CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Cracks on Pinch Roll and Their Formation Mechanism
Literature Overview
This study examines the cracking phenomena observed in overlay layers applied to pinch rolls used in hot rolling mills. Pinch rolls operate under severe conditions involving high temperature, cyclic loading, and abrasive contact with hot steel slabs, making them prime candidates for hardfacing overlay. However, the overlay layers frequently develop cracks that lead to premature failure. The literature investigates the formation mechanisms of these cracks through metallurgical analysis, thermal simulation, and stress analysis, providing a comprehensive understanding that can guide improved overlay procedures.
Crack Classification and Formation Mechanisms
The study identifies three primary crack types in pinch roll overlay layers: transverse cracks perpendicular to the weld direction, longitudinal cracks parallel to the weld direction, and radial cracks extending from the weld toe into the base metal. Each crack type has a distinct formation mechanism rooted in the interaction between thermal stresses, residual stresses, and microstructural evolution.
| Crack Type | Location | Primary Cause | Typical Depth | Prevention Strategy |
|---|---|---|---|---|
| Transverse Cracks | Overlay layer surface | Thermal contraction stress + high carbon equivalent of base metal | 0.5–2 mm | Preheating base metal to 200–300°C; using low hydrogen flux |
| Longitudinal Cracks | Between overlay passes | Hydrogen embrittlement + martensitic transformation | 1–3 mm | Post-weld heat treatment at 550–650°C; controlling hydrogen in consumables |
| Radial Cracks | Base metal near weld toe | High residual stress concentration + base metal toughness reduction | 2–5 mm | Reducing heat input; using compatible filler metal with lower carbon equivalent |
Thermal Stress Analysis
The formation of transverse cracks is primarily attributed to the mismatch between the thermal expansion coefficients of the overlay material and the base metal. During cooling, the overlay layer contracts more than the base metal, generating tensile stresses that exceed the fracture toughness of the overlay material, particularly at the surface where cooling rates are highest. The study's thermal simulation reveals peak thermal stresses exceeding 800 MPa in the overlay layer during cooling from 900°C to 400°C, which is sufficient to initiate cracking in high-hardness martensitic overlay materials.
Hydrogen-Induced Cracking Mechanism
Longitudinal cracks between overlay passes are strongly associated with hydrogen accumulation. The study demonstrates that hydrogen from the flux or moisture on the base metal surface diffuses into the overlay layer during welding and becomes trapped at microstructural boundaries during martensitic transformation. The hydrogen concentration in the overlay layer reaches critical levels (above 2.5 ppm) when interpass temperature is maintained below 150°C, as the hydrogen diffusion rate decreases with temperature, preventing escape. This finding underscores the importance of adequate preheating and controlled cooling rates in pinch roll overlay applications.
Base Metal Influence
The study provides critical insight into how base metal properties influence crack susceptibility. Pinch rolls are typically fabricated from medium-carbon steel or low-alloy steel with carbon equivalent values between 0.45% and 0.65%. The higher carbon equivalent of the base metal increases the hardenability of the heat-affected zone, creating a brittle martensitic structure near the weld toe that serves as a crack initiation site for radial cracks. The study recommends that base metals with carbon equivalent above 0.55% require preheating to at least 250°C and post-weld heat treatment to reduce hardness in the HAZ below 350 HV.
Process Optimization Recommendations
Based on the mechanistic understanding, the study proposes a comprehensive overlay procedure for pinch rolls that addresses all three crack types simultaneously. The recommended procedure includes: preheating the base metal to 250–300°C using induction heating; applying a transition layer of low-carbon stainless steel (such as 308L) before the hardfacing overlay to reduce dilution and carbon pickup; maintaining interpass temperature between 150–200°C to allow hydrogen escape while preventing excessive grain growth; and performing post-weld heat treatment at 600°C for 2 hours to relieve residual stresses and temper martensite.
Engineering Practice Integration
In hot rolling mill operations, pinch roll overlay failures can cause unplanned shutdowns costing significant production losses. The study's findings support implementing a preventive maintenance program where pinch rolls are inspected for overlay cracks using magnetic particle testing before each major maintenance interval. Engineers should also consider the thermal cycling history of the pinch roll when planning overlay repairs, as repeated thermal cycling can degrade the base metal toughness and increase crack susceptibility. The integration of crack monitoring with predictive maintenance systems can significantly extend pinch roll service life and reduce unplanned downtime.
Study Insights
The most significant contribution of this study is the systematic correlation between crack type, formation mechanism, and process parameter. Rather than treating overlay cracking as a monolithic problem, the study provides engineers with a diagnostic framework that allows them to identify the root cause of specific crack patterns and apply targeted corrective measures. This mechanistic approach is far more effective than trial-and-error parameter adjustment and should be adopted as standard practice in pinch roll overlay repair operations.