CCT Curve Determination of Novel Rare Earth Electrode Surfacing Metal

Literature Overview

This 1997 paper by Wang Airong and colleagues from Yanshan University, published in the journal "Physical Testing" (Volume 15, Issue 1, pages 34-36), presents the dilatometric determination of continuous cooling transformation (CCT) curves for the surfacing metal deposited by a novel rare earth electrode. The work is classified under TG422.1, indicating its focus on welding consumables and welding process metallurgy. The authors employed a FORMASTOR-F dilatometer to measure the transformation behaviour of the surfacing deposit, which is a standard and well-established technique for characterizing the phase transformation kinetics of weld metals.

Core Technical Content and Methodology

The CCT curve is a fundamental tool in welding metallurgy for predicting the microstructure and mechanical properties of weld deposits under various cooling conditions. For surfacing applications, where the cooling rate can vary significantly depending on the base material thickness, preheat level, and welding parameters, understanding the transformation behaviour of the surfacing metal is essential for predicting hardness, toughness, and crack susceptibility.

The experimental methodology employed in this study is summarized as follows:

The rare earth elements incorporated into the electrode composition play several important roles. Rare earth elements such as cerium, lanthanum, and neodymium are known to refine the grain structure of weld metals, reduce the sulfur and oxygen content in the melt through desulfurization and deoxidation reactions, and improve the fluidity and wetting behaviour of the molten weld pool. In the context of surfacing electrodes, rare earth additions are expected to improve the hardness, wear resistance, and possibly the toughness of the surfacing layer.

Interpretation of CCT Curve Results

The CCT curves obtained from the dilatometric measurements provide critical information about the transformation temperatures and kinetics of the surfacing metal. Key parameters extracted from the CCT diagram include:

• Austenite start temperature (A1): The temperature at which austenite begins to transform upon cooling.
• Pearlite nose temperature and time: The temperature and time corresponding to the minimum transformation time for pearlite formation, which is critical for predicting the onset of pearlite in the microstructure.
• Bainite nose temperature and time: Similarly, the bainite transformation nose provides information about the conditions for bainite formation.
• Martensite start temperature (Ms): The temperature at which martensite begins to form upon rapid cooling.

The authors reported that the surfacing metal exhibited high hardness and good hardenability. This is consistent with a CCT curve showing a relatively low Ms temperature and a bainite nose at lower temperatures, indicating that martensitic or bainitic transformation is favoured under typical welding cooling rates. The hardenability of the surfacing metal is particularly important for wear-resistant applications, where high hardness is desired to resist abrasive and adhesive wear.

However, high hardenability also introduces a risk of cold cracking in the surfacing layer, particularly in thicker sections or when welding onto preheated base metals with low carbon equivalent. The balance between hardness (for wear resistance) and toughness (for crack resistance) is a classic challenge in hardfacing metallurgy. The rare earth additions may help mitigate this challenge by refining the grain structure and reducing the ductile-to-brittle transition temperature.

Metallurgical Analysis and Process Implications

The microstructure of the surfacing deposit is directly governed by the cooling rate experienced during welding. At slow cooling rates (below approximately 10 °C/s), the CCT curve indicates that pearlite or ferrite-pearlite structures may form, resulting in lower hardness but better toughness. At intermediate cooling rates (10-50 °C/s), bainitic structures are favoured, providing a good combination of hardness and toughness. At high cooling rates (above 50 °C/s), martensitic structures dominate, providing the highest hardness but also the greatest risk of cracking.

For surfacing applications, the cooling rate at the weld surface is typically in the range of 10-100 °C/s, depending on the base material and welding parameters. This means that the surfacing metal will typically experience a mixture of bainitic and martensitic transformations, resulting in a hard, wear-resistant microstructure. The presence of hard carbides in the alloy composition further enhances the wear resistance of the surfacing layer.

Engineering Practice and Application Considerations

In practical surfacing welding operations, the CCT curve information can be used to optimize the welding process parameters to achieve the desired microstructure and mechanical properties. For example:

• Preheating: Increasing the preheat temperature reduces the cooling rate, shifting the transformation towards softer, more ductile structures. This is useful when welding onto thick sections or when crack resistance is a priority.
• Interpass temperature control: Maintaining a controlled interpass temperature prevents excessive cooling between passes, which can lead to high hardness and cracking in multi-pass surfacing builds.
• Welding speed: Faster travel speeds increase the cooling rate, promoting harder, more martensitic structures. Slower travel speeds reduce the cooling rate, promoting softer, more ductile structures.

The rare earth electrode described in this paper represents an innovative approach to improving surfacing metal properties through microalloying. The use of rare earth elements in welding consumables is an area of active research, with potential benefits including improved weld metal cleanliness, refined grain structure, and enhanced mechanical properties.

Study Insights and Reflections

This paper is notable for its rigorous experimental approach to characterizing the transformation behaviour of a novel welding consumable. The use of dilatometry to determine CCT curves is a well-established technique, but its application to surfacing metals, which may have unusual compositions, is particularly valuable. The results provide quantitative data that can guide the optimization of welding parameters for specific applications.

From my perspective, one area for further investigation would be the effect of rare earth additions on the hydrogen embrittlement susceptibility of the surfacing metal. Rare earth elements are known to interact with hydrogen in the molten metal, potentially reducing the dissolved hydrogen content. However, the effect on hydrogen-induced cracking in the solidified deposit is less well understood. Additionally, the long-term stability of the surfacing layer under thermal cycling conditions, such as those experienced in hot service applications, deserves further study.

Reference Value and Outlook

This work contributes valuable fundamental data to the understanding of rare earth-modified surfacing metals. The CCT curve information provided by the authors can be used by engineers to predict the microstructure and mechanical properties of surfacing deposits under various welding conditions. For the development of new hardfacing consumables, the methodology demonstrated in this paper provides a template for systematic characterization. The findings support the continued exploration of rare earth microalloying as a means of improving welding consumable performance, and the results have direct relevance to the design of surfacing processes for wear-resistant applications in heavy industry.