JP2011235318A - Method for surface treatment of die-casting die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To substantially provide a nitrogen compound layer that causes heat check and wear, and on the other hand, a large amount of nitrogen can be introduced into a mold, and as a result, heat check resistance and wear resistance are excellent. To provide a surface treatment method capable of giving a die casting mold.
A nitriding 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 a nitrogen compound on the mold design surface, and discharging the ammonia gas from the heating furnace. It includes a compound decomposition step of introducing an atmospheric gas and heat-treating to decompose the nitrogen compound, and a shot peening step of performing shot peening on the mold design surface. Here, the nitriding step is a step of forming a nitride layer including at least a compound layer having a thickness in the range of 2 to 7 microns.
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Description

  The present invention relates to a surface treatment method for a die casting mold provided by applying compressive residual stress to a mold design surface by shot peening.

  In die-cast molding, in which molten metal is injected, solidified, and the molding cycle is repeated, fine heat checks (thermal cracks) are likely to occur on the design surface of the mold due to the thermal history given by the molding cycle, and mechanical contact occurs. Wear is also likely to occur. Such a heat check develops into cracks and damages the mold, and wear reduces the dimensional accuracy of the molded product. Therefore, surface nitriding treatment that increases the hardness of the mold design surface and shot peening that applies compressive residual stress are performed to improve the heat check resistance and wear resistance and extend the life of the mold. Can be broken.

  The surface nitriding treatment in the mold is often performed by gas nitriding mainly from the viewpoint of ease of processing and cost. In such a method, ammonia gas is decomposed at a high temperature, and the generated nitrogen is diffused from the mold design surface into the mold to provide a diffusion hardened layer. On the other hand, shot peening in a mold is performed by a method in which small spheres mainly made of ceramic or hard metal of 1 mm or less are accelerated by a projection device and sprayed onto a mold design surface. A compressive residual stress is given to the mold design surface by work hardening by collision of small spheres.

  For example, Patent Document 1 discloses that after a nitriding treatment is performed on a mold design surface to form a nitrogen diffusion hardened layer, shot peening is further performed to give a high compressive residual stress to the surface. By applying the nitriding treatment and shot peening in combination, the life of the mold can be greatly increased.

  By the way, it is also known that a compound layer having poor plastic deformability is formed on the surface of the nitrogen diffusion hardened layer in the nitriding treatment. Since such a compound layer grows into cracks due to heat check and causes wear due to peeling, a nitriding method has been proposed in which this compound layer is not formed or is formed as thin as possible.

  For example, in Patent Document 2, ammonia gas nitriding is performed in a relatively low temperature range of 450 to 530 ° C., and thereafter, the supply of ammonia is reduced or stopped, and nitrogen is internally diffused at a processing temperature of 550 to 590 ° C. A two-stage process for heat treatment is disclosed. In ammonia gas nitriding in a relatively low temperature range, the compound layer is formed thin. On the other hand, the depth of the nitrogen diffusion layer is also reduced. Therefore, the nitrogen in the nitrogen diffusion layer is diffused deep into the mold by heat treatment to obtain a thick nitrogen diffusion layer while being a thin compound layer.

  Similarly, in Patent Document 3, ammonia gas nitriding is performed under reduced pressure at a temperature of less than 570 ° C., and thereafter, the supply of ammonia is reduced or stopped, and nitrogen is internally diffused at a processing temperature of 570 ° C. to 650 ° C. A two-stage process for heat treatment is disclosed. In gas nitriding under such reduced pressure, it is stated that the nitrogen compound layer is obtained in a thin and non-porous state, and that the depth of the nitrogen diffusion layer is increased by heat treatment.

JP 2004-148362 A Japanese Patent Laid-Open No. 10-306364 Japanese Patent Laid-Open No. 11-100635

  In the ammonia gas nitriding to form a thin nitrogen compound layer as disclosed in Patent Documents 2 and 3, the absolute amount of nitrogen supplied to the mold is small, and the nitrogen in the nitrogen diffusion layer is diffused deeper by heat treatment. If it is made to do, sufficient hardness cannot be given to a nitrogen diffusion layer.

  The present invention has been made in view of such circumstances, and the object of the present invention is to substantially not provide a nitrogen compound layer that causes heat check and wear, while on the other hand, nitrogen inside the mold It is an object of the present invention to provide a surface treatment method capable of providing a die-casting mold having excellent heat check resistance and wear resistance.

  The present inventor has found that the outermost nitrogen compound formed by various nitriding treatments such as gas soft nitriding, gas oxynitriding, and plasma nitriding can be decomposed relatively easily by heat treatment. And, while examining the manufacturing method of the mold that does not substantially give the nitrogen compound layer, it is possible to increase the amount of nitrogen supplied to the mold by generating nitrogen by such decomposition and diffusing this into the mold I thought of it.

  Accordingly, a surface treatment method for a die casting mold according to the present invention is a surface treatment method for a die casting mold provided by applying compressive residual stress to a mold design surface, and a gas containing at least ammonia gas is introduced into a heating furnace. Then, a nitriding step for forming a nitride layer including a compound layer made of a nitrogen compound on the mold design surface, an ammonia gas is discharged from the inside of the heating furnace, and an atmosphere gas is introduced for heat treatment to decompose the nitrogen compound. A compound decomposing step, and a shot peening step of performing shot peening on the mold design surface, wherein the nitriding step forms the nitride layer including at least the compound layer having a thickness in the range of 2 to 7 microns. It is a step to perform.

  According to this invention, the nitrogen compound is formed in the compound decomposition step by controlling the nitrogen compound layer formed by nitriding performed by introducing a gas containing at least ammonia gas into the heating furnace to a predetermined thickness. As a result, the nitrogen compound layer is not substantially provided, and the amount of nitrogen supplied to the mold can be increased by the nitrogen generated thereby, thereby providing a nitrogen diffusion layer having high hardness. In addition, the nitrogen compound layer that substantially disappears contains a lot of voids and absorbs and dissipates the collision energy of the shot in shot peening. However, it is possible to apply compressive residual stress by shot peening by controlling the nitrogen compound layer to a predetermined thickness in the nitriding step. That is, a die-cast mold having excellent wear resistance and heat check resistance can be provided by high hardness and high compressive residual stress.

  In the above-described invention, the compound decomposition step may be characterized in that the heat treatment is performed at a temperature at least higher than that of the nitriding step. According to this invention, it is possible to give a higher hardness and higher compressive residual stress, and it is possible to give a die casting mold that is more excellent in wear resistance and heat check resistance.

It is a perspective view of a test piece. It is a figure which shows the steel type and surface treatment conditions of a test piece. It is a figure which shows a heating / cooling test apparatus. It is an expanded sectional view which shows the change of the surface vicinity of a test piece. It is a figure which shows the change of the nitrogen concentration of the surface vicinity of a test piece. It is a graph which shows the relationship between a compound layer thickness and the number of heat checks (HC). It is a graph which shows the relationship between a compound layer thickness and a residual stress. It is a photograph of the section of a test piece (Example 11).

  The details of the surface treatment method of the die casting mold according to the present invention will be described through the results of verification tests as shown in FIGS. The demonstration test was performed by preparing a cylindrical test piece 1 (see FIG. 1) corresponding to a die-casting mold and performing various surface treatments for evaluation.

  A cylindrical test piece 1 having an outer diameter D1 = 15 mm, an inner diameter D2 = 3 mm, and a length L = 20 mm as shown in FIG. 1 was prepared. Test piece 1 was processed from a round bar material equivalent to SKD61. Instead of the SKD61 equivalent material, the test piece 1 was processed from a round bar material equivalent to SKD7 for Example 9, and from a round bar material equivalent to SKH51 for Example 10. These are summarized in FIG.

  Next, ammonia gas was introduced into the furnace while heating the test piece 1 in the heating furnace, and the outer peripheral surface of the test piece 1 was subjected to gas soft nitriding treatment. As shown in FIG. 2, the nitride layer 5 (see FIG. 4) including the compound layer 2 of each compound layer thickness was formed by giving each condition of processing temperature, nitriding time, and gas mixture ratio. Instead of the gas soft nitriding treatment, the gas nitronitriding treatment was carried out for Example 6, and the plasma nitriding treatment was carried out for Example 7 and Comparative Example 5.

  Next, after exhausting the ammonia gas from the heating furnace, nitrogen is introduced as an atmospheric gas, and the test piece 1 is heat-treated in the same heating furnace as it is to be diffused, and a compound layer produced by nitriding as described later 2 (see FIG. 4) was completely decomposed. The temperature and time of this diffusion treatment are also shown in FIG.

  Next, shot peening was performed by projecting amorphous small spheres having a diameter of 0.05 mm to 0.2 mm on the outer peripheral surface of the test piece 1 with a projection pressure of 0.3 MPa.

  About the test piece 1 which gave the said process, the residual stress of the outer peripheral surface of the longitudinal direction center part vicinity was measured.

  Further, the test piece 1 was repeatedly subjected to a heating / cooling test using a test apparatus 20 as shown in FIG. Specifically, the small-diameter portion 22a of the support portion 22 of the test apparatus 20 was inserted into the through hole 1a of the test piece 1, and the test piece 1 was sandwiched and fixed from above and below by the holder 23. The outer peripheral surface of the test piece 1 is heated from room temperature to 700 ° C. for 4 seconds with the high-frequency coil 21, cooled by cooling water 24 from a water outlet (not shown) to room temperature for 3 seconds, and dried by air blow for 1 second. It was. Such a heating, cooling and drying cycle was repeated 1000 times in total, and the test piece 1 was removed from the test apparatus 20. The test piece 1 removed from the test apparatus 20 was cut in the vicinity of the central portion in the longitudinal direction with a plane perpendicular to the central axis, and after filling with resin, the cut surface was mirror-polished. The cut surface was observed with an optical microscope (100 times), and the number of heat checks (HC) generated on the outer peripheral surface of the test piece 1 was measured.

  A part of the test piece 1 after the above nitriding treatment is taken out of the furnace and the thickness of the compound layer 2 (see FIG. 4) described later is measured. The test piece 1 taken out from the furnace was cut in a plane perpendicular to the central axis in the vicinity of the central portion in the longitudinal direction, the cut surface was mirror-polished and observed with an optical microscope, and the thickness of the compound layer 2 was measured.

  By the way, in the nitriding treatment, as shown in FIGS. 4 (a) and 5 (a), activated nitrogen in the gas phase diffuses from the outer peripheral surface of the test piece 1 to the inside (base material) 4, A nitride layer 5 is formed in the vicinity of the outer peripheral surface. The nitride layer 5 is composed of the outermost nitrogen compound layer 2 and the nitrogen diffusion layer 3 on the inner side. The compound layer 2 is made of a complex nitride of Fe or Cr and is a very brittle layer. Note that the growth rate of plasma nitriding is much slower than that of gas nitriding. The nitrogen diffusion layer 3 is a solid solution layer of nitrogen containing nitride that has been dispersed and precipitated.

  In the diffusion process subsequent to the above nitriding process, the depth of the nitrided layer 5 is increased as shown in FIGS. 4B and 5B. Specifically, the nitrogen flux supplied from the gas phase through the outer peripheral surface of the test piece 1 decreases, and the nitrogen in the nitrogen diffusion layer 3 mainly diffuses into the interior 4 of the test piece 1. Here, when the nitrogen compound in the compound layer 2 is decomposed, the nitrogen generated thereby diffuses into the inside of the test piece 1, but the concentration of nitrogen contained in the compound (see reference numeral 3 a in FIG. 5A). ) Is significantly higher than the concentration of nitrogen contained in a nitrogen solid solution such as the nitrogen diffusion layer 3 (see reference numeral 3b in FIG. 5A), so that the nitrogen diffusion layer 3 having a significantly large amount of nitrogen is obtained. (See reference numeral 31 in FIG. 5B). The case where only the nitrogen diffusion layer 3 obtained by the nitriding treatment is subjected to the diffusion treatment is indicated by reference numeral 32 in FIG.

  On the other hand, when the nitrogen compound in the compound layer 2 is decomposed, the surface layer 2 ′ containing a large amount of voids is formed due to the volume shrinkage. Such a surface layer 2 'absorbs and dissipates shot collision energy in shot peening, thereby inhibiting the formation of compressive residual stress. Details will be described later.

  The results of the above measurement will be described below. First, the relationship between the number of heat checks (HC) after repeated heating / cooling tests and the thickness of the compound layer 2 is shown in FIG.

  It can be seen that the number of heat checks decreases as the thickness of the compound layer 2 is increased, and the heat check resistance can be improved. That is, in Comparative Examples 1 and 5 in which the thickness of the compound layer 2 was formed as thin as 1.5 μm and 1.0 μm, the number of heat checks was 597 and 441, respectively. On the other hand, in Examples 1 to 14 in which the thickness of the compound layer 2 was increased to 2 to 7 μm, the number of heat checks was greatly reduced to 13 to 257. In particular, in Example 11 where the diffusion treatment was performed at a temperature lower than the temperature of the nitriding treatment, the number of heat checks was significantly reduced.

  As described above, when the thickness of the compound layer 2 is increased, the amount of nitrogen compound decomposed by the diffusion treatment increases, so the amount of nitrogen in the nitrogen diffusion layer 3 is increased and the hardness after the diffusion treatment is increased. This increases the wear resistance and heat check resistance.

  On the other hand, the number of heat checks increases rapidly when the thickness of the compound layer 2 is increased to a predetermined value or more, and it can be seen that the heat check resistance can be greatly reduced. That is, in Comparative Examples 2, 3, and 4 in which the thickness of the compound layer 2 was formed as thick as 8.0, 9.0, and 10.0 μm, the number of heat checks was 706, 707, and 840, respectively. Compared to Examples 1 to 14, it increased rapidly.

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

  It can be seen that the compressive residual stress can increase its absolute value when the thickness of the compound layer 2 is increased. That is, in Comparative Examples 1 and 5 in which the thickness of the compound layer 2 was formed as thin as 1.5 μm and 1.0 μm, the compressive residual stress was −965 MPa and −993 MPa, respectively. On the other hand, in Examples 1 to 14 in which the thickness of the compound layer 2 was formed to be as thick as 2 to 7 μm, the compressive residual stress was −1350 MPa to −1755 MPa, and the absolute value thereof was greatly increased.

  On the other hand, it is also found that the absolute value of the compressive residual stress can be drastically lowered when the thickness of the compound layer 2 is increased to a predetermined value or more. That is, in Comparative Examples 2, 3, and 4 in which the thickness of the compound layer 2 was formed as thick as 8.0, 9.0, and 10.0 μm, the compressive residual stresses were −1298 MPa, −1251 MPa, and −938 MPa, respectively. Thus, the absolute value was significantly lower than in Examples 1 to 14.

  As described above, when the thickness of the compound layer 2 is increased to a predetermined value or more, the absolute value of the compressive residual stress is greatly reduced, and the heat check resistance is greatly reduced. This is because of the surface layer 2 ′ (see FIG. 4B) containing many voids formed by decomposition of the compound of the compound layer 2. That is, by increasing the thickness of the compound layer 2, the surface layer 2 ′ is also thickly formed by the diffusion treatment, and the formation of compressive residual stress by shot peening is rapidly inhibited, and as a result, the heat check resistance is greatly reduced. It is.

  Incidentally, FIG. 8A is an optical micrograph of the cut surface of the test piece 1 after nitriding in Example 11. FIG. FIG. 8B is an optical micrograph of the cut surface of the test piece 1 after the diffusion treatment was applied following the nitriding treatment in Example 11. In the former, the compound layer 2 and the nitrogen diffusion layer 3 are observed. On the other hand, in the latter, an increase in the thickness of the nitrogen diffusion layer 3 is observed, and a surface layer 2 ′ containing black voids is observed particularly on the side close to the nitrogen diffusion layer 3 due to decomposition of the compound.

  From the above, the compressive residual stress due to shot peening can be given satisfactorily by limiting the thickness of the compound layer 2, that is, the thickness of the surface layer 2 ′ including voids remaining after the compound is decomposed. It is possible to provide a die-casting mold having excellent heat check resistance. In particular, the thickness of the compound layer 2 (surface layer 2 ′ remaining after the compound is decomposed) is preferably 2 to 7 μm under general shot peening conditions as in the examples.

  By the way, the difference between nitriding treatments such as gas soft nitriding, gas oxynitriding and plasma nitriding as in Examples 6 and 7, and SKD61 equivalent material, SKD7 equivalent material and SKH51 as in Example 9 and Example 10 are used. Regarding the difference in equivalent steel materials, the relationship between the thickness of the compound layer 2 in FIGS. 6 and 7, the number of heat checks, and the compressive residual stress is aligned on the same plot line. That is, it is suggested that the method of this embodiment can be applied regardless of differences in nitriding treatment and differences in general mold materials.

  As described above, the surface treatment method of the die casting mold according to the present embodiment is such that nitrogen is applied to the mold design surface by nitriding treatment such as gas soft nitriding, gas impregnating nitriding, plasma nitriding which introduces at least ammonia gas into the heating furnace. A nitriding step for forming a nitride layer including a compound layer made of a compound, a compound decomposing step for discharging ammonia gas from the heating furnace and introducing an atmospheric gas to decompose nitrogen compounds, and a mold design surface A shot peening step for performing shot peening. Here, the nitriding step is a step of forming a nitride layer including at least a compound layer having a thickness in the range of 2 to 7 microns.

  By controlling the nitrogen compound layer formed by nitriding performed by introducing a gas containing at least ammonia gas into the heating furnace to a predetermined thickness, the nitrogen compound is decomposed in the compound decomposition step. A nitrogen diffusion layer having a high hardness can be provided without substantially providing a compound layer and increasing the amount of nitrogen supplied to the mold by nitrogen generated thereby. In addition, the nitrogen compound layer that substantially disappears remains as a layer containing a large amount of voids, and absorbs and dissipates the shot collision energy in shot peening. However, by controlling the nitrogen compound layer to a predetermined thickness in the nitriding step, compressive residual stress due to shot peening can be applied to the nitrogen diffusion layer. That is, a die-cast mold having excellent wear resistance and heat check resistance can be provided by high hardness and high compressive residual stress.

  Up to this point, the representative embodiments according to the present invention and the modifications based thereon have been described, but the present invention is not necessarily limited thereto. That is, those skilled in the art will be able to find various alternative embodiments and modifications without departing from the scope of the appended claims.

DESCRIPTION OF SYMBOLS 1 Test piece 2 Compound layer 3 Nitrogen diffusion layer 5 Nitride layer 20 Test apparatus

Claims (2)

A die casting surface treatment method provided by applying compressive residual stress to the mold design surface,
A nitriding 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 a nitrogen compound on the mold design surface;
A compound decomposing step of decomposing the nitrogen compound by discharging ammonia gas from the inside of the heating furnace and introducing an atmosphere gas to heat treatment;
Shot peening step of performing shot peening on the mold design surface,
The surface treatment method of a die casting mold, wherein the nitriding step is a step of forming the nitride layer including at least the compound layer having a thickness in a range of 2 to 7 microns. 2. The surface treatment method of a die casting mold according to claim 1, wherein in the compound decomposition step, the heat treatment is performed at a temperature lower than that of the nitriding step.