CCT Curve Determination of Overlay Layer Metal from Novel Rare Earth Electrodes

Literature Overview

This paper by Wang Airong, Ren Xuejun, Yang Qingxiang, Mu Junhua, and Wu Haoquan from Yanshan University was published in the journal "Physical Testing" in 1997 (Volume 15, Issue 1, pp. 34-36). The study investigates the continuous cooling transformation (CCT) behavior of overlay weld metal deposited using a novel rare earth-containing electrode. The authors employed a FORMASTOR-F dilatometer to measure the CCT curves of the overlay layer, revealing that the resulting metal exhibits high hardness and good hardenability. This work is significant because it provides fundamental metallurgical data that underpins the practical performance of rare earth modified hardfacing electrodes.

Scientific Background and Motivation

Rare earth elements (REEs), particularly cerium (Ce), lanthanum (La), and their alloys, have been widely recognized for their beneficial effects on welding metallurgy. In the context of overlay welding, rare earth additions can refine the grain structure, modify the morphology of reinforcing phases (such as carbides), and improve the toughness and wear resistance of the deposited metal. However, the precise influence of rare earth elements on the solidification and transformation behavior of overlay weld metals has not been fully characterized through systematic CCT analysis.

The overlay welding process creates a unique thermal cycle characterized by rapid heating and relatively fast cooling rates, which can produce non-equilibrium microstructures. Understanding the CCT behavior is critical for predicting the as-deposited microstructure and mechanical properties, as well as for optimizing post-weld heat treatment parameters.

Experimental Methodology

The authors used a FORMASTOR-F dilatometer, a well-established instrument for measuring dimensional changes in metals during thermal cycling. The experimental procedure involved:

1. Specimen preparation: Overlay weld metal specimens were machined from multi-layer deposited plates to obtain representative samples of the overlay zone.
2. Dilatometry: Specimens were heated to a homogeneous temperature (typically 850-1000°C for hardfacing alloys), held for sufficient austenitization, and then cooled at various rates (from air cooling to water quenching equivalents).
3. CCT curve construction: The dilatation data were converted to transformation start (Ms) and finish (Mf) temperatures, as well as transformation start and finish points for pearlite and bainite transformations, to construct the complete CCT diagram.

Key Findings and Metallurgical Analysis

The CCT analysis revealed several important characteristics of the rare earth modified overlay metal:

The refinement of the microstructure by rare earth additions is attributed to the following mechanisms:

• Grain refinement: Rare earth elements act as heterogeneous nucleation sites during solidification, producing finer austenite grains that increase the transformation temperature spread and enhance hardenability.
• Carbide modification: REEs modify the morphology and distribution of carbide phases (such as M7C3 and M23C6 in high-chromium alloys), preventing coarse carbide network formation at grain boundaries.
• Desulfurization and deoxidation: REEs form stable sulfides and oxides, reducing the detrimental effects of impurities on grain boundary cohesion and crack susceptibility.

Implications for Overlay Welding Practice

The CCT data obtained in this study have direct practical implications for the design and application of rare earth modified hardfacing electrodes:

1. Process optimization: Knowledge of the CCT curve allows engineers to predict the microstructure that will form at different cooling rates, enabling optimization of welding parameters (current, travel speed, interpass temperature) to achieve the desired combination of hardness and toughness.
2. Heat treatment design: The CCT data provide a basis for designing post-weld tempering treatments that can relieve residual stresses while maintaining adequate hardness. For example, tempering at 200-300°C can reduce brittleness in martensitic hardfacing without significantly reducing hardness.
3. Electrode development: The results support the continued development of rare earth modified hardfacing electrodes for demanding applications, providing metallurgical justification for their use in critical wear-resistant overlay applications.
4. Quality control: The high hardenability and hardness of the overlay layer should be verified through hardness testing and metallographic examination during production to ensure consistency.

Study Insights and Reflections

This work exemplifies the importance of fundamental metallurgical research in supporting practical welding applications. While the practical benefits of rare earth modified electrodes were known empirically, the CCT analysis provides the scientific foundation for understanding why these electrodes perform better and how to optimize their use. For engineers involved in overlay welding, this paper reinforces the principle that material selection and process design should be guided by metallurgical understanding, not just empirical trial and error.

The methodology of using dilatometry to characterize overlay weld metals is straightforward and cost-effective, and it can be applied to any new hardfacing composition under development. The FORMASTOR-F dilatometer remains a standard instrument in welding research laboratories worldwide, and the data it produces are essential for building predictive models of weld metal properties.

One limitation of this study is that it focuses solely on the overlay layer metal and does not examine the heat-affected zone (HAZ) or the base metal response to the welding thermal cycle. In practice, the HAZ properties are critical for overall joint performance, and future work should integrate HAZ CCT analysis with overlay layer characterization to provide a complete picture of the weldment metallurgy.