38CrMoAl material and vacuum nitriding method

Abstract

The invention discloses a 38CrMoAl material and a vacuum nitriding method, and relates to the field of nitriding processes of special-shaped parts, and the nitriding method comprises the steps that S1, after a 38CrMoAlA steel part is put into a furnace, the furnace is filled with air, heating is conducted, pre-oxidation is conducted, and a pre-oxidation workpiece is obtained; s2, vacuumizing, refilling nitrogen, and raising the temperature to a nitriding temperature; s3, after the temperature is increased to the nitriding temperature, ammonia cracking gas is introduced into the furnace for nitriding treatment; and S4, nitrogen removal. And the problem of furnace temperature uniformity in the vacuum nitriding process can be effectively improved. According to different nitriding stages, the concentration of nitrogen atoms in the furnace can be automatically and accurately controlled, the ammonia gas consumption is reduced, and the nitriding layer depth and the nitriding layer quality are improved.

Description

Technical Field

[0001] The present application relates to a 38CrMoAl material and a vacuum nitriding method, especially the nitriding process technology for complex-shaped parts with special structures. Background Art

[0002] Gas nitriding is to introduce ammonia gas (NH3) into a heat-resistant tank, which decomposes into active nitrogen atoms and hydrogen gas under certain temperature and pressure. The active nitrogen atoms react with the active metal atoms on the surface layer of the material through chemical and physical adsorption to form metal nitrides. When the nitrogen atom concentration in the surface layer reaches a certain level, it will diffuse into the material interior under the drive of the concentration difference. After maintaining a certain temperature for a certain period of time, a metal nitride layer with a certain depth is finally formed inside the material. Taking steel materials as an example, due to different nitrogen atom concentrations, ε (Fe2-3N, nitrogen content 4.55% - 11.0%) → ε + γ' → γ' (Fe4N, nitrogen content 5.5% - 5.9%) → γ' + α → α (matrix) will be formed from the surface to the interior. Gas nitriding has the advantages of small workpiece deformation, high hardness of the formed nitride layer, controllable nitrided layer position, low working temperature, etc. It has high wear resistance and corrosion resistance, and the core of the workpiece can maintain a certain toughness, and is widely used in gears and rod-shaped parts requiring high hardness and wear resistance. 38CrMoAlA material is an ideal nitriding material. However, currently, ion nitriding or nitriding under atmospheric conditions is usually adopted. Ion nitriding has high requirements for the part structure, and for structures with blind holes and sharp corners, nitriding cannot be completed completely at these blind hole and sharp corner positions. Gas nitriding under atmospheric environment has the drawback of surface oxidation. Summary of the Invention

[0003] The technical problem solved by the present application is: overcoming the deficiencies of the prior art, providing a 38CrMoAl material and a vacuum nitriding method to improve the nitrided layer depth and nitrided layer quality.

[0004] Furthermore, it can effectively improve the problem of furnace temperature uniformity during the vacuum nitriding process. For different nitriding stages, the nitrogen atom concentration in the furnace can be automatically and accurately controlled, saving the consumption of ammonia gas.

[0005] The present invention has carried out research on the vacuum nitriding process technology, mastered the influence law of different process gas flow rates on the change of ammonia decomposition rate (nitrogen potential, K N value), and mastered the method of adjusting the gas flow rate to control the K N value in different temperature and different decomposition rate ranges. Through a large number of experiments, it is verified that the process plan and quality control measures are mature and feasible, which can effectively improve the nitrided layer depth and nitrided layer uniformity, significantly improve the nitrided layer quality, and improve the production efficiency.

[0006] Among them, the vacuum nitriding process and equipment in the present invention mainly involve the following technical principles:

[0007] (1) Vacuum nitriding furnace and nitrogen potential control principle

[0008] The vacuum precision nitriding furnace consists of a furnace body, atmosphere panel, ammonia cracker, post-combustion device, cooling system, measurement and control system, sensor, pump system and medium supply system. The structure of the furnace body is as follows Figure 1 As shown, the furnace body 11 is provided with a furnace, a furnace bed support 111 is provided in the furnace, and the workpiece 12 is placed on the furnace bed support 111 in the furnace. The furnace is connected to the outside atmosphere through a balance valve 13, and the air inlet 14 and the air outlet 15 are both connected to the furnace. During nitriding, the ammonia cracking gas enters the furnace body from the air inlet 14, and the ammonia cracking gas is stirred by the circulating fan 18. The ammonia cracking gas flows along the outside of the air guide tube 19 to the front side of the furnace, and then flows back along the inside of the air guide tube. After the reaction of the workpiece 12, the waste gas is burned and discharged through the air outlet 15 to the afterburning device. The cooling system 16 is used to cool the outside of the furnace body. The heating method adopts external heating, that is, the heating body is arranged outside the inner shell of the furnace. During the nitriding process, when the pressure in the furnace is higher than the outside atmospheric pressure, the balance valve is opened to keep the pressure in the furnace balanced with the outside atmospheric pressure. For 38CrMoAlA steel parts, vacuum is evacuated and nitrogen is backfilled after loading the furnace, and the temperature is raised to the nitriding temperature and filled with ammonia cracking gas.

[0009] (2) Types and functions of process gases

[0010] The atmosphere panel is responsible for supplying process gas into the vacuum precision nitriding furnace. The atmosphere panel includes: liquefied petroleum gas pipeline, pre-oxidation air pipeline, NH3 pipeline, and N2 pipeline. The types and functions of process gases are shown in Table 1.

[0011] Table 1 Types and functions of process gases

[0012]

[0013]

[0014] (3) Structure and measurement principle of hydrogen probe

[0015] The hydrogen partial pressure measured by the hydrogen probe in the vacuum precision nitriding furnace is converted into the nitriding coefficient K by the calculation software provided by the system. N (nitrogen potential), adjusting the ratio of process gas introduced into the furnace can control K N value, and K N The value can be converted into the main process parameter of the original nitriding system - ammonia decomposition rate through the formula. Since the hydrogen probe can continuously measure the hydrogen partial pressure in the furnace, it can control K N The value can achieve precise control of the nitriding process, and then achieve precise control of the depth, hardness and brittleness of the nitriding layer. Ammonia (NH3) and nitrogen (N2) are introduced into the furnace, and the K is controlled by adjusting the ratio of ammonia and nitrogen. N value.

[0016] The structure and measurement principle of the hydrogen probe are as follows: Figure 2 and Figure 3 As shown. The exterior of the hydrogen probe consists of a protective tube 21 that passes through the furnace wall and extends into the furnace, and a measuring head 22 connected to the outside of the furnace. Inside the protective tube is a quartz tube 23 and a measuring tube 24 with a closed end. The measuring tube is made of a special material, and only hydrogen in the furnace gas can diffuse through the material into the measuring tube. The measuring head draws a small amount of furnace gas from the furnace through the quartz tube and continuously out of the furnace along the measuring tube. The head of the measuring head is connected to the measuring tube, and a small section of the end is left open to be closed by the pressure transmitter.

[0017] During the measurement process, when the measuring tube is surrounded by furnace gas, since only hydrogen can enter the measuring tube, the hydrogen partial pressure P in the tube is i (H2) and the hydrogen partial pressure P of the furnace gas outside the tube a (H2) will establish a balance, so that the absolute hydrogen partial pressure P(H2) in the furnace gas can be measured by the pressure transmitter connected to the end of the measuring head. Since the following reactions exist in most nitriding processes:

[0018]

[0019] Where [N] is the active nitrogen atom, that is, the nitrogen atom that participates in the diffusion process into the material during the nitriding process. Since there are only two gas phases in the reaction and the pressure in the furnace is constant (balanced with the external atmospheric pressure), it is an isothermal and isobaric process. Therefore, the nitriding coefficient K N It can be expressed by the following formula:

[0020]

[0021] Since the ratio of partial pressure of gas can be converted into volume ratio in the isothermal and isobaric process, and the pressure inside the furnace is the same as the external atmospheric pressure, the hydrogen content (volume percentage) in the furnace gas can be obtained by the following formula:

[0022] V(H2)[%V]=(P(H2)P atm )×100%

[0023] Among them, P atm =P 炉压 Since atmospheric pressure varies with altitude and climate, furnace pressure is also an indicator that needs to be monitored. The volume of the gas introduced can be obtained through the flow sensor on the gas flow meter of each process gas on the atmosphere panel, and the instantaneous K of the gas in the furnace can be obtained using the calculation software provided by the system. N value.

[0024] For the denitriding process or certain high-temperature and high decomposition rate (low nitrogen potential) process, since a large number of active nitrogen atoms are enriched on the material surface, far exceeding the adsorption capacity of the material surface, in addition to the diffusion process of nitrogen atoms into the material inside the material surface, the following reactions also exist:

[0025]

[0026] In fact, this reaction exists in all nitriding processes. Only in the nitriding process with high nitrogen potential, diffusion is the main process, and the active nitrogen atoms consumed by this reaction can be ignored compared with the nitrogen atoms participating in diffusion. Since it belongs to an isothermal and isobaric process, introducing nitrogen gas can affect the direction of this reaction, and further affect the direction of the ammonia decomposition reaction, thereby achieving the purpose of controlling the K N value.

[0027] The technical solution provided by this application is as follows:

[0028] A vacuum nitriding method for 38CrMoAl material, including:

[0029] S1: After the 38CrMoAlA steel parts are loaded into the furnace, the inside of the furnace is air, and pre-oxidation is carried out by heating to obtain pre-oxidized workpieces;

[0030] S2: Vacuum is pumped and nitrogen is backfilled, and the temperature is raised to the nitriding temperature;

[0031] S3: After the temperature is raised to the nitriding temperature, ammonia cracking gas is filled into the furnace for nitriding treatment;

[0032] S4: Denitriding.

[0033] Further, in the S1, the pre-oxidation by heating includes: heating to 420 - 460 °C and lasting for 60 - 80 min.

[0034] Further, in the S2, the nitriding temperature is 540 ± 5 °C.

[0035] Further, in the S3, the nitriding treatment includes: the nitriding treatment time is 45 - 50 h, and the ammonia decomposition rate of the ammonia cracking gas is 25% - 35%.

[0036] Further, in the S3, the nitriding treatment includes: in the first 15 - 16 h of the nitriding treatment time, the ammonia decomposition rate of the ammonia cracking gas is 25%; in the middle 15 - 16 h of the nitriding treatment time: the ammonia decomposition rate of the ammonia cracking gas is 30%; in the last 15 - 16 h of the nitriding treatment time: the ammonia decomposition rate of the ammonia cracking gas is 35%.

[0037] Further, in the S4, the denitriding treatment includes: the denitriding heating temperature: 520 ± 10 °C, the denitriding treatment time is 3 - 4 h, and the ammonia decomposition rate of the ammonia cracking gas is > 70%;

[0038] A 38CrMoAl material, prepared by the vacuum nitriding method of a 38CrMoAl material according to any one of the above.

[0039] In summary, the present application includes at least the following beneficial technical effects:

[0040] If the existing air nitriding method is directly used for the 38CrMoAl material, the nitrided layer is too shallow and the thickness of the nitrided layer is uneven; because there is a relatively dense oxide layer on the surface of the 38CrMoAl material, in this regard, the present application adds: a pre-oxidation step, through which a loose multi-microporous structure is formed on the surface to facilitate the diffusion of active nitrogen atoms during the subsequent nitriding process, thereby increasing the uniformity and depth of the nitrided layer;

[0041] During the nitriding process, the ammonia decomposition rate is segmented to increase the depth of the nitrided layer; the optimal parameters of the ammonia decomposition rate and vacuum nitriding are invented, and this method fills the blank of the vacuum gas nitriding process method. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a schematic structural diagram of a vacuum precision nitriding furnace body;

[0043] Figure 2 It is a schematic structural diagram of a hydrogen probe;

[0044] Figure 3 It is a schematic structural diagram of a hydrogen probe.

[0045] Description of the reference numerals: 11, furnace body; 111, furnace bed support; 12, workpiece; 13, balance valve; 14, air inlet; 15, air outlet; 16, cooling system; 17, heating element; 18, circulation fan; 19, air guide cylinder;

[0046] 21, protection tube; 22, measuring head; 23, quartz tube; 24, measuring tube; 25, pressure transmitter. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0047] To make the objectives, technical solutions, and advantages of the present application clearer, the following will further describe in detail the disclosed embodiments of the present application with reference to the accompanying drawings.

[0048] A vacuum nitriding method for a 38CrMoAl material disclosed in this embodiment includes:

[0049] S1: After the 38CrMoAlA steel parts are loaded into the furnace, the inside of the furnace is air, and pre-oxidation is carried out by heating to obtain pre-oxidized workpieces; wherein, the pre-oxidation conditions are heating to 420 - 460 °C after the workpieces are loaded into the furnace and maintaining for 60 - 80 min.

[0050] S2: Evacuate the air and refill with nitrogen, then heat up to the nitriding temperature of 540 - 560 °C;

[0051] S3: After heating up to the nitriding temperature, introduce ammonia cracking gas into the furnace for nitriding treatment; among them, the nitriding treatment includes a nitriding temperature of 540 - 560 °C and an adjusted ammonia decomposition rate of 25% - 35%;

[0052] S4: Denitriding.

[0053] Preparation Example

[0054] A total of 15 pieces of 38CrMoAl bars are used After the bars are quenched and tempered according to the process specifications (930 °C × 40 min, oil cooling + 680 °C × 90 min, air cooling), 3 pieces are used for machining tensile specimens; 2 pieces are used for machining impact specimens, machined to 10.5 × 10.5 × 55; 10 pieces are used for machining nitriding specimens, machined to 10.5 × 10.5 × 55, unit mm, and subjected to stabilizing tempering (500 °C × 100 min, AC), then machined to 10 × 10 × 50, unit mm, with a surface roughness Ra0.8. Each example or comparative example uses tensile specimens, impact specimens, and nitriding specimens, and nitriding is carried out according to the corresponding vacuum nitriding method. The nitrided tensile specimens obtained are used to detect the strength and plasticity indexes of the whole (nitrided layer and matrix), the nitrided impact specimens obtained are used to detect the impact toughness of the whole (nitrided layer and matrix), and the nitrided nitriding specimens obtained are used to detect the surface hardness, nitrided layer depth, and nitrided layer brittleness indexes of the nitrided layer.

[0055] Examples 1 - 8

[0056] A vacuum nitriding method for 38CrMoAl material, including:

[0057] S1: Tensile specimens, impact specimens, and nitriding specimens of 38CrMoAlA steel are all loaded into the furnace. The inside of the furnace is air, and pre-oxidation is carried out by heating to obtain pre-oxidized workpieces; among them, the pre-oxidation conditions are all heating to 450 °C and lasting for 75 min;

[0058] S2: Evacuate the air and refill with nitrogen, then heat up to the nitriding temperature;

[0059] S3: After heating up to the nitriding temperature, introduce ammonia cracking gas into the furnace for nitriding treatment;

[0060] S4: Denitriding.

[0061] Among them, the nitriding temperatures and nitriding treatment conditions of Examples 1 - 8 are different, where:

[0062] ① Keep the ammonia decomposition rate unchanged during nitriding. The ammonia decomposition rate is 25% - 35% (25% in the first 15 h, 30% in the middle 15 h, 35% in the last 15 h); denitriding: 520 °C × 3 h, ammonia decomposition rate: > 70%. The nitriding temperatures are set at 530 °C, 540 °C, 550 °C, and 560 °C respectively. Study the influence of nitriding temperature on mechanical properties and nitrided layer depth.

[0063] ② On the premise of ensuring the same nitriding temperature, adjust the ammonia decomposition rate respectively to study the influence of ammonia decomposition rate on mechanical properties and nitrided layer depth.

[0064] The specific nitriding temperature and nitriding treatment conditions are shown in the process parameters in Table 3.

[0065] Comparative Example 1

[0066] The difference from Example 6 is that this comparative example does not perform the pre-oxidation step of S1.

[0067] Detect the nitrided workpieces obtained from the examples and comparative examples. The mechanical property data of 38CrMoAlA are shown in Table 2. The nitrided layer property data of 38CrMoAlA are shown in Table 3.

[0068] Table 3 Nitrided layer property data of 38CrMoAlA

[0069]

[0070]

[0071] Table 2 Mechanical property data of 38CrMoAlA

[0072]

[0073]

[0074] Analysis of test data:

[0075] It can be seen from Table 2 that after experiencing different nitriding processes, the mechanical properties of 38CrMoAlA material do not change significantly. The tensile strength, yield strength, elongation, reduction of area, and impact energy are all at the same level, indicating that the holding process of gas nitriding will not affect the mechanical properties of the base material; the mechanical properties of the base material after nitriding treatment with different parameters are at the same level.

[0076] During the nitriding process, keep the ammonia decomposition rate unchanged in Table 3. The ammonia decomposition rate is 35% - 45% (the first 15 hours: 35%; the middle 15 hours: 40%; the last 15 hours: 45%); for denitriding: 520°C × 3 hours, and the ammonia decomposition rate is >70%. The nitriding temperatures are set at 530°C, 540°C, 550°C, and 560°C respectively. It can be seen that the hardness and brittleness meet the requirements at these four temperatures, but the depth of the nitrided layer first increases and then decreases with the increase of the nitriding temperature, and only 540°C meets the requirements.

[0077] While keeping the nitriding holding temperature of 540°C unchanged, adjust the ammonia decomposition rate between 15% and 50%. From the test results, it can be seen that when the ammonia decomposition rate is between 15% and 50%, the hardness and brittleness meet the requirements, but the decomposition rate is between 25% and 35% (the first 15 hours: 25%; the middle 15 hours: 30%; the last 15 hours: 35%); for denitriding: 520°C × 3 hours, and the ammonia decomposition rate is >70%, and the depth of the nitrided layer is the best.

[0078] According to Comparative Example 1 and Example 6, after pre-oxidation treatment, the brittleness, hardness, and case depth of the nitrided layer meet the requirements. Compared with the comparative example without pre-oxidation, only the brittleness meets the requirements, and the hardness and case depth do not meet the requirements.

[0079] In summary, through multiple batches of tests, the vacuum nitriding method for 38CrMoAlA material has been mastered.

[0080] Within a certain temperature range, the lower the temperature, the deeper the depth of the nitrided layer. When the ammonia decomposition rate is between 25% and 35%, the depth of the nitrided layer is the best.

[0081] The vacuum nitriding parameters for 38CrMoAlA material are: after pre-pumping vacuum, the nitriding holding temperature is 540°C, control the ammonia decomposition rate at 25% - 35% (the first 15 hours: 25%; the middle 15 hours: 30%; the last 15 hours: 35%); for denitriding: 520°C × 3 hours, and the ammonia decomposition rate is >70%

[0082] The content not described in detail in the specification of this application belongs to the well-known technology of those skilled in the art.

[0083] The above has described this application in detail in combination with specific embodiments and exemplary examples, but these descriptions should not be construed as limitations on this application. Those skilled in the art understand that without departing from the spirit and scope of this application, various equivalent substitutions, modifications, or improvements can be made to the technical solutions and implementation manners of this application, and these all fall within the scope of this application. The protection scope of this application is subject to the appended claims.

Claims

1. A vacuum nitriding method for 38CrMoAl material, characterized in that: include: S1: After the 38CrMoAlA steel parts are loaded into the furnace, the furnace is filled with air and heated for pre-oxidation to obtain pre-oxidized workpieces; the pre-oxidation conditions are: heating to 420-460°C for 60-80 minutes; S2: evacuate and backfill with nitrogen, and heat to nitriding temperature; S3: After the temperature is raised to the nitriding temperature, ammonia cracking gas is charged into the furnace for nitriding treatment; S4: Nitrogen withdrawal.

2. A vacuum nitriding method for 38CrMoAl material according to claim 1, characterized in that: In S1, heating for pre-oxidation includes: heating to 430-450° C. for 60-80 minutes.

3. A vacuum nitriding method for 38CrMoAl material according to claim 1, characterized in that: In S2, the nitriding temperature is 540±5°C.

4. The vacuum nitriding method of 38CrMoAl material according to claim 1, characterized in that: In S3, the nitriding treatment includes: the nitriding treatment time is 45 to 50 hours, and the ammonia decomposition rate of the ammonia cracking gas is 25% to 35%.

5. The vacuum nitriding method of 38CrMoAl material according to claim 1, characterized in that: In S3, the nitriding treatment includes: in the first 15 to 16 hours of the nitriding treatment time, the ammonia decomposition rate of the ammonia cracking gas is 25%, in the middle 15 to 16 hours of the nitriding treatment time: the ammonia decomposition rate of the ammonia cracking gas is 30%; in the last 15 to 16 hours of the nitriding treatment time: the ammonia decomposition rate of the ammonia cracking gas is 35%.

6. A vacuum nitriding method for 38CrMoAl material according to claim 1, characterized in that: In S4, the denitrification treatment includes: denitrification heating temperature: 520±10° C., denitrification treatment time: 3 to 4 hours, and ammonia decomposition rate of ammonia cracking gas is >70%.

7. A 38CrMoAl material, characterized in that: The 38CrMoAl material is prepared by the vacuum nitriding method according to any one of claims 1 to 6.

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