CLADDING TECHNOLOGY SHANXI CO., LTDResearch on Weld Overlay Cracking in Pressurized Gasifier Valves
Literature Overview
This study by Wu Aiping from Tsinghua University, published in 1993 in the journal Welding (焊接), addresses the cracking problem encountered during weld overlay of valves used in pressurized gasification furnaces. Gasifier valves operate in extremely harsh environments characterized by high temperatures, high pressures, abrasive gas-solid mixtures, and corrosive atmospheres containing hydrogen sulfide and other reactive species. The overlay layer must provide both wear resistance and corrosion resistance while maintaining structural integrity under cyclic thermal and mechanical loading.
Technical Background
Service Environment of Gasifier Valves
| Parameter | Typical Conditions |
|---|---|
| Operating temperature | 400–1200 °C (depending on gasifier type) |
| Operating pressure | 2.0–8.0 MPa |
| Gas composition | CO, H2, CO2, H2S, H2O, N2 |
| Erosion severity | Severe (gas-solid two-phase flow) |
| Corrosion type | Hot corrosion, oxidation, sulfidation |
| Cycle frequency | Continuous operation with occasional shutdowns |
Common Overlay Materials for Gasifier Valves
| Material Type | Composition | Application |
|---|---|---|
| Stellite 6 | Co-Cr-W (Co bal., Cr 21%, W 5.6%) | High-temperature erosion resistance |
| Stellite 21 | Co-Cr-W-Mo | Enhanced hot corrosion resistance |
| Ni-Cr-Mo alloy | Ni bal., Cr 20%, Mo 10% | General high-temperature service |
| Cr-Ni stainless steel | Cr 25%, Ni 20% | Moderate temperature oxidation resistance |
| Tungsten carbide composite | WC-Co or WC-Ni | Abrasive wear resistance |
Crack Classification and Analysis
Types of Cracks Observed
| Crack Type | Location | Orientation | Primary Cause |
|---|---|---|---|
| Hot cracking | Weld metal (centerline) | Parallel to weld axis | Low-melting-phase segregation at solidification |
| Cold cracking | HAZ | Transverse to weld | Hydrogen embrittlement + martensitic transformation |
| Reheat cracking | HAZ (post-PWHT) | Parallel to weld axis | Precipitation at prior austenite grain boundaries |
| Thermal fatigue cracking | Overlay surface | Random | Cyclic thermal stress exceeding fatigue limit |
| Stress corrosion cracking | Overlay surface | Perpendicular to tensile stress | Residual stress + corrosive environment |
Root Cause Analysis Using Fishbone Diagram Approach
The cracking problem can be systematically analyzed considering the following categories:
Material factors:
- High carbon content in filler metal promoting carbide formation
- Sulfur and phosphor impurities promoting hot cracking
- High carbon equivalent of base metal promoting HAZ hardening
Process factors:
- Excessive heat input causing grain coarsening
- Inadequate preheating leading to rapid cooling
- Poor flux or shielding gas quality introducing hydrogen
- Excessive restraint from valve body geometry
Design factors:
- Incompatible material selection for joint
- Insufficient weld geometry consideration for stress relief
- Inadequate overlay thickness for service conditions
Metallurgical Investigation
Metallographic Findings
Typical microstructural observations in cracked overlay welds include:
- Hot cracking: Liquid films of low-melting eutectics (Fe-S, Fe-P, or Ni-S systems) segregated at interdendritic regions during solidification.
- Cold cracking: Hard martensitic structure in HAZ with hardness exceeding 400 HV, accompanied by hydrogen concentration at prior austenite grain boundaries.
- Reheat cracking: Precipitation of M23C6 carbides at grain boundaries during PWHT, reducing boundary cohesion.
Chemical Analysis of Cracked Zones
| Element | Base Metal | Weld Metal | HAZ |
|---|---|---|---|
| C (%) | 0.15–0.25 | 0.03–0.08 | 0.20–0.35 |
| Mn (%) | 0.8–1.2 | 0.5–1.0 | 0.8–1.2 |
| Cr (%) | 0.5–1.0 | 15–25 | 0.5–1.0 |
| Ni (%) | 0.05–0.10 | 8–15 | 0.05–0.10 |
| S (%) | < 0.010 | < 0.005 | < 0.010 |
| P (%) | < 0.020 | < 0.010 | < 0.020 |
Preventive Measures and Process Optimization
Comprehensive Anti-Cracking Strategy
| Measure | Implementation Details | Effectiveness |
|---|---|---|
| Low-carbon filler metal | Use filler with C < 0.04% | Reduces HAZ hardness |
| Adequate preheating | 250–400 °C depending on base metal CE | Reduces cooling rate |
| Hydrogen control | Oven-dry flux at 300 °C for 2+ hours | Eliminates hydrogen source |
| Post-weld heat treatment | 650–750 °C, 2–4 hours | Relieves residual stress, transforms martensite |
| Multi-pass welding | Multiple thin passes instead of single thick pass | Improves microstructure, reduces stress |
| Post-weld machining | Machine overlay surface after PWHT | Removes HAZ, eliminates surface cracks |
| Design modification | Add stress-relief grooves or reduce restraint | Allows plastic deformation |
Recommended Welding Parameters for Gasifier Valve Overlay
| Parameter | Specification |
|---|---|
| Process | SAW (submerged arc welding) or ESW (electroslag welding) |
| Filler metal | Low-carbon Co-Cr-W alloy or Ni-Cr-Mo alloy |
| Preheat temperature | 300 °C (uniform, verified by thermocouple) |
| Interpass temperature | 250–400 °C |
| Heat input | 20–40 kJ/cm (ESW) or 8–15 kJ/cm (SAW) |
| Travel speed | 100–200 mm/min (SAW) |
| PWHT | 700 °C × 3 hours (furnace), cool at ≤ 100 °C/h |
| Post-PWHT hardness | ≤ 350 HV (weld metal), ≤ 250 HV (HAZ) |
| NDT | 100% PT + MT of overlay surface; UT for bond strength |
Quality Control Procedures
Inspection Protocol for Gasifier Valve Overlay
| Stage | Inspection Method | Acceptance Criteria |
|---|---|---|
| Pre-weld | Visual + PT of base metal surface | No surface defects |
| During welding | Thermocouple monitoring | Preheat/interpass within specification |
| Post-weld (before PWHT) | PT + MT | No cracks, no surface discontinuities |
| Post-PWHT | Hardness survey | Within specified limits |
| Post-PWHT | PT + MT (repeat) | No new cracks |
| Post-machining | PT of finished surface | No defects |
| Final | Hydrostatic test | No leakage at 1.5× design pressure |
Engineering Lessons and Reflections
The cracking problem in gasifier valve overlay welding illustrates several fundamental principles:
- Material compatibility is paramount: The selection of overlay material must consider not only wear and corrosion resistance but also metallurgical compatibility with the base metal to minimize cracking susceptibility.
- Process control is non-negotiable: Even the best material selection will fail if welding parameters are not tightly controlled, particularly preheat and interpass temperature.
- Post-weld treatment is essential: For high-carbon-equivalent base metals, PWHT is not optional but mandatory for crack prevention.
- Systematic approach is required: Crack prevention must address all contributing factors simultaneously rather than relying on any single measure.
Conclusion
This early but foundational study provides valuable insights into the cracking mechanisms and prevention strategies for weld overlay applications in extreme service environments. The systematic analysis approach—combining metallurgical examination, chemical analysis, and process parameter evaluation—establishes a methodology that remains applicable to modern overlay welding challenges. Engineers working on gasifier valve or similar high-temperature, high-pressure equipment should adopt the comprehensive anti-cracking strategy presented here, recognizing that welding quality in these applications demands the same level of rigor as nuclear-grade fabrication. The key lesson is that crack prevention is a multi-factorial challenge requiring coordinated control of material selection, process parameters, thermal treatment, and quality inspection throughout the entire fabrication sequence.