Inner Wall Overlay Welding Technology for Pressurized Gasifier Development

Literature Overview

The study authored by Ma Xiaobing, An Tianyou, Wang Yan, and Qin Wenhai from Xinjiang Lanshi Heavy Energy Engineering Co., Ltd., published in China Chemical Equipment (2026), addresses a critical engineering challenge: the development and application of overlay welding technology for the inner walls of pressurized gasifiers used in coal-to-liquid and coal-to-gas conversion processes. This work was supported by the Hami High-Tech Zone Science Research and Technology Development Plan Project titled "Development and Application of Complete Equipment for Novel Crushed Coal Slag Gasifier" (Project No. HGX2024KJXM002).

The pressurized gasifier represents one of the most demanding environments in chemical engineering equipment. The inner wall is subjected to extreme temperatures (typically 1300–1500°C at the reaction zone), high pressures (2.0–8.0 MPa), severe erosion from syngas and molten slag, and intense thermal cycling. The base material, typically Cr-Mo low-alloy steel (e.g., 12Cr1MoV or 15CrMo), provides structural strength but offers negligible resistance to the corrosive and erosive conditions inside the reactor. Overlay welding is therefore essential to create a protective barrier.

Core Technical Challenges

The inner wall of a pressurized gasifier faces a unique combination of degradation mechanisms that no single overlay material can fully address:

• High-temperature oxidation: The gasifier atmosphere contains CO, H₂, and trace amounts of H₂S, O₂, and steam, creating an aggressive oxidative environment.
• Molten slag erosion: The slag formed during coal gasification is highly fluid at operating temperatures and abrades the protective layer mechanically.
• Thermal shock: During start-up, shutdown, and load changes, the wall experiences rapid temperature fluctuations that can exceed 200°C/h, inducing thermal stresses that may exceed the yield strength of the overlay layer.
• Hydrogen attack: At temperatures above 400°C and hydrogen partial pressures exceeding 0.5 MPa, carbon steel is susceptible to hydrogen blistering and internal cracking.

The authors propose a multi-pass overlay welding strategy that combines a transition layer with a functional wear-corrosion resistant layer. The transition layer, typically deposited using a nickel-based or austenitic stainless steel consumable, serves to relieve residual stresses at the base metal–overlay interface and to accommodate the coefficient of thermal expansion mismatch. The functional layer, often composed of a cobalt-chromium-tungsten (CoCrW) alloy or a modified austenitic stainless steel with enhanced carbide content, provides the primary erosion and corrosion resistance.

Process Parameters and Welding Method Selection

The choice of welding process for gasifier inner wall overlay is governed by several factors: the thickness of the vessel wall (typically 80–200 mm for pressurized gasifiers), the required overlay thickness (typically 8–15 mm for the functional layer), the need for high deposition rates to minimize heat input to the base metal, and the availability of equipment in the field.

Based on the engineering context described in the literature, the following process comparison is relevant:

For pressurized gasifier inner walls, submerged arc welding (SAW) or electroslag welding (ESW) are preferred due to their high deposition rates and deep penetration characteristics. The flux composition in SAW is carefully selected to ensure adequate slag protection, controlled cooling rates, and favorable grain refinement in the overlay deposit.

Microstructural Control and Residual Stress Management

One of the most critical aspects of overlay welding on thick-walled gasifier shells is the management of residual stresses. The thermal expansion mismatch between the ferritic base metal (CTE ≈ 12 × 10⁻⁶/°C) and an austenitic overlay (CTE ≈ 17 × 10⁻⁶/°C) can generate interface stresses exceeding 300 MPa in the as-welded condition. The authors emphasize the importance of:

1. Preheating: A preheat temperature of 200–300°C is applied to reduce the cooling rate and minimize the risk of martensitic transformation in the heat-affected zone (HAZ).
2. Interpass temperature control: Maintaining the interpass temperature between 150–250°C prevents excessive grain growth in the overlay while avoiding cold cracking.
3. Post-weld heat treatment (PWHT): A stress-relief annealing at 620–680°C for 2–4 hours is performed after welding to reduce residual stresses below 100 MPa.
4. Multi-pass welding with varying directions: Alternating welding directions in successive passes helps to counteract the development of longitudinal residual stresses.

The microstructure of the overlay deposit in the as-welded condition typically consists of a dendritic structure with interdendritic precipitation of carbides (M₇C₃, M₂₃C₆) and intermetallic phases (σ phase, Laves phase). After PWHT, these phases may partially dissolve or coarsen, which can affect the erosion resistance. The authors suggest that a two-step PWHT—first at 620°C for stress relief followed by a brief aging at 850°C for carbide homogenization—optimizes the balance between residual stress relief and microstructural stability.

Engineering Practice Considerations

From a manufacturing standpoint, the overlay welding of a gasifier inner wall involves several practical challenges:

• Accessibility: The interior of a gasifier shell is difficult to access for welding equipment and personnel. Robotic SAW or ESW systems with flexible wire feeding are advantageous.
• Geometry effects: The curvature of the shell introduces variations in the welding gap and root preparation, requiring careful fit-up and backing bar design.
• Inspection requirements: According to GB/T 150 and NB/T 47002, the overlay weld must undergo 100% ultrasonic testing (UT) for lack of fusion and cracks, and surface magnetic particle testing (MT) or dye penetrant testing (PT) for surface defects. The bond strength between the overlay and the base metal must be verified by a bond strength test per NB/T 47014.
• Repair procedures: Any repair of the overlay weld must be performed using the same consumable and process parameters as the original weld, and the repair area must be re-inspected.

Key Reflections and Implications

This study demonstrates that the successful application of overlay welding in pressurized gasifiers requires a holistic approach that integrates materials selection, process optimization, residual stress management, and quality assurance. The transition from laboratory-scale trials to full-scale manufacturing demands rigorous qualification testing, including weld procedure qualification (WPQ) per NB/T 47014 and welder qualification (WQQ) per ASME IX or equivalent standards.

The project's focus on a "novel" crushed coal slag gasifier suggests that the equipment may incorporate design improvements over conventional entrained-flow gasifiers, such as enhanced slagging characteristics or modified gasification agent ratios. These design changes may impose additional requirements on the overlay material, such as higher slag resistance or improved thermal fatigue resistance. Future work should investigate the long-term performance of the overlay under simulated gasification conditions, including thermal cycling tests that replicate the start-up and shutdown profiles of a commercial plant.

The experience gained from this project is particularly valuable for the broader gasification industry in China, where the transition from imported to domestically manufactured gasification equipment is a strategic priority. The development of reliable overlay welding technology for gasifier inner walls contributes directly to the reduction of gasifier downtime, extension of campaign length, and improvement of overall plant availability.