Fracture analysis and prevention of 45# steel forged shaft parts

45# steel is widely used in low-grade forged shaft parts because of its low price, convenient source, good processing performance, high hardness after quenching, good strength and toughness, high strength, and certain wear resistance after quenching and tempering. However, during the 45# steel forged shaft heat treatment process, due to the material itself, improper thermal processing, and unreasonable heat treatment process arrangement, it is often prone to heat treatment fracture or early failure in work, resulting in product scrapping and seriously affecting production.

 

1. Diesel engine crankshaft heat treatment cracks and prevention

A diesel engine factory produces a batch of diesel engine crankshafts. The factory uses round steel forging. To catch up with the construction period, the processing procedure is: blanking → forging → rough machining → quenching and tempering → finishing → inspection and storage. After quenching, some crankshafts cracked at the cranks, resulting in product scrapping.
It can be seen from the analysis of the process arrangement that because there is no annealing or normalizing after forging, the forging stress generated by the steel during forging is not eliminated well. Therefore, during heat treatment and quenching, the stress generated by the quenching is superimposed with the stress in the original forged shaft. When the superimposed stress exceeds the strength limit of the material, cracks are generated at the surface stress concentration of the 45# steel crankshaft.
Aiming at the causes of cracks in 45# steel forging crankshaft, normalizing heat treatment of 45# steel forging crankshaft is carried out, which not only eliminates the stress generated by forging but also prepares the microstructure for the quenching and tempering heat treatment of crankshaft. After normalizing, 45# steel crankshaft forgings no longer produce cracks after quenching.

2. 45# steel forged shaft parts heat treatment process improvement

A mechanical factory produces several forged shaft sleeves with an outer diameter of 140 mm, an inner diameter of 100 mm, and a length of 320 mm. If it is made of round steel, only the inner hole is cut off, more than 50 % of the material is wasted, and a lot of processing time is wasted. Later, the factory adopted a steel pipe with a slightly larger outer diameter and a slightly smaller inner diameter, saving many steel materials and a lot of processing time. To improve the strength of the forged shaft sleeve, the quenching, and tempering heat treatment is adopted. The heat treatment process is quenching heating temperature of 840 °C, water cooling, tempering at 550 °C, holding for 2h, and air cooling; the hardness of the forged shaft sleeve is 25-30 HRC after tempering, which meets the technical requirements.
However, two of the 12 shaft sleeves in the first heat treatment batch had cracks. Analysis of its causes:

• (1) Due to the hollow sleeve in quenching cooling, the inner and outer wall cooling is not uniform; the external cooling is fast, and the inner wall cooling is slow, resulting in internal and external stress not being uniform, so the sleeve cracks.
• (2) The internal stress generated during the rolling process of the steel sleeve needs to be better eliminated. Therefore, the residual stress after rolling and the quenching stress after heat treatment are superimposed. When the superimposed stress exceeds the strength limit of the steel, the 45# steel sleeve will break.

Given the above reasons, two process improvements are adopted:

• (1) Before quenching, the bushing is first subjected to stress relief annealing at a heating temperature of 700 °C for 1h and then air-cooled after discharge.
• (2) When the forged shaft sleeve is quenched and cooled, it should be vertically into the water and move up and down continuously, which accelerates the cooling rate of the inner wall of the forged shaft sleeve, reduces the inconsistency of the cooling rate of the inner and outer walls of the steel sleeve, and thus reduces the cooling stress of the inner and outer walls.

By adopting the above two process measures, the forged shaft sleeve will no longer crack after heat treatment, which improves the product quality and ensures the normal production of the forged shaft sleeve.

3. Heat treatment process improvement of large 45# steel forging shaft

We performed quenching and tempering heat treatment in a heat treatment plant on a 320 mm × 1600 mm large forging shaft. Although several process measures were taken in the quenching and cooling process: the large forging shaft was pre-cooled in the air when removed from the resistance furnace, and the quenching temperature was appropriately reduced. In the cooling stage, water cooling → air cooling → water cooling → air cooling → water cooling was taken, and the water temperature was increased, but the forging shaft still appeared cracks. This is because the temperature difference between the inside and outside of the large forged shaft during quenching is large, and the uneven cooling rate leads to excessive internal and external stress.
We improved the heat treatment process, using normalizing instead of quenching and tempering to treat large forging shafts. The microstructure and properties of the quenched and tempered forged shaft and the normalized forged shaft were checked, and it was found that the microstructure and properties of the two types of forged shafts were the same. Therefore, the larger diameter forged shaft can be normalized instead of quenching and tempering heat treatment, which solves the quality problem of large forged shafts and ensures that large forged shaft quenching does not produce cracks. The large diameter forged shaft with high surface wear resistance can be normalized after forging. After processing and forming, the surface medium frequency induction quenching of the large-diameter forged shaft can be carried out, which can greatly improve the service life of the forged shaft.

4. The heat treatment process improvement of slender forged shaft

A batch of 20mm × 300mm 45# steel slender forged shafts was treated in a mechanical plant, and the surface hardness of the forged shaft was 45-50HRC. To reduce the heat treatment deformation of the slender forged shaft, we adopted subcritical quenching, that is, the heating temperature of the forged shaft is 800 °C, water-cooled → oil-cooled double-liquid quenching, and then 200 °C for 2h and then air-cooled. The deformation of the forged shaft is reduced, the hardness is slightly reduced, and the heat treatment hardness is 43-48 HRC. However, in the process of use, although the hardness is low, early fracture still occurs.
Cause analysis: Through metallographic examination, it was found that there was massive residual ferrite in the tempered martensite structure, which reduced the fracture resistance of the forged shaft so that the forged shaft had early fracture failure during use, seriously affecting the normal production.
Later, when dealing with such forged shafts, we appropriately increased the quenching heating temperature (830 °C), cooled in water, and straightened when the forged shaft was cooled to 150-200 °C. This is because when the 45# steel forged shaft is at 200-150 °C, the austenite to martensite transformation is more intense, the phase transformation super plasticity effect is obvious, and the straightening is easier. After straightening, the forged shaft is tempered at 200 °C. The deformation of the forged shaft is very small, and the hardness is 45-50 HRC, which meets the technical requirements, and no early fracture failure occurs during use.

5. Heat treatment of inferior 45# steel forged shaft

Cracks were produced in batches during the quenching heat treatment of a batch of 45# steel forged shafts in a machinery factory. Various measures were taken, such as reducing the quenching temperature, increasing the effluent temperature, and tempering in time, but there were still many forged shaft fractures. The chemical composition analysis and metallographic examination showed that the carbon content of this batch of materials was high, w_C = 0.58 %, w_S = 0.3 %, which seriously exceeded the standard. Metallographic examination found that the steel contains more non-metallic inclusions. Excessive sulfur content and many non-metallic inclusions in steel are the main reasons for the fracture of the 45# steel forged shaft during heat treatment.
For this batch of axles, we can only take all scrapped measures. Later, we suggested that the factory should strictly control the threshold of steel entering the factory and carry out chemical composition inspection and metallographic examination on each batch of steel entering the factory to ensure the quality of products from the source and prevent the generation of waste products.

6. Conclusion

The shaft is an important part of construction machinery, which mainly plays a role in transmitting power and bears loads of different sizes and forms, such as bending, torsion, fatigue, impact, etc. Therefore, generally, forged shafts need higher mechanical properties, and the economy of material selection should also be considered.
45# steel is generally the preferred steel for the forged shaft. To improve the mechanical properties of the 45# steel forged shaft, the forged shaft needs to be quenched and tempered. However, due to the poor hardenability of 45# steel, it is often prone to cracks when quenched in water. Therefore, we must first control the steel quality in the factory. Secondly, during the heat treatment, according to different conditions, a reasonable heat treatment process is adopted to reduce the early fracture of the 45# steel forged shaft in the heat treatment process and use, improve the service life of the forged shaft, and ensure normal production.
Author: Zhao Changsheng