Defect Analysis and Remediation of TIG Weld Joints in Small-Diameter Dissimilar Steel Heat-Transfer Tubes

Literature Overview

The study by Dou Huaiwu, published in 2005 and associated with Guodian Jingyuan Power Generation Co., Ltd., addresses a practical and recurring challenge in power plant maintenance: the identification, analysis, and remediation of defects in TIG (GTAW) weld joints of small-diameter dissimilar steel tubes used in heat-absorbing surfaces (radiant tubes, convection tubes) of boilers and heat exchangers. Dissimilar steel welds — such as carbon steel to stainless steel, or low-alloy steel to austenitic stainless steel — are common in heat-transfer applications where different sections of a tube require different corrosion resistance or thermal properties.

Technical Background and Defect Classification

Small-diameter tubes (typically 25–57 mm OD) present unique welding challenges due to their thin walls (1.5–4 mm), limited accessibility for root pass preparation, and the difficulty of maintaining adequate shielding gas coverage on the internal surface. Dissimilar steel welds further compound these challenges through differential thermal expansion, potential formation of intermetallic compounds at the fusion boundary, and increased susceptibility to hot cracking.

Defect Analysis Methodology

The study employs a systematic defect analysis approach that combines macroscopic examination, metallographic analysis, and mechanical property testing. The typical methodology follows a structured sequence:

1. Visual and dimensional inspection of the weld joint to identify surface indications such as undercut, excessive reinforcement, and visible cracking.
2. Non-destructive testing (NDT) using magnetic particle testing (MT) for surface defects and ultrasonic testing (UT) for subsurface discontinuities.
3. Destructive testing on representative samples, including macro-etching, microstructural examination, and hardness profiling across the weld cross-section.
4. Mechanical property evaluation including tensile testing, bend testing, and impact testing of the weld joints.

Typical Defect Mechanisms in Dissimilar Steel TIG Welds

The formation of intermetallic phases at the fusion boundary between carbon steel and austenitic stainless steel is a well-documented concern. During welding, the dilution of the base metal into the weld metal creates a composition gradient that can lead to the formation of brittle iron-chromium carbides (M23C6, M7C3) in the HAZ of the carbon steel side. These phases are particularly detrimental at elevated temperatures, where they embrittle the microstructure and reduce the creep strength of the joint.

The study identifies hydrogen-induced cracking (HIC) as a predominant defect mechanism in the carbon steel HAZ of dissimilar welds. The hydrogen originates from moisture in the base metal surface, flux contamination, or the filler metal itself. In small-diameter tubes, the high cooling rate (often exceeding 100°C/s) promotes the formation of hard, brittle martensite in the carbon steel HAZ, which combined with residual hydrogen, leads to delayed cracking.

Remediation Strategies

The remediation approach presented in the study follows a systematic repair protocol:

For dissimilar steel welds in heat-transfer tubes, the selection of filler metal is critical. When joining carbon steel to austenitic stainless steel (e.g., 20# steel to 304/321), a hyper-manganese austenitic filler metal (such as ER309L or ER309MoL) is typically specified to compensate for base metal dilution and maintain adequate corrosion resistance in the weld metal. The dilution ratio in small-diameter tube welds can be as high as 40–60% depending on the joint geometry and welding parameters.

Engineering Practice Integration

From a cladding and bimetal pressure vessel perspective, this study highlights several transferable lessons. First, the importance of rigorous NDT protocols for dissimilar metal welds cannot be overstated — the same principles apply to the bond-line inspection of weld-overlay clad plates. Second, the management of intermetallic phase formation at dissimilar interfaces is directly relevant to clad plate quality, where the clad-base metal interface must be free of brittle phases to ensure long-term structural integrity. Third, the systematic defect analysis methodology — combining NDT, metallography, and mechanical testing — provides a comprehensive quality assurance framework that should be adopted for all critical bimetal joints.

Study Insights and Reflections

This practical, field-oriented study demonstrates that even in well-established welding applications, systematic defect analysis and remediation protocols remain essential for maintaining equipment reliability. The focus on small-diameter dissimilar steel tubes is particularly relevant to engineers working on heat exchanger tubesheets, tube-to-tubesheet joints, and clad heat exchanger fabrication, where similar metallurgical challenges arise. The study reinforces the principle that welding process optimization must always be considered in the context of the specific joint geometry, base metal combination, and service conditions — a lesson that applies equally to cladding and overlay welding operations. The remediation strategies presented, particularly the emphasis on appropriate filler metal selection and controlled thermal cycles, provide a practical reference for engineers addressing similar defects in bimetal pressure vessel fabrication.