Zhuojin Pipe Fitting Co., LtdEffect of Preheating on Overlay Welding Cracks in Pinch Rolls
Literature Overview
This paper by Wang Lihua and Hui Yuanbo, published in Hot Working Technology (2006, Vol. 35, No. 19, pp. 23–25), investigates the influence of preheating on both cold cracks and hot cracks in pinch roll overlay welding. Pinch rolls are critical components in rolling mills, subjected to extreme mechanical and thermal loading during operation. Their overlay repair is essential for maintaining production continuity, yet cracking during and after overlay welding remains a persistent quality challenge. The authors conducted experimental investigations to establish the relationship between preheating temperature and crack susceptibility, addressing both cold cracking (hydrogen-induced, delayed) and hot cracking (solidification cracking) mechanisms.
Core Technical Content
Crack Classification and Mechanisms
The paper identifies two distinct crack types with different formation mechanisms:
| Crack Type | Primary Mechanism | Timing | Driving Force |
|---|---|---|---|
| Cold crack | Quenched hard martensitic structure + hydrogen embrittlement | After welding (delayed) | Hydrogen diffusion into high-stress, high-hardness HAZ |
| Hot crack | Liquid film formation at grain boundaries + welding stress | During solidification | Thermal contraction stress exceeding liquid film strength |
Cold Crack Mechanism
Cold cracking in pinch roll overlay welding occurs due to the combination of:
- Quenched hard structure: Rapid cooling of the weld metal and heat-affected zone produces martensitic or bainitic microstructures with high hardness and low ductility.
- Hydrogen presence: Hydrogen from moisture in the flux, base material surface contamination, or the welding atmosphere diffuses into the weld and HAZ.
- Residual stress: Thermal contraction and phase transformation generate tensile stresses that exceed the material's fracture toughness.
The three factors—hardness, hydrogen, and stress—must coexist for cold cracking to occur. Preheating primarily addresses the first factor by reducing cooling rates and promoting softer microstructures.
Hot Crack Mechanism
Hot cracking occurs during solidification when:
- Liquid film formation: Low-melting-point phases (sulfides, silicates) form liquid films at grain boundaries in the last-solidifying regions.
- Welding stress: Thermal contraction generates tensile stresses that pull apart the liquid film boundaries.
- Insufficient plasticity: The partially solidified microstructure cannot accommodate the imposed strain without fracture.
Preheating affects hot cracking by reducing thermal gradients and thermal stresses, thereby lowering the tensile stress component of the cracking equation.
Preheating Strategy Analysis
Effect on Cold Cracks
Preheating reduces cold crack susceptibility through multiple mechanisms:
- Slower cooling rate: Preheating elevates the base material temperature, reducing the thermal gradient between the weld pool and the base material. This slows the cooling rate through the martensitic transformation range, allowing more time for diffusional processes and reducing the volume fraction of martensite.
- Reduced hardness: Slower cooling promotes the formation of bainite or tempered martensite rather than untempered martensite, reducing hardness and increasing ductility.
- Hydrogen diffusion: Higher base material temperature provides thermal energy for hydrogen to diffuse out of the weld and HAZ before the structure becomes too hard and brittle for hydrogen escape.
Effect on Hot Cracks
Preheating reduces hot crack susceptibility by:
- Reduced thermal stress: Lower thermal gradients result in lower thermal stresses during solidification.
- Modified solidification behavior: Higher initial temperature can modify the solidification path and reduce the fraction of low-melting-point phases at grain boundaries.
Technical Recommendations
Based on the experimental findings, the following preheating guidelines can be derived for pinch roll overlay welding:
| Base Material Condition | Recommended Preheat Temperature | Rationale |
|---|---|---|
| High-carbon steel, thick section | 200–300°C | Reduce cooling rate below martensite start temperature |
| Medium-carbon steel, moderate thickness | 150–250°C | Moderate cooling rate reduction |
| Low-carbon steel, thin section | 100–200°C | Primarily for hydrogen diffusion and stress reduction |
Engineering Practice Integration
Pinch rolls in rolling mills typically have a hardened outer layer (chrome-plated or case-hardened) over a tough steel core. Overlay welding onto such components introduces additional complexity:
- The hardened surface layer has limited plasticity, making it susceptible to cracking under welding stresses.
- The thermal mass of the roll is large, meaning that preheating must be substantial to effectively reduce cooling rates.
- Multi-pass overlay welding requires interpass temperature maintenance to prevent cracking between passes.
The paper's findings support the industry practice of preheating pinch rolls before overlay welding, but the specific temperature should be determined based on the base material composition, section thickness, and overlay alloy selection.
Study Insights
This paper provides a clear mechanistic framework for understanding how preheating addresses two fundamentally different crack types. The key insight is that preheating is not a universal cure-all; its effectiveness depends on the specific crack mechanism being addressed. For cold cracks, preheating is highly effective because it directly reduces hardness and facilitates hydrogen escape. For hot cracks, preheating is beneficial but may need to be supplemented by other measures such as filler metal selection (low-sulfur, low-phosphorus consumables) and welding parameter optimization. Engineers should approach preheating as one element of a comprehensive crack prevention strategy rather than a standalone solution.