Zhuojin Pipe Fitting Co., LtdDevelopment of Rare Earth Anti-Cracking Overlay Welding Electrodes
Literature Overview
This paper by Yang Qingxiang and colleagues (1997, Welding Technology, Vol. 26, No. 2) from Yanshan University presents the development of rare earth-containing anti-cracking overlay welding electrodes specifically designed for the repair of hot rolling rolls and hot work tool steels. The research addresses a critical industrial problem: the susceptibility of high-carbon, high-alloy overlay deposits to hot cracking during welding due to the large thermal gradients and high cooling rates inherent in the process.
Core Technical Content
Overlay welding of hot rolling rolls and hot work tool steels presents unique challenges because these substrates typically have high carbon content (0.4–0.8%), high hardenability, and low thermal conductivity. When a hard alloy overlay is deposited, the combination of high cooling rate, thermal stress, and dilution from the base metal creates severe cracking susceptibility in both the weld metal and the heat-affected zone.
Electrode Composition Design
The electrode design philosophy centers on two key elements: rare earth oxides and nickel. The rare earth oxides (primarily La₂O₃ and CeO₂) serve multiple metallurgical functions:
| Component | Function | Mechanism |
|---|---|---|
| La₂O₃ | Grain refinement | Heterogeneous nucleation, reduces grain size |
| CeO₂ | Deoxidation | Reacts with dissolved oxygen, reduces oxide inclusions |
| Ni | Crack resistance | Solid solution strengthening, reduces thermal stress |
| Ni + RE synergy | Anti-cracking | Combined effect on grain morphology and inclusion modification |
| RE oxides | Inclusion modification | Converts brittle MnS to rounder, more ductile RE-Mn-S inclusions |
The rare earth elements modify the morphology and distribution of inclusions in the weld metal. Without rare earth addition, the weld metal contains elongated, brittle manganese sulfide inclusions that act as crack initiation sites under thermal stress. With rare earth addition, these inclusions are transformed into more equiaxed, dispersed rare earth-manganese-sulfide compounds that have reduced crack initiation capability.
Anti-Cracking Mechanism
The anti-cracking effect operates through multiple mechanisms:
- Grain refinement: Rare earth oxides act as heterogeneous nucleation sites, reducing grain size and increasing the number of grain boundaries available to deflect propagating cracks.
- Inclusion modification: The transformation of brittle elongated inclusions to rounded RE-containing inclusions reduces stress concentration factors.
- Solid solution strengthening: Nickel in solid solution with iron increases the ductility of the austenitic or martensitic weld metal, accommodating thermal strain without cracking.
- Reduced cooling rate sensitivity: The combined Ni-RE system broadens the solidification temperature range, reducing the susceptibility to hot cracking.
Application Performance
The developed electrodes enable overlay repair of hot rolling rolls and hot work tool steels without preheating or post-weld heat treatment. This is a significant process simplification because:
- Elimination of preheating reduces thermal distortion and process time
- Elimination of post-weld heat treatment reduces residual stress relaxation time and equipment requirements
- Reduced hydrogen cracking risk because the absence of preheating is compensated by the electrode's low hydrogen content design
The practical application results demonstrate successful repair of hot rolling rolls in production environments, with service life comparable to or exceeding conventionally repaired components.
Engineering Practice Integration
This research has direct applicability to several industrial scenarios:
- Steel mill roll repair: Hot rolling rolls require frequent overlay repair due to wear; the ability to repair without preheating significantly reduces downtime.
- Die and mold repair: Hot work tool steels used in forging and extrusion dies can be repaired on-site without the need for controlled cooling.
- Pipeline component repair: For carbon and low-alloy steel pipelines with hardfacing requirements, the anti-cracking electrodes provide a reliable solution for field repair operations.
From a process optimization perspective, the electrode parameters should be carefully controlled:
| Process Parameter | Recommended Range | Rationale |
|---|---|---|
| Current | 80–120 A | Moderate heat input to control dilution |
| Travel speed | 50–100 mm/min | Adequate penetration without excessive dilution |
| Electrode diameter | 3.2–4.0 mm | Suitable for repair applications |
| Electrode drying temperature | 300–350 °C | Moisture removal to prevent hydrogen cracking |
| Interpass temperature | ≤150 °C | Prevents excessive softening of base metal |
| Number of passes | 2–3 | Reduces dilution in subsequent passes |
Key Questions and Reflections
A critical question is the long-term stability of the anti-cracking effect under repeated thermal cycling. Hot rolling rolls experience repeated heating and cooling during production runs, which could potentially alter the microstructure of the overlay deposit. Engineers should monitor the overlay layer condition after extended service and consider periodic re-overlay when hardness or crack resistance degrades.
The synergistic effect of rare earth and nickel is well-demonstrated in this study, but the optimal ratio requires further investigation. Excessive rare earth addition may promote the formation of brittle rare earth oxides in the weld metal, while excessive nickel increases cost without proportional benefit. A systematic optimization study varying the RE:Ni ratio would provide more precise design guidelines.
The elimination of preheating is a significant process advantage, but it should be validated under all service conditions. For very thick sections or extremely cold environments, some preheating may still be necessary to prevent cold cracking in the heat-affected zone. Engineers should develop site-specific procedures that account for ambient temperature, section thickness, and base metal composition.
Study Insights and Implications
The most significant contribution of this research is the demonstration that rare earth modification can fundamentally alter the crack susceptibility of overlay weld metal, enabling process simplification in industrial repair applications. This principle extends beyond the specific electrode compositions studied here; rare earth modification is a universal metallurgical tool that can be applied to various overlay welding systems, including submerged arc overlay, gas metal arc overlay, and plasma arc overlay processes.