CLADDING TECHNOLOGY SHANXI CO., LTDCrack Formation and Mechanism in Cladding of Intermediate Rollers
Overview of the Topic
The cladding of intermediate rollers (pinch rolls) in rolling mills is a critical engineering challenge because these components operate under extreme thermal cycling, mechanical loading, and abrasive contact with hot metal strips. The literature on crack formation in cladding layers of intermediate rollers provides valuable insights into the metallurgical and thermomechanical factors that lead to failure. This study note synthesizes the key findings and connects them to practical fabrication strategies.
Core Mechanisms of Cracking
Cracks in cladding layers on intermediate rollers can be broadly classified into three categories based on their formation stage: hot cracks (solidification cracks), reheat cracks, and cold cracks (hydrogen-assisted). Understanding the dominant mechanism is essential for selecting the correct countermeasures.
Hot Cracks and Solidification Cracks
Hot cracks form during solidification of the weld overlay when the molten pool contracts and the surrounding solidified material restricts shrinkage. The key factors include:
- Dendritic solidification morphology with low solid fraction in the interdendritic regions, which cannot accommodate the shrinkage strain
- High sulfur and phosphorus content in the base material or filler metal, which segregates to the interdendritic regions and forms low-melting eutectics
- Excessive heat input leading to wide weld beads with prolonged solidification time and increased susceptibility to hot cracking
| Crack Type | Formation Stage | Primary Cause | Typical Location |
|---|---|---|---|
| Hot crack | Solidification | Low-melting eutectic segregation, restraint stress | Center of weld bead, interdendritic regions |
| Reheat crack | Post-weld heat treatment | Grain boundary precipitation, stress relaxation | HAZ grain boundaries |
| Cold crack | Post-weld cooling | Hydrogen diffusion, martensitic transformation | HAZ, weld root |
Reheat Cracks
Reheat cracks are particularly relevant for rollers that undergo post-weld stress relief. They occur when the microstructure in the heat-affected zone contains precipitates along grain boundaries that reduce intergranular cohesion. The literature highlights that chromium and molybdenum-rich phases at grain boundaries are primary contributors.
Cold Cracks and Hydrogen Embrittlement
Cold cracks are the most insidious because they can appear hours or even days after welding. The classic three-factor model applies:
- Diffusible hydrogen from the filler metal, flux, or atmospheric moisture
- Hard, brittle microstructure (typically martensite) in the HAZ or weld metal
- Residual stress exceeding the yield strength of the local material
The intermediate roller application is particularly prone to cold cracking because the high-carbon or high-alloy steels used for the roller body often have limited preheat capability due to thermal distortion concerns, while the overlay material may be hydrogen-sensitive.
Process Parameters and Countermeasures
The literature provides several practical recommendations for crack mitigation:
- Preheat temperature: Maintain a minimum preheat of 150–250°C depending on the carbon equivalent of the base material. For high-carbon steel rollers (CE > 0.6), preheat above 250°C is advisable.
- Interpass temperature control: Keep the interpass temperature above 150°C to prevent rapid cooling and martensite formation in the HAZ.
- Heat input optimization: Use a moderate heat input range of 15–25 kJ/mm to balance dilution and solidification rate. Too low heat input promotes martensite; too high promotes grain coarsening and hot cracking.
- Filler metal selection: Choose low-hydrogen consumables (H₂ content < 5 mL/100g) and ensure proper storage and baking of electrodes.
- Post-weld heat treatment: Perform stress relief at 580–620°C for 2 hours per 25 mm thickness to reduce residual stresses without promoting reheat cracking.
Engineering Practice Implications
In my experience with intermediate roller fabrication, the most common failure mode is cold cracking at the weld root or in the HAZ of the roller body. A practical approach that has proven effective includes:
- Thorough surface preparation to remove rust, paint, and moisture
- Application of a low-carbon, high-toughness transition layer (e.g., E8018-D1 or equivalent) before the final overlay
- Use of a backing plate or root treatment to ensure full penetration and avoid slag inclusion at the root
- Implementation of a two-pass strategy: first pass for bonding, second pass for wear resistance
The study also emphasizes the importance of non-destructive testing. Magnetic particle inspection (MT) and ultrasonic testing (UT) are mandatory for detecting surface and subsurface cracks. The acceptance criteria should follow NB/T 47013 or equivalent standards, with zero tolerance for linear indications exceeding 25 mm in length.
Summary and Reflections
The study of crack formation in intermediate roller cladding reinforces the principle that crack prevention is a multi-factor problem requiring coordinated control of material, process, and inspection. The dominant mechanism varies with the specific application, base material composition, and service environment. Engineers should adopt a systematic approach: identify the likely crack mechanism through metallurgical analysis, select appropriate countermeasures based on the dominant factor, and verify through rigorous NDT. The integration of FMEA into the fabrication process planning stage can significantly reduce the probability of crack-related failures in service.