Microstructure and Properties of Hot-Wire TIG Overlay Welds on BHW35 Steel

Literature Overview

This research focuses on the application of hot-wire TIG (GTAW with hot-wire feed) overlay welding technology on BHW35 steel, a medium-carbon alloy steel commonly used in wear-resistant components and pressure vessel applications. The study examines the microstructural evolution, mechanical properties, and wear resistance of the overlay welds, with particular attention to the effects of hot-wire feed parameters on the resulting weld quality. BHW35 steel, with its carbon equivalent of approximately 0.45-0.55%, presents significant welding challenges due to its susceptibility to cracking and hardness variations, making the selection of overlay process parameters critical for achieving a high-quality, crack-free weld deposit.

Core Technical Findings

Hot-Wire TIG Process Characteristics

Hot-wire TIG overlay welding combines the precision of conventional TIG welding with the increased deposition rate of hot-wire feed, where the filler wire is preheated by an induction heater or resistance heating element before entering the arc. This preheating reduces the amount of energy required to melt the wire, allowing for higher deposition rates while maintaining the low dilution and narrow heat-affected zone advantages of the TIG process. The following table summarizes the process parameters studied:

The hot-wire TIG process achieves deposition rates approximately 2-3 times higher than conventional TIG while maintaining lower base metal dilution, which is particularly advantageous for overlay applications where the overlay composition must be preserved.

Microstructural Analysis

The overlay welds deposited on BHW35 steel using hot-wire TIG exhibit a columnar dendritic structure with finer dendrite arm spacing compared to conventional TIG deposits, attributed to the higher cooling rates achieved with the faster travel speeds. The microstructure consists primarily of ferrite and pearlite phases in the overlay, with the specific phase fractions depending on the filler wire composition. When using a low-carbon filler wire such as ER70S-6, the overlay microstructure is predominantly acicular ferrite, which provides excellent toughness and crack resistance. The base metal heat-affected zone (HAZ) exhibits a narrower width compared to conventional TIG, with reduced hardness peaks, indicating lower susceptibility to cracking.

Mechanical Properties

The hot-wire TIG overlay demonstrates superior impact toughness compared to both the conventional TIG overlay and the base BHW35 steel, which is attributed to the finer microstructure and lower carbon equivalent of the overlay deposit. The wear resistance is also improved, making the overlay suitable for applications involving sliding or abrasion wear.

Process Analysis and Key Technical Points

Effects of Hot-Wire Preheat Temperature

The wire preheat temperature is a critical parameter that influences the weld pool dynamics, solidification behavior, and resulting microstructure. Higher preheat temperatures reduce the arc energy required to melt the wire, allowing for lower arc currents and faster travel speeds. However, excessively high preheat temperatures can lead to:

• Increased spatter and arc instability
• Reduced arc force and poor weld bead profile
• Potential oxidation of the preheated wire surface, leading to increased porosity

The optimal preheat temperature range of 400-600°C provides a good balance between deposition efficiency and weld quality, as demonstrated in the study.

Crack Resistance of the Overlay

BHW35 steel has a carbon equivalent (CE) of approximately 0.45-0.55%, which places it in the high-hardenable category with significant susceptibility to hydrogen-induced cracking and low-temperature cracking. The hot-wire TIG process offers several advantages in terms of crack resistance:

1. Lower arc current reduces the peak temperature in the base metal, limiting the formation of hard, brittle martensite in the HAZ.
2. Higher travel speed reduces the heat input per unit length, further limiting the HAZ width and hardness.
3. The preheated wire acts as a heat source, promoting a more uniform temperature distribution and reducing thermal gradients.
4. The lower dilution ratio ensures that the overlay composition remains low in carbon, reducing the susceptibility of the overlay to cracking.

The study confirms that no cracks were observed in the hot-wire TIG overlay welds, even at travel speeds as low as 20 mm/min, whereas conventional TIG welds exhibited cracking at comparable parameters.

Engineering Practice Integration

Application to Pressure Vessel and Wear Component Repair

Hot-wire TIG overlay welding on BHW35 steel is particularly suitable for the following applications:

• Repair of worn surfaces on pressure vessel internals, such as support plates, baffles, and nozzle reinforcements.
• Overlay of wear-resistant layers on rotating equipment components, including pump shafts, valve seats, and impeller surfaces.
• Restoration of dimensional accuracy on machined surfaces that have been worn beyond specification.

The low dilution and controlled heat input of the hot-wire TIG process make it ideal for overlaying on thick-section BHW35 steel components where the risk of cracking is high. The process can be applied in the field with appropriate shielding arrangements, making it suitable for in-situ repair of large pressure vessels and equipment.

Quality Control Considerations

The following quality control measures are recommended for hot-wire TIG overlay welds on BHW35 steel:

• Visual inspection of the overlay surface for uniformity, lack of undercut, and absence of surface defects.
• Magnetic particle testing (MT) of the overlay surface and the HAZ for detection of surface and near-surface cracks.
• Ultrasonic testing (UT) of the overlay-to-base interface for detection of lack of fusion and delamination.
• Hardness mapping across the overlay, interface, and HAZ to verify the hardness distribution and identify any abnormally hard zones.
• Impact testing of qualification welds to confirm the toughness of the overlay and HAZ at the minimum service temperature.

Key Questions and Reflections

The study highlights the potential of hot-wire TIG overlay welding as a superior alternative to conventional TIG for overlay applications on high-carbon equivalent steels. However, several questions remain for further investigation. The long-term performance of the overlay under cyclic loading and thermal fatigue conditions is not fully characterized. Additionally, the scalability of the hot-wire TIG process for large-area overlay applications, such as entire pressure vessel shells or large wear plates, requires further development of multi-wire and multi-arc configurations. The cost of the hot-wire feed equipment and the associated consumables must also be evaluated against the benefits of improved weld quality and productivity.

Study Insights and Implications

This research demonstrates that hot-wire TIG overlay welding is a highly effective process for applying wear-resistant and crack-resistant overlay layers on BHW35 steel. The combination of low dilution, controlled heat input, and high deposition rate makes this process particularly suitable for applications where the base metal is susceptible to cracking and the overlay composition must be preserved. Engineers working with high-carbon equivalent steels should consider hot-wire TIG as a preferred overlay process, provided that appropriate equipment and qualified procedures are available. The study underscores the importance of process parameter optimization and rigorous quality control in achieving reliable overlay welds on challenging base materials.