Hard Alloy Cladding Application on 125m3 Blast Furnace Charging Bell and Hopper Welding

Literature Overview and Background

This technical document, published in Shandong Metallurgy in 2000, addresses the application of hard alloy weld overlay on a 125 m³ blast furnace charging bell and hopper at Laiwu Iron and Steel Group's sintering plant. Blast furnace charging systems are subjected to severe abrasive wear from iron ore, coke, and sinter material flow. The charging bell and hopper are critical components where material impacts and slides at high velocity, leading to rapid surface degradation. The study by Ma Lingyun focuses on selecting appropriate hard alloy cladding materials and optimizing the welding process to extend service life while ensuring structural integrity.

Core Technical Content

The 125 m³ blast furnace charging bell typically experiences erosive wear patterns concentrated at the bell lip, hopper throat, and transition zones. The base material is generally low-carbon steel such as Q235 or Q345, which provides adequate strength but insufficient hardness to resist abrasive wear. Hard alloy cladding introduces carbide-forming elements to achieve surface hardness exceeding 500 HV while maintaining a ductile substrate.

Cladding Material Selection

Parameter	Specification
Base material	Q235 / Q345 low-carbon steel
Cladding material type	Cast hard alloy / surfacing electrode
Target hardness	500-650 HV
Typical cladding thickness	3-6 mm
Key alloying elements	Cr, Mo, V, W (carbide formers)
Carbon equivalent	Controlled to prevent base metal cracking

The selection of hard alloy cladding material must balance hardness, wear resistance, and weldability. High carbon and high chromium compositions produce fine chromium carbides (Cr7C3 and Cr23C6) that provide excellent abrasion resistance. However, excessive carbon content raises the carbon equivalent (CE) and increases cold cracking susceptibility in the base metal during cooling.

Welding Process Parameters

The study describes the use of manual arc surfacing (SMAW) with special surfacing electrodes. Key process considerations include:

Preheating: The base metal requires preheating to 150-250 °C to reduce thermal gradient and prevent hydrogen-induced cracking. The carbon equivalent of the base steel and the dilution from the cladding layer both contribute to cracking risk.
Interpass temperature control: Maintained between 150-250 °C to prevent excessive cooling rates in the heat-affected zone (HAZ).
Layer arrangement: Typically two to three layers are deposited, with the first layer acting as a transition layer to control dilution and reduce residual stress.
Post-weld treatment: Stress-relieving annealing at 550-650 °C for 2-4 hours per 25 mm thickness is recommended to reduce residual stresses and prevent delayed cracking.

Defect Analysis and Countermeasures

Common defects in hard alloy cladding on blast furnace components include:

Defect Type	Cause	Countermeasure
Cracks in HAZ	High CE, hydrogen, rapid cooling	Preheat, low-hydrogen electrodes, PWHT
Cracks in overlay	Excessive carbon, brittle carbide network	Multi-layer deposition, dilution control
Poor bond strength	Base metal contamination, insufficient penetration	Surface preparation, proper root pass technique
Excessive dilution	Large wire diameter, low current density	Use smaller diameter consumables, higher travel speed

The transition layer technique is critical. A low-carbon, high-ductility transition layer is deposited between the base metal and the hard alloy layer. This layer reduces the carbon gradient at the interface, minimizes dilution of the hard alloy by the base metal, and provides a buffer zone that absorbs residual stresses.

Engineering Practice Insights

In blast furnace charging system applications, the geometry of the bell and hopper presents significant welding accessibility challenges. The curved surfaces and confined spaces require careful planning of weld sequence and positioning. The welding procedure must be qualified according to relevant standards, with emphasis on both the base metal weldability and the cladding layer bond strength.

The service life improvement achieved through hard alloy cladding is substantial. Without cladding, the charging bell lip may require replacement every 6-12 months due to abrasive wear. With proper hard alloy cladding, service intervals can be extended to 24-36 months or longer, representing significant economic savings in both material and downtime costs.

A key engineering insight from this literature is the importance of understanding the wear mechanism. In the charging bell, the dominant wear mode is erosive-abrasive wear caused by material impact at angles of 30-60 degrees. The cladding material must be selected to resist this specific wear mechanism, which favors materials with fine, evenly distributed carbides rather than coarse carbide structures.

Study Reflections and Conclusions

This literature provides valuable practical guidance for cladding applications in metallurgical equipment. The key takeaway is that hard alloy cladding success depends on the integrated consideration of material selection, process parameter optimization, and defect prevention. The transition layer concept is particularly important for high-carbon cladding on low-carbon base metals, as it addresses the fundamental metallurgical incompatibility between the two materials. Engineers working on similar blast furnace or material handling equipment should pay close attention to preheat temperature, interpass temperature, and post-weld heat treatment, as these factors are critical to preventing cracking in both the base metal HAZ and the cladding layer itself.