Zhuojin Pipe Fitting Co., LtdMicrostructural Evolution of the Heat-Affected Zone in Single-Sided Overlay Welding of Membrane-Type Water Wall Tubes
Literature Overview
The paper by Chen Chuanbao, Wei Mingzhen, Zhou Jian, and Xue Feng, published in Modern Transportation and Metallurgical Materials (2021, Vol. 1, No. 5, pp. 78-83), investigates the effect of welding heat input on the microstructure of the heat-affected zone (HAZ) in single-sided overlay welding of membrane-type water wall tubes used in coal-fired boilers. The study combines ANSYS-based thermal simulation with experimental metallographic analysis to evaluate three different heat input levels and their impact on the HAZ microstructure.
Background and Engineering Significance
Membrane-type water wall tubes are a critical component of modern coal-fired boilers, where they absorb radiant heat from the furnace while providing structural integrity to the boiler wall. The tubes are typically made of low-alloy steel such as 20G, 12Cr1MoV, or P91, and they may require overlay welding to repair surface defects or to apply a corrosion-resistant or oxidation-resistant layer. Single-sided overlay welding is necessary because the water wall tubes are installed in a configuration where access is limited to one side.
The HAZ is the region of the base metal that is heated to a temperature between the Ac1 and Ac3 transformation temperatures but is not melted. The microstructure and mechanical properties of the HAZ are directly influenced by the welding heat input, which determines the peak temperature, cooling rate, and the width of the affected zone.
Thermal Simulation Results
The ANSYS-based thermal simulation was performed for three heat input levels. The simulation results reveal the following key observations:
| Parameter | Low Heat Input | Medium Heat Input | High Heat Input |
|---|---|---|---|
| Peak temperature at heat source | Approaches melting point | Approaches melting point | Approaches melting point |
| Post-heating temperature at heat source position | Rapidly drops to ~600°C | Rapidly drops to ~600°C | Rapidly drops to ~600°C |
| Maximum pool temperature | Lower | Intermediate | Higher (increases with heat input) |
| Temperature gradient across pool | Moderate | Moderate | Higher (gradient >100°C) |
| Cooling rate | Faster | Moderate | Slower |
| Pool depth | Shallower | Intermediate | Deeper |
| HAZ width | Narrower | Intermediate | Wider |
The simulation results confirm that increasing heat input leads to a deeper weld pool, a wider HAZ, and a slower cooling rate. These thermal characteristics directly influence the microstructural evolution in the HAZ.
Microstructural Analysis
The experimental metallographic analysis of the HAZ for the three heat input levels revealed distinct microstructural differences:
| Heat Input Level | Dominant Microstructure | Ferrite Content | Pearlite Content | Widmanstätten Ferrite |
|---|---|---|---|---|
| Low | Fine pearlite + fine ferrite | Lower | Higher | Not observed |
| Medium | Coarse pearlite + coarse ferrite | Moderate | Moderate | Not observed |
| High | Coarse ferrite + reduced pearlite | Higher | Lower | Observed |
The key microstructural trends are:
- Ferrite content increases with heat input: The slower cooling rate at higher heat inputs allows more time for the austenite-to-ferrite transformation, resulting in a higher fraction of ferrite in the final microstructure.
- Pearlite content decreases with heat input: The reduced cooling rate shifts the transformation kinetics toward ferrite formation, reducing the amount of pearlite that forms.
- Widmanstätten ferrite appears at high heat input: The Widmanstätten ferrite morphology is characterized by lath-like or needle-like ferrite plates that grow across the prior austenite grain boundaries. This morphology is associated with coarse grain growth in the HAZ and can have detrimental effects on toughness.
Implications for Mechanical Properties and Service Performance
The microstructural evolution described above has direct implications for the mechanical properties of the HAZ:
- Hardness: The transition from pearlite to ferrite generally reduces hardness, as ferrite is softer than pearlite. However, the presence of Widmanstätten ferrite can locally increase hardness due to its fine, plate-like morphology.
- Toughness: Widmanstätten ferrite is particularly detrimental to toughness, as it provides crack initiation sites and promotes intergranular fracture. The appearance of Widmanstätten ferrite at high heat input is therefore a significant concern for service performance.
- Strength: The reduction in pearlite content and the increase in ferrite content may reduce the yield strength of the HAZ, potentially creating a soft zone that is susceptible to deformation under service loads.
Process Optimization Recommendations
Based on the simulation and experimental results, the following process optimization recommendations can be made:
- Minimize heat input: The lowest feasible heat input should be used to minimize the HAZ width and avoid the formation of Widmanstätten ferrite.
- Control preheating: Preheating should be limited to the minimum level required to prevent cracking, as excessive preheating further reduces the cooling rate and promotes coarse grain growth.
- Post-weld heat treatment: If high heat input is unavoidable, a post-weld heat treatment (PWHT) should be performed to normalize the HAZ microstructure and eliminate Widmanstätten ferrite.
- Multi-pass welding: Using multiple passes with lower heat input per pass can help control the HAZ width and cooling rate, although this increases the total welding time.
Study Insights and Reflections
This paper provides a valuable integration of numerical simulation and experimental metallography for understanding the HAZ microstructural evolution in single-sided overlay welding. The use of ANSYS for thermal simulation is a practical and efficient approach that allows rapid evaluation of different heat input scenarios without the need for extensive experimental trials.
The identification of Widmanstätten ferrite as a critical microstructural concern at high heat input is particularly important for boiler water wall applications, where the tubes are subjected to high temperatures and cyclic thermal loading. Widmanstätten ferrite can significantly reduce the fatigue life of the HAZ under these conditions, making it essential to control the heat input to avoid its formation.
One area for further investigation is the quantitative relationship between heat input and the onset of Widmanstätten ferrite. The paper identifies the qualitative trend but does not provide a quantitative threshold. Establishing a critical heat input value above which Widmanstätten ferrite appears would provide a clear process window for welders and process engineers. Additionally, the effect of welding speed, torch angle, and travel direction on the HAZ microstructure should be evaluated, as these parameters also influence the thermal cycle experienced by the base metal.
In summary, this paper demonstrates the value of combining thermal simulation with metallographic analysis for optimizing the welding process of membrane-type water wall tubes, and it highlights the critical importance of controlling heat input to prevent detrimental microstructural evolution in the HAZ.