CLADDING TECHNOLOGY SHANXI CO., LTDFailure Analysis of Cracking in Surface Cladding Layer of Support Rollers
Overview of the Failure Case
This study note examines a documented failure case involving cracking in the surface cladding layer of a support roller used in heavy-duty rolling mill applications. Support rollers operate under extreme cyclic loading, high contact stress, and often elevated temperatures, making the integrity of their overlay layers critical to continuous production. The cracking observed was intergranular in nature, propagating along prior austenite grain boundaries within the cladding microstructure, which suggests a metallurgical origin rather than purely mechanical overload.
Root Cause Analysis
Through metallographic examination and fractographic analysis, several contributing factors were identified. The primary root cause was identified as improper heat input during the welding overlay process, which resulted in excessive grain growth in the dilution zone between the base metal and the cladding layer. Secondary factors included inadequate preheating of the roller surface, residual stresses from multi-pass overlay welding, and hydrogen-induced cracking susceptibility in the high-carbon martensitic microstructure of the cladding material.
| Failure Parameter | Observed Value | Acceptable Range | Assessment |
|---|---|---|---|
| Preheat Temperature | 80 °C | 200–300 °C | Insufficient |
| Interpass Temperature | 280 °C | ≤250 °C | Excessive |
| Heat Input per Pass | 3.8 kJ/mm | ≤2.5 kJ/mm | Excessive |
| Post-Weld Cooling Rate | 8 °C/s | 3–6 °C/s | Too rapid |
| Hardness of Cladding | 620 HV | 450–550 HV | Over-hardened |
Metallurgical Investigation
The cladding layer exhibited a fully martensitic microstructure with retained austenite content exceeding 15%, indicating incomplete austenite transformation during cooling. The prior austenite grain size was measured at ASTM No. 1 (average grain diameter approximately 200 μm), which is far coarser than the recommended ASTM No. 5–7 range for high-stress applications. Coarse grain boundaries provide preferential paths for crack initiation and propagation under cyclic loading conditions.
The dilution zone showed a gradient of microstructural transition from ferrite-pearlite in the base metal through a mixed ferrite-martensite region into the fully martensitic cladding. This gradient creates a hardness mismatch of approximately 200 HV across a narrow transition band, which concentrates stress during operational loading and accelerates fatigue crack initiation at the interface.
Engineering Countermeasures
Based on the failure analysis findings, the following corrective measures were recommended and subsequently implemented in subsequent overlay operations:
- Increase preheat temperature to 250 °C and maintain interpass temperature below 200 °C to reduce cooling rates and minimize martensite formation.
- Reduce heat input per pass to below 2.0 kJ/mm by adjusting welding current and travel speed parameters.
- Implement post-weld heat treatment (PWHT) at 580–620 °C for a minimum of 2 hours per inch of cladding thickness to temper martensite and relieve residual stresses.
- Select a cladding material with lower carbon equivalent (CE ≤ 0.45) to reduce hydrogen cracking susceptibility.
- Apply a two-layer strategy: a transition layer of lower-carbon material followed by the functional overlay layer to reduce interface hardness gradient.
Reflections and Engineering Implications
This failure case underscores a fundamental principle in weld overlay engineering: the mechanical properties of the cladding layer must be viewed in conjunction with the thermal-metallurgical compatibility of the entire layered system. A cladding material that achieves excellent hardness and wear resistance in isolation may become a liability if the welding parameters produce excessive hardness, coarse grain structures, or high residual stresses. The lesson for practitioners is that overlay welding parameter selection must always be validated through post-weld hardness profiling, microstructural examination of the dilution zone, and residual stress measurement before the component is released for service.