Strip Cladding Process for Lock Hopper Cone Sections

Application Context and Process Requirements

Lock hoppers in coal-fired power plants and chemical processing facilities are cone-shaped pressure vessels that transport coal or other materials between pressure zones. The inner surface of the cone is subjected to severe abrasive wear from falling material, necessitating a hardfacing overlay. The conical geometry presents unique challenges for strip cladding, including variable curvature, accessibility constraints, and the need to maintain dimensional accuracy for proper assembly and operation.

The strip cladding process, also known as electroslag welding (ESW) strip cladding, is selected for its high deposition rate, low dilution, and ability to produce thick, dense overlay layers with minimal porosity. For lock hopper cones, the typical overlay thickness is 6-12 mm of Cr-Mo hardfacing alloy or Stellite-type alloy, depending on the abrasiveness of the material being handled.

Process Design for Conical Geometry

The conical geometry requires careful process planning to ensure uniform overlay thickness and quality throughout the cone. The key design considerations include:

• Welding direction: The overlay should be applied in a spiral pattern from the apex to the base, or in circumferential passes with overlap. The spiral approach provides better coverage but requires more complex fixture design.
• Fixture design: A rotary fixture is essential to rotate the cone while the welding gun remains stationary. The fixture must accommodate the cone's taper angle and provide adequate clamping force to prevent distortion.
• Gun positioning: The welding gun must maintain a constant angle relative to the cone surface throughout the travel. This requires a CNC-controlled positioner or a custom-designed guide rail system.

The variation in parameters across the cone reflects the changing geometry and the need to maintain consistent heat input per unit length. Thicker sections require more passes and higher parameters, while the apex region, with its sharper curvature, requires lower parameters to prevent sagging and poor wetting.

Material Selection for Lock Hopper Application

The selection of overlay material depends on the specific service conditions:

For most lock hopper applications, Cr15Mo or Cr20Mo provides the best balance of wear resistance and cost. However, in applications where moisture is present (wet coal or slurry), Stellite 6 is preferred due to its superior corrosion resistance in aggressive environments.

Process Challenges and Solutions

Thermal Distortion Control

The conical geometry is particularly susceptible to thermal distortion during ESW overlay. The differential thermal expansion between the overlay and base material can cause the cone to warp, affecting dimensional accuracy. Countermeasures include:

• Apply symmetric passes on opposite sides of the cone to balance thermal input.
• Use a backing plate with thermal mass to absorb excess heat.
• Apply the overlay in multiple thin passes rather than a single thick pass.
• Allow controlled cooling between passes (interpass temperature < 200 °C).

Porosity Prevention

Porosity is a common defect in ESW overlay, caused by gas entrapment in the slag or base metal. Prevention strategies include:

• Thorough surface preparation: grind to bare metal and clean with solvent.
• Use low-hydrogen flux with proper drying (300 °C for 2 hours).
• Maintain adequate slag coverage to prevent air entrapment.
• Avoid welding over rust, paint, or mill scale.

Crack Prevention at the Apex

The apex of the cone is the most challenging region due to the sharp curvature and limited space for slag accumulation. The high thermal gradient in this region can cause cracking. Solutions include:

• Use a smaller diameter strip (3 mm) for the apex region.
• Apply a transition layer of 309L or E309 before the hardfacing layer.
• Reduce current and increase travel speed to lower the thermal input.
• Consider machining the apex region flat before overlay application.

Quality Control and Inspection

The quality control plan for lock hopper cone overlay includes:

• Visual inspection: 100% inspection of all overlay surfaces for undercut, spatter, and surface irregularities.
• Magnetic particle testing (MT): 100% of overlay surfaces for surface and near-surface cracks.
• Ultrasonic testing (UT): 10% of overlay surfaces for subsurface defects and bond integrity.
• Hardness survey: Grid pattern of hardness readings across the overlay surface, with minimum 5 readings per square meter.
• Dimensional check: Verify cone dimensions after overlay and machining to ensure assembly compatibility.

Engineering Practice and Lessons Learned

A key lesson from practical implementation is that the dimensional tolerance of the cone before overlay must be carefully considered. The overlay adds thickness to the inner surface, which affects the internal volume and, consequently, the material handling capacity of the lock hopper. The design must account for the overlay thickness in the initial cone geometry to avoid reducing the effective volume.

Another important consideration is the post-overlay machining. The overlay surface must be machined to achieve the required surface finish and dimensional accuracy. The machining allowance should be at least 2 mm beyond the required final thickness to account for surface irregularities from the welding process. For cones with tight dimensional tolerances, a semi-automated machining process is recommended to ensure uniform thickness removal.

The overall study of strip cladding for lock hopper cones confirms that this process is highly effective for providing wear protection, provided that the process parameters are carefully adapted to the conical geometry and the quality control plan is rigorously implemented. The productivity advantage of ESW over manual hardfacing processes is substantial, with deposition rates 3-5 times higher than manual methods, making it the preferred process for large-scale production of lock hopper components.