Microstructure and Properties of Inconel 625 Overlay Welded Boiler Waterwall Tubes

Literature Overview

This study examines the microstructure evolution and mechanical performance of Inconel 625 alloy overlay welds applied to boiler membrane waterwall tubes. The research addresses a critical challenge in ultra-supercritical coal-fired power plants, where waterwall tubes are subjected to extreme thermal gradients, high-temperature oxidation, and molten ash corrosion. The overlay welding process selected for this investigation is GTAW-based multi-pass welding, which is well-suited for the relatively thin-walled geometry of waterwall tubes. The primary objective is to understand how the dilution ratio, cooling rate, and solidification mode affect the final microstructure and, consequently, the long-term service reliability of the overlay layer.

Core Technical Points

The study highlights several critical microstructural features that govern overlay performance. The Inconel 625 overlay weld metal exhibits a predominantly dendritic columnar structure near the fusion line, transitioning to equiaxed grains toward the cap. The key alloying elements—niobium and molybdenum—precipitate as NbC and Mo-rich phases, which provide solid solution strengthening and precipitation hardening respectively. The dilution ratio between the base carbon steel and the Inconel 625 overlay is identified as the most sensitive parameter, with values exceeding 40 percent leading to excessive formation of brittle intermetallic phases such as sigma phase and Laves phase at the bond line.

Microstructural Analysis

Parameter	Base Metal	Overlay Layer	Bond Line
Grain Structure	Ferrite-Pearlite	Dendritic columnar + equiaxed	Fine columnar with Laves phase risk
Hardness (HV)	180-220	280-340	300-380
Dilution Ratio	N/A	15-35%	Critical zone
Sigma Phase Risk	None	Low	Moderate at high dilution
Oxidation Resistance (1000°C)	Poor	Excellent	Depends on dilution

The bond line is identified as the weakest link in the overlay system. When the cooling rate exceeds 20 K/s, the Laves phase (Fe2Mo) precipitates preferentially at the interface, which is inherently brittle and susceptible to intergranular fracture. The study recommends limiting the dilution ratio to below 30 percent through careful control of the first pass weld geometry and heat input.

Process Parameters and Engineering Practice

The overlay welding process parameters were optimized through systematic trial welding. The recommended GTAW parameters include: DCEN polarity, welding current of 120-160 A, arc voltage of 12-16 V, travel speed of 3-5 mm/s, and interpass temperature below 150°C. A single-layer multi-pass technique is preferred to maintain dilution within acceptable limits. The first pass acts as the critical bond pass, and its geometry directly determines the dilution level for subsequent passes.

In engineering practice, waterwall tubes are typically manufactured from 20G or 12Cr1MoV steel. The overlay is applied to the tube inner surface to protect against corrosion by fly ash and sulfur compounds. The service temperature range is 550-650°C, with peak thermal flux exceeding 100 kW/m² during normal operation. Post-weld heat treatment is generally not applied to waterwall tubes due to the risk of distortion, but the overlay must be designed to withstand the thermal cycling without cracking.

Common Defects and Countermeasures

Defect Type	Root Cause	Countermeasure
Bond line cracking	High dilution, Laves phase formation	Reduce first-pass width, control cooling rate
Porosity	Hydrogen pickup from moisture	Dry shielding gas, preheat base metal
Overlap	Excessive travel speed	Reduce travel speed, increase current
Crater cracking	Incomplete filler deposition	Backfill crater, use proper termination technique
Insufficient fusion	Low heat input	Increase current, reduce travel speed

Study Insights and Reflections

This literature provides valuable insight into the metallurgical behavior of Inconel 625 overlay welds under boiler service conditions. The emphasis on dilution control is particularly important for engineers who frequently encounter bond-line failures in the field. The recommended practice of using a narrow first pass to minimize base metal melting is a practical approach that can be readily implemented in production welding procedures. The study also reinforces the importance of metallographic examination of the bond line, as visual inspection alone cannot detect the presence of brittle intermetallic phases that compromise long-term integrity. For engineers involved in ultra-supercritical boiler design, this work underscores the necessity of overlay procedure qualification that specifically addresses dilution control and bond-line microstructural integrity.