Patent Application
20120031249
The present invention is related to a method for surface reforming by refining the texture of a surface layer part in a ferrous material, also related to a ferrous material having a microscopic texture, especially related to a profitable method for manufacturing tool steels and blades having microscopic texture. Furthermore, the expression “refining a texture” means the refinement of crystal grains of a base metal material and the refinement of carbides existing in the base metal material.
An international application PCT/JP2008/067565, filed on Sep. 26, 2008 is incorporated herein by reference.
A following published paper is incorporated herein by reference: Yoshiaki Morisada, Hidetoshi Fujii, Tadashi Mizuno, Genryu Abe, Toru Nagaoka, Masao Fukusumi, “Modification of nitride layer on cold-work tool steel by laser melting and friction stir processing”, Surface & Coatings Technology 204 (2009) 386-390
The demands for improved function and prolonged service life of cutting tools, blades, and the like are upraised in various fields of industry and health care. From the point of sharpness, there is not only desire of having high hardness of the material that forms cutting tools and blades, but also requisite to refine the texture of the material for making a sharp cutting edge.
It is well known that the mechanical properties (such as hardness, strength) of metal material are largely influenced by the size of the diameter of the crystal which forms the metal. Generally, the smaller the diameter of crystal grain, the higher the mechanical properties of metal material. Although the methods for refining the crystal grain of the metal such as ECAP (Equal Channel Angular Pressing) or ARB (Accumulative Roll Bonding) etc. had been developed, (Japan Laid-open Patent Publication No. 2003-096551, Japan Laid-open Patent Publication No. 2000-073152), there still are problems that the refinement of ferrous material especially tool steel used for cutting tools and blades is extremely difficult. The technology of obtaining tool steel with microscopic texture by solidifying metallic powder of severe deformation has been published (New Energy and Industrial Technology Development Organization Nanometal technology project', report of “Reach and Development on Super Strengthened and Super Anticorrosive Tool Steel by Nano Texture Control”), however it is not easy to obtain a material having necessary size for making cutting tools and blades by this method.
Furthermore, at the condition that there are demands of high hardness, high strength, and high wear resistance for various tools, blades, or die and mold, the carbide generating elements such as Cr, Mo, W, V etc., are added into the base material of ferrous material which forms tools and the like. The carbides are separated and diversified in the base material. Because large carbides may lead the sharpness of the cutting tools and blades decline with the shortage of the service life, the refining of the carbides is also important in aspect to the improved function, and prolonged service life of the cutting tools and blades.
From the point of view mentioned above, there were inventors who devised a method to refine texture of metal material through using locally melting of material surface by laser beam. (Japan Laid-open Patent Publication No. 2005-146378). According to this technique, it is possible to refine the carbides in the surface layer part of the metal material. However, the refined carbides were separated from grain boundary of crystal grain of base metal material, and remarkably declined the strength of the grain boundary. Thus the desire of significantly improved properties and prolonged service life of cutting tools, and blades can not be achieved.
An aspect of the present invention is a method of modifying a ferrous material. The method includes a first step and a second step. The first step includes making carbide particles in a portion of a base material smaller. The second step includes nitriding at least a part of said portion.
Another aspect of the present invention is a ferrous material. The ferrous material contains a base material and an area. In this area, an average size of carbide particles is smaller than that in the base material. This area contains a nitrided portion.
Another aspect of the present invention is a ferrous material, which contains an area where a micro-Vickers hardness with a load of 100 g at a depth of 60 μm from a surface of the ferrous material is more than one half of a micro-Vickers hardness with a load of 100 g at a depth of 20 μm from a surface of the ferrous material. It is preferable that the micro-Vickers hardness with a load of 100 g at a depth of 20 μm from a surface of the ferrous material is more than 1000 Hv in said area.
The method for refining the texture of a ferrous material according to the present invention, comprises a first step in which the surface layer part in the ferrous material is locally and rapidly heated by a laser beam to form a melt reservoir which is then rapidly solidified to form a carbide refined region; and a second step in which the carbide refined region formed in the first step is subjected to a friction agitation process to form a texture refined region. Further, in the first step microplasma welding may be utilized during the surface layer part of the ferrous material is locally and rapidly heated as well as rapidly solidified.
The second step is a step that the carbide refined region formed in the first step is subjected to a friction agitation process. The said friction agitation process employs a friction agitation joining method which was devised in 1991 at TWI (The Welding Institute) England, as surface reforming method of metal material. The friction agitation joining is a kind of technique comprising press a rotating cylindrical tool at high speed into a joining region (a protruding called as “probe” is located on the bottom of the tool, press the said probe into); agitate a joined material softened by friction to complete the join while scanning along the direction of the joining region. In general the region that is agitated by rotating tool is called as “agitation part”, wherein mechanical properties are improved with homogeneity of material as well as decrease of crystal grain diameter by the joining condition. The technique which employs improvement of mechanical properties with homogeneity of material as well as decrease of crystal grain diameter by means of friction agitation for surface reforming, is friction agitation process, and is largely studied in recent years.
A tool steel having microscopic texture of the present invention as illustrating in
The blade of the present invention as illustrating in
The ferrous material 50 contains two areas, a refined area 54 and a non-refined area 56. The refined area 54 is a part of the base material 52, in which a refining process, such as the laser melting treatment and/or the friction stir processing as described above, was applied. In the case of
In the refined area 54, an average size of the carbide particles is smaller than that in the base material 52. More specifically, an average size of the carbide particles in the refined area 54 is smaller than that in the non-refined area 56. This enhances the strength, hardness and durability of the steel in the refined area 54. Furthermore, this also enhances permeability of nitrogen in the refined area 54 as described later.
In this respect, it is desirable that the average size of the carbide particles in the refined area 54 is less than one fifth of that in the non-refined area 56, which is base material 52 outside of the refined area 54. It is more desirable that the average size of the carbide particles in the refined area 54 is less than one tenth of that in the non-refined area 56. Such characteristic maximizes the above effect. One indicator of the average size of the carbide particles is an average area of the carbide particles observed on the vertical cross section of the ferrous material 50 under optical microscope or transmission electron microscope. Although not limited, a minimal ratio of the average size of the carbide particles in the refined area 54 to that in the non-refined area 56 can be set as one a hundred thousandth.
In the ferrous material 50, a nitrided portion 58 is formed on a surface part of the base material 52. The nitrided portion 58 is a part of the base material 52, in which nitrogen is doped into the base material 52. In this embodiment, the nitrided portion 58 is exposed to a surface of the ferrous material 50. This characteristic provides an ideal property as a blade after the ferrous material 50 is processed into a blade.
As shown in
In this respect, it is preferable that the depth of the nitrided portion 58a from the surface of the ferrous material is more than 1.2 times deeper than the depth of the nitrided portion 58b from the surface of the ferrous material. It is more preferable that the depth of the nitrided portion 58a from the surface of the ferrous material is more than 1.4 times deeper than the depth of the nitrided portion 58b from the surface of the ferrous material. Such characteristic is ideal for forming a blade from the ferrous material 50. The nitrided portion 58 is often visible on the vertical cross section of the ferrous material 50 under optical microscope because the color or texture of the nitrided portion 58 is often different from that outside of the nitrided portion 58 in the base material 52. In such case, one indicator to measure the depth of the nitrided portion 58 is to regard the portion where the color or the texture changes as a border of the nitrided portion 58. Although not limited, a maximum ratio of the depth of the nitrided portion 58a to that of the nitrided portion 58b can be set as 10.
The nitrided portion 58 contains nitride compounds. Of the nitride compounds, γ′-Fe4N is most preferred as a component of the nitrided portion 58 particularly in a crystal phase. In other word, it is desirable that the nitride compound contained in the nitrided portion 58 in a largest percentage is γ′-Fe4N. This compound provides better strength and durability. Major components of the nitrided portion 58 can be determined by X-ray diffraction.
The ferrous material 50 in this embodiment is ideal for forming into a blade because the nitrided portion 58a is very strong, hard and durable.
The nitrided portion 58a is located at the edge 62. In other word, the edge 62 is made of the nitrided portion 58a. This arrangement adds strength and durability to the edge 62. Thereby, the blade 60 can cut or slice an object more sharply. And, the blade 60 becomes dull less quickly.
A ferrous material should have an area that has following characteristics. 1) A micro-Vickers hardness close to the surface is large. 2) The micro-Vickers hardness decreases gradually as it goes deeper from the surface. Such ferrous material is optimal as a material for a blade because it adds strength, hardness and durability to the blade. In the previous embodiment, the nitrided portion 58a may have such characteristics. Also, some of the ferrous materials referred in
In the first view point, it is preferable that a micro-Vickers hardness at the depth of 20 is more than 1000 Hv. It is more preferable that a micro-Vickers hardness at the depth of 20 μm is more than 1100 Hv. Such property adds stability and sharpness to the ferrous material when it is formed into a blade.
In the second view point, it is preferable that a micro-Vickers hardness at a depth of 60 μm is more than one half of a micro-Vickers hardness at a depth of 20 μm. It is more preferable that a micro-Vickers hardness at a depth of 60 μm is more than two thirds of a micro-Vickers hardness at a depth of 20 μm. This property is a good indicator that the micro-Vickers hardness doesn't drop suddenly as it goes deeper from the surface. If the ferrous material fulfills such property, strength and durability of the ferrous material become larger.
In this respect, it is preferable that a micro-Vickers hardness at a depth of 60 μm is more than 500 Hv. It is more preferable that a micro-Vickers hardness at a depth of 60 μm is more than 667 Hv. If the ferrous material fulfills such property, it implies that the micro-Vickers hardness close to the surface is large and the micro-Vickers hardness decreases gradually as it goes deeper from the surface.
Furthermore, it is desirable that the ferrous material fulfills following properties. First, it is preferable that a micro-Vickers hardness at a depth of 100 μm is more than one half of a micro-Vickers hardness at a depth of 20 μm. In this respect, it is preferable that the micro-Vickers hardness at the depth of 100 μm is more than 500 Hv. Such ferrous material has a large micro-Vickers hardness in a deep portion of the ferrous material. Such property adds further strength and durability to the ferrous material.
It is desirable that a micro-Vickers hardness decreases gradually from a shallow point (20 μm) through a middle-depth point (60 μm) to a deep point (100 μm). A good indicator of such gradual decrease is that a micro-Vickers hardness at the depth of 60 μm is within ±20% of an average of a micro-Vickers hardness at the depth of 20 μm and a micro-Vickers hardness at the depth of 100 μm. Furthermore, it is preferable that a micro-Vickers hardness at the depth of 60 μm is within ±15% of an average of a micro-Vickers hardness at the depth of 20 μm and a micro-Vickers hardness at the depth of 100 μm. The ferrous material having such property is strong and resistant to breakage against mechanical force even if the ferrous material is shaped to be thin. Thus, such ferrous material is ideal as a material for a blade.
In the same viewpoint, it is preferable that a micro-Vickers hardness at the depth of 60 μm is less than 90% of a micro-Vickers hardness at the depth of 20 μm. It is also preferable that a micro-Vickers hardness at the depth of 100 μm is less than 70% of a micro-Vickers hardness at the depth of 20 μm. Although not limited, maximum values of micro-Vickers hardnesses at depths of 20, 60 and 100 μm can be set as 10000, 9000 and 7000 Hv respectively.
Above ferrous material can be obtained by a following method.
[Step 1] Modifying microstructures in a portion of the base material 52.
[Step 2] Nitriding at least a part of the portion where the microstructures were modified.
Actually, prior to initiating the first step, it is usual to prepare the base material 52. Thus, the base material 52 is explained first. A steel that contains carbon is optimally used as the base material 52. Furthermore, it is desirable that the steel contains chromium and molybdenum. Although other steels can be used, optimal steels for the base material 52 are tool steels, particularly cold work tool steels.
It is preferable that the base material 52 contains at least 1.2 mass % of carbon. It is more preferable that the base material 52 contains at least 1.4 mass % of carbon. Generally speaking, carbon reacts to nitrogen and forms compounds which reduce the strength of the steel. Contrary to the general knowledge, the inventors discovered that the structural change brought by the Step 1 prevents the steel from being weak even if it contains a large amount of carbon. Rather, it increases the strength of the steel even after the Step 2. Although not limited, a maximum content of carbon can be set as 15 mass %.
It is preferable that the base material 52 contains at least 10 mass % of chromium. It is more preferable that the base material 52 contains at least 11 mass % of chromium. While chromium increases permeability of nitrogen to be doped, it reacts with nitrogen and the formed compounds reduce strength of the steel. The inventors discovered that the structural change brought by the Step 1 prevents the steel from being weak. Therefore, a larger content of chromium does not decrease the strength of the steel but rather increases. Although not limited, a maximum content of chromium can be set as 40 mass %.
It is preferable that the base material 52 contains at most 1.5 mass % of molybdenum. It is more preferable that the base material 52 contains at most 1.0 mass % of molybdenum. While molybdenum increases permeability of nitrogen, it decreases corrosion and rust resistances. The inventors discovered that the structural change brought by the Step 1 facilitates nitrogen permeation in the steel without increasing the content of molybdenum. Although not limited, a minimum content of molybdenum can be set as 0.001 mass %.
This step makes crystal particles particularly carbide particles in a portion of the base material 52 smaller. In the case of
In this respect, it is desirable that by this step an average diameter of the particles is made less than one fifth of that in the base material 52 before this step. It is more desirable that by this step an average diameter of the particles is made less than one tenth of that in the base material 52 before this step. Such small particles maximize the above effect. Furthermore, it is preferable that the average diameter of the particles is made 10 nm-1 μm. One indicator of the average diameter of the particles is an average diameter of the particles observed on the vertical cross section of the ferrous material under an optical microscope or a transmission electron microscope. Although not limited, a minimal ratio of the average diameter of the particles before the Step 1 to after the Step 1 can be set as one ten thousandth.
Ways of making the carbide particles smaller include 1) melting and solidifying a portion of the base material 52, 2) applying a friction on a portion of the base material 52, and 3) welding a portion of the base material 52 by microplasma. The first method, melting and solidifying a portion of the base material, is preferable. As described later, it increases nitriding efficiency. It also increases micro-Vickers hardness combined with nitriding. This method was explained in detail above with
[Step 2] Nitriding at Least a Part of the Portion where the Microstructures were Modified.
This step dopes nitrogen in the base material 52. Thereby, the ferrous material 50 becomes stronger, harder and more durable. In the case of
One good way to nitride the base material 52 is to expose the base material 52 in an active gas containing a nitrogen atom. Examples of the gas containing a nitrogen atom include ammonia, ammonia derivatives, hydrazine and hydrazine derivatives. Examples of ammonia derivatives and hydrazine derivatives are compounds in which at least one of the hydrogen atoms of ammonia or hydrazine is replaced by a carbohydrate group. Ammonia is most preferably used for nitriding the base material 52. Ammonia diffuses efficiently in the base material 52. Thus, it can nitride the base material 52 with larger depth. In addition, it can nitride the base material 52 more homogeneously.
An inert gas can be mixed with the gas containing a nitrogen atom. Examples of the inert gas include nitrogen gas and argon gas. Ratio of the inert gas and the gas containing a nitrogen atom can be 1:10 to 10:1.
Nitriding can be preferably performed at 300-800° C. for 3-8 hours. In such condition, micro-Vickers hardness tends to be large at a portion close to the surface of the base material 52 and tends to decrease gradually as it goes deeper from the surface of the base material 52.
It is preferable to initiate the Step 2 within 72 hours after finishing the Step 1. It is more preferable to initiate the Step 2 within 24 hours after finishing the Step 1. This can bring more homogeneous nitriding.
Prior to the Step 2 after the Step 1, the base material 52 can be pretreated to activate its surface. An example of the pretreatment includes exposing the base material 52 in an acid gas such as hydrogen disulfide gas. This increases the efficiency of nitrogen doping.
After the Step 2, the ferrous material 50 can be formed into the blade 60 so that the part of the base material 52, whose microstructure was modified and which was nitrided, is located at the edge 62.
In the above embodiments, the ferrous material was used to form the blade. It is not to mention that the ferrous material of the above embodiments can be used for other purposes such as producing drill bits or other tools. The ferrous material in the above embodiments is suitably used as a material for producing articles, for which strength, hardness or durability is needed.
Examples of the present invention will be described below with reference to the accompanying drawings. The specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all modifications are intended to be included within the scope of present invention. Further, the treated material e.g. DC 53, used in the embodiments is general-purpose cold-work steel which is a kind of tool steel with excellent malleability.
In a DC 53 plate material, there is formed a carbide refined region by using semiconductor laser (output: 1 kW). The laser beam is just focused at the surface of the DC 53 plate material (the diameter of laser beam on the surface of the DC 53 plate material is about 1 mm), and the speed of the laser scan is 1000 mm/min. In order to make the carbide refined region formed by each laser scan at least overlap partially, the radiating position of the laser beam will vertically move a distance of 0.7 mm along the laser scan direction after each laser scan is finished, and performs totally 5 times of laser scan. The photo of the obtained sample is depicting in
Table 1 indicates Vickers hardness of the region melted and rapidly solidified by the radiation of laser beam from surface towards depth direction. Vickers hardness is measured under the condition that the loading is 2.94N (300 gf) with maintaining time of 15 seconds. The Vickers hardness of the untreated region is at level of 200-300 Hv, but the Vickers hardness of the region subjected to laser beam treatment is enhanced to around 500 Hv
The DC 53 plate material is subjected to laser beam treatment. After the carbide refined region is formed in the DC 53 plate material, the said carbide refined region is subjected to friction agitation process. A semiconductor laser (output: 1 kW) is used to form the carbide refined region, and is just focused on the surface of DC 53 plate material (the diameter of the laser beam on the surface of DC 53 plate material is about 1 mm). Yet the scanning speed of the laser is 1200 mm/min. In order to make the carbide refined region formed by each laser scan at least overlap partially, the radiating position of the laser beam will vertically move a distance of 0.7 mm along the laser scan direction after each laser scan is finished, and performs totally 15 times of laser scan. In the friction agitation process a super hard alloy tool which is cylinder shape with 10 mm of diameter is used. The said tool rotating at a speed of 400 rpm is pressed into the carbide refined region with 2600 kg of loading. The moving speed of the tool is 400 mm/min, and argon gas is flowed in to prevent the tool and the samples from oxidation. Moreover, the insert position of the tool is at the center of the carbide refined region; it should be noted that the untreated DC 53 plate material should not be agitate with the tool.
The DC 53 plate material is subjected to laser beam treatment. After the carbide refined region is formed in the DC 53 plate material, the said carbide refined region is subjected to friction agitation process. A semiconductor laser (output: 1 kW) is used to form the carbide refined region, and is just focused on the surface of DC 53 plate material (the diameter of the laser beam on the surface of DC 53 plate material is about 1 mm). Yet the scanning speed of the laser is 1200 mm/min. In order to make the carbide refined region formed by each laser scan at least overlap partially, the radiating position of the laser beam will vertically move a distance of 0.7 mm along the laser scan direction after each laser scan is finished, and performs totally 15 times of laser scan. In the friction agitation process a super hard alloy tool which is cylinder shape with 10 mm in diameter is used. The said tool rotating at a speed of 400 rpm is pressed into the carbide refined region with 2600 kg of loading. The moving speed of the tool is 400 mm/min, and argon gas is flowed in to prevent the tool and the samples from oxidation. Moreover, the insert position of the tool is adjusted to lead about half of the tool to touch the untreated DC 53 plate material from the carbide refined region; therefore the tool agitates the untreated DC 53 plate material as well as the carbide refined region simultaneously.
Further, because the friction agitation process is performed on laser beam treated region as well as untreated region simultaneously, a texture refined region may also exist beyond the carbide refined region. In addition, rougher and larger carbides may exist in surface vicinity of the texture refined region. It is regarded that rough and large carbides which exist in untreated DC 53 plate material by plastic flow due to the friction agitation process may mix into the texture refined region. In this embodiment because a cylindrical tool (without probe) is used in the friction agitation process, the press power of the tool is small for the carbide refined region and the influence of friction agitation can not extend to the whole area of the carbide refined region.
The DC 53 plate material is subjected to laser beam treatment. After the carbide refined region is formed in the DC 53 plate material, the said carbide refined region is subjected to friction agitation process. A semiconductor laser (output: 1 kW) is used to form the carbide refined region, and is just focused on the surface of DC 53 plate material (the diameter of the laser beam on the surface of DC 53 plate material is about 1 mm). Yet the scanning speed of the laser is 1200 mm/min. In order to make the carbide refined region formed by each laser scan at least overlap partially, the radiating position of the laser beam will vertically move a distance of 0.7 mm along the laser scan direction after each laser scan is finished, and performs totally 15 times of laser scan. In the friction agitation process a super hard alloy tool which is cylinder shape with 10 mm in diameter is used. The said tool rotating at a speed of 400 rpm is pressed into the carbide refined region with 2600 kg of loading. The moving speed of the tool is 400 min/min, and argon gas is flowed to avoid oxidation of the tool and the samples. After that, the region that is subjected to the friction agitation process (the texture refined region) is fabricated as a cutting edge, and then a plane is done. Again, a carbide refined region which is not subjected to the friction agitation process is fabricated as a cutting edge to make a plane for comparison.
A veneer board called LVL is cut with the fabricated plane to perform valuation of the characteristics of the plane. The cutting condition is as follow: cutting speed is 96 mm/min, cutting depth is 0.15 mm, angle of blade lathe is 35°, and angle of cutting edge of the blade is 31°. After cutting 5 pieces of LVL board in length of 1.8 m, observe the shape of cutting edge by optical microscope.
The DC 53 plate material is subjected to laser beam treatment. After the carbide refined region is formed in the DC 53 plate material, the said carbide refined region is subjected to friction agitation process. A semiconductor laser (output: 1 kW) is used to form the carbide refined region, and is just focused on the surface of DC 53 plate material (the diameter of the laser beam on the surface of DC 53 plate material is about 1 mm). Yet the scanning speed of the laser is 1200 mm/min. In order to make the carbide refined region formed by each laser scan at least overlap partially, the radiating position of the laser beam will vertically move a distance of 0.7 mm along the laser scan direction after each laser scan is finished, and performs totally 15 times of laser scan. In the friction agitation process a super hard alloy tool which is cylinder shape, 10 mm in diameter, is used. The said tool rotating at a speed of 400 rpm is pressed into the carbide refined region with 2600 kg of loading. The moving speed of the tool is 400 mm/min, and argon gas is flowed in to prevent the tool and the samples from oxidation. Afterward, the region subjected to the friction agitation process (the texture refined region) is fabricated as a cutting edge, and a blade (veneer slicer) for carpenter-use is made.
A cedar log is cut with the fabricated veneer slicer to perform the evaluation of the characteristics of the veneer slicer. The cutting condition is as follow: cutting speed is 23 mm/min, cutting depth is 0.3 mm, and angle of cutting edge of the blade is 20°. After cutting about 17 m, observe the shape of cutting edge by optical microscope.
The DC 53 plate material is subjected to laser beam treatment. After the carbide refined region is formed in the DC 53 plate material, the said carbide refined region is subjected to friction agitation process. A semiconductor laser (output: 1 kW) is used to form the carbide refined region, and is just focused on the surface of DC 53 plate material (the diameter of the laser on the surface of DC 53 plate material is about 1 mm). Yet the scanning speed of the laser is 1200 mm/min. In order to make the carbide refined region formed by each laser scan at least overlap partially, the radiating position of the laser beam will vertically move a distance of 0.7 mm along the laser scan direction after each laser scan is finished, and performs totally 15 times of laser scan. In the friction agitation process a super hard alloy tool which is cylinder shape, 10 mm in diameter, is used. The said tool rotating at a speed of 400 rpm is pressed into the carbide refined region with 2600 kg of loading. The moving speed of the tool is 400 mm/min, and argon gas is flowed in to prevent the tool and the samples from oxidation. Afterward, the region subjected to the friction agitation process (the texture refined region) is fabricated as a cutting edge, and then a scalpel is made.
General copy-paper (woodfree paper) is cut off by using the fabricated scalpel as well as scalpel on the market. Evaluation of the characteristics of the scalpels is performed by means of observing the amount of paper cut off and changes of cutting edge shape. A bundle of 950 g copy-paper of 210 pieces is put on the top of a scalpel (the angle between cutting edge and copy-paper is 15°). Calculate the number of pieces of the copy-paper cut off during the said bundle is moved at a speed of 3000 mm/min. Cut off test about one scalpel is performed 20 times continuously; the change of the number of pieces cut off is observed. Yet, Cut off test about one sort of scalpel is performed 6 times of the 20 times continuous cut off test.
Table 2 and Table 3 respectively indicate the number of pieces cut off concerning fabricated scalpel and scalpel on the market. As to the whole cut off test, the number of pieces cut off by the fabricated scalpel is more than the number of pieces cut off by the scalpel on the market. Further, the number of pieces cut off by the scalpel on the market decreases with increase of the number of times of the cut off test; on the contrary, the number of pieces cut off by the fabricated scalpel hardly decreases. From this result, it is demonstrated that the fabricated scalpel is not only sharp but also durable.
A commercially available plate of SKD11, which is the representative cold-work tool steel, was used. The chemical composition of the SKD11 is shown in Table 4.
The surface of the SKD11 plate was melted by multi-pass laser heating (1 kW, LASERLINE LDF-1000-750) to produce a rapidly solidified zone. The scanning rate of the laser beam and the beam diameter at the surface of the plate were 1000 mm/min and 1 mm, respectively. The overlap between the beam paths was 0.3 mm. The as-received SKD11 and the laser treated SKD11 were modified by friction stir processing (FSP). The FSP tool made of hard metal (WC—Co) had a columnar shape (Φ12 mm) without a probe. The shape of the tool end was flat. The FSP tool without a probe was effective to form the large treated area on the SKD11 plate which had a high plastic deformation resistance. A constant tool rotating rate of 400 rpm was adopted and the constant travel speed was 400 mm/min. A tool tilt angle of 3° was used. The process was conducted by a single pass. Nitriding was carried out using a mixture of nitrogen (flow rate: 1 L/h) and ammonia (flow rate: 3 L/h) at 540° C. for 5 h. Hydrogen disulfide gas was used to activate the surface of the SKD11 plate for 1 h at the beginning of the nitriding.
Transverse sections of the as-received and the variously treated SKD11 specimens were mounted and then mechanically polished. The microstructures of the samples were observed by optical microscopy and transmission electron microscope (TEM) (JEOL JEM-2100) at an accelerating voltage of 200 kV. The crystal phase of the samples was identified by X-ray diffraction (XRD) (Rigaku RINT2500V). The microhardness was measured using a micro-Vickers hardness tester (Akashi HM-124) with a load of 100 g.
The representative SKD11 plate treated by the combination of laser melting and FSP before nitriding is shown in
XRD patterns for the nitrided samples with and without the compound layer are shown in
Compared with the results of The First, Second, Third, Fourth, Fifth and Sixth Examples, it is expected that blades made from the laser-treated and nitrided SKD11 will be sharper, stronger and more durable than blades made from the nitrided SKD11 without pretreatment. Furthermore, it is also expected that blades made from the FSPed and nitrided SKD11 and blades made from the laser-treated, FSPed and nitrided SKD11 will be sharper, stronger and more durable than the blades made from the laser-treated and nitrided SKD11.
The microstructural control of the nitrided case on the SKD11 plate by laser melting and FSP was studied. The obtained results can be summarized as follows.
(1) The diffusion zone of uniform depth and microstructure without any local formation of the nitride particles can be obtained for the SKD11 by the combined pretreatment of laser melting and FSP.
(2) The crystal phase of the nitride is not influenced by the microstructural modification by laser melting and FSP. The compound layer is γ′-Fe4N, and the nitride particles in the diffusion zone are ε-Fe2-3N and CrN for all the nitrided samples.
(3) The microstructural modification of the SKD11 leads to differences in the thickness and the microhardness of the diffusion zone for the nitrided samples. The FSP before the nitriding increases the thickness and decreases the change in the microhardness from the surface.
