Microstructure and Properties of BHW35 Steel Hot-Wire TIG Overlay Weld Joint

Literature Overview

This 2013 paper by Wu Guangfeng, published in Welding, examines the microstructure and corrosion resistance of overlay welds produced using hot-wire TIG (H-TIG) welding technology, depositing a stainless steel corrosion-resistant layer on a steel substrate. Funded by the Chongqing University Excellent Achievement Transformation Program, this research addresses the growing demand for corrosion-resistant overlay solutions in chemical processing, marine, and environmental protection applications.

Core Technical Findings

The H-TIG process combines the precision of TIG welding with the increased deposition rate of hot-wire technology, where the wire is preheated by an auxiliary induction coil before entering the arc. This preheating reduces the thermal shock on the base metal while enabling higher deposition rates and improved metallurgical compatibility between dissimilar materials.

Multi-Layer Microstructural Evolution

The study reveals a characteristic alternating microstructure in multi-pass overlay welds:

Layer Position	Microstructure	Hardness Trend	Metallurgical Character
Top overlay layer	Dendritic crystals	Moderate	Full overlay composition
Inter-layer boundary	Columnar crystals	Lower (valley)	Partial dilution
Transition zone	Mixed dendritic/columnar	Higher (peak)	Significant dilution
Fusion zone	Coarse grains	Variable	Maximum dilution
Base metal	Original structure	Baseline	Unchanged composition

The wave-like hardness variation with depth is a direct consequence of the dilution gradient in multi-pass overlay welding. Each new pass creates a new dilution zone at its interface with the previous layer, resulting in the characteristic alternating pattern.

Corrosion Resistance Analysis

Electrochemical testing in 5% HCl solution reveals a clear hierarchy of corrosion resistance among the different zones of the overlay joint:

Zone	Corrosion Resistance (High to Low)	Primary Protection Mechanism
Transition layer	Highest	High Cr content from dilution creates passive film
Top overlay layer	Second highest	Full alloy composition with adequate Cr
Fusion zone	Third	Reduced Cr content, possible Cr-depleted zones
Base metal	Lowest	Carbon steel, no Cr protection

The counterintuitive finding that the transition layer exhibits superior corrosion resistance compared to the top overlay layer is attributed to the dilution effect, which increases the chromium content at the interface. This is a critical observation for engineers designing multi-pass overlay procedures, as it suggests that the intermediate layers may provide superior corrosion protection in certain applications.

Intergranular Corrosion Performance

The overlay layer demonstrates good intergranular corrosion resistance, which is a critical requirement for austenitic stainless steel overlay applications. This performance is achieved through:

Adequate chromium content maintained above the minimum threshold of 12% Cr for passivity
Controlled carbon levels that minimize Cr₂₃C₆ precipitation at grain boundaries during welding
Proper heat input management that avoids prolonged exposure in the sensitization temperature range of 450–850°C

Process Parameters and H-TIG Advantages

The hot-wire TIG process offers distinct advantages over conventional TIG for overlay applications:

Parameter	Conventional TIG	Hot-Wire TIG	Benefit
Wire preheating	None	200–400°C preheat	Reduced thermal stress on base metal
Deposition rate	0.5–2 kg/h	2–6 kg/h	Improved productivity
Dilution control	Moderate	Good	Preheated wire melts more uniformly
Weld pool fluidity	Standard	Enhanced	Better wetting and fusion
Equipment complexity	Low	Moderate	Requires induction heating coil

Engineering Practice Implications

For engineers considering H-TIG overlay welding for corrosion-resistant applications:

Substrate preparation: The base steel should be thoroughly cleaned and, in some cases, pre-coated with a compatible alloy layer to minimize excessive dilution at the first pass.
Multi-pass strategy: The wave-like hardness profile suggests that an odd number of passes may be preferred, ensuring that the final surface layer is a full-composition overlay rather than a dilution zone.
Corrosion testing: Engineers should not assume that the top layer always provides the best corrosion protection. Testing of intermediate layers may reveal superior performance in specific service environments.
Application suitability: This technology is particularly well-suited for repairing carbon steel equipment in corrosive environments where full replacement is not economically viable, such as chemical reactor internals, heat exchangers, and marine structures.

Study Insights

This research highlights an important and sometimes overlooked aspect of overlay welding: the metallurgical complexity of multi-pass joints and the non-uniform distribution of properties through the overlay thickness. Engineers must understand that an overlay weld is not simply a homogeneous coating but a complex multi-zone structure with varying composition, microstructure, and performance characteristics at different depths. This understanding is essential for proper design, qualification, and service life prediction of overlay-welded components.