Method for heat treating flat wire stock for chain

Abstract

This invention relates to the field of chain manufacturing, specifically to a heat treatment method for flat wire materials used in chains. Addressing the problems of existing processes such as sensitivity to the surface activity of incoming materials, abrupt changes in the diffusion gradient, excessive decarburization and deformation, and insufficient batch consistency, a synergistic path of "manganese phosphating—pre-oxidation—segmented carbon potential carburizing—oil quenching—low-temperature tempering" is proposed. First, a controllable manganese phosphating film is constructed on the surface of the flat wire material, and a stable precursor phase is formed through pre-oxidation. Then, during the carburizing stage, it is transformed in situ into a nano-micro composite interface, which, in conjunction with segmented carbon potential, achieves a gradual adjustment of carbon flux from rapid to stable. Finally, appropriate quenching and tempering releases the phase transformation stress. This method obtains flat wire materials with a smooth microstructure gradient, stable surface, and controlled dimensional changes without requiring additional thermal cycles. It significantly reduces decarburization and deformation, improves the wear resistance and early elongation performance of the mating components, and enhances batch consistency.

Description

Technical Field

[0001] This invention relates to the field of surface treatment technology for chain raw materials, and specifically to a heat treatment method for flat wire materials used in chains. Background Technology

[0002] Chain drive systems in automobiles, motorcycles, and construction machinery are subjected to alternating loads and boundary lubrication conditions over long periods. The key mating pairs have stringent requirements regarding the surface hardness gradient, core toughness, and dimensional stability of the materials. The industry commonly employs a heat treatment route of "carburizing (or carbonitriding) – quenching – tempering" to obtain a matched structure of surface martensite and cementite, and a core tempered troostite, thereby improving wear resistance and fatigue performance. However, most existing publicly available technologies design furnace temperature and carbon potential programs at the finished part stage (after geometric shaping of pins, rollers, etc.), rarely considering cross-process surface activity and carburizing compatibility design from the raw material stage (such as flat wire for chains). This results in room for improvement in batch-to-batch consistency, decarburization, and deformation control.

[0003] Taking the published Chinese patent document CN106086771A as an example, this document proposes a gas carburizing heat treatment process: at a carburizing temperature of approximately 900℃, with a given flow rate of propane, methanol, and nitrogen, the carbon potential is controlled in stages, and time periods such as boosting, diffusion, and heat preservation are set. Subsequently, oil quenching completes the formation of the hardened layer, thereby achieving stable control of the surface hardness and carburizing depth of the pin. This scheme optimizes the matching of temperature, time, and atmosphere at the part level, but its technical focus is still on optimizing the carburizing window at the part stage. It does not systematically discuss the "pre-adjustment" effect of the surface state of the incoming material (oxidation, residue, pre-film) and the interface structure of the raw material stage on the subsequent carbon flux. In other words, CN106086771A focuses its process improvement efforts on atmosphere control and time programming within the carburizing furnace, without establishing a clear coupling relationship between upstream surface engineering (such as phosphating film phase, film weight, film thickness, and precursor phase stability) and the carburizing carbon potential curve. It also fails to propose an integrated "material-interface-carburizing compatible" control framework for upstream incoming materials such as flat wire.

[0004] In actual mass production, flat wire materials for chains typically undergo processes such as blanking, punching, cold heading, and rolling before entering heat treatment. If a controllable and stable surface activity and anti-decarburization interface are lacking at the raw material stage, on the one hand, it will amplify the sensitivity of carburizing to furnace gas drift (oxygen probe deviation, CO / CO2 conversion error) and sealing, causing overshoot or undershoot of carbon potential supply, resulting in surface oversaturation, high residual austenite, or steep gradient changes in the carburized layer; on the other hand, for slender or thin-section blanks (typically flat wire materials), it will also cause bending, ellipticing, and twisting caused by phase transformation stress and thermal stress, increasing the pressure of subsequent dimensional correction and sorting, and limiting yield and cycle time. Existing part-oriented process optimization (including lifting, diffusion section duration allocation, heat preservation and cooling paths, etc.) is certainly important, but without the coordination of raw material side interface buffering and carbon release capacity, the hardness curve after carburizing often shows a steep attenuation in the first stage, amplified surface decarburization, or significant batch-to-batch differences, making it difficult to achieve wear resistance, low deformation, and consistency with low process complexity.

[0005] In summary, while existing technologies have improved the carburizing-quenching process for finished parts, they lack source interface engineering and carbon potential coupling design for flat wire materials, making it difficult to simultaneously achieve gradient smoothness, low deformation, and batch consistency. Therefore, there is an urgent need for a collaborative technology for the raw material stage of flat wire materials that can simultaneously achieve gradient smoothness, shallow decarburization, low deformation, and improved batch consistency without adding additional thermal cycles. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the present invention introduces manganese-based phosphating and pre-oxidation stabilizing film into the flat wire material for chain before heat treatment, and coordinates it with the segmented carbon potential carburizing-quenching process to achieve controllable surface carbon flux, thereby reducing decarburization and deformation, improving batch consistency and wear resistance life.

[0007] To achieve the above-mentioned technical objectives, the present invention provides the following technical solution.

[0008] The first invention provides a heat treatment method for flat wire material for chains, comprising the following steps:

[0009] S1. Pretreatment: The flat yarn is subjected to alkaline degreasing, water washing, acid irrigation and activation, and then water washing again in sequence.

[0010] S2. Place the flat wire material treated in S1 into a manganese phosphating solution and treat it at 88-96°C for 6-12 minutes, while controlling the ferrous ion content (Fe) in the manganese phosphating solution during the treatment. 2+ The concentration was 0.25–0.45 g / L, the redox potential (ORP) with Ag / AgCl reference was +380–+430 mV, the free acidity (FA) was 1.0–1.4 pt, and the total acidity (TA) was 14–20 pt.

[0011] S3. Rinsing and drying: Rinse and dry the phosphated flat wire material to form a manganese phosphate film with a weight of 12-22 g / m² and a thickness of 0.8-1.5 μm on its surface;

[0012] S4. Pre-oxidation: The flat yarn material treated with S3 is kept at 350-450℃ for 5-15 minutes in air or a micro-oxygen atmosphere;

[0013] S5. Carburizing: Gas carburizing is carried out at 880-930℃. The gas carburizing is controlled by segmented carbon potential Cp. The carbon potential Cp of the lifting section is 0.95-1.10wt%C and held for 30-60min. The carbon potential Cp of the diffusion section is 0.80-0.90wt%C and held for 60-140min. The carbon potential Cp is the carbon potential of the carburizing furnace gas, expressed as a mass fraction.

[0014] S6. Quenching and tempering: Quench and cool the carburized flat wire in oil at 60-80℃, and then temper it at 180-220℃ for 1-2 hours, or use a two-stage tempering: first temper at 190-210℃ for 0.3-0.8 hours, and then temper at 170-190℃ for 0.8-1.5 hours.

[0015] Preferably, the flat wire is a carburized steel flat wire, with the steel grade being 20CrMnTi and / or 10B21, and the flat wire is supplied in an annealed state with an initial hardness of HB160-200.

[0016] Preferably, the manganese-based phosphating solution contains 20-40 g / L of Mn(H2PO4)2 and 2-6 g / L of an accelerator, wherein the accelerator is selected from one or more of nitrites, nitrates, or nitrogen-containing organic salts; the manganese-based phosphating solution further contains Ni. 2+ 0.2–0.8 g / L and organic leveling agent 0.1–0.5 g / L.

[0017] Preferably, the pre-oxidation is carried out in a micro-oxygen atmosphere with an oxygen volume fraction of 2-10%, and the pre-oxidation temperature is 380-420℃, with a holding time of 8-12 minutes.

[0018] Preferably, the carburizing is carried out at 900°C, with the carbon potential Cp in the lifting section being 0.98–1.06 wt%C and held for 35–55 min, and the carbon potential Cp in the diffusion section being 0.82–0.88 wt%C and held for 90–120 min.

[0019] Preferably, the quenching uses 70℃ mineral-based quenching oil, and the quenching oil is stirred by an impeller to achieve an oil flow rate of 0.3 to 0.8 m / s.

[0020] Preferably, in the manganese-based phosphating step, Fe... 2+Electrochemical sensor and Ag / AgCl reference ORP online instrument for Fe 2+ The concentration and ORP are monitored online, and the FA and TA are adjusted in a closed loop through an automatic acid replenishment and accelerator replenishment unit to ensure that the membrane weight of the manganese phosphating film is 12-22 g / m² and the membrane thickness is 0.8-1.5 μm.

[0021] Secondly, the present invention also provides a flat wire material for chains, which is obtained by the heat treatment method described in the first aspect, and has an effective diffusion depth (ECD) of 0.85 to 0.98 mm, a surface decarburization layer depth of no more than 10 μm, and a residual austenite mass fraction of no more than 12%.

[0022] Thirdly, the present invention further provides a chain sleeve blank, which is obtained by cold forming of the flat wire material described in the second aspect.

[0023] This invention involves forming a dense manganese phosphate film in a manganese phosphate bath before the flat wire material enters the carburizing process, and controlling the Fe... 2+ The film phase and grains are stabilized with ORP (Ag / AgCl reference), followed by pre-oxidation at 350–450 °C to generate FePO4 / Fe2O3 precursor phases within the film. When segmented carbon potential carburizing at 880–930 °C is carried out, the precursor phase is transformed in situ into an ultrathin Fe3P–Fe2O3 composite layer, serving as a "carbon flux buffer-carbon release" interface. This, combined with the carbon potential curve from the Cp-increasing segment to the diffusion segment, allows the surface carbon supply to transition from rapid to stable and from the surface to the interior in a continuous manner, thereby obtaining a smooth carbon concentration gradient and a predictable microstructure evolution path. This process is also compatible with the subsequent phase transformation stress release processes of oil quenching and low-temperature tempering.

[0024] Compared with existing technologies, this invention achieves significant beneficial effects. While ensuring high surface hardness, the flat wire material forms a smooth hardening layer gradient, significantly suppressing residual austenite fluctuations caused by surface decarburization and oversaturation of the diffusion layer. Compared with the unphosphated control, the decarburized layer depth can be controlled to ≤10μm, the total deformation is reduced by ≥15%, the batch-wide consistent dimensional dispersion converges, and the post-phosphating HV curve is monotonic, reproducing consistency between laboratory and mass production. After downstream cold heading, punching, and forming into chain parts, early wear and elongation of the mating components decrease, anti-galling ability is improved, and the entire process chain is shortened, demonstrating improved wear life and enhanced dimensional stability. Detailed Implementation

[0025] To better understand the technical solution of the present invention, specific embodiments are given below. The following embodiments are used to illustrate the feasibility of the present invention, and all embodiments and comparative examples were completed under the same or comparable equipment and testing conditions. Unless otherwise stated, process conditions not specified are conventional conditions in the art.

[0026] I. Raw materials, equipment, preparation process, testing and terminology explanation

[0027] 1. Raw materials

[0028] Flat wire steel grade: preferably 20CrMnTi or 10B21; preferably supplied in annealed condition, with an original hardness of HB160–200; take (n=60) samples from each group.

[0029] 2. Key Equipment

[0030] Manganese phosphating tank: with Fe 2+ Electrochemical sensor and Ag / AgCl reference ORP online instrument; automatic acid replenishment and accelerator replenishment unit.

[0031] Pre-oxidation furnace: Controlled atmosphere box furnace (air / micro-oxygen), heating rate 5–20℃ / min.

[0032] Carburizing furnace: Gas carburizing furnace, with oxygen probe and infrared CO / CO2 analysis, carbon potential (Cp) control.

[0033] Quenching: Oil cooling at 60-80℃, equipped with a stirring device.

[0034] Tempering: Forced convection electric furnace.

[0035] Fe in the manganese phosphating tank 2+ Both the electrochemical sensor and the online redox potential meter are connected to an industrial controller, which performs sampling on Fe at a set period. 2+ Concentration and ORP are collected and recorded online, and a closed-loop regulation is formed in conjunction with the acid replenishment unit and the accelerator replenishment unit. Preferably, the Fe... 2+ The electrochemical sensor undergoes two-point or multi-point calibration using a standard solution after tank opening, solution replacement, or continuous operation for a certain period. The online ORP instrument uses an Ag / AgCl reference electrode, which is periodically maintained and zero-point verified to reduce the impact of electrode drift on tank control decisions. Furthermore, the controller performs noise reduction filtering and over-limit alarms on the online signal. When the detected value exceeds the preset upper or lower limits, it automatically triggers replenishment or process interlocks to prevent fluctuations in the tank solution state from causing coarsening of phosphating film grains, abnormal film thickness, or film weight dispersion.

[0036] 3. Processing methods

[0037] A heat treatment method for flat wire material for chains includes the following steps:

[0038] S1 Pretreatment: The flat yarn is subjected to alkaline degreasing, water washing, acid irrigation and activation, and then water washing again in sequence;

[0039] S2 manganese phosphating: The manganese phosphating solution is treated at 88-96℃ for 6-12 min to form a manganese phosphating film.

[0040] S3 Rinsing and Drying: Obtain a manganese-based phosphate film with a weight of 12–22 g / m² and a thickness of 0.8–1.5 μm. After S3, the phosphate film is sampled and judged to ensure that the workpieces entering S4 and S5 have comparable interface conditions. Preferably, the film weight and film thickness of the same batch of workpieces are measured by weighing and metallographic method according to a preset sampling ratio. When the film weight or film thickness is lower than the lower limit, it indicates insufficient film formation, and it is preferable to return to S2 for rephosphating or appropriately extend the phosphating time. When the film weight or film thickness is higher than the upper limit, it indicates over-film formation, and it is preferable to shorten the phosphating time, adjust the bath window, or perform light surface activation before phosphating to bring the film back to the target range. By setting the process judgment and rework path for film weight and film thickness, the dispersion of carbon flux boundary conditions in the subsequent carburizing process can be significantly reduced, and the gradient consistency and dimensional stability of the hardened layer can be improved.

[0041] S4 pre-oxidation: Holding at 350–450℃ for 5–15 min; in S4 pre-oxidation, the air atmosphere can be obtained directly through natural air intake or forced air supply within the furnace; the micro-oxygen atmosphere is preferably obtained by mixing inert gas and oxygen-containing gas, for example, using nitrogen as a diluent gas and mixing it with air or oxygen in a certain proportion to stabilize the oxygen volume fraction within the furnace within a preset range. Preferably, the pre-oxidation furnace is equipped with an oxygen content monitoring unit, adjusting the flow ratio of inert gas to oxygen-containing gas according to the monitoring value to suppress the thickening of oxide scale and surface roughening caused by excessive oxidation. Further, the pre-oxidation heating rate can be controlled at 5–20℃ / min, and after holding, furnace cooling or controlled cooling can be used to ensure that the workpiece surface is in a stable oxidized state before entering the carburizing heating stage, thereby forming a uniform precursor phase or transition layer of the phosphating film under the action of pre-oxidation, providing a controllable interfacial reaction basis for the subsequent carburizing stage.

[0042] S5 Carburizing: Gas carburizing is performed at 880–930℃ in stages, with the carbon potential Cp in the rising stage being 0.95–1.10 wt% C for 30–60 min, and the carbon potential Cp in the diffusion stage being 0.80–0.90 wt% C for 60–140 min. The carbon potential Cp is the equilibrium carbon content achievable on the workpiece surface by the carburizing furnace atmosphere. It is calculated from the oxygen probe signal inside the furnace combined with CO and CO2 analysis results, and is controlled in a closed loop by the control system through adjusting the ratio of enriched gas to carrier gas. Preferably, the carburizing furnace employs a carrier gas and enriched gas supply system. The carrier gas can be an endothermic atmosphere or other base atmosphere suitable for gas carburizing, and the enriched gas can be a hydrocarbon enriched gas or a carbon-containing enriching medium. The control system adjusts the amount of enriched gas added according to the Cp deviation, and fine-tuning of the carbon potential can be achieved by introducing small amounts of air or carbon dioxide. Furthermore, when switching carbon potential between the lifting section and the diffusion section, it is preferable to set a steady-state transition time so that the furnace atmosphere composition reaches the new target carbon potential before the holding time of that section is included. This reduces surface oversaturation or undercarbonization caused by the transient carbon potential switching, thereby obtaining a monotonous and smooth hardness gradient and a stable effective diffusion layer depth.

[0043] S6 Quenching and Tempering: Oil quenching at 60–80℃, followed by tempering at 180–220℃ for 1–2 hours, or a two-stage tempering process: first tempering at 200℃ for 0.5 hours, then tempering at 180℃ for 1.0 hour. During S6 oil quenching, mineral-based quenching oil preheated to the target temperature is preferred, and a stable oil flow is formed by impeller stirring to reduce uneven cooling caused by differences in vapor film stages. The oil flow rate can be directly measured at specified measuring points using a flow meter, or calculated by converting impeller speed and tank structure parameters, and the oil flow rate is stabilized within the set range by adjusting the impeller speed. Furthermore, after tempering, air cooling or controlled cooling is preferred to avoid introducing secondary thermal stress through rapid cooling. When using two-stage tempering, after the first stage of tempering, the furnace can be briefly refrigerated and the temperature reduced to the second stage tempering temperature before timing, to reduce the dispersion of tempering effect caused by temperature fluctuations, thereby further improving the consistency of batch dimensions and the dispersion of curvature.

[0044] 4. Terminology and Parameter Definitions

[0045] In this specification, unless otherwise stated, the meanings of the relevant terms and parameters are as follows:

[0046] (1) Oxidation-reduction potential (ORP): refers to the oxidation-reduction potential of the bath solution measured with Ag / AgCl reference electrode as a reference, which is used to characterize the change in the oxidation capacity of the bath solution;

[0047] (2) Free acidity FA and total acidity TA: Both are expressed as points pt. The points are the volume of standard alkali solution consumed when titrating to the specified endpoint, which is used to characterize the acidity and buffering capacity of the bath solution.

[0048] (3) Ferrous ion Fe 2+ : refers to the mass concentration of ferrous ions in the phosphating bath, expressed in g / L, used to characterize the catalytic / activation conditions of the film-forming reaction;

[0049] (4) Carbon potential Cp: refers to the equivalent carbon content characterization value when the carburizing furnace atmosphere reaches equilibrium with the workpiece surface, expressed as wt%C, which is converted from oxygen probe and CO and CO2 analysis data and used for carburizing control.

[0050] (5) Membrane weight and membrane thickness: Membrane weight is the mass of the phosphating membrane per unit area, expressed in g / m². 2 The film thickness is the metallographic thickness of the phosphating film, expressed in μm.

[0051] (6) Effective penetration depth (ECD): refers to the depth along the cross-sectional microhardness curve from the surface inward, at the position corresponding to the microhardness dropping to 550 HV, in mm;

[0052] (7) Decarburization layer depth: refers to the depth of the surface carbon content reduction layer as measured by the specified metallographic or standard methods, in μm;

[0053] (8) Residual austenite content: refers to the residual austenite content measured by X-ray diffraction, expressed as a percentage;

[0054] (9) Intra-batch discrete 3σ: refers to three times the standard deviation of the test results of the same batch of samples, used to characterize intra-batch consistency.

[0055] II. Examples, Comparative Examples, and Tests

[0056] 1. Implementation Examples and Comparative Examples

[0057] The following lists the key process parameters for the embodiments and comparisons. Process steps not listed are as described in the heat treatment method above.

[0058] Example 1

[0059] Flat wire material: 20CrMnTi;

[0060] Manganese-based phosphating solution: Accelerator 4g / L (a mixture of nitrite and nitrate);

[0061] Phosphating conditions: 90℃ × 10 min;

[0062] Tank control index: Fe 2+ 0.35 g / L; ORP +400 mV (Ag / AgCl); Free acidity FA 1.2 pt; Total acidity TA 16 pt;

[0063] Pre-oxidation conditions: 400℃ × 10 min;

[0064] Carburizing (900℃): Carbon potential segmentation—Raising segment Cp 1.02wt%C × 45min; Diffusion segment Cp 0.85wt%C × 100min;

[0065] Quenching and tempering: quenching in mineral oil at 70℃; tempering at 200℃ for 0.5h and then at 180℃ for 1.0h.

[0066] Example 2

[0067] Flat wire material: 10B21;

[0068] The manganese-based phosphating formulation, conditions, and tank control parameters are the same as in Example 1;

[0069] Pre-oxidation: 380℃ × 12 min;

[0070] Carburizing (900℃): Lifting section Cp 0.98wt%C × 40min, Diffusion section Cp 0.82wt%C × 90min;

[0071] Quenching and tempering: Same as in Example 1.

[0072] Example 3

[0073] Flat wire material: 20CrMnTi;

[0074] Manganese-based phosphating solution: Mn(H2PO4) 230 g / L; accelerator 4 g / L; Ni 2+ 0.5 g / L; Organic leveling agent 0.2 g / L;

[0075] Phosphating conditions: 92℃ × 8 min;

[0076] Tank control index: Fe 2+ 0.30g / L, ORP+410mV, FA1.1pt, TA18pt;

[0077] Pre-oxidation conditions: 420℃×8min.

[0078] Carburizing (900℃): Lifting section Cp 1.00wt%C × 40min, diffusion section Cp 0.85wt%C × 110min.

[0079] Quenching and tempering: Same as in Example 1.

[0080] Comparative Example A

[0081] The comparative examples are for reference only and are not part of the technical solution of this invention.

[0082] Flat wire material: 20CrMnTi; no phosphating, only degreasing / activation; no pre-oxidation;

[0083] Carburizing (900℃): Lifting section Cp 1.02wt%C × 45min, Diffusion section Cp 0.85wt%C × 100min;

[0084] Quenching and tempering: Same as in Example 1.

[0085] Comparative Example B

[0086] Flat wire material: 20CrMnTi;

[0087] Manganese-based phosphating: The formulation, conditions, and tank control parameters are the same as in Example 1;

[0088] No pre-oxidation is performed;

[0089] Carburizing (900℃): Lifting section Cp 1.02wt%C × 45min, Diffusion section Cp 0.85wt%C × 100min;

[0090] Quenching and tempering: Same as in Example 1.

[0091] Comparative Example C

[0092] Flat wire material: 20CrMnTi;

[0093] Manganese-based phosphating: The formulation and conditions are the same as in Example 1; the tank control parameters are the same as in Example 1;

[0094] Pre-oxidation conditions: 400℃ × 10 min;

[0095] Carburizing (900℃): 1.20wt%C × 45min in the boosting section, and 0.95wt%C × 90min in the diffusion section (set as an excessively high carbon potential window to examine the risk).

[0096] Quenching and tempering: Same as in Example 1.

[0097] Examples and comparative examples were used to determine the comparison system: Comparative example A tested for the absence of phosphating; Comparative example B tested for the absence of pre-oxidation; Comparative example C tested for the risk of excessively high carbon potential. Example 3 investigated the effects of Ni²⁺ and leveling agents on film grain size and uniformity.

[0098] 2. Detection Method

[0099] Phosphating film weight: weighing method, GB / T9792-2003; film thickness: metallographic method.

[0100] Decarburized layer: GB / T224–2019.

[0101] Microhardness: GB / T4340.1-2024; Effective penetration depth (ECD): the depth at which the hardness curve drops to 550 HV.

[0102] Curvature: Measured on a three-point fixed-distance platform, results are expressed in mm / m; intra-batch discreteness is characterized by 3σ.

[0103] Metallography: GB / T13298-2015; Residual austenite: XRD.

[0104] Wear of the mating parts: The flat wire was cold-formed into short sleeve samples (Φ7.94×8) using the same mold, and the wear volume was tested in a four-ball friction machine (load 392N, speed 1200rpm, oil-based lubricant 46#, 60min) (ASTM D4172 approximate conditions, for relative comparison).

[0105] 3. Phosphating film quality and bath solution control results: the average of each group with n=10 is taken, and the results are shown in Table 1.

[0106] Table 1. Results of Online Control of Phosphating Film and Bath Solution

[0107]

[0108] As can be seen from Table 1, Example 3, due to the addition of The addition of a leveling agent refines the grains and slightly reduces the film thickness, which is beneficial for the steady-state transformation during subsequent heating.

[0109] 4. Hardness gradient and effective carburizing results, HV0.5, carburizing at 900℃, as shown in Table 2.

[0110] Table 2. Cross-sectional microhardness and effective infiltration results

[0111]

[0112] As shown in Table 2, the hardness curves of Examples 1 and 3 are monotonous and smooth, with ECD close to 0.9-0.95 mm; Comparative Example A has a shallower ECD and a steeper decline in the early part of the curve due to the lack of manganese phosphating-pre-oxidation interface adjustment; Comparative Example C has an excessively high Cp, resulting in a supersaturated surface layer, significantly higher residual austenite, and an excessively deep gradient, which increases the risk of subsequent deformation and embrittlement.

[0113] 4. The results of the decarburized layer depth and surface microstructure are shown in Table 3.

[0114] Table 3 Decarburized layer depth and surface microstructure

[0115]

[0116] As shown in Table 3, the precursor phase formed by manganese phosphating and pre-oxidation of the present invention is transformed in situ into a carbon flux buffer interface during carburizing, which effectively suppresses surface decarburization and step gradient. Decarburization is obvious without phosphating (Comparative Example A). Although there is improvement without pre-oxidation (Comparative Example B), it is still inferior to the examples.

[0117] 5. Dimensional stability and batch consistency, see Table 4.

[0118] Table 4. Curvature and Three-Standard Deviation (3σ) within a Batch (n=60)

[0119]

[0120] As shown in Table 4, compared with Comparative Example A, the total deformation of Example 1 was reduced by approximately 23%, while that of Example 3 was... The leveling agent further converges to 3σ. In contrast, due to excessively high Cp, the comparative example introduces phase transformation stress and structural anomalies, significantly amplifying the curvature and dispersion.

[0121] 6. Wear comparison of mating parts (short sample after forming), see Table 5.

[0122] Table 5. Wear volume and early elongation of four balls (relative values)

[0123]

[0124] As shown in Table 5, the wear volume of Examples 1 and 3 is significantly lower than that of the comparative example, reflecting the improvement of wear and early elongation of the mating pair due to the smooth gradient of the hardened layer and the stability of the surface structure; the oversaturated surface and high residual austenite of the comparative example C resulted in the worst wear and early elongation.

[0125] 7. Summary of Overall Results

[0126] Based on the results in Tables 1 to 5, it can be seen that in the process chain of manganese phosphating—pre-oxidation—segmented carbon potential carburizing—oil quenching—low-temperature tempering, the present invention, through the synergistic control of the phosphating film quality and the carburizing carbon potential window, enables the flat wire material used in the chain to obtain a stable hardened layer gradient, shallow decarburization, and low deformation, and can further meet the requirements for mating wear and early elongation of cold-formed sleeve blanks. To facilitate understanding of the feasibility and control points of the present invention, the following is a summary explanation from three aspects: product implementation method, window deviation risk, and overall effect.

[0127] 7.1 Product Implementation Method

[0128] The flat wire material for chains prepared by the methods described in Examples 1 to 3 has a cross-sectional microhardness curve that shows a monotonous and smooth downward trend (see Table 2). The effective diffusion depth (ECD) is preferably 0.85–0.98 mm, the surface decarburization layer depth is preferably no greater than 10 μm (see Table 3), and the residual austenite content is preferably no greater than 12% (see Table 2). This ensures both surface wear resistance and contact fatigue resistance while also taking into account core toughness and dimensional stability.

[0129] The flat wire material can be further cold-formed to prepare chain sleeve blanks: the flat wire material is cut or punched into blanks according to a preset blanking length, and then cold-forged or cold-extruded to form the sleeve shape and complete the sizing. Because the hardened layer obtained by the present invention has a smooth gradient and shallow decarburization, the cold-formed sample has a lower wear volume and smaller early elongation under the relative comparison conditions of four-ball friction (see Table 5). At the same time, the mean curvature of the flat wire material after heat treatment and the batch dispersion (3σ) converge significantly (see Table 4), which is beneficial to meet the consistency requirements of subsequent punching, cold forging and assembly of the chain.

[0130] 7.2 Risk of Process Window Deviation

[0131] When critical steps are missing or the window of opportunity is missed, foreseeable quality risks can easily be introduced:

[0132] (1) No phosphating and no pre-oxidation (Comparative Example A): During the carburizing process, the surface carbon flux boundary conditions are unstable, which easily leads to the deepening of decarburization and the abrupt change of the carburized layer transition zone (see Table 3). The hardness curve attenuates sharply in the first part, the ECD is shallow and the residual austenite is high (see Table 2), which leads to the increase of curvature and intra-batch dispersion (see Table 4), and the deterioration of wear volume and early elongation (see Table 5).

[0133] (2) Phosphating but no pre-oxidation (Comparative Example B): Although it is an improvement over Comparative Example A, there is still a problem of insufficient control of decarburization and diffusion layer transition zone (see Table 3). The hardness gradient and consistency are still inferior to the example (see Table 2 and Table 4), indicating that pre-oxidation is necessary as a link in the process chain.

[0134] (3) Excessive carbon potential (Comparative Example C): When the carbon potential of the lifting and diffusion sections exceeds the preferred window, the surface layer is prone to carbon supersaturation, resulting in a significant increase in residual austenite and excessive diffusion layer (see Table 2). The superposition of phase transformation stress and abnormal structure significantly amplifies the mean and dispersion of bending (see Table 4), and the wear volume and early elongation are the worst (see Table 5), indicating that excessively high carbon potential windows should be avoided (for example, the carbon potential of the lifting section reaches or exceeds about 1.20 wt% C).

[0135] 7.3 Overall Effect

[0136] In summary, the technical effect of this invention is not achieved through optimization of a single parameter, but rather through a synergistic improvement resulting from interface engineering and segmented carbon potential, primarily manifested in:

[0137] 1) Necessity and mechanism of interface engineering: The conversion film formed by manganese phosphating (film weight in the range of 12-22 g / m², film thickness in the range of 0.8-1.5 μm) together with the subsequent pre-oxidation (350-450℃, 5-15 min) constitutes a stable interface precursor layer, which makes the carbon flux boundary conditions in the carburizing stage more controllable, thereby effectively suppressing surface decarburization and reducing the step change in the transition zone of the carburized layer, making the hardness curve smoother, and the decarburized layer depth preferably not greater than 10 μm (see Table 3).

[0138] 2) Synergistic window of segmented carbon potential: By adopting two-stage carbon potential control of the lifting stage and the diffusion stage, a stable and repeatable effective carburization depth ECD (preferably about 0.85 to 0.98 mm) can be obtained under carburizing conditions of 880 to 930℃, while taking into account the control of residual austenite and deformation. When the carbon potential exceeds the preferred window, it will introduce the risk of increased residual austenite, excessively deep carburization, and amplified deformation dispersion (Comparative Example C).

[0139] 3) Improved dimensional stability and batch consistency: Under the parameters of this invention, the mean curvature of the flat wire material after heat treatment and the batch 3σ are significantly converged (the dispersion level is reduced by about 20-40% compared with comparative example A, see Table 4), which meets the dimensional and matching requirements of subsequent cold heading and punching of the chain.

[0140] 4) Optimize the phosphating system to further improve consistency: Ni can be added to the phosphating solution. 2+ With the addition of a small amount of organic leveling agent (e.g., 0.2–0.8 g / L) and a small amount of organic leveling agent (e.g., 0.1–0.5 g / L), the phosphating film grains can be further refined and the film uniformity can be improved, thereby further converging the 3σ within the batch (the improvement of Example 3 compared to Example 1 is shown in Tables 1 and 4).

[0141] Therefore, the examples and comparative examples together demonstrate that the present invention, through the linkage control of manganese phosphating—pre-oxidation—segmented carbon potential carburizing—oil quenching—low temperature tempering, achieves a smooth hardened layer gradient, shallow decarburization, low deformation and good consistency without increasing additional thermal cycles, and can effectively improve the wear of the mating parts and early elongation performance of the formed sleeve blank.

[0142] The above description is merely a preferred embodiment of the present invention, used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Through the description of the disclosed embodiments, those skilled in the art can clearly understand and implement the present invention. All equivalent substitutions, modifications, or variations made within the spirit and scope of the present invention should be covered within the scope of protection of the present invention. The technical ideas and principles of the present invention can be applied to other similar materials and heat treatment systems, and should not be limited to the parameters or equipment forms listed in the specific embodiments herein. Therefore, the present invention will not be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novelty disclosed herein.

Claims

1. A heat treatment method for a flat wire material for a chain, characterized by, Includes the following steps: S1. The flat yarn is subjected to alkaline degreasing, water washing, acid irrigation and activation, and then water washing in sequence; S2, the flat wire material treated in S1 is placed in a manganese phosphating solution, and is treated at 88-96℃ for 6-12min, and the ferrous ion Fe 2+ concentration is 0.25-0.45g / L, the oxidation-reduction potential ORP with Ag / AgCl reference is +380 to +430mV, the free acidity FA is 1.0-1.4pt, and the total acidity TA is 14-20pt. S3, rinsing and drying: rinsing and drying the phosphated flat wire to form a manganese phosphating film with a film weight of 12-22 g / m 2 and a film thickness of 0.8-1.5 μm on the surface thereof. S4. The flat yarn treated in S3 is kept at 350-450℃ for 5-15 minutes in air or a slightly oxygenated atmosphere. S5. Gas carburizing is carried out at 880-930℃. The gas carburizing is controlled by segmented carbon potential Cp. The carbon potential Cp of the lifting section is 0.95-1.10wt%C and held for 30-60min. The carbon potential Cp of the diffusion section is 0.80-0.90wt%C and held for 60-140min. The carbon potential Cp is the carbon potential of the carburizing furnace gas, expressed as a mass fraction. S6. Quench and cool the carburized flat wire in oil at 60-80℃, then temper it at 180-220℃ for 1-2 hours, or use a two-stage tempering process: first temper at 190-210℃ for 0.3-0.8 hours, and then temper at 170-190℃ for 0.8-1.5 hours.

2. The heat treatment method according to claim 1, characterized by, The flat wire material is carburized steel flat wire material, the steel grade is 20CrMnTi and / or 10B21, and the flat wire material is supplied in the annealed state with an original hardness of HB160~200.

3. The heat treatment method according to claim 1, characterized by, The manganese-based phosphating solution contains 20-40 g / L of Mn(H2PO4)4 and 2-6 g / L of an accelerator, wherein the accelerator is selected from one or more of nitrites, nitrates or nitrogen-containing organic salts.

4. The heat treatment method according to claim 3, characterized by, The manganese-based phosphating liquid further comprises Ni 2+ 0.2-0.8 g / L and an organic leveling agent 0.1-0.5 g / L.

5. The heat treatment method according to claim 1, characterized by, The pre-oxidation is carried out in a micro-oxygen atmosphere with an oxygen volume fraction of 2-10%, and the pre-oxidation temperature is 380-420℃, with a holding time of 8-12 minutes.

6. The heat treatment method according to claim 1, characterized by, The carburizing is carried out at 900℃, with the carbon potential Cp in the lifting section being 0.98–1.06 wt%C and held for 35–55 min, and the carbon potential Cp in the diffusion section being 0.82–0.88 wt%C and held for 90–120 min.

7. The heat treatment method according to claim 1, characterized by, The quenching process uses 70℃ mineral-based quenching oil, and the quenching oil is agitated by an impeller to achieve an oil flow rate of 0.3 to 0.8 m / s.

8. The heat treatment method according to claim 1, characterized by, The manganese-based phosphating step is carried out by Fe 2+ The electrochemical sensor and Ag / AgCl reference ORP online instrument monitor the concentration of Fe 2+ The concentration of Fe and TA are monitored online by ORP, and closed-loop adjustment of FA and TA is carried out by automatic acid and accelerator supplement units to stabilize the film weight and thickness of the manganese-based phosphating film within the range defined in claim 1.

9. A flat strand material for a chain, characterized by It is prepared by the heat treatment method according to any one of claims 1 to 8, and its effective diffusion depth ECD is 0.85 to 0.98 mm, the surface decarburization layer depth is not greater than 10 μm, and the retained austenite mass fraction is not greater than 12%; wherein the effective diffusion depth ECD is the depth at which the microhardness curve of the flat wire cross section drops to 550 HV.

10. A chain sleeve blank, characterized by It is obtained by cold forming of the flat wire material as described in claim 9.

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