CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Continuous Casting Machine Pilot Chain
Overview and Context
This 2005 technical paper from Shanghai Meishan Iron and Steel Co. Ltd. addresses the weld overlay repair of pilot chains used in continuous casting machines. Pilot chains are critical components that serve as the initial solidification seed during the continuous casting process, and they are subjected to extreme thermal cycling, mechanical abrasion from molten steel contact, and chemical attack from flux and slag. The authors from the Technology Center and Steelmaking Plant of Meishan Steel present a systematic approach to extending the service life of these chains through overlay welding, which is a highly practical solution given the cost and lead time associated with full chain replacement.
Technical Approach and Process Parameters
The pilot chain operates under conditions where the working surface contacts molten steel at temperatures exceeding 1500 °C, with thermal shock cycles occurring at every casting cycle. The overlay repair process must therefore produce a surface layer that offers excellent thermal fatigue resistance, oxidation resistance, and wear resistance while maintaining adequate bond strength to the base chain material.
| Parameter | Typical Specification |
|---|---|
| Base material | Alloy steel chain (typically 45# or 50Cr steel) |
| Overlay material | Cast iron or high-carbon steel welding electrode |
| Welding process | SMAW (shielded metal arc welding) |
| Electrode type | E5015 or specialized high-carbon overlay electrode |
| Preheat temperature | 150–250 °C |
| Interpass temperature | < 250 °C |
| Overlay layer thickness | 3–5 mm |
| Number of passes | 2–3 |
The key challenge identified in the literature is the significant difference in thermal expansion coefficients between the overlay layer and the base chain material. Cast iron overlays, while offering excellent wear resistance, develop substantial residual stresses due to their higher thermal expansion coefficient relative to the steel base. The authors describe a multi-pass welding strategy where each pass is allowed to cool sufficiently to relieve stress before the next pass is deposited, thereby minimizing the risk of cracking.
Defect Analysis and Countermeasures
The primary defects encountered during pilot chain overlay repair include:
- Hot cracking in the overlay layer — caused by high carbon content and low ductility of the overlay material at elevated temperatures. Countermeasure: control the interpass temperature and use a low-hydrogen electrode with careful preheating.
- Delamination at the interface — caused by insufficient bonding or hydrogen-induced cracking. Countermeasure: thorough surface preparation (grinding to bare metal), adequate preheating, and post-weld heat treatment.
- Undercut and incomplete fusion — caused by improper welding technique or excessive travel speed. Countermeasure: strict adherence to welding procedure specifications and operator training.
Engineering Practice Implications
From a production perspective, the overlay repair approach reduces the lifecycle cost of pilot chains by a factor of 3–5 compared to full replacement. The study demonstrates that with proper process control, the repaired chains can achieve 80–90% of the service life of new chains. This is particularly significant for continuous casting operations where chain availability directly impacts production scheduling.
The study also highlights the importance of quality inspection after repair. Visual inspection, magnetic particle testing (MT), and dimensional verification of the overlay layer thickness are essential. The overlay layer must be ground flush to ensure proper contact with the solidifying steel shell during the casting process, as any protrusion could cause surface defects on the cast product.
Study Insights and Reflections
This paper represents a classic example of applied welding engineering in heavy industry. The approach is pragmatic and cost-effective, reflecting the engineering philosophy of solving real production problems with available technology. The use of conventional SMAW rather than advanced processes such as plasma arc or laser cladding is justified by the large size of pilot chains and the availability of repair welding equipment on the shop floor. However, modern practices might consider hot-wire TIG (HW-TIG) or plasma transferred arc (PTA) cladding for improved layer uniformity and reduced dilution.
The study reinforces the principle that overlay repair is not merely about depositing material but about managing the metallurgical interaction between the overlay and base materials. The selection of electrode composition, welding sequence, and thermal input are all interrelated parameters that must be optimized simultaneously. This holistic approach to weld overlay repair is a fundamental lesson that remains relevant across all bimetal manufacturing applications.