stiffness and which is subjected to a gas plasma-assisted treatment, including implantation of foreign metal ions directly into the austenitic surface of the bar shaped doctor element. A product made of a bar of austenitic steel, however, regardless the surface hardness of the bar, has not a doctoring efficiency comparable with that of a doctor or coater blade, which, besides hardness and excellent wear resistance in the working part of the tool, has an adequate flexibility, which allows the tool to be pressed with a high pressure of contact, but at the same time resiliently, against a printing or ductor roller or against a running web.
Another prior art problem is the so called “burr”, i.e. tiny projections which may develop on the trailing side of the tool, which may cause scratches or other damages on gravure printing cylinders and anilox rolls for flexographic printing, as well as defects on the web material, respectively, or other damages or defects of various kinds if the “burr” material would come loose.
It is the purpose of the present invention to address the above mentioned complex of problems in order to provide better blade shaped tools of the doctor and coater type of blade tools than according to prior art. Particularly, the invention aims at providing a thin, improved blade shaped tool which advantageously combines a high wear resistance with a high stiffness against bending, allowing the tool the be pressed with a high contact pressure against the object to be doctored or coated. This can be achieved therein that the invention is characterized in what is stated in the appending patent claims. Other characteristic features, objects and advantages of the invention will be apparent from the following detailed description and from the achieved results which also will be disclosed.
In the following description of the invention, reference will be made to the accompanying drawings, in which
The four blade shaped tools according to the invention, whose profiles are shown in
The steels in the tools 1-4 have in their hardened and tempered condition a hardness between 500 and 700 HN (Hardness Vickers). The body 5 of the tools 1-4, i.e. their main portion, has a thickness which typically is between 0.07 and 0.4 mm as far as doctor blades is concerned, while coater blades typically has a thickness between 0.3 and 0.6 mm. The width is typically about 10-100 mm. The length can be up to about three to four meters, or more in exceptional cases, but may also, as far as doctor blades for printeries are concerned, be as short as 10 cm.
According to the invention, each of the two wide sides of the blade shaped tool has a surface layer 6a and 6b, respectively,
The surface layers 6a, 6b consist of components from the base material of the doctor blade, i.e. the strip steel which the doctor blade is made mainly of and of hard products which have been incorporated in the surface layers. The hard products in said surface layers consist, according to the invention, mainly of hard products of reaction between, on one hand, one or more of the agents nitrogen, oxygen, and carbon and, on the other hand, one or more reactive metals that react with said agents, said agents and metals, respectively, having been implanted through plasma-immersion-ion-implantation. A Swedish terminology in the region of plasma-immersion-ion-implantation has not yet been standardized. English is the common language, but even English has no terminology standards in this field. Presently, professionals use a number of definitions of different variants of the technique existing in this field, such as Plasma Immersion Ion Implantation (PIII), Plasma Immersion Ion Implantation and Deposition (PIIID), Metal Plasma Immersion Ion Implantation and Deposition (MePIII) and others, and also various combinations of said variants with their specific acronyms. Detailed information about this technique, which is employed according to the invention, can be found in “Handbook of Plasma Immersion and Deposition; Edited by André Anders”. The base is the PIII-technique according to which the workpiece—as far as the invention is concerned, the workpiece is the steel strip/the tool that shall be treated—is placed directly in a plasma in a vacuum-chamber and is subjected to high, pulsating negative potential. When carrying out the method in practice, a large number of steel strips in the form of rolls are placed in the vacuum-chamber, each roll containing a steel strip having a length of one or two kilometers or more. The number of rolls may amount to the order of ten or twenty in each batch. A plasma sheath is formed around each of all these workpieces, even within the rolls between the winding layers, so that the pulsating potential causes ions to bombard all surfaces of every one of the strips. The process in other words is very rational. By employing one of the variants of this technique, on one hand nitrogen, oxygen and/or carbon atoms, and on the other hand metal atoms, preferably of one or more of the metals vanadium, molybdenum, tungsten, chromium, zirconium, titanium, and aluminum are caused to be ionized, i.e. to form ions, which bombard the workpieces/the steel strips, such that the reactive atoms are implanted in the steel, where they react with one another and possibly also with reactive agents which already exist in the steel material.
In the surface layers 6a, 6b of the tool 1, 2, 3, 4 according to the invention,
According to a preferred embodiment of the invention, the surface layers 6a, 6b which contain said products which increase the hardness, are very thin. However, they have a thickness of at least 100 nm (nanometers), preferably at least 200 nm and as a maximum about 2 μm. Performed experiments have indicated that the thickness should be in the range 200-2000 nm. A preferred thickness may be at least 300 nm. The hardness of layers of material having so small thickness dimensions, however, can hardly be measured through conventional methods of hardness determination, such as for example Vickers. But it is possible to get an apprehension about the hardness through comparative studies of the bending strength of tools of the invention and of conventionally manufactured tools.
It is also possible to make an image of the structure and thickness of the surface layers by means of advanced microscopy, to some extent also their compositions, and in some cases their stratification in zones which have different contents of hard, implanted products.
Further, it has been possible to make a chemical characterization of the surface layers 6a, 6b and of the base material, i.e. of the body 5, by means of energy-dispersive X-ray spectroscopy, EDS.
According to a conceived development of the invention, there exist beneath the very hard but at the same time very thin surface layers 6a, 6b,
Now, hardness increasing products are established in an outermost surface layer of the nitreded and precipitation hardened layers 7a, 7b through Plasma Immersion Ion Implantation of one or more metals together with nitrogen, carbon and/or oxygen so that the hardness increasing products are formed as has been described in the foregoing.
Firstly, it can contribute to the formation of a comparatively hard matrix for those particles which are implanted into the layers 6a′, 6 b′ in a subsequent step in order to increase the hardness still more, in which hard matrix the hard particles can be secured more safely. Secondly, the nitrided, comparatively thick layers 7a, 7b under the very thin surface layers 6a′, 6b′ form a support for the latter ones. Normally, very hard and very thin surface layers run a risk to be damaged if they rest like a thin skin directly on a much softer underlayer. According to the described embodiment of the invention, the very hard but thin surface layers 6a′, 6b′ are supported by the not that hard but thicker layer 7a and 7b, respectively, which reduces said risk. Then, under the nitrided layers 7a, 7b comes the steel material 5 which is hardened and tempered to a hardness of between 500 and 700 HV.
According to a conceivable development of the invention, no thermal treatment of the nitrided layers 7a, 7b is performed before the surface layers 6a′, 6 b′ are applied. Instead, the Plasma Immersion Ion Implantation of the surface layers 6a′, 6b′ is performed at an elevated temperature near although lower than the tempering temperature of the steel and for so long period of time that the desired diffusion of nitrogen into the steel will be achieved during this phase. According to a variant of this embodiment, a thermal treatment of the nitrided layers is performed before the surface layers 6a′, 6 b′ are applied, whereupon an additional thermal treatment takes place during a Plasma Immersion Ion Implantation of the surface layers 6a′, 6b′ at an elevated temperature.
Still another developed form of the invention is applicable in the cases when the strip steel is of a type which allows precipitation of secondary carbides, e.g. because the steel contains a substantial amount of retained austenite. Conventionally, cold rolled strip steels are hardened and tempered in a continuous process, which may cause some types of strip steel to contain a certain amount of retained austenite after hardening and tempering. The steel grades Sanprint (SANDVIK) and Uddeholm Hotvar, which are listed in Table 1, are examples of this type of steels which may make precipitation of secondary carbides during a second tempering operation possible, during which the retained austenite at least partly is transformed into secondary carbides and newly formed martenite. This is applied in said developed form of the invention according to any of the following procedures, namely
Irrespective of which of the above procedures that is applied, there is obtained as a positive side effect that grain coarsening of the steel is counteracted.
According to still another conceivable development, the hard surface layers 6a, 6b,
In the following, some examples of the invention shall be described and achieved results be reported from tests of some embodiments of doctor blades of the invention that have been performed.
In the example there was used as a doctor blade material a low-alloyed, cold-rolled, hardened and tempered strip steel of grade Sandvik 20C2 having the nominal chemical composition which is stated in Table 1. The strip thickness was 0.20 mm. The strip was ground to a profile according to
The microstructure of a sample of the surface treated strip material was studied by means of scanning electrone microscope.
The chemical characterization of the base material,
Thus, a doctor blade made in accordance with the invention of the surface treated strip material of the above mentioned steel grade, was subjected to a comparative field test in a flexo printing machine with a ceramic ink transferring plate cylinder, which was laser beam engraved to a line density of 220 lines/cm. When the plate cylinder had rotated a distance against the doctor blade according to the invention corresponding to about 2.2×106 running meters (2 200 000 metres), the doctor blade was considered to be worn out according to the criteria which were valid in that printing office. Doctor blades of the same steel grade, manufactured according to conventional practice, i.e. in a mode including cold rolling, hardening and tempering, normally has a useful life corresponding to about 200 000 running meters in the same printing machine, using the same ink, in the same printing office. Thus, with the doctor blade of the invention, the working life span was extended eleven times, demonstrating a dramatic improvement of the tool's resistance to wear.
In this case there was used a stainless, martensitic chromium steel of grade Sandvik 13C26 but containing a somewhat lower content of silicon and manganese than according to Table 1; 0.3 Si and 0.55 Mn. The pretreatment was the same as in the foregoing example; cold-rolling to 0.2 mm thickness, hardening, tempering, and grinding to the profile shown in
The purpose was to evaluate the importance of the duration of the treatment of the strip material in the furnace with reference to achieved springiness in terms of springback capacity of the doctor blade. It was judged that the bending test which has been described above with reference to
The test also demonstrates that a prolonged duration of treatment, and thence probably an increased amount of hard products in the doctor blade material, gives a significant improvement of a feature which is very important for a strip shaped doctor blade, namely a high bending strength in combination with an adequate flexibility and ductility.
The two materials were also studied in a scanning electron microscope and by means of energy-dispersive X-ray spectroscopy, EDS. The microscope-photographic images,
The microstructure of the surface layers could not be determined by means of scanning electron microscopy, but when applying atomic number contrast imaging, the layer which is farthest out, which is indicated by the upper arrow on the image,
As a doctor blade material in this case there was used an alloyed but not stainless, cold-rolled, hardened, and tempered, strip-shaped tool-steel with the following nominal composition in weight-%:0.55 C, 1.0 Si, 0.75 Mn, 2.6 Cr, 2.25 Mo, 0.85 V, balance iron and normally existing impurities. Also this strip material had a thickness of 0.2 mm and was ground to the same profile,
The purpose of this example what is to illustrate the importance of performing the Plasma Immersion Ion Implantation in accordance with a preferred embodiment of the invention, in which the treatment in a first step is performed with nitrogen in the furnace but without any doping metals, while in a second step also doping metals are employed.
The same type of alloyed but not stainless tool steel was used as in Example 3. Also the pre-treatment of the strip material was the same; cold-rolling, hardening, and tempering. The strip which had been pre-treated in that way, was subjected, in two steps, to Plasma Immersion Ion Implantation treatment in the same furnace as according to the foregoing examples. In the two steps, the following parameters for the treatment in the furnace were applied:
In bending tests, which were performed in the same way as in tests which have been described in the foregoing, the test sample sprung back 140°, Ex 4,
It should in this connection also be noted that in no case any crevices were observed in the hard surfaces layers of the tools according to the invention, neither in connection with the field tests nor in connection with the bending tests. In the surface layers, thus a high hardness is combined with a good toughness. While the hard hardness may be attributed to the hard products which have been provided in the surface layers, the good toughness of the surface layers—without binding the invention to any specific theory—depend on the fact that the surface layers are integrated with the original base material, an opinion which is supported by the results from energy dispersive X-ray spectroscopy (EDS) studies, but also because the hardness decreases successively from the very outside of the wide side surfaces of the tools towards the centre, without abrupt steps where high tensions could be concentrated.
In summary, a number of conclusions can be drawn from the experiences from the manufacturing of a number of different tool materials according to the invention, employing strip steels of different chemical compositions, and from a number of tests of the manufactured materials, namely:
that the hard surface layers contain—for the achievement of desired hardness, wear resistance, bending strength and ductility—hard products of reaction between on one hand one or more of the elements nitrogen, oxygen, and carbon, and on the other hand one or more metals which may react with said elements, which elements and metals, have been implanted into the surface layers of the steel strip through Plasma Immersion Ion Implantation, and
that the hard surface layers have a thickness of at least 100 nm (nanometer), preferably at least 200 nm and suitably at least 300 nm. The experiences also show that the thickness may as great as 2 000 nm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1100200-3 | Mar 2011 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE12/50284 | 3/15/2012 | WO | 00 | 9/9/2013 |
