In the course of routine road testing for a new vehicle model, a catastrophic failure occurred within the engine assembly. The component in question was the crankshaft, manufactured from nodular cast iron grade QT700-2. The fracture originated at the fourth connecting rod journal, leading to a complete severance. As the lead materials engineer on this investigation, my objective is to determine the root cause of this failure through a systematic, first-person analytical journey. This report details the methodologies employed, the data gathered, and the conclusions drawn to prevent recurrence. The nodular cast iron crankshaft, a critical component subjected to complex multi-axial loading, must exhibit exceptional fatigue resistance, and its failure warrants a thorough dissection.
The crankshaft is the backbone of an internal combustion engine, converting the linear motion of pistons into rotational torque. It operates under an environment of severe cyclic stresses—combining bending, torsion, and vibration. High-stress concentrations are inherent at the fillet radii transitioning between the journal pins and the crank webs. Therefore, the material’s integrity, the quality of surface treatments, and the precision of machining in these regions are paramount. The use of nodular cast iron, specifically QT700-2, is common due to its favorable combination of castability, strength, wear resistance, and damping capacity, largely imparted by its spherical graphite nodules within a pearlitic matrix. However, any deviation in microstructure, residual stress state, or geometry can significantly compromise its fatigue life.
Materials and Analytical Methods
Our investigation protocol was designed to examine the failure from multiple angles: fractography, metallography, chemical composition, mechanical properties, and residual stress analysis. The failed crankshaft and an identical, unfailed unit from the same production batch were made available for comparative study.
| Analysis Type | Method/Standard | Equipment/Technique | Purpose |
|---|---|---|---|
| Macro/Micro Fractography | Visual & Scanning Electron Microscopy (SEM) | Quanta 200 SEM | Identify crack initiation site, failure mode, and fracture features. |
| Metallographic Examination | Optical Microscopy | Sample Mounting, Polishing, Etching (4% Nital) | Assess graphite morphology, matrix structure, and heat treatment layer. |
| Optical Emission Spectrometry | Spectrometer | Verify conformity to QT700-2 specification. | |
| Mechanical Properties | Tensile Testing, Hardness Testing | MTS 322 Tester, Vickers/Brinell Hardness Testers | Measure tensile strength, yield strength, elongation, and hardness profile. |
| Surface Residual Stress | X-ray Diffraction (XRD) | Xstress-3000 X-ray Stress Analyzer |
Results and Observations
1. Fractographic Analysis
Macroscopic observation of the fracture surface revealed a classic fatigue failure pattern. Multiple crack initiation sites were identified at the edge of the rolled fillet radius of the fourth connecting rod journal. The fracture origin zones showed slight wear, likely from post-fracture rubbing. The crack propagation region exhibited clear beach marks (clam shell patterns) radiating from the initiation points. The final fast fracture zone was relatively small, indicative of high-cycle fatigue under predominantly elastic stresses.
SEM examination provided higher-resolution details. The crack initiation zones exhibited a quasi-cleavage morphology, with no evidence of inherent casting defects such as shrinkage porosity, slag inclusions, or large clusters of degenerate graphite. This is a positive indicator of the basic quality of the nodular cast iron casting. The propagation zone unequivocally confirmed the fatigue mechanism, showing striations associated with cyclic crack advancement.
A critical observation was made on the fillet surface itself. The bottom of the rolled fillet appeared unusually smooth, lacking the typical machining marks. However, near the edge—precisely where the cracks initiated—faint circumferential machining marks were still visible. This suggests a potential non-uniformity in the rolling process or the initial machining of the fillet geometry.
2. Metallurgical and Microstructural Analysis
Samples were extracted from both the fractured (fourth) and an unfractured (first) connecting rod journal for comparative analysis.
Graphite Morphology: The graphite nodularity was excellent, rated at Grade 1 (≥90% sphericity). The nodule size was Grade 6 (3-6 mm at 100x magnification). This is a prerequisite for good mechanical properties in nodular cast iron.
Matrix Structure: The microstructure consisted of a pearlitic matrix with a small amount of ferrite and minor carbides. The pearlite content was consistently above 98% at the surface and core of both journals, meeting the expectations for QT700-2. No significant microstructural banding or inhomogeneity was observed in the bulk.
Induction Hardening Layer: The journals undergo localized induction hardening. Cross-sectional analysis revealed the depth and profile of this hardened case.
$$ \text{Case Depth (Fourth Journal)}: \approx 1.5-2.0 \text{ mm} $$
$$ \text{Case Depth (First Journal)}: \approx 1.3-2.0 \text{ mm} $$
However, the width of the unhardened zone on the journal surface between the hardened tracks was not symmetrical, varying from 3.5mm to 5.5mm. More importantly, the hardened case did not show a distinct plastic deformation layer from the subsequent fillet rolling operation at the surface. This was consistent across both journals.
| Location | Graphite Rating | Matrix Structure | Pearlite Content | Note |
|---|---|---|---|---|
| 4th Journal Fillet (Surface) | 1级 (≥90%) | Pearlite + Ferrite + Carbides | ≥98% | No visible rolling deformation layer. |
| 1st Journal Fillet (Surface) | 1级 (≥90%) | Pearlite + Ferrite + Carbides | ≥98% | No visible rolling deformation layer. |
| 4th Journal (Core) | 1级 (≥90%) | Pearlite + Ferrite + Carbides | ≥98% | Uniform structure. |
3. Chemical Composition and Mechanical Properties
The chemical composition of the nodular cast iron was largely within the specified range for QT700-2, with the exception of Carbon, which measured slightly low. It is noted that in nodular cast iron, a significant portion of carbon exists as graphite, which can sometimes lead to underestimation in certain analytical techniques.
| Element | C | Si | Mn | P | S |
|---|---|---|---|---|---|
| Measured (wt.%) | 3.24 | 2.37 | 0.40 | 0.026 | 0.0071 |
| QT700-2 Typical Range | 3.60-3.85 | 1.90-2.60 | 0.20-0.70 | ≤0.05 | ≤0.03 |
The tensile properties and hardness far exceeded the minimum requirements for QT700-2, confirming that the bulk material possessed adequate strength.
| Property | Sample 1 | Sample 2 | QT700-2 Requirement |
|---|---|---|---|
| Tensile Strength, Rm (MPa) | 970 | 999 | ≥ 770 |
| Yield Strength, Rp0.2 (MPa) | 589 | 617 | ≥ 480 |
| Elongation, A (%) | 7.5 | 8.0 | ≥ 2 |
| Core Hardness (HB) | 268-275 | 280-291 | 240-320 |
| Surface Hardness (HRC) | 55.5-56.0 | 54.5-55.5 | 45-60 |
4. Residual Stress Measurement
Surface residual stress, particularly compressive stress, is crucial for inhibiting fatigue crack initiation. Measurements were taken at the fillet radii using X-ray diffraction.
| Measurement Location | Residual Stress (MPa) | Comment |
|---|---|---|
| 1st Journal Fillet (A) | -407.6 ± 22.8 | High compressive stress. |
| 4th Journal Fillet (B) – Unbroken side | -333.8 ± 24.7 | Moderate compressive stress. |
| 4th Journal Fillet (C) – Fractured side | -63.3 ± 8.5 | Low compressive stress (likely relaxed). |
The significant relaxation of compressive stress at location (C) is a consequence of the fracture releasing the stored elastic energy. The comparison between locations (A) and (B) suggests a potential variance in the effectiveness of the fillet rolling process between different journals.
Discussion: Synthesizing the Failure Mechanism
Integrating all findings, the failure scenario becomes clear. This was a high-cycle torsional fatigue failure of the nodular cast iron crankshaft. The primary cause was not a gross deficiency in the base material, which met or exceeded standard mechanical and microstructural specifications. Instead, the failure stemmed from a confluence of factors related to manufacturing process and stress state.
1. Stress Concentration and Multi-Site Initiation: The cracks initiated at the very edge of the rolled fillet radius, a point of high stress concentration. The presence of multiple initiation sites and radial “step” marks between them is characteristic of failure under a high principal torsional stress. The fatigue life Nf under cyclic stress is governed by equations like the Basquin or Coffin-Manson laws, but the presence of a stress concentrator (notch) drastically reduces the effective fatigue strength. The stress concentration factor Kt for the fillet geometry is critical:
$$ \sigma_{local}^{max} = K_t \cdot \sigma_{nominal} $$
Where a higher Kt leads to a much higher local stress, accelerating crack initiation.
2. Inadequate Surface Enhancement at the Critical Location: The fillet rolling process is intended to induce deep compressive residual stresses and work-harden the surface, thereby improving fatigue resistance. Our analysis showed two issues: (a) The residual compressive stress on the unfractured side of the fourth journal (-334 MPa) was lower than that on the first journal (-408 MPa), indicating process inconsistency. (b) More critically, the metallographic cross-section showed no visible plastically deformed layer at the fillet surface. This suggests the rolling parameters (force, feed rate) may have been insufficient to properly work the material at the precise edge where the machining marks were still visible. This left a potentially “softer,” less compressive zone exactly where the stress was highest.
3. The Role of Residual Stress: Compressive residual stress σres superimposes on the applied cyclic stress σa, effectively lowering the mean stress and retarding crack initiation and growth. The modified stress intensity factor range ΔK can be conceptually considered as:
$$ \Delta K_{eff} \propto Y \cdot (\sigma_{a} – \sigma_{res}) \cdot \sqrt{\pi a} $$
where Y is a geometric factor and a is crack length. Insufficient compressive σres at the fillet edge directly increases ΔKeff, dramatically reducing fatigue life.
4. Microstructure and Material Considerations: While the bulk microstructure was acceptable, the slight carbon deficiency, though analytically uncertain, could theoretically impact the hardenability and the stability of the pearlitic matrix under cyclic loading. The uniformity of the induction hardening case, particularly its coverage and the smoothness of its transition, is also vital for the nodular cast iron component’s performance.
Conclusion and Recommendations
The investigation concludes that the QT700-2 nodular cast iron crankshaft failed due to torsional fatigue, with cracks initiating at multiple points along the edge of the fourth connecting rod journal’s fillet radius. The root cause was the insufficient generation of a protective, deep compressive residual stress layer and potential micro-notches (machine marks) at this critically stressed location during the fillet rolling operation, exacerbated by high operational loads.
To prevent future failures and enhance the durability of the nodular cast iron crankshafts, the following measures are recommended:
- Optimize and Control the Fillet Rolling Process: Reevaluate and rigorously validate the rolling parameters (force, roller profile, feed speed) to ensure consistent and adequate plastic deformation is achieved across all journals, especially at the fillet edge. Implement 100% in-process monitoring of rolling force or torque.
- Improve Fillet Machining Preparation: Ensure the pre-rolled fillet geometry and surface finish are uniform and free of sharp transitions or pronounced machine marks that can act as stress raisers. Consider a finishing operation before rolling.
- Supplement with Shot Peening: Implement a controlled shot peening process after fillet rolling. This can enhance and homogenize the surface compressive stress field, improve surface finish, and provide an additional safety factor against fatigue. The Almen intensity and coverage must be specified.
- Enhance Quality Assurance: Introduce periodic destructive audits to measure actual case depth, residual stress profile, and the presence of a deformed layer. Perform regular chemical analysis to ensure consistent melt chemistry for the nodular cast iron.
This failure underscores that for high-integrity components like crankshafts made from nodular cast iron, achieving superior fatigue performance relies not just on specifying the correct grade, but on meticulously controlling every subsequent manufacturing process that influences the final surface condition and stress state.