JPH0128809B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface treatment method for improving the properties of high-speed steel, particularly its cutting performance.

General high-speed steel contains a large amount of primary carbides made of alloying elements such as W, Mo, Cr, and V (around 10% by weight in the normal quenched state), which has a great effect on improving cutting performance. is giving. Many studies on improving machinability have focused on the form of the primary carbide described above, and can basically be divided into the following two streams.

(1) Increasing the amount of primary carbides Adding large amounts of carbide-forming elements such as Mo, W, and V along with a corresponding amount of C to increase the amount of primary carbides to improve wear resistance. It is said that V has the greatest effect in forming hard MC type carbides. In 1968, according to the JIS standard, high C-high V series SKH53, SKH54,
SKH57 etc. were added, and after that, products with further increased or adjusted amounts of alloying elements are being developed.
However, in any case, products with increased primary carbides tend to cause segregation, and angular coarse carbides also increase, resulting in an extreme deterioration of toughness.Furthermore, the amount of alloying elements is large, resulting in a rise in price. For this reason, it is currently only used for special purposes (machining difficult-to-cut materials).

(2) Reduction of segregation and refinement of primary carbides In order to improve the cutting performance of high-speed steel,
In addition to wear resistance, it is also necessary to improve toughness. In particular, for steels containing a large amount of primary carbides, such as high-V high-speed steels, improving toughness is particularly important, and for this purpose, it is necessary to reduce the segregation of primary carbides and refine the crystals. It is valid. The ESR method and the powder metallurgy method are typical improvement measures that follow this basic idea. However, in the former ESR method, 1
Although the segregation of secondary carbides is considerably suppressed, the carbide grain size tends to become coarser, and it is less effective for small parts that originally have a uniform distribution of carbides, and the manufacturing process is complicated, so the price rises. In addition, in the case of the latter powder metallurgy method, if there is no segregation of primary carbides and the particles are made finer, toughness can be considerably improved, but if the composition is the same as that of ordinary melted materials, wear resistance tends to deteriorate. However, the manufacturing process is complicated and the product cost is high.

As described above, in the conventional method, when trying to improve the wear resistance of high-speed steel, toughness is sacrificed, and when trying to improve toughness, economical problems arise. Therefore, there is a need for the development of a technology that can increase wear resistance without deteriorating toughness and that can be produced easily and at low cost.

Considering these circumstances, the present inventors believed that it would be difficult to achieve essential improvements only by adjusting the amount of alloy components, and therefore aimed to improve the surface properties by devising a post-forming surface treatment method. We have been conducting intensive research. The present invention was completed as a result of such research, and its configuration consists of a laser beam applied to the surface of hardened high-speed steel, an absorbed energy density of 2000 J/cm ² or more, and a relative movement between the beam and the workpiece. Speed (hereinafter referred to as beam movement speed) 2 to 5 m/
The gist is that the steel surface is melted by irradiation for more than 10 minutes, the primary carbides in the surface layer are dissolved in the matrix, and then the solid-dissolved carbides are finely precipitated by tempering.

In the present invention, after the high-speed steel is once quenched,
By irradiating the surface with a high-energy-density laser beam at a moving speed higher than a certain speed, the primary carbide on the surface is dissolved in the matrix, and it contains almost no primary carbide that has a negative effect on toughness. In addition to forming a surface layer, the solid-dissolved carbide is finely precipitated by subsequent tempering, and the amount of secondary hardening is greatly increased, thereby improving wear resistance and ultimately cutting performance. .

The structure and effects of the present invention will be clarified below as the experiment progresses.

Lasers generally have very high energy density and can locally heat and cool the surface of a material. In particular, the structure of the rapidly solidified layer obtained by melting only the surface is significantly different from the normal heat-treated structure in terms of the morphology of primary carbides, solid solution precipitation state of alloying elements, crystal grain size, etc. is expected to have the potential to significantly improve mechanical properties including hardness. Therefore, first of all, as high-speed steel,
A hardened material of SKH9 was selected, and the internal structure of the laser beam irradiation area and the unaffected base material area were compared using an optical microscope when laser beam irradiation treatment was performed under the following conditions.

[Irradiation conditions]

Heat source: CO ₂ laser output: 3KW to 5KW Energy density: 2300 to 4300J/cm ² Beam movement speed: 5m/sec Cooling: Natural cooling The results are shown in Figure 1 (a photomicrograph in place of a drawing of the laser irradiation area). As shown in Fig. 2 (micrograph substituted for drawing of the base metal), primary carbides, which are often observed in the normal heat-treated material (base material: Fig. 2), are found in the laser irradiated part (Fig. 1). It has completely disappeared, and the primary carbide is dissolved in the matrix by laser irradiation, resulting in an extremely uniform crystal structure.

Further, when the relationship between the tempering temperature and the secondary hardening of this laser irradiation treated material was investigated, the results shown in FIG. 3 were obtained. This figure shows that in the part of the base material that is not affected by laser irradiation, the hardness decreases as the tempering temperature increases, especially when the tempering temperature is about
When the temperature exceeds 600℃, the hardness decreases rapidly. On the other hand, a different tendency was observed in the laser irradiated area.
Tempering at 500 to 600°C caused rapid secondary hardening, and the tempering hardness of the final product surface layer (laser irradiation area) increased to Hv1100, and it was confirmed that the tempering softening resistance was also excellent. Figure 4 shows the tempering temperature when the laser beam output is 3KW or 5KW.
The same process as above was performed except that the temperature was set at 550℃.
This is an experimental graph that investigated the relationship between distance from the surface and hardness. From this figure, a surface hardening layer is obtained over a depth of about 700 μm from the laser irradiated surface when the laser output is 3KW, and about 1200 μm when the laser power is 5KW, and as the laser power is increased, the secondary hardening layer becomes deeper. Can be done. However, the hardness of the hardened layer shows a constant value of approximately Hv1100, and almost no influence of laser output on hardness is observed.

Figure 5 is an experimental graph comparing the wear resistance of the hardened surface layer (laser treated area) and base metal (normally heat treated area) for high cutting steel that was hardened at 550℃ for 1 hour after laser irradiation. . The test conditions were as follows.

〔Test conditions〕

Test method: Okoshi type wear test Compatible material: SUJ2, 1.5mm width x 30mm diameter (H _R C =
60) Friction distance: 400m Load: 3.3Kg As is clear from this figure, the surface hardening layer created by laser irradiation showed excellent wear resistance, especially in the high-speed wear region.

FIG. 6 is an experimental graph obtained by conducting a continuous cutting test under the following conditions using a cutting tool made using the laser irradiated material of the present invention and a conventional heat-treated material.

[Cutting conditions]

Bit installation angle: 0-15-6-6-15-15-
R0.4 Overhang: 34mm Depth of cut: 1.5mm Feed: 0.2mm Work material: SNCM8 (H _R C32) Lubrication: None Life judgment: Complete life Figure 7 shows similar cutting conditions (however, the workpiece The material was grooved with a width of 10 mm at every 90° angle, and the life judgment was performed at V _B max = 0.6 mm).

As is clear from FIGS. 6 and 7, the material subjected to laser irradiation and tempering according to the method of the present invention has superior machinability compared to conventional heat-treated materials. It can be seen that it has excellent performance especially in interrupted cutting where a high degree of toughness is required.

As described above, in the present invention, by irradiating the surface with a laser beam after the hardening process, the precipitation of coarse primary carbides in the surface layer is eliminated, and the subsequent tempering process at a specific temperature significantly reduces the secondary hardening. This can significantly improve machinability by increasing both hardness and toughness. However, these effects tend to vary ^considerably depending on the absorbed energy density and beam movement speed during laser beam irradiation. Must be set to at least 1 minute. However, the absorbed energy density refers to a value calculated from the following formula.

Absorbed energy density (J/cm ² ) = Output (KW) x 3.6 x 10
⁶ / [Beam movement speed (cm/min) x Beam diameter (cm) x 60
(minutes)] x absorption rate The reason why the laser irradiation conditions were set in this way is as follows. That is, regarding the absorbed energy density, it was clarified in Figure 5 above that the larger the density, the deeper the hardened layer can be. When we examined this in more detail, we obtained the results shown in Figure 8. As is clear from this figure, in order to form a hardened layer on the surface layer by laser irradiation, the absorbed energy density must be at least 2000 J/cm ² or more. If the absorbed energy density is higher than this, the hardened layer after tempering will become deeper and the machinability will improve, but this irradiation effect reaches a saturation state at about 4000 J/ ^cm2 , and if the absorbed energy density is higher than that, the hardened layer will become deeper and the machinability will improve. Increasing the output power of the laser generator merely increases the output power of the laser generator. On the other hand, Figure 9 shows
This is an experimental graph showing the relationship between the beam movement speed, the volume fraction of carbide as-treated by the laser, and the hardness when the subsequent tempering treatment (550°C x 1 hr) is performed. In order to make significant use of this, the beam movement speed must be set to 200cm/min or higher. However, if the beam movement speed is too fast, it will not be possible to provide a predetermined absorption energy density to the laser irradiated area, and a cured layer with satisfactory hardness will not be obtained. Moreover, not only is the depth of the hardened layer insufficient, but also the surface tends to become uneven, and when these unevenness are removed by polishing during finishing, the hardened layer becomes even shallower. For this reason, the beam movement speed must be kept below 500 cm/min.

By the way, the technology of using laser beam irradiation to increase the surface hardness of high-speed steel was disclosed in Japanese Patent Application Laid-Open No. 1986-
It is also disclosed in Publication No. 62119. However, as is clear from the description in the published specification, this technique involves irradiating the surface of a hardened and tempered high-speed steel tool with a laser beam to rapidly heat and self-cool it, and the present invention is similar to that of the present invention. After quenching, laser irradiation is performed to make the structure of the surface layer almost free of carbides [carbide size: 1 μm or less: Fig. 1,
The same volume ratio is 2% or less: Fig. 9], and there is no idea of dramatically increasing the surface hardness by secondary hardening by tempering treatment after laser irradiation. Incidentally, in the present invention, it is essential to set the beam movement speed to 200 to 500 cm/min or more in order to obtain the above-mentioned tissue, but the same movement speed shown in the disclosed invention is extremely slow at 40 cm/min. Even if the surface hardness of the final product is determined by the disclosed invention, it is at most H _R
C69 (about 1000 Hv), whereas in the present invention
It can be raised to about Hv1100.

The present invention is roughly constructed as described above, and it has become possible to provide a high-speed steel that can significantly improve hardness without reducing toughness and has excellent cutting performance.

[Brief explanation of drawings]

Figures 1 and 2 are micrographs that are used as drawings to show the steel structure, Figure 3 is a graph that shows the relationship between tempering temperature and hardness, and Figure 4 is a graph that shows the relationship between distance from the laser irradiation surface and hardness. , Figure 5 is a graph showing the relationship between friction speed and specific wear amount, Figure 6 is a graph showing the relationship between impact number and cutting speed, Figure 7 is a graph showing cutting speed and cutting time, and Figure 8 is absorption. FIG. 9 is a graph showing the relationship between the energy density and the depth of the molten layer (hardened layer), and FIG. 9 is a graph showing the relationship between the beam moving speed, hardness, and carbide volume fraction.

Claims (1)

[Claims] 1 The surface of hardened high-speed steel is irradiated with a laser beam at an absorbed energy density of 2000 J/cm ² or more and a beam movement speed of 2 to 5 m/min to melt the steel surface and transform the primary carbide in the surface layer into a matrix. A method for surface treatment of high-speed steel, characterized by finely precipitating carbides dissolved in the solid solution by dissolving the carbides in the solid solution, and then subjecting the carbides to a tempering treatment.