After 7305 hours of operation of a diesel engine in our company, cracks were found on the piston skirt of the third cylinder.
To determine the cause of the cracking, we dissected the piston skirt and conducted a comprehensive analysis and evaluation of the fracture’s chemical composition, mechanical properties, macrostructure, macro morphology, and microstructure.
1. Technical requirements for piston skirt
Our company uses die forging for the piston skirt, made of material 4032 in the T6 delivery state, which undergoes solid solution heat treatment. The chemical composition of the material is compliant with GB/T3190.
The production process involves forging, solid solution, artificial aging, and machining. After forging, solid solution, and artificial aging, the mechanical properties of the piston skirt are: HBS=100~125 (10/1000), σb≥280MPa, and δ5≥1%.
The quarter section’s macrostructure must not have segregation, cracks, pores, or inclusions. The metal flow direction generally follows the contour of the forging without flow through or folding.
For quality control, a sample is taken at the end of the tensile sample, which is then examined using a microscope with 100x or 400x magnification. The sample must not contain harmful defects, such as inclusions, segregation, or overburning.
2. Process of crack discovery
The following is an analysis of the photographs provided for inspection of the piston skirt.
Fig. 1 displays a photograph of the piston skirt with visible cracks. The cracks are transverse in nature and are located on the outer diameter of the piston. The cracks are more than 1/4th the diameter of the piston and have penetrated the wall thickness.
Fig. 2 depicts the crack morphology of the inner cavity. The crack extends from the inside of the piston through the pinhole and to the outer surface.
Fig. 3 shows a photograph of the piston skirt opening along the crack with an external force. The purpose is to analyze the fracture morphology of the crack.
Fig. 4 is a photograph of the piston skirt dissected along mutually perpendicular center lines. The purpose of the dissection is to test and analyze the mechanical properties of the piston skirt, including low magnification fiber streamline.
(1) The chemical composition of the piston skirt is inspected, and the material is 4032 GB / T3190. The inspection results are shown in Table 1.
Table 1 chemical composition (mass fraction) (%)
|Element||Measured value||Requirement||Conformity determination|
Conclusion: the chemical composition meets the requirements of 4032 in GB / T3190.
(2) The mechanical properties were tested and the results are shown in Table 2.
Table 2 mechanical property test
|Project||Measured value||Requirement||Conformity determination|
|Tensile strength / MPa||352.1||≥280||coincident|
|Yield strength /MPa||333.0||–||–|
|Elongation after fracture (%)||4.6||≥1||coincident|
Conclusion: the mechanical properties are qualified and meet the design requirements.
(3) The macrostructure was inspected, and the fracture was analyzed.
As shown in Figure 5, the metal fiber’s flow direction is distributed roughly along the forging contour, with no signs of flow-through or folding, indicating a normal macrostructure.
Figure 6 reveals that the crack initiation point on the piston skirt is located at the sharp corner where the small oil hole and piston pin oil groove meet.
No evident plastic deformation is observed in the fracture morphology. However, typical fatigue bands are visible in the macrostructure, and the fatigue arc’s center points towards the sharp corner of the oil hole.
Figure 7 displays the crack end morphology on the piston skirt’s outer surface. The fracture exhibits fatigue bands in the middle and unstable jumping ridge lines in the transient fracture area near the free surface.
Figure 8 illustrates the front macro morphology of the sharp corner of the oil hole in the fatigue source area. The macro morphology indicates that there are no burr flash and original crack on the burr at the sharp corner of the oil hole.
Finally, Figure 9 shows that the fracture extension area is dominated by cleavage morphology.
(4) The fracture was observed under an optical microscope, and the fiber structure was analyzed.
In Fig. 10, a photograph of the metallographic specimen under an optical microscope is shown. The microstructure consists of α+ (α+ Si) + strengthening phase and impurity phase, exhibiting a normal microstructure with no metallurgical or heat treatment defects.
Fig. 11 displays a photograph of the cross-section specimen under the scanning electron microscope, revealing that the fracture originates at the sharp corner of the oil hole processing.
Fig. 12 illustrates the morphology of the fracture initiation source area, demonstrating that the dimple morphology dominates the final fracture area of the fracture.
3. Results and analysis
(1) Analysis of inspection results
According to the chemical composition analysis, the chemical composition of the piston skirt satisfies the standards of material 4032 in GB/T3190.
The mechanical properties of the piston skirt meet the design requirements of the product.
The metallographic structure and macrostructure of the piston skirt are normal, and no metallurgical, heat treatment, or forging defects were detected.
The crack originates from the acute angle surface formed by the intersection of the oil hole of the piston skirt and the oil groove of the piston pin hole.
The crack spreads laterally along the piston skirt and extends from the inside to the outside, indicating a typical fatigue crack.
(2) Cracking reason analysis
After the piston skirt is machined, the oil groove of the oil hole and the piston pin hole form an acute angle. There are still unclean burrs, and many original cracks can be seen on the curled burrs. These cracks cause the piston skirt to fatigue and crack during operation.
The specific cracking process analysis is as follows:
The sharp edge is a stress concentration area, and the small crack is subjected to external force at the stress concentration area, causing the sharp edge to become a fatigue source area.
The fatigue source area is highly sensitive to notches, and the final notch (crack) extends and propagates under stress concentration, resulting in the crack of the piston skirt. Therefore, the cracks are fatigue cracks caused by burrs on the sharp corners of the oil holes.
In the actual production process, we should inspect the parts for pits, cracks, burrs, and other defects on the surface of the piston skirt before the start of each process, such as forging, heat treatment, machining, and assembly.
If any defects are found, they should be cleaned before entering the next process. This will help avoid scrapping of parts due to the expansion of defects in subsequent production or use.