Method for reducing cracking of large end of differential half axle gear cold forging

Abstract

The invention discloses a method for reducing cracking of the large end of a differential half axle gear cold forging.The process comprises the specific steps of sawing, normalizing treatment, spheroidizing annealing treatment, turning, shot blasting treatment, phosphorus saponification treatment, molybdenum disulfide powder coating and tooth profile cold extrusion. The situation that granular bainite appears in a spheroidized structure is avoided, the problems that the material hardness cannot be reduced, and the plastic deformation capacity of the material cannot be improved are solved, the cold forging forming quality is improved, and the defects that tooth profiles are not filled fully and large ends crack are overcome.

Description

Technical Field

[0001] The invention relates to the technical field of automobile parts production, in particular to a method for reducing cracking of the big end of a cold forged differential half-shaft gear. Background Art

[0002] In recent years, China has witnessed rapid development and increasing penetration of new energy vehicles. Electric vehicles (EVs) feature high motor speeds, exceeding 20,000 rpm, and offer excellent acceleration performance, with 0-100 km / h acceleration times now dropping to under 4 seconds. These performance requirements place higher demands on automotive gears, requiring high tooth root fatigue strength to withstand the shock, vibration, and noise generated by high-speed operation and acceleration. Consequently, the demand for gear steel is increasing, requiring increased strength for both gears and shafts. The increasing use of higher-strength materials for various gears makes forging, especially cold forging, more challenging. Cold forging is a manufacturing process that forms metal at room temperature using a die and a press. It offers advantages such as high strength, high precision, and high material utilization. However, cold forging is associated with high forming forces and high die costs. Reducing costs and increasing efficiency has been a major area of ​​focus for the industry, with spheroidization of the blank playing a key role.

[0003] Currently, the materials used for passenger car differential spur gears are generally low-carbon alloy steels such as 20CrMo, 20MnCr5, and 20CrMnTiH. The forging of differential spur gears typically utilizes a combined warm-cold forming process, where the tooth profile is formed by warm forging followed by cold finishing. This process has disadvantages such as oxidation and decarburization of the blank during heating, high equipment investment, the need for automated mass production, and the high tonnage of the cold-finished tooth surface. Many manufacturers also utilize a one-step cold forging process for the forging of differential spur gears. These gear steels require a spheroidizing annealing softening treatment to improve the metal's plasticity and fluidity.

[0004] While existing technology can improve the plastic fluidity of metal materials through spheroidizing annealing and softening, if spheroidization is poor, the deformation ratio increases as the diameter of the tooth's large end increases, which can easily lead to quality issues such as large end cracking. Some manufacturers increase subsequent machining allowances to turn away cracks, but these microcracks can extend into the interior of the forging, leaving the product at risk of premature cracking and failure. Summary of the Invention

[0005] Purpose of the invention: In order to address the deficiencies of the prior art, the present invention provides a method for reducing cracking at the big end of cold forged differential axle gears, optimizes the die structure design, reduces material flow resistance, increases the fluidity of the metal material, and fundamentally solves quality problems such as incomplete tooth filling and cracks easily occurring at the big end of the tooth profile. The structure is uniform, so that the product does not have abnormal structure.

[0006] Technical Solution: To achieve the above objectives, the present invention provides a method for reducing the cracking of the big end of a cold forged differential axle gear. The specific steps of the process are as follows: Step 1: Sawing According to the drawing requirements, cylindrical bars with limited diameter specifications are selected, and then cut into blanks by a cutting machine; Step 2: Normalizing The blank is placed in a box-type resistance furnace for heating at 870~950℃ for 3.5h~4h; then air-cooled to 570~650℃ for isothermal treatment for 5h~6h, and then taken out of the furnace for air-cooling to room temperature to eliminate granular bainite and homogenize the structure; Step 3: Spheroidizing annealing The blank after normalizing is subjected to spheroidizing annealing treatment. The blank in the box-type resistance furnace is heated again at a temperature of 720°C~760°C and kept at this temperature for 3h~4h. Then, the cooling system switch in the box-type resistance furnace is turned on to rapidly cool the blank to 650°C~660°C. After keeping this temperature for 5h~6h, the blank is cooled to 400°C~450°C without blowing air and then taken out of the furnace to increase the spheroidization rate and reduce the hardness. Step 4: Turning The softened blank is turned into an end portion and an outer diameter of a defined size by a lathe; Step 5: Shot blasting The blank after turning is shot blasted with a drum shot blasting machine. The diameter of the shot blasting particles in the shot blasting machine is 1.2~1.5mm, and the shot blasting time is 25~30min to remove the attachments on the surface of the bar, improve the surface roughness of the workpiece, and improve the adhesion of the phosphorus saponified lubricating film. Step 6: Phosphorus saponification treatment The blank after shot blasting is subjected to phosphorus saponification treatment, the phosphating temperature is 70℃~85℃, the time is 15min~20min, and the phosphating layer with a thickness of 10μm~15μm is formed; the saponification temperature is 70℃~85℃, the time is 5min~8min, and the saponification layer with a thickness of 5μm~8μm is formed; Phosphating and saponifying the shot-blasted blanks. Zinc phosphate is used for phosphating. Due to its high density and excellent adhesion, a stable zinc phosphate layer can withstand pressures exceeding 2000 MPa. The aqueous solvent used for phosphating contains the original zinc phosphate (Zn(H,PO)) and phosphoric acid (HPO), which causes a series of chemical reactions in the blank during phosphating. The phosphate layer serves as a support layer for the saponified lubricant, which is then applied to the surface. The lubricant support layer firmly bonds the lubricant to the outer surface of the blank. The lubricant coating acts as a sliding layer, reducing friction between the extruded part and the die. This lubricant layer must not be allowed to fall off, otherwise scratches and grooves caused by cold welding will immediately appear on the workpiece, leading to die scratches.

[0007] Step 7: Molybdenum disulfide powder coating The surface of the phosphorus saponified blank is lubricated and coated with an ultrafine molybdenum disulfide solution, with the coating area being the upper half of the blank; Step 8: Cold extrusion tooth shape After the upper toothed die and the lower die are opened, the lubricated blank is placed between the upper toothed die and the lower die. The lower punch is installed at the axis of the lower die. The upper toothed die is sleeved on the outer side of the inner punch of the upper toothed die. The upper toothed die and the inner punch of the upper toothed die above the blank move downward along with the slide of the press. The press continues to move downward after the upper toothed die and the lower die are closed, so that the blank is deformed into a formed forging under the extrusion of the lower punch. After the extrusion is completed, the press returns upward with the upper tooth-shaped die, disengages from the lower die, and the press ejector pushes out the formed forging after the tooth extrusion is completed; The lower concave die is provided with a die entry opening, and the die entry opening is provided with an inclined surface.

[0008] As a further preferred embodiment of the present invention, in step 1, the diameter of the blank is 5 mm to 8 mm smaller than the inner diameter of the lower die.

[0009] As a further preferred embodiment of the present invention, in step three, the blank is placed in a box-type resistance furnace for heating again.

[0010] As a further preferred embodiment of the present invention, in step three, the cooling system switch is a nitrogen circulation switch, and the cooling rate is 30°C / h~80°C / h.

[0011] As a further preferred embodiment of the present invention, in step 4, the shot blasting machine is a drum-type shot blasting machine.

[0012] As a further preferred embodiment of the present invention, in step 5, the diameter of the shot blasting particles in the shot blasting machine is 1.2 mm to 1.5 mm. As a further preferred embodiment of the present invention, in step seven, the concentration of the ultrafine molybdenum disulfide solution is greater than 15%.

[0013] As a further preferred embodiment of the present invention, in step seven, the surface friction coefficient μ of the blank after surface lubrication treatment is 0.05-0.06.

[0014] As a further preferred embodiment of the present invention, in step eight, the angle of the inclined surface provided on the die inlet is 30° to 60°.

[0015] As a further preferred embodiment of the present invention, the angle of the inclined surface increases as the difficulty of extrusion forming the blank increases.

[0016] Beneficial effects: Compared with the prior art, the method for reducing the cracking of the big end of the cold forged differential axle gear of the present invention has the following advantages: (1) By optimizing the mold structure design, the material flow resistance is reduced, the fluidity of the metal material is increased, and the cause of the cracks is fundamentally solved; (2) The structure obtained by this process is a uniformly distributed ferrite matrix and spheroidized pearlite, avoiding the appearance of granular bainite, so that the product does not have abnormal structure; (3) This process solves the problem of excessive softening of materials due to low carbon content after traditional normalizing and annealing, effectively reduces the defective rate of materials, ensures that the materials have better mechanical properties in subsequent processing, and improves the yield of materials; (4) The bevel gear forgings obtained by cold forging the blank through this process have good forming quality, full tooth filling, and basically no cracking at the big end, which reduces the cost and is suitable for large-scale automated production; (5) Through the provided lubrication treatment process, the lubrication layer has strong bonding strength and uniform thickness, which reduces the risk of mold vent blockage; (6) By utilizing this process, the cold forging quality can be significantly improved, and the problems of incomplete tooth filling, large end cracking, and other defects can be reduced. Figures in the specification

[0017] Figure 1 It is a schematic diagram of the process flow of the present invention; Figure 2 This is a full cross-sectional view of the cold extrusion tooth die; DETAILED DESCRIPTION

[0018] The present invention will be further illustrated below with reference to specific embodiments.

[0019] The invention discloses a method for reducing cracking of the big end of a cold forged differential half-shaft gear. The specific steps of the process include sawing, normalizing, spheroidizing annealing, turning, shot blasting, phosphorus saponification, molybdenum disulfide powder coating and cold extrusion of tooth profile.

[0020] Before sawing, the raw materials need to be inspected upon arrival at the factory. The raw materials for gear steel are generally low-carbon alloy steels such as 20CrMo, 20MnCr5, 20CrMnTiH, and 20CrNiMo. They are required to have a normalized structure of 1 to 3 levels, a grain size ≥ 5 levels, a hardness of HB140 to 190, and no granular bainite, mixed crystals, Widmanstätten structure, and other undesirable structures. If the requirements are not met, isothermal normalizing treatment is required after cutting. Example

[0021] Step 1: Saw the material According to the drawing requirements, a cylindrical bar with a limited diameter is selected, and then the blank 1 is obtained by sawing it through a blanking machine; Step 2: Normalizing The blank 1 after being cut is placed in a box-type resistance furnace for heating at a temperature of 870°C and a holding time of 3.5 hours; air-cooled to 570°C for isothermal treatment for 5 hours, and then taken out of the furnace for air-cooling to room temperature to eliminate granular bainite and homogenize the structure; Step 3: Spheroidizing annealing The blank 1 in the box-type resistance furnace is heated again at a temperature of 720°C and a holding time of 3 hours. Then, the nitrogen circulation cooling system in the box-type resistance furnace is turned on and the cooling rate is 30°C / h to rapidly cool the blank to 650°C. After holding for 5 hours, the blank is cooled to 400°C without blowing air and then taken out of the furnace, so that the spheroidization rate is improved and the hardness is reduced. Step 4: Turning The softened blank 1 is turned into an end portion and an outer diameter of a defined size by a lathe; Step 5: Shot blasting The blank 1 after turning was shot blasted using a drum shot blasting machine. The diameter of the shot blasting particles in the shot blasting machine was 1.2 mm, and the shot blasting time was 25 min. This was to remove the attachments on the surface of the bar, increase the surface roughness of the workpiece, and improve the adhesion of the phosphorus saponified lubricating film. Step 6: Phosphorus saponification treatment The blank 1 after shot blasting was subjected to phosphorus saponification treatment at a phosphating temperature of 70°C for 15 minutes to form a phosphating layer with a thickness of 10 μm; the saponification temperature was 70°C for 5 minutes to form a saponified layer with a thickness of 5 μm; Step 7: Molybdenum disulfide powder coating The surface of the phosphorus saponified blank is lubricated and coated with an ultrafine molybdenum disulfide solution. The concentration of the ultrafine molybdenum disulfide solution is greater than 15%. The coating area is the upper half of the blank. The surface friction coefficient μ of the blank after the surface lubrication treatment is 0.05. Step 8: Cold extrusion tooth shape After the upper toothed die 2 and the lower die 4 are opened, the billet after surface lubrication treatment is placed between the upper toothed die 2 and the lower die 4. The lower punch 5 is installed at the axis position of the lower die 4. The upper toothed die 2 is sleeved on the outer side of the upper toothed die inner punch 1. The upper toothed die 2 and the upper toothed die inner punch 1 located above the billet 1 move downward along with the slide of the press. The press continues to move downward after the upper toothed die 2 and the lower die 4 are closed, so that the billet is deformed into a formed forging 3 under the extrusion action of the lower punch 5. After the extrusion is completed, the press machine returns upward with the upper tooth-shaped die 2, disengages from the lower die 4, and the press machine ejector pushes out the formed forging 3 after the tooth extrusion is completed; The lower concave die 4 is provided with a die entry 6, and the die entry 6 is provided with a 30° inclined surface, and the angle of the inclined surface increases as the difficulty of the blank being extruded increases. Example

[0022] Step 1: Saw the material According to the drawing requirements, cylindrical bars with limited diameter specifications are selected, and then cut into blanks by a cutting machine; Step 2: Normalizing The blank is placed in a box-type resistance furnace for heating at 928°C for 3.8 hours; air-cooled to 615°C for isothermal treatment for 5.6 hours, and then air-cooled to room temperature to eliminate granular bainite and homogenize the structure; Step 3: Spheroidizing annealing The blank in the box-type resistance furnace is heated again at a temperature of 735°C and held at this temperature for 3.5 hours. The nitrogen circulation cooling system in the box-type resistance furnace is then turned on, and the cooling rate is 60°C / h to rapidly cool the blank to 655°C. After holding the blank for 5.5 hours, the blank is cooled to 425°C without blowing air and then taken out of the furnace, thereby increasing the spheroidization rate and reducing the hardness. Step 4: Turning The softened blank 1 is turned into an end portion and an outer diameter of a defined size by a lathe; Step 5: Shot blasting The blank 1 after turning was shot blasted using a drum shot blasting machine. The diameter of the shot blasting particles in the shot blasting machine was 1.4 mm, and the shot blasting time was 28 minutes. This was to remove the attachments on the surface of the bar, increase the surface roughness of the workpiece, and improve the adhesion of the phosphorus saponified lubricating film. Step 6: Phosphorus saponification treatment The blank 1 after shot blasting was subjected to phosphorus saponification treatment at a phosphating temperature of 75°C for 18 minutes to form a phosphating layer with a thickness of 12 μm; the saponification temperature was 75°C for 6 minutes to form a saponified layer with a thickness of 6 μm; Step 7: Molybdenum disulfide powder coating The surface of the phosphorus saponified blank is lubricated and coated with an ultrafine molybdenum disulfide solution. The concentration of the ultrafine molybdenum disulfide solution is greater than 15%. The coating area is the upper half of the blank. The surface friction coefficient μ of the blank after the surface lubrication treatment is 0.05. Step 8: Cold extrusion tooth shape After the upper toothed die 2 and the lower die 4 are opened, the billet after surface lubrication treatment is placed between the upper toothed die 2 and the lower die 4. The lower punch 5 is installed at the axis position of the lower die 4. The upper toothed die 2 is sleeved on the outer side of the upper toothed die inner punch 1. The upper toothed die 2 and the upper toothed die inner punch 1 located above the billet 1 move downward along with the slide of the press. The press continues to move downward after the upper toothed die 2 and the lower die 4 are closed, so that the billet is deformed into a formed forging 3 under the extrusion action of the lower punch 5. After the extrusion is completed, the press machine returns upward with the upper tooth-shaped die 2, disengages from the lower die 4, and the press machine ejector pushes out the formed forging 3 after the tooth extrusion is completed; The lower concave die 4 is provided with a die entry 6, and the die entry 6 is provided with a 50° inclined surface, and the angle of the inclined surface increases as the difficulty of the blank being extruded increases. Example

[0023] Step 1: Saw the material According to the drawing requirements, cylindrical bars with limited diameter specifications are selected, and then cut into blanks by a cutting machine; Step 2: Normalizing The blank is placed in a box-type resistance furnace for heating at 950°C for 4 hours; air-cooled to 650°C for isothermal treatment for 6 hours, and then taken out of the furnace for air-cooling to room temperature to eliminate granular bainite and homogenize the structure; Step 3: Spheroidizing annealing The blank in the box-type resistance furnace is heated again at 760°C and held for 4 hours. Then the nitrogen circulation cooling system in the box-type resistance furnace is turned on and the cooling rate is 80°C / h to quickly cool the blank to 660°C. After holding for 6 hours, the blank is cooled to 450°C without blowing air and then taken out of the furnace to increase the spheroidization rate and reduce the hardness. Step 4: Turning The softened blank 1 is turned into an end portion and an outer diameter of a defined size by a lathe; Step 5: Shot blasting The blank 1 after turning was shot blasted using a drum shot blasting machine. The diameter of the shot blasting particles in the shot blasting machine was 1.5 mm, and the shot blasting time was 30 min. This was to remove the attachments on the surface of the bar, increase the surface roughness of the workpiece, and improve the adhesion of the phosphorus saponified lubricating film. Step 6: Phosphorus saponification treatment The blank 1 after shot blasting was subjected to phosphorus saponification treatment at a phosphating temperature of 85°C for 20 minutes to form a phosphating layer with a thickness of 15 μm; the saponification temperature was 85°C for 8 minutes to form a saponified layer with a thickness of 8 μm; Step 7: Molybdenum disulfide powder coating The surface of the phosphorus saponified blank is lubricated and coated with an ultrafine molybdenum disulfide solution. The concentration of the ultrafine molybdenum disulfide solution is greater than 15%. The coating area is the upper half of the blank. The surface friction coefficient μ of the blank after the surface lubrication treatment is 0.06. Step 8: Cold extrusion tooth shape After the upper toothed die 2 and the lower die 4 are opened, the billet after surface lubrication treatment is placed between the upper toothed die 2 and the lower die 4. The lower punch 5 is installed at the axis position of the lower die 4. The upper toothed die 2 is sleeved on the outer side of the upper toothed die inner punch 1. The upper toothed die 2 and the upper toothed die inner punch 1 located above the billet 1 move downward along with the slide of the press. The press continues to move downward after the upper toothed die 2 and the lower die 4 are closed, so that the billet is deformed into a formed forging 3 under the extrusion action of the lower punch 5. After the extrusion is completed, the press machine returns upward with the upper tooth-shaped die 2, disengages from the lower die 4, and the press machine ejector pushes out the formed forging 3 after the tooth extrusion is completed; The lower concave die 4 is provided with a die entry 6, and the die entry 6 is provided with a 60° inclined surface, and the angle of the inclined surface increases as the difficulty of the blank being extruded increases.

[0024] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for reducing the cracking of the big end of a cold forged differential axle gear, characterized in that: The specific steps of this process are as follows: Step 1: Sawing According to the drawing requirements, cylindrical bars with limited diameter specifications are selected, and then cut into blanks by a cutting machine; Step 2: Normalizing The blank is placed in a box-type resistance furnace for heating at 870~950℃ for 3.5h~4h; air-cooled to 570~650℃ for isothermal treatment for 5h~6h, and then taken out of the furnace for air-cooling to room temperature; Step 3: Spheroidizing annealing The billet after normalizing is subjected to spheroidizing annealing treatment at a heating temperature of 720°C~760°C and a holding time of 3h~4h. Then, the cooling system switch in the box-type resistance furnace is turned on to rapidly cool the billet to 650°C~660°C. After holding for 5h~6h, the billet is cooled to 400°C~450°C without blowing air and then taken out of the furnace. Step 4: Turning The softened blank is turned into an end portion and an outer diameter of a defined size by a lathe; Step 5: Shot blasting Use a shot blasting machine to shot blast the blank after turning, and the shot blasting time is 25min~30min; Step 6: Phosphorus saponification treatment The blank after shot blasting is subjected to phosphorus saponification treatment, the phosphating temperature is 70℃~85℃, the time is 15min~20min, and the phosphating layer with a thickness of 10μm~15μm is formed; the saponification temperature is 70℃~85℃, the time is 5min~8min, and the saponification layer with a thickness of 5μm~8μm is formed; Step 7: Molybdenum disulfide powder coating The surface of the phosphorus saponified blank is lubricated and coated with an ultrafine molybdenum disulfide solution, with the coating area being the upper half of the blank; Step 8: Cold extrusion tooth shape After the upper toothed die (2) and the lower die (4) are opened, the billet after surface lubrication treatment is placed between the upper toothed die (2) and the lower die (4), the lower punch (5) is installed at the axis position of the lower die (4), the upper toothed die (2) is sleeved on the outer side of the punch (1) inside the upper toothed die, the upper toothed die (2) and the punch (1) inside the upper toothed die above the billet (1) move downward along with the slide of the press, and the press continues to move downward after the upper toothed die (2) and the lower die (4) are closed, so that the billet is deformed into a formed forging (3) under the extrusion action of the lower punch (5); After the extrusion is completed, the press machine returns upward with the upper toothed die (2) and disengages from the lower die (4), and the press machine ejects the formed forging (3) after the tooth extrusion is completed; The lower concave die (4) is provided with a die entry opening (6), and the die entry opening (6) is provided with an inclined surface.

2. The method for reducing big end cracking of cold forged differential axle gears according to claim 1, characterized in that: In step 1, the diameter of the blank is 5mm~8mm smaller than the inner diameter of the lower die.

3. The method for reducing big end cracking of cold forged differential axle gears according to claim 1, characterized in that: In step three, the blank is placed in a box-type resistance furnace for heating again.

4. The method for reducing big end cracking of a cold forged differential axle gear according to claim 1, characterized in that: In step 3, the cooling system switch is a nitrogen circulation switch, and the cooling rate is 30°C / h~80°C / h.

5. The method for reducing big end cracking of cold forged differential axle gears according to claim 1, characterized in that: In step 4, the shot blasting machine is a drum-type shot blasting machine.

6. The method for reducing big end cracking of a cold forged differential axle gear according to claim 1, characterized in that: In step 5, the diameter of the shot blasting particles in the shot blasting machine is 1.2mm~1.5mm.

7. The method for reducing big end cracking of a cold forged differential axle gear according to claim 1, characterized in that: In step seven, the concentration of the ultrafine molybdenum disulfide solution is greater than 15%.

8. The method for reducing big end cracking of cold forged differential axle gears according to claim 1, characterized in that: In step seven, the surface friction coefficient μ of the blank after surface lubrication treatment is 0.05-0.

06.

9. The method for reducing big end cracking of cold forged differential axle gears according to claim 1, characterized in that: In step eight, the angle of the inclined surface provided on the die inlet (6) is 30° to 60°.

10. The method for reducing big end cracking of a cold forged differential axle gear according to claim 9, characterized in that: The angle of the inclined surface increases as the difficulty of extrusion forming the blank increases.

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