CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis of Cracking in Weld Overlay Layer of Rubber Extruder Screw Ridge
Literature Overview
This study note presents a failure analysis of cracking in the weld overlay layer applied to the screw ridge of a rubber extruder. Rubber extruders operate under conditions of high temperature (150-300 °C), high shear stress, and abrasive contact with rubber compounds containing fillers such as carbon black and silica. The screw ridge, which is the primary wear surface, is often repaired by weld overlay to restore dimensions and improve wear resistance. However, cracking in the overlay layer can lead to catastrophic failure, contaminating the rubber product and causing unplanned downtime. This case provides valuable insights into the metallurgical and mechanical factors that contribute to overlay layer cracking.
Failure Investigation and Root Cause Analysis
The investigation employed a systematic approach combining macroscopic examination, metallographic analysis, and fracture surface analysis. The overlay layer was applied using a hardfacing alloy, typically a cobalt-based or chromium-carbide-based material, with a target hardness of 50-65 HRC. The substrate was a medium-carbon alloy steel, such as 40Cr or 42CrMo, with a base hardness of 25-32 HRC.
The failure mode was identified as transverse cracking perpendicular to the screw axis, with cracks extending through the full thickness of the overlay layer and sometimes propagating into the substrate. The metallographic examination revealed the following features:
| Microstructural Feature | Location | Interpretation |
|---|---|---|
| Martensitic transformation | Overlay base and heat-affected zone | High carbon dilution from substrate |
| Carbide networks | Bond interface | Excessive interfacial carbon concentration |
| Grain boundary cracking | Overlay surface | Thermal fatigue from repeated heating cycles |
| Microvoids | Near bond interface | Incomplete wetting of substrate surface |
The root cause analysis identified three contributing factors:
- Excessive carbon dilution: The high-carbon hardfacing alloy combined with carbon from the substrate created a carbon-rich zone at the bond interface. This zone underwent martensitic transformation during cooling, generating high residual stresses and reduced ductility.
- Thermal fatigue: The repeated thermal cycling during extruder operation (150-300 °C) caused cyclic thermal strains in the overlay layer. The high hardness and low thermal conductivity of the hardfacing material amplified these thermal strains, leading to progressive crack initiation and propagation.
- Inadequate stress relief: The absence of post-weld stress relief treatment left high residual tensile stresses in the overlay layer, which acted as driving forces for crack propagation.
Remedial Measures and Process Optimization
Based on the failure analysis, several corrective measures were implemented:
Material Selection Modification
The overlay material was changed from a single hardfacing alloy to a multi-layer system:
| Layer | Material | Function |
|---|---|---|
| Transition layer | Ni-Fe alloy (low carbon) | Buffer against carbon dilution |
| Intermediate layer | Cr-C alloy (medium hardness, 45-50 HRC) | Balance hardness and toughness |
| Surface layer | Co-Cr-C alloy (high hardness, 55-60 HRC) | Wear resistance |
Process Parameter Optimization
The welding parameters were adjusted to minimize heat input and reduce dilution:
- Reduced current by 15-20% compared to original parameters.
- Increased travel speed by 10-15% to reduce heat affected zone width.
- Applied a backing bar to ensure full penetration of the root layer.
- Maintained interpass temperature at 150-200 °C (previously uncontrolled).
Post-Weld Treatment
A mandatory post-weld stress relief treatment was implemented at 600-650 °C for 2 hours, followed by controlled cooling in the furnace. This treatment reduced residual stresses by approximately 60-70%, significantly improving the fatigue resistance of the overlay layer.
Prevention Strategy and Monitoring Program
To prevent recurrence, a comprehensive prevention strategy was developed using the PDCA cycle:
- Plan: Establish a documented repair procedure with defined material specifications, welding parameters, and inspection criteria.
- Do: Train welding operators on the specific requirements of screw ridge repair, including surface preparation, layer thickness control, and interpass temperature monitoring.
- Check: Implement in-process inspection including hardness testing after each layer and visual examination of all welds before stress relief.
- Act: Conduct periodic overlay condition monitoring during extruder operation using ultrasonic testing to detect early-stage cracking before it becomes critical.
The monitoring program includes ultrasonic thickness measurement of the overlay layer at intervals of 500 operating hours and dye penetrant testing at each scheduled maintenance interval. Any detected cracking is immediately addressed by removal of the affected area and reapplication of the overlay following the optimized procedure.
Study Insights and Engineering Implications
This failure analysis case is a powerful demonstration of how material selection, process control, and post-weld treatment interact to determine the long-term performance of a weld overlay repair. The most critical insight is that high hardness alone does not guarantee service life; the overlay layer must possess adequate toughness to withstand thermal cycling and mechanical loading. The multi-layer approach with a nickel-based transition layer proved essential in preventing the carbon dilution problem that caused the original failure. From an engineering practice standpoint, this case reinforces the principle that every weld overlay repair should be designed as a system, not as a single operation. The selection of each layer material, the welding parameters for each pass, and the post-weld treatment must all be coordinated to achieve the desired performance. Furthermore, the case highlights the importance of post-repair monitoring, as overlay layers can degrade over time due to thermal fatigue, and early detection of degradation allows for preventive intervention before catastrophic failure occurs.