CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of BHW35 Steel Hot-Wire TIG Overlay Weld Joints
Literature Overview and Background
This study focuses on the microstructural evolution and mechanical properties of hot-wire TIG (HWT) overlay weld joints deposited on BHW35 steel, a high-strength low-alloy steel widely used in heavy machinery, construction equipment, and pressure vessel applications. Hot-wire TIG welding represents an advanced variant of conventional GTAW in which a filler wire is preheated by a separate heating element before entering the arc, enabling significantly higher deposition rates while maintaining the narrow, low-dilution weld profile characteristic of TIG welding. The research addresses a practical engineering need: how to achieve efficient overlay cladding on thick-section BHW35 steel components while ensuring adequate toughness and resistance to hydrogen-induced cracking.
Process Parameters and Welding Configuration
The study employs a systematic approach to optimizing the HWT process parameters for overlay applications on BHW35 steel. The key process variables include arc current, wire feed speed, preheating temperature of the filler wire, travel speed, and shielding gas flow rate. The table below summarizes the typical process windows investigated:
| Parameter | Range Investigated | Optimal Value |
|---|---|---|
| Arc current | 120–200 A | 160 A |
| Wire feed speed | 3.0–5.5 m/min | 4.2 m/min |
| Wire preheat temperature | 400–800 °C | 650 °C |
| Travel speed | 150–300 mm/min | 220 mm/min |
| Shielding gas (Ar) | 10–20 L/min | 15 L/min |
| Interpass temperature | 80–150 °C | 120 °C |
The preheating of the filler wire is the defining feature of HWT and the primary factor distinguishing it from conventional TIG overlay. By preheating the wire to 600–700 °C, the effective heat input at the weld pool increases substantially, resulting in deeper penetration and higher deposition rates—typically 2 to 3 times that of conventional TIG—without widening the weld bead excessively.
Microstructural Analysis
Metallographic examination of the HWT overlay weld joints reveals a distinct microstructural gradient from the fusion line to the weld center. Near the fusion boundary, fine acicular ferrite and granular bainite dominate due to rapid cooling from the BHW35 substrate. Moving toward the weld center, the microstructure transitions to a coarser bainitic and ferritic structure, with occasional martensite-austenite (M-A) constituents forming in regions of higher carbon and alloy enrichment. The overlay layer deposited on the BHW35 base metal shows a dilution rate of approximately 15–25%, which is notably lower than what would be achieved with submerged arc welding or gas metal arc welding under comparable deposition rate conditions.
The presence of M-A islands is a critical observation for engineers, as these constituents can act as crack initiation sites under impact loading. The study demonstrates that controlling the interpass temperature within the 100–150 °C range minimizes M-A island formation while maintaining adequate weld toughness. Excessive interpass temperatures above 200 °C lead to grain coarsening and a significant reduction in Charpy V-notch impact energy at the fusion line.
Mechanical Properties and Performance Evaluation
The mechanical properties of the HWT overlay joints are evaluated through tensile testing, hardness profiling, and Charpy impact testing. The results demonstrate that the overlay deposit achieves a yield strength of approximately 550–620 MPa with elongation of 18–22%, which is compatible with the BHW35 base metal properties. Hardness measurements across the weld cross-section show a maximum hardness of 240–260 HV at the fusion zone, gradually decreasing to 200–220 HV at the weld center. This hardness gradient is acceptable for most structural applications but requires attention when the overlay is intended for wear-resistant service.
| Test Location | Yield Strength (MPa) | Elongation (%) | Charpy CVN at 20 °C (J) | Hardness (HV) |
|---|---|---|---|---|
| Base metal (BHW35) | 450–510 | 20–24 | 85–110 | 190–210 |
| Fusion zone | 520–580 | 16–20 | 60–80 | 240–260 |
| Weld center | 550–620 | 18–22 | 50–70 | 200–220 |
The Charpy impact energy values indicate that the HWT overlay joints maintain acceptable toughness at ambient temperature, which is essential for pressure vessel and structural applications where low-temperature service may be encountered. The study also evaluates the susceptibility to hydrogen-induced cracking through delayed cracking tests, confirming that the HWT process, with its relatively low hydrogen pickup and controlled cooling rates, produces joints with low susceptibility to cold cracking.
Engineering Practice and Process Advantages
The hot-wire TIG process offers several advantages for overlay applications on BHW35 and similar high-strength steels. The narrow weld profile enables precise control of dilution, which is critical when depositing corrosion-resistant or wear-resistant alloys onto structural substrates. The process is also well-suited for robotic automation, making it attractive for high-volume production environments such as power plant maintenance and heavy equipment manufacturing. However, engineers must be aware that the preheating mechanism adds complexity to the welding setup and requires careful maintenance of the heating element to ensure consistent wire temperature throughout the deposition sequence.
The study's findings support the use of HWT for overlay repair of BHW35 steel components in mining equipment, hydraulic cylinders, and structural weldments where both wear resistance and toughness are required. The process is particularly advantageous for repairing components with limited access or where excessive heat input could compromise the mechanical properties of the base metal. Post-weld stress relief treatment at 550–600 °C is recommended to reduce residual stresses and improve the long-term service life of the overlay joint.
Study Insights and Conclusions
This literature provides a comprehensive understanding of how the hot-wire TIG process influences the microstructure and properties of overlay weld joints on BHW35 steel. The key takeaway for practicing engineers is that HWT represents a viable alternative to conventional overlay methods when high deposition rates and low dilution are simultaneously required. The process parameter optimization data presented in the study can serve as a starting point for qualification procedures under NB/T 47014 or ASME IX. The microstructural insights, particularly regarding M-A island control and hardness gradient management, are directly applicable to welding procedure specification development. Ultimately, the research reinforces the principle that process selection in overlay welding must be driven by a clear understanding of the metallurgical consequences of each variable, and HWT offers a compelling balance of efficiency, quality, and versatility for industrial cladding applications on high-strength steel substrates.