Method for regulating nitrogen permeation layer of die-casting mold

Abstract

The application relates to a method for regulating a nitrogenized permeation layer of a die-casting die, which comprises the following steps: pretreating and partitioning the die-casting die; placing the pretreated die into a nitrogenizing device, introducing nitrogen and hydrogen into the nitrogenizing device, the volume ratio of the nitrogen and the hydrogen being 4-5:1, and heating the nitrogenizing device to a first temperature interval and then performing insulation; monitoring and calculating the nitrogenizing reaction rate in the nitrogenizing device in real time, starting a partitioned heating program according to the partitioning of the die-casting die, and implementing partitioned heating on different regions of the die-casting die; adjusting the volume ratio of the nitrogen and the hydrogen to 1:1, maintaining the temperature of each region set in the partitioned heating program, maintaining the process parameters in this stage until the thickness and hardness of the nitrogenized layer reach target values; and cooling the die to a furnace-out temperature under a protective atmosphere after the nitrogenizing treatment is completed. The method is favorable for improving the hardness distribution uniformity, thermal fatigue resistance, wear resistance and overall service life of the die-casting die.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of surface treatment of die casting molds, in particular to a method for regulating a nitrided permeation layer of a die casting mold. BACKGROUND

[0002] Die casting molds are key equipment in die casting production, and their working conditions need to withstand repeated erosion and rapid cooling and heating of high-temperature molten metal, which puts extremely high requirements on the heat fatigue resistance, wear resistance and corrosion resistance of the mold surface. As a mature surface strengthening technology, gas nitriding is widely used in the surface treatment of die casting molds. By forming a high-hardness nitrided layer on the mold surface, the service life of the mold can be significantly improved.

[0003] Traditional gas nitriding processes usually use fixed ammonia decomposition rates or fixed ratios of nitrogen-hydrogen mixed gas and constant treatment temperatures. This static process method can achieve certain results when treating molds with simple structures and uniform wall thicknesses. However, with the development of manufacturing, the shapes of die castings are becoming more complex, and the corresponding molds also exhibit features of irregular shapes, i.e., the molds themselves contain different structural regions with large differences in thermal capacity and heat dissipation conditions, such as deep cavities, thin walls, sharp corners, and thick and solid mold frames. When using traditional static nitriding processes to treat such irregular molds, the windows for achieving optimal nitriding kinetics conditions for each part of the mold are different under fixed temperature and gas phase composition. The thin-walled, sharp-cornered regions are prone to excessive nitriding due to rapid heating and strong nitrogen atom adsorption, resulting in the formation of a thick brittle white layer or even peeling; while the thick regions are prone to insufficient nitriding due to large thermal inertia and slow heating, resulting in a shallow permeation layer, which cannot meet the requirements for hardness and wear resistance. This unevenness in nitriding layer depth and hardness directly leads to performance imbalance of each part of the mold during service, and thermal fatigue cracks are easily generated in weak areas first.

[0004] Therefore, the gas nitriding method in the prior art cannot solve the core technical problem of achieving uniformity and controllability of the depth, hardness and microstructure of the nitriding gradient permeation layer in a single nitriding cycle for die casting irregular molds with complex geometric features. This seriously restricts the further improvement of the performance and service life of high-end die casting molds. SUMMARY

[0005] Therefore, it is necessary to provide a method for regulating a nitrided permeation layer of a die casting mold, which can effectively improve the nitriding uniformity of die casting irregular molds with complex geometric features and improve the performance and service life of the die casting mold.

[0006] The technical scheme is as follows: a method for regulating a nitrided permeation layer of a die casting mold, the method comprising the following steps:

[0007] Pretreating the die casting mold and partitioning it;

[0008] The pre-processed mold is placed in a nitriding device, nitrogen and hydrogen are introduced into the nitriding device, the volume ratio of the nitrogen to the hydrogen is 4-5:1, and the nitriding device is heated to a first temperature interval and then is kept at the first temperature interval;

[0009] The nitriding reaction rate in the nitriding device is monitored and calculated in real time, wherein, when the reaction rate reaches a first preset threshold, the volume ratio of the nitrogen to the hydrogen is adjusted to 2.5-3.5:1, and a partition heating program is started according to the partition of the die casting mold, and different regions of the die casting mold are subjected to partition heating;

[0010] The volume ratio of the nitrogen to the hydrogen is adjusted to 1:1, and the temperature of each region set in the partition heating program is maintained, and the process parameters in this stage are maintained until the thickness and hardness of the nitriding layer reach target values;

[0011] After the nitriding treatment is completed, the mold is cooled to a furnace discharge temperature under a protective atmosphere.

[0012] In one of the embodiments, the real-time monitoring and calculation of the nitriding reaction rate comprises:

[0013] The pressure change rate ΔP / Δt in the nitriding device and / or the ammonia decomposition rate α in the furnace gas are monitored;

[0014] Based on the pressure change rate and / or the ammonia decomposition rate, the reaction rate R is calculated through a pre-established reaction kinetics model, wherein R=k1*(ΔP / Δt) / P0+k2*α, the pressure change rate ΔP / Δt is in units of Pa / s, P0 is a reference pressure in units of Pa, k1 and k2 are weight coefficients, and k1+k2=1, and the above calculation adopts dimensionless processing.

[0015] In one of the embodiments, the step of pre-treating the die casting mold specifically comprises:

[0016] The die casting mold is surface cleaned, and is wiped with anhydrous ethanol after ultrasonic cleaning;

[0017] After cleaning, the die casting mold is subjected to a stress relief annealing treatment, the annealing temperature is 600-650℃, and the temperature is kept for 0.5-1.5h and then is cooled to room temperature.

[0018] In one of the embodiments, the partition of the die casting mold comprises a complex cavity region, a deep hole region, an edge and corner stress concentration region, and a plane region, and in the partition heating program, the temperature for partition heating is set as follows: the complex cavity region: 490-500℃; the deep hole region: 430-460℃; the edge and corner stress concentration region: 480-490℃; and the plane region: 500-510℃.

[0019] In one of the embodiments, the step of placing the pre-processed mold into the nitriding equipment, introducing nitrogen and hydrogen into the nitriding equipment, the volume ratio of the nitrogen to the hydrogen being 4-5:1, and heating the nitriding equipment to a first temperature interval and then maintaining the temperature in the first temperature interval, the first temperature interval being 340-360°C, and the maintaining time being 1.5-2h, and maintaining the furnace pressure at 0.12-0.15MPa.

[0020] In one of the embodiments, the step of adjusting the volume ratio of the nitrogen to the hydrogen to 1:1, and maintaining the temperature in each interval set in the interval heating procedure, and maintaining the process parameters in this stage until the thickness and hardness of the nitriding layer reach the target values, specifically includes:

[0021] adjusting the volume ratio of the nitrogen to the hydrogen to 1:1, and maintaining the temperature in each interval set in the interval heating procedure;

[0022] receiving feedback signals from the online hardness monitoring system, and proportionally reducing the nitrogen flow when the monitored surface hardness value exceeds a second preset threshold value;

[0023] terminating the nitriding treatment when the thickness and hardness of the nitriding layer reach the target values.

[0024] In one of the embodiments, the step of receiving feedback signals from the online hardness monitoring system, and proportionally reducing the nitrogen flow when the monitored surface hardness value exceeds a second preset threshold value, the hardness detection being multi-point detection of at least 3 detection points in each region, the second preset threshold value being 650-680HV, and the nitrogen ratio being reduced by 5%-10% when the hardness is too high.

[0025] In one of the embodiments, the protective atmosphere is a hydrogen or nitrogen atmosphere.

[0026] In one of the embodiments, the step of placing the pre-processed mold into the nitriding equipment, introducing nitrogen and hydrogen into the nitriding equipment, the volume ratio of the nitrogen to the hydrogen being 4-5:1, and heating the furnace body of the nitriding equipment to a first temperature interval and then maintaining the temperature in the first temperature interval, the purity of the nitrogen being ≥99.99%, the purity of the hydrogen being ≥99.95%, and the total flow of the nitrogen-hydrogen mixed gas being 0.8-1.2m 3 / h.

[0027] In one of the embodiments, the step of cooling the mold to the discharge temperature under the protective atmosphere after the nitriding treatment is completed, the cooling method being:

[0028] reducing to 250°C at a rate of 4-6°C / min, and maintaining the temperature for 1h; reducing to below 100°C at a rate of 8-10°C / min, and maintaining the protective atmosphere of the nitrogen-hydrogen volume ratio of 1:1 throughout the process.

[0029] The beneficial effects of the technical solution are as follows:

[0030] The regulating method for the nitrided permeation layer of the die casting die has the beneficial effects that, in the treatment process, by combining the dynamically adjusted nitrogen-hydrogen gas ratio with the partitioned temperature rising program based on the die structure, and accurately controlling the process conversion point according to the real-time monitored nitriding reaction rate, the problems of uneven nitriding layer depth and non-ideal hardness caused by the differences in heat capacity and heat dissipation conditions of each part of the special-shaped die casting die in the traditional nitriding process can be effectively solved, so that a high-quality nitriding layer with uniform thickness and dense structure can be synchronously obtained on the entire die surface, which is beneficial to improving the hardness distribution uniformity, thermal fatigue resistance, wear resistance and overall service life of the die casting die. BRIEF DESCRIPTION OF DRAWINGS

[0031] The accompanying drawings, which form a part of this application, are intended to provide further understanding of the application and are incorporated herein for a description of the application and are not intended to limit the application unduly.

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the accompanying drawings needed to be used in the embodiment description will be briefly introduced. Obviously, the accompanying drawings in the following description are only some embodiments of the present application, and other accompanying drawings can be obtained by those skilled in the art without any creative effort.

[0033] Figure 1 The working flow chart of the regulating method for the nitrided permeation layer of the die casting die described in an embodiment. DETAILED DESCRIPTION

[0034] In order to make the above-mentioned purposes, features and advantages of the present application more apparent and easy to understand, the specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. In the following description, many specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be implemented in many different ways than those described herein, and those skilled in the art can make similar improvements without departing from the scope of the present application, so the present application is not limited to the specific embodiments disclosed below.

[0035] In the description of the present application, it should be understood that the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application.

[0036] In addition, the terms "first", "second", etc. are used only for descriptive purposes and are not to be construed as indicating or implying relative importance or an ordered ranking of the indicated technical features. Thus, features defined with "first", "second" can include at least one of the features explicitly or implicitly. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise explicitly specified and limited.

[0037] In the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connecting", "fixing" and the like should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements, unless otherwise explicitly limited. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0038] In the present application, unless otherwise explicitly specified and limited, the first feature is "on" or "under" the second feature, which can be direct contact between the first and second features, or indirect contact between the first and second features through an intermediate medium. Moreover, the first feature "above", "above" and "above" the second feature can be directly above or obliquely above the first feature, or only indicate that the horizontal height of the first feature is higher than that of the second feature. The first feature "below", "below" and "below" the second feature can be directly below or obliquely below the first feature, or only indicate that the horizontal height of the first feature is less than that of the second feature.

[0039] It should be noted that when an element is referred to as "fixed to" or "provided on" another element, it can be directly on another element or there can be a middle element. When an element is considered to be "connected" to another element, it can be directly connected to another element or there can be a middle element. The terms "vertical", "horizontal", "up", "down", "left", "right" and similar expressions used herein are for illustrative purposes only and are not the only embodiment.

[0040] Referring to Figure 1 , Figure 1 A work flow chart of a method for controlling the nitrogen permeation layer of a die casting mold is shown, the method for controlling the nitrogen permeation layer of the die casting mold comprising the following steps:

[0041] S10: Pretreatment and partitioning of the die casting mold;

[0042] S20: placing the pretreated mold into a nitriding device, introducing nitrogen and hydrogen into the nitriding device, the volume ratio of nitrogen to hydrogen being 4-5:1, and heating the nitriding device to a first temperature interval and then maintaining the temperature;

[0043] S30: monitoring and calculating the nitriding reaction rate in the nitriding device in real time, wherein when the reaction rate reaches a first preset threshold, the volume ratio of nitrogen to hydrogen is adjusted to 2.5-3.5:1, and a zoned heating program is started according to the zones of the die casting mold to implement zoned heating of different regions of the die casting mold;

[0044] S40: adjusting the volume ratio of nitrogen to hydrogen to 1:1 and maintaining the temperature of each zone set by the zoned heating program, and maintaining the process parameters at this stage until the thickness and hardness of the nitriding layer reach target values;

[0045] S50: after the nitriding treatment is completed, the mold is cooled to the furnace discharge temperature under a protective atmosphere.

[0046] The above-mentioned method for controlling the nitriding permeation layer of a die casting mold can effectively solve the problem of uneven nitriding layer depth and unsatisfactory hardness caused by differences in heat capacity and heat dissipation conditions of different parts of a special-shaped die casting mold in traditional nitriding by combining the dynamically adjusted nitrogen-hydrogen gas ratio with the zoned heating program based on the mold structure and accurately controlling the process switching point according to the real-time monitored nitriding reaction rate, so that a high-quality nitriding layer with uniform thickness and dense structure can be obtained on the entire mold surface, which is beneficial to improving the hardness distribution uniformity, thermal fatigue resistance, wear resistance and overall service life of the die casting mold.

[0047] Alternatively, according to different die casting molds, the ratio of nitrogen to hydrogen in step S20 can be 4:1, 4.2:1, 4.5:1, 4.8:1, 5:1 or any ratio range within the interval. In step S30, the volume ratio of nitrogen to hydrogen can be 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, 3:1, 3.1:1, 3.2:1, 3.3:1, 3.4:1, 3.5:1 or any ratio range within the interval.

[0048] In one embodiment, the real-time monitoring and calculation of the nitriding reaction rate comprises: monitoring the pressure change rate ΔP / Δt and / or ammonia decomposition rate α in the furnace gas in the nitriding equipment; based on the pressure change rate and / or ammonia decomposition rate, calculating the reaction rate R through a pre-established reaction kinetics model, wherein R=k1*(ΔP / Δt) / P0+k2*α, the pressure change rate ΔP / Δt is in units of Pa / s, P0 is a reference pressure in units of Pa, k1 and k2 are weight coefficients, and k1+k2=1, and the above calculation is dimensionless. The pressure change rate directly reflects the volume change of the gas generated by the decomposition of ammonia, and the ammonia decomposition rate represents the level of nitrogen potential. The integration of these two parameters through the weight coefficient can more comprehensively evaluate the reaction kinetics state. This implementation makes the process stage transition no longer dependent on fixed time parameters, but is dynamically triggered according to the actual reaction process. In this way, the key time point of the formation of the surface compound layer and the diffusion of nitrogen atoms becoming the control step can be accurately captured, providing accurate timing control for subsequent adjustment of gas ratio and start of zoned heating, thereby avoiding the problem of shallow nitriding layer caused by inaccurate transition period judgment.

[0049] Specifically, the reference pressure P0 is set to 0.1 MPa, which matches the conventional nitriding initial pressure. The weight coefficients are adjusted according to the mold material: when the mold is H13 steel, k1=0.6, k2=0.4, so that the pressure change has a more significant effect on the reaction rate. When the material of the die casting mold is 3Cr2W8V steel, k1=0.4, k2=0.6, and the ammonia decomposition rate has a greater impact on nitrogen penetration. The monitoring frequency is to collect pressure and ammonia decomposition rate data every 10 seconds, and the reaction rate R is calculated by automatically substituting the formula through the PLC system.

[0050] Further, the first preset value is 0.8-0.9, for example, when R≥0.8, the subsequent process adjustment is triggered.

[0051] It should be noted that the first preset threshold R is used to determine whether the nitriding reaction has entered the nitrogen atom diffusion control stage from the surface active adsorption stage, so as to decide whether to perform gas ratio adjustment and start the zoned heating program. The setting basis mainly includes: material characteristics: different mold steels (such as H13, 3Cr2W8V) have different adsorption and diffusion capabilities for nitrogen, and the threshold needs to be adjusted accordingly. Process target: target nitriding layer depth, hardness, and uniformity of microstructure. Test data: through previous process tests, it is determined that the best nitriding effect can be obtained by converting the process at a certain reaction rate.

[0052] For example, for H13 steel, the nitrogen adsorption capacity is strong, and it is necessary to transfer to the diffusion stage in time after the surface reaction is basically completed to avoid surface over-nitriding, so the R value is set to 0.85. For 3Cr2W8V steel, nitrogen diffusion is slow, and the surface activation stage needs to be extended to ensure that nitrogen atoms penetrate sufficiently. Therefore, the R value is set to 0.9 to improve wear resistance. For P20 steel, the nitriding layer requires thin and hard, and needs to enter the diffusion control stage as soon as possible to avoid deep nitriding, so the R value is set to 0.8 to prevent sticking. For high-alloy die steel, the alloying element is more, and the nitriding is difficult, and it is necessary to transfer to the diffusion stage after the surface reaction is sufficient to avoid too many brittle phases in the surface layer, so the R value is set to 0.88 to achieve uniform depth and prevent embrittlement.

[0053] In one embodiment, step S10: pretreatment is performed on the die casting mold, specifically including:

[0054] S11: cleaning the surface of the die casting mold, using ultrasonic cleaning and then wiping with anhydrous ethanol;

[0055] S12: after cleaning, the die casting mold is subjected to stress relief annealing treatment, the annealing temperature is 600-650°C, and after holding for 0.5-1.5h, it is cooled to room temperature.

[0056] Specifically, ultrasonic cleaning uses cavitation effect to completely remove oil and particulate matter on the surface of the mold and cavity, and anhydrous ethanol wiping further removes residual polar contaminants and quickly evaporates to ensure surface cleanliness. Stress relief annealing releases residual stress generated by machining by heating the mold to below the recrystallization temperature and holding. In this way, the absolutely clean surface ensures uniform adsorption of active nitrogen atoms at the beginning of nitriding, which can avoid uneven penetration layer caused by pollution; in addition, it is also beneficial to eliminate internal stress, thereby reducing the risk of deformation and size out-of-tolerance of the mold during nitriding heat treatment.

[0057] Further, the ultrasonic cleaning parameters are: power 200-300W, cleaning medium is alkaline cleaner, pH=10-11, cleaning time is 15-20min, and after cleaning, rinse with deionized water for 3 times. Anhydrous ethanol wiping is done with a dust-free cloth, wiping in the same direction to avoid secondary pollution. Stress relief annealing uses a box-type resistance furnace, the heating rate is 10°C / min, and after holding, it is cooled to room temperature with the furnace, the cooling rate is ≤5°C / min to avoid stress generated by rapid cooling.

[0058] In one embodiment, the sub-zones of the die casting mold include a complex cavity zone, a deep hole zone, a corner stress concentration zone, and a flat zone. In the sub-zone temperature rising program, the temperature of the sub-zone temperature rising is set as: complex cavity zone: 490-500℃; deep hole zone: 430-460℃; corner stress concentration zone: 480-490℃; flat zone: 500-510℃. The above sub-zone mode is based on the compensation mechanism of heat conduction and nitrogen atom diffusion dynamics. For the corner area with good heat dissipation condition and small heat capacity, a relatively low temperature is used to inhibit excessive nitriding; for the deep hole zone with slow heat dissipation and large heat capacity, a higher temperature is used to promote the diffusion of nitrogen atoms to the inside. This differentiated temperature field implementation can offset the thermal differences caused by the geometry of the mold. Through precise temperature compensation, the mold parts reach similar nitriding kinetic conditions, which is beneficial to realize the uniformity of the overall nitriding layer depth of the complex shape mold, while avoiding brittle peeling at sharp corners due to excessive nitriding.

[0059] Specifically, the sub-zone division is based on thermal simulation analysis through a three-dimensional mold model. The region with a heat capacity ≥500 J / (kg·℃) is marked as a complex cavity zone, the hole structure region with a depth-diameter ratio ≥5 is marked as a deep hole zone, the region with a fillet radius ≤3 mm is marked as a corner stress concentration zone, and the rest is marked as a flat zone. The temperature rising equipment uses a multi-zone temperature control nitriding furnace. Each zone is equipped with an independent thermocouple and heating pipe. The temperature control accuracy is ±5℃. When the sub-zone temperature rising starts, the temperature rises to the set temperature at a rate of 5℃ / min to avoid thermal stress caused by sudden temperature rise.

[0060] In one embodiment, step S20: the pre-processed mold is placed in a nitriding device, nitrogen and hydrogen are introduced into the nitriding device, the volume ratio of nitrogen to hydrogen is 4-5:1, and the nitriding device is heated to a first temperature interval and then maintained. The first temperature interval is 340-360℃, and the maintenance time is 1.5-2h, and the furnace pressure is maintained at 0.12-0.15MPa. Such temperature interval is the pre-nitriding stage, which can ensure sufficient atomic activity for surface reaction, and can avoid excessive and rapid growth of nitrides due to high temperature, and appropriate holding time ensures that the mold as a whole reaches temperature balance and completes surface activation. Controlling the furnace pressure in this range is beneficial to maintaining a stable gas flow state and ensuring the uniformity of the atmosphere. Moreover, through the optimized combination of temperature, time and pressure parameters, it is beneficial to realize the uniform activation of the mold surface, form the active surface layer necessary for high-quality nitriding, and further improve the nitriding effect in the later stage.

[0061] Further, the temperature rising process rises from room temperature to 340-360℃ at a rate of 12℃ / min. Nitrogen-hydrogen mixed gas is continuously introduced during the temperature rising process to exclude air in the furnace. The furnace pressure is controlled by the cooperation of gas flow and exhaust valve during the holding stage, and the pressure fluctuation is controlled within ±0.01MPa. The ammonia decomposition rate in the furnace is recorded 10min before the end of the holding stage.

[0062] In one embodiment, step S40: adjust the volume ratio of nitrogen to hydrogen to 1:1, and maintain the temperature of each zone set by the zoning temperature program, maintain the process parameters in this stage until the nitriding layer thickness and hardness reach the target value, specifically comprising:

[0063] S41: adjust the volume ratio of nitrogen to hydrogen to 1:1, and maintain the temperature of each zone set by the zoning temperature program;

[0064] S42: receive feedback signals from the online hardness monitoring system, when the monitored surface hardness value exceeds the second preset threshold, reduce the nitrogen flow proportionally;

[0065] S43: after the thickness and hardness of the nitriding layer reach the target value, end the nitriding process.

[0066] When the nitrogen to hydrogen ratio is adjusted to 1:1, the nitrogen atom penetration rate and diffusion rate reach the best balance, which ensures the growth of the nitriding layer thickness and avoids the increase of brittleness caused by the excessive precipitation of nitrides such as Fe2N; the online hardness system captures the surface hardness changes in real time, and reduces the nitrogen flow when the hardness exceeds the threshold. Too high nitrogen ratio will lead to too fast penetration of nitrogen atoms, forming supersaturated nitrides, resulting in high hardness and low toughness. By proportional adjustment, the amount of nitride precipitation can be precisely controlled. In this way, the hardness and microstructure of the final nitriding layer can be precisely controlled, avoiding the formation of brittle layers due to excessive nitriding, while ensuring the consistency of the results of different batches, especially suitable for surface treatment of precision die casting molds with strict performance requirements.

[0067] Further, in step S41: the nitrogen to hydrogen ratio switching is realized by mass flow controllers, and the switching process lasts for 5 minutes to avoid fluctuations in the furnace atmosphere caused by sudden changes in flow. In step S42: the online hardness monitoring system uses a portable ultrasonic hardness tester, which collects data every 30 seconds. When the hardness value exceeds 650-680 HV, the flow adjustment program is started. In step S43: the target thickness is set to 0.15-0.25 mm, and the target hardness is 600-650 HV. When the monitoring data reaches the target value for three consecutive times, it is determined that the nitriding is complete.

[0068] In one embodiment, step S42: receiving a feedback signal from the online hardness monitoring system, when the monitored surface hardness value exceeds a second preset threshold, proportionally reducing the nitrogen flow rate, the hardness detection is a multi-point detection of at least 3 detection points per region, the second preset threshold is 650-680HV, when the hardness is too high, the nitrogen proportion is reduced by 5%-10%. The multi-point detection of at least 3 detection points per region can avoid the contingency of single-point detection, ensure the overall uniformity of the region hardness, especially for complex cavity regions and other regions prone to hardness fluctuations. The second preset threshold of 650-680HV is determined based on the use requirements of the die casting mold. Hardness above this threshold will increase the brittleness of the nitriding layer and easily crack in thermal fatigue cycles. The 5%-10% reduction in nitrogen flow rate has been verified by process, which can effectively reduce the hardness without affecting the continuity and density of the nitriding layer. In this way, the early failure of the mold caused by local high hardness can be avoided, the consistency of the hardness of each region is improved, the stress is uniform when the mold bears impact load, the service life is prolonged, and the proportional control of the flow rate adjustment ensures the accuracy of the process adjustment and avoids the hardness from being too low due to the large adjustment range.

[0069] It should be noted that the second preset threshold is used to determine whether the surface hardness of the nitriding layer is too high, so as to decide whether to reduce the nitrogen flow rate to adjust the nitriding rate. The main basis for setting it includes: performance requirements: die casting molds usually require a surface hardness of 600-650HV, and excessive hardness will increase brittleness and easily cause cracking. Material brittleness limit: different steels have different brittleness transition hardness, which needs to be avoided to exceed the limit. Structural risk: edge, thin-walled and other regions are prone to brittleness due to excessive nitriding, and a safety threshold needs to be set.

[0070] For example, for a complex cavity region, the cavity structure is complex and prone to stress concentration, and the hardness should not be too high to avoid cracking during service, so the second preset threshold is set to 650-660HV; for a deep hole region, the heat dissipation is slow, the nitriding is prone to be deep, and the hardness threshold is appropriately increased to compensate for the diffusion deficiency, so the second preset threshold is set to 660-670HV; for the edge stress concentration region, the nitriding is prone to be excessive, and a lower threshold is set to inhibit the formation of a brittle layer, so the second preset threshold is set to 640-650HV; for the planar region, the heat dissipation is uniform, and the hardness target can be appropriately increased to improve wear resistance, so the preset threshold is set to 670-680HV.

[0071] Further, the hardness on-line detection method is that 5 detection points are uniformly arranged according to the cavity profile in the complex cavity area, including the cavity bottom, side wall and corner, 1 detection point is arranged at the orifice, 1 / 2 depth of the hole and the hole bottom in the deep hole area, and 3 detection points are uniformly arranged in the corner stress concentration area and the plane area. The sensor probe is perpendicular to the mold surface during detection, the contact pressure is 0.5N, and the average value is taken after removing the abnormal value of the detection data. The flow adjustment is automatically executed through the PLC system, and the total flow is maintained at 0.8~1.2m 3 / h.

[0072] In one embodiment, the protective atmosphere is a hydrogen or nitrogen atmosphere. After the nitriding treatment is completed, the mold surface temperature is high, which is easy to react with oxygen in the air to generate Fe3O4, Fe2O3 and other oxides, which destroys the structure of the nitriding layer. Hydrogen as a reducing atmosphere can react with the oxide film on the mold surface, avoiding oxidation and promoting the conversion of residual unstable nitrides in the nitriding layer. Nitrogen is an inert atmosphere that can isolate air and prevent oxygen from contacting the mold surface, serving as a simple protective role. This is conducive to avoiding the oxidation failure of the nitriding layer during the cooling process, ensuring the integrity and density of the nitriding layer, and protecting the smoothness of the mold surface, reducing subsequent polishing and processing procedures, and reducing production costs.

[0073] Specifically, if the mold material contains alloy elements such as chromium and molybdenum, hydrogen atmosphere is preferred, and the oxygen content in the furnace is ≤0.1%, and an explosion-proof device is installed. When nitrogen atmosphere is selected, the atmosphere is introduced immediately after the nitriding treatment is completed, the nitriding reaction gas is turned off, the protective atmosphere is switched, and the mold is cooled to the discharge temperature, which is conducive to reducing production costs.

[0074] In one embodiment, step S20: the pretreated mold is placed in a nitriding device, nitrogen and hydrogen are introduced into the nitriding device, the volume ratio of nitrogen to hydrogen is 4~5:1, the furnace body of the nitriding device is heated to a first temperature interval and then insulated, the purity of nitrogen is ≥99.99%, the purity of hydrogen is ≥99.95%, and the total flow of the nitrogen-hydrogen mixed gas is 0.8~1.2m 3 / h.

[0075] High-purity nitrogen and hydrogen can avoid the entry of impurity gases such as gas, moisture and carbon dioxide into the furnace. Oxygen will react with the mold surface to form an oxide film, hindering the penetration of nitrogen atoms. Moisture will cause pinholes and porosity in the nitriding layer. Carbon dioxide will react with hydrogen to form carbon monoxide, affecting the stability of the reaction atmosphere. The total flow is 0.8~1.2m 3The design of the gas flow rate can ensure sufficient gas circulation in the furnace, timely discharge of reaction by-products such as excess hydrogen generated by ammonia decomposition, and stable pressure in the furnace, thereby avoiding energy waste or pressure loss of control due to excessive flow rate. This is conducive to improving the purity and efficiency of the nitriding reaction, reducing defects such as oxidation points, loose layers and nitrided layers caused by impurities, and improving the compactness of the nitrided layer. Stable gas flow rate provides a guarantee for precise control of process parameters, which is conducive to reducing the risk of process failure caused by gas fluctuations.

[0076] Further, the protective gas is provided in the following manner: nitrogen is treated by a molecular sieve drying device, and the water content is ≤10 ppm; hydrogen is treated by a deoxidizing device, and the oxygen content is ≤5 ppm. A mass flow controller is used to adjust the flow rate of nitrogen and hydrogen, and the total flow rate is displayed in real time by a flow meter. Data is recorded once an hour to ensure that the flow rate is maintained at 0.8-1.2 m 3 / h, with a fluctuation of not more than ±0.05 m 3 / h.

[0077] In one embodiment, after the nitriding treatment is completed, the mold is cooled to the furnace discharge temperature under a protective atmosphere, and the cooling method is as follows:

[0078] The temperature is reduced to 250℃ at a rate of 4-6℃ / min, and the temperature is maintained for 1h; the temperature is reduced to below 100℃ at a rate of 8-10℃ / min, and a protective atmosphere with a nitrogen-hydrogen volume ratio of 1:1 is maintained throughout the process.

[0079] The segmented cooling can avoid thermal stress generated in the mold due to excessive cooling rate. In the first stage, the temperature is reduced to 250℃ at a rate of 4-6℃ / min, which can promote the further diffusion of nitrogen atoms in the nitrided layer, making the structure more uniform, and can also avoid stress concentration caused by excessive temperature difference. The temperature is maintained at 250℃ for 1h to eliminate residual stress generated during cooling and stabilize the structure of the nitrided layer. In the second stage, the temperature is reduced to below 100℃ at a rate of 8-10℃ / min. At this time, the mold temperature is relatively low, and rapid cooling will not produce obvious stress, and the mold can be discharged below 100℃ to avoid condensation of moisture in the air on the surface of the mold at room temperature. The 1:1 nitrogen-hydrogen ratio is maintained throughout the process to continuously protect the surface of the mold and avoid oxidation during cooling. This can minimize mold deformation and stress retention during cooling, reduce deformation, avoid difficulties in mold assembly or size precision failure caused by deformation; at the same time, the integrity of the nitrided layer is protected to ensure that the hardness and thickness of the nitrided layer after cooling meet the target requirements.

[0080] The technical features of the above embodiments can be combined in any manner. To make the description concise, not all possible combinations of the technical features in the above embodiments are described, but as long as the combinations of the technical features do not exist contradictions, they should be considered as the scope of the present disclosure.

[0081] The above embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the patent scope of the application. It should be noted that for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, which are all within the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for controlling the nitriding penetration layer of a die-casting mold, characterized in that, The method for controlling the nitriding penetration layer of the die-casting mold includes the following steps: The die-casting mold is pre-treated and divided into zones, including complex cavity zone, deep hole zone, edge stress concentration zone and planar zone; The pretreated mold is placed in a nitriding device, and nitrogen and hydrogen are introduced into the nitriding device. The volume ratio of nitrogen to hydrogen is 4~5:

1. The temperature inside the nitriding device is raised to the first temperature range and then kept at that temperature. Real-time monitoring and calculation of the nitriding reaction rate within the nitriding equipment specifically includes: monitoring the pressure change rate ΔP / Δt and / or the ammonia decomposition rate α in the furnace gas within the nitriding equipment; and calculating the reaction rate R based on the pressure change rate and / or ammonia decomposition rate using a pre-established reaction kinetic model, where R = k1*(ΔP / Δt) / P0 + k2*α, the pressure change rate ΔP / Δt is in Pa / s, P0 is the reference pressure in Pa, and k1 and k2 are weighting coefficients that satisfy k1 + k2 = 1. The above calculations are dimensionless. When the reaction rate reaches the first preset threshold, the volume ratio of nitrogen to hydrogen is adjusted to 2.5~3.5:1, and a zone heating program is started according to the zones of the die-casting mold. The zone heating program sets the temperature for each zone as follows: complex cavity zone: 490~500℃; deep hole zone: 430~460℃; corner stress concentration zone: 480~490℃; planar zone: 500~510℃. The volume ratio of nitrogen to hydrogen is adjusted to 1:1, and the temperature of each zone is maintained as set in the partitioned heating program. The process parameters at this stage are maintained until the nitrided layer thickness and hardness reach the target values. After the nitriding process is completed, the mold is cooled to the oven temperature under a protective atmosphere.

2. The method for controlling the nitriding penetration layer of a die-casting mold according to claim 1, characterized in that, The steps involve pre-treating the die-casting mold, specifically including: The die-casting mold is cleaned by ultrasonic cleaning followed by wiping with anhydrous ethanol. After cleaning, the die-casting mold is subjected to stress-relieving annealing at a temperature of 600℃~650℃, held for 0.5~1.5 hours, and then cooled to room temperature.

3. The method for controlling the nitriding penetration layer of a die-casting mold according to claim 1, characterized in that, Steps: Place the pretreated mold in a nitriding device, introduce nitrogen and hydrogen into the nitriding device, with a volume ratio of nitrogen to hydrogen of 4~5:1, and heat the nitriding device to a first temperature range of 340~360°C for 1.5~2 hours, maintaining the furnace pressure at 0.12~0.15MPa.

4. The method for controlling the nitriding penetration layer of a die-casting mold according to claim 1, characterized in that, Steps: Adjust the volume ratio of nitrogen to hydrogen to 1:1, and maintain the temperature of each zone as set in the zone heating program. Maintain the process parameters at this stage until the nitrided layer thickness and hardness reach the target values. Specifically, this includes: Adjust the volume ratio of nitrogen to hydrogen to 1:1 and maintain the temperature of each zone as set in the partition heating program. Receive feedback signals from the online hardness monitoring system, and reduce the nitrogen flow rate proportionally when the monitored surface hardness value exceeds the second preset threshold. Once the thickness and hardness of the nitrided layer reach the target values, the nitriding process is completed.

5. The method for controlling the nitriding penetration layer of a die-casting mold according to claim 4, characterized in that, Steps: Receive feedback signals from the online hardness monitoring system. When the monitored surface hardness value exceeds the second preset threshold, reduce the nitrogen flow rate proportionally. Hardness detection is multi-point detection with at least 3 detection points per area. The second preset threshold is 650~680HV. When the hardness is too high, reduce the nitrogen ratio by 5%~10%.

6. The method for controlling the nitriding penetration layer of a die-casting mold according to claim 1, characterized in that, The protective atmosphere is a hydrogen or nitrogen atmosphere.

7. The method for controlling the nitriding penetration layer of a die-casting mold according to claim 1, characterized in that, Steps: Place the pretreated mold in a nitriding device, and introduce nitrogen and hydrogen gas into the device. The volume ratio of nitrogen to hydrogen is 4-5:

1. Heat the furnace body of the nitriding device to the first temperature range and maintain the temperature. The nitrogen purity is ≥99.99%, the hydrogen purity is ≥99.95%, and the total flow rate of the nitrogen-hydrogen mixture is 0.8-1.2 m³ / h. 3 / h.

8. The method for controlling the nitriding penetration layer of a die-casting mold according to claim 1, characterized in that, Steps: After nitriding, cool the mold to the oven temperature under a protective atmosphere. The cooling method is as follows: The temperature was lowered to 250℃ at a rate of 4~6℃ / min and held for 1 hour; then lowered to below 100℃ at a rate of 8~10℃ / min, maintaining a protective atmosphere with a nitrogen-hydrogen volume ratio of 1:1 throughout the process.

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