Microstructure Investigation of the Heat-Affected Zone in Single-Side Overlay Welding of Membrane-Type Water Walls

Literature Overview

This study by Chen Chuanbao, Wei Mingzhen, Zhou Jian, and Xue Feng from the School of Materials Science and Engineering, Southeast University, published in Modern Transportation and Metallurgical Materials (Vol. 1, No. 5, 2021, pp. 78-83), investigates the microstructural evolution in the heat-affected zone (HAZ) of membrane-type water wall tubes subjected to single-side overlay welding. The research combines ANSYS transient thermal simulation with experimental metallographic analysis to elucidate how varying heat input levels influence HAZ microstructure, including ferrite content, pearlite fraction, and the appearance of Widmanstätten ferrite. This work is directly relevant to boiler tube engineering, where overlay welding is used to enhance corrosion resistance or repair damaged sections of water wall tubes in coal-fired and biomass power plants.

Core Technical Findings

Transient Temperature Field Simulation Results

The ANSYS-based thermal simulation reveals critical characteristics of the thermal cycle in single-side overlay welding of membrane-type water walls:

Simulation Parameter	Heat Input Level 1	Heat Input Level 2	Heat Input Level 3
Heat input (J/mm)	450	650	850
Peak pool temperature (°C)	~1500	~1600	~1650
Temperature differential (°C)	<100	>100	>100
Estimated cooling rate t800-500 (s)	1.8	3.2	5.1
HAZ width (mm)	3.2	4.8	6.5
Pool depth (mm)	2.1	3.0	3.8

A key finding is that after the heat source passes, the temperature rapidly drops to approximately 600°C, which corresponds to the critical range for microstructural transformation in low-carbon and low-alloy steels. The cooling rate through this range (t800-500) is the primary metallurgical parameter governing HAZ microstructure.

Microstructural Evolution with Increasing Heat Input

Microstructural Feature	Low Heat Input (450 J/mm)	Medium Heat Input (650 J/mm)	High Heat Input (850 J/mm)
Ferrite content (%)	45	58	72
Pearlite content (%)	55	38	23
Widmanstätten ferrite	Absent	Trace	Abundant
Grain size (μm)	25-35	35-50	50-70
Hardness (HV)	185-210	165-190	145-170

The progressive increase in ferrite content and decrease in pearlite with rising heat input is consistent with classical welding metallurgy principles. Slower cooling rates allow more complete austenite-to-ferrite transformation, reducing the pearlite fraction. The appearance of Widmanstätten ferrite at higher heat inputs is particularly significant from a fracture toughness perspective, as Widmanstätten ferrite acts as a preferential crack propagation path and significantly degrades Charpy impact energy.

Implications for Water Wall Tube Integrity

Membrane-type water walls operate under extreme thermal and mechanical loads, including cyclic thermal stress, internal pressure, and erosion from flue gas. The HAZ microstructure directly influences:

Thermal fatigue resistance: Widmanstätten ferrite reduces thermal fatigue life by providing crack initiation sites at ferrite-pearlite interfaces.
Creep resistance: Coarse grain boundaries in high-heat-input HAZ reduce creep strength at elevated operating temperatures.
Corrosion resistance: Heterogeneous microstructures create galvanic cells that accelerate localized corrosion in water-wall environments.

Engineering Practice Integration

Recommended Process Windows for Membrane Water Wall Overlay Welding

Based on the study's findings, the following process parameters are recommended for maintaining acceptable HAZ microstructure:

Heat input: Maintain below 600 J/mm to limit Widmanstätten ferrite formation and control grain growth.
Preheat temperature: 100-150°C for low-carbon steel tubes (SA-192, SA-213 T2) to moderate cooling rates without excessively widening the HAZ.
Interpass temperature: Control between 150-250°C to manage thermal cycling without promoting coarse grain growth.
Post-weld heat treatment: Stress relief at 550-600°C for 1-2 hours to transform any retained austenite and reduce residual stresses.

Connection to Pipe and Fitting Welding Practice

The principles established in this study extend broadly to overlay welding on steel pipes and fittings:

Single-side welding constraints: In membrane water walls, the tube geometry often restricts access to one side only. This asymmetrical heat input creates uneven thermal profiles that must be managed through careful parameter selection.
Thermal cycling effects: Repeated thermal cycles during multi-pass overlay welding compound the HAZ damage, making heat input control even more critical.
Residual stress management: The rapid cooling to ~600°C followed by further cooling generates significant longitudinal residual stresses that can reach 200-300 MPa in thin-walled tubes.

Key Questions and Reflections

The study raises an important question about the optimal balance between deposition rate and HAZ quality. In industrial settings, productivity often drives operators toward higher heat inputs, which this research clearly shows degrades HAZ microstructure. The appearance of Widmanstätten ferrite at heat inputs above 650 J/mm represents a critical threshold that should be treated as a process control limit.

Another reflection concerns the applicability of these findings to modern advanced materials. The study focuses on conventional low-carbon steel water wall tubes, but contemporary boilers increasingly employ high-strength low-alloy (HSLA) and austenitic stainless steel tubes. The HAZ metallurgical behavior of these materials under overlay welding conditions differs significantly, and similar systematic studies are warranted.

The combination of thermal simulation with experimental validation provides a methodology that can be readily adapted for process development on new materials or geometries. For engineers involved in pipe repair and overlay welding qualification, this approach offers a practical framework for establishing process windows without relying solely on trial-and-error experimentation.