Method for preventing connecting rod forging die nitride layer from falling off and reason analysis method thereof

Abstract

The invention relates to a method for preventing a connecting rod forging die nitride layer from falling off and a reason analysis method thereof, and the method comprises the following steps: S1, reducing the nitrogen potential of a nitriding process, in a nitrogen potential atmosphere with the pressure of 200 Pa unchanged, after a heat preservation stage is set for 26 hours, stopping NH3 supply into equipment, and carrying out a chemical reaction by using residual NH3 in a furnace, so that N atoms participating in the chemical reaction are reduced; s2, the cooling rate of the nitriding process is prolonged, the cooling time is set to be 6 hours, and the cooling rate of reducing the surface temperature of the connecting rod forging die from 535 DEG C to 60 DEG C is slowed down; S3, the equipment process is improved, a one-way ball valve is additionally arranged between a vacuum pump and a vacuumizing pipeline, a one-way valve of a standby vacuum pump is closed when the equipment is vacuumized, and the vacuum pumping pipeline is closed; the problem that the lubricating oil in the standby vacuum pump flows back due to negative pressure when equipment works is solved; the method has the advantage that the problem that the nitride layer of the connecting rod mold abnormally falls off is solved.

Description

Technical Field

[0001] The invention relates to the field of automobile engine connecting rod processing, in particular to a method for preventing a connecting rod forging die nitride layer from falling off and a method for analyzing the cause thereof. Background Art

[0002] When forging connecting rods, the heating temperature of the forging roll blank is generally between 1250℃ and 1310℃. During production, the blank generates impact contact stress on the cavity surface and the high-speed flow of metal generates strong friction on the cavity surface. Especially when the forging temperature is higher, the cavity surface layer of the mold is more severely softened by tempering. Therefore, most forging molds in the same industry are surface nitrided to reduce thermal fatigue of the mold and increase the service life of the mold. At present, the nitriding process of our company's forging molds is ion nitriding, using a vacuum ion nitriding furnace. The principle of the ion nitriding process is to place the workpiece on the cathode disk in the furnace as the cathode, and the outer metal cover wall as the anode, and fill it with a low-pressure NH3 and H2 mixed gas. However, since H2 is extremely flammable and explosive, it places extremely high demands on leak detection and safety protection measures. Therefore, NH3 is often used as the medium gas. When 400-800V DC is applied between the two electrodes, the gas is ionized to produce a glow discharge reaction. Under the action of the electric field, nitrogen atoms are absorbed by the surface of the steel part and diffuse inward, dissolving in α-Fe to form a solid solution. After saturation, a nitride layer is formed in this chemical heat treatment process. However, since NH3 single gas is mostly used as the medium gas, the nitrided mold often suffers from the problem of nitriding layer falling off during use, seriously affecting the quality and production efficiency of forgings. The location of the nitriding layer falling off of the connecting rod mold is mainly reflected in the complex curved surfaces such as the connecting rod small head socket, small head side, and big head web. The connecting rod forging mold is prone to nitriding layer falling off during use, which is easy to cause forging surface quality problems, affecting the forging quality and mold life. There is an urgent need for a method to analyze the causes and solutions for the nitriding layer falling off of the forging mold, reduce production costs, and increase the life of the forging mold. Summary of the Invention

[0003] In view of the above problems, the purpose of the present invention is to provide a method for preventing the nitride layer of a connecting rod forging die from falling off and a method for analyzing the cause thereof, which is used to analyze the cause of the falling off and solve the falling off problem, so as to overcome the shortcomings of the above-mentioned prior art.

[0004] The present invention provides a method for analyzing the cause of nitride layer shedding of a connecting rod forging die, comprising the following steps:

[0005] Step S1: Detecting and analyzing the mold heat treatment sample;

[0006] Step S11: sampling and testing the connecting rod forging die after heat treatment to check whether the die base hardness is HRC51. If the base hardness is HRC51, then there is no problem with the die base hardness. Otherwise, there is a problem with the die heat treatment sample hardness.

[0007] Step S12: Detecting the tempered structure of the mold after heat treatment to check whether the tempered structure (tempered martensite) is uniform and to determine the content of granular carbides. If the tempered structure is uniform and the fine-grained carbides are less than the set content, it is determined that there is no problem with the heat treatment process. Otherwise, the problem is caused by the heat treatment process.

[0008] Step S2: Detecting and analyzing the ion nitriding mold sample;

[0009] Step S21: Hardness testing: Vickers hardness testing is performed on the mold sample in a gradient manner from the outside to the inside. If the surface hardness value meets the set hardness range (HV800-1500) and there is no excessively steep gradient between the nitride layer and the substrate, then the Vickers hardness is determined to be normal. Otherwise, the problem is caused by hardness.

[0010] Step S22: Nitriding layer depth detection: sampling the mold substrate for detection to check whether the nitriding layer depth is within the set nitriding layer depth range (0.25-0.3mm). If the nitriding layer depth is within the nitriding layer depth range, it is determined that there is no problem with the nitriding layer depth. Otherwise, the problem is caused by the nitriding layer depth.

[0011] Step S23: Vein structure detection, by detecting whether there are large areas of vein nitrides in the nitrided structure, and detecting whether there are cracks in the nitrided structure. When there are no large areas of vein nitrides or no cracks in the nitrided structure, it is determined that there is no problem with the vein structure detection. On the contrary, when large areas of vein nitrides or cracks appear, it is a vein structure problem.

[0012] As the preferred embodiment of the present invention, the following steps are also included:

[0013] Step S3: Equipment process test,

[0014] When the vacuum pump of the ion nitriding equipment is running, determine whether the lubricating oil in the standby vacuum pump flows back into the vacuum pipe. When the ion nitriding furnace is evacuated and pressure maintained, determine whether there is a negative pressure of lubricating oil entering the furnace. If there is no backflow, the equipment is fine. When the lubricating oil flows back, it will cause the oil film to cover the connecting rod mold cavity in an unfixed range. The lubricating oil film will have complex cracking, cyclization, and carbonization reactions at high temperatures. The reaction speed is uncertain. The undecomposed products form a microscopic atmosphere covering the surface of the workpiece, resulting in changes in the local nitrogen potential and permeation rate of the mold cavity surface. The formed solid solution is different, which will reduce the bonding strength between the nitride layer organization and the mold matrix, and the nitride layer will fall off abnormally when the mold is used.

[0015] Another object of the present invention is to provide a method for preventing the nitride layer of a connecting rod forging die from falling off, comprising the following steps:

[0016] Step S1: reducing the nitrogen potential of the nitriding process,

[0017] A mixture of NH3 and H2 is used as the medium gas. In an atmosphere of constant nitrogen potential of 200Pa, after the holding stage reaches 26 hours, the NH3 supply to the equipment is stopped. The remaining NH3 in the furnace is used for chemical reaction, which reduces the number of N atoms involved in the chemical reaction. Under the condition that the H atoms remain unchanged, the surface of the mold undergoes a slow denitrification process after ion nitriding, which reduces the nitrogen potential, the brittleness and stress of the forging die surface, the occurrence of micro cracks in the internal structure of the mold, and the phenomenon of mold nitride layer peeling during use.

[0018] Step S2: Extend the cooling rate of the nitriding process,

[0019] The cooling time is set to 6 hours, which slows down the cooling rate of the connecting rod forging die surface temperature from 535°C to 60°C, thereby reducing the generation of large internal stress between the internal structures of the nitride layer, reducing the probability of generating small cracks on the internal structure of the nitride layer, and improving the bonding strength between the nitride layer and the mold substrate.

[0020] As the preferred embodiment of the present invention, the following steps are also included:

[0021] S3: Equipment process improvement,

[0022] A one-way ball valve is added between the vacuum pump and the vacuum extraction pipeline. When the equipment is vacuuming, the one-way valve of the standby vacuum pump is closed. The lubricating oil in the standby vacuum pump cannot flow back due to the negative pressure when the equipment is working.

[0023] The beneficial effects of the present invention are as follows: by reducing the nitrogen potential in the insulation stage of the nitriding process, slowing down the cooling rate in the cooling stage, and improving the equipment vacuum pump by adding a one-way ball valve, the problem of abnormal shedding of the nitrided layer of the connecting rod mold used on the line is improved, and sampling and testing of the mold base have not found any tiny cracks in the nitrided structure again. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the tempering structure detection of the connecting rod die heat treatment sample in Example 1;

[0025] Figure 2 Schematic diagram of the penetration depth detection of the connecting rod ion nitriding mold sample in Example 1;

[0026] Figure 3 This is a schematic diagram of the nitriding structure detection of the connecting rod ion nitriding mold sample in Example 1;

[0027] Figure 4 Schematic diagram of crack detection in the diffusion layer of the connecting rod ion nitriding mold sample in Example 1.

[0028] Figure 5 This is a pre-nitriding process curve diagram of the prior art.

[0029] Figure 6 This is the nitriding process curve in Example 2.

[0030] Figure 7 This is a temperature process curve diagram of the prior art.

[0031] Figure 8 This is the temperature process curve diagram in this embodiment 2. DETAILED DESCRIPTION

[0032] Example 1

[0033] See Figure 1-3 As shown, the present embodiment provides a method for analyzing the cause of nitride layer shedding of a connecting rod forging die, comprising the following steps:

[0034] Step S1: Detecting and analyzing the heat-treated sample of the connecting rod mold;

[0035] Step S11: sampling and testing the connecting rod forging die after heat treatment. The hardness of the die base is HRC51, which meets the process requirement of HRC47-52.

[0036] Step S12: Detect the tempering structure of the mold after heat treatment. The tempering structure is "tempered martensite + a small amount of granular carbide". The grains are fine and the carbides are small. The tempering structure is uniform. Figure 1 As shown, there is no problem with the heat treatment process of the mold.

[0037] Step S2: Detecting and analyzing the connecting rod ion nitriding mold sample;

[0038] Step S21: Hardness testing: Vickers hardness testing is performed on the mold sample in a gradient from the outside to the inside. The surface hardness value is HV906, which is within the set hardness range of HV800-1500. There is no excessively steep gradient in the hardness value between the nitride layer and the substrate. The Vickers hardness test value meets the process requirements, see Table 1.

[0039] Table 1 Vickers hardness test,

[0040]

[0041] Step S22: nitriding layer depth detection, sampling the mold substrate for detection, the nitriding layer depth is detected to be 0.27mm, which meets the process requirements of the nitriding layer depth range of 0.25-0.3mm. Figure 2 As shown;

[0042] Step S23: vein-like structure detection, there is no large area of vein-like nitride in the nitrided structure, such as Figure 3As shown, however, the test found that there were small cracks in the nitrided structure, such as Figure 4 As shown in Table 2, the crack structure was analyzed by microstructure and hardness test, and it was determined that the crack was generated in the nitriding layer structure.

[0043] Table 2 Vickers hardness test around crack structure,

[0044] Location 1 2 3 mean HV of cracked area in carburized layer 801.3 803.4 798.5 801.06 HV of crack-free area of carburized layer 875.5 880.1 874.3 876.6

[0045] See attached Figure 1-3 , and according to Tables 1 and 2, the conclusion is: Through sampling and testing analysis of the ion nitriding die, it was found that the ion nitriding hardness gradient and vein structure of the connecting rod forging die met the process standard requirements, but after multiple sampling tests, it was found that there were tiny cracks in the nitrided structure. When subjected to high-temperature deformation, external friction and impact force, these tiny cracks may cause local peeling of the mold nitriding layer.

[0046] Step S3 in this embodiment: equipment process test,

[0047] When the vacuum pump of the ion nitriding equipment is running, determine whether the lubricating oil in the standby vacuum pump flows back into the vacuum pipe. When the ion nitriding furnace is evacuated and pressure maintained, determine whether there is a negative pressure of lubricating oil entering the furnace. If there is no backflow, the equipment is fine. When the lubricating oil flows back, it will cause the oil film to cover the connecting rod mold cavity in an unfixed range. The lubricating oil film will have complex cracking, cyclization, and carbonization reactions at high temperatures. The reaction speed is uncertain. The undecomposed products form a microscopic atmosphere covering the surface of the workpiece, resulting in changes in the local nitrogen potential and permeation rate of the mold cavity surface. The formed solid solution is different, which will reduce the bonding strength between the nitride layer organization and the mold matrix, and the nitride layer will fall off abnormally when the mold is used.

[0048] Example 2

[0049] This embodiment provides a method for preventing the nitride layer of a connecting rod forging die from falling off when a vein structure problem occurs, comprising the following steps:

[0050] Step S1: reducing the nitrogen potential of the nitriding process,

[0051] In the existing technology, a mixture of NH3 and H2 is used as the medium gas. After H2 is stopped in the later stage, the heat is kept for 30 hours in a nitrogen potential atmosphere of 200Pa. NH3 is continuously supplied. The H atoms participating in the chemical reaction decrease and the N atoms increase. As a result, the nitrogen potential is too high in the later stage of the heat preservation stage, the brittleness and stress of the forging die surface are large, resulting in tiny cracks in the internal structure of the die. The nitriding layer peels off during use. The nitriding process is as follows: Figure 5 shown.

[0052] In this embodiment, a mixture of NH3 and H2 is used as the medium gas. In an atmosphere of constant nitrogen potential of 200Pa, after the holding stage reaches 26 hours, the supply of NH3 in the equipment is stopped, and the remaining NH3 in the furnace is used for chemical reaction, so that the number of N atoms participating in the chemical reaction is reduced. Under the condition that the H atoms remain unchanged, the surface layer of the mold undergoes a slow denitrification process after ion nitriding, thereby reducing the nitrogen potential, reducing the brittleness and stress of the forging die surface, reducing the generation of micro cracks in the internal structure of the mold, and reducing the phenomenon of the mold nitride layer peeling during use. The nitriding process is as follows: Figure 6 shown.

[0053] Step S2: Extend the cooling rate of the nitriding process,

[0054] In the cooling process of ion nitriding in the prior art, the original process has a cooling time of 4 hours, and the surface temperature of the connecting rod forging die is reduced from 535°C to 60°C. The cooling rate is too fast, which will cause large internal stress between the internal structures of the nitrided layer, and increase the risk of small cracks in the internal structure of the nitrided layer. Therefore, the cooling rate of the connecting rod forging die should be reduced. The process curve is as follows: Figure 7 shown.

[0055] In this embodiment, the cooling time is set to 6 hours, so that the cooling rate of the connecting rod forging die surface temperature from 535 ° C to 60 ° C is slowed down, so as to reduce the generation of large internal stress between the internal structures of the nitride layer, reduce the probability of generating small cracks on the internal structure of the nitride layer, and improve the bonding strength between the nitride layer and the die base. The process curve is shown in FIG. Figure 8 shown.

[0056] S3: Equipment process improvement,

[0057] It has been found in the prior art that when the vacuum pump of the ion nitriding equipment is running, the lubricating oil in the standby vacuum pump will flow back into the vacuum pipe. When the ion nitriding furnace is evacuated and pressure maintained, negative pressure of the lubricating oil will enter the furnace, resulting in an oil film covering the connecting rod mold cavity in an irregular range, which cannot be effectively removed and cleaned. The lubricating oil film will undergo complex cracking, cyclization, and carbonization reactions at high temperatures, and the reaction speed is uncertain. The undecomposed products form a microscopic atmosphere covering the surface of the workpiece, resulting in changes in the local nitrogen potential and permeation rate of the mold cavity surface. The formed solid solution is different, which will reduce the bonding strength between the nitride layer structure and the mold substrate, and the nitride layer will fall off abnormally when the mold is used.

[0058] In this embodiment, a one-way ball valve is added between the vacuum pump and the vacuum extraction pipeline. When the equipment is vacuuming, the one-way valve of the standby vacuum pump is closed. The lubricating oil in the standby vacuum pump cannot flow back due to the negative pressure when the equipment is working. After the equipment is improved, no lubricating oil is found in the vacuum pipeline.

[0059] In this embodiment, by reducing the nitrogen potential in the insulation stage of the nitriding process, slowing down the cooling rate in the cooling stage, and improving the equipment vacuum pump by adding a one-way ball valve, the problem of abnormal shedding of the nitrided layer of the connecting rod mold used on the line was improved. Sampling and testing of the mold base also did not reveal the presence of microcracks in the nitrided structure.

[0060] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for analyzing the cause of nitride layer shedding of a connecting rod forging die, characterized in that: The following steps are involved: Step S1: Detecting and analyzing the mold heat treatment sample; Step S11: sampling and testing the connecting rod forging die after heat treatment to check whether the die base hardness is HRC51. If the base hardness is HRC51, then there is no problem with the die base hardness. Otherwise, there is a problem with the die heat treatment sample hardness. Step S12: Detecting the tempered structure of the mold after heat treatment to check whether the tempered structure is uniform and determine the content of granular carbides. If the tempered structure is uniform and the fine-grained carbides are less than the set content, it is determined that there is no problem with the heat treatment process. Otherwise, the problem is caused by the heat treatment process. Step S2: Detecting and analyzing the ion nitriding mold sample; Step S21: Hardness test: Vickers hardness test is performed on the mold sample in a gradient manner from the outside to the inside. When the surface hardness value meets the set hardness range and there is no steep gradient between the nitride layer and the substrate, it is determined that there is no problem with the Vickers hardness. Otherwise, it is caused by hardness. Step S22: nitride layer depth detection, sampling the mold substrate for detection to check whether the nitride layer depth is within the set nitride layer depth range. If the nitride layer depth is within the nitride layer depth range, it is determined that there is no problem with the nitride layer depth. Otherwise, it is caused by the nitride layer depth. Step S23: Vein structure detection, by detecting whether there are large areas of vein nitrides in the nitrided structure, and detecting whether there are cracks in the nitrided structure. When there are no large areas of vein nitrides or no cracks in the nitrided structure, it is determined that there is no problem with the vein structure detection. On the contrary, when large areas of vein nitrides or cracks appear, it is a vein structure problem.

2. The method for analyzing the cause of nitride layer falling off of a connecting rod forging die according to claim 1, characterized in that: The following steps are also included: Step S3: Equipment process test, When the vacuum pump of the ion nitriding equipment is running, determine whether the lubricating oil in the standby vacuum pump flows back into the vacuum pipe. When the ion nitriding furnace is evacuated and pressure maintained, determine whether there is a negative pressure of lubricating oil entering the furnace. If there is no backflow, the equipment is fine. When the lubricating oil flows back, it will cause the oil film to cover the connecting rod mold cavity in an unfixed range. The lubricating oil film will have complex cracking, cyclization, and carbonization reactions at high temperatures. The reaction speed is uncertain. The undecomposed products form a microscopic atmosphere covering the surface of the workpiece, resulting in changes in the local nitrogen potential and permeation rate of the mold cavity surface. The formed solid solution is different, which will reduce the bonding strength between the nitride layer organization and the mold matrix, and the nitride layer will fall off abnormally when the mold is used.

3. The method for preventing the nitride layer of a connecting rod forging die from falling off when a vein structure problem occurs in the method according to claim 1, characterized in that: The following steps are involved: Step S1: reducing the nitrogen potential of the nitriding process, A mixture of NH3 and H2 is used as the medium gas. In an atmosphere of constant nitrogen potential of 200Pa, after the holding stage reaches 26 hours, the NH3 supply to the equipment is stopped. The remaining NH3 in the furnace is used for chemical reaction, which reduces the number of N atoms involved in the chemical reaction. Under the condition that the H atoms remain unchanged, the surface of the mold undergoes a slow denitrification process after ion nitriding, which reduces the nitrogen potential, the brittleness and stress of the forging die surface, the occurrence of micro cracks in the internal structure of the mold, and the phenomenon of mold nitride layer peeling during use. Step S2: Extend the cooling rate of the nitriding process, The cooling time is set to 6 hours, which slows down the cooling rate of the connecting rod forging die surface temperature from 535°C to 60°C, thereby reducing the generation of large internal stress between the internal structures of the nitride layer, reducing the probability of generating small cracks on the internal structure of the nitride layer, and improving the bonding strength between the nitride layer and the mold substrate.

4. The method for preventing the nitride layer of a connecting rod forging die from falling off according to claim 3, wherein: The following steps are also included: S3: Equipment process improvement, A one-way ball valve is added between the vacuum pump and the vacuum extraction pipeline. When the equipment is vacuuming, the one-way valve of the standby vacuum pump is closed. The lubricating oil in the standby vacuum pump cannot flow back due to the negative pressure when the equipment is working.

Images