I. Operating Environment of Valves and Main Causes of Breakage
Valves in marine diesel engines operate for extended periods under high-temperature and high-stress conditions. Therefore, it is crucial to select appropriate materials during the design and manufacturing processes to ensure they can withstand these harsh working environments.
When the engine is running, the combustion of the air-fuel mixture in the cylinder generates extremely high temperatures. The valves are subjected to high-speed impacts from hot, corrosive exhaust gases, with temperatures reaching 700-900°C. Although heat can be partially dissipated through the valve guides and seats to the cylinder head, the cooling effect is limited.
Additionally, the sudden entry of fresh air causes instantaneous cooling of the valves, resulting in a temperature difference of up to 650-800°C. This alternating exposure to extreme temperatures leads to fatigue and creep failure, significantly degrading the mechanical properties and wear resistance of the valves. Moreover, the valves are susceptible to erosion at high temperatures, leading to deformation and potential failure due to leakage or cracking.
The uneven temperature distribution within the valve, particularly in the valve disc area, where temperature differences can reach 150-200°C, generates thermal stresses that cause thermal deformation. This poor heat transfer increases the working temperature of the valve, creating a vicious cycle that further exacerbates the problem. Under heavy loads and long-term exposure to high temperatures and pressures, the structural integrity of the valve is easily compromised, leading to various failure-related issues. In severe cases, this can result in accidents.
A specific case study involves a marine diesel engine that failed after accumulating 22,390 hours of operation and 2,116 hours post-major repair. The rocker arm pushrod of cylinder A2 was found bent and deformed, and both the intake and exhaust valves had fractured. This paper analyzes the damaged valves, examining their material composition and microstructure morphology, and provides insights into the causes of their failure based on the findings.
Analysis of Valve Failure Modes
Valve failure is closely related to the valve mechanism and associated components. Based on extensive experience, different failure phenomena of valves have been summarized, and the main failure modes are categorized as follows:
1. Valve Fracture
This includes fractures at various locations:
1) **Lock Groove Fracture**: Fractures occurring in the lock groove and its surrounding area.
2) **Fracture at the Friction Welded Joint**: Manifested as false welding, incomplete welding, or the presence of welding crack zones due to improper friction welding and connection. Under high stress during operation, this can lead to fractures.
3) **Stem Fracture**: Fractures occurring along the valve stem.
4) **Valve Head Drop**: Fractures at the neck of the stem, leading to the detachment of the valve head.
5) **Head Block Drop**: Partial detachment or loss of the head at the large plane, neck arc, or conical surface, which can be further divided into radial fractures and chord block fractures [3].
2. Abnormal Valve Wear
This includes several types of wear:
1) **Excessive Conical Surface Wear**: Excessive wear of the valve conical surface due to excessive friction with the valve seat ring during contact.
2) **Uneven Conical Surface Wear**: Uneven wear caused by uneven seating during the closing process.
3) **Abnormal Wear at the Valve Stem End**: Problems such as excessive wear or pitting at the valve stem end during use.
4) **Abnormal Wear Along the Valve Stem**: Excessive wear, uneven wear, or seizing caused by abnormal contact between the valve stem and the guide tube.
3. Valve Deformation
This refers to distortion of the valve head caused by factors such as impact from foreign objects during valve movement. It also includes:
1) **Conical Surface Erosion**: Caused by mismatch between the valve conical surface and the valve seat ring, leading to poor cylinder sealing. In this case, the conical surface and head are eroded by gas, overheated, cracked, and oxidized. Premature ignition in the cylinder can also cause significant overheating and erosion of the valve head area.
2) **Conical Surface Corrosion**: Cracking and damage caused by the accumulation of corrosive media from combustion products on the conical surface and neck of the valve head.
3) **Surfacing Layer Failure**: Improper surfacing and subsequent processing techniques during surfacing on the conical surface or rod end can result in welding defects such as porosity, shrinkage cavities, cracking, and false welding, leading to valve failure.
4) **Valve Drop-off**: The valve falls into the cylinder and becomes ineffective due to issues with other components such as the valve spring, valve spring upper seat, and valve lock plate.
5) **Seat Ring Dislodgement**: The valve seat ring comes out of the valve seat ring hole in the cylinder head during operation due to factors such as assembly, processing, and thermal fatigue.
4. Other Failure Modes
1) **Valve Spring Breakage**: Valve springs break due to defects or unreasonable structure.
2) **Cylinder Head Cracking**: Cracking and leakage of the cylinder head during operation due to poor cooling or carbon deposits in the cylinder.
3) **Piston Crown Melting**: Overheating and melting of the piston crown during use due to poor heat transfer, carbon deposits in the cylinder, or abnormal fuel injectors.
4) **Crankshaft Bearing Seizure**: Seizure of the journal due to abnormal lubrication and unreasonable contact.
5) **Timing Disorder**: Timing errors caused by abnormalities in timing gears or camshafts.
6) **Abnormal Valve Clearance**: Excessive dynamic clearance due to abnormal wear of valve mechanism components.
7) **Pushrod and Tappet Failure**: Breakage and deformation of the pushrod and tappet during operation.
The following provides a detailed description of the manifestation and causes of these failure modes, supporting failure prevention and cause analysis.
III. Valve Failure Experiment Detection
During the research process, faulty valve components were inspected, with particular focus on the head end and rod end (bent section) of the valve stem for submission and analysis. Various inspection methods were comprehensively applied to enhance the reliability of the results.
1. Overview of the Samples
Four samples were submitted for inspection:
- One intact valve stem
- Two broken valve stems
- One failed cylinder head
As shown in Figure 6, material analysis and comparative examination of the morphology and metallographic conditions of the fracture surfaces of both intact and failed valves were conducted to support failure cause analysis and serve as a reference for failure detection and prevention.
The two sections of the failed valve stem were defined as follows:
- **Section A (Intact Section)**: The valve stem was intact on both the front and back.
- **Section B (Failed Section)**: The mushroom head of the valve stem on the front was missing, and the spring and sleeve on the back were also absent. The valve stem in section B was bent, broken, and jammed inside the sleeve. Relevant conditions are illustrated in Figure 4.
Figure 4 Sample for Inspection
2. Inspection Basis and Equipment
The inspection was conducted in accordance with the following standards:
- Metallographic Examination Methods (GB/T 13298-2015)
- General Rules for Electron Beam Microanalysis of Defects in Steel Materials (GB/T 21638)
The inspection equipment utilized includes:
- LEICA M205A Stereo Microscope (SCW034)
- ZEISS Axioplan2 Optical Microscope (SCW026)
- JSM Scanning Electron Microscope (SCW030)
3. Inspection Results
Initially, a visual inspection was performed. It was observed that within the damaged hole on the front side of the cylinder head, a broken valve stem was present. On the opposite side, the spring component remained intact, whereas on the side with the fracture damage, the spring component was missing, as illustrated in Figure 5.
Figure 5 Macroscopic morphology of the failed part
Disassembly and Analysis of the Cylinder Part
The cylinder part was laterally disassembled, and the bent valve stem was carefully extracted. Upon inspection, it was observed that the front surface of the cylinder part exhibited severe wear. The outer ring of the cylinder showed signs of fracture, with the initial fracture originating from the surface of the ring. On the opposite side where the valve stem had broken, a failure related to the tube fracture was identified. Specifically, a crack was found at the other end of the tube, which extended to the outer frame, causing additional cracking, as illustrated in Figure 6.
Figure 6 Macroscopic morphology of the failed part and the location and fracture morphology of the casing fracture
Analysis of Fracture Surfaces in Section B
From the汇总 and analysis of the failed parts in section B, it is evident that there are two distinct fractures with three corresponding fracture surfaces. These three fracture surfaces are designated as follows:
- Fracture Surface B1
- Fracture Surface B2
- Fracture Surface B3
The locations of these fracture surfaces are illustrated in Figure 7.
Figure 7 Macroscopic morphology of the valve stem at part B of the failed component
Observation of Fracture Surfaces under Stereomicroscope
The fracture surfaces were examined using a macroscopic stereomicroscope. It was observed that the fracture morphology exhibited severe damage due to impact wear. The fracture surfaces B2 and B3 are associated with each other. On the B3 fracture surface, localized fatigue striations were identified, although they are faint and barely discernible. The fracture origin is located at the edge, as illustrated in Figures 8 and 9.
Figure 8 Macroscopic stereoscopic morphology of the origin site of the valve stem head fracture
Figure 9 Macroscopic stereoscopic morphology of the valve stem fracture at part B of the failed component
Comparison of Surface Damage and Fracture Origin in Three Valve Stems
A comparative analysis of the three valve stems revealed that the locations of surface damage exhibited a high degree of similarity. Furthermore, there was a clear positional correlation between the fracture origin and the areas of surface damage, as illustrated in Figure 10.
Figure 10 Macroscopic Stereoscopic Morphology of the Valve Stem Head
Impact-induced deformation is observed at the end of the head spring, and metal shavings are adhered to the inner wall of the spring. This damage corresponds to the surface damage on the valve stem, as illustrated in Figure 11.
Figure 11 Macroscopic Stereoscopic Morphology of the Spring Retainer Area at the Valve Stem Head
4. Inspection Conclusion
The fractured valve stem submitted for inspection can be primarily divided into three sections: the head end, the bent stem section, and the valve seat. Near the fracture surface of the valve stem, fine transverse cracks with depths ranging from 4 to 10 μm were observed. These cracks result in stress concentration at their roots. Under the influence of a corrosive environment and repetitive motion, fatigue cracking occurs, ultimately leading to the fracture of the valve stem head.
Following the fracture of the valve stem head, the movement of the valve stem becomes unbalanced, causing operational jamming, bending, and friction against the sleeve. This leads to cracks and eventual failure of the sleeve. Upon comparison with an intact valve stem, it was determined that these fine cracks are machining marks from turning operations. These machining marks serve as initial defects that contribute to the initiation of fractures. Component spectral analysis revealed the presence of corrosive oxides within the fine cracks, consistent with those found on the valve surface. The presence of these oxides accelerates material corrosion and promotes crack propagation.