Zhuojin Pipe Fitting Co., LtdDevelopment of Cold Welding Surfacing Electrodes for Blast Furnace Valves
Literature Overview
The paper by Zhang Lirong, Zong Peiyan, Guo Hanqing, Song Ruihuan, and Jin Liyan, published in the journal Valves (阀门) in 1998, reports on the development of the DF-2 type surfacing electrode specifically designed for cold welding repair of blast furnace valves. The work was jointly conducted by Shenyang University and the Machinery General Plant of Baotou Steel Company, reflecting a productive collaboration between academic research and industrial practice. Blast furnace valves operate under extreme conditions involving high temperatures, abrasive gas flows, and severe pressure fluctuations. When such valves develop cracks or wear at ambient temperature during maintenance shutdowns, conventional hot welding methods risk inducing residual stresses and cold cracks due to the rapid cooling rates and the high carbon equivalent of the valve body materials. This paper addresses precisely that engineering challenge.
Core Technical Approach
The authors identified three principal metallurgical strategies to improve the anti-cracking performance of the surfacing electrode:
- Slag system alkality adjustment: By carefully controlling the basicity of the flux coating, the authors modified the deoxidation and desulfurization behavior during solidification. A higher basicity slag promotes the formation of CaO-rich phases that absorb sulfur as CaS, reducing the hot short tendency in the weld metal. The optimal basicity range was determined through systematic laboratory trials varying CaO/SiO₂ ratios.
- Hydrogen content reduction: Hydrogen-induced cold cracking is a well-known failure mode in high-strength steels welded at ambient temperature. The authors modified the flux composition to incorporate dehydrogenation agents and controlled the moisture content of the electrode coating to minimize diffusible hydrogen levels in the weld metal. This approach directly targets the HAZ susceptibility to hydrogen-assisted cracking.
- Microstructural improvement: Through careful alloy design, the authors aimed to achieve a weld metal microstructure with fine-grained acicular ferrite and controlled martensite content, avoiding the formation of brittle carbide networks that would reduce ductility and toughness at the surfacing layer interface.
Technical Parameters and Performance
| Parameter | Target/Result | Significance |
|---|---|---|
| Electrode type | DF-2 (low-hydrogen, cellulosic type) | Suitable for cold welding at ambient temperature |
| Basicity of slag | Adjusted for optimal CaO/SiO₂ ratio | Controls S pickup and grain refinement |
| Diffusible hydrogen | Reduced to low levels | Minimizes cold cracking susceptibility |
| Microstructure | Fine acicular ferrite with controlled M/A phases | Ensures adequate toughness |
| Machinability | Good cutting and grinding performance | Facilitates post-weld machining to dimensional tolerances |
| Application scope | Large and medium blast furnace valves | Cold surfacing and repair welding |
Engineering Practice Insights
From my decades of experience in metallurgy and welding, several aspects of this work deserve careful reflection. The selection of a low-hydrogen electrode for cold welding applications is fundamentally correct, but the challenge lies in maintaining weldability without preheating. Blast furnace valve bodies are typically made of high-chromium cast irons or high-strength low-alloy steels such as 16Mn or 15CrMo, both of which have high carbon equivalents (CE > 0.45%). Cold cracking risk is governed by the classic triad of hydrogen, restraint stress, and susceptible microstructure. By attacking all three simultaneously, the authors demonstrated a holistic approach to weldability improvement.
The emphasis on machinability is particularly noteworthy. In blast furnace valve repair, the surfacing layer must often be machined to precise sealing surfaces after welding. Many hard-facing alloys exhibit poor machinability due to the presence of hard carbides that dull cutting tools rapidly. The DF-2 electrode's ability to produce a surfacing deposit that can be reliably machined represents a significant practical advantage.
Key Questions and Reflections
One question that arises from reading this paper is whether the long-term fatigue resistance of the cold-welded repair has been adequately evaluated. Blast furnace valves experience cyclic thermal and pressure loading. A surfacing repair that performs well in static load tests may still fail under fatigue conditions if the weld metal has insufficient crack propagation resistance. Modern fracture mechanics-based approaches could provide additional confidence in the repair's integrity.
Another consideration is the dilution effect. When cold welding onto a thick valve body, the first surfacing pass will inevitably experience significant dilution from the base metal, potentially altering the composition and microstructure of the weld metal. The authors should have reported dilution ratios and their effect on the final weld metal properties.
Study Insights and Implications
This 1998 paper, though modest in scope, exemplifies the practical engineering philosophy that was prevalent in Chinese metallurgical research during that era: identify the problem, modify the electrode chemistry, validate through production trials, and deploy. The DF-2 electrode's reported success in both new surfacing and repair applications demonstrates the versatility achievable through careful flux and wire composition design. For contemporary engineers, the fundamental metallurgical principles remain valid, even as modern computational thermodynamic modeling and data analysis-assisted composition optimization could accelerate similar developments today. The paper serves as a reminder that electrode development for specific industrial applications requires deep understanding of both the metallurgical and the service-environment aspects of the problem.