Raw material defects | Machining special operations and practical cases 109

Material: D6AC steel.

Failure background: A newly developed cylinder, the main process flow of the cylinder is: cutting → heating → punching → rough turning → heat treatment → fine turning. After fine turning, small cracks were found on the outer surface of the cylinder as shown in Figure 5-73.

Figure 5-74 Microscopic image of crack

Figure 5-75 Crack microstructure

Cause analysis: From the failure characteristics, it can be seen that the cracks are hairline cracks. Hairline cracks are a kind of macroscopic defects in steel. They are caused by inclusions or bubbles, looseness, etc. in the steel being extended along the forging direction during the processing and deformation of the steel. They are linear defects among crack defects. Hairline cracks seriously endanger the mechanical properties of steel, especially fatigue strength.

Cause of failure: There are metallurgical defects in the steel, which cause hairline cracks on the outer surface of the cylinder.

Corrective measures: Increase the final carbon content to prevent the molten steel from being over-oxidized. Use bottom blowing of argon in the later stage of smelting to prevent the molten steel from absorbing nitrogen and reducing the sulfur content in the molten steel, so as to avoid the reaction of iron sulfide with iron to form hot brittle eutectic.

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