Using Forging Technology to Improve Strength of Powder Metallurgical Gear Parts

I The application of new powder forging technology in gear forming and processing

The overall forging process of powder: powder loading - filling powder - powder sealing - powder pressing - pressure drop - pressure guide - powder loading. As an important transmission component, gears play a crucial role in automobiles. The density and hardness of gears are closely related to the performance and preparation process of materials. Advanced pressing technology has increased the density of powder compacts and improved the performance of powder forged products; At the same time, the dimensional accuracy of the parts can be improved, and the shape can also be more complex. Next, we will first discuss the new powder forging process and its impact on gears.

1、Sweden has developed a high-speed pressing process for powder forged gears

 The development of this process makes it possible to develop high-density and large powder forged parts exceeding 5 kg. It enables the powder to be compressed within 20 ms, and multiple presses within 300 ms can further increase density.

High speed pressing, as a mass production method, can break through the limitations of current powder forging. Traditional pressing requires high forming pressure, which is limited by the tonnage of the press, while high-speed pressing is not limited by this limitation. The powder density based on pre alloying and diffusion alloying can reach 7.4-7.7 g/cm3, and this new manufacturing technology has recently been introduced into the powder forging industry.

The densification of high-speed pressing is mainly achieved through the strong impact wave generated by the hydraulic controlled hammer, and the quality of the hammer and the speed at which it is pressed determine the magnitude of the impact energy and the degree of densification. Due to the use of hydraulic control, the safety performance is high. By appropriate process control, it is possible to avoid non axial rebound causing micro defects in the compact.

For high-speed pressing, multiple presses are possible, while traditional presses do not significantly increase the density of repeated presses after the first press. Because the impact energy of 4 kJ is the same as the impact energy of 2 kJ twice, its compression density is the same. Therefore, a medium press can be used to achieve high density through multiple presses. Multiple impact pressing can also be completed quickly, as the interval time between each impact is less than 300 ms. This type of press can accurately control the stroke and impact energy of the hammer using a computer, and the production process of the parts pressed by it is generally consistent with traditional forming processes.

The density of traditional powder compacts is distributed low in the middle and high at both ends, which can easily cause excessive shrinkage in the middle after molding and affect the dimensional accuracy of the parts. The density distribution of high-speed pressed parts is relatively uniform. After forming and processing, the difference in size between the middle and end parts will be smaller, which will improve the consistency of part dimensions.

If high-speed forming is combined with other processes, the performance of the material will be greatly improved. The ASTALOY CrM pre alloyed powder containing 0.4% carbon has a compact density of 7.5 g/cm3 after high-speed compression. The tensile strength reaches 1220 MPa after high-temperature forming and processing at 1250 ℃, and 1380 MPa after forming and hardening at 1120 ℃. From this, it can be seen that the performance of high-speed pressed parts has reached a high level.

As a process between traditional powder forming and powder forging, high-speed pressing has obvious advantages. Due to its good cost-effectiveness, it has a wide range of applications. Specifically, its advantages include high and evenly distributed density, high productivity, the ability to produce several kilograms of large parts, small elastic aftereffect, and high accuracy. It can produce parts with a large aspect ratio (up to 6 0).

The high-speed pressing technology is currently under continuous development, and in the early stages of development, it could only form simple straight barrel parts without steps. However, more complex parts that can form one step have been developed. However, for other parts with more complex shapes, production is currently not possible, which is also an important reason for the limitations of high-speed pressing technology.

2、Gear forming, working hardening, and forming, working hardening are processes that combine the forming process of powder forging with the quenching heat treatment process to improve material properties, in order to reduce costs. The forming and hardening process can eliminate the post forming heat treatment process, while achieving high strength and hardness performance, thereby reducing production costs. In addition, quenching can generate high residual internal stress and cause deformation of the parts, making it difficult to control the dimensional tolerance of the parts. The forming and hardening process can minimize deformation due to the fact that the cooling rate after forming is much lower than that of quenching. Therefore, the forming work hardening process is suitable for large and complex shaped parts that are difficult to handle.

Formed work hardening steel is generally used to manufacture medium to high density parts. In general, the main alloying elements for forming and hardening iron powder include molybdenum, manganese, chromium, copper, and nickel. Materials containing these alloying elements have sufficiently high hardenability to harden during forming, processing, and cooling. After forming and hardening, the metallographic structure of the alloy is mostly martensite, in addition to a small amount of fine pearlite, bainite, and residual austenite; There may be a small amount of nickel rich zone depending on the molding temperature and time. According to the actual conditions of forming and processing and the specific requirements of the parts, appropriate chemical composition can be mixed to obtain the required hardness and performance after cooling.

According to literature reports, a large number of formed and hardened gears have been applied to transmission mechanisms such as automobiles. Compared with traditional processes, it reduces production costs but does not reduce any performance. These gears have high dimensional accuracy, low noise, high strength, good wear resistance and corrosion resistance. The gears of Ningbo Dongmu (NB TM) Company are hardened through forming and processing, with a density greater than 7.0g/cm3. After tempering, the hardness is greater than HRC40. Compared with traditional methods, the cost is reduced by 10% and the danger of quenching deformation is reduced.

3、High temperature forming processing is an important measure to improve strength. By high-temperature forming processing, a portion of oxides can be reduced, atomic diffusion rate can be increased, and composition uniformity can be increased. This can enable full spheroidization of pores and larger pore spacing, making it suitable for new powder forging materials such as high-speed steel, stainless steel, and high-temperature alloys. This can improve the density, mechanical properties, axial/rotational bending fatigue strength, corrosion resistance, and physical properties of the parts.

However, there are also some drawbacks, such as increased equipment loss, increased energy consumption, increased furnace maintenance costs, reduced productivity, increased part deformation, reduced coaxiality of parts, low cooling rate, and other process issues. Therefore, high-temperature forming and processing of powder forged parts will add some additional costs.

For iron-based materials, high-temperature forming processing is suitable for the following situations: ① materials require high-temperature forming processing, such as new silicon-containing iron-based materials and high-performance stainless steel; ② High temperature forming processing is the most effective or only method that can meet the requirements; ③ High temperature forming processing can reduce processes or other equipment, such as changing secondary pressing to primary pressing; ④ Pre alloy or pre mixed powder forming processing, at this time, due to the reduction of some oxides, the degree of alloying increases, the hardening performance improves, and the mechanical properties improve. One important reason for the unstable performance of formed gears is the segregation of the mixed powder. By high-temperature forming processing, the influence of segregation can be significantly reduced or eliminated. High temperature forming is necessary for some materials, and on the other hand, existing materials do not fully realize their potential when forming at lower temperatures. To fully develop the potential of these materials, which requires them to have high apparent hardness, exceptional impact resistance, and tensile strength, high-temperature forming processing must also be used. Powder forged parts with these properties will have strong competitiveness; Although according to foreign analysis, high-temperature forming processing will increase costs by approximately 10% to 15%.

II Surface densification process of gears

Achieving high density is the main method to improve the performance of powder forged parts, but recent studies have shown that heat treatment and post processing can also have a significant impact on the quality of parts. The failure of gears is mostly due to surface contact fatigue, and increasing surface density can improve fatigue performance. Gears that have undergone surface carburization and surface heat treatment (high-frequency or laser heat treatment) have high external hardness (carbon content), good wear resistance, low core hardness (carbon content), and good toughness.

Due to the presence of pores, the surface contact fatigue strength of powder forged parts is often worse than that of cast and rolled steel processing, and after surface densification treatment, the surface in contact with the rolling die at the teeth almost reaches full density. After surface densification, the gear teeth have a porous surface and the core is a porous body; The production cost of gears is relatively low because only the surface of the gears bears external stress; Under the repeated rolling of the rolling die, the tooth shape and accuracy of the formed and processed gears have been improved. By surface densification, the dimensional accuracy of gears can be further improved. The surface densification depth exceeds 0.7mm, and the surface contact fatigue strength of gears can be significantly improved through surface densification.

In addition, the surface roughness of the gears meets the standard of "mirror surface", resulting in lower noise during gear operation. After appropriate heat treatment, the bending fatigue strength and contact fatigue strength of this type of gear with no holes on its surface completely reach the level of carburized steel. The manufacturing process of the above-mentioned gears is as follows: forming (high density; forming processing (controlling cooling speed); Machining; Surface densification; Heat treatment (control of heat treatment deformation). Surface densification technology has the advantages of no pores in the teeth, low surface roughness, high wear resistance, low noise, good corrosion resistance, high dimensional accuracy, and good fatigue characteristics of the parts. These factors are undoubtedly essential for high-quality gears. This also indicates that the surface contact fatigue performance of formed gears with a density of only 7.56 g/cm3 after surface densification treatment is slightly higher than that of cast and rolled steel.

1、Spur gear, helical gear powder forging mold mechanism

1.1 Helical gear powder forging mold

Figure 1

1- Tooth shaped upper punching die; 2- Core rod; 3- Helical gear pressing die; 4- Tooth shaped female die; 5- Tooth shaped lower punching die; 6- Mold pad; 7- Unidirectional thrust ball bearing; 8- Bearing seat

Helical gear pressing die adopts rotary pressing, and the pressing die rotates on one side and decreases on the other side with the helical angle of the female die during pressing. When pressing straight teeth, the upper touch punch forces the powder inside the teeth to move in the direction of pressing along the inner straight teeth of the female touch: when pressing oblique teeth, the gear powder moves downwards along the groove of the inner oblique teeth of the female touch. At the same time, the upper touch punch with teeth rotates and descends like a screw inserted into a nut. Therefore, rotary pressing is often used to press the helical gear blank. During the pressing and demolding process, there must be relative rotation between the punching and the concave during the relative movement up and down to ensure the smooth pressing and demolding process.

The design of the rotary press mold for the helical gear blank, the helical gear blank: the rotary press mold, during pressing, the mold punch rotates on one side and descends on the other side with the spiral angle of the female mold. When pressing the bevel gear blank, the powder at the teeth cannot move straight down along the pressing direction, but will move downward along the bevel groove in the female mold. At the same time, the upper die punch with teeth rotates and descends like a screw screwed into a nut. During the pressing and demolding process, the die punch and female mold must have relative rotation while moving up and down. Therefore, it is required that the die punch and female mold can rotate, and usually a flat ball bearing mold seat is installed.

1.2 Design of the rotary press mold for the powder forging of spur gears

The press mold for spur gears is a bidirectional press mold, where the die punch and the female mold move relative up and down without the need for relative rotation. 1.3 Calculation of pressing force: The total pressing force during the molding process is equal to the sum of net pressure and external friction force. The study of the quantitative relationship between unit pressing pressure and compact density has been the main content of theoretical research on powder forming in the past 60 years.

F=F1+F2 F: Pressing force.

F1: Net pressure.

F2: Overcoming the friction between the powder and the wall.

Calculation of demolding force: After removing the pressing pressure, the side pressure will decrease by 35-77% due to the elastic aftereffect in the height direction. Under low speed and high unit compression pressure conditions, plastic metal powder is prone to "mold buildup"; Poor surface quality, poor lubrication, and excessively high mold temperature of the mold exacerbate the phenomenon of mold nodules. In severe cases, the demolding pressure exceeds the pressing pressure, resulting in mold strain. Non lubricating plastic metal powder should be avoided from high-pressure pressing F detachment= μ Static P side residual S side residual P side residual=E ∑ R residual (m2-1)/2R P side residual=j ξ 0 ρ Among them: ∑ R residual: the remaining deformation on the radius of the female mold after pressure relief; j: The ratio of residual lateral pressure to lateral pressure is determined by the stiffness of the mold; m: The ratio of the outer diameter to the inner diameter of the female mold; ρ： When the relative density of the compact is 0.80~0.85, m=2-4, which can be roughly estimated as follows: for iron base: P-side residue=0.18~0.20P for copper base: P-side residue=0.20~0.22P