CLADDING TECHNOLOGY SHANXI CO., LTDSecondary Weld Overlay Technology for TCS-TI Wear-Resistant Strip
Literature Overview
The study of secondary weld overlay technology for TCS-TI wear-resistant strip addresses a critical engineering challenge in the manufacturing of heavy-duty wear parts such as crusher hammers, mill liners, and mining equipment components. The TCS-TI strip is a composite material consisting of a high-carbon, high-chromium martensitic wear-resistant layer bonded to a ductile backing substrate, providing a combination of extreme hardness and impact toughness that is difficult to achieve through single-material approaches. The secondary weld overlay process involves re-depositing a fresh overlay layer onto the worn surface of an in-service TCS-TI component, effectively restoring its functional dimensions and wear resistance without requiring complete replacement of the part.
Core Technical Points
The fundamental principle of secondary weld overlay relies on achieving proper metallurgical bonding between the new overlay material and the existing substrate while managing residual stresses that accumulate during the re-welding process. Unlike virgin manufacturing, secondary overlay must contend with pre-existing residual stresses, potential surface contamination, and possible microstructural degradation of the base material from prior thermal exposure. The process typically employs submerged arc welding or gas metal arc welding with carefully selected consumables that match or exceed the hardness and wear characteristics of the original overlay layer.
Key process parameters include preheating temperature, interpass temperature control, welding current and voltage, travel speed, and layer thickness management. The preheating temperature is particularly critical for secondary overlay operations because the base material may have undergone thermal cycling during service, potentially introducing microcracks or phase transformations that affect weldability. Typical preheat temperatures range from 200 °C to 400 °C depending on the carbon equivalent of the base material and the thickness of the component being repaired.
| Process Parameter | Recommended Range | Purpose |
|---|---|---|
| Preheat temperature | 200–400 °C | Reduce cooling rate, prevent cracking |
| Interpass temperature | 250–350 °C | Maintain ductility, control HAZ hardness |
| Arc voltage | 28–34 V | Control heat input and bead profile |
| Travel speed | 150–250 mm/min | Balance dilution and deposition rate |
| Layer thickness | 3–6 mm per pass | Ensure adequate dilution control |
| Post-weld cooling | Controlled to ≤50 °C/hr | Prevent martensitic transformation in HAZ |
Metallurgical Considerations
The microstructure of the secondary overlay layer is heavily influenced by the thermal history of the base material and the dilution between the new weld metal and the existing substrate. In the case of TCS-TI strip, the original overlay layer typically contains hard carbides such as M7C3 and MC-type carbides distributed in a martensitic matrix, with hardness values ranging from 58 to 65 HRC. During secondary overlay, the dilution between the new weld metal and the existing surface material can alter the carbide distribution and matrix microstructure, potentially reducing the effective hardness of the new overlay layer.
The heat-affected zone (HAZ) in secondary overlay presents unique challenges compared to initial manufacturing. The repeated thermal cycling can cause grain coarsening in the HAZ, reduce the tempering resistance of the martensitic structure, and potentially initiate microcracking along prior austenite grain boundaries. Metallographic examination of successfully executed secondary overlay repairs typically shows a gradient transition from the new overlay layer through a tempered martensitic HAZ into the unaffected base material, with no evidence of cracking or excessive softening.
Defect Analysis and Countermeasures
Common defects encountered in secondary weld overlay operations include lack of fusion between the new overlay and the existing surface, undercuts at the weld toe, hot cracks in the weld metal, and cold cracks in the HAZ. The following table summarizes these defects along with their root causes and recommended countermeasures.
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Lack of fusion | Surface contamination, insufficient penetration | Thorough surface preparation, increased heat input |
| Hot cracking | High sulfur/phosphorus in base material | Use of low-sulfur consumables, controlled solidification |
| Cold cracking | High carbon equivalent, rapid cooling | Preheating, post-weld heat treatment |
| Excessive dilution | Too high travel speed or wire feed rate | Optimize welding parameters, use multi-pass technique |
| Porosity | Hydrogen contamination from moisture | Dry consumables, shield gas flow rate control |
Engineering Practice Insights
In practical applications, the decision to perform secondary overlay rather than replace the entire component is driven by economic considerations, availability of replacement parts, and the remaining structural integrity of the base material. A systematic assessment protocol should be followed before initiating secondary overlay, including visual inspection for existing cracks, ultrasonic testing for subsurface defects, hardness mapping to identify areas of excessive softening, and dimensional measurement to determine the required build-up thickness.
The repair procedure should follow a structured approach: initial surface preparation by grinding to remove all contaminated and degraded material, thorough cleaning to remove oxide scale and foreign matter, application of a compatible primer coat if required, and then the sequential deposition of overlay layers with appropriate interpass temperature control. Post-weld heat treatment is often essential to relieve residual stresses and temper the martensitic weld metal to achieve the desired balance of hardness and toughness.
Summary and Reflections
The secondary weld overlay technology for TCS-TI wear-resistant strip represents a mature yet demanding repair methodology that requires careful attention to metallurgical compatibility, process parameter control, and quality assurance. The key insight from studying this technology is that successful secondary overlay is not merely a repetition of the original manufacturing process but rather a controlled repair operation that must account for the altered metallurgical state of the component. Engineers working with these components should develop comprehensive repair procedures that include thorough pre-repair assessment, validated welding procedures, and rigorous post-repair inspection protocols to ensure that the repaired component meets or exceeds the performance requirements of the original design.