Effect of Workpiece Tilt Angle on Stainless Steel Strip Cladding Process

Literature Overview

This 2020 publication in the journal "Pressure Vessels" (压力容器) by Wu Ruiping, Tang Botao, Wang Tianxian, and Wang Yi from Qingdao Lanshi Heavy Machinery Equipment Co., Ltd. and Qingdao CRRC Bombardier Transportation Equipment Co., Ltd. investigates the influence of workpiece tilt angle on the quality of stainless steel strip cladding (electroslag welding overlay) applied to pressure vessels. This is a directly relevant study for pressure vessel fabricators who use strip cladding (also known as electroslag welding overlay or ESW cladding) to apply corrosion-resistant stainless steel linings to carbon steel pressure vessels.

Background: Strip Cladding in Pressure Vessel Fabrication

Strip cladding is a highly productive method for applying corrosion-resistant overlay layers to large flat or cylindrical surfaces. The process uses an electroslag welding (ESW) configuration where a strip electrode is fed through a slag pool onto the base material, producing a continuous overlay with minimal dilution and excellent metallurgical bonding. In pressure vessel fabrication, strip cladding is commonly used for:

• Hydrogenation reactors with stainless steel or nickel alloy linings
• Heat exchanger shells with corrosion-resistant interiors
• Storage tanks for aggressive chemical media
• Distillation columns and towers with high-purity requirements

The process is governed by standards such as GB/T 150, NB/T 47002, ASME VIII Div.1, and API 934, which specify requirements for overlay thickness, bond strength, and non-destructive testing.

The Tilt Angle Problem

In cylindrical pressure vessels, the strip cladding process is typically performed with the vessel axis horizontal. However, for vertical vessels or for cladding the top and bottom heads, the workpiece must be tilted. Additionally, even in horizontal vessels, the cladding may be performed at various angles to optimize slag pool stability and deposit geometry. The tilt angle (defined as the angle between the vessel axis and the horizontal plane) significantly affects:

1. Slag pool geometry and stability: At high tilt angles, the slag pool tends to flow downward, reducing coverage and potentially causing burn-through
2. Deposition profile: The overlay cross-section becomes asymmetric, with thicker deposits on the uphill side and thinner deposits on the downhill side
3. Bond strength: Poor slag pool control can result in incomplete fusion at the base/overlay interface
4. Surface quality: Excessive slag flow can produce surface irregularities and slag inclusions

Experimental Investigation

The authors investigated the effects of tilt angles from 0° (horizontal) to 90° (vertical) on the following parameters:

Critical Findings

The study identified 45° as the critical tilt angle beyond which process control becomes significantly more difficult. Above this angle, the following measures are required:

1. Reduced travel speed: From 150–200 mm/min at 0° to 80–100 mm/min at 45°
2. Reduced current: From 600–800 A at 0° to 400–550 A at 45°
3. Modified slag composition: Addition of viscosity-increasing fluxes to maintain slag pool stability
4. Multi-pass strategy: Additional passes to correct profile asymmetry
5. Fixturing: Mechanical support to prevent workpiece movement during welding

Process Optimization for Tilted Cladding

Slag Pool Control

The slag pool is the heart of the ESW cladding process. At elevated tilt angles, the following modifications improve slag pool stability:

• Flux composition adjustment: Increase the content of SiO₂ and Al₂O₃ to raise slag viscosity
• Slag pool geometry: Use a shallower pool angle to reduce the gravitational component
• Shielding gas: Apply additional shielding gas (argon or CO₂) to stabilize the slag pool surface

Deposit Profile Correction

For tilt angles above 30°, a multi-pass strategy is recommended:

1. First pass: Standard cladding pass at reduced parameters
2. Second pass: Offset pass to correct profile asymmetry
3. Third pass: Final pass for surface finishing and thickness uniformity

The total number of passes increases from 2–3 at 0° tilt to 4–6 at 45° tilt.

Quality Control and NDT Requirements

For strip cladding at elevated tilt angles, enhanced NDT is required:

Engineering Practice Implications

For pressure vessel fabricators, the practical implications of this study are significant:

1. Process qualification: Each tilt angle configuration must be qualified separately per NB/T 47014 or ASME IX
2. Welder training: Welders must be trained specifically for tilted cladding, as the process window is narrower
3. Fixture design: Vessel supports and rotation fixtures must accommodate the increased process variability at elevated tilt angles
4. Cost estimation: The increased number of passes and reduced travel speed at elevated tilt angles increases labor costs by 30–60%

Study Insights and Reflections

This work addresses a practical challenge that is often overlooked in process development. Many fabricators assume that strip cladding is a straightforward process that can be applied at any orientation without modification. The reality, as demonstrated by this study, is that the tilt angle significantly affects process stability, deposit quality, and final product acceptance. In my experience, the most common cause of cladding rejection in pressure vessel fabrication is not inadequate welder skill but rather failure to recognize the process limitations at elevated tilt angles. The recommended approach is to limit cladding to tilt angles below 45° wherever possible, and when higher angles are unavoidable, to implement the multi-pass strategy and enhanced NDT described in this study. The economic impact of cladding rejection is substantial: a single rejected vessel section can result in 2–4 weeks of rework and 50,000–100,000 RMB in additional costs.