In industrial production, compressors can be regarded as the heart of power machinery, and the crankshaft is the backbone of the compressor. The role of the crankshaft is to convert the reciprocating motion of the piston into rotational motion and to output all the power externally. Due to its extremely complex working environment, it is subjected to complex alternating loads such as periodically varying gas forces, inertial forces of reciprocating motion mass, and centrifugal forces of rotational acceleration, as well as the action of vibration-induced additional stresses, resulting in torsional and transverse and longitudinal vibrations, and bearing tensile, compressive, and bending loads. Therefore, the stress situation is complex and prone to fatigue failure. This imposes high requirements on the comprehensive mechanical properties of the crankshaft, such as strength, rigidity, toughness, and wear resistance. For large six-throw crankshafts, the production difficulty is self-evident due to the complex shape of the forgings and strict technical requirements.
The throws of a large six-throw crankshaft are evenly distributed at a 120° angle. This requires that the angle must be accurate during forging, otherwise it will affect the overall performance of the crankshaft, which is a major difficulty in its forging. In addition, the internal quality requirements for six-throw crankshafts are extremely strict. Ultrasonic testing, low-magnification, and metallographic inspection items all need to meet high standards. To ensure these technical requirements are met, corresponding technical measures need to be taken in multiple aspects such as smelting, forging, and post-forging heat treatment.
Due to the strict requirements for the low-magnification structure and non-metallic inclusions of six-throw crankshafts, to ensure their quality, a vacuum refining process is adopted, namely electric furnace rough refining of molten steel + LFV ladle refining. During the vacuum refining process, the vacuum degree and vacuum holding time must be strictly controlled. The vacuum degree should be controlled below 100 pascals, with a holding time of 15 to 20 minutes. This can effectively control the internal quality of the ingot from the source, reduce inclusions and segregation in the steel, and control S, P ≤ 0.015%, [H] ≤ 3ppm.
The purpose of heating is to obtain a single austenitic structure in the steel, increase plasticity, reduce deformation resistance, and at the same time allow molecular diffusion and homogenization of composition. Heating temperature and holding time are the main control parameters of the heating process. For the first few heats, the temperature should be as high as possible without overheating, selecting 1220–1250°C; for the last heat, 1150–1180°C should be chosen. This ensures that the crankshaft can obtain finer grains under the required remaining forging ratio, while also ensuring that the formation level of Widmanstätten structure meets the technical standard requirements.
The forging process of six-throw crankshafts is a key link in ensuring their performance and quality. To meet the high requirements of the crankshaft under complex working conditions, the compilation of the forging process needs to take into account multiple factors such as material properties, shape complexity, and internal quality control. The following are the specific compilation contents for the forging process of six-throw crankshafts:
During the operation, it is essential to ensure that the tongs do not press off-center. This ensures that the axis of the ingot does not tilt during subsequent deformation processes, preventing the poorer metallurgical quality parts of the center of the large ingot from flowing to the dangerous cross-section.
Based on the mechanical properties and inspection requirements of the forging, a forging ratio of K≥4 is selected. A 14t ingot cannot meet the forging ratio requirement by direct drawing out alone, so a upsetting process is added. Ensuring a sufficiently large upsetting diameter to increase the forging ratio enhances the comprehensive performance indicators of the crankshaft. After upsetting, the holding time is increased by 50% to improve the compaction effect of the next process.
The degree of forging penetration of the ingot, the initiation of internal cracks, and the forming quality of the forging are all directly related to the dimensional changes of the deformed blank. This is reflected in the drawing out process in terms of relative feed rate (anvil width ratio), material width ratio, and reduction amount, which should be effectively controlled.
Drawing out a rectangular cross-section blank with a flat anvil is essentially the result of local upsetting of a rectangular blank plus the influence of the rigid end. Since the deformation zone directly under the anvil is affected by its adjacent free end (undeformed end), compared with the separate upsetting of a square column with a flat anvil: when the anvil width ratio A/H < 0.5, it increases the axial tensile stress σx at the center of the deformed body, while the deformation at the center of the blank is small, which is not conducive to center compaction. When A/H > 0.5, it reduces the axial tensile stress σx at the center of the deformed body. When A/H ≥ 0.6–0.8, an axial compressive stress acts, and the stress state at the center of the blank is good. The deformation zone appears as a single-drum shape on the side, with a large deformation in the middle region of the blank, which is conducive to the welding of metallurgical defects.
The wide anvil flat drawing out method is used to ensure the internal quality of the drawing out deformed body. It employs a large feed rate, that is, full anvil feed, A=W (W is the upper anvil width); and a large reduction rate, with each reduction rate εH1 and εH2 around 20%. During the drawing out process, the longitudinal anvil width ratio W/H is controlled between 0.6 and 0.8. For most of the drawing out deformation time, the instantaneous anvil width ratio in one direction, that is, the axial direction, can reach the optimal parameter value of 0.7. The material width ratio in the transverse direction is controlled at 0.5 < B/H < 2. After a 90° rotation, it ensures that the blank undergoes compressive deformation in different directions. A 180° rotation allows the blank to deform sufficiently in one direction, achieving a more uniform internal structure and properties of the forging. During the drawing out process, the blank remains in a flat state, with a flattened appearance in the transverse cross-section. According to the tensile stress theory of rigid-plastic mechanics, under these conditions, the deformation of the blank mainly occurs in the center part of the blank. As the reduction amount increases, the easy deformation area expands. Moreover, due to the restriction of the adjacent free end (undeformed end) of the blank center, the center is axially compressed, thereby enhancing the compaction effect of the center. This causes the porosity and inclusions in the center of the ingot to be fully crushed and compacted. Therefore, the wide anvil flat drawing out method not only ensures sufficient deformation of the blank center, allowing the inevitable porosity inside the ingot to be welded in advance, but also avoids the occurrence of RST effect and uneven surface deformation.
The wide anvil flat drawing out method uses upper and lower wide flat anvils (anvil width 800mm). According to the WHF method pressing program and specific deformation data, the blank is drawn out at high temperature. According to the widening coefficient formula in the WHF method: α=0.78-0.14H–3, △b=α•△H (α: widening coefficient; H: height before pressing; △H: height pressing amount; △b: width widening amount), the process parameters for each deformation are determined, and the cross-sectional dimensions for each drawing out process are calculated.
The specific operating procedure is as follows: Using upper and lower 800mm wide anvils, first draw the blank after upsetting to 1500mm. The full anvil feed reduction rate is 20%.
Start the first flat drawing out: Flip 180° and press to a cross-section of 950mm×1810mm.
Flip 90° and start the second flat drawing out: Flip 180° and press to a cross-section of 950mm×1400mm.
Flip 90° and press to a cross-section of: φ1150mm, open the blank to a regular hexagonal cross-section of 800mm.
Perform angle presetting, forge the throws and throw diameters in different parts and directions, and correct the throw angles to produce the finished product.
To ensure the internal quality of the crankshaft forging, the following precautions are proposed for forging operations:
When forging a regular hexagonal forging blank, it is required that the hexagon be regular to ensure the accuracy of dimensions and angles, and to avoid angle loss of control during subsequent deformation. When pressing the throw plate, press the diagonal direction of the regular hexagon so that they form a 60° angle with each other. When dividing the material, control the material size to prevent the throw from disengaging. Pay attention to the rotation direction throughout the forging process to prevent the three throw angles from being pressed in the wrong direction, causing angle errors that cannot be repaired.
Due to the many forging heats and complex operation control of the six-throw crankshaft, to avoid defects such as coarse grains, mixed grains, Widmanstätten structure, and white spots in the forged crankshaft after forging, the post-forging six-throw crankshaft adopts isothermal annealing treatment to achieve the purpose of refining grains, adjusting structure, preventing white spots, and improving post-forging machinability.
In summary, the production of large six-throw crankshafts is a complex and meticulous process that requires strict control at multiple stages. From the source control of the vacuum refining process, to the precise parameter setting of the heating process, to the fine operation of the forging process, and finally to the post-forging heat treatment, every stage is crucial. By adopting the above processes, we have successfully produced high-quality large six-throw crankshaft forgings. Even if there are individual angle deviations, they can be corrected to meet the process requirements. This not only proves the feasibility and reliability of the process, but also provides strong technical support for the production of large six-throw crankshafts. In the future, with the continuous progress of technology and the continuous optimization of processes, we hope to further improve production efficiency and product quality to meet the needs of more high-end industrial applications.
