Development of Cold Weld Overlay Electrodes for Blast Furnace Valves

Literature Overview

This 1998 publication by Zhang Lirong, Zong Peiyan, Guo Hanqing, Song Ruihuan, and Jin Liyan from Shenyang University and Baotou Steel Mechanical Works addresses a specific and demanding industrial application: the development of cold welding overlay electrodes suitable for blast furnace valve repair and maintenance. Blast furnace valves operate under extreme conditions—high temperatures, abrasive slurries, corrosive gases, and cyclic mechanical loading—making them critical components whose failure can halt entire production lines. The authors present the metallurgical design, welding process development, and field performance evaluation of specialized overlay electrodes for this severe service environment.

Operating Environment and Failure Mechanisms of Blast Furnace Valves

Blast furnace valves are subjected to a uniquely harsh combination of degradation mechanisms that distinguish them from conventional industrial valves:

• Thermal cycling: Valve surfaces experience temperatures ranging from ambient to 400-600°C depending on the furnace section, causing thermal fatigue and thermal stress cracking
• Abrasive wear: Slag, dust, and particulate materials in the gas stream erode valve faces and seats at rates that can exceed 1-2 mm per month in severe locations
• Chemical corrosion: Reduction gases (CO, H₂), sulfur compounds, and alkali metals attack the valve surface
• Thermal shock: Rapid temperature changes during furnace shutdown and restart cycles
• Mechanical fatigue: Cyclic opening and closing under high differential pressure

The combination of these mechanisms means that conventional repair welding deposits, while providing temporary dimensional restoration, fail prematurely due to cracking, spalling, or excessive wear. The overlay electrode must therefore provide a composite of hardness, toughness, thermal stability, and corrosion resistance that no single material system can easily achieve.

Electrode Design and Metallurgical Considerations

The development of the cold welding overlay electrode involves careful selection of the filler metal composition to balance competing requirements. The authors describe a multi-layer approach where different electrode types are used for the transition layer and the functional overlay layer.

The metallurgical challenge is particularly acute at the weld metal/base metal interface. Blast furnace valve bodies are typically fabricated from carbon steel or low-alloy steel (such as Q345 or 15CrMo), which have low carbon content and limited hardenability. When a high-carbon, high-alloy overlay is deposited directly onto such a substrate, the rapid cooling in the base material can produce hard martensitic microstructures with high susceptibility to cracking. The transition layer electrode, rich in nickel, acts as a diffusion barrier and stress-relief zone, reducing the risk of interfacial cracking by:

1. Diluting carbon and alloy elements from the base material at the interface
2. Promoting austenitic or ferritic-austenitic microstructures that are more ductile and crack-resistant
3. Reducing the thermal gradient at the interface through the higher thermal conductivity of nickel-based alloys

Welding Process Parameters

The cold welding designation indicates that the valve does not require preheating before overlay application, which is a significant practical advantage for field repair operations where controlled preheating equipment may not be available. However, achieving successful cold welding of high-alloy deposits onto carbon steel requires careful process control:

A critical process parameter that the authors emphasize is the control of dilution. In multi-layer overlay welding, the dilution from the base material into the first weld pass can be as high as 30-50%, significantly altering the intended composition of the overlay. Subsequent passes have lower dilution (10-20%) as the preceding weld metal acts as the new base. The authors recommend using a more dilution-resistant electrode for the first pass and reserving the high-performance overlay electrode for subsequent passes where dilution is naturally lower.

Performance Evaluation and Field Results

The developed electrode system was evaluated through both laboratory testing and field service trials at Baotou Steel's blast furnace operations. The evaluation criteria included:

• Wear life: Measured as the number of operating hours before valve face wear exceeded 2 mm
• Crack resistance: Assessed through visual and MT examination after thermal cycling simulation
• Corrosion resistance: Evaluated through immersion testing in simulated blast furnace gas condensate
• Bond strength: Measured by shear testing of weld metal/base metal interface

The field results demonstrated that the multi-layer overlay approach extended valve service life by a factor of 3-5 compared to conventional single-pass repair welding. The nickel-based transition layer proved essential in preventing the interfacial cracking that was the primary failure mode of earlier repair attempts.

FMEA Analysis of Overlay Failure Modes

Applying Failure Mode and Effects Analysis (FMEA) to the overlay process reveals several critical failure modes:

Key Questions and Reflections

The 1998 publication raises several questions that remain relevant today. First, the literature does not address the application of newer overlay technologies such as plasma transferred arc (PTA) welding or laser cladding, which have since become available and offer superior control over dilution and microstructure. Second, the environmental impact of nickel-based transition electrodes is not discussed, yet modern regulations increasingly restrict the use of nickel-containing materials in certain applications. Third, the long-term performance of the overlay after extended thermal cycling is not fully characterized—field data beyond 5000 operating hours would strengthen the technical case.

The engineering insight that stands out most clearly is the recognition that successful overlay welding is not merely a matter of selecting a hard or wear-resistant filler metal. The metallurgical compatibility between the overlay and the base material, the thermal management of the welding process, and the systematic approach to multi-layer deposition are equally important. This philosophy—treating the overlay as a system rather than a single material—remains the foundation of modern cladding technology.

Study Insights and Implications

This publication exemplifies the practical, problem-driven approach to welding technology development that characterized Chinese industrial engineering in the 1990s. The collaboration between academic researchers (Shenyang University) and industry practitioners (Baotou Steel) produced a solution that was both metallurgically sound and practically deployable. For contemporary engineers, the key lessons are: (1) always design the overlay system as a multi-layer structure with attention to interfacial metallurgy; (2) the cold welding designation must be backed by rigorous process qualification, not assumed; (3) field performance data, while valuable, should be supplemented with controlled laboratory testing to isolate individual failure mechanisms; and (4) the FMEA approach to overlay process development should be adopted systematically to identify and mitigate failure modes before they manifest in service.