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Analysis on internal crack of 42CrMo forging shaft during quenching

42CrMo medium carbon alloy steel has excellent combination of toughness and impact properties after quenching and tempering. Because of its excellent comprehensive mechanical properties, it has been widely used in engineering construction. Our company applies 42CrMo forging as an indispensable shaft structural part in the construction of a project, and the size of the workpiece is Ф 130mm, 500mm long. Internal cracks were found during sawing the connecting bar after heat treatment.

Materials and technology

The processing procedure of cracked shaft is: round steel blanking → forging → normalizing → rough machining → ultrasonic flaw detection → quenching and tempering → finishing.

The heat treatment process is shown in Figure 1, normalizing at 880 ℃ for 3 h and air cooling. The results show that the hardness and mechanical properties meet the requirements of 241-286HBs and mechanical properties.

20210624230155 88314 - Analysis on internal crack of 42CrMo forging shaft during quenching

Fig.1 normalizing and tempering heat treatment process

After heat treatment, the test bar was sawed on the workpiece with connected test bar, and the crack shown in Fig. 2 was found after sawing. In order to observe the overall appearance of the crack, the vertical crack at the crack source shown in Fig. 2 is sawed as shown in Fig. 3. Samples were taken from the crack source and near the surface of the workpiece for analysis.

20210624230354 93931 - Analysis on internal crack of 42CrMo forging shaft during quenching

Figure 2

20210624230420 71064 - Analysis on internal crack of 42CrMo forging shaft during quenching

Figure 3

Analysis process

In order to find out the cause of internal crack, the technological process, macro morphology, chemical composition and metallographic structure of the product were analyzed.
(1) Firstly, the technological process was analyzed. There were four forging shafts in this batch, one of which was with connecting bar. The ultrasonic flaw detection of the four shafts before heat treatment was qualified, and the internal crack as shown in Fig. 2 was found when sawing the connecting bar after heat treatment, so it can be judged that it was the crack produced during heat treatment. No internal cracks were found in the other three shafts by ultrasonic testing.
(2) By observing the crack morphology, the crack first forms in the center of the workpiece, and then extends to both sides to form internal crack. In general, due to the combined effect of thermal stress and structural stress, most of the shaft parts form tensile stress on the surface and compressive stress on the center during quenching. Therefore, most of the shaft parts crack during quenching, which is formed on the surface first and then spread to the center. This time, the crack source is in the center of the workpiece, and it is rare for it to extend outward to the near surface. Therefore, we take a test block at the crack source of the center of the workpiece for testing, and we also take a reference block near the surface of the workpiece for testing.
(3) First, test the chemical composition, and the results are shown in the attached table. The carbon content at the center crack source of the workpiece exceeds the standard, and the carbon content near the surface is within the normal range. Other chemical components are in the normal range.

Chemical composition of 42CrMo steel (%)

Element C Si Mn Cr Mo
Standard value 0.380.45 0.170.37 0.500.80 0.901.20 0.150.25
Near surface 0.426 0.239 0.589 0.998 0.164
Central District 0.472 0.241 0.599 1.021 0.161

(4) For metallographic analysis, the longitudinal metallographic sample was cut from the cracked part of the workpiece. After grinding and polishing, it was eroded with 4% nitric acid alcohol solution, and the microstructure was observed under the optical microscope.
According to the metallographic analysis of the sample taken from the crack source as shown in Fig. 2, the metallographic sample 500 times from the crack source is shown in FIG. 4A. The sample is in the center of the workpiece, and the diameter of the workpiece is 130mm. By observing the metallographic structure in FIG. 4A, it is found that in addition to some massive ferrites, there are lath martensite retained sorbite and feathery upper bainite retained tempered structure, which are not normal mixed structure of ferrite and pearlite, and the structure in the center of the workpiece is disordered. This kind of mixed coarse structure has high brittleness and low fracture resistance, If tensile stress is formed in these areas during quenching, quenching cracks are easily formed.

According to the metallographic analysis of samples taken near the surface as shown in Fig. 2, the metallographic samples taken near the surface as 500 times as shown in Fig. 4B. The fine tempered sorbite near the surface is the normal quenching and tempering structure, which indicates that the heat treatment process is reasonable.

20210624230458 25990 - Analysis on internal crack of 42CrMo forging shaft during quenching

(a) Crack source 500 × 
20210624230520 13905 - Analysis on internal crack of 42CrMo forging shaft during quenching
(b) 500 near the surface ×
Figure 4

Conclusion

Combined with the results of chemical composition analysis and metallographic analysis, due to the excessive carbon content in some areas of the center, the brittle upper bainite structure is produced in these areas during quenching, and the abnormal structure stress in the center is produced during quenching. The compressive stress formed in the center turns into tensile stress, and the coarse mixed structure formed in the center is more brittle, The fracture resistance is poor, so the crack source is formed at these parts when the workpiece is quenched. During the quenching process, the crack expands to the near surface, thus forming the internal crack as shown in Fig. 2.
The local carbon content in the center of the material is very harmful. When raw materials enter the factory to detect chemical composition, according to the national standard, the sampling position will not be taken in the center, so the material defects will not be found, so the hidden danger is buried for the subsequent processing and use. Such material defects need to be paid attention to in the production process of steel plant.

Author: Xin yongmu

Source: Network Arrangement – China Flanges Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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