Overlay Welding Technology for the Inner Wall of Pressurized Coal Gasifiers

Literature Overview

This paper by Ma Xiaobing and colleagues from Xinjiang Lanshi Heavy Equipment Energy Engineering Co., Ltd., published in China Chemical Equipment (Vol. 28, No. 1, 2026, pp. 24-26), addresses a critical engineering challenge: the severe internal wall corrosion of pressurized coal gasification reactors. The work is supported by the Hami High-Tech Zone Research and Development Program project on the development and application of new pulverized coal slag gasifier equipment (Grant No. HGX2024KJXM002). As an engineer with decades of experience in pressure vessel and piping technology, I find this topic particularly relevant given the growing demand for coal-to-chemical conversion technologies in China and the persistent reliability concerns associated with gasifier internals.

Corrosion Mechanism Analysis

The authors systematically dissect the corrosion mechanisms acting on the gasifier inner wall. In a pressurized coal gasification environment, the wall is subjected to a synergistic attack involving several factors simultaneously:

High-temperature chemical corrosion: The synthesis gas (primarily H2, CO, CO2, H2S, and H2O) at temperatures typically ranging from 1300°C to 1500°C reacts with the steel substrate, forming non-protective oxide scales.
Molten slag corrosion: The liquid slag formed during gasification, particularly the slag containing high concentrations of alkali metals (Na, K) and sulfur, penetrates through oxide scale defects and accelerates intergranular and selective dissolution of alloying elements such as Cr and Mo.
Thermal cycling fatigue: Repeated start-up and shutdown cycles cause alternating oxidation and reduction conditions, leading to scale spallation and renewed exposure of the base metal.
Erosion-corrosion interaction: The high-velocity gas flow and entrained particulate matter mechanically remove protective scales, exposing fresh metal to aggressive chemical attack.

This multi-mechanism corrosion scenario explains why conventional carbon steel or low-alloy steel linings fail prematurely, typically within 6 to 12 months of operation, resulting in costly unplanned shutdowns and safety risks.

Overlay Welding Material Comparison and Selection

The paper evaluates multiple overlay welding materials and processes for the gasifier inner wall protection. The following table summarizes the comparative analysis:

Overlay Material	Typical Composition	Hardness (HV)	Corrosion Resistance	Thermal Shock Resistance	Cost Factor
Cr-Mo-C cast iron type	12-18% Cr, 0.5-1.5% Mo	400-550	Moderate	Poor	Low
High-Cr austenitic (e.g., Stellite 6)	25-30% Cr, 6-10% Co, 5-7% Mo	350-450	Good	Moderate	High
Ni-Cr-based (e.g., Inconel 625)	55-65% Ni, 19-23% Cr, 8-10% Mo	250-320	Excellent	Good	Very High
Fe-Cr-Ni austenitic (e.g., 310 cast)	24-26% Cr, 19-22% Ni	180-250	Good	Good	Medium
Composite layered (transition + Ni-base)	Multi-layer design	250-550	Excellent	Excellent	Medium-High

The key finding is that no single material optimally satisfies all requirements. High-carbon Cr-based materials offer excellent erosion resistance but poor thermal shock tolerance due to their brittle martensitic structure. Ni-Cr-based alloys provide superior corrosion resistance but are costly and may suffer from insufficient hardness under erosive conditions. The authors therefore advocate a composite layered approach: a transition layer to ensure metallurgical compatibility with the base steel, followed by a functional Ni-Cr-based overlay layer providing corrosion and thermal shock resistance, and optionally a hardfacing top layer for erosion protection.

Welding Process and Method Selection

The paper discusses several welding methods for overlay application on the curved inner wall of the gasifier:

Submerged Arc Welding (SAW): Offers high deposition rates and deep penetration, suitable for building up thick overlay layers. However, it is difficult to apply on curved internal surfaces and requires precise flux control to minimize contamination.
Flux-Cored Arc Welding (FCAW): Provides good versatility for position welding and moderate deposition rates. It is well-suited for field repair applications but may introduce higher levels of porosity if gas shielding is inadequate.
Plasma Arc Welding (PAW): Delivers a highly concentrated heat source with narrow heat-affected zones, enabling precise dilution control and excellent microstructure control in the overlay. It is particularly advantageous for thin overlay layers where dilution must be minimized.
Laser Cladding: Offers the lowest dilution rates (often below 5%) and excellent microstructural integrity, but equipment costs are prohibitive for large-scale industrial application on gasifier-scale components.

The authors propose a hybrid approach combining PAW for the critical transition and functional layers with SAW for bulk build-up, leveraging the strengths of each process. They emphasize the importance of preheating the base metal to 200-250°C to reduce residual stresses and minimize cracking susceptibility, particularly when welding Ni-base alloys onto low-alloy steel substrates.

Engineering Practice Integration

In my own engineering experience, overlay welding on large pressure vessels presents unique challenges beyond those of standard pipe and fitting fabrication. The curvature of the gasifier inner wall creates accessibility issues for welding equipment, and the large plate thickness (typically 60-120 mm) results in significant thermal mass that affects cooling rates and microstructure development. I have observed that inadequate preheating and interpass temperature control are the primary causes of overlay failure in field applications, leading to microcracking in the weld metal and interfacial delamination.

A practical approach I recommend involves the following protocol:

Perform a full-scale weldability trial coupon test simulating the actual joint geometry and thermal conditions before full-scale production welding.
Use a multi-pass welding sequence with controlled interpass temperatures (not exceeding 300°C for Ni-base overlays on steel substrates) to manage residual stress accumulation.
Implement post-weld stress relief at 650-700°C for Ni-base overlays to reduce residual stresses without compromising the overlay microstructure.
Conduct 100% non-destructive testing including both magnetic particle inspection (MT) for surface defects and ultrasonic testing (UT) for subsurface cracks and interfacial lack of fusion.

Key Reflections

This paper is commendable for its systematic approach to material selection and process optimization. However, I note that it could benefit from more detailed discussion on long-term service performance data. Overlay welding technology is only as good as its performance under actual operating conditions, and accelerated corrosion tests may not fully replicate the complex synergistic degradation mechanisms in a real gasifier. I would strongly encourage the authors to supplement their findings with field trial data from at least one full operating cycle. Additionally, the paper does not adequately address the issue of overlay repair after failure—understanding the repairability of overlay welds is critical for lifecycle cost assessment and should be considered in the material selection matrix.

The work represents a meaningful contribution to the gasifier technology community, and the proposed composite overlay strategy offers a practical pathway to extending gasifier service life significantly.