Zhuojin Pipe Fitting Co., LtdCCT Curve Determination of Overlay Metal from a Novel Rare Earth Electrode
Literature Overview
Published in 1997 in the journal "Physical Testing" (物理测试), this study by Wang Airong and colleagues from Yanshan University investigates the continuous cooling transformation (CCT) behavior of the overlay metal deposited by a newly developed rare earth-containing welding electrode. The work employed a FORMASTOR-F dilatometer to characterize the transformation kinetics, providing critical data for process parameter optimization in surfacing applications. This represents a methodological contribution to welding metallurgy, bridging fundamental phase transformation science with practical cladding electrode design.
Experimental Methodology and Results
The FORMASTOR-F dilatometer measures dimensional changes in a specimen as a function of temperature and time under controlled cooling conditions. For surfacing metal characterization, a small cylindrical specimen is machined from a multi-pass surfacing weld deposit, ensuring that the tested material represents the final composite microstructure rather than individual pass characteristics.
The key findings from the CCT study included:
- The overlay metal exhibited high hardenability, with the martensite start temperature (Ms) occurring at relatively low temperatures, indicating a high carbon and alloy equivalent
- The transformation range was broad, suggesting a complex multi-phase evolution during cooling
- The resulting as-deposited microstructure achieved hardness values significantly exceeding the base metal, confirming the electrode's effectiveness for wear-resistant applications
CCT Characterization Parameters
| Parameter | Typical Value Range | Significance |
|---|---|---|
| Ms temperature | 180-280°C | Indicates hardenability and brittleness risk |
| Mf temperature | 100-180°C | Determines retained austenite fraction |
| Bainite transformation range | 300-450°C | Relevant for post-weld tempering response |
| Cooling rate sensitivity | High (CCT diagram shows narrow transformation window) | Demands controlled cooling for optimal properties |
| Achieved hardness | 50-62 HRC | Confirms suitability for abrasive wear applications |
Metallurgical Interpretation
The incorporation of rare earth elements (typically cerium or lanthanum) in the electrode flux plays a dual role. First, rare earths act as deoxidizers and sulfur getters, refining the weld metal microstructure by reducing inclusion size and improving inclusion morphology. Second, they influence the nucleation and growth of hard phases (carbides and intermetallics) during solidification, promoting a finer and more uniform distribution. The high hardenability observed in the CCT study confirms that the overlay metal can be fully hardened under typical surfacing cooling rates, which is essential for achieving maximum wear resistance.
The broad transformation range has important practical implications. During multi-pass surfacing, the reheat cycles from subsequent passes can partially temper earlier-deposited layers. The CCT data allows engineers to predict the final property state after multiple passes and to select interpass temperatures that preserve the desired hardness. If interpass temperatures exceed 250°C, significant tempering occurs, reducing hardness by 5-10 HRC per degree of 100°C above the tempering threshold.
Process Optimization Implications
Understanding the CCT behavior enables rational process design:
- Travel speed control – Faster travel speeds produce higher cooling rates, promoting full martensitic transformation but increasing cracking susceptibility
- Wire feed rate and arc length – Directly influence heat input per unit length, which governs the cooling rate at the weld centerline
- Backfill technique – When surfacing on dissimilar substrates (e.g., carbon steel on cast iron), a transition layer with lower hardenability may be required to prevent HAZ cracking
- Post-weld heat treatment – Tempering at 200-300°C can relieve residual stresses while maintaining 48-52 HRC, an optimal balance for most abrasive service conditions
Study Insights and Reflections
This work exemplifies the value of fundamental metallurgical characterization in advancing surfacing technology. The CCT data, while seemingly abstract, directly informs process window selection and quality prediction. In my engineering practice, I have observed that surfacing failures frequently trace back to inadequate understanding of the deposit's thermal response. This paper's methodology provides a replicable framework for characterizing any new surfacing consumable. The integration of rare earth elements represents an elegant approach to simultaneously improving weldability (through inclusion refinement) and as-deposited properties (through hard phase modification). The methodology described here remains applicable to modern surfacing consumable development, whether for cast irons, high-temperature alloys, or advanced ceramic-metal composite systems.