Manufacturing Process of Gasifier Shell with S31603 Inner Wall Overlay Welding

Literature Overview

This paper by Wang Puxuan, published in Shanxi Chemical Engineering (2017, Vol. 37, No. 2, pp. 100-103), describes the manufacturing process for a gasifier shell requiring S31603 (equivalent to UNS S31603, a low-carbon 316L austenitic stainless steel) overlay welding on the inner wall. The author works at Shanxi Fengxi Chemical Equipment Co., Ltd., a manufacturer specializing in large-scale chemical process vessels. The paper addresses the significant engineering challenge of fabricating a large-diameter pressure vessel that combines a carbon steel or low-alloy steel outer shell with a corrosion-resistant austenitic stainless steel overlay layer on the interior surface exposed to harsh gasification syngas environments.

Core Technical Content

The gasifier shell is subjected to extreme conditions including high temperature (typically 1300-1500 °C at the coal injection zone), high pressure (6.5-8.0 MPa), and severe corrosion from hydrogen sulfide, carbon monoxide, carbon dioxide, and trace halides in the syngas. The S31603 overlay provides resistance to intergranular corrosion and pitting corrosion in this environment while the carbon steel shell provides structural strength at lower cost.

The manufacturing strategy involves dividing the large shell into multiple pre-welded segmental assemblies (group weldments) before final circumferential and longitudinal joint assembly. This modular approach reduces the overall welding volume in a single continuous operation, thereby minimizing cumulative distortion and residual stress buildup.

Key Manufacturing Sequence

1. Material preparation and cutting: Shell plates are cut with CNC flame or plasma cutting, followed by edge preparation for both the base weld and the overlay weld.
2. Segmental assembly: Individual shell panels are tacked and weld-fitted into segmental blocks (typically 30°-90° arc segments).
3. Base weld deposition: Root, fill, and cap welds are deposited on the carbon steel shell using matching low-alloy steel consumables (e.g., E7018 electrodes or ER80S-G flux-cored wire).
4. S31603 overlay welding: Multiple passes of austenitic stainless steel overlay are applied on the inner surface, typically requiring 2-3 layers with total thickness of 6-10 mm.
5. Heat treatment: Post-weld stress relief (PWSR) is performed at 580-620 °C for the base metal welds, while the overlay layer requires careful thermal cycling to avoid sensitization.
6. Final assembly: Segmental assemblies are joined into the complete shell with circumferential welds, followed by final overlay repair on the weld cap.

Overlay Welding Process Parameters

Distortion Control Strategies

The paper emphasizes several distortion control measures that are critical for maintaining dimensional accuracy in large-diameter gasifier shells:

• Symmetric welding sequence: Welding is performed in a symmetric pattern around the shell circumference to balance thermal contraction forces.
• Mechanical clamping and restraining: Heavy-duty clamps and backing bars are used to restrain radial and axial movement during welding.
• Multi-pass with alternating direction: Each overlay pass is alternated in direction to cancel out directional distortion.
• Mechanical machining before and after welding: The paper notes extensive use of CNC machining to bring components within tolerance before final welding, reducing the correction welding volume and associated distortion.
• Thermal cutting allowance: Cutting templates include generous machining allowances (typically 3-5 mm) to accommodate post-weld dimensional changes.

Heat Treatment Considerations

The heat treatment of the combined carbon steel shell with S31603 overlay presents a unique challenge. The austenitic stainless steel overlay must not be held in the sensitization temperature range (450-850 °C) for extended periods, as this would precipitate chromium carbides at grain boundaries and cause intergranular corrosion. However, the carbon steel base metal welds require stress relief at 580-620 °C.

The solution adopted is:

• Perform PWSR for the entire vessel at 580-620 °C with a holding time of no more than 2 hours per 25 mm of wall thickness.
• Use a rapid heating rate (≤ 100 °C/h) and controlled cooling rate to minimize the time spent in the sensitization range.
• Alternatively, apply the overlay after PWSR of the base welds, using a lower interpass temperature and accepting slightly higher residual stress in the overlay.

Engineering Practice Insights

From a quality control perspective, the following inspection requirements are essential:

A critical concern in this application is the dilution ratio between the carbon steel base metal and the S31603 overlay. Excessive dilution (> 30%) can reduce the overlay's corrosion resistance by lowering the chromium and molybdenum content below the required minimum. The paper's approach of using multiple thin overlay passes with low heat input helps maintain dilution below 20%.

Key Reflections

The manufacturing philosophy presented in this paper reflects a mature engineering approach where process planning is driven by the final quality requirements rather than being a simple sequence of welding operations. The decision to divide the shell into segmental assemblies is particularly noteworthy, as it transforms an intractable large-scale welding problem into manageable smaller operations. This modular approach also facilitates parallel manufacturing, reducing overall project duration.

The emphasis on mechanical machining as a distortion control measure is pragmatic. In large vessel fabrication, achieving tight dimensional tolerances through welding alone is extremely difficult and costly. By accepting moderate welding distortion and correcting it through machining, the manufacturer achieves a better balance between welding quality and dimensional accuracy.