Analysis of Surface Defects in Diesel Engine Valve Body Forgings

During the trial production process of diesel engine valve body forgings, small linear defects were visible to the naked eye on their surface. Macroscopic and microscopic observations and composition analysis were conducted on the defect location to determine the defective nature of the valve body and solve the quality problem of the forging. The results show that the defect belongs to a folding defect caused by unreasonable metal flow. The defect was resolved by modifying the billet groove and replacing the billet specifications.

Our unit has developed a new diesel engine valve body forging type made of 4Cr10Si2Mo steel. 4Cr10Si2Mo is a chromium silicon molybdenum Martensite steel with high thermal strength, which is often used to make intake valves for internal combustion engines and exhaust valves for light load engines. 4Cr10Si2Mo steel has a high alloy element content, poor thermal conductivity, and a narrower forging temperature range than ordinary materials. Therefore, special attention should be paid to the forging process, monitoring the temperature and cooling method afterward. During the trial production process, it was found that there were small linear defects visible to the naked eye on the surface of the forging. We conducted analysis and research on the defect location to determine the nature of defects and solve the quality problem of valve body forgings.

1. Trial production of forgings

The valve body part diagram is shown in Figure.1, and the valve body forging is shown in Figure.2. Two blank making grooves were designed according to the cross-section of the blank, and the blank making mold is shown in Figure.3. Firstly, place the blank into the mold groove shown in Figure.3 (a) for the first press bending to make the blank. Then, please remove the blank, flip it 90 °, and translate it into the mold groove shown in Figure.3 (b) for the second press bending. Finally, place it into the final forging groove for final forging.
According to the diameter diagram of the blank, select the material specification as 60mm × 66mm, the forging temperature is 1100-900 ℃, and the cooling method after forging is gray cooling. Twenty pieces were trial produced, and small linear defects were found on the surface of each forging visible to the naked eye. One piece was selected and sampled from the defect site for chemical composition, metallographic structure, and other analyses to determine the nature of defects and solve the quality of valve body forgings.

Figure.1 Valve Body Parts Drawing (mm)

Figure.2 Valve Body Forging Drawing (mm)

Figure.3 Diagram of Billet Making Mold

2. Defect analysis

2.1 Macro analysis of defects

Through macroscopic photo observation of the defects, it was found that they were located on the rod of the valve body forging, and the defects were linear and consistent with the fiber flow line of the raw material, as shown in Figure.4.

Figure.4 Defect morphology of valve body forgings

2.2 Chemical composition analysis

Sample the defective and normal parts separately for component analysis, as shown in Table 1. From Table 1, it can be seen that the chemical composition meets the standard requirements.
Table.1 Chemical Composition Analysis Results of Samples (Mass Fraction,%)

2.3 High magnification analysis of defects

Cutting horizontally along the strip like defect, it was found that there is oxidation inside the defect, and the end is blunt, as shown by the arrow in Figure.5 (a). Continuing to observe the defect area, decarburization was found on both sides of the arrow area shown in Figure.5, as shown by the arrow in Figure.5 (b)

Continuing observation along the linear defect, decarburization was found on both sides, as shown by the arrow in Figure.5 (c).

Figure.5 High magnification morphology of defects

2.4 Process Analysis

After analyzing and searching for information, it is believed that the forging temperature of 1100-900 ℃ and the cooling method after forging are reasonable. Looking up process records, both the final forging temperature and initial forging temperature meet the process requirements, and there is no problem of surface cracks caused by the forging process temperature being lower than the final forging temperature. The gray cooling method after forging is reasonable and will not produce surface cracks.

2.5 Analysis of the Causes of Defect Formation

High magnification inspection of the dissected part of the forging found that the metal grains at the defect area were small and uniform, without any signs of overheating or burning. After observing the defect area at high and low magnification, it was found that the end of the defect is round and blunt, with signs of confluence, and oxidation is visible inside the defect. Obviously, oxidation decarburization layers on both edges indicate that the defect is folded. Usually, folding is caused by unreasonable metal flow during the forging process, by the convection and convergence of two or more strands of metal; Or by a rapid and massive flow of metal that carries the surface metal of adjacent parts, the two converge to form; Or formed due to bending or refluxing of deformed metal, or partially deformed metal being pressed into another part of the metal.
Through analysis of the forging, it was found that the metal distribution of the billet was unreasonable. During the forging process, there was a local shortage of material, while the excess material in other parts flowed back and formed folding. The forging is formed using a quasi 60mm bar material. When hammering in the final forging cavity, due to the large cross-sectional area of the billet, it cannot fit with the mold during initial forging. During the rapid striking process, the metal flow is unreasonable, resulting in two or more strands of metal converging and forming folds.

3. Measures and Verification

After recalculating and analyzing the mold design process, the secondary bending angle of the blank mold groove is believed to be relatively small. By recalculating the cross-section of the blank, the secondary bending angle was increased from 30 ° to 44 °. The optimized shape of the blank is shown in Figure.6.

Figure.6 Optimized shape of the billet

Recalculate through the diameter diagram of the blank, select a new material specification, and ultimately select a bar with a size of 50mm. Fifty pieces were produced using this specification, with no surface defects. They were all qualified through magnetic particle testing. The photo of the trial produced forging is shown in Figure.7.

Figure.7 Trial produced forging after process optimization

4. Conclusion

This article analyzes the linear surface defects of diesel engine valve body forgings during the trial production process and ultimately determines that the defect is folding. The defect was solved by modifying the billet groove and replacing the billet specifications, which can provide a reference for forming similar forgings.
Author: Gao Lianke