CLADDING TECHNOLOGY SHANXI CO., LTDInvestigation of Weld Overlay Cracks on Pressure Gasifier Valves
Literature Overview
This study, authored by Wu Aiping from Tsinghua University, was published in the journal Welding in 1993. Although the publication date is relatively early, the technical content remains highly relevant to modern practice. The paper investigates the cracking phenomenon observed during weld overlay on pressure gasifier valves, which are critical components in coal gasification systems. These valves operate under high temperature, high pressure, and highly corrosive conditions involving hydrogen, carbon monoxide, and hydrogen sulfide. The weld overlay is applied to provide wear and corrosion resistance to the valve sealing surfaces, and any cracking in the overlay or the heat-affected zone can lead to valve failure and catastrophic gas leakage.
Technical Background
Pressure gasifier valves are typically fabricated from high-strength alloy steels such as 12Cr1MoV or 9Cr1Mo, which possess excellent high-temperature strength and creep resistance. The weld overlay material is usually a hardfacing alloy such as Stellite 6 (Co-Cr-W alloy) or a nickel-based alloy, selected for its resistance to erosion and corrosion in the gasifier environment. The cracking problem arises from the combination of the following factors:
- High carbon equivalent of the base metal: 12Cr1MoV has a carbon equivalent (CEV) of approximately 0.45–0.55, which is above the threshold for cold cracking susceptibility.
- Large mismatch between base metal and overlay: The coefficient of thermal expansion of the cobalt-based overlay (approximately 13 × 10⁻⁶/°C) is significantly different from that of the 9Cr1Mo base metal (approximately 12 × 10⁻⁶/°C), leading to high residual stresses at the bond line.
- High hydrogen content in the gasifier environment: The hydrogen present in the gasifier atmosphere can diffuse into the weld metal and HAZ during operation, promoting hydrogen-assisted cracking.
- Complex geometry of the valve: The curved surfaces and tight corners of the valve create stress concentration sites that are susceptible to cracking.
Crack Analysis
The study classifies the observed cracks into two categories:
| Crack Type | Location | Morphology | Mechanism |
|---|---|---|---|
| Cold crack (hydrogen crack) | HAZ and weld metal | Intergranular or transgranular, often branching | Hydrogen embrittlement combined with residual stress |
| Hot crack (solidification crack) | Weld metal center | Interdendritic, along grain boundaries | Low melting point phase segregation during solidification |
The cold cracks are the predominant type and are the focus of the investigation. The authors use metallographic examination and fracture surface analysis to determine the crack initiation sites and propagation paths. The findings indicate that:
- Cold cracks initiate at the bond line between the base metal and the first pass of the overlay, where the hardness is highest (typically 450–550 HV) and the hydrogen concentration is highest.
- The cracks propagate in an intergranular fashion along the prior austenite grain boundaries of the HAZ, which are weakened by carbide precipitation during welding.
- The cracks are often delayed, appearing 12–48 hours after welding, which makes them difficult to detect during immediate post-weld inspection.
Process Parameters and Countermeasures
The study recommends the following process modifications to eliminate the cracking problem:
- Increased preheat temperature: The preheat temperature should be raised from the conventional 100 °C to 250–300 °C to reduce the cooling rate below the critical rate for martensite formation and to promote hydrogen diffusion out of the weld zone.
- Low-hydrogen consumables: The use of low-hydrogen welding consumables is mandatory. For SMAW, electrodes with hydrogen diffusibility below 5 mL/100 g should be used. For SAW, low-hydrogen flux must be dried at 300 °C for 2 hours prior to use.
- Multi-pass cladding with low dilution: The first pass should be applied with a low heat input to minimize dilution from the high-carbon base metal. Subsequent passes should be applied with a controlled interpass temperature not exceeding 300 °C.
- Post-weld heat treatment: A stress-relief anneal at 650–700 °C for 2–4 hours is recommended to reduce residual stresses and temper any martensite that has formed in the HAZ.
- Delayed NDT: A waiting period of at least 24 hours after the final welding operation is recommended before performing the final NDT inspection to ensure that delayed cracks have had time to nucleate.
Engineering Practice and Lessons Learned
The experience from this study has been applied to the fabrication of gasifier valves in multiple coal gasification plants. The following lessons have been learned:
- The preheat temperature is the single most important parameter for preventing cold cracking in high-carbon equivalent steel weld overlay.
- The use of low-hydrogen consumables alone is insufficient; the preheat and PWHT must be implemented in conjunction.
- The complex geometry of the valve requires careful planning of the welding sequence to minimize the accumulation of residual stress.
- The NDT inspection must include both surface (MT or PT) and subsurface (UT) methods to detect cracks at the bond line.
Study Insights and Implications
Although this study was published in 1993, its technical content remains highly relevant. The fundamental mechanisms of cold cracking in high-carbon equivalent steel weld overlay have not changed, and the countermeasures recommended in this study are still the standard practice today. The study serves as a reminder that the prevention of welding cracks requires a systematic approach that addresses all contributing factors simultaneously, rather than relying on any single countermeasure. For engineers involved in the fabrication of high-pressure valves and similar components, this study provides a valuable reference for process development and quality assurance.