Microstructure and Properties of Cladding on Stretch-Bend Rolls

Overview of the Study

This literature examines the microstructural characteristics and mechanical properties of cladding layers applied to stretch-bend rolls, which are critical components in cold rolling mills used for producing thin strip and sheet products. Stretch-bend rolls are subjected to extreme contact stresses, abrasive wear from the strip surface, and thermal cycling from the rolling process. The cladding layer must provide superior wear resistance while maintaining adequate toughness to withstand the high contact pressures and occasional impact loading from strip edge defects.

Microstructural Characteristics of the Cladding Layer

The study investigates several cladding approaches for stretch-bend rolls, including hardfacing with high-carbon martensitic alloys, composite cladding with carbide particles, and multilayer cladding with alternating hard and tough layers. The most commonly used cladding material is a high-carbon martensitic alloy containing 2 to 4 percent carbon with alloying additions of chromium, molybdenum, and vanadium. The deposited microstructure consists of retained austenite with dispersed M7C3 and M23C6 carbides, providing a combination of hardness and toughness.

The microstructure of the cladding layer shows a distinct gradient from the surface to the substrate interface. The surface region, produced by the last pass, exhibits fine carbide distribution and high hardness due to rapid solidification. The intermediate region shows coarser carbides and slightly lower hardness due to the thermal effects of subsequent passes. The interface region, closest to the base roll material, shows the highest dilution and the lowest hardness, with a microstructure approaching that of the base material.

The carbide morphology is particularly important for wear resistance. In high-carbon martensitic cladding, the M7C3 carbides form as fine, elongated particles aligned with the solidification direction, providing good wear resistance through particle support. In composite cladding with tungsten carbide additions, the WC particles provide extremely high hardness but can create stress concentration sites if not properly bonded to the matrix. The bond quality between WC particles and the Ni-Cr-C matrix is critical and is influenced by the powder composition, melting characteristics, and solidification conditions.

Mechanical Properties and Wear Performance

The mechanical properties of cladding layers for stretch-bend rolls are evaluated through hardness profiling, indentation testing, and wear testing under simulated rolling conditions. Hardness profiles show that the surface hardness of high-carbon martensitic cladding reaches 850 to 950 HV, providing excellent resistance to abrasive wear from strip surface contaminants. The hardness decreases to 600 to 700 HV at the substrate interface due to dilution effects.

Contact fatigue resistance is a critical property for stretch-bend roll cladding, as the rolls are subjected to cyclic contact stresses from the rolling process. The cladding layer must have sufficient toughness to resist crack initiation and propagation under repeated loading. High-carbon martensitic cladding with 15 to 25 percent retained austenite provides a favorable balance of hardness and toughness, with the retained austenite transforming to martensite under contact loading, providing work hardening and crack arrest.

Wear testing under simulated rolling conditions shows that high-carbon martensitic cladding provides 2 to 4 times the wear resistance of the unclad base roll material. The wear mechanism transitions from abrasive wear at low contact pressures to adhesive and fatigue wear at high contact pressures. The cladding layer effectively resists abrasive wear through the hard carbide phase and adhesive wear through the ductile retained austenite phase.

The thermal stability of the cladding layer is also important, as stretch-bend rolls experience temperature variations from ambient conditions to 200 to 300 degrees Celsius during operation. The high-carbon martensitic cladding retains its hardness at elevated temperatures, with less than 10 percent hardness loss at 300 degrees Celsius. In contrast, austenitic cladding shows significant softening at elevated temperatures due to carbide dissolution and grain growth.

Engineering Practice and Application Considerations

The practical application of cladding on stretch-bend rolls requires careful consideration of the roll geometry, operating conditions, and maintenance schedule. The cladding thickness is typically 3 to 8 millimeters, with thicker layers used for heavier gauge rolling and thinner layers for lighter gauge applications. The cladding is applied to the roll barrel surface, with the bearing journals and shaft ends left unclad to maintain dimensional accuracy for bearing fit.

The welding process used for stretch-bend roll cladding is typically submerged arc welding (SAW) or gas metal arc welding (GMAW) with automated travel. SAW provides high deposition rates and good penetration, making it suitable for thick cladding layers. GMAW offers better control over dilution and is preferred for thinner layers or when precise dilution control is required. The choice of process depends on the cladding thickness, required dilution control, and production volume.

Post-weld grinding is essential to restore the roll surface to the required dimensional accuracy and surface finish. The amount of grinding depends on the cladding thickness and the precision of the welding process. Automated welding with precise travel speed and wire feed control can reduce grinding by 30 to 50 percent compared to manual methods. The final surface finish should meet the specification for the particular rolling application, typically Ra 0.4 to 1.6 micrometers for precision rolling.

A critical consideration is the residual stress state of the cladding layer. The thermal cycling of the welding process introduces tensile residual stresses at the surface, which can promote fatigue crack initiation during service. Post-weld stress relief at 550 to 600 degrees Celsius for 2 to 4 hours is recommended to reduce residual stresses to acceptable levels. However, this temperature range must be carefully controlled to avoid excessive tempering of the martensitic microstructure, which would reduce hardness.

Study Insights and Reflections

The most significant finding of this study is that high-carbon martensitic cladding provides an optimal balance of wear resistance, toughness, and thermal stability for stretch-bend roll applications. The retained austenite phase plays a crucial role in providing toughness and work hardening under contact loading, making it an essential component of the microstructure rather than an undesirable phase to be eliminated.

The study also highlights the importance of dilution control in achieving consistent cladding properties. The interface region, where dilution is highest, often represents the weakest link in the cladding layer and is the most likely site of failure initiation. Minimizing dilution through process optimization and multi-pass strategies is essential for achieving reliable long-term performance.

The economic analysis of cladding versus roll replacement must consider not only the initial cost but also the extended service life and reduced downtime. A well-executed cladding repair can extend roll life by 3 to 5 times compared to an unclad roll, with a total cost that is significantly lower than purchasing a new roll. The return on investment is particularly favorable for rolls operating in severe wear conditions or where downtime costs are high.

In conclusion, this study provides comprehensive insights into the microstructural and mechanical behavior of cladding layers applied to stretch-bend rolls. The findings offer practical guidance for selecting cladding materials, optimizing welding processes, and ensuring reliable long-term performance. Engineers responsible for roll maintenance should consider cladding as a cost-effective and technically sound approach to extending roll service life, provided that the process is carefully controlled and the resulting properties are verified through appropriate testing.