Cladding Process for Rolling Mill Rolls

Literature Overview

This 2005 publication in the journal Welding Technology by Ding Jie from Tianjin Sino-German Vocational and Technical College and Shen Qianhui from Tianjin Steel Pipe Company addresses the cladding process for rolling mill rolls, a critical application in the steel and metal forming industry. Rolling mill rolls are subjected to extreme mechanical loads, thermal cycling, and abrasive wear during the hot rolling of steel products. The cladding of roll surfaces with wear-resistant and thermally stable materials is a well-established practice for extending roll life and maintaining product quality. This study provides practical guidance on the selection and application of cladding processes for rolling mill rolls.

Rolling Mill Roll Operating Conditions

Rolling mill rolls operate under a combination of severe mechanical, thermal, and chemical loading conditions that vary depending on the specific rolling application:

The combination of high contact pressures, sliding velocities, and elevated temperatures creates a complex tribological environment that promotes multiple wear mechanisms simultaneously. The primary wear mechanisms on hot rolling rolls include abrasive wear from oxide scale removal, adhesive wear from contact with the hot steel strip, and thermal fatigue from repeated heating and cooling cycles.

Cladding Materials for Rolling Mill Rolls

The selection of cladding material for rolling mill rolls depends on the specific application and the dominant wear mechanism:

For hot rolling applications, Fe-Ni-Cr and Ni-Cr (stellite) systems are commonly used because of their excellent thermal stability and resistance to thermal fatigue. These materials maintain their mechanical properties at elevated temperatures and resist the formation of deep thermal cracks that would propagate into the roll body.

For cold rolling applications, high-carbon chromium systems (Fe-Cr-C) are often preferred because of their very high hardness and excellent resistance to abrasive and adhesive wear. The high hardness provides resistance to plastic deformation under the high contact pressures encountered in cold rolling.

Cladding Processes for Rolling Mill Rolls

Several cladding processes are used for rolling mill rolls, each with distinct advantages and limitations:

Electroslag Welding (ESW) Cladding

Electroslag welding is the most widely used process for cladding large-diameter rolling mill rolls. The process involves feeding a consumable electrode into a slag pool that melts the base metal and electrode material simultaneously. The resulting weld deposit is deposited in a single pass with excellent composition control and minimal dilution.

The advantages of ESW cladding include high deposition rates, excellent composition control, low dilution, and the ability to clad large diameters in a single pass. The disadvantages include high equipment cost, the need for roll rotation during welding, and limited flexibility for complex geometries.

Submerged Arc Welding (SAW) Cladding

Submerged arc welding is another commonly used process for rolling mill roll cladding, particularly for smaller diameter rolls or when higher flexibility is required. SAW offers good deposition rates and excellent weld quality, but typically requires multiple passes to achieve the desired cladding thickness.

SAW cladding is particularly suitable for cladding the working surfaces of rolls where the cladding thickness does not exceed 10 mm. The process offers good flexibility for handling different roll diameters and can be performed on both horizontal and vertical roll axes.

Plasma Transferred Arc (PTA) Cladding

Plasma transferred arc cladding is used for high-quality cladding applications where excellent composition control and surface finish are required. PTA uses a plasma arc to melt a powder feedstock, producing a weld deposit with very low dilution and excellent microstructure.

PTA cladding is particularly suitable for precision rolls where surface finish and dimensional accuracy are critical. The process offers excellent control over cladding composition and thickness, but at a significantly higher cost per kilogram of deposit compared to ESW or SAW.

Quality Control for Roll Cladding

The quality of roll cladding is critical for roll performance and product quality. The following quality control measures are recommended:

Non-Destructive Testing

Mechanical Property Verification

The mechanical properties of the cladding should be verified by:

1. Hardness testing: Vickers hardness should be measured at multiple locations across the cladding surface and at different depths to verify composition uniformity and hardness distribution.
2. Metallographic examination: Cross-sectional samples should be examined to verify the microstructure, bond quality, and absence of internal defects.
3. Wear testing: Laboratory wear testing under simulated rolling conditions can provide valuable data on expected service life.

Engineering Practice Reflections

In my experience with rolling mill roll cladding, the selection of the cladding process should be driven by the specific application requirements and the economics of the operation. For large hot rolling mills with high production volumes, ESW cladding offers the best balance of cost and performance. For precision cold rolling applications, PTA cladding provides the surface quality and dimensional accuracy required for high-quality product formation.

The success of roll cladding depends not only on material and process selection but also on proper surface preparation, careful control of welding parameters, and thorough quality inspection. Roll surfaces should be machined to remove scale and contamination before cladding, and the cladding should be machined to final dimensions after welding. Post-weld stress-relief heat treatment is recommended for thick claddings to reduce residual stresses and prevent cracking during service.

Summary

The cladding of rolling mill rolls is a well-established practice that significantly extends roll life and improves product quality. The selection of cladding material and process should be driven by the specific operating conditions, including temperature, pressure, and wear mechanism. ESW and SAW are the most commonly used processes for large-diameter rolls, while PTA is preferred for precision applications. Quality control measures including non-destructive testing, hardness verification, and metallographic examination are essential for ensuring satisfactory cladding performance. Engineers should develop comprehensive welding procedure specifications and maintain rigorous quality assurance practices to ensure consistent cladding quality and long-term roll reliability.