Selection of Underlay Cladding Materials for Ash Lock Valve Sealing Surfaces

Literature Overview and Background

This technical paper, published in Welding Technology in 1997, addresses the selection of underlay cladding materials for ash lock valve sealing surfaces. The study is authored by Liu Jie and Fan Guangwei from Taiyuan Heavy Machinery Institute and Taiyuan Iron and Steel Group. Ash lock valves are critical components in blast furnace gas cleaning systems, where they control the flow of dry dust (ash) while maintaining gas-tight seals. The sealing surfaces are subjected to abrasive wear from ash particles, thermal cycling, and chemical corrosion. The underlay cladding material serves as a metallurgical buffer between the base steel and the final hard alloy overlay, and its selection is critical to the overall performance and durability of the cladding system.

Core Technical Content

Ash Lock Valve Operating Environment

The ash lock valve sealing surface experiences a combination of wear, corrosion, and thermal stress. The ash particles are sharp and angular, causing severe abrasive wear. The gas environment contains sulfur compounds that can cause corrosion of unprotected steel surfaces. The thermal cycling between hot ash and cooler gas phases introduces additional fatigue stresses.

Underlay Material Selection Criteria

The underlay layer serves multiple critical functions:

1. Dilution control: The underlay dilutes the base metal into the overlay, reducing the carbon and alloy content at the interface.
2. Crack resistance: The underlay provides a ductile buffer that absorbs residual stresses and prevents crack propagation from the base metal into the hard overlay.
3. Metallurgical compatibility: The underlay creates a gradual transition in composition between the base steel and the hard alloy overlay, reducing the risk of brittle intermetallic formation.
4. Bond strength: The underlay ensures adequate mechanical and metallurgical bonding between the base metal and the overlay.

Comparison of Underlay Material Options

The selection depends on the specific operating conditions and the final overlay material. For ash lock valve applications, where the primary concern is abrasive wear resistance and the environment is mildly corrosive, a low-carbon martensitic stainless steel underlay (such as 12Cr or 13Cr with low carbon) is often the optimal choice. This material provides adequate dilution control, good crack resistance, and reasonable cost.

Welding Process for Ash Lock Valve Cladding

The recommended welding sequence is:

1. Surface preparation: Machine the sealing surface to remove all damaged material. Clean thoroughly to remove rust, oil, and contaminants.
2. Preheat: Preheat to 150-200 °C to reduce thermal gradient.
3. Underlay layer: Deposit 2-3 mm of underlay material using SMAW or SAW. Use low-hydrogen consumables to minimize hydrogen pickup.
4. Overlay layer: Deposit 3-5 mm of hard alloy overlay material. The overlay material should contain Cr, Mo, and possibly Co to achieve hardness of 45-55 HRC.
5. Post-weld treatment: Stress-relief annealing at 550-600 °C for 2 hours.
6. Machining: Machine the sealing surface to final dimensions and surface finish (Ra ≤ 3.2 μm).

Defect Analysis and Countermeasures

The underlay thickness is a critical parameter. Too thin an underlay fails to adequately buffer the base metal, while too thick an underlay wastes material and increases cost without significant benefit. A thickness of 2-3 mm is generally optimal for most applications, providing sufficient dilution control and crack resistance.

Engineering Practice Insights

The ash lock valve application highlights the often-overlooked importance of the underlay layer in cladding systems. Many practitioners focus on the overlay material selection while neglecting the underlay, which is equally critical to overall performance. The underlay material must be selected based on the specific combination of base metal, overlay material, and operating conditions.

A practical approach to underlay selection is to consider the carbon equivalent of the base metal and the dilution expected from the overlay. If the base metal has a high CE (above 0.6%), a ductile, low-carbon underlay is essential to prevent cracking. If the base metal has a low CE (below 0.4%), a more austenitic or nickel-based underlay may be appropriate to maximize dilution control.

The ash lock valve sealing surface also requires attention to geometric tolerances. The cladding process introduces thermal distortion, and the final machining must compensate for this. The welding sequence should be planned to minimize distortion, with balanced welding from both sides of the valve body where possible.

Study Reflections and Conclusions

This literature provides valuable guidance on underlay material selection for cladding applications in metallurgical equipment. The key insight is that the underlay is not merely a sacrificial layer but a critical engineering component that determines the metallurgical compatibility, crack resistance, and overall durability of the cladding system. Engineers should approach underlay selection as a systematic engineering decision, considering the base metal properties, overlay material characteristics, operating environment, and cost constraints. The ash lock valve case demonstrates that even in relatively simple applications, careful underlay selection and process optimization can significantly extend service life and reduce maintenance costs.