Archive for year: 2024

Large steel forgings generally refer to products with ingot weight ≥150t and single forging weight ≥90t. The representative grades are generally 25Cr2Ni4MoV, 30Cr2Ni4MoV, 40Cr3MoV, 45Cr4NiMoV, 12Cr2Mo1, 12Cr2Mo1V, 20MnNiMo and other grades of forging materials. Large steel forgings are generally used to manufacture the main components of industrial equipment such as power, shipbuilding, metallurgy, and petrochemicals. Because they are used as main components, the performance and quality requirements of large steel forgings are generally extremely strict, and the forging method and process of steel forgings play a decisive role in performance and quality.

In my country, there are three processing standards for large steel forgings, namely GB/T 37464-2019 “Quenching and tempering of large steel forgings”, GB/T 37558-2019 “Post-forging heat treatment of large steel forgings”, and GB/T 37559-2019 “Normalizing and annealing of large steel forgings”. These three standards play a major role in guiding the processing technology of large steel forgings. Only by mastering the basic forging methods and process flow of steel forgings can we better break through and develop new processes. Therefore, it is necessary to understand the typical forging methods and process flow of large steel forgings.

For large steel forgings, the main purpose of forging is to form and improve the internal defects and organization of the material to ensure the shape and size of the forgings and improve the internal quality. There are four common typical forging methods, namely wide flat anvil high temperature strong pressure method (WHF forging method), center compaction method, center tensile stress free forging method (FM forging method), wide large anvil compaction forging method (KD forging method). The process flow is divided into large shaft process flow, cylinder forging process flow, and pancake forging process flow according to the type of forging. The following is a detailed introduction to the relevant methods or processes.

1. Wide flat anvil high temperature strong pressure method (WHF forging method)

Wide flat anvil high temperature strong pressure method (WHF forging method) is a forging process method that uses high temperature and large deformation conditions on a wide flat anvil to forge the porosity defects in the steel ingot. During processing, it is recommended to use an anvil width ratio of 0.6 to 0.8 and a reduction rate of about 20%. Under this parameter, the stress and strain distribution in the billet is more reasonable, and the center, pores and loose structures will be effectively compacted. There should be about 10% overlap of the anvil width in the middle of the two pressed parts, and pay attention to the staggered anvil when turning and pressing, so as to achieve the purpose of uniform compaction of the billet.

2. Center compaction method

The center compaction method is to chamfer the steel ingot, forge it into a square cross-section billet, and then heat it to 1220℃~1250℃ (initial forging temperature) and keep it warm. Then take it out of the furnace and use surface air cooling, blast or spray cooling to 720℃~780℃ (final forging temperature). A layer of “hard shell” is formed on the surface of the steel ingot. At this time, the temperature of the core of the steel ingot is still maintained at 1050℃~1100℃, and the temperature difference between the inside and outside is about 230℃~270℃. Use a narrow flat anvil to pressurize the steel ingot longitudinally, and use the wrapping effect of the surface low-temperature hard shell to achieve the purpose of significantly compacting the core.

 

3. Center tensile stress-free forging method (FM forging method)

The center tensile stress-free forging method (FM forging method) is that the lower anvil is a wide flat anvil and the upper anvil is a narrow anvil. The billet deforms between the asymmetric flat anvils, and asymmetric deformation occurs inside the forging billet, and the stress state of each part also changes. When the billet is deformed, the part that forms tensile stress moves to the bottom of the billet, while the central part is subjected to compressive stress, which has a significant effect on the internal pore defects of the forged steel ingot. The recommended process parameters of the FM forging method are an upper anvil width ratio of 0.6 and a reduction rate of 14% to 15%.

4. Wide anvil compaction forging method (KD forging method)

The upper and lower anvils of the wide anvil compaction forging method (KD forging method) are both V-shaped anvils. Taking advantage of the fact that the steel ingot has sufficient plasticity under long-term high temperature conditions, it can be forged with a wide anvil and a large reduction rate on limited equipment. The use of upper and lower V-shaped anvils for forging is conducive to improving the metal plasticity of the forging surface, increasing the three-dimensional compressive stress state of the core, ensuring the concentricity of the forging, and effectively forging the internal defects of the steel ingot. The process parameters here recommend the upper and lower V-shaped anvil opening angle α=135°, the anvil width ratio of 0.4~0.8 (the best is 0.6), and the reduction rate of about 20%. When forging, attention should be paid to staggering the anvil and turning it 90° to ensure uniform compaction of the core of the forging and high drawing efficiency.

5. Introduction to the process flow of different forged steel parts

(1) The process flow of large shaft forgings is: clamping jaws → main deformation → material separation → finished product. The main deformation can be carried out by wide flat anvil high temperature strong pressure method, center compaction method, center tensile stress free forging method, wide anvil compaction forging method, or other suitable forging methods to ensure that the forging has a sufficient forging ratio and that the entire cross section is fully forged. If the cross-sectional size of the forging is large, the forging ratio can be increased by increasing the number of upsetting and drawing times, or other methods can be used to increase the forging ratio.

(2) The process flow of large cylindrical forgings is generally: blanking → upsetting → punching → hole expansion. For long large cylindrical forgings, the mandrel drawing process can be added after punching. In order to ensure that the forging has a sufficient forging ratio, the forging ratio can be increased by increasing the drawing or upsetting and drawing before blanking, or other methods can be used to increase the forging ratio.

(3) The process flow of large pancake forgings is generally: jaw pressing → main deformation → blanking → upsetting, expansion. The main deformation of pancake forgings can refer to the process flow of large shaft forgings. After the pancake hollow forgings are upset and expanded, punching procedures should be added. When expanding pancake forgings, reasonable expansion methods can be used to ensure the compaction effect and uniform deformation of pancake forgings.

The above is an introduction to the common forging methods and process flow of large forged steel parts. Quality assurance technology is very important in the manufacturing of large forged steel parts, but large forged steel parts are typically multi-variety and small-batch production, so in many cases, it is difficult for enterprises to realize automated processing for their forging production. The forging process of large forged steel parts is a systematic process, which also requires us to master the forging equipment and accessories operation process, the ingot performance meets the requirements, and the risk control of the forging process. All of this requires us to reasonably select the forging deformation scheme and the distribution of deformation amount of each fire according to the actual production and operation, the internal quality requirements of the enterprise, and the actual scientific research results.

The machining sequence is arranged according to the principle of base surface first, rough first then fine, main first then secondary. For general parts of gear shaft, after the center hole is prepared, the outer circle is processed first, and then other parts are processed, and it is important to separate rough and fine processing. In the gear shaft processing technology, heat treatment is used as a sign, rough processing before quenching and tempering treatment, semi-finishing before quenching treatment, and fine processing after quenching. After separating the stages in this way, the fine processing of the main surface is guaranteed, and the stress during processing of other surfaces will not affect the accuracy of the main surface.
When arranging the order of gear shaft processes, the following points should also be noted.

(1) The rough machining of the tooth profile of the shaft should be arranged after the semi-finishing of the outer circles of the gear shaft. Because the gear shaft has a relatively large workload and is difficult to process, the processing position should be appropriately placed a little later to increase the positioning height of the positioning reference. The tooth profile finishing should be arranged after all the outer circles of the part are processed, so as to eliminate the heat treatment deformation caused by the local quenching of the tooth profile.
(2) The processing sequence of the outer circle surface should be to process the large diameter outer circle first, and then the small diameter outer circle, so as not to reduce the rigidity of the workpiece at the beginning.
(3) The processing of secondary surfaces such as keyways on gear shafts should generally be arranged after the outer circle finishing or rough grinding and before the outer circle finishing. If the keyway is milled before finishing, on the one hand, before finishing, the vibration caused by intermittent cutting will affect the processing quality and easily damage the tool; on the other hand, the size requirements of the keyway are also difficult to guarantee. These surface processing should not be arranged after the main surface finishing, so as not to damage the main surface accuracy.

Large gear rings are important and indispensable parts of equipment, just like gears. Although the two are similar, their main function is to transmit and change the direction of force. However, in addition to this, the protection of the main body of the machine system by the drive of the gear ring cannot be ignored.
The outer teeth of the large gear ring are divided into two types of tooth shapes: drum teeth and straight teeth. It can change the contact conditions of the teeth and improve the ability to transmit torque, thereby meeting the requirements of cost protection and extending the service life. The large gear ring is a thin-walled ring forging. It is easy to have problems such as uneven wall thickness, folding, and ellipse during the forging process. Therefore, the quality of the casting can be improved by controlling the details of the casting process.
Therefore, it can be said that the large gear ring is a very important component for protecting the service life of the machine. So when the large gear is damaged, how should we repair it?

First, the steel castings should be processed. Different processing methods should be used for large gears with different degrees of damage, and the damaged parts of the large gear ring should be processed;

Then, welding should be carried out. Before welding, it should be noted that there should be no impurities such as dirty oil, rust, slag, etc. near the welding part, otherwise it will affect the welding effect;

After the welding treatment, there may still be relatively small pores or sand holes on the surface of the steel casting. After removing these defects, the area should be repaired.

After the welding treatment, the damaged large gear ring can be used normally again, which not only saves the production cost of the enterprise, but also helps to reduce the loss of resources, which can be said to kill two birds with one stone.