Pin shaft for telescopic boom of crane and heat treatment process of pin shaft

Abstract

The invention discloses a pin shaft for a telescopic boom of a crane and a heat treatment process of the pin shaft. The heat treatment process comprises the following steps: cleaning and protecting treatment: carrying out cleaning and protecting treatment on the surface of the pin shaft; in-furnace flushing treatment is conducted, specifically, the treated pin shaft is loaded into a furnace, and then nitrogen is introduced into the furnace for flushing; staged heating treatment is conducted, specifically, staged heating treatment is conducted on the pin shaft in the furnace; the multi-stage nitriding treatment is carried out on the heated pin shaft according to the following process parameters: first-stage nitriding: the temperature is raised to 490-500 DEG C, the heat preservation is more than or equal to 240 minutes, and the nitrogen potential is more than or equal to 7; the temperature is controlled to be 490-500 DEG C, heat preservation is conducted for 720-1200 min, and the nitrogen potential is smaller than or equal to 2.5; the temperature is controlled to be 490-500 DEG C, heat preservation is conducted for 180 min or longer, and the nitrogen potential is smaller than or equal to 1.5; the temperature is controlled to be 490-500 DEG C, heat preservation is conducted for more than or equal to 120 min, and the nitrogen potential is larger than or equal to 4 and smaller than or equal to And cooling treatment is conducted, specifically, the pin shaft subjected to nitriding is cooled. According to the method, the nitriding layer depth, the surface hardness and the tensile strength are optimized, and the quality of the pin shaft is improved.

Description

Technical Field

[0001] The present invention relates to the field of heat treatment technology, in particular to a pin shaft for a crane telescopic arm and a heat treatment process thereof. Background Art

[0002] Large-scale, high-strength crane telescopic boom pins are typically made from 300mm diameter 40CrNiMoA hot-rolled bars through hot forging, heat treatment normalizing, rough turning, finish turning, heat treatment quenching and tempering, salt bath nitriding, and surface chrome plating. The salt bath composition currently used in the salt bath nitriding process can generally only be processed above 500°C. However, the heat treatment quenching and tempering high-temperature tempering temperature for large-scale, high-strength crane telescopic boom pins is generally around 500°C. When the workpiece is salt bath nitrided at a salt bath temperature above 500°C, its tensile strength is 910-1020 MPa, which does not meet the technical strength requirements. Furthermore, when the salt bath temperature is below 500°C, the salt bath nitriding efficiency decreases significantly, and the nitrided layer consists of only a diffusion layer and a very thin compound layer, with a depth of 0.1-0.3mm and a surface hardness of 523-650 HV, which does not meet the technical requirements for nitriding high-strength crane telescopic boom pins.

[0003] Among them, the technical requirements for nitriding and strength of the telescopic arm pin of high-strength cranes are: nitriding layer depth 0.3~0.5mm, surface hardness 551~751HV, and tensile strength ≥980MPa.

[0004] When the depth of the nitrided layer on the high-strength crane telescopic boom pin obtained through the aforementioned salt bath nitriding treatment does not meet the standard and is directly put into use, the pin is prone to rapid wear, resulting in dimensional deviations or functional failure; it is prone to fracture due to fatigue crack propagation, shortening its service life. When the nitrided surface hardness of the crane telescopic boom pin does not meet the standard, pitting or uniform corrosion is prone to occur on the workpiece surface, scratches and peeling appear on the surface, and dimensional accuracy is difficult to maintain, affecting assembly or measurement accuracy. Dimensional deviations lead to seal failure or jamming, which is prone to fatigue cracks and leads to premature fracture. When the tensile strength of the high-strength crane telescopic boom pin is lower than the standard requirement before being put into use, the pin can directly break during use, causing serious accidents. Summary of the Invention

[0005] To overcome the shortcomings and deficiencies of the prior art, the present invention aims to provide a pin for a crane telescopic arm and a heat treatment process thereof, utilizing a gas nitriding process to optimize the depth of the nitriding layer, surface hardness, and tensile strength, thereby improving the quality of the pin. This objective is achieved through the following technical solutions:

[0006] A heat treatment process for a pin shaft for a crane telescopic arm comprises the following steps:

[0007] Cleaning and protective treatment: clean and protect the surface of the pin shaft;

[0008] Furnace flushing treatment: Place the pin shaft after the above treatment into the furnace, and then introduce nitrogen into the furnace for flushing;

[0009] Stage heating treatment: the pin in the furnace is heated in stages;

[0010] Multi-stage nitriding treatment: The pin after heat treatment is subjected to multi-stage nitriding treatment according to the following process parameters:

[0011] The first stage of nitriding: heating to 490-500℃ and keeping it for ≥240min, nitrogen potential ≥7;

[0012] Second stage nitriding: control the temperature at 490-500℃ for 720-1200min, nitrogen potential ≤ 2.5;

[0013] The third stage of nitriding: control the temperature at 490-500℃ for ≥180min, and the nitrogen potential ≤1.5;

[0014] The fourth stage of nitriding: control the temperature at 490-500℃ for ≥120min, 4≤nitrogen potential≤6;

[0015] Cooling treatment: Cool the pin after nitriding.

[0016] Furthermore, the multi-stage nitriding treatment further includes a fifth stage of nitriding, and the process parameters of the fifth stage of nitriding include: cooling with the furnace to ≤480°C, 4≤nitrogen potential≤6.

[0017] Furthermore, the cleaning of the pin surface specifically includes: soaking and cleaning various parts of the pin with anhydrous ethanol and / or industrial diesel.

[0018] Furthermore, the protection of the pin surface specifically includes: protecting the threaded hole of the pin with bolts or a coating agent, and covering the non-nitrided part of the pin with aluminum foil for protection.

[0019] Furthermore, the step of introducing nitrogen into the furnace for flushing specifically includes: flushing time ≥ 50 min, nitrogen flow rate ≥ 10 ml / min.

[0020] Furthermore, the staged heating treatment specifically includes:

[0021] The first stage of heating: heating to ≤350℃ and keeping warm for ≥30min, with nitrogen flow rate ≥10ml / min;

[0022] Second stage heating: heating to ≥450℃ and keeping warm for ≥30min, with nitrogen flow rate ≥10ml / min;

[0023] The third stage of heating: heat to ≥480℃ and keep warm for ≥60min, introduce ammonia and control the nitrogen potential to ≥7.

[0024] Furthermore, the cooling treatment includes a furnace cooling stage, and the furnace cooling stage specifically includes: the pin is furnace cooled to ≥450°C, and the nitrogen flow rate is ≥10ml / min.

[0025] Furthermore, the cooling treatment also includes a rapid cooling stage after the furnace cooling stage, and the rapid cooling stage specifically includes: rapidly cooling the pin to ≥80°C and introducing nitrogen at a flow rate of ≥10ml / min.

[0026] The present invention also provides a pin shaft for a telescopic arm of a crane, wherein the pin shaft is obtained by the heat treatment process described above.

[0027] Furthermore, the nitriding layer of the pin has a depth of 0.35 to 0.48 mm, a surface hardness of 620 to 736 HV, and a tensile strength of 1000 to 1045 MPa.

[0028] The beneficial effects of the present invention are as follows: the required pin is obtained through a cleaning and protective treatment, a furnace flushing treatment, a staged heating treatment, a multi-stage nitriding treatment and a cooling treatment in sequence; the multi-stage nitriding process (nitrogen potential 7→2.5→1.5→4~6) is combined with temperature control to precisely control the thickness and gradient of the compound layer and the diffusion layer, thereby improving the surface hardness while avoiding excessive generation of a brittle phase, optimizing the depth of the nitriding layer, the surface hardness and the tensile strength, and achieving a balance among hardness, toughness and corrosion resistance. DETAILED DESCRIPTION

[0029] Specific embodiments of the present invention are described in detail below. Obviously, the described embodiments are only some of the embodiments of the present invention, not all of them. Based on the description of the present invention, all other embodiments derived by persons of ordinary skill in the art without inventive effort are also within the scope of protection of the present invention.

[0030] The present invention provides a heat treatment process for a pin shaft for a crane telescopic arm, comprising the following steps:

[0031] Cleaning and protective treatment: clean and protect the surface of the pin shaft;

[0032] Furnace flushing treatment: Place the pin shaft after the above treatment into the furnace, and then introduce nitrogen into the furnace for flushing;

[0033] Stage heating treatment: the pin in the furnace is heated in stages;

[0034] Multi-stage nitriding treatment: The pin after heat treatment is subjected to multi-stage nitriding treatment according to the following process parameters:

[0035] The first stage of nitriding: heating to 490-500℃ and keeping it for ≥240min, nitrogen potential ≥7;

[0036] Second stage nitriding: control the temperature at 490-500℃ for 720-1200min, nitrogen potential ≤ 2.5;

[0037] The third stage of nitriding: control the temperature at 490-500℃ for ≥180min, and the nitrogen potential ≤1.5;

[0038] The fourth stage of nitriding: control the temperature at 490-500℃ for ≥120min, 4≤nitrogen potential≤6;

[0039] Cooling treatment: Cool the pin after nitriding.

[0040] In this embodiment, the required pin is obtained through cleaning and protection treatment, furnace flushing treatment, stage heating treatment, multi-stage nitriding treatment and cooling treatment. The multi-stage nitriding process (nitrogen potential 7→2.5→1.5→4~6) is combined with temperature control to accurately control the thickness and gradient of the compound layer and the diffusion layer, thereby improving the surface hardness while avoiding excessive formation of brittle phases, optimizing the depth of the nitriding layer, surface hardness and tensile strength, and achieving a balance between hardness, toughness and corrosion resistance.

[0041] Specifically, the first stage of nitriding (high-temperature and high-nitrogen potential nitriding) quickly forms a dense surface compound layer through high nitrogen potential (≥7, generally not exceeding 15) at high temperature, thereby suppressing the initial brittle loose layer; the second stage of nitriding (long-term low-nitrogen potential diffusion) reduces the nitrogen potential, prolongs the holding time, promotes the diffusion of active nitrogen atoms into the deep layer of the matrix, reduces the surface brittleness, and improves the toughness of the matrix; the third stage of nitriding (ultra-low nitrogen potential surface passivation) passivates the surface under an ultra-low nitrogen potential environment, eliminates loose defects, and reduces surface porosity; the fourth stage of nitriding (medium-high nitrogen potential repair and re-nitriding) supplements nitrogen atoms through medium-high nitrogen potential secondary nitriding, repairs existing cracks, suppresses the expansion of new cracks, and optimizes the density of the nitrided layer.

[0042] This embodiment solves the existing contradiction between hardness, brittleness and layer depth through the coordinated regulation of the nitrogen potential sequence of "high → low → ultra-low → medium-high" and temperature, optimizes the depth of the nitriding layer, surface hardness and tensile strength, and achieves a balance between hardness, toughness and corrosion resistance.

[0043] The nitriding temperature is generally above 480°C, and ammonia gas is introduced to achieve a significant nitriding effect. The actual control accuracy of the nitrogen potential is within ±0.5; any smaller accuracy is unattainable and meaningless in practical applications. The pin is made of 300mm diameter 40CrNiMoA hot-rolled bar. After hot forging, heat treatment normalizing, rough turning, finish turning, heat treatment quenching and tempering, and gas nitriding, it meets the nitriding technical requirements for high-strength crane telescopic boom pins. The chemical composition of the 40CrNiMoA raw material is: C: 0.42%, Si: 0.25%, Mn: 0.72%, P: 0.010%, S: 0.008%, Cr: 0.78%, Mo: 0.21%, Ni: 1.45%, and the remaining elements are Fe.

[0044] Furthermore, the multi-stage nitriding treatment includes a fifth stage, nitriding. The process parameters for this fifth stage include: furnace cooling to ≤480°C, and a nitrogen potential of 4 ≤ ≤6. This fifth stage, characterized by slow cooling combined with a medium-to-high nitrogen potential, stabilizes the carburized layer structure and refines the grain size. It also prevents oxidation of the workpiece (pin) during the cooling process, further improving the performance of the pin after nitriding.

[0045] Furthermore, cleaning the pin surface involves soaking and cleaning all parts of the pin with anhydrous ethanol and / or industrial diesel. Anhydrous ethanol, a polar solvent, effectively removes grease and polar residues, evaporating quickly and leaving no residue. Industrial diesel, a non-polar solvent, is suitable for removing heavy oil contamination and requires subsequent drying to prevent carbonization. This cleaned pin surface enhances nitrogen adsorption, promoting the uniform formation of a compound layer and preventing residual grease from carbonizing at high temperatures (>400°C), forming carbon black or carbides that could contaminate the furnace and interfere with nitrogen potential control.

[0046] Furthermore, pin surface protection specifically involves protecting the pin's threaded holes with bolts or coatings, and covering non-nitrided areas with aluminum foil. Pins typically require protection in threaded holes, mating surfaces, and weld areas to prevent increased brittleness caused by nitriding (e.g., threaded holes prone to chipping after nitriding) while also preserving subsequent processing performance. Cleaning and protection, combined with ensuring the reliability of the nitriding process, are key pretreatment steps for increasing workpiece life and reducing scrap.

[0047] Furthermore, nitrogen is introduced into the furnace for flushing, specifically including: flushing time ≥ 50 min, nitrogen flow rate ≥ 10 ml / min; nitrogen is introduced to exhaust air in the furnace to avoid oxidation of the pin shaft surface.

[0048] Furthermore, the staged heating treatment specifically includes:

[0049] The first stage of heating: heating to ≤350℃ and keeping warm for ≥30min, with nitrogen flow rate ≥10ml / min;

[0050] Second stage heating: heating to ≥450℃ and keeping warm for ≥30min, with nitrogen flow rate ≥10ml / min;

[0051] The third stage of heating: heat to ≥480℃ and keep warm for ≥60min, introduce ammonia and control the nitrogen potential to ≥7.

[0052] In this embodiment, the first stage of heating is low-temperature preheating: slowly heating to below 350°C to eliminate the internal stress remaining from machining or the previous process, and avoid deformation caused by stress release in the subsequent high-temperature stage; by low-temperature insulation (≥30min), the moisture or volatile substances (such as cleaning agent residues) adsorbed on the surface of the workpiece are evaporated to prevent vaporization and contamination of the furnace atmosphere at high temperature; nitrogen (flow rate ≥10ml / min) is introduced to replace the air in the furnace, initially establishing an oxygen-free environment to avoid oxidation of the workpiece; this heating stage reduces the deformation of the workpiece and the risk of surface oxidation, providing a clean environment for subsequent nitriding. The second stage of heating is medium-temperature transition: gradually heating from 350°C to 450°C to avoid thermal gradient differences caused by sudden temperature rise and ensure uniform temperature of the overall workpiece; continuously introducing nitrogen to completely eliminate trace oxygen in the furnace to prevent oxidation side reactions when ammonia decomposes. The third stage is heating for high-temperature nitriding preparation: the temperature is raised to above 480°C. At this temperature, the thermal decomposition activity of ammonia (NH3) is significantly enhanced, releasing a large number of active nitrogen atoms (N*), laying the foundation for efficient nitriding in the subsequent first nitriding stage (490-500°C); ammonia is introduced and the nitrogen potential is controlled (≥7) to establish a high nitrogen concentration gradient in advance and accelerate the formation of surface nitrides.

[0053] This embodiment adopts a gradual heating process of low-temperature preheating → medium-temperature transition → high-temperature nitriding preparation. The staged heating rate reduces thermal stress and avoids cracks or warping; the gradual heating promotes the gradient diffusion of nitrogen atoms from the surface to the core, forming a smooth hardness transition; through the staged switching of nitrogen and ammonia, the atmosphere in the furnace is precisely controlled to avoid defects in the nitriding layer caused by gas switching disturbances.

[0054] Furthermore, the cooling process includes a furnace cooling stage, specifically: the pin is furnace-cooled to ≥450°C at a nitrogen flow rate of ≥10 ml / min. In this embodiment, furnace cooling gradually reduces the pin's temperature, reduces thermal stress, and thus maintains its dimensional stability. Continuous nitrogen flow prevents air ingress and prevents oxidation discoloration on the pin's surface.

[0055] The cooling process also includes a rapid cooling phase after the furnace cooling stage. Specifically, the pin is rapidly cooled to ≥80°C while introducing nitrogen at a flow rate of ≥10ml / min. This rapid cooling inhibits nitride coarsening, refines surface grains, and improves surface hardness, while also preventing nitrogen atom escape caused by slow cooling. It significantly shortens the nitriding cycle, and continues to introduce nitrogen to prevent air ingress and the resulting oxidation discoloration on the pin surface.

[0056] The present invention also provides a pin shaft for a telescopic arm of a crane, and the pin shaft is obtained by the above-mentioned heat treatment process.

[0057] Furthermore, the pin's nitriding layer has a depth of 0.35 to 0.48 mm, a surface hardness of 620 to 736 HV, and a tensile strength of 1000 to 1045 MPa. After gas nitriding, the pin's nitriding layer depth, surface hardness, and tensile strength meet the technical requirements for nitriding and strength of high-strength crane telescopic arm pins: a nitriding layer depth of 0.3 to 0.5 mm, a surface hardness of 551 to 751 HV, and a tensile strength of 980 MPa or higher. This eliminates the adverse effects of salt bath nitriding temperatures on the strength and nitriding performance of these large, high-strength pins.

[0058] The present invention obtains the required pin shaft through cleaning and protection treatment, furnace flushing treatment, stage heating treatment, multi-stage nitriding treatment and cooling treatment in sequence. The multi-stage nitriding process (nitrogen potential 7→2.5→1.5→4~6) is combined with temperature control to accurately control the thickness and gradient of the compound layer and the diffusion layer, thereby improving the surface hardness while avoiding excessive generation of brittle phase, optimizing the depth of the nitriding layer, surface hardness and tensile strength, and achieving a balance between hardness, toughness and corrosion resistance.

[0059] Examples and Comparative Examples

[0060] The gas nitriding heat treatment process for the pin shaft for the crane telescopic arm includes the following steps:

[0061] S0: Workpiece (pin) cleaning: use anhydrous ethanol or industrial diesel to soak and clean all parts of the workpiece;

[0062] S1: Workpiece cleaning and protection. After S0 is completed, the threaded holes and non-nitrided parts are covered with bolts, coating agents or aluminum foil. After completion, the workpiece is loaded into the furnace according to the furnace loading diagram.

[0063] S2: Nitrogen flushing in the furnace. After S0 and S1 are completed, the workpiece is loaded into the furnace and sent into the heating chamber. Nitrogen is used to flush the air in the furnace. The flushing time is not less than 50 minutes, and the nitrogen flow rate is not less than 10 ml / min.

[0064] S3: Heating and holding 1 (first stage heating): after S0, S1, and S2 are completed, the workpiece is heated to a temperature not higher than 350°C and then held for a period of not less than 30 minutes. At the same time, nitrogen is introduced into the furnace at a nitrogen flow rate of not less than 10 ml / min.

[0065] S3: Heating and holding 2 (second stage heating): after S0, S1 and S2 are completed, the workpiece is heated to a temperature of not less than 450°C and then held for not less than 30 minutes. At the same time, nitrogen is introduced into the furnace at a nitrogen flow rate of not less than 10 ml / min.

[0066] S4: heating and holding 3 (third stage heating). After S0, S1, S2, and S3 are completed, the workpiece is heated to a temperature of not less than 480°C and then held for not less than 60 minutes. At the same time, ammonia gas is introduced into the furnace with a nitrogen potential of not less than 7.

[0067] S5: Nitriding 1 (first stage nitriding), after S0, S1, S2, S3, and S4 are completed, the workpiece will be heated to a temperature of not less than 490°C and not more than 500°C and then kept warm for not less than 240 minutes. At the same time, ammonia gas will be introduced into the furnace with a nitrogen potential of not less than 7;

[0068] S6: Nitriding 2 (second stage nitriding), after S0, S1, S2, S3, S4, and S5 are completed, the workpiece is kept at a temperature of not less than 490°C and not more than 500°C, with a holding time of not less than 720 minutes and not more than 1200 minutes. At the same time, ammonia gas is introduced into the furnace, and the nitrogen potential is not higher than 2.5;

[0069] S7: Nitriding 3 (third stage nitriding), after S0, S1, S2, S3, S4, S5, and S6 are completed, the workpiece is then controlled to a temperature of not less than 490°C and not more than 500°C, and then kept warm for not less than 180 minutes. At the same time, ammonia gas is introduced into the furnace, and the nitrogen potential is not higher than 1.5;

[0070] S8: Nitriding 4 (the fourth stage of nitriding): After S0, S1, S2, S3, S4, S5, S6 and S7 are completed, the workpiece is kept at a temperature of not less than 490°C and not more than 500°C for not less than 120 minutes. At the same time, ammonia gas is introduced into the furnace with a nitrogen potential of not more than 6 and not less than 4.

[0071] S9: Nitriding 5 (fifth stage nitriding): After S0, S1, S2, S3, S4, S5, S6, S7, and S8 are completed, the workpiece is cooled with the furnace to a temperature not higher than 480°C, and ammonia is introduced into the furnace at the same time, with a nitrogen potential not higher than 6 and not lower than 4;

[0072] S10: Cooling with the furnace. After S0, S1, S2, S3, S4, S5, S6, S7, S8, and S9 are completed, the workpiece is cooled with the furnace to a temperature not lower than 450°C, and the nitrogen flow rate is not lower than 10 ml / min;

[0073] S11: Rapid cooling. After S0, S1, S2, S3, S4, S5, S6, S7, S8, S9, and S10 are completed, the cooling pump is turned on to rapidly cool the workpiece in the furnace to no less than 80°C; at the same time, nitrogen is introduced into the furnace with a nitrogen flow rate of no less than 10 ml / min;

[0074] S12: After S0, S1, S2, S3, S4, S5, S6, S7, S8, S9, S10 and S11 are completed, the workpiece is removed from the furnace using a furnace removal device.

[0075] The workpiece is heat treated according to the above heat treatment process steps, and the corresponding experimental data of the embodiment and comparative example are as follows:

[0076] Table 1-1 Experimental data and test data of Examples 1 to 6

[0077]

[0078] Table 1-2 Experimental data and test data of Examples 7 to 13

[0079]

[0080] Table 2-1 Experimental data and test data of comparative examples 1 to 7

[0081]

[0082] Table 2-2 Experimental data and test data of comparative examples 8 to 14

[0083]

[0084] Through the analysis of the above experimental data, we can know that:

[0085] 1. Comparative Examples 1-4 (for changes in parameters in the nitriding stage 1): The tensile strength of Comparative Example 1 (nitriding temperature 510 ° C) is only 910 MPa, which is significantly lower than that of the embodiment (1000-1045 MPa), proving that the nitriding temperature of 1 exceeding 500 ° C will lead to a decrease in matrix strength; the depth of the nitriding layer of Comparative Example 2 (nitriding time 200 min) is only 0.28 mm, and the surface hardness is 547 HV, which is much lower than that of the embodiment (0.35-0.48 mm, 620-736 HV), indicating that the nitriding time of 1 is insufficient (<240 min) and a sufficient compound layer cannot be formed; the depth of the nitriding layer of Comparative Example 3 (nitriding potential 6, <7) is only 0.28 mm. The surface hardness is 541HV, which means that when the nitrogen potential is less than 7, the active nitrogen atoms are insufficient and the high-hardness surface layer cannot be effectively formed. In comparative example 4 (nitriding temperature 480°C), the nitriding layer depth is only 0.18mm and the surface hardness is 528HV, both of which are lower than the technical requirements. When the temperature is lower than 490°C, the diffusion rate of nitrogen atoms is greatly reduced and an effective nitriding layer cannot be formed. In addition, the low temperature causes the decomposition rate of ammonia to decrease, the supply of active nitrogen atoms is insufficient, and the formation of the compound layer is limited. The nitriding stage 1 needs to be strictly controlled at 490-500°C, the nitrogen potential ≥7, and the time ≥240min, otherwise the nitriding layer depth and hardness will not meet the standards, and the high temperature will damage the matrix strength.

[0086] 2. Comparative Examples 5-8 (for changes in parameters in the nitriding 2 stage): Comparative Example 5 (nitriding 2 temperature 510 ° C) has a tensile strength of 923 MPa, which is slightly lower than that of the embodiment (1000-1045 MPa), indicating that too high a temperature (> 500 ° C) will still weaken the matrix strength; Comparative Example 8 (nitriding 2 time 660 min, < 720 min) has a nitriding layer depth of 0.25 mm and a surface hardness of 534 HV, which are significantly lower than those of the embodiment, proving that insufficient time leads to incomplete deep diffusion; Comparative Example 6 (nitriding 2 nitrogen potential 3.5, > 2.5) has a nitriding layer depth of 0.52 mm (exceeding the standard), but a surface hardness of 729 HV and tensile strength of 1032MPa still meet the standards, indicating that too high nitrogen potential will excessively increase the thickness of the carburized layer, which may cause brittleness risk; in comparative example 7 (nitriding temperature 480℃), the nitriding layer depth is only 0.26mm, and the surface hardness is 539HV, both of which are lower than the technical requirements. When the temperature is lower than 490℃, the diffusion rate of nitrogen atoms is greatly reduced, and an effective carburized layer cannot be formed. In addition, the low temperature causes the decomposition rate of ammonia to decrease, the supply of active nitrogen atoms is insufficient, and the formation of the compound layer is limited; the nitriding stage 2 needs to maintain a temperature of 490-500℃, a nitrogen potential ≤2.5, and a time of 720-1200min to balance the carburized layer depth and matrix toughness.

[0087] 3. Comparative Examples 9-11 (for changes in parameters in the nitriding 3 stage): Comparative Example 9 (nitriding 3 temperature 510°C) has a surface hardness of 693HV and a tensile strength of 918MPa, both lower than those in the embodiment, indicating that overtemperature (>500°C) causes nitrogen atoms to escape, and the hardness and strength decrease simultaneously; Comparative Example 10 (nitriding 3 nitrogen potential 2.5, >1.5) has a surface hardness of 719HV and a nitrided layer depth of 0.57mm (exceeding the standard), but the tensile strength of 1022MPa meets the standard, indicating that excessive nitrogen potential will prolong surface activity and may increase the risk of brittleness; Comparative Example 11 (nitriding 3 time 150min, <180min) has a nitrided layer depth of 0.28mm and a surface hardness of 549HV. Insufficient verification time leads to insufficient surface passivation and increased porosity; the nitriding 3 stage needs to strictly maintain a temperature of 490-500°C, a nitrogen potential ≤1.5, and a time ≥180min to achieve surface passivation and control brittleness.

[0088] 4. Comparative Examples 12-14 (for parameter changes in the nitriding stage 4): Comparative Example 12 (nitriding temperature 510°C) has a tensile strength of 952 MPa, which is lower than that of the embodiment (1000-1045 MPa), indicating that the temperature is too high and the repair effect is weakened; Comparative Example 14 (nitriding potential 7, >6) has a nitrided layer depth of 0.57 mm (exceeding the standard) and a surface hardness of 741 HV, but the tensile strength of 1025 MPa still meets the standard, indicating that excessive nitrogen potential will lead to excessive back-infiltration and may cause brittleness; Comparative Example 13 (nitriding time 90 min, <120 min) has a surface hardness of 526 HV, which is significantly lower than that of the embodiment, proving that insufficient time leads to incomplete repair of microcracks; the nitriding stage 4 needs to control the temperature at 490-500°C, the nitrogen potential at 4-6, and the time at ≥120 min to repair defects and optimize corrosion resistance to avoid excessive nitriding.

[0089] In summary, the temperature, nitrogen potential, and time of multi-stage nitriding must be strictly matched. Exceeding the limit for a single parameter will destroy the balance between hardness, layer depth, and strength.

[0090] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with this profession can make some changes or modifications to the technical contents disclosed above without departing from the scope of the technical solution of the present invention, which are equivalent embodiments of equivalent changes. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of protection of the technical solution of the present invention.

Claims

1. A heat treatment process for a pin shaft for a crane telescopic arm, characterized in that: The following steps are involved: Cleaning and protective treatment: clean and protect the surface of the pin shaft; Furnace flushing treatment: Place the pin shaft after the above treatment into the furnace, and then introduce nitrogen into the furnace for flushing; Stage heating treatment: the pin in the furnace is heated in stages; Multi-stage nitriding treatment: The pin after heat treatment is subjected to multi-stage nitriding treatment according to the following process parameters: The first stage of nitriding: heating to 490-500℃ and keeping it for ≥240min, nitrogen potential ≥7; Second stage nitriding: control the temperature at 490-500℃ for 720-1200min, nitrogen potential ≤ 2.5; The third stage of nitriding: control the temperature at 490-500℃ for ≥180min, and the nitrogen potential ≤1.5; The fourth stage of nitriding: control the temperature at 490-500℃ for ≥120min, 4≤nitrogen potential≤6; Cooling treatment: Cool the pin after nitriding.

2. The heat treatment process for the pin shaft for the crane telescopic arm according to claim 1, characterized in that: The multi-stage nitriding treatment further includes a fifth stage of nitriding, and the process parameters of the fifth stage of nitriding include: cooling with the furnace to ≤480° C., 4≤nitrogen potential≤6.

3. The heat treatment process for the pin shaft for the crane telescopic arm according to claim 1, characterized in that: The cleaning of the pin shaft surface specifically includes: soaking and cleaning various parts of the pin shaft with anhydrous ethanol and / or industrial diesel.

4. The heat treatment process for the pin for the crane telescopic arm according to claim 1, characterized in that: The protection of the pin surface specifically includes: protecting the threaded hole of the pin with bolts or a coating agent, and covering the non-nitrided part of the pin with aluminum foil for protection.

5. The heat treatment process for the pin for the crane telescopic arm according to claim 1, characterized in that: The step of introducing nitrogen into the furnace for flushing specifically includes: flushing time ≥ 50 min, nitrogen flow rate ≥ 10 ml / min.

6. The heat treatment process for the pin for the crane telescopic arm according to claim 1, characterized in that: The staged heating treatment specifically includes: The first stage of heating: heating to ≤350℃ and keeping warm for ≥30min, with nitrogen flow rate ≥10ml / min; Second stage heating: heating to ≥450℃ and keeping warm for ≥30min, with nitrogen flow rate ≥10ml / min; The third stage of heating: heat to ≥480℃ and keep warm for ≥60min, introduce ammonia and control the nitrogen potential to ≥7.

7. The heat treatment process for the pin for the crane telescopic arm according to claim 1, characterized in that: The cooling treatment includes a furnace cooling stage, and the furnace cooling stage specifically includes: cooling the pin to ≥450° C. with a nitrogen flow rate of ≥10 ml / min.

8. The heat treatment process for the pin for the crane telescopic arm according to claim 7, characterized in that: The cooling treatment further includes a rapid cooling stage after the furnace cooling stage, and the rapid cooling stage specifically includes: rapidly cooling the pin to ≥80° C. and introducing nitrogen at a flow rate of ≥10 ml / min.

9. A pin shaft for a telescopic arm of a crane, characterized in that: The pin is obtained by the heat treatment process according to any one of claims 1 to 8.

10. The pin shaft for the telescopic arm of a crane according to claim 9, wherein: The nitriding layer of the pin has a depth of 0.35 to 0.48 mm, a surface hardness of 620 to 736 HV, and a tensile strength of 1000 to 1045 MPa.

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