High-strength qt800 cast iron piston with built-in cooling oil cavity

Abstract

The application discloses a high-strengthened QT800 cast iron piston with a built-in cooling oil cavity and belongs to the technical field of cast iron pistons. The piston comprises an integrally cast piston head, a piston ring belt, a piston pin seat and a piston skirt. A combustion chamber is arranged on the outer end face of the piston head, and an inner cavity is arranged on the inner end face of the piston head. The whole piston is made of Cu-Mo micro-alloyed QT800-2 nodular cast iron. The inner part of the piston head is integrated with a closed annular cooling oil cavity. The overall cylinder diameter of the piston is adapted to 81mm. The Cu-Mo micro-alloyed QT800-2 nodular cast iron material and the small-cylinder-diameter special closed annular cooling oil cavity can solve the technical defects of poor heat dissipation, large high-temperature creep, insufficient strength, obvious stress concentration, poor durability and incapability of adapting to 18MPa small-cylinder-diameter high-strengthened working conditions.

Description

Technical Field

[0001] This invention belongs to the field of cast iron piston technology, and particularly relates to a high-strength QT800 cast iron piston with a built-in cooling oil chamber. Background Technology

[0002] Small-bore (80mm class) high-performance diesel engines currently typically experience burst pressures reaching 18MPa, subjecting the pistons to long-term high-frequency, high-amplitude mechanical shocks coupled with thermal loads. Currently, small-bore cast iron pistons in the industry generally employ a structure without an internal cooling oil chamber, relying solely on open-type cooling via connecting rod oil injection, which presents significant technical shortcomings.

[0003] Conventional HT250 and QT600 cast iron pistons have insufficient high-temperature strength. Under high explosion pressure of 18MPa, the temperature of the piston top, combustion chamber throat and first ring groove area can easily exceed 400℃. Long-term operation can easily lead to faults such as widening of ring groove creep, oil coking, piston ring sticking, and thermal fatigue cracks, resulting in increased oil consumption, power loss and a significant reduction in the overall durability of the machine.

[0004] Existing piston structures with cooling oil chambers are mostly used in large-bore steel or aluminum alloy pistons. For 81mm small-bore cast iron pistons, problems exist such as unreasonable oil chamber layout, low oil filling rate, easy oil splashing and loss, severe stress concentration, and difficulty in casting. These issues make them unsuitable for demanding operating conditions with limited space and high loads. Furthermore, conventional cast iron materials have poor high-temperature creep resistance and thermal fatigue resistance, failing to meet the requirements of continuous high-load operation at 18MPa, thus hindering performance upgrades and durability improvements in small-bore, high-strength diesel engines. Summary of the Invention

[0005] The purpose of this invention is to provide a high-strength QT800 cast iron piston with a built-in cooling oil chamber in order to solve the above-mentioned problems.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a high-strength QT800 cast iron piston with a built-in cooling oil cavity, comprising an integrally cast piston head, piston rings, piston pin seat and piston skirt, wherein a combustion chamber is provided on the outer end face of the piston head and an inner cavity is provided on the inner end face of the piston head, the piston as a whole is made of Cu-Mo micro-alloyed QT800-2 ductile iron, the piston head integrates a closed annular cooling oil cavity, and the overall piston diameter is adapted to 81mm.

[0007] As a further description of the above technical solution: The composition of Cu-Mo microalloyed QT800-2 ductile iron is as follows: C 3.40%~3.70%, Si 2.30%~2.70%, Mn 0.40%~0.60%, Cu 0.60%~0.90%, Mo 0.15%~0.25%, S≤0.015%, P≤0.035%, Mg 0.035%~0.055%.

[0008] As a further description of the above technical solution: The metallographic structure of Cu-Mo microalloyed QT800-2 ductile iron has a spheroidization grade of 1 to 2, a spheroidization rate of ≥90%, and a pearlite matrix ratio of ≥85%.

[0009] As a further description of the above technical solution: The piston is integrally cast using resin sand core, and then subjected to normalizing and low-temperature tempering treatments after casting.

[0010] As a further description of the above technical solution: The closed annular cooling oil chamber is located between the combustion chamber and the first annular groove of the piston ring belt. The closed annular cooling oil chamber has a trapezoidal flat structure with a narrow top and a wide bottom. The top wall thickness of the closed annular cooling oil chamber is 6.0 to 6.5 mm. The bottom of the closed annular cooling oil chamber has a diagonally arranged directional oil inlet and oil return hole. The oil inlet is directly opposite the oil injection direction of the connecting rod, and the oil return hole is arranged at a low position.

[0011] As a further description of the above technical solution: The inner wall of the closed annular cooling oil chamber is provided with a micro-waveform flow guiding structure.

[0012] As a further description of the above technical solution: The combustion chamber has a shallow ω-shaped cross-section, and the throat of the combustion chamber is provided with a large rounded corner with R≥3.5mm. The first annular groove is strengthened by medium-frequency local quenching.

[0013] As a further description of the above technical solution: The piston pin seat adopts an inner and outer double-flared elliptical pin hole structure, and the connection between the piston pin seat and the closed annular cooling oil cavity and the inner cavity adopts a large arc transition.

[0014] As a further description of the above technical solution: The piston skirt is treated with phosphating and coated with a graphite solid lubricant composite coating, and the top surface of the piston head is coated with an anodized coating.

[0015] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: In this invention, by using Cu-Mo microalloyed QT800-2 ductile iron material and a dedicated closed annular cooling oil chamber for small cylinder diameters, the technical defects of existing technologies, such as poor heat dissipation, large high-temperature creep, insufficient strength, obvious stress concentration, poor durability, and inability to adapt to the 18MPa small cylinder diameter high-strength working conditions, are specifically solved. Attached Figure Description

[0016] Figure 1 This is a cross-sectional view of a high-strength QT800 cast iron piston with a built-in cooling oil chamber.

[0017] Legend: 1. Piston head; 2. Piston ring band; 3. Piston pin seat; 4. Piston skirt; 5. Combustion chamber; 6. Inner cavity; 7. Closed annular cooling oil chamber; 8. First annular groove; 9. Throat. Detailed Implementation

[0018] 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. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Please see Figure 1 This invention provides a technical solution: a high-strength QT800 cast iron piston with a built-in cooling oil chamber, comprising an integrally cast piston head 1, piston ring 2, piston pin seat 3, and piston skirt 4. A combustion chamber 5 is provided on the outer end face of the piston head 1, and an inner cavity 6 is provided on the inner end face of the piston head 1. The piston as a whole is made of Cu-Mo micro-alloyed QT800-2 ductile iron. The piston head 1 integrates a closed annular cooling oil chamber 7. The overall cylinder diameter of the piston is 81mm, and the rated maximum burst pressure is 18MPa. The composition of Cu-Mo microalloyed QT800-2 ductile iron is as follows: C 3.40%~3.70%, Si 2.30%~2.70%, Mn 0.40%~0.60%, Cu 0.60%~0.90%, Mo 0.15%~0.25%, S≤0.015%, P≤0.035%, Mg0.035%~0.055%. By relying on Cu to stabilize the pearlite structure and Mo to enhance high-temperature creep resistance, it solves the problem of high-temperature strength decay in conventional ductile iron. The yield strength at 350℃ is not less than 380MPa, meeting the requirements of coupled high mechanical load and high-temperature thermal load conditions of 18MPa. The metallographic structure of Cu-Mo microalloyed QT800-2 ductile iron has a spheroidization grade of 1-2, a spheroidization rate of ≥90%, and a pearlite matrix content of ≥85%. The piston is integrally cast using resin sand core, and then subjected to normalizing and low-temperature tempering treatments after casting. The closed annular cooling oil chamber 7 is located between the combustion chamber 5 and the first annular groove 8 of the piston ring 2. The closed annular cooling oil chamber 7 has a trapezoidal flat structure with a narrow top and wide bottom, which increases the oil storage volume and heat exchange area and avoids the problem of interference in small space oil chambers. The top wall thickness of the closed annular cooling oil chamber 7 is 6.0-6.5mm, which takes into account both heat dissipation efficiency and structural strength and eliminates local stress concentration. The bottom of the closed annular cooling oil chamber 7 has diagonally arranged directional oil inlet holes and oil return holes to expand the oil facing area, improve the oil capture rate, and effectively suppress oil splashing and loss. The oil inlet hole is directly facing the oil spray direction of the connecting rod, and the oil return hole is arranged at a low position to ensure smooth oil circulation inside the oil chamber and no dead corners for oil accumulation, so as to continuously remove heat from the piston top and ring. Specifically, the closed annular cooling oil chamber 7 and the hole system are machined and cast, and then the oil inlet and return holes are precision machined to ensure that the channels are smooth and unobstructed and match the oil injection path of the connecting rod; the burrs on the inner wall of the oil chamber are trimmed to ensure smooth oil circulation.

[0020] The inner wall of the closed annular cooling oil chamber 7 is provided with a micro-waveform flow guiding structure to extend the oil retention heat exchange time and further improve heat dissipation efficiency. The cross-section of the combustion chamber 5 has a shallow ω-shaped structure to eliminate thermal stress concentration. The throat 9 of the combustion chamber 5 is provided with a large rounded corner with R≥3.5mm. The first ring groove 8 is strengthened by medium-frequency local quenching, and the hardness is controlled at 42~48HRC to improve the high-temperature wear resistance and creep resistance of the ring groove and avoid high-temperature deformation and piston ring jamming. Specifically, medium-frequency local quenching is performed on the first annular groove 8 area to precisely control the quenching hardness and the depth of the hardened layer; The piston pin seat 3 adopts an inner and outer double-flared elliptical pin hole structure to optimize the stress distribution of the pin hole and reduce the peak stress at the edge of the pin hole under a high explosion pressure of 18MPa. The piston pin seat 3, the closed annular cooling oil chamber 7, and the inner cavity 6 all adopt a large arc transition at the connection. Combined with the radial reinforcing rib structure of the inner cavity 6, the overall rigidity of the piston is guaranteed while reducing weight and preventing mechanical fatigue cracking. Specifically, the transition fillets of the throat 9 and piston pin seat 3 are precision machined and polished to eliminate stress concentration sharp corners; The piston skirt 4 is treated with phosphating and sprayed with a graphite solid lubricant composite coating to reduce cylinder friction loss, adapt to the low thermal expansion characteristics of cast iron piston, and control the cold cylinder clearance at 0.060~0.090mm to effectively avoid cold engine knocking and cylinder scoring failures. The top surface of the piston head 1 is sprayed with an anodized coating to further block high temperature radiation inside the cylinder and reduce the overall working temperature of the piston. Specifically, after the piston skirt 4 is phosphated, a graphite solid lubricant coating is sprayed on and dried and cured; an anodized coating is sprayed on the top surface of the piston head 1 as needed to complete the finished product preparation.

[0021] 1. By using a closed annular cooling oil chamber, trapezoidal cross section, microwave flow guidance on the inner wall, and directional opposing oil hole structure, the defects of traditional open heat dissipation without oil chamber, such as low heat exchange efficiency, easy oil splashing, uneven cooling, and high temperature coking are completely solved.

[0022] Compared to traditional open-type oil injection cooling, the closed oil chamber of this invention can stably store oil, extend the oil heat exchange residence time, and achieve targeted and continuous cooling of the high-temperature zone of the piston top and the first ring groove, improving heat exchange efficiency by more than 40%. It can stably control the highest temperature of the piston top surface within 430°C and the temperature of the first ring groove within 320°C, fundamentally suppressing failure problems such as high-temperature creep widening of the ring groove, piston ring sticking, carbon deposits, and oil leakage, and significantly reducing the overall oil consumption rate and failure rate of the engine.

[0023] 2. By using Cu-Mo microalloyed QT800 ductile iron formulation and high pearlitic metallographic structure design, the shortcomings of ordinary QT600 and HT250 cast iron, such as low high-temperature strength, poor creep resistance, poor thermal fatigue resistance, and inability to withstand 18MPa explosive pressure coupling thermal load, are solved.

[0024] The alloy material of this invention has a high-temperature yield strength of ≥380MPa at 350℃ and strong high-temperature structural stability. Compared with conventional cast iron, it effectively avoids thermal fatigue cracks and plastic deformation at the piston top, throat and pin seat under high explosion pressure conditions, making the piston mechanical safety factor stable at ≥1.4. It is fully compatible with 81mm small cylinder diameter and long-term continuous high load conditions of 18MPa, and significantly improves the overall durability of the piston.

[0025] 3. By using a compact, conformal, thin-walled, and uniform design for the small-diameter oil cavity and a large rounded corner transition structure, the industry problems of large-diameter oil cavity structures being unable to adapt to small-diameter cylinders, spatial interference, thin wall thickness, molding difficulties, and low yield rates are solved.

[0026] The oil chamber of this invention is custom-designed for a narrow 81mm space, with a compact layout and no structural interference. The top wall thickness is uniform and controllable, which greatly improves heat dissipation capacity while ensuring the overall structural rigidity and strength of the piston. It solves the problems of easy breakage, low oil filling rate and inability to mass-produce when the oil chamber of the traditional large cylinder diameter is transplanted to a small cylinder diameter. It takes into account both high performance and casting processability, and is suitable for mass industrial production.

[0027] 4. Through the composite reinforcement structure of large rounded corners of the combustion chamber, medium-frequency quenching of the first annular groove, double-flared elliptical pin holes, and internal cavity reinforcing ribs, multiple stress concentration sources are effectively eliminated, and the fatigue resistance under thermo-mechanical coupling conditions is greatly improved.

[0028] By opening a cooling oil chamber and hollowing out the piston to reduce weight, the problem of reduced stiffness caused by the hollow structure is compensated, the peak stress at the pin hole, throat and ring groove is reduced, fatigue cracking under high cyclic alternating loads is avoided, and the defects of insufficient stiffness and easy failure of conventional modified oil chamber pistons are solved.

[0029] 5. By using phosphate coating on the piston skirt and graphite lubrication composite coating, and matching the low thermal expansion characteristics of QT800 cast iron, the problems of small cylinder clearance, large cold start friction, and easy cylinder knocking and scoring of cast iron pistons are solved.

[0030] It effectively reduces piston reciprocating friction loss, improves overall mechanical efficiency, and ensures dimensional stability, low operating noise, and good overall consistency under both hot and cold operating conditions.

[0031] 6. The overall structure has a high cost-performance ratio and strong adaptability, filling the technical gap of pistons with 81mm small cylinder diameter and 18MPa level high-strength cast iron cooling oil chamber.

[0032] Compared to steel pistons and ADI's high-end ductile iron pistons, this invention achieves lower casting and machining costs while meeting high-load durability requirements. Compared to traditional cast iron pistons, it offers a significant performance improvement, perfectly meeting the upgrade and iteration needs of non-road high-strength small-bore diesel engines, and possesses extremely high engineering promotion value and market application prospects.

[0033] 1. Significantly improved heat dissipation performance and significantly reduced heat load: The built-in closed conformal cooling oil chamber replaces the traditional open oil injection cooling system, improving the oil circulation heat exchange efficiency by more than 40%. It can control the maximum temperature of the piston top surface to within 430℃ and the temperature of the first ring groove to within 320℃, completely solving the problems of high-temperature carbon buildup, ring groove creep, and thermal cracking in small-diameter, high-strength pistons.

[0034] 2. High mechanical strength, suitable for 18MPa high explosion pressure conditions: Made of Cu-Mo alloyed QT800 material, it has excellent high temperature creep resistance and thermal fatigue resistance. Combined with the pin seat, ring groove and combustion chamber all-round reinforced structure, the mechanical safety factor is ≥1.4, which can withstand the peak explosion pressure of 18MPa for a long time, and the service life is greatly improved.

[0035] 3. Adaptable to small cylinder diameter conditions and has good processability: The oil cavity structure is specifically adapted to the narrow space of an 81mm small cylinder diameter. The layout is compact and interference-free. It is formed by one-piece sand core casting, which does not require complicated subsequent processing. It has strong mass production stability and solves the industry pain points of difficult forming of small cylinder diameter oil cavity pistons and high defect rate.

[0036] 4. Improved overall performance and stability: The piston dimensions are stable and the ring grooves are not easily deformed, effectively controlling oil consumption and reducing engine failure rate; the low thermal expansion characteristics combined with the lubricating coating result in low operating noise and low friction loss, balancing power and economy.

[0037] 5. Controllable cost and suitable for mass production: Compared with steel pistons and ADI ductile iron pistons, the QT800 alloy cast iron solution of this invention has a lower cost and its performance far exceeds that of conventional QT600 pistons. It has a significant cost-performance advantage and is suitable for mass production of small and medium diameter high-strength diesel engines.

[0038] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high-strength QT800 cast iron piston with a built-in cooling oil chamber, comprising an integrally cast piston head (1), piston ring belt (2), piston pin seat (3), and piston skirt (4), wherein a combustion chamber (5) is provided on the outer end face of the piston head (1), and an inner cavity (6) is provided on the inner end face of the piston head (1), characterized in that, The piston is made of Cu-Mo microalloyed QT800-2 ductile iron. The piston head (1) has an integrated closed annular cooling oil chamber (7). The overall cylinder diameter of the piston is 81mm.

2. The high-strength QT800 cast iron piston with a built-in cooling oil chamber according to claim 1, characterized in that, Composition of Cu-Mo microalloyed QT800-2 ductile iron: C 3.40%~3.70%, Si 2.30%~2.70%, Mn 0.40%~0.60%, Cu 0.60%~0.90%, Mo 0.15%~0.25%, S≤0.015%, P≤0.035%, Mg 0.035%~0.055%.

3. A high-strength QT800 cast iron piston with a built-in cooling oil chamber according to claim 2, characterized in that, The metallographic structure of Cu-Mo microalloyed QT800-2 ductile iron has a spheroidization grade of 1 to 2, a spheroidization rate of ≥90%, and a pearlite matrix ratio of ≥85%.

4. A high-strength QT800 cast iron piston with a built-in cooling oil chamber according to claim 1, characterized in that, The piston is integrally cast using resin sand core, and then subjected to normalizing and low-temperature tempering treatments after casting.

5. A high-strength QT800 cast iron piston with a built-in cooling oil chamber according to claim 1, characterized in that, The closed annular cooling oil chamber (7) is located between the combustion chamber (5) and the first annular groove (8) of the piston ring belt (2). The cross-section of the closed annular cooling oil chamber (7) is a trapezoidal flat structure that is narrow at the top and wide at the bottom. The top wall thickness of the closed annular cooling oil chamber (7) is 6.0 to 6.5 mm. The bottom of the closed annular cooling oil chamber (7) has a diagonally arranged directional oil inlet and oil return hole. The oil inlet is directly opposite the oil injection direction of the connecting rod, and the oil return hole is arranged at a low position.

6. A high-strength QT800 cast iron piston with a built-in cooling oil chamber according to claim 5, characterized in that, The inner wall of the closed annular cooling oil chamber (7) is provided with a micro-waveform flow guiding structure.

7. A high-strength QT800 cast iron piston with a built-in cooling oil chamber according to claim 1, characterized in that, The cross-section of the combustion chamber (5) is a shallow ω-shaped structure. The throat (9) of the combustion chamber (5) is provided with a large rounded corner with R≥3.5mm. The first annular groove (8) is subjected to medium-frequency local quenching strengthening treatment.

8. A high-strength QT800 cast iron piston with a built-in cooling oil chamber according to claim 1, characterized in that, The piston pin seat (3) adopts an inner and outer double-flared elliptical pin hole structure, and the connection between the piston pin seat (3) and the closed annular cooling oil chamber (7) and the inner cavity (6) adopts a large arc transition.

9. A high-strength QT800 cast iron piston with a built-in cooling oil chamber according to claim 1, characterized in that, The piston skirt (4) is treated with phosphating and coated with a graphite solid lubricant composite coating, and the top surface of the piston head (1) is coated with an anodized coating.

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