The analysis of failed metal parts must follow the logic of "macroscopic first, then microscopic; phenomenon first, then essence; qualitative first, then quantitative". Its core lies in identifying the failure mode (e.g., fracture, corrosion, wear, deformation) through multi-dimensional testing, then tracing the root cause of failure (design, material, process, service environment, etc.), and finally providing a basis for improvement solutions. Below is a systematic analysis framework covering 6 core steps:
The prerequisite for failure analysis is to grasp the "full-life-cycle information" of the part; otherwise, it is easy to deviate from the correct direction. Key information to collect includes:
- Basic Part Information
- Part name, application (e.g., shaft, gear, pressure vessel), structural design drawing (focus on stress concentration areas such as fillets and holes);
- Material grade (e.g., 45 steel, 304 stainless steel, TC4 titanium alloy) and original performance parameters (hardness, tensile strength, corrosion resistance, etc.).
- Manufacturing and Processing Processes
- Forming processes (casting, forging, welding, 3D printing), heat treatment processes (quenching and tempering, solution aging), surface treatment (chrome plating, carburizing);
- Whether there were defects during processing (e.g., welding pores, forging cracks, heat treatment deformation).
- Service and Operating Conditions
- Working load (static/dynamic/impact load, load magnitude and direction);
- Environmental parameters (temperature: room/high/low temperature; medium: air, water, oil, acid-base solution, dust; presence of vibration or fatigue cycles);
- Operating status before failure (e.g., whether abnormal noise, leakage, or precision degradation occurred; whether failure was sudden or gradual).
- Historical Maintenance Records
- Whether the part has undergone repair or replacement; whether similar failures occurred before; whether there was improper operation during maintenance (e.g., overload use, insufficient lubrication).
Macroscopic analysis involves observing the appearance, fracture, and deformation characteristics of the failed part with the naked eye or a low-magnification magnifier (≤100x) to initially identify the failure type and key areas. It serves as the "navigation" for subsequent microscopic analysis. Focus on the following dimensions:
- Localization of the Failure Site
- Whether the failure occurred in a "stress-sensitive area" (e.g., shaft shoulder, keyway, thread root), "process weak area" (e.g., weld joint, casting riser), or "material defect area" (e.g., inclusions, porosity);
- Example: If a shaft fractures at the shaft shoulder fillet, it is likely related to stress concentration; if a pipeline leaks at the weld, welding quality should be prioritized for inspection.
- Observation of Appearance Characteristics
- Fracture Failure: Observe the fracture color (presence of oxide color to determine if fracture occurred at high temperature), flatness (flat = brittle fracture, rough = ductile fracture), and presence of radial lines (a typical feature of fatigue fracture, with the starting point of radial lines being the crack source);
- Corrosion Failure: Identify the corrosion type (pitting: local small holes; uniform corrosion: overall thinning; intergranular corrosion: cracking along grain boundaries; stress corrosion: accompanied by cracks and corrosion traces);
- Wear Failure: Observe whether the worn surface has abrasive scratches (abrasive wear), adhesion marks (adhesive wear, e.g., "seizure" of metal surfaces), or fatigue spalling (contact fatigue, e.g., spalling of gear tooth surfaces);
- Deformation Failure: Measure key dimensions of the part (e.g., shaft diameter, plate flatness) to determine if they exceed tolerances (e.g., "thermal deformation" at high temperatures, "plastic deformation" under overload).
- Verification of Macroscopic Mechanical Properties
- Sample the "non-failed area" of the failed part to test hardness, tensile strength, yield strength, etc., and compare with design requirements to determine if failure was caused by substandard material properties (e.g., insufficient hardness after heat treatment).
After narrowing the scope through macroscopic analysis, microscopic testing methods are used to observe the material’s microstructure, fracture details, and element distribution, revealing the "microscopic mechanism" of failure (e.g., brittle fracture due to coarse grains, cracking due to intergranular corrosion). Common methods and application scenarios are as follows:
- If SEM observes a large number of "dimples" (pit-like features) on the fracture, it indicates ductile fracture, which may be caused by overload (load exceeding the material’s yield strength);
- If the fracture has "cleavage planes" (flat small crystal planes) or "intergranular fracture" (cracks propagating along grain boundaries), it indicates brittle fracture, which may be caused by low temperature, material inclusions, or intergranular corrosion;
- If the fracture has "fatigue striations" (parallel stripes), it indicates fatigue fracture, which may be caused by repeated alternating loads (e.g., rotational vibration of a shaft) or surface crack sources (e.g., machining scratches).
The failure mechanism refers to the "physical/chemical process leading to part failure". It is necessary to combine macroscopic + microscopic analysis results to clarify the core cause of failure. Common failure mechanisms and corresponding scenarios are as follows:
- Mechanical Failure Mechanisms
- Overload fracture: Load exceeds the material’s ultimate strength, with dimples on the fracture;
- Fatigue fracture: Repeated alternating loads, with fatigue striations + crack sources on the fracture;
- Plastic deformation: Load exceeds the material’s yield limit, or material softening at high temperatures;
- Wear: Material loss due to surface contact friction (abrasive wear, adhesive wear, contact fatigue wear).
- Chemical Failure Mechanisms
- Corrosion: Chemical reaction between metal and environmental medium (e.g., carbon steel rusting in humid environments, stainless steel pitting in Cl⁻ environments);
- Oxidation: Reaction between metal and oxygen at high temperatures (e.g., steel forming oxide scale above 800℃, leading to reduced dimensional accuracy).
- Thermal Failure Mechanisms
- Thermal softening: High temperatures cause a decrease in material strength/hardness, leading to deformation or fracture;
- Thermal fatigue: Repeated heating-cooling cycles cause thermal stress cycles and crack formation (e.g., boiler pipes, engine blocks).
Based on the failure mechanism, further investigate the root cause from 5 links: "design, material, manufacturing, use, maintenance", avoiding stopping at "phenomenon description":
- Design Link
- Defects: Stress concentration design (e.g., excessively small fillet radius), insufficient safety factor (load calculation error), improper material selection (e.g., using ordinary carbon steel instead of stainless steel in corrosive environments);
- Example: A chemical pipeline using Q235 steel (not acid-resistant) to transport hydrochloric acid resulted in corrosion leakage, with the root cause being "incorrect material selection".
- Material Link
- Defects: Substandard material composition (e.g., insufficient alloying element content), internal inclusions (e.g., sulfide inclusions in steel), metallurgical defects (e.g., casting porosity, forging cracks);
- Example: A gear made of 20CrMnTi steel fractured at the tooth root due to excessive sulfur content during smelting, leading to inclusions.
- Manufacturing Link
- Defects: Incorrect heat treatment process (e.g., insufficient hardness due to low quenching temperature), improper welding process (e.g., incomplete penetration, pores), surface processing defects (e.g., turning scratches leading to fatigue crack sources);
- Example: A shaft part cracked on its own during storage due to excessive internal stress caused by not tempering in time after quenching.
- Use Link
- Defects: Overload operation (e.g., crane overload), operation beyond temperature/pressure limits (e.g., boiler pressure exceeding design value), abnormal environment (e.g., no rust prevention in humid environments);
- Example: A motor shaft suffered fatigue fracture due to excessive alternating loads caused by excessive equipment vibration.
- Maintenance Link
- Defects: Insufficient lubrication (accelerating bearing wear), failure to clean corrosive media in time (e.g., unremoved scale on pipeline inner walls, intensifying corrosion), improper repair (e.g., introducing new cracks during welding repair).
The ultimate goal of analysis is to solve the problem. Targeted and implementable improvement measures should be proposed based on the root cause, with common directions as follows:
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