Microstructure and Properties of BHW35 Steel Hot Wire TIG Overlay Welding Joint

Literature Overview

This paper by Wu Guangfeng (2013, Welding, Issue 11) investigates the microstructure and corrosion resistance of overlay welds deposited on steel substrates using hot wire TIG (H-TIG) welding with stainless steel consumables. The study, funded by the Chongqing University Outstanding Achievement Transformation Program, addresses the multi-layer overlay welding scenario and examines how the microstructure evolves through successive passes, with particular attention to corrosion behavior in aggressive acidic environments.

Core Technical Points

Hot Wire TIG Process Characteristics

Hot wire TIG welding differs from conventional TIG in that the filler wire is preheated by a separate resistance heating element before entering the arc. This provides several advantages for overlay applications:

The preheated wire enters the arc pool at elevated temperature, reducing the total arc energy required for melting and allowing higher deposition rates. This is particularly advantageous for overlay welding where large volumes of weld metal must be deposited.

Multi-Layer Microstructural Evolution

The multi-pass overlay produces a characteristic alternating pattern of dendritic and columnar structures:

The wave-like hardness variation with depth is explained by the thermal cycling effect: each subsequent pass reheats the previous pass, causing partial recrystallization and grain refinement in the transition zones between passes. These reheated regions develop higher hardness due to grain refinement and potentially different precipitate distributions.

Corrosion Performance in 5% HCl

The electrochemical testing in 5% hydrochloric acid solution reveals the following corrosion resistance ranking:

The transition layer's superior corrosion resistance is attributed to the thermal cycling from subsequent passes, which homogenizes the microstructure and eliminates the coarse dendritic segregation typical of as-welded austenitic stainless steel deposits. This is a significant finding because it suggests that the intermediate layers in a multi-pass overlay may actually outperform the final surface layer in terms of corrosion resistance.

Intergranular Corrosion Resistance

The overlay layer demonstrates good resistance to intergranular corrosion, which is critical for austenitic stainless steel overlay welds. This indicates that chromium carbide precipitation at grain boundaries (the mechanism responsible for sensitization in 304/316 stainless steels) is minimal or that the carbon content in the deposit is sufficiently low to prevent sensitization during welding cooling.

Engineering Practice Integration

Application Scenarios

BHW35 steel with stainless steel overlay is commonly used in:

1. Chemical processing equipment — reactor shells, heat exchanger tubes, and pipe fittings exposed to chloride-containing environments
2. Marine applications — propeller blades, shafts, and underwater structures requiring corrosion resistance with adequate mechanical strength
3. Food processing equipment — surfaces requiring both corrosion resistance and wear resistance
4. Pulp and paper industry — equipment exposed to acidic cooking liquors

Quality Control Considerations

For production-quality overlay welds using H-TIG:

Process Optimization Recommendations

Based on the findings:

1. Multi-pass strategy is essential — single-pass deposits will have uniform dendritic structure without the beneficial thermal cycling effects observed in the transition layers.
2. Interpass temperature control — maintaining interpass temperature between 150–250°C optimizes the balance between thermal cycling benefits and avoiding excessive grain growth.
3. Layer thickness control — keeping individual pass thickness between 1–2 mm ensures adequate thermal cycling of the previous layer without excessive dilution.
4. Post-weld treatment — solution annealing at 1050–1100°C followed by water quenching can further homogenize the microstructure and dissolve any chromium carbide precipitates that may have formed during welding.

Key Questions and Reflections

An important engineering question arises from the corrosion ranking: if the transition layers have superior corrosion resistance compared to the top layer, should the design philosophy be reconsidered? In practice, the top layer is what contacts the corrosive environment, so its properties are most critical. However, this finding suggests that the final layer could benefit from a post-weld solution treatment to dissolve dendritic segregation and achieve corrosion resistance comparable to the transition layers.

The study also raises the question of long-term corrosion behavior. Short-term electrochemical testing in 5% HCl provides useful comparative data, but real service conditions may involve variable temperatures, flow velocities, and mixed chemical environments. Crevice corrosion and pitting initiation times may differ significantly from the general corrosion rates measured in this study.

Study Insights and Implications

This research provides valuable evidence that multi-pass overlay welding creates microstructural heterogeneity that can be either beneficial or detrimental depending on the application requirement. For corrosion-critical applications, the wave-like hardness variation and the superior corrosion resistance of transition layers suggest that the deposit thickness should be designed to ensure that the topmost layer has undergone sufficient thermal cycling from subsequent passes — or that a final solution treatment is applied. The hot wire TIG process, with its high deposition rate and good dilution control, is well-suited for producing thick overlay layers on carbon steel substrates where both corrosion and wear resistance are required. Engineers should note that the fusion zone remains the weakest link in the corrosion resistance chain, and any design that relies on the overlay for full corrosion protection must ensure complete coverage of the base metal surface.