Manufacturing Process for Inner Wall S31603 Weld Overlay on Gasifier Shell

Literature Overview

Gasifiers are critical components in coal-to-chemical and coal-to-gas conversion processes, where they operate at extreme temperatures (1300–1600 °C) and pressures (2.0–8.0 MPa) with highly corrosive slag and gas environments. The inner wall of the gasifier shell is subjected to direct contact with molten slag and hot gas, necessitating a corrosion-resistant overlay layer. This study note focuses on the manufacturing process for S31603 (316L equivalent) inner wall weld overlay on gasifier shells, examining the challenges of large-diameter cylindrical geometry, high-temperature service, and the stringent quality requirements.

Gasifier Operating Environment

The inner wall of a gasifier experiences a complex combination of thermal, mechanical, and chemical stresses:

Stress Type	Magnitude	Effect on Overlay
Thermal cycling	25–1400 °C	Thermal fatigue cracking
Internal pressure	2.0–8.0 MPa	Hoop stress, 150–350 MPa
Slag erosion	Continuous	Material loss, surface degradation
Corrosive gas	H2S, HCl, HF, CO2	Chemical attack, pitting
Slag adhesion	High-viscosity slag	Mechanical damage during cleaning

The S31603 overlay is selected for its excellent resistance to pitting and crevice corrosion due to its molybdenum content (2–3%), combined with the low carbon content (≤0.03%) that minimizes sensitization and intergranular corrosion. However, S31603 has a relatively low melting point (1375–1400 °C) compared to the gasifier operating temperature, which means the overlay layer operates near its melting point during peak conditions, requiring careful design of the thermal barrier.

Base Material and Welding Challenges

The gasifier shell is typically fabricated from 15CrMoR or 12Cr1MoVR, which are low-alloy steels with good high-temperature strength but limited corrosion resistance. The welding of S31603 overlay onto these base materials presents several challenges:

Dilution: The high carbon and alloy content of the base metal dilutes the overlay, reducing corrosion resistance
Cracking susceptibility: The high thermal expansion mismatch between austenitic overlay and ferritic base creates residual stresses
Geometry: The large diameter (typically 4000–6000 mm) and thick wall (60–100 mm) create significant thermal gradients
Access: Internal welding requires specialized equipment and positioning

Process Design

Pre-Weld Preparation

The base surface must be thoroughly prepared to ensure good fusion:

Machining of the base surface to remove scale, rust, and previous welds (depth 3–5 mm)
Grinding to a smooth finish with Ra ≤ 6.3 μm
Cleaning with acetone or similar solvent to remove contaminants
Preheat to 150–200 °C using induction heating or electric blankets

Welding Process Selection

For the inner wall overlay of a gasifier shell, the following processes are evaluated:

Process	Deposition Rate	Quality	Cost	Applicability
SAW (Submerged Arc)	Very High	Good	Low	Bulk deposition, flat sections
GMAW (MIG)	Moderate	Good	Moderate	Complex geometry, transition layers
PTA (Plasma Transfer Arc)	Low	Excellent	High	Final layer, precision overlay
ESW (Electroslag)	Very High	Good	Moderate	Very thick overlay, large sections

A multi-layer approach is typically employed:

Transition layer: 2–3 mm of 309L or 310S using GMAW to buffer dilution
Bulk layer: 5–8 mm of 316L using SAW for high efficiency
Final layer: 2–3 mm of S31603 using PTA for maximum corrosion resistance

Parameter Control

Layer	Process	Current	Voltage	Speed	Wire/Powder
1 (Transition)	GMAW	200–250 A	26–30 V	100–150 mm/min	ER309L, 1.2 mm
2 (Bulk)	SAW	500–600 A	35–40 V	180–220 mm/min	ER316L, 2.0 mm
3 (Final)	PTA	250–300 A	20–24 V	80–120 mm/min	S31603 powder

Thermal Management

The thermal management of the gasifier shell overlay is critical due to the large thermal mass and the risk of distortion:

Preheat: 150–200 °C for the base material, applied uniformly using induction heating
Interpass temperature: 150–250 °C, monitored with embedded thermocouples
Weld sequencing: Symmetrical welding pattern to minimize distortion
Post-weld cooling: Controlled cooling rate of 50–100 °C/hour using insulation blankets

The residual stress distribution in the overlay layer is a concern, as tensile stresses can promote cracking during service. Post-weld heat treatment (PWHT) at 620–650 °C for 2 hours per 25 mm of wall thickness is recommended to relieve residual stresses. However, PWHT must be carefully controlled to avoid sensitization of the austenitic overlay, which would reduce corrosion resistance.

Defect Prevention and Detection

Common defects in gasifier shell overlays and their prevention:

Defect	Cause	Prevention	Detection
Cracking in overlay	High residual stress, hydrogen	PWHT, low-hydrogen consumables	MT, UT
Poor fusion	Inadequate cleaning, low heat input	Thorough surface prep, parameter optimization	UT, PT
Porosity	Contaminated base, moisture	Clean base, dry consumables	RT, UT
Excessive dilution	High base metal mixing	Transition layer, parameter control	Metallography
Distortion	Uneven heating, large thermal mass	Symmetrical welding, preheat, cooling control	Visual, dimensional

Engineering Practice Case

A gasifier shell with diameter 5200 mm and wall thickness 80 mm required an 8 mm S31603 inner wall overlay. The base material was 15CrMoR. The manufacturing process involved:

Internal machining of 4 mm from the base surface
Preheat to 180 °C using induction heating
GMAW transition layer of 2 mm using ER309L
SAW bulk layer of 5 mm using ER316L with flux
PTA final layer of 1 mm using S31603 powder
PWHT at 640 °C for 6 hours
Full UT inspection of the overlay

The resulting overlay had a dilution of 12% in the final layer, hardness of 180–200 HV, and excellent corrosion resistance in simulated gasifier slag environment. The shell passed all inspection requirements and was commissioned successfully.

Study Insights

The manufacturing of S31603 inner wall overlays on gasifier shells requires a holistic approach that integrates material selection, process design, thermal management, and quality control. The transition layer is not merely an optional step but a critical requirement for ensuring adequate corrosion resistance in the final overlay. Engineers must carefully balance deposition efficiency with quality requirements, recognizing that the final PTA layer, while expensive, is essential for achieving the required corrosion performance. The experience from this literature emphasizes the importance of process qualification and the need for detailed weld mapping to ensure traceability and consistent quality across the entire shell circumference.