Overview
Forging is one of the most common processes used with metals. It is a kind of hot working. Simply put, forging can give metals a wide variety of shapes. It can also give metals some special properties.
For the forging of superalloys, there are many things that need to be paid attention to. Due to the special properties of superalloys, many factors such as temperature, heating rate, and forging method should be paid attention to when forging superalloys.
In this article, we will first introduce forging and different forging methods. Next, we will introduce the difficulty and precautions for forging superalloys. I believe this article will help you have a deeper understanding of this process.
What is Forging?
Forging is an ancient metalworking process. It is the process of deforming a metal material into a desired shape by applying strong pressure to it. Forging usually occurs in a high-temperature environment, making the metal softer and easier to deform. Forging can be divided into two main types: hot forging and cold forging.
Hot forging is performed at high temperatures, usually when the metal has reached its plasticity optimum (recrystallization temperature). The metal is heated to high temperatures and then its shape is changed by hammering or pressure. This method rearranges the metal particles, increasing their strength.
Cold forging is a forging process performed at room temperature or slightly higher temperature. Although cold forging generally requires more force than hot forging, it helps produce more detailed and accurate parts. Cold forging can also increase the strength of a metal and retain its specific mechanical properties.
Forging is widely used in manufacturing to produce a wide range of products from small components to large structures. This ancient and reliable technique not only improves the quality and strength of metal products, but also provides a strong and durable foundation for many items in our daily lives.
Different Forging Methods
Open Die Forging
Open die forging refers to using a large die to punch out a specific shape. First, the metal block is heated, then it is placed on a large mold. The two sides of the mold open up and form an "opening." Then use huge force to quickly close the mold. This extrudes the metal into the desired shape. Open die forging is typically used to manufacture relatively simple, high-volume products, such as vehicle parts.
Closed Die Forging
Closed die forging involves clamping a piece of metal between two dies. The metal block is heated and placed between two molds, which are complementary in shape, like two pieces of a puzzle. The molds come together to shape the metal into the designed shape. Closed die forging is typically used to create more complex and precise products because the shape of the mold can be more detailed and complex. For example, aircraft engine parts may be manufactured by closed-die forging.
The advantages of closed die forging are that it is the most accurate, saves the most materials, and can make small or high-precision parts. The disadvantage is that the mold is the most expensive and complex, and requires strict control of the size and position of the blank.
Characteristics of Superalloys
Low Hot Working Plasticity
Superalloys are alloyed to a higher degree than ordinary stainless steel and carbon steel. In other words, superalloys contain more chemical elements and have more complex compositions.
These elements are added to superalloys for two purposes: solid solution strengthening and precipitation strengthening. Either effect will reduce the plasticity of the alloy.
Since forging is mainly performed at high temperatures, and solid solution strengthening and precipitation strengthening will also occur in superalloys at high temperatures. Therefore, their plasticity at high temperatures is lower than that of ordinary metals and the forging of superalloys will be more difficult.
For precipitation-strengthened alloys, it is recommended to forge above the precipitation phase solution temperature to improve the plasticity during forging. The table below shows the amounts of precipitated elements for different Nimonic alloys as well as their dissolution temperatures.
| Alloy | Al% + Ti% | Dissolution Temperature of γ" Phase °C |
|---|---|---|
| Nimonic 80A | 2.7 ~ 3.8 | 820 ~ 910 |
| Nimonic 90 | 2.8 ~ 4.0 | 910 ~ 970 |
| Nimonic 105 | 5.0 ~ 8.0 | 1060 ~ 1080 |
| Nimonic 115 | 9.0 | 1150 |
| Nimonic 118 | 9.0 | 1160 |
Narrow Hot Working Temperature Range
In previous articles we mentioned that the temperature range of hot working needs to be controlled below the initial melting temperature and above the recrystallization temperature.
First, there are some elements with lower melting points in superalloys, such as aluminum and titanium. They significantly lower the melting point of superalloys. In addition, trace elements such as phosphorus, silicon, sulfur, and boron will also lower the melting point of the alloy. Therefore, the initial melting temperature of most superalloys is lower. The following table shows the relationship between the initial melting temperature of the alloy and the aluminum and titanium content:
| Al% + Ti% | Initial Melting Temperature °C |
|---|---|
| 1.5 | 1400 |
| 3.4 | 1360 |
| 4.5 | 1350 |
| 4.6 | 1350 |
| 5.0 | 1345 |
| 7.4 | 1280 |
| 10.3 | 1260 |
Secondly, there are a large number of solid solution strengthening elements in superalloys. These elements increase the binding force between atoms, making it more difficult for atoms to diffuse. Therefore, the recrystallization temperature of superalloys is higher. The following table shows the effect of the amount of solid solution elements on the recrystallization temperature:
| Cr% + W% + Mo% + Nb% + Co% |
Initial Melting Temperature °C |
|---|---|
| 20.0 | 950 |
| 27.0 | 1000 |
| 36.0 | 1080 |
| 37.5 | 1100 |
In summary, the recrystallization temperature of superalloys is higher and the initial melting temperature is lower. Therefore, its hot working temperature range is narrower. This makes it more difficult to control the temperature of superalloys during forging.
Low Thermal Conductivity
The higher number of alloying elements in a superalloy also results in its lower thermal conductivity. When heating, uneven temperatures often occur.
This situation will first cause the crystal structure inside the alloy to be uneven. This has an adverse effect on the properties of the alloy.
What's more serious is that uneven temperature of the alloy will lead to inconsistent expansion stress inside the alloy, which can easily cause cracks inside the alloy.
Therefore, the heating speed of superalloys should be controlled when heating to make the temperature inside the alloy more uniform.
Precautions for Forging Superalloys
Reasonable Temperature
Above we introduced that the hot working temperature range of superalloys is very narrow. Therefore, more care should be taken to determine a more suitable forging temperature when forging. At the same time, drastic changes in temperature need to be avoided during the forging process.
In addition, when forging and heating, the heating rate of the alloy should be strictly controlled to avoid cracks caused by uneven internal temperature of the alloy. After forging is completed, the cooling rate of the material should also be as slow as possible in order to obtain a more uniform structure.
Deformation Degree
Because superalloys have lower plasticity at high temperatures. During the forging process, if the deformation is too high, it can easily lead to brittle fracture of the material.
Depending on the grade, the forging deformation of the superalloy should be controlled within the range of 3% to 25%.
For some specific superalloys, grain size is also an important indicator. In this case, the amount of forging deformation needs to be as high as possible, but still needs to be controlled within a reasonable range. During final forging deformation, lower heating temperature and larger deformation degree are beneficial to obtain uniform and fine grain structure.
Forging Pressure
Because superalloys have low plasticity and high strength, they are more difficult to deform under the same forging pressure. Therefore, superalloys should use larger tonnage pressure processing equipment. The main reference indicators that affect forging pressure are tensile strength and yield strength.
For die forging, if the equipment capacity is insufficient, the forging temperature should be appropriately increased to reduce the forging pressure.
Deformation Speed
One of the characteristics of hot working is that it has both work hardening and recrystallization effects, which are two completely opposite effects. Simply put, work hardening increases the strength and decreases the plasticity of the material, while recrystallization increases the plasticity and decreases the strength of the material.
Therefore, when forging, both effects should be considered simultaneously to achieve the best balance of material properties. The deformation speed is the key. The following are three cases of deformation speed:
- At very low deformation speed, recrystallization has sufficient time to offset work hardening. In this case the material is more plastic and less strong.
- When the deformation speed increases, recrystallization does not have time to eliminate the effects of work hardening. The material is stronger and less plastic at this speed.
- When the deformation speed further increases, the deformation produces internal thermal effects that increase the recrystallization rate. This situation in turn allows the material to obtain balanced properties.
During forging, the third deformation speed is generally used. In hot rolling, the first deformation speed is generally used. They can all achieve relatively stable performance.
FAQ
What is the difference between forging and casting?
Forging and casting are two different metal processing methods. Forging is the process of plastically deforming metal through pressure, while casting is the process of melting and pouring the metal into a liquid state and solidifying it into shape.
The advantage of forging is that it can eliminate defects within the metal, refine the grains, and improve strength and plasticity. The disadvantage of forging is that it cannot produce parts with complex shapes, the cost is higher, and the process requirements are also higher.
The advantage of casting is that it can produce parts with complex shapes, with lower cost and simpler process. The disadvantage of casting is that it is prone to defects such as inclusions, shrinkage cavities, and segregation, and the structure is coarse.
What is the difference between forging and hot rolling?
Hot rolling is the process of extruding heated steel ingots through a press.
Compared with forging, hot rolling has the advantages of lower cost, higher efficiency, and the ability to produce large-sized metal materials. The disadvantages of hot rolling are poor surface quality, easy generation of oxide scale, lower precision and lower performance.
When is it necessary to use the forging process?
Generally speaking, the forging process is suitable for metal parts that require high strength, high toughness, and high reliability, such as engine shafts, gears, etc. Forging can also produce parts with complex shapes.
Further Reading
Conclusion
Forging is a process that uses strong pressure to deform metal materials at high temperatures. Through forging, materials can obtain specific shapes, better properties, and finer grains. According to different forging methods, forging can be divided into free forging, open die forging and closed die forging.
For superalloys, forging should more strictly control temperature, deformation amount, pressure and deformation speed.
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