A composite strengthening process for high-performance nodular cast iron crankshaft surface

Abstract

The application discloses a kind of high-performance nodular cast iron crankshaft surface composite strengthening process, it is related to metal material preparation technical field, based on alloying design and after casting heat treatment crankshaft blank, the matrix structure of this blank is pearlite or lower bainite, tensile strength ≥1000 MPa, elongation ≥5%, impact value >40 J / cm², surface hardness is 25 HRC~35 HRC, before carrying out composite strengthening process, through rough machining, crankshaft blank is turned and milling processing, process crankshaft main journal, connecting rod neck and fillet contour, then drill oil hole, using intermediate frequency induction heating equipment carries out surface quenching process to make crankshaft journal surface hardening, again through numerical control rolling machine tool carries out rolling process, using roller to implement plastic deformation to crankshaft fillet position, introduce residual compressive stress in fillet surface layer, finally shot blasting, polishing and cleaning, realize the strengthening of crankshaft surface, the scheme cooperates intermediate frequency induction quenching and rolling strengthening process, to realize the effective strengthening operation to crankshaft surface.

Description

Technical Field

[0001] This invention relates to the field of metal material preparation technology, and in particular to a composite strengthening process for the surface of a high-performance ductile iron crankshaft. Background Technology

[0002] The crankshaft is the core component in an engine that converts the reciprocating motion of the piston into rotational motion. Its working conditions are extremely harsh, subjecting it to periodically changing gas combustion pressure, reciprocating inertial forces, and centrifugal forces. With the rapid development of internal combustion engine technology towards "high power density, lightweight, and low emissions," almost stringent requirements have been placed on the comprehensive mechanical properties of the crankshaft: on the one hand, the main journals and connecting rod journals must possess extremely high surface hardness and wear resistance to resist fretting wear of the bearings; on the other hand, the crankshaft fillet areas, as the areas of highest stress concentration, must possess extremely high bending fatigue strength to prevent high-cycle fatigue fracture.

[0003] Ductile iron, due to its excellent casting processability, good vibration damping, low cost, and the ability to achieve high strength through alloying, has gradually replaced traditional forged steel and become the mainstream choice for crankshafts in medium and heavy-duty engines. Although its tensile strength has approached or partially reached the level of forged steel, in practical engineering applications, the strength of the material itself alone cannot meet the service requirements under high burst pressure, and surface strengthening processes are necessary. Existing surface strengthening processes for ductile iron crankshafts commonly include: 1. chemical heat treatment; 2. medium-frequency induction hardening; 3. fillet rolling, etc.

[0004] Among these processes, ion nitriding or gas soft nitriding in chemical heat treatment are commonly used. These processes can form a high-hardness nitride layer on the crankshaft surface, significantly improving wear resistance and anti-galling capabilities. However, crankshafts treated only with nitriding have limited improvement in bending fatigue limit, making it difficult to meet the reliability requirements of high-pressure engines. Furthermore, for the strengthening method of medium-frequency induction hardening, to prevent quenching cracks caused by stress concentration at the transition between the journal and shoulder (fillet), the process usually requires the hardened zone to avoid the fillet area. This results in the fillet area, where stress is most concentrated, being in a soft matrix state, becoming a major area prone to fatigue fracture. Finally, there is the fillet rolling process. Fillet rolling is an effective means of improving crankshaft fatigue strength. By applying plastic deformation to the fillet with rollers, deep residual compressive stress can be introduced. However, in existing technologies, rolling is often treated as an independent process or the last process, neglecting its synergistic relationship with the preceding heat treatment processes.

[0005] In summary, existing technologies often achieve crankshaft surface strengthening through a single strengthening process or a simple combination of processes. This cannot simultaneously resolve the contradiction between "journal wear resistance" and "rounded corner fatigue resistance" in ductile iron crankshafts, and also presents technical problems such as poor process compatibility and difficulty in quality control. Summary of the Invention

[0006] To address the above shortcomings, this invention provides a composite strengthening process for the surface of high-performance ductile iron crankshafts, combining medium-frequency induction hardening and rolling strengthening processes to achieve effective strengthening of the crankshaft surface. The specific technical solution is as follows:

[0007] A composite strengthening process for the surface of a high-performance ductile iron crankshaft, using a crankshaft blank that has undergone alloying design and post-cast heat treatment, with a matrix structure of pearlite or lower bainite, tensile strength ≥1000 MPa, elongation ≥5%, impact value >40 J / cm², and surface hardness of 25 HRC~35 HRC, includes the following steps:

[0008] S1 Rough machining: The crankshaft blank is turned and milled to produce the main journal, connecting rod journal and fillet contour of the crankshaft. The journal diameter is reserved with a fine grinding allowance of 0.20mm~0.30mm, the fillet is reserved with a machining allowance of 0.10mm~0.20mm, and the dimensional accuracy of the fillet radius R is controlled within ±0.10mm.

[0009] S2 Oil Hole Drilling: Drill oil holes in the crankshaft blank;

[0010] S3 journal surface hardening: A medium-frequency induction heating device is used to carry out surface hardening process on the main journal and connecting rod journal of the crankshaft. The medium-frequency induction hardening process parameters are as follows: power supply is set to 350KW, working frequency is set to 45Hz~55Hz, single-station heating time is 15s~20s, quenching liquid with a concentration of 18~22% is used as the cooling medium for spray cooling, and low-temperature tempering is performed after quenching. The tempering temperature is 180℃~220℃, and the holding time is 1.5h~2.5h, so that the journal surface hardness reaches 52HRC~60HRC and the hardened layer depth is 2mm~5.0mm.

[0011] S4 Roller Hardening: The roller hardening process is carried out using a CNC roller hardening machine. Rollers are used to plastically deform the fillet area of ​​the crankshaft. The rolling force is 30KN, the number of rolling times is 1-3 times, and the rolling is 3-5 turns. Residual compressive stress is introduced into the fillet surface layer.

[0012] S5 Semi-finishing and Finishing: The crankshaft after surface hardening is precision ground to remove the oxide layer and deformation layer on the journal and fillet surfaces, so that the dimensional accuracy of the fillet radius R reaches ±0.05mm and the surface roughness Ra≤0.8μm;

[0013] S6 shot peening: Shot peening is performed on the entire crankshaft or at least the rounded corners after fine grinding and rolling to make the residual compressive stress on the crankshaft surface ≥500 MPa;

[0014] S7 Polishing and Cleaning: The crankshaft after shot peening is polished to make the surface roughness Ra of the journal and fillet ≤ 0.4μm, and then cleaned and dried.

[0015] Preferably, the power supply is set to 350KW, the working frequency is set to 50Hz, the single-station heating time is 16s, the tempering temperature is 200℃, and the heat preservation time is 1.5h.

[0016] Preferably, a quenching fluid with a concentration of 20% is used as the cooling medium for jet cooling.

[0017] Preferably, the rolling pressure is 30KN, the rolling cycle is 1, and the rolling is 4 revolutions.

[0018] Preferably, dynamic balancing correction is performed after step S5, and the dynamic balancing accuracy level is G1.6.

[0019] Preferably, the cleaning step in S7 uses an alkaline ultrasonic cleaning solution with a pH value of 9.5 to 11.0 and a cleaning temperature of 50°C to 60°C.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. This invention provides a composite strengthening process for the surface of high-performance ductile iron crankshafts. This composite strengthening process is based on ductile iron blanks that have undergone high-performance alloying design, eliminating the need for traditional normalizing. Crankshaft blanks with tensile strength ≥1000 MPa, elongation ≥5%, impact value >40 J / cm², and surface hardness of 25 HRC~35 HRC are perfectly suited for subsequent rolling processes, ensuring no brittle cracking occurs during rolling and shot peening. After this composite strengthening process, the crankshaft fatigue test strength is increased by more than 70%.

[0022] 2. The composite strengthening process provided by this invention combines a crankshaft surface hardening process based on medium-frequency induction quenching with a rolling strengthening process that uses rollers to plastically deform the crankshaft fillet parts. This process provides differentiated strengthening for different key parts of the crankshaft, avoiding the performance imbalance caused by traditional single treatment methods. Furthermore, through precise coordination of process parameters, it enables the wear resistance of the journal and the fatigue resistance of the fillet to be simultaneously and stably improved on the same crankshaft. This fundamentally solves the technical bottleneck of the prior art where these two aspects cannot be simultaneously addressed, significantly improving the overall reliability and service life of the crankshaft. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0024] Figure 1 This is a flowchart of the composite reinforcement process of the present invention; Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0026] In the description of this invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0027] In the description of this invention, "several" means one or more, "multiple" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. Where the terms "first," "second," and "third" are used for descriptive purposes and to distinguish technical features, they should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the sequential relationship of the indicated technical features.

[0028] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances. Furthermore, the technical features involved in the different embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0029] Example

[0030] like Figure 1As shown, this invention provides a composite strengthening process for the surface of a high-performance ductile iron crankshaft. The crankshaft blank used is a ductile iron part that has undergone alloying design and post-cast heat treatment. Its matrix structure is pearlite or lower bainite, tensile strength ≥1000 MPa, elongation ≥5%, impact value >40 J / cm², and surface hardness of 25 HRC~35 HRC. The process includes the following steps:

[0031] S1 Rough machining: The crankshaft blank is turned and milled to produce the main journal, connecting rod journal and fillet contour of the crankshaft. The journal diameter is reserved with a fine grinding allowance of 0.20mm~0.30mm, the fillet is reserved with a machining allowance of 0.10mm~0.20mm, and the dimensional accuracy of the fillet radius R is controlled within ±0.10mm.

[0032] S2 Oil Hole Drilling: Drill oil holes in the crankshaft blank;

[0033] S3 journal surface hardening: A medium-frequency induction heating device is used to carry out surface quenching process to harden the surfaces of the crankshaft main journal and connecting rod journal. The medium-frequency induction quenching process parameters are as follows: power supply set to 350KW, working frequency set to 45Hz~55Hz, single-station heating time to 15s~20s, using a quenching liquid with a concentration of 18~22% as the cooling medium for spray cooling, preferably using a quenching liquid with a concentration of 20% as the cooling medium for spray cooling, followed by low-temperature tempering at a tempering temperature of 180℃~220℃ and a holding time of 1.5h~2.5h, so that the journal surface hardness reaches 52HRC~60HRC and the hardened layer depth is 2mm~5.0mm. Preferably, the power supply set to 350KW, working frequency set to 50Hz, single-station heating time to 16s, tempering temperature to 200℃, and holding time to 1.5h.

[0034] S4 Roller Hardening: A CNC roller hardening machine is used to perform the roller hardening process. Rollers are used to plastically deform the crankshaft fillet area. The rolling force is 30KN, the number of rolling times is 1-3 times, and the rolling is 3-5 circles. Residual compressive stress is introduced into the fillet surface. Preferably, the rolling force is 30KN, the number of rolling times is 1 time, and the rolling is 4 circles.

[0035] S5 Semi-finishing and Finishing: The crankshaft after surface hardening is precision ground to remove the oxide layer and deformation layer on the journal and fillet surfaces, so that the dimensional accuracy of the fillet radius R reaches ±0.05mm and the surface roughness Ra≤0.8μm;

[0036] Furthermore, after finishing, dynamic balancing is performed, with a dynamic balancing accuracy grade of G1.6.

[0037] In some preferred embodiments, after the crankshaft undergoes induction heat treatment to form a journal hardened layer and surface plastic deformation treatment to form a fillet plastic deformation layer, its material structure and residual stress distribution change significantly, which can easily lead to increased mass eccentricity and rotational imbalance, thereby affecting vibration stability and fatigue life under high-speed operation. By performing dynamic balancing correction after finishing and strictly controlling the dynamic balancing accuracy level to G1.6, the mass deviation caused by the non-uniform distribution of the hardened layer and plastic deformation layer during the composite strengthening process can be effectively compensated, the rotational imbalance can be accurately eliminated, and the crankshaft can maintain excellent dynamic stability under high-speed conditions, significantly improving the overall machine reliability and service life. At the same time, this process step is closely connected with the aforementioned strengthening process, forming a complete technical closed loop from microstructure strengthening to macro dynamic balance, realizing the synergistic optimization of wear resistance, fatigue resistance and rotational stability.

[0038] S6 shot peening: Shot peening is performed on the entire crankshaft or at least the rounded corners after fine grinding and rolling to make the residual compressive stress on the crankshaft surface ≥500 MPa;

[0039] S7 Polishing and Cleaning: The crankshaft after shot peening is polished to make the surface roughness Ra of the journal and fillet ≤ 0.4μm, and then cleaned and dried.

[0040] Furthermore, the cleaning step uses an alkaline ultrasonic cleaning solution with a pH value of 9.5 to 11.0 and a cleaning temperature of 50°C to 60°C.

[0041] In some preferred embodiments, after the crankshaft undergoes induction heat treatment and surface plastic deformation treatment, its surface inevitably retains contaminants such as quenching fluid, processing oil, and metal debris. If these contaminants are not removed, they will affect the accuracy of subsequent non-destructive testing, coating adhesion, and long-term service performance. By using an alkaline ultrasonic cleaning solution with a pH value of 9.5 to 11.0 and controlling the cleaning temperature between 50°C and 60°C, the cleaning solution effectively saponifies grease and dissolves quenching medium residues in a strongly alkaline environment. At the same time, the ultrasonic cavitation effect, combined with the thermal energy, significantly enhances the penetration and peeling ability to tiny gaps and complex curved surfaces, thereby achieving efficient and thorough removal of contaminants from the surface of the reinforced layer. Furthermore, this pH and temperature range will not cause corrosion or structural degradation to the already formed hardened layer and plastic deformation layer, ensuring the integrity and stability of the composite reinforcement treatment effect. This provides a key guarantee for the high-precision testing, reliable packaging, and long-term service performance of the crankshaft.

[0042] In summary, this invention provides a composite strengthening process for the surface of high-performance ductile iron crankshafts. This composite strengthening process is based on ductile iron blanks that have undergone high-performance alloying design, eliminating the need for traditional normalizing. Crankshaft blanks with tensile strength ≥1000 MPa, elongation ≥5%, impact value >40 J / cm², and surface hardness of 25 HRC~35 HRC are perfectly suited for subsequent rolling processes, ensuring that no brittle cracking occurs during rolling and shot peening. After this composite strengthening process, the crankshaft fatigue test strength is improved by more than 70%. The composite strengthening process provided by this invention is based on the combination of a crankshaft surface hardening process of medium frequency induction quenching and a rolling strengthening process of plastic deformation of the crankshaft fillet parts using rollers. Differentiated strengthening is carried out on different key parts of the crankshaft. This not only avoids the performance imbalance caused by traditional single treatment methods, but also achieves synchronous and stable improvement of the wear resistance of the journal and the fatigue resistance of the fillet on the same crankshaft through precise coordination of process parameters. This fundamentally solves the technical bottleneck that the two cannot be achieved at the same time in the prior art, and significantly improves the overall reliability and service life of the crankshaft.

[0043] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A composite strengthening process for the surface of a high-performance ductile iron crankshaft, characterized in that, The crankshaft blank used is a ductile iron part that has undergone alloying design and post-cast heat treatment. Its matrix structure is pearlite or lower bainite, tensile strength ≥1000 MPa, elongation ≥5%, impact value >40 J / cm², and surface hardness 25 HRC~35 HRC. The process includes the following steps: S1 Rough machining: The crankshaft blank is turned and milled to produce the main journal, connecting rod journal and fillet contour of the crankshaft. The journal diameter is reserved with a fine grinding allowance of 0.20mm~0.30mm, the fillet is reserved with a machining allowance of 0.10mm~0.20mm, and the dimensional accuracy of the fillet radius R is controlled within ±0.10mm. S2 Oil Hole Drilling: Drill oil holes in the crankshaft blank; S3 journal surface hardening: A medium-frequency induction heating device is used to carry out surface hardening process on the main journal and connecting rod journal of the crankshaft. The medium-frequency induction hardening process parameters are as follows: power supply is set to 350KW, working frequency is set to 45Hz~55Hz, single-station heating time is 15s~20s, quenching liquid with a concentration of 18~22% is used as the cooling medium for spray cooling, and low-temperature tempering is performed after quenching. The tempering temperature is 180℃~220℃, and the holding time is 1.5h~2.5h, so that the surface hardness of the journal reaches 52HRC~60HRC and the hardened layer depth is 2mm~5.0mm. S4 Roller Hardening: The roller hardening process is carried out using a CNC roller hardening machine. Rollers are used to plastically deform the fillet area of ​​the crankshaft. The rolling force is 30KN, the number of rolling times is 1-3 times, and the rolling is 3-5 turns. Residual compressive stress is introduced into the fillet surface layer. S5 Semi-finishing and Finishing: The crankshaft after surface hardening is precision ground to remove the oxide layer and deformation layer on the journal and fillet surfaces, so that the dimensional accuracy of the fillet radius R reaches ±0.05mm and the surface roughness Ra≤0.8μm; S6 shot peening: Shot peening is performed on the entire crankshaft or at least the rounded corners after fine grinding and rolling to make the residual compressive stress on the crankshaft surface ≥500 MPa; S7 Polishing and Cleaning: The crankshaft after shot peening is polished to make the surface roughness Ra of the journal and fillet ≤ 0.4μm, and then cleaned and dried.

2. The composite strengthening process for the surface of a high-performance ductile iron crankshaft according to claim 1, characterized in that, The power supply is set to 350KW, the operating frequency is set to 50Hz, the single-station heating time is 16s, the tempering temperature is 200℃, and the holding time is 1.5h.

3. The composite strengthening process for the surface of a high-performance ductile iron crankshaft according to claim 1, characterized in that, A 20% quenching fluid was used as the cooling medium for jet cooling.

4. The composite strengthening process for the surface of a high-performance ductile iron crankshaft according to claim 1, characterized in that, The rolling pressure is 30KN, the rolling cycle is 1, and the rolling is 4 revolutions.

5. The composite strengthening process for the surface of a high-performance ductile iron crankshaft according to claim 1, characterized in that, After step S5, dynamic balancing correction is performed, with a dynamic balancing accuracy level of G1.

6.

6. The composite strengthening process for the surface of a high-performance ductile iron crankshaft according to claim 1, characterized in that, The cleaning step in S7 uses an alkaline ultrasonic cleaning solution with a pH value of 9.5 to 11.0 and a cleaning temperature of 50°C to 60°C.

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