Internal Wall Cladding Technology for Thick-Walled Pressure Vessels

Literature Overview and Research Background

Thick-walled pressure vessels — such as hydrogenation reactors, polymerization reactors, and high-pressure separators — often require internal corrosion-resistant cladding to protect expensive alloy construction materials from aggressive process media. The cladding is applied to the internal surface of vessels with wall thicknesses typically ranging from 50 mm to 300 mm, presenting unique challenges related to heat input management, distortion control, residual stress, and inspection accessibility. This study examines the technology, methodology, and quality assurance requirements for internal wall cladding of thick-walled pressure vessels, covering process selection, parameter optimization, and post-weld inspection strategies.

Core Technical Findings

Process Selection for Thick-Walled Internal Cladding

Process	Typical Wall Thickness Range	Cladding Thickness	Advantages	Limitations
Strip cladding (strip surfacing)	30–150 mm	2–6 mm	High deposition rate; good flatness	Limited to accessible surfaces; equipment size
Submerged arc welding (SAW) overlay	20–200 mm	3–10 mm	High deposition rate; low dilution	Requires flux handling inside vessel
Electroslag welding (ESW) overlay	50–400 mm	5–25 mm	Very high deposition rate; good for thick layers	Requires vertical orientation; complex setup
Flux-cored arc welding (FCAW) overlay	10–100 mm	2–5 mm	Flexible; good for complex geometries	Lower deposition rate; higher fume
Laser cladding	10–80 mm	1–5 mm	Low heat input; precise control	Low deposition rate; expensive equipment
TIG overlay (multi-pass)	5–50 mm	1–3 mm	Low dilution; fine control	Very low deposition rate

For thick-walled vessels requiring 5–15 mm of overlay, a hybrid approach combining strip cladding for the bulk deposition followed by TIG or laser cladding for surface finishing is often optimal.

Heat Input and Thermal Management

The thick base material acts as a heat sink, which can be advantageous (reducing distortion) but also challenging (requiring higher heat input for adequate fusion):

Base Wall Thickness	Recommended Heat Input	Preheat Temperature	Interpass Temperature
50–80 mm	0.5–0.8 kJ/mm	100–150°C	≤200°C
80–150 mm	0.8–1.2 kJ/mm	150–250°C	≤250°C
150–250 mm	1.0–1.5 kJ/mm	200–300°C	≤300°C
250–350 mm	1.2–1.8 kJ/mm	250–350°C	≤300°C

The thick base material also creates a significant thermal gradient through the wall thickness, which can lead to:

Transverse thermal stresses that may cause cracking in the overlay layer.
Differential cooling rates between the surface (rapid cooling) and interior (slow cooling).
Potential for hydrogen-induced cracking in the heat-affected zone of low-alloy steel bases.

Dilution and Metallurgical Compatibility

Dilution is a critical parameter in internal cladding because it directly affects the corrosion resistance of the overlay:

Overlay Material	Base Material	Acceptable Dilution	Maximum Dilution
304L SS	Q345R	≤20%	≤25%
316L SS	Q345R	≤15%	≤20%
321 SS	15CrMoR	≤15%	≤20%
Inconel 625	15CrMoR	≤15%	≤20%
Hastelloy C276	15CrMoR	≤10%	≤15%
Monel 400	Q345R	≤15%	≤20%

Low-dilution processes (laser cladding, TIG with minimal travel speed, SAW with high travel speed) are preferred for critical corrosion service where dilution must be minimized.

Process Implementation Methodology

Pre-Welding Preparation

Surface preparation: The internal surface must be cleaned to SA 2.5 grade (near-white metal blast) or equivalent to ensure proper bonding. Residual scale, rust, or oil contamination significantly reduces bond strength.
Geometry assessment: For vessels with internal nozzles, supports, or other attachments, the cladding sequence must be planned to avoid entrapment of flux or shielding gas.
Access planning: For large vessels, internal access for welders and equipment must be planned. This includes:

Scaffolding or mobile platforms.
Lighting (minimum 200 lux at work surface).
Ventilation (mechanical extraction for flux fumes).
Equipment positioning (strip cladding machines, SAW equipment).

Thermal simulation: For thick-walled vessels, a thermal simulation should be performed to predict:

Peak temperatures at various depths.
Cooling rates (particularly for HAZ hydrogen cracking assessment).
Residual stress distribution.
Distortion magnitude and