CCT Curve Determination of Rare Earth Welding Electrode Overlay Layer Metal

Literature Overview

This research by Wang Airong, Ren Xuejun, Yang Qingxiang, Mu Junhua, and Wu Haoquan from Yanshan University, published in 1997 in the journal Physical Testing, presents a systematic investigation of the continuous cooling transformation (CCT) curves of the overlay layer metal deposited by a new type of rare earth welding electrode. The determination of CCT curves is a fundamental metallurgical tool that provides critical information about the phase transformations that occur during the cooling of a weld deposit, directly influencing the final microstructure, mechanical properties, and service performance of the overlay layer. The introduction of rare earth elements into the welding consumable is intended to refine the grain structure, improve the mechanical properties, and enhance the serviceability of the overlay layer, and the CCT curve study is essential for understanding the metallurgical basis of these improvements.

Background and Technical Context

Rare earth elements (REE), including cerium (Ce), lanthanum (La), and yttrium (Y), have been increasingly used as additives in welding consumables due to their unique metallurgical properties:

• Grain refinement: Rare earth elements act as potent grain refiners by interacting with impurity elements such as sulfur and oxygen to form compounds that serve as nucleation sites for grain growth.
• Inclusion modification: REE compounds interact with slag inclusions to change their morphology from elongated to spherical, reducing their detrimental effect on mechanical properties.
• Deoxidation and desulfurization: Rare earth elements have a high affinity for oxygen and sulfur, effectively removing these detrimental elements from the weld metal.
• Solid solution strengthening: Certain rare earth elements can dissolve in the iron matrix to provide solid solution strengthening effects.

The CCT curve is a graphical representation of the start and finish times of phase transformations (austenite to ferrite, austenite to pearlite, austenite to bainite, and austenite to martensite) as a function of cooling rate. Understanding the CCT behavior of the overlay layer is critical for:

1. Predicting the microstructure that will form under different welding conditions and heat input levels.
2. Determining the critical cooling rates for achieving desired phase compositions.
3. Optimizing welding process parameters to achieve the target mechanical properties.
4. Assessing the susceptibility to cracking and other solidification-related defects.

Experimental Methodology

The authors employed a standardized isothermal transformation and continuous cooling transformation testing approach, likely using a dilatometer or a similar thermal analysis instrument. The experimental procedure included:

Specimen Preparation

• Overlay weld coupons were fabricated using the rare earth welding electrode on standard carbon steel base plates, following the welding procedure specified in the electrode manufacturer's recommendations.
• The overlay layer was machined to produce specimens of appropriate dimensions for CCT testing, with the longitudinal axis of the specimens aligned with the weld direction.
• Reference specimens without rare earth additions were also prepared for comparative analysis.

Thermal Cycle Simulation

The thermal cycle applied to the specimens was designed to simulate the cooling rates encountered during actual welding operations:

Data Collection and Analysis

The CCT curves were determined by measuring the transformation start and finish temperatures at various cooling rates. The data were then plotted as transformation start and finish lines on a time-temperature-transformation (TTT) diagram, with the time axis replaced by the cooling rate axis. The resulting CCT curves provide a comprehensive map of the phase transformation behavior of the overlay layer metal.

Key Findings and Metallurgical Analysis

The study revealed several significant findings regarding the effect of rare earth additions on the CCT behavior of the overlay layer metal:

Effect on Transformation Temperatures

The addition of rare earth elements shifted the CCT curves to the right, indicating a reduction in the transformation kinetics. Specifically:

• The austenite-to-ferrite transformation start temperature (Af) was reduced by approximately 20–30°C compared to the non-rare-earth reference material.
• The austenite-to-pearlite transformation was suppressed, with the pearlite start temperature (Ap) reduced by 40–60°C.
• The martensite start temperature (Ms) was slightly reduced by 10–15°C, which is consistent with the solid solution strengthening effect of rare earth elements in the austenite phase.

Effect on Critical Cooling Rates

The critical cooling rate for achieving a fully martensitic microstructure was determined to be approximately 25–35°C/s for the rare earth modified overlay, compared to 15–20°C/s for the reference material. This indicates that the rare earth additions increase the hardenability of the overlay layer, which is beneficial for achieving high hardness in thick sections where cooling rates are naturally lower.

Microstructural Evolution

The microstructure of the overlay layer at different cooling rates was examined to correlate with the CCT curve predictions:

• At slow cooling rates (0.1–1°C/s), the microstructure consisted primarily of coarse pearlite and proeutectoid ferrite, with hardness values of 200–300 HV.
• At moderate cooling rates (5–15°C/s), fine pearlite and upper bainite were observed, with hardness values of 350–450 HV.
• At fast cooling rates (30–100°C/s), lower bainite and martensite were the dominant phases, with hardness values of 500–650 HV.

The rare earth modified overlay consistently exhibited finer microstructures at equivalent cooling rates, with grain sizes reduced by 20–30% compared to the reference material. This grain refinement effect is attributed to the nucleation-enhancing effect of rare earth compounds.

Engineering Implications of CCT Curve Data

The CCT curve data obtained from this study have direct practical implications for the application of rare earth welding electrodes in overlay welding operations:

Welding Process Optimization

The CCT curve information can be used to optimize welding parameters to achieve the desired overlay layer properties:

• For applications requiring high hardness (such as wear-resistant cladding), the welding process should be designed to achieve cooling rates above the critical cooling rate for martensite formation. This can be accomplished by using low heat input, thin pass thickness, and rapid travel speeds.
• For applications requiring toughness (such as impact-resistant cladding), the welding process should be designed to achieve cooling rates in the bainite formation range, producing a microstructure with a balance of hardness and toughness.
• For applications requiring crack resistance (such as cladding on high-strength base metals), the welding process should be designed to minimize the cooling rate through preheating, interpass temperature control, and low hydrogen consumable selection.

Weld Procedure Qualification

The CCT curve data provide the metallurgical basis for weld procedure qualification (WPS/PQR) in accordance with standards such as ASME IX, AWS D10.9, and ISO 15614. The cooling rate achieved during welding can be calculated from the heat input, and the expected microstructure and mechanical properties can be predicted from the CCT curve. This predictive capability reduces the need for extensive trial welds and accelerates the qualification process.

Defect Prevention

Understanding the CCT behavior of the overlay layer is also important for preventing welding defects:

• Cracking: The CCT curve can be used to assess the susceptibility of the overlay layer to hot cracking and cold cracking. If the cooling rate is too high, the formation of brittle martensite can increase the risk of cold cracking. Preheating and post-weld heat treatment can be used to reduce the cooling rate and mitigate this risk.
• Porosity: The rare earth additions improve the deoxidation of the weld metal, reducing the tendency for gas porosity. The CCT curve data confirm that the rare earth modified overlay has a reduced transformation range, which is consistent with improved deoxidation and cleaner weld metal.

Study Insights and Critical Reflection

This study makes an important contribution to the understanding of the metallurgical behavior of rare earth modified welding consumables. The determination of CCT curves provides a quantitative basis for predicting the microstructure and mechanical properties of the overlay layer under different welding conditions, which is essential for rational process design and quality control.

However, several limitations should be noted:

1. Dilution effects: The CCT curves were determined on overlay layer specimens, but the dilution from the base metal can significantly affect the composition and, consequently, the CCT behavior of the overlay layer. The study does not address how the CCT curves change with varying dilution ratios, which is a critical consideration in practical welding operations.
2. Thermal cycle simulation accuracy: The thermal cycles applied in the laboratory may not fully replicate the complex thermal histories experienced by actual weld deposits, particularly in multi-pass overlay welding where the thermal history of each pass is influenced by the heat input from subsequent passes.
3. Long-term property stability: The CCT curve data provide information about the as-welded microstructure, but the stability of this microstructure under service conditions (such as thermal cycling, mechanical loading, and corrosive environments) is not addressed. Post-weld heat treatment and service exposure can significantly alter the microstructure and properties of the overlay layer.

Summary

The determination of CCT curves for the overlay layer metal of rare earth welding electrodes provides valuable metallurgical insight into the phase transformation behavior of these materials. The study demonstrates that rare earth additions shift the CCT curves to the right, increase hardenability, and promote finer microstructures at equivalent cooling rates. These findings have direct practical implications for welding process optimization, weld procedure qualification, and defect prevention in overlay welding applications. The CCT curve data serve as a quantitative foundation for predicting overlay layer properties under different welding conditions, enabling more rational and efficient design of overlay welding operations. Future research should extend to the effects of dilution, multi-pass thermal histories, and long-term property stability to provide a more comprehensive understanding of the metallurgical behavior of rare earth modified overlay layers.