Leakage occurred during the sealing hydraulic pressure test of cylinder head in a gas engine. After the test, it was found that the spark plug Bush was cracked. The lining material is cast brass ZCuZn38(H62) , an Non-ferrous metal alloy widely used in industry. The technological process is: CASTING COPPER BAR → hot pressure processing → machining → fitting trial run.

1.Test method

The chemical composition of the cracked bushing was analyzed by Oblf Direct-reading Spectrometer, and the sample was cut by wire-cutting method. Microscopical observation and microcomponent analysis of the crack were carried out using Zeiss Scanning electron microscope 18 with an x Ray Spectrometer.

2. Test results

2.1 The chemical composition of the bushing was determined by direct reading spectroscopy. The results are shown in Table 1. It meets the requirements of GB/t 1176 -- 2013《 cast copper and Copper Alloy 》 .

2.2 macro view

There are two cracks in the bushing, which extend from the bushing shoulder to the bottom of the bushing shoulder. The crack extends to the inside of the shoulder and gradually narrows, as shown in Fig. 1, indicating that the crack originates at the outside of the shoulder and extends downward and inwards.

2.3 microscopic observations

Scanning Electron microscopy showed that there was a layer of floc near the outer surface of the bushing, as shown in figures 2A and 2B. It can be seen that the whole fracture surface shows Brittle Fracture: Rock Candy intergranular fracture, a small amount of partial transgranular, and there are corrosion products and small corrosion pits in the grain boundary. The corrosion products mainly include O, Cu, Zn and Al, as shown in Fig. 2C and Fig. 2D. The freshly torn parabolic dimple is clear and clean, and the fracture morphology is normal, as shown in figures 2E and 2F. The original fracture shows a lamellar strip along the axial direction.

Along the axial grinding, polishing, corrosion, observation of cracks and microstructure, as shown in figure 3. The crack is irregular and continuous, with branch and branch, and the tail is sharp and Dendritic, which is consistent with the typical characteristics of the stress corrosion crack. The metallographic structure is phase + very few point-like phase, and there is obvious slip line in the structure, and the striated structure of the crack Bush is obvious.

2.4 Finite Element Analysis

The interference fit of the bushing is 0.069 ~ 0.100mm, the assembly stress of the bushing is analyzed by finite element method (FEM) . Fig. 4 is the first principal stress Nephogram of the bushing assembly state, and the bright color area is the tensile stress area. It can be seen that the tensile stress of the outer wall above the bushing shoulder, the outer wall of the Chamfer and the inner surface of the thin neck is larger, which is consistent with the actual position of the crack origin. Although the internal surface tensile stress of bushing is large, there is no sealant and no stress corrosion condition.

3.Conclusion and analysis

After forming by hot pressure at 650 ~ 800 °C, the Bush blank is naturally cooled to room temperature in air. All brass has a brittle zone between 200 °C and 700 °C, and the processing temperature of hot pressure should not be lower than 700 °C. The lower limit temperature is lower, the material is easy to crack, the difficulty of blank forming is increased, and the residual stress is larger. In the process of machining, the bushing is in frequent contact with the cutting tool, under the force, and the size changes, which must produce a certain amount of residual stress, so that the lining in a certain period of time in a humid, corrosive environment. There is an interference fit between the bushing and the cylinder head. After the pressure assembly, there is a large tensile stress on the outer wall above the bushing shoulder and the chamfered outer wall, contains the Amino catalyst, provides the weak corrosion working environment for the bushing. When the cylinder head is tested for water pressure seal, the outer wall of the bushing shoulder is stressed, and the cracks grow rapidly, while the inner surface of the bushing neck has high tensile stress, but it does not touch the sealant, and the stress corrosion condition is poor.

4. Conclusion

1) the mechanism of bushing crack is stress corrosion cracking and intergranular brittle cracking. 2) The bushing crack concentrated in a certain month, before the batch crack failure, through the above analysis, confirmed that bushing quality problems, bushing processing residual stress is too large, resulting in bushing after the assembly of additional tensile stress and sealant (or cutting fluid in the process) weak corrosion environment occurred stress corrosion cracking. 

Source: Metalworking (hot working) magazine, by Guan Huihuan, Zhang Huaiqing, Xu Jidong

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