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Analysis of the causes of surface cracking of 45# steel pins

The surface hairline of 45# steel pins after machining was analyzed by chemical composition analysis, metallurgical microscopy, scanning electron microscopy and energy spectrometer. The results show that the large inclusions in the raw material are the main cause of the hairline, and the inclusions extend in the rolling direction during the rolling process and are exposed during the subsequent machining process. The generation of such defects can be reduced by techniques such as protective casting and slag roll prevention.

45# steel is a medium-carbon high-quality carbon steel, which has excellent comprehensive mechanical properties after normalization or quenching, and is widely used in the machinery manufacturing industry [1,2]. After tempering, it has high strength and plasticity, and has certain toughness, which can be used to manufacture various parts such as gears, tie rods, pins, crankshafts, etc. Most of these parts require high wear resistance and high hardness on the surface of the product, while the heart must have high strength and toughness. A company uses 45# round bar to produce pins for construction machinery, and its production process is: round steel → surface turning → tempering → straightening → finishing. In one production process, one pin was found to have cracks on the surface after turning and processing.
This paper analyzed the macroscopic shape, chemical composition, metallographic organization, scanning electron microscopy and energy spectrum of the defective sample to determine the cause of the cracks. At the same time, reasonable preventive measures are proposed to avoid the recurrence of such defects.

1. Macroscopic analysis

The macroscopic shape of the pin defect sample is shown in Fig. 1, the crack is near the small end of the pin, about 3 mm from the end, extending along the axial direction, the crack is thin and straight, the length is about 3-4 mm.
20220730115628 17632 - Analysis of the causes of surface cracking of 45# steel pins
Figure.1 Macroscopic appearance of the crack defect

2. Physical and chemical examination

2.1 Chemical composition analysis

The chemical composition analysis of the pin defect sample, the results are shown in Table 1, it can be seen from the table, the material composition are within the national standard requirements, and no abnormalities, indicating that the chemical composition is not the direct cause of cracking.
Table.1 Chemical composition of 45# steel pin (mass fraction, %)

Measured value and requirements C Si Mn P S Cr Ni Cu
Measured value 0.45 0.24 0.7 0.02 0.002 0.023 0.005 0.03
GB/T 699 requirements 0. 42-0.50 0.17-0.37 0.50-0.80 ≤0.035 ≤0.035 ≤0.25 ≤0.30 0.25

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Figure.2 Microstructure at the crack
(a), (b) before corrosion; (c), (d) after corrosion

2.2 Metallographic examination

In the defective samples taken at the crack, in the transverse microscopic observation, corrosion before and after the morphology as shown in Figure 2. From Figure 2 (a), 2 (b) can be seen, the depth of the crack is very shallow, only about 0.035mm; the bottom of the crack is rounded and smooth, no inclusions in and around the crack exists. After corrosion, there is no decarburization phenomenon at the crack, and the organization of the crack is consistent with the matrix organization, pearlite + ferrite, as shown in Figures 2 (c) and 2 (d).

2.3 Scanning electron microscopy and energy spectrum analysis

The defective sample was observed by SEM, and the crack length was about 3.66mm and width was about 0.09mm, as shown in Figure 3. The A and B areas in Fig. 3 were enlarged to observe the crack shape, as shown in Fig. 4 and Fig. 5. It can be found from the figure that part of the black line shape is grooved cracks, and the other part has material residue in the black line gap; through the energy spectrum analysis, it is found that the material in the grooved cracks and the residue in the hairline gap contains magnesium, aluminum, calcium and other elements, these materials belong to magnesium aluminum spinel.

2.4 Analysis of the results

From the chemical composition and metallographic organization of the pin, the composition and organization are normal, and no factors leading to the hairline were found.
20220730120316 42004 - Analysis of the causes of surface cracking of 45# steel pins
Figure.3 SEM morphology of cracks
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Figure.4 Magnified view of the cracked A-zone (a) and energy spectrum (b)
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Figure.5 Magnified shape of the B zone of the crack (a) and energy spectrum (b)
Table.2 Results of energy spectrum analysis in A and B regions

Spectrogram position C 0 Mg Si Al S Ca Zr Fe The sum
Spectrum 1 A 6. .04 45.6 2.97 21.11 0.36 20.37 3.55 100
Spectrum 2 B 4. .83 13. 62 1.76 0.22 10.68 7.86 1.19 59. 84 100

The results of SEM and energy spectrum analysis show that the hairline is thin and long, and the energy spectrum analysis shows that there are inclusions containing calcium, aluminum and magnesium in the hairline. The inclusions belong to magnesium-aluminum spinel type. Liu Junhui et al [4] also found similar inclusions when studying the causes of hairlines on the surface of 45# steel piston rods. These inclusions are mainly from the steelmaking process, using aluminum deoxidation of steel, after refining will produce a large number of magnesium and aluminum spinel inclusions. Among them, aluminum is mainly used for deoxidation and refining grain, magnesium oxide generally comes from the involvement of refining slag and refractory erosion. From the results of energy spectrum analysis in Table 2, the magnesium content of inclusions is low, which should be from the involvement of refining slag. Some research shows [5,6], calcium and magnesium aluminum spinel inclusions always exist in the steel generated by high alkalinity refining slag, so in the continuous casting process must pay attention to avoid such inclusions into the continuous casting billet, through the protection of casting and prevent the roll slag, crystallizer inclusions floating and other techniques can avoid such inclusions into the continuous casting billet, to achieve clean steel generation.

3. Conclusions

  • 1) The presence of large inclusions in the raw material is the main cause of pin cracking.
  • 2) The generation of such inclusions can be reduced by techniques such as protected casting and prevention of slag rolls and floating of inclusions in the crystallizer.

Authors: Zhong Fanghua, Li Juan, Liu Nianfu, Zhong fan, Wu Xuexing

Source: China Flanges Manufacturer – Yaang Pipe Industry (

(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.)

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