CLADDING TECHNOLOGY SHANXI CO., LTDCracking Mechanism and Prevention in Weld Overlay of Pressurized Gasification Furnace Valves
Overview of the Technical Issue
Pressurized gasification furnace valves operate under extreme conditions involving high temperature, high pressure, and corrosive gas media such as syngas, hydrogen sulfide, and carbon monoxide. The valve trim components, particularly the seat and plug surfaces, are typically subjected to weld overlay cladding with hardfacing alloys or corrosion-resistant materials to extend service life. However, cracking in the overlay layer and heat-affected zone (HAZ) remains one of the most persistent and challenging defects encountered in such applications. This study note examines the root causes of overlay cracking, the metallurgical mechanisms involved, and practical countermeasures drawn from both literature findings and engineering experience.
Metallurgical Analysis of Cracking Mechanisms
Cracking in weld overlay deposits on gasification furnace valves can be broadly classified into three categories based on formation timing and mechanism. Hot cracking occurs during solidification when the molten pool contracts and is restrained by the surrounding solid metal. Cold cracking, also known as hydrogen-induced delayed cracking, develops hours or even days after welding due to the combination of hydrogen diffusion, residual stress, and microstructural susceptibility. Reheat cracking or tempered cracking may appear during post-weld heat treatment when the material passes through a critical temperature range.
The base material of gasification furnace valves is often a low-alloy steel such as 12Cr1MoV or 20CrMo, which is inherently prone to cold cracking due to its high hardenability. When a hardfacing overlay alloy with high carbon and high alloy content is deposited, the dilution between base metal and overlay creates a complex microstructure with high hardness and low toughness. The rapid cooling rate in valve components, which are typically thick and heavily constrained, exacerbates residual stress accumulation.
| Cracking Type | Formation Temperature | Primary Cause | Typical Location |
|---|---|---|---|
| Hot Cracking | 1200-1400°C | Low-melting intermetallics, restraint | Weld centerline, grain boundaries |
| Cold Cracking (HIC) | Room temperature (delayed) | Hydrogen diffusion + residual stress + martensite | HAZ, fusion line |
| Reheat Cracking | 500-650°C | Stress relief during PWHT | Prior austenite grain boundaries |
Key Technical Findings from Literature Study
The literature identifies several critical factors that contribute to overlay cracking in gasification furnace valve applications. First, the carbon equivalent (CE) of the base material plays a decisive role. For 12Cr1MoV steel, the CE typically ranges from 0.35 to 0.45, placing it well above the threshold where preheat becomes mandatory. Second, the hydrogen content in the deposited metal must be controlled below 5 mL/100g to prevent delayed cracking. Third, the residual stress in thick valve bodies can exceed 400 MPa without proper stress relief procedures.
A particularly important finding is that the dilution rate between the base metal and the overlay layer significantly affects the final hardness and crack susceptibility. When dilution exceeds 30%, the resulting microstructure becomes overly hard and brittle, with martensite content exceeding 80%. The literature recommends limiting dilution to below 20% by selecting appropriate welding parameters and potentially using a transition layer.
Process Countermeasures and Engineering Recommendations
Based on the cracking mechanisms identified, the following countermeasures are recommended for preventing overlay cracking in gasification furnace valve fabrication:
- Preheat and Interpass Temperature Control: Preheat the valve body to 200-250°C for 12Cr1MoV base material, maintaining interpass temperature between 150-250°C to slow cooling rates and reduce hydrogen diffusion rate.
- Low-Hydrogen Consumables: Use low-hydrogen flux-cored wire or coated electrodes with guaranteed hydrogen diffusion rate below 5 mL/100g. Store electrodes at 300°C in ovens and use within 4 hours of removal.
- Multi-Pass Strategy with Transition Layer: Apply a transition layer of medium-carbon stainless steel or Ni-base alloy before the final hardfacing layer to reduce dilution and create a diffusion barrier.
- Post-Weld Heat Treatment: Perform stress relief at 580-620°C for a duration of 2 hours per 25 mm thickness, with controlled cooling rate below 100°C/hour.
- Welding Sequence Optimization: Use balanced welding sequences to minimize distortion and residual stress concentration. Divide the overlay into segments and weld in a balanced pattern.
Practical Engineering Insights
In my engineering practice, I have observed that the most effective approach to preventing overlay cracking combines multiple strategies rather than relying on a single countermeasure. The integration of proper preheat, low-hydrogen consumables, controlled welding parameters, and post-weld stress relief creates a synergistic effect that significantly reduces cracking probability. Metallographic examination of successfully fabricated valves confirms that when dilution is controlled below 20% and hydrogen content is maintained below 5 mL/100g, the overlay layer exhibits a balanced microstructure of austenite and carbide particles with hardness in the range of 40-50 HRC, providing excellent wear and corrosion resistance without cracking susceptibility.
The study reinforces the principle that weld overlay quality is fundamentally a function of process control rather than material selection alone. Even the best hardfacing alloy will crack if the welding parameters and thermal management are inadequate. Engineers must adopt a systematic approach that considers the entire fabrication sequence from material preparation through to final inspection, treating each step as critical to the prevention of cracking defects.