CLADDING TECHNOLOGY SHANXI CO., LTDFailure Analysis of Support Roller Surface Cladding Layer Cracking
Background and Failure Description
Support rollers used in hot rolling mills, paper machines, and steel processing lines are subjected to extreme thermal cycling, mechanical loading, and corrosive environments. To extend their service life, the surface of these rollers is often clad with wear-resistant or corrosion-resistant alloy layers using various cladding techniques such as submerged arc welding (SAW), gas metal arc welding (GMAW), or plasma transferred arc (PTA) welding. However, cracking of the cladding layer remains a persistent failure mode that leads to premature roller replacement and production downtime. This study note examines the metallurgical and mechanical root causes of cladding layer cracking in support rollers, based on metallographic analysis and field investigation data.
Metallurgical Analysis of Cracking
Crack Morphology and Initiation Sites
Metallographic examination of failed cladding layers typically reveals that cracks initiate at the bond line interface or within the overlay layer itself. The crack morphology varies depending on the cladding material and the welding process used. In the case of stainless steel overlay layers on carbon steel or low-alloy steel base metals, cracks are often intergranular, propagating along the grain boundaries of the overlay layer. In nickel-based alloy cladding, cracks tend to be transgranular, indicating a different failure mechanism.
| Crack Type | Location | Morphology | Typical Cause |
|---|---|---|---|
| Intergranular | Overlay layer grain boundaries | Wavy, branching | Sensitization, thermal cycling |
| Transgranular | Through overlay grains | Straight, cleavage-like | Thermal stress, mechanical overload |
| Bond line | Overlay-base interface | Mixed mode | Lack of fusion, thermal mismatch |
| Surface | Near surface of overlay | Radial, short | Thermal fatigue, spalling |
Microstructural Evolution
The microstructure of the cladding layer undergoes significant changes during the welding process and subsequent thermal cycling in service. The heat-affected zone (HAZ) at the bond line can develop a martensitic or bainitic structure, which is hard and brittle. During thermal cycling in service, this hard phase can undergo transformation-induced cracking. Additionally, chromium carbide precipitation at grain boundaries in stainless steel overlay layers leads to sensitization, reducing the resistance to intergranular stress corrosion cracking (IGSCC).
Residual Stress Distribution
Residual stresses are a primary driver of cladding layer cracking. The residual stress distribution in a clad roller is complex, with compressive stresses in the overlay layer and tensile stresses in the base metal near the bond line. During thermal cycling, the differential thermal expansion between the overlay and base metal generates additional cyclic stresses that can exceed the yield strength of the overlay layer, leading to fatigue cracking.
Process-Related Factors
Welding Procedure Qualification
The welding procedure specification (WPS) must be properly qualified according to NB/T 47014 or ASME IX. Key parameters include the welding current, voltage, travel speed, number of passes, interpass temperature, and post-weld heat treatment. An improperly qualified WPS can lead to excessive residual stresses, poor metallurgical bonding, and cracking susceptibility.
| Parameter | Typical Value | Impact on Cracking Risk |
|---|---|---|
| Interpass temperature | 150–250 °C | Low: high cooling rate, increased cracking risk |
| Number of passes | 2–5 | More passes: lower residual stress per pass |
| Post-weld stress relief | 580–620 °C, 2 h per 25 mm | Absent: high residual stress, cracking risk |
| Preheat temperature | 100–200 °C | Low: hydrogen cracking risk in HAZ |
| Dilution rate | 5–15% | High: reduced overlay composition, lower cracking resistance |
Base Metal Condition
The mechanical properties and microstructure of the base metal roller significantly influence the cracking behavior of the cladding layer. If the base metal has been cold-worked or has a high hardness due to previous heat treatment, the thermal mismatch between the base and overlay increases, raising the cracking risk. Surface defects such as scratches, dents, or residual scale on the base metal can act as crack initiation sites.
Service-Related Factors
Thermal Cycling
Support rollers in hot rolling mills experience repeated thermal cycling from ambient temperature to temperatures as high as 600–800 °C. Each thermal cycle generates cyclic thermal stresses in the cladding layer. Over time, these cyclic stresses lead to thermal fatigue cracking, particularly at the bond line and at grain boundaries in the overlay layer.
Mechanical Loading
In addition to thermal cycling, support rollers are subjected to mechanical loads from the material being processed. These loads can be static, dynamic, or impact in nature. The combination of mechanical loading and thermal cycling creates a multiaxial stress state that accelerates crack initiation and propagation.
Corrosive Environment
In applications involving corrosive media, the cladding layer must resist stress corrosion cracking (SCC) and intergranular stress corrosion cracking (IGSCC). If the overlay layer is sensitized due to improper welding parameters or thermal cycling, the chromium content at grain boundaries can be depleted, leading to IGSCC in the presence of chloride ions.
Failure Analysis Methodology
A systematic failure analysis approach should be employed to identify the root cause of cladding layer cracking. The recommended methodology includes the following steps:
- Visual examination of the crack location, direction, and morphology.
- Metallographic examination of the crack path, microstructure, and inclusion distribution.
- Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analysis of the crack surface and nearby microstructure.
- Hardness testing across the bond line and overlay layer to identify the hard and brittle zones.
- Residual stress measurement using X-ray diffraction or hole-drilling methods.
- Review of welding procedure qualification records and production parameters.
Countermeasures and Design Improvements
Based on the failure analysis findings, the following countermeasures can be implemented to prevent or mitigate cladding layer cracking:
| Countermeasure | Description | Effectiveness |
|---|---|---|
| Post-weld stress relief | Heat treatment at 580–620 °C | Reduces residual stress by 70–90% |
| Multi-pass welding | Increase number of passes | Distributes residual stress, reduces peak stress |
| Overlay composition optimization | Use crack-resistant alloy composition | Reduces hot and cold cracking susceptibility |
| Base metal preparation | Grinding, preheating, surface cleaning | Eliminates crack initiation sites |
| Thermal barrier layer | Apply a transition layer between base and overlay | Reduces thermal mismatch |
| Service temperature control | Limit maximum operating temperature | Reduces thermal cycling amplitude |
Summary
The cracking of support roller cladding layers is a multifactorial failure mode driven by the interaction of metallurgical, process, and service factors. A thorough failure analysis, combining metallographic examination, microstructural characterization, and residual stress measurement, is essential to identify the root cause and implement effective countermeasures. The key insight is that cracking prevention requires a holistic approach that addresses both the welding process and the service environment, rather than relying on any single corrective action. Engineers should adopt a systematic failure analysis methodology and maintain detailed records of welding procedures, material certifications, and service conditions to enable continuous improvement in cladding quality and roller service life.