CLADDING TECHNOLOGY SHANXI CO., LTDFailure Analysis of Overlay Weld Cracking in Remanufactured Large Hot-Rolled Support Rollers
Literature Overview
This study note presents a detailed failure analysis of overlay weld cracking in large hot-rolled support rollers that have been remanufactured through the weld overlay process. Large support rollers in hot rolling mills experience severe thermal, mechanical, and chemical loading conditions, and their remanufacture through overlay welding is a critical maintenance activity that directly impacts mill availability and product quality. The cracking of the overlay layer represents a serious failure mode that can lead to catastrophic roller failure, production downtime, and safety hazards.
Failure Description and Investigation Methodology
The failure analysis employed a systematic methodology following the principles of root cause analysis. The investigation included visual examination, macrographic and micrographic analysis, hardness profiling, mechanical property testing, chemical composition analysis, and fractographic examination using scanning electron microscopy. The overlay weld was applied using submerged arc welding (SAW) or gas metal arc welding (GMAW) with a hardfacing alloy, typically a martensitic or austenitic composition, onto the surface of a low-alloy steel roller body.
The cracking pattern observed was characterized by transverse and longitudinal cracks in the overlay layer, with some cracks extending into the heat-affected zone (HAZ) and occasionally reaching the base metal. The crack initiation sites were predominantly located at the weld toe or at the interface between the overlay layer and the HAZ. The cracking was classified as a combination of hot cracking and cold cracking, with the relative contribution of each mechanism depending on the specific alloy system and welding parameters used.
Metallurgical Analysis and Root Cause Identification
| Analysis Method | Finding | Significance |
|---|---|---|
| Macrograph examination | Cracks predominantly in weld toe region | Indicates stress concentration at geometry transition |
| Micrograph examination | Intergranular cracking in HAZ | Suggests HAZ softening and microstructural degradation |
| Hardness profile | Hardness peak in HAZ (450-550 HV) | Indicates excessive hardening from martensitic transformation |
| SEM fractography | Cleavage and intergranular fracture features | Mixed-mode failure with both brittle and ductile mechanisms |
| Chemical analysis | Sulfur and phosphorus inclusions in base metal | Suggests inclusion-induced crack initiation |
| Dilution analysis | 15-25% base metal dilution in first weld pass | Excessive dilution affecting overlay composition |
The root cause analysis identified several contributing factors. The primary factor was the excessive heat input during the overlay welding process, which resulted in a wide and coarse HAZ with significant microstructural degradation. The high carbon equivalent of the base metal, combined with the rapid cooling rates at the weld toe, promoted the formation of hard and brittle martensitic microstructures in the HAZ. These brittle microstructures were susceptible to cracking under the residual stresses generated by the welding process.
Welding Process Parameters and Defect Analysis
The welding process parameters used during the remanufacturing operation were reviewed and found to be outside the recommended ranges for the specific material combination. The heat input was approximately 25 to 35 kilojoules per millimeter, which is significantly higher than the recommended maximum of 15 to 20 kilojoules per millimeter for this type of application. The interpass temperature was not adequately controlled, with temperatures reaching 350 to 450 degrees Celsius between passes, which promoted further grain coarsening in the HAZ.
The following table summarizes the recommended versus actual welding parameters:
| Parameter | Recommended | Actual | Deviation |
|---|---|---|---|
| Heat input (kJ/mm) | 10-20 | 25-35 | Excessive |
| Interpass temperature (°C) | 100-200 | 350-450 | Excessive |
| Welding current (A) | 200-300 | 350-450 | Excessive |
| Travel speed (mm/min) | 200-350 | 120-200 | Too slow |
| Preheat temperature (°C) | 150-250 | 0-50 | Insufficient |
Countermeasures and Engineering Recommendations
Based on the failure analysis findings, the following countermeasures were recommended for future remanufacturing operations. First, the welding parameters must be optimized to reduce the heat input and control the interpass temperature within the specified limits. Second, the base metal surface must be thoroughly cleaned and preheated to reduce the cooling rate and minimize the formation of brittle microstructures in the HAZ. Third, a multi-pass welding strategy with lower heat input per pass should be employed to achieve the required overlay thickness while maintaining acceptable microstructural characteristics.
Additionally, the use of a suitable flux or shielding gas composition can help control the dilution ratio and improve the metallurgical quality of the overlay layer. Post-weld heat treatment, such as stress relief annealing, should be performed to reduce the residual stresses in the weld and HAZ. The overlay layer should be inspected using non-destructive testing methods, including magnetic particle testing (MT) and ultrasonic testing (UT), to detect any residual defects before the roller is returned to service.
Study Insights and Reflections
This failure analysis underscores the critical importance of proper welding procedure qualification and strict adherence to welding parameters in the remanufacturing of critical components. The cracking of the overlay layer was not caused by a single factor but by a combination of excessive heat input, inadequate preheating, and insufficient interpass temperature control. The engineer must recognize that the remanufacturing of large rollers is not a simple overlay application but a complex welding operation that requires careful consideration of the material properties, welding process parameters, and post-weld treatment. A systematic approach to welding procedure development, including thorough material characterization, welding procedure qualification testing, and comprehensive post-weld inspection, is essential to prevent recurrence of this failure mode.