CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Technology Research on the Inner Wall of Pressurized Gasifiers
Literature Overview
This paper examines the weld overlay technology applied to the inner wall of pressurized gasifiers, which are critical components in coal-to-liquid and coal-to-gas conversion processes. The inner wall of a pressurized gasifier is subjected to extreme conditions: temperatures exceeding 1200 °C, high concentrations of hydrogen and carbon monoxide, and aggressive chemical attack from molten slag and syngas. The overlay layer must provide corrosion resistance, thermal stability, and mechanical integrity under these severe conditions, making the selection of overlay material, welding process, and quality control strategy a matter of paramount engineering importance.
The study focuses on the application of nickel-based alloy overlay, specifically Inconel 625 (UNS N06625) and Incoloy 800H (UNS N08810), onto a low-alloy steel substrate (typically 15CrMoR or 12Cr1MoV per GB/T 150). The welding processes evaluated include submerged arc welding (SAW) with flux-cored wire, plasma transferred arc (PTA) cladding, and laser cladding, each with distinct advantages and limitations for this application.
Material Selection and Overlay System Design
The selection of the overlay material is governed by the operating environment of the gasifier. The following table summarizes the key material properties and their relevance to gasifier service:
| Property | Inconel 625 | Incoloy 800H | 15CrMoR Base |
|---|---|---|---|
| Cr content (wt%) | 20–23 | 19–23 | 0.8–1.1 |
| Ni content (wt%) | 52–58 | 27–33 | — |
| Mo content (wt%) | 8–10 | — | — |
| C content (wt%) | ≤0.10 | ≤0.10 | ≤0.17 |
| Thermal conductivity (W/m·K) | 11.4 | 26.1 | 33.5 |
| Thermal expansion (μm/m·K, 20–800 °C) | 13.2 | 16.5 | 14.5 |
| Recommended overlay thickness (mm) | 6–10 | 8–12 | — |
The dilution issue is the central challenge in this application. When overlaying nickel-based alloys onto low-alloy steel using SAW, the dilution ratio can reach 40–60% in the first pass, which significantly reduces the chromium and nickel content in the overlay layer and compromises its corrosion resistance. The study demonstrates that a multi-pass strategy with a transition layer (typically a nickel-iron alloy such as Stellite 6 or a 309L stainless steel) between the base and the final overlay layer can reduce the dilution in the top layer to below 20%, ensuring that the overlay meets the required corrosion resistance criteria.
Welding Process Comparison and Process Parameters
The study compares three welding processes for the gasifier inner wall overlay application:
| Process | SAW (Flux-Cored Wire) | PTA Cladding | Laser Cladding |
|---|---|---|---|
| Deposition rate | High (5–10 kg/h) | Moderate (2–5 kg/h) | Low (0.5–2 kg/h) |
| Dilution ratio | 30–60% | 15–35% | 5–15% |
| Heat input | High | Moderate | Low |
| Distortion control | Difficult | Moderate | Excellent |
| Equipment cost | Low | Moderate | High |
| Suitable for large areas | Yes | Limited | No |
| Overlay thickness per pass | 2–4 mm | 1.5–3 mm | 0.5–1.5 mm |
The study concludes that a hybrid approach combining SAW for the bulk of the overlay and PTA for the final functional layer offers the best balance of productivity, cost, and performance. The SAW process is used for the first 4–6 passes to build up the bulk of the overlay layer, followed by 2–3 PTA passes to deposit the final corrosion-resistant layer with controlled dilution and fine microstructure.
The process parameters for the SAW overlay include a current range of 400–600 A, voltage of 30–38 V, travel speed of 200–350 mm/min, and a flux composition optimized for low hydrogen content and good slag fluidity. The interpass temperature is strictly controlled at 100–150 °C to minimize the risk of cold cracking and to maintain the required microstructural refinement.
Quality Control and Inspection Strategy
Given the criticality of the gasifier inner wall overlay, the quality control strategy must be comprehensive and aligned with applicable standards such as GB/T 150, NB/T 47002, and ASME VIII Div.2. The inspection protocol includes:
- Visual inspection (VT) of the entire overlay surface for undercut, porosity, and surface defects.
- Ultrasonic testing (UT) of the overlay layer for internal defects, including lack of fusion, slag inclusion, and cracking. The testing is performed in accordance with NB/T 47013 using phased array ultrasonic testing (PAUT) for enhanced sensitivity to planar defects.
- Radiographic testing (RT) of a representative number of welds for volumetric defects, with acceptance criteria per GB/T 3323.
- Magnetic particle testing (MT) of the overlay surface for surface-breaking cracks, particularly in areas of high residual stress such as weld starts and stops.
- Hardness testing across the overlay cross-section to verify the hardness gradient and detect any regions of excessive dilution.
- Spectroscopic analysis (OES or XRF) of the overlay surface to confirm the chemical composition and dilution level.
- Bend testing of coupon specimens to verify the bond strength between the overlay layer and the base metal.
The study reports that the bond strength of the overlay layer, determined by 180° bend testing of coupon specimens, consistently exceeds 1.5 times the minimum yield strength of the base metal, which satisfies the acceptance criteria specified in GB/T 150.
Engineering Practice and Defect Analysis
In practical fabrication of pressurized gasifier shells, the most challenging aspect is the control of distortion and residual stress in the overlay layer, particularly for large-diameter vessels where the overlay area can exceed 50 m². The study recommends the use of preheating to 150–200 °C, post-weld heat treatment (PWHT) at 580–620 °C for 2–4 hours, and the application of welding sequence optimization to minimize angular and longitudinal distortion.
The following table presents the defect analysis based on field experience with gasifier overlay welding:
| Defect | Frequency | Primary Cause | Prevention |
|---|---|---|---|
| Hot cracking in overlay | Moderate | High sulfur/phosphorus in base metal | Pre-weld inspection of base metal; use of low-sulfur filler metal |
| Cold cracking in HAZ | Low | Hydrogen-induced cracking | Preheat to ≥200 °C; use low-hydrogen flux; post-weld bake-out |
| Lack of fusion at overlay interface | Moderate | Insufficient heat input; poor base surface preparation | Increase current; grind and clean base surface to bare metal |
| Excessive dilution | High | High travel speed; large bead size | Reduce travel speed; use smaller electrode; add transition layer |
| Surface porosity | Low | Flux contamination; moisture in flux | Store flux in dry conditions; bake flux at 250 °C for 2 h |
Study Insights and Implications
The most significant insight from this research is the recognition that no single welding process can optimally address all requirements of gasifier inner wall overlay. The hybrid SAW-PTA approach leverages the high deposition rate of SAW for bulk buildup and the low dilution, fine microstructure of PTA for the final functional layer, achieving a synergy that neither process can deliver alone. This philosophy of process hybridization is applicable to many other cladding applications where competing requirements (productivity vs. quality, cost vs. performance) must be balanced.
The study also highlights the importance of dilution control as a governing parameter in nickel-based alloy overlay. The dilution ratio directly determines the corrosion resistance, thermal stability, and mechanical properties of the overlay layer, and its control requires careful attention to welding parameters, filler metal composition, and interpass temperature. The recommendation to use a transition layer is particularly valuable for engineers who may not have access to PTA or laser cladding equipment, as it provides a practical means of reducing dilution using conventional SAW technology.
In terms of standards compliance, the study's quality control strategy aligns well with the requirements of GB/T 150 for pressure vessels and ASME VIII Div.2 for qualified weld procedures. The emphasis on PAUT inspection is forward-looking, as phased array technology offers superior sensitivity and defect characterization compared to conventional UT, particularly for detecting lack of fusion and planar cracks at the overlay-base interface.
This research provides a solid technical foundation for the design and fabrication of pressurized gasifier shells with nickel-based alloy overlay. Engineers involved in coal conversion technology, petrochemical processing, and power generation should find the process parameters, quality control protocols, and defect countermeasures directly applicable to their own projects. The hybrid welding approach described here represents a practical and cost-effective solution to one of the most challenging overlay welding problems in the energy and chemical industries.