After 7305h operation of a diesel engine of our company, cracks were found on the piston skirt of the third cylinder.
In order to find out the cause of cracking, we dissected the piston skirt and comprehensively analyzed and judged the chemical composition, mechanical properties, macrostructure, macro morphology and microstructure of the fracture.
1. Technical requirements for piston skirt
The piston skirt of our company is die forging.
The material used is 4032.
The delivery state of the material is T6 (solid solution heat treatment).
The chemical composition of the material complies with GB / T3190.
The production process is: forging → solid solution → artificial aging → machining.
The mechanical properties of piston skirt after forging, solid solution and artificial aging are: HBS = 100 ～ 125 (10 / 1000), σ b≥280MPa， δ 5≥1%。
In addition, the macrostructure of the quarter section is not allowed to have segregation, cracks, pores and inclusions.
The metal flow direction is generally distributed along the contour of the forging, and there is no flow through and folding;
Take a sample at the end of the tensile sample, and use a microscope with 100 times or 400 times to check.
No harmful defects such as inclusions, segregation and overburning are allowed.
2. Process of crack discovery
Fig. 1 is a photo of the piston skirt submitted for inspection.
Obvious cracks can be seen in the photo, which extend transversely along the outer diameter of the piston, exceeding 1 / 4 of the outer diameter of the piston and penetrating the wall thickness of the piston.
Fig. 2 shows the crack morphology of the inner cavity.
The crack passes through the piston pin hole and extends from the inside to the outside along the pin hole.
Fig. 3 is a photo of the piston skirt opening along the crack with external force, for the purpose of analyzing the fracture morphology of the crack.
Fig. 4 is a photograph of the piston skirt dissected along mutually perpendicular center lines;
The purpose is to test and analyze the mechanical properties of piston skirt and 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.
It can be seen from Fig. 5 that the flow direction of the metal fiber is roughly distributed along the contour of the forging, there is no flow through and folding, and the macrostructure is normal;
It can be seen from Fig. 6 that the crack initiation end of the piston skirt is located at the sharp corner formed by the small oil hole and the piston pin oil groove.
No obvious plastic deformation is found in the fracture morphology.
Typical fatigue bands can be seen in the macro. The fatigue arc center points to the sharp corner of the oil hole;
Fig. 7 is the end morphology of the crack on the outer surface of the piston skirt.
It can be seen from the morphology that fatigue bands can be seen in the middle of the fracture, and unstable jumping ridge lines can be seen in the transient fracture area near the free surface.
Fig. 8 is the front macro morphology of the sharp corner of the oil hole in the fatigue source area. From the macro morphology, it can be seen that there are no burr flash and original crack on the burr at the sharp corner of the oil hole.
It can be seen from Fig. 9 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.
Fig. 10 is a photograph of the metallographic specimen under an optical microscope.
From the photograph, it can be seen that the microstructure is: α+ ( α+ Si) + strengthening phase and impurity phase, normal microstructure, no metallurgical and heat treatment defects.
Fig. 11 is a photograph of the cross-section specimen under the scanning electron microscope.
It can be seen from the photograph that the fracture originates at the sharp corner of the oil hole processing.
Fig. 12 shows the morphology of the fracture initiation source area.
It can be seen that the morphology of the final fracture area of the fracture is dominated by the dimple morphology.
3. Results and analysis
(1) Analysis of inspection results
The results of chemical composition analysis show that the chemical composition of piston skirt meets the requirements of material 4032 in GB / T3190.
The mechanical properties of piston skirt meet the requirements of product design.
The metallographic structure and macrostructure are normal, and no metallurgical, heat treatment and forging defects are found.
The crack originates from the acute angle surface intersected by the oil hole of the piston skirt and the oil groove of the piston pin hole.
The crack extends laterally along the piston skirt and extends from the inside to the outside. It is 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 forms an acute angle, and there are still unclean burrs, and many original cracks can be seen on the curled burrs.
These cracks make the piston skirt fatigue crack during the operation of the piston skirt.
Specific cracking process analysis is as follows:
The sharp edge is the stress concentration part, and the small crack is subjected to external force at the stress concentration part, so that the sharp edge becomes the fatigue source area;
The fatigue source region is highly sensitive to the notch, and the final notch (crack) extends and emits under the stress concentration state, resulting in the crack of the piston skirt.
Cracks are fatigue cracks caused by the burrs on the sharp corners of the oil holes.
Therefore, in the actual production process, we should clean the parts before the start of each process.
For example, the piston skirt should be inspected for pits, cracks, burrs and other defects on the surface of the parts before the start of each process of forging, heat treatment, machining and assembly.
If so, it should be cleaned before entering the next process.
Avoid parts scrapping due to the expansion of defects in subsequent production or use.