Zhuojin Pipe Fitting Co., LtdCrystallization Cracking in Submerged Arc Overlay Welding of Pressurized Gasifier Valves
Literature Overview
This 1993 study by Wu Aiping from Tsinghua University, published in the journal Welding, investigates the causes of crystallization cracks in submerged arc welding (SAW) overlay deposits on pressurized gasifier valves. Although published over three decades ago, the fundamental metallurgical mechanisms described remain highly relevant to modern overlay welding practice, particularly in the context of high-pressure gasification systems where valve integrity is critical.
Core Technical Findings
The study identified high silicon (Si) content in the weld metal as the primary cause of crystallization cracking during SAW overlay welding of gasifier valve components. Crystallization cracks, also known as hot cracks, form during the solidification of the weld metal when the interdendritic liquid films are subjected to tensile stresses that exceed the strength of the partially solidified microstructure.
The mechanism is as follows: silicon promotes the formation of low-melting-point eutectics at grain boundaries during solidification. In the presence of sulfur and other impurities, Si-rich phases can form liquid films along the interdendritic regions at temperatures below the solidus. When the weld metal contracts during cooling, these liquid films are unable to accommodate the thermal strain, resulting in crack initiation and propagation along the interdendritic paths.
Cracking Mechanism Analysis
Crystallization cracking is governed by three factors: the susceptibility of the weld metal composition, the magnitude of the solidification strain, and the拘束度 (restraint) of the weldment. In the context of gasifier valve overlay welding, all three factors contribute:
| Factor | Contribution to Cracking |
|---|---|
| High Si content | Forms low-melting eutectics at interdendritic boundaries |
| High restraint | Valve geometry restricts free contraction during cooling |
| Thick overlay deposits | Large thermal gradients and high residual stresses |
The valve component is typically a thick-walled, high-restraint structure. When a thick overlay deposit is applied by SAW, the large heat input creates a steep thermal gradient from the molten pool to the cooler substrate. The resulting thermal contraction is constrained by the surrounding material, generating tensile stresses in the solidifying weld metal. If the interdendritic liquid films are weakened by Si-rich eutectics, these stresses will exceed the local strength, causing cracks.
Preventive Measures and Weld Material Selection
The study's primary recommendation is to select low-Si welding consumables to prevent crystallization cracking. This is a straightforward but effective approach: by reducing the Si content in the filler metal, the formation of Si-rich low-melting eutectics at interdendritic boundaries is minimized, thereby increasing the solidification crack resistance of the weld metal.
Additional preventive measures that can be applied in practice include:
- Multi-pass welding with thin layers: Reducing the heat input per pass lowers the thermal gradient and the magnitude of solidification stresses.
- Preheating and interpass temperature control: Maintaining a moderate preheat (typically 150-250°C for carbon steel valve bodies) reduces the cooling rate and the thermal strain.
- Weld design optimization: Using grooves or bevels to reduce the cross-sectional area of each pass and to allow for more uniform heat distribution.
- Post-weld heat treatment: Stress relief annealing can reduce residual stresses and improve the overall integrity of the overlay deposit.
Engineering Practice Reflections
This study, while old, illustrates a fundamental principle in overlay welding: the composition of the weld metal must be tailored not only for the desired properties of the overlay (hardness, corrosion resistance, wear resistance) but also for the processability and crack resistance of the deposit. In the case of gasifier valve overlay welding, the requirement for a wear-resistant or corrosion-resistant surface may tempt engineers to use high-alloy consumables with elevated Si content. However, the study demonstrates that this can lead to catastrophic cracking if the process parameters are not adjusted accordingly.
The lesson for modern practice is that crack susceptibility should be evaluated as part of the consumable selection process, not as an afterthought. For thick overlay deposits on high-restraint components, low-Si, low-S welding consumables should be the default choice unless there is a specific metallurgical reason to use high-Si materials.