KR20130069545A - A method for surface treatment of a die-casting die - Google Patents

Abstract

The present invention does not substantially provide the mold with a nitrogen compound layer that causes heat check or abrasion, while introducing a large amount of nitrogen into the mold, resulting in a die cast mold having excellent heat check resistance and wear resistance. It is to provide a surface treatment method which can be performed. The method includes a nitriding treatment step of introducing a gas containing at least ammonia gas into a heating furnace to form a nitride layer including a compound layer composed of at least nitrogen compounds on a design surface of a die casting mold, and ammonia gas from the heating furnace. A compound decomposition step of decomposing the nitrogen compound by discharging the gas by introducing an atmosphere gas into the heating furnace and performing a heat treatment; and a shot peening treatment step of performing a short peening treatment on the surface of the mold. The thickness of the compound layer included in the nitrogen layer formed in the nitriding treatment step is in the range of 2 μm to 7 μm.

Description

Surface treatment method of die cast mold {A METHOD FOR SURFACE TREATMENT OF A DIE-CASTING DIE}

TECHNICAL FIELD This invention relates to the surface treatment method of the die-cast die provided by giving compressive residual stress to the design surface of a die-cast die using shot peening.

In diecast molding which repeats the molding cycle including injection of the molten metal, solidification, and mold removal of the molded article, a fine heat check is likely to be formed on the design surface of the diecast mold due to the thermal history imparted by the molding cycle. Therefore, wear due to mechanical contact is also likely to occur. Such heat checks grow into cracks or cracks, which damage the mold, and wear may degrade the dimensional accuracy of the molded article. Therefore, in order to improve heat check resistance and abrasion resistance and prolong the life of the product, treatments such as surface nitriding treatment for increasing the hardness of the design surface of the mold and short peening treatment for imparting compressive residual stress to the design surface are performed.

Surface nitriding in a metal mold | die is mainly performed by gas nitriding. The main reason is the ease of processing and the cost of processing. By gas nitriding, ammonia gas is decomposed under high temperature, and the generated nitrogen diffuses from the design surface of the mold into the mold, thereby providing a diffusion hardened layer.

The shot peening process in a metal mold | die is performed by accelerating the small sphere which consists mainly of ceramic or hard metal of 1 mm or less with a projection apparatus, and projects it on the design surface of a metal mold | die. Compression residual stress is given to the design surface of a metal mold | die based on the work hardening process by the collision of a small sphere.

For example, Patent Document 1 discloses that, first, surface nitriding treatment is performed on a design surface of a mold to form a nitrogen diffusion hardened layer, followed by shot peening to impart high compressive residual stress to the surface of the diffusion hardened layer. It is. By combining the surface nitriding treatment and the shot peening treatment, the product life of the mold can be significantly extended.

It is known that the nitriding treatment forms a compound layer lacking plastic deformability on the surface of the nitrogen diffusion hardened layer. Since such a compound layer causes crack or crack growth by heat check, and wear by peeling, the nitriding treatment method which prevents formation of a compound layer or forms it as thin as possible is proposed.

For example, Patent Literature 2 discloses a two-stage treatment in which ammonia gas nitriding is first performed in a relatively low temperature range of 450 to 530 ° C. Then, while supply of ammonia is reduced or stopped, nitrogen diffuses internally in the process temperature range of 550-590 degreeC. In general, ammonia gas nitriding under a relatively low temperature range forms a thin compound layer. However, the depth of the nitrogen diffusion layer also becomes shallow. Accordingly, nitrogen in the nitrogen diffusion layer is diffused deeply into the mold to provide a thick nitrogen diffusion layer, while keeping the compound layer thin.

Similarly, Patent Document 3 discloses a two-stage treatment in which ammonia gas nitriding treatment is first performed under a reduced pressure at a temperature of less than 570 ° C. Then, while supply of ammonia is reduced or stopped, nitrogen diffuses internally in the process temperature range of 570 degreeC-650 degreeC. Patent document 3 describes that the nitrogen compound layer can be made thin and non-porous, while the depth of the nitrogen compound layer can be made deeper than the depth of the nitrogen diffusion layer by heat treatment.

Japanese Laid-Open Patent Publication No. 2004-148362 Japanese Unexamined Patent Publication No. 10-306364 Japanese Patent Application Laid-Open No. 11-100655

As disclosed in Patent Literatures 2 and 3, in the ammonia gas nitriding process for forming the nitrogen compound layer thinly, since the absolute amount of nitrogen supplied to the mold is small, it is intended to diffuse the nitrogen in the nitrogen diffusion layer deeper into the mold by heat treatment. In this case, the nitrogen compound layer cannot have sufficient hardness.

This invention is made | formed in view of such a situation. It is an object of the present invention to provide a surface treatment method capable of introducing a large amount of nitrogen into a mold while substantially not providing a nitrogen compound layer that causes heat check or wear to the mold. As a result, a die cast mold excellent in heat check resistance and wear resistance can be produced.

The inventors of the present invention have found that the outermost layer made of nitrogen compounds formed by various nitriding treatments such as gas soft nitriding treatment, gas immersion nitriding treatment, and plasma nitriding treatment can be easily decomposed by heat treatment. While investigating a method for manufacturing a mold that does not substantially provide a nitrogen compound layer to the mold, the inventors of the present invention found that nitrogen is generated by the decomposition, and the amount of nitrogen supplied to the mold is diffused by dispersing the generated nitrogen into the mold. It was designed to increase.

Therefore, in the die-casting surface treatment method of the present invention provided by applying a compressive residual stress to the design surface of a die-cast die by using a shot peening treatment, the die is introduced by introducing a gas containing at least ammonia gas into a heating furnace. A nitriding treatment step of forming a nitriding layer on the design surface of the cast die, the nitride layer comprising a compound layer made of at least nitrogen compounds by nitriding treatment, for example, gas soft nitriding, gas immersion nitriding, and plasma nitriding; A compound decomposition step of decomposing the nitrogen compound by discharging the ammonia gas and introducing an atmosphere gas into the heating furnace to perform a heat treatment, and a short peening treatment step of performing a short peening treatment on the surface of the mold. The thickness of the compound layer included in the nitrogen layer formed in the nitriding treatment step is in the range of 2 μm to 7 μm.

According to this method, by introducing a gas containing at least ammonia gas into the heating furnace, the thickness of the nitrogen compound layer formed by nitriding is controlled to a predetermined thickness, so that the nitrogen compound is decomposed in the compound decomposition step. As a result, the nitrogen compound layer is substantially not provided. Therefore, nitrogen is generated instead and the amount of nitrogen supplied to the mold is increased, thereby providing a nitrogen diffusion layer having a high hardness. However, the nitrogen compound layer substantially lost remains as a layer containing a large amount of voids, so as to absorb and disperse the impact energy of the shot in the shot peening treatment. However, in the nitriding treatment step, the thickness of this nitrogen compound layer is also controlled to a predetermined thickness so that the compressive residual stress due to the shot peening treatment can be applied to the nitrogen diffusion layer. Therefore, it is possible to manufacture a die-cast die having excellent wear resistance and heat check resistance based on high hardness and high compressive residual stress.

In the above method, the compound decomposition step is preferably performed at a temperature lower than the temperature of the nitriding treatment step. In this case, higher hardness and higher compressive residual stress can be applied to the die cast mold, and a die cast mold having better wear resistance and heat check resistance can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS It is a perspective view of one test piece used for embodiment of the surface treatment method of the die cast metal mold of this invention.
It is a figure which shows the steel grade and surface treatment conditions of the test piece shown in FIG.
It is a figure which shows an example of the heating and cooling test apparatus used for embodiment of the surface treatment method of the die-cast die of this invention.
4 is a change in the vicinity of the surface of the test piece after the nitriding treatment step (FIG. 4A) and a change in the vicinity of the surface of the test piece after the compound decomposition step (FIG. 4) in the die-casting surface treatment method of the present invention. It is an expanded sectional drawing which shows (b)).
FIG. 5 is an enlarged cross-sectional view illustrating a change in nitrogen concentration near the surface of the test piece of FIGS. 4A and 4B.
6 is a graph showing the relationship between the compound layer thickness and the number of heat checks (HC) for a test piece to which the surface treatment method of the die cast mold of the present invention is applied.
7 is a graph showing the relationship between the compound layer thickness and the residual stress for the test piece to which the shot peening treatment step is applied in the surface treatment method of the die cast mold of the present invention.
8 is a cross-sectional photograph of a test piece (eleventh embodiment) after the nitriding treatment step.

The surface treatment method of the die cast die of the present invention will be described in detail with reference to the results of the demonstration test as shown in FIGS. 1 to 8. In the proof test, cylindrical test pieces 1 (see Fig. 1) corresponding to die cast molds are prepared, and various surface treatments are performed to evaluate them.

A cylindrical test piece 1 having an outer diameter D1 of 15 mm, an inner diameter D2 of 3 mm, and a length L of 20 mm is prepared. Each test piece 1 is processed from a round bar stock of Japanese Industrial Standards (JIS), which is an alloy tool steel equivalent to SKD61. Instead of the SKD61 equivalent, the test piece 1 in the ninth and tenth embodiments is processed from a round bar stock equivalent to SKD7 and an alloy tool steel equivalent to SKH51. Steel classes of the embodiment and the comparative example are collectively shown in FIG. 2.

Next, ammonia gas is introduce | transduced into a heating furnace, heating each test piece 1 in a heating furnace, and the gas soft nitriding process of the peripheral surface of the test piece 1 is carried out (nitriding process step). Instead of the gas soft nitriding treatment, the test piece of the sixth embodiment is subjected to gas percolation nitriding treatment. In addition, all the test pieces of a 7th embodiment and a 5th comparative example are plasma nitridated. In embodiment and a comparative example, the kind of nitriding process, the kind of gas, temperature, and time are put together in Table 2, and are shown.

Next, after discharging the ammonia gas from the inside of the heating furnace, nitrogen is introduced into the heating furnace as the atmosphere gas, while the test piece is continuously heated and diffused in the heating furnace. Therefore, the nitrogen compound is completely decomposed in the compound layer 2 (refer to FIG. 4) generated by the nitriding treatment as described later (compound decomposition step). The temperature and time of the diffusion treatment are also collectively shown in FIG. 2.

For example, a small sphere made of an amorphous alloy each having a diameter of 0.05 mm to 0.2 mm is projected on the outer circumferential surface of the test piece 1 at a projection pressure of 0.3 MPa to perform a shot peening treatment (short peening treatment step).

About the test piece 1 processed by the said process, the residual stress of the outer peripheral surface near a longitudinal center part is measured.

In addition, as shown in FIG. 3, heat check resistance is evaluated by repeatedly performing a heating and cooling test on each test piece 1 with the test apparatus 20. More specifically, the small diameter portion 22a of the support member 22 of the test apparatus 20 is inserted into the through hole 1a of the test piece 1, and the test piece 1 is placed from the top and the bottom of the holder ( 23) to fix it in place. The outer circumferential surface of the test piece 1 is heated from room temperature to 700 ° C. for 4 seconds with the high frequency coil 21. Then, the cooling water 24 is sprayed from a water jet (jet orifice) to cool the test piece 1 to room temperature for 3 seconds. Next, the heated and cooled test piece 1 is dried by air blow for 1 second. This cycle of heating, cooling, and drying is repeated 1000 times in total, and the test piece 1 is removed from the test apparatus 20. The test piece 1 removed from the test apparatus 20 cuts the vicinity of the center part of the longitudinal direction to the plane perpendicular | vertical to the center axis. Then, the test piece 1 is embedded in resin, and the cut surface is mirror polished. The cut surface of a test piece is observed using a 100-fold optical microscope, and the number of heat checks HC generated on the outer peripheral surface of the test piece 1 is measured.

In addition, a part of test piece 1 after said nitriding process is removed from a heating furnace, and the thickness of the compound layer 2 (refer FIG. 4) is measured as mentioned later. The test piece 1 removed from the heating furnace is cut in a plane perpendicular to the central axis near the longitudinal center part thereof. Then, the cut surface of the cut test piece 1 is mirror polished. Next, the polished cut surface is observed with an optical microscope, and the thickness of the compound layer 2 is measured.

In the nitriding treatment, as shown in Figs. 4A and 5A, activated nitrogen in the gas phase diffuses from the outer peripheral surface of the test piece 1 to the inside (4, base material), The nitride layer 5 is formed in the vicinity of the outer circumferential surface. The nitride layer 5 consists of the nitrogen compound layer 2 which is the outermost layer, and the nitrogen diffusion layer 3 inside it. The compound layer 2 consists of a composite nitride of Fe and Cr, and is a very weak layer. For reference, the growth rate of the compound layer in the plasma nitriding treatment is much slower than the growth rate of the compound layer in the gas nitriding treatment. The nitrogen diffusion layer 3 is a solid solution layer of nitrogen containing dispersed and precipitated nitride.

In the diffusion process subsequent to the above-mentioned nitriding treatment, as shown in FIGS. 4B and 5B, the depth of the nitride layer 5 is enlarged. More specifically, the flux of nitrogen supplied to the test piece 1 from the gas phase through the outer circumferential surface decreases, and the nitrogen of the nitrogen diffusion layer 3 mainly diffuses into the test piece 1. At this time, when the nitrogen compound of the compound layer 2 is decomposed and nitrogen is generated, the generated nitrogen also diffuses into the test piece 1. The concentration of nitrogen contained in the compound (see symbol '3a' of FIG. 5 (a)) is the concentration of nitrogen contained in nitrogen solid solution such as nitrogen diffusion layer 3 (see symbol '3b' of FIG. 5 (b)). Since it is much higher than), a nitrogen diffusion layer 3 containing a large amount of nitrogen can be obtained (see reference numeral 31 in Fig. 5B). In Fig. 5B, reference numeral 32 denotes a case where only the nitrogen diffusion layer 3 obtained only by the nitriding treatment is diffused.

On the other hand, when the composite nitride of the compound layer 2 decomposes, it becomes the surface layer 2 'containing a large amount of voids due to the volume shrinkage. This surface layer 2 'absorbs and dissipates the collision energy of the shot in the shot peening treatment, and the formation of the compressive residual stress is inhibited. Details thereof will be described later.

The result of the above measurement is demonstrated below. 6 shows a relationship between the number of heat checks HC and the thickness of the compound layer 2 after repeated heating and cooling tests.

It can be seen that the number of heat checks decreases as the thickness of the compound layer 2 is increased, thereby improving the heat check resistance. That is, the thin compound layer 2 with a thickness of 1.5 mu m in the first comparative example has 597 heat checks, and the thin compound layer 2 with a thickness of 1.0 mu m in the fifth comparative example has 441 heat checks. In contrast, the thick compound layer 2 having a thickness of 2 μm to 7 μm in the first to fourteenth embodiments is greatly reduced to 13 to 257 heat checks.

In particular, the heat check number in the eleventh embodiment in which the diffusion treatment (heating treatment) is performed at a temperature lower than the temperature of the nitriding treatment is greatly reduced. Therefore, it can be seen that the heat treatment in the compound decomposition step is preferably performed at a temperature lower than the temperature of the nitriding treatment step.

As described above, as the thickness of the compound layer 2 is increased, the amount of nitrogen compounds decomposed by the diffusion treatment increases, so that the amount of nitrogen in the nitrogen diffusion layer 3 can be increased, and the diffusion treatment Abrasion resistance can be improved by increasing after hardness, and heat check resistance can be improved.

On the other hand, it can be seen that the number of heat checks increases rapidly as the thickness of the compound layer 2 is increased to a predetermined thickness or more, so that the heat check resistance can be greatly reduced. That is, the number of heat checks by formation of the thick compound layer 2 of thickness 8.0 micrometers, 9.0 micrometers, and 10.0 micrometers in the 1st, 2nd, and 4th comparative example is 706, 707, and 840, respectively. This sharply increased in comparison with the first to fourteenth embodiments.

Regarding the above, Fig. 7 shows a relationship between the residual stress applied to the test piece 1 by the shot peening treatment and the thickness of the compound layer 2. In FIG. 7, the compressive stress is represented by a negative value.

It can be seen that the absolute value of the compressive residual stress may be increased by increasing the thickness of the compound layer 2. That is, the compressive residual stress due to the formation of the thin compound layer 2 having a thickness of 1.5 m and 1.0 m in the first and fifth comparative examples is -965 MPa and -993 MPa, respectively. In contrast, the compressive residual stress due to the formation of the thick compound layer 2 having a thickness of 2 μm to 7 μm in the first to fourteenth embodiments greatly increased from -1350 MPa to -1755 MPa. This was significantly increased compared to the first and fifth comparative examples.

On the other hand, it can be seen that the absolute value of the compressive residual stress may decrease rapidly as the thickness of the compound layer 2 is increased to a predetermined thickness or more. That is, the compressive residual stress due to the formation of the thick compound layer 2 having a thickness of 8.0 μm, 9.0 μm, and 10.0 μm in the second, third, and fourth comparative examples is -1298 MPa, -1251 MPa, and -938 MPa, respectively. .

As mentioned above, when the thickness of the compound layer 2 is increased more than predetermined thickness, the absolute value of a compressive residual stress will fall largely, and therefore, the heat resistance check resistance will fall largely. This is due to the surface layer 2 '(see FIG. 4B) containing a large amount of voids formed by decomposition of the compound of the compound layer. More specifically, by forming the thick compound layer 2, the surface layer 2 'is thickened by the diffusion treatment, and the formation of the compressive residual stress due to the shot peening treatment is rapidly inhibited. It is greatly reduced.

FIG. 8A is an optical micrograph of the cut surface of the test piece 1 after nitriding treatment in the eleventh embodiment. FIG. 8B is an optical micrograph of the cut surface of the test piece 1 after diffusion treatment following the nitriding treatment in the eleventh embodiment. In the former, the compound layer 2 and the nitrogen diffusion layer 3 can be observed. In the latter, it is possible to observe the nitrogen diffusion layer 3 having an increased thickness, and the surface layer 2 'including black pores, particularly near the nitrogen diffusion layer 3, by decomposing the compound.

Based on the above, by limiting the thickness of the compound layer 2 and hence the thickness of the surface layer 2 'including the voids remaining after the compound is decomposed, the compressive residual stress due to the shot peening treatment is good. In addition, it is possible to obtain a die cast mold having excellent heat resistance. In particular, under the general conditions of the shot peening treatment as in the embodiment, the thickness of the compound layer 2 (surface layer 2 'remaining after the compound is decomposed) for achieving the present object is 2-7 mu m.

Regardless of the difference between the types of nitriding treatments such as gas soft nitriding, gas immersion nitriding, and plasma nitriding, as in the sixth and seventh embodiments, the thickness and heat check number of the compound layer 2 of FIGS. 6 and 7 or The relationship between the compressive residual stresses is aligned on the same plot line. Moreover, in 9th and 10th embodiment, it does not matter also with the difference between the steel materials of SKD61 equivalent material, SKD7 equivalent material, and SKH51 equivalent material. In other words, these results suggest that the method of the present invention can be applied irrespective of differences between kinds of nitriding treatments and differences between general mold materials.

As mentioned above, embodiment of the surface treatment method of the die-cast die of this invention introduce | transduces the gas containing at least ammonia gas into a heating furnace, and nitriding process, for example, gas soft nitriding, to the design surface of a die-cast die. And a nitriding treatment step of forming a nitride layer including at least a compound layer made of a nitrogen compound by gas immersion nitriding and plasma nitriding, and ammonia gas is discharged from the heating furnace, and an atmospheric gas is introduced into the heating furnace to perform the heat treatment. Thereby including a compound decomposition step of decomposing the nitrogen compound and a shot peening treatment step of performing a shot peening treatment on the surface of the mold. The thickness of the compound layer included in the nitrogen layer formed in the nitriding treatment step is in the range of 2 μm to 7 μm.

By introducing a gas containing at least ammonia gas into the heating furnace, the thickness of the nitrogen compound layer formed by the nitriding treatment is maintained at a predetermined thickness so that the nitrogen compound is decomposed in the compound decomposition step. As a result, the nitrogen compound layer is substantially not provided, and thus nitrogen can be generated instead to increase the amount of nitrogen supplied to the mold, thereby providing a nitrogen diffusion layer having a high hardness. However, the nitrogen compound layer substantially lost remains as a layer containing a large amount of voids, so as to absorb and disperse the impact energy of the shot in the shot peening treatment. However, in the nitriding treatment step, the thickness of the nitrogen compound layer is also maintained at a predetermined thickness so that the compressive residual stress due to the shot peening treatment can be imparted to the nitrogen diffusion layer. Therefore, it is possible to manufacture a die-cast die having excellent wear resistance and heat check resistance based on high hardness and high compressive residual stress.

Representative embodiments of the present invention and modifications thereof have been described, but the present invention is not limited thereto. Those skilled in the art will be able to find various alternative embodiments and modifications without departing from the scope of the appended claims.

For example, the foregoing description refers to specific shapes and dimensions of each test piece, but this is only intended to facilitate the description and is not intended to limit the invention.

1: test piece
2: compound layer
3: nitrogen diffusion layer
20: test device

Claims (2)

A surface treatment method of a die cast mold provided by applying compressive residual stress to the design surface of the die cast mold using shot peening treatment,
A nitriding treatment step of introducing a gas containing at least ammonia gas into a heating furnace to form a nitride layer including a compound layer made of at least a nitrogen compound on a design surface of the die cast mold;
A compound decomposition step of decomposing the nitrogen compound by discharging ammonia gas from the heating furnace and introducing an atmosphere gas into the heating furnace to perform a heat treatment;
A short peening treatment step of performing a short peening treatment on the surface of the mold;
The thickness of the compound layer included in the nitrogen layer formed in the nitriding treatment step is in the range of 2㎛ to 7㎛ range surface treatment method of a die cast die. The method of claim 1,
In the compound decomposition step, the heat treatment is performed at a temperature lower than the temperature of the nitriding treatment step.

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