TECHNICAL FIELD

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.

BACKGROUND OF THE INVENTION

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, Α1 ₂ 0 ₃ , which they normally do. The ALOa-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. BRIEF DISCLOSURE OF THE INVENTION

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.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description of the invention, reference will be made to the

accompanying drawings, in which

Fig. l shows a known profile of a doctor blade in the form of a blade shaped tool with rounded edges;

Fig. 1A shows in cross section an encircled and enlarged edge portion of the tool of Fig. 1 after wearing for a period of time;

Fig. 2 shows a known profile of a blade shaped tool with a beveled edge intended for doctor blades and coater blades; Fig. 3 shows a known profile of a blade shaped tool with a lamella tip suitable for doctor blades;

Fig. 4 shows a known profile of a blade shaped tool with a cross ground edge for creping doctor blades;

Fig. 5 shows schematically, enlarged and not according scale, the surface regions of the tool which have been encircled in Fig. 1, according to a first embodiment of the invention;

Fig. 6 shows, enlarged, in the same mode as in Fig. 5, the surfaced regions of the tool according to a second embodiment of the invention;

Fig. 7 shows a printing doctor blade, on which "burr" has been formed, i.e. tiny projections;

Fig. 8 schematically illustrates a simple device for testing the bending strength of a tool according to the invention;

Fig. 9 shows a picture made through scanning electron microscopy of a section of a doctor blade according to the invention, made of a low-alloy martensitic strip steel, showing in cross section a hard surface layer on a wide side of the blade and the carbide rich microstructure of the base material of the blade beneath the surface layer;

Fig. 10 illustrates the chemical characterization of the base material and of the surface layer of the sample shown in Fig. 9, employing energy-dispersive X-ray spectroscopy, EDS, image A showing the spectrum of the base material and image B showing the spectrum of the surface layer;

Fig. 1 1 shows the lighter layer which through scanning electron microscopy was observed on the surface of a tool made of a stainless, martensitic strip steel, which had been treated for 16 hours in a plasma-immersion-ion-implantation furnace, where the obtained surface layer appears to contain the same type of carbides as the steel base material beneath the surface but without the martensitic structure of the body,

Fig.12 shows the same type of steel in the region of the surface and in the base material beneath the surface after 30 hours of treatment in the same furnace;

Fig. 13 is an atomic number contrast image which illustrates how the surface layer of a steel which has been bombarded by metal atoms with different atomic weight consists of two zones; an outer, darker zone which is enriched with lighter, implanted atoms, and an inner, lighter zone which is enriched with heavier atoms;

Fig. 14 shows the lighter, hard surface layer of a doctor blade made of a medium alloyed tool steel, and, beneath the surface layer, the carbide rich, martensitic microstructure of the tool steel;

Fig. 15 is an atomic number contrast image of the same tool steel as in Fig. 14, showing a light transition zone and light carbides which are rich in molybdenum and darker carbides which are rich in vanadium, which probably have been precipitated from existing retained austenite during the plasma-immersion-ion-implantation process, contributing to the hardness and bending resistance of the material; and

Fig. 16 shows a more conventional microstructure picture of a section of the same tool steel as in Fig. 14 and Fig. 15 with a typical martensitic structure, which contains dark needles of martensite, light lamella of cementite and tiny temper carbides, which have been precipitated from existing retained austenite during the plasma-immersion-ion- implantation process. DETAILED DESCRIPTION OF THE INVENTION

The four blade shaped tools according to the invention, whose profiles are shown in Fig. 1 - Fig. 4, are designated 1, 2, 3, and 4, respectively. As a base material in the tools, i.e. in the doctor blades, coater blades, or other tools according to the invention, there is used a hardenable, cold-rolled strip steel, for example any of those types of material which today conventionally are used for the manufacturing of doctor and/or coater blades, such as cold-rolled strips of carbon steel, low-alloyed hardenable steels or stainless, martensitic chromium steels. Among the large number of types of cold-rolled steel strips which presently are available on the market-place and which can be used as a base material for the blade shaped tools according to the invention, reference can be made to strip steels manufactured by BOHLER-UDDEHOLM Precision Strip AB, Sweden, and SANDVIK AB, Sweden, which have nominal chemical compositions according to Table 1.

Table

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, Fig. 5, which contains hardness increasing products which are precipitated in the matrix of the steel near the surface although to a certain depth. All surfaces of the tool, according to the embodiment, are covered by such surface layers, which are very hard. It is particularly important that the two wide sides 10a and 10b are covered by the hard surface layers 6a, 6b, which are well integrated with the steel body 5, such that they will possess a high tensile strength. This, in combination with an adequate hardness of the body 5 of the tool, affords the tool a high bending strength according to elementary science of the strength of materials, which in turn makes it possible to press the tool with a high pressure of contact against e.g. a printing cylinder, without the tool being bent - deflected - to an unacceptable degree. It is also important that the hard surface layers 6a, 6b on the wide sides of the steel body extend as far as to the working edge of the tool in order to provide a good wear-resistance to the edge. On the other hand it is not important that the edge initially is covered by the same hard surface layer as the wide sides. Fig. 1 A illustrates schematically the appearance of the working edge portion when its outermost tip - e.g. of the embodiment according to Fig. 1 - has been worn down. The material of the body 5 of the doctor blade 1 is exposed, but the very hard surface layers 6a and 6b, which contact the printing cylinder, reduce wear. The two hard surface layers also prevent that the edge portion of the doctor blade, which is pressed against the printing cylinder, is hollowed out or that material is pressed out by the cylinder to form burr 9, a phenomenon which is illustrated in Fig. 7. Burr impairs the quality of the printed product and is therefore a problem when doctor blades according to prior art are used, but also that problem can be eliminated through the present invention. 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 (PHI), Plasma Immersion Ion Implantation and Deposition (PII1D), 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 Andre Anders". The base is the Pill-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, Fig. 5, therefor hard products are formed, which may comprise or contain e.g. products generated through reactions between various implanted elements and/or between implanted elements and elements which exist originally in the steel, such as reaction products in the form of nitrides, carbides, and oxides, among them for example one or more of the following: titanium nitride (TiN) and aluminum nitride (A1N); vanadium carbide (VC), tungsten carbide (WC), molybdenum carbide (Mo2C Mo6C) and chromium carbide (Cr3C2); and zirconium oxide (Zr02), aluminum oxide (A1203) and chromium oxide (Cr203), respectively. Also mixed carbides as well as mixtures e.g. of type carbonitrides, such as titaniumcarbonitndes and others are conceivable and shall be included in the list of conceivable hardness promoting products, which may exist as discrete particles or as complex agglomerates of precipitated phases, such as new martensite and/or ferrite formed from retained austenite. It should be mentioned in this connection that the above list contains only a few selected but not restricting examples of hard reaction products which according to the invention may be implanted in the hard surface layers on the two wide sides of the tool.

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 μηι. 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. Fig. 8 shows schematically a simple device 12 used for such studies, which give clear and distinct proofs of the improvements of bending strength which have been achieved according to the invention. These results also imply an important progress in the technique which is based on the use of doctor and/or coater blades. The improvement of the bending strength is particularly significant as it is combined with a pronounced improvement of the wear resistance of the tool which has been evidenced through field tests. When the bending strength of a strip shaped tool according to the invention shall be tested by means of the device 12, a first end portion 14 of a piece 13 of the strip is placed in a slot 15 in a holder 16, such that the opposite, second end portion 17 , which is longer than said first end portion 14, will stick up. The operator then folds down the second end to a stop, as is shown schematically in the drawing, suitably by hand power, as is indicated by an arrow, upon which he allows the test specimen to spring back. The degree of springback is a measure of the bending strength. 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, Fig. 6, which contain said hardness increasing particles, underlying layers 7a, 7b which are harder than the base material of the body but not as hard as the surface layers but possibly thicker than them. More specifically, the underlying layers 7a, 7b may be characterized as nitrified layers, i.e. layers of steel containing a high content of nitrides, i.e. compounds of nitrogen and nitride forming metals which may exist in the steel alloy. These layers are established prior to the Plasma Immersion Ion Implantation of the or those metal/metals which in combination with carbon, nitrogen and/or oxygen form the hardness increasing products in the surface layers 6a, 6b. More specifically, the nitrified layers 7a, 7b according to this conceived development of the invention, are established through Plasma Immersion Ion Implantation of nitrogen in a surface layer of the tool at a temperature which may be lower than the tempering temperature of the steel, before the very thin surface layers 6a, 6b which contain said hardness increasing products are established in a subsequent process. However, nor may nitrogen penetrate into the steel to any great depth during a low temperature treatment of that kind, but through a thermal post-treatment at a higher temperature, but below the tempering temperature of the steel, the nitrogen may be caused to diffuse deeper into the steel. The diffusion may be enhanced by using a strip steel which has a high content of nitride forming metals, such as chromium,

molybdenum, titanium, zirconium, vanadium and/or tungsten and others, wherein the nitrogen may diffuse to a depth of at least 0.1 μιη, preferably to at least 0.5 μηι (500 nm) already at a temperature of about 300 °C. Through selection of a steel having a tempering temperature which is not lower than about 500 °C, a thermal treatment at a temperature of up to 480 °C is conceivable in order to promote the diffusion of nitrogen to even greater depths and in any case to between at least 1 μηι but not more than 10 μηι, if so desired, but to maximum 30 % of the strip thickness, and thence also making the precipitation of metal nitrides in the steel more efficient. Therefore, in the nitride hardened layers 7a, 7b, the hardness may be increased with another 100-2500 HV-units (Hardness Vickers units) in excess of the base material's hardness of 500-700 HV. However, the nitrided layers exhibit a hardness gradient counted from the surface layers, because the amount of nitrogen which has diffused into the steel successively is lower and lower in the direction of diffusion. At least in a zone of the nitrided layers, however, the hardness should amount to between 1000 and 3000 HV. In this connection should be mentioned that thicker nitrided layers than 30 % of the thickness of the strip are not desired, because that would cause that the tool 1 , 2, 3, 4, which typically is very thin, could be too brittle.

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. Fig. 6 schematically illustrates the result of the multi-step treatment. In thin surface layers 6a', 6b' which form part of the initially nitride hardened layers, now very hard particles in the form of Me-nitrides, oxides, and/or carbides are precipitated, where Me represents nitride, oxide, and/or carbide forming metals. Under the surface layers 6a', 6b', there are the said layers 7a and 7b, respectively, which have a thickness of at least 0.1 μηι (100 nm), preferably at least 0.5 μπι 500), and suitably at least 1 μηι (1000 nm), however not more than 30 % of the thickness of the strip, and which contain

precipitated nitrides of a type and in an amount which afford said layers an additional hardness, so that they at least in a zone of the layers attain a hardness of between 1000 and 3000 HV. The nitriding such as it has been described above has two desired effects. 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

a) that the nitriding is performed through Plasma Nitriding at a temperature which provides a secondary tempering of the steel, including precipitation of secondary carbides and formation of newly formed fine martensite, at the same time as nitrogen diffuses into the steel strip and nitrides are precipitated, or

b) that the nitriding through Plasma Nitriding is not performed at a temperature which provides a secondary tempering according to a) but at a lower temperature, while instead the subsequent Plasma Immersion Ion Implantation of the surface layers 6a', 6b' is performed at a temperature which provides a secondary tempering of the steel, including precipitation of secondary carbides and formation of newly formed martensite, at the same time as nitrogen diffuses into the steel strip and nitrides are precipitated, or

c) that the nitriding is performed through Plasma Nitriding at a temperature which provides some but not a complete secondary tempering of the steel, including precipitation of secondary carbides and formation of newly formed fine martensite, at the same time as nitrogen diffuses into the steel strip and nitrides are precipitated as according to a), and that the subsequent Plasma Immersion Ion Implantation of the surface layers 6a', 6b' is performed at a temperature is performed at a temperature which provides a complementary and thence essentially complete secondary tempering of the steel, including precipitation of secondary carbides and formation of newly formed fine martensite, at the same time as the nitrogen also during this phase may diffuse into the steel strip and nitrides be precipitated.

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, Fig. 5, or 6a', 6b', Fig. 6, may be coated with a sliding layer which reduces the friction against the roller, cylinder of corresponding which the doctor/coater blade is pressed against, e.g. a molybdenum layer which may be applied electrolytically according to any known technique, or a layer of Teflon (PTFE).

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. Example 1

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 Fig. 3. Ten rolls, each roll containing a strip with a length of 1 000 meters, were arranged to form a stack which was placed in a Vacuum Plasma Implantation and Condensation Furnace. In this furnace the work-piece, i.e. the strip material, was subjected to surface treatment through Ion Implantation and

Condensation by Ion Discharging during a Plasmaphase. For the treatment in the furnace the following parameters applied:

Pressure 4 x lO ^"3 ton-

Temperature 160 °C

Duration of treatment 16 hours

Frequence 25 kHz

Doping metal C, Mo, Ti, Cr, V

Gas N ₂

The microstructure of a sample of the surface treated strip material was studied by means of scanning electrone microscope. Fig. 9 shows the material greatly magnified. The lighter material a is the surface layer (designated 6a and 6b in the schematic drawing, Fig. 5), the thickness of which could be measured to about 350 nm. The microstructure of the base material b (designated 5 in Fig. 5), which contains major iron carbides minor carbides of not identified type, is visible under the surface layer. The hardness of the substrate, i.e. the base material or body 5, was measured to 595 HV.

The chemical characterization of the base material, Fig. 10 A, and of a surface layer, Fig. 10 B, was determined by energy-dispersive X-ray spectroscopy, EDS, which indicates the presence of different elements by measuring the energy of incoming X-ray quanta. The result of the investigation is visualized as a spectrum, in which the area of the peaks are related to the amount of the different elements. In both spectra, the elements iron, chromium, manganese and silicon of the base material dominate. The reason why the test indicates so high content of the base elements, however, may be that it cannot be avoided, due to the thinness of the surface layers, that also the underlying steel has influenced the test. In the surface layer, the right hand image B, however, there are, besides the elements which exist in the base material, also significant amounts of the implanted doping metals molybdenum and vanadium, and an increased amount of chromium which also was implanted. The test thus shows that the surface layer has been enriched on these elements, which in the surface layer have been combined with carbon and/or nitrogen to form very small carbides, nitrides and/or carbonitrides, the presence of which could not be detected by scanning electron microscopy. Indirectly, however, the presence of these hard products could be registered through the remarkable increase of the wear resistance of the tool that was achieved.

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 x 10 ⁶ 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.

Example 2

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 Fig. 3. Also this material was then subjected to Plasma Immersion Ion Implantation in the same furnace as in the foregoing example with the following parameters being applied for the treatment in the furnace.

Pressure 4 x 10 ^"3 torr

Temperature 300 °C

Duration of treatment A first batch A treated in the furnace for 16 hours and second batch B for 30 hours

Frequence 25 kHz

Doping metal C, Mo, Ti, Cr, V

Gas N ₂ 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 Fig. 8 should be an adequate, comparative method for obtaining a comprehension about the importance of the duration of treatment. A first batch of strips, test A, was therefor treated for 16 hours, while a second batch of strips of the same kind, test B, was treated for 30 hours but in other respects in the same mode as the first batch. When subjected to the bending test, a test specimen from the first batch A sprung back to a permanent angle of about 85° , Ex 2A, while a specimen from the second batch B sprung back to a permanent angle of about 120°, Ex 2B. The starting material, i.e. the same type of normal doctor blade material, which had not been subjected to any Plasma Immersion Ion Implantation sprung back to only about 63°, Fig. 8, Ex 2N. This simple test thus indicates that the tool material according to the invention not only has an improved bending strength but also a good ductility, when compared with the conventional material, if ductility is defined as ability to retain strength and freedom from cracks when shape is altered.

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, Fig. 1 1 and Fig. 12 (after 16 and 30 hours treatment, respectively, in the Plasma Immersion Ion Implantation furnace), are made so that also the surface of the specimens on top of the surface layers are viewed. The darker parts represent the base material, i.e. the steel in the tool body under the surface layers. There, one can, especially in Fig. 11, see clearly the typical microstructure of the stainless steel in the base material/the body with larger chromium carbides and smaller, unidentified carbides. The thicknesses after 16 and 30 hours treatment were measured to about 320 nm and 590 nm, respectively. The EDS-analysis gave the same information as in Example 1, namely that Mo, V, and Cr were enriched in the surface layers. The hardness of the base material (body 5) was measured to 580 HV after 16 hours as well as after 30 hours of treatment. 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, Fig. 13, became dark. This means that that outer layer has a lower mean atomic weight, which in turn means that the lighter atoms Ti and V, i.e. the doping metals titanium and vanadium have been enriched in that layer in the form of carbides. Under the darker layer one can discern a transition zone which is lighter and indicated by the lower arrow. It is assumed that this may indicate that this zone is enriched on the heavier doping metal molybdenum, and in that case probably in the form of M6C-carbides and/or -nitrides. Provided these assumptions are true, the stratification implies that there has been established an outer, very hard zone, which contains i.a. extremely hard titanium-carbides and/ or -nitrides, and an inner, not as hard zone, which contains not so extremely hard molybdenum-carbides and/or -nitrides. In other words, already in the surface layer there may be what can be termed de-escalation or phasing out of hardness, which is a desirable effect.

Example 3

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, Fig. 3, as in Examples 1 and 2. The material was treated in the same furnace and at the same pressure, period of time, frequency, and with the same gas, N ₂ , as in Example 1, but the temperature was higher, 400 °C, and as doping metals there were in this case used Mo, Ti, Cr, V, and B. Through the treatment there was established an about 380 nm thick surface layer; the light layer in Fig. 14. Also the surface of the layer is visible on the image. Under the layer, the carbide-rich microstructure of the tool-steel is visible. Fig. 15 shows an atomic number contrast image of the same treated steel. One can see on the image a light transition zone and molybdenum-rich carbides of M6C-type in the form of round particles, which are white on the image, and vanadium-rich carbides of MC-type which are dark, because their mean atomic number is low. Fig. 16 shows a section of the base material, which has a typically martensitic structure. Adjacent to needles of martensite Ma there are lamella of cementite Ce, which have been precipitated during the tempering of the steel, and also small tempering carbides X, which have been precipitated from existing retained austenite during the Plasma Immersion Ion Implantation treatment.

Example 4

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:

Step 1 Step 2

Pressure 4 10 ^"3 torr 4 x 10 ^"3 torr

Temperature 400 °C 140 °C

Duration of treatment 10 hours 16 hours

Frequence 25 kHz 25 kHz

Doping metal None Mo, Ti, Cr, B, B

Gas N ₂

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, Fig.8. A test sample of the same steel grade in a cold-rolled, hardened and tempered condition but not subjected to Plasma-Immersion-Ion-Implantation sprung back 100°, Ex 4N, Fig.8. The two-step treatment thus provided a material, which had the best springiness of the tested materials, but a sample of this tool steel on the other hand had a comparatively good bending strength already in its normal condition, Ex 4N. The greatest improvement of the bending resistance was gained through the long term treatment of the stainless steel, Ex 2B, Fig. 8.

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.