Cladding Technology for Steel Rolling Assistant Rollers and Deep Bending Rollers

Literature Overview

This 2007 publication by Ma Yaodong, Fu Dingmei, Shen Fenggang, and Liu Jingfeng, jointly authored by Dalian Huarui Heavy Industry Special Spares Manufacturing Co. and the Welding Research Institute of MCC Group, addresses the restoration and performance enhancement of critical rolling mill components: assistant rollers (used to support and guide the strip during rolling) and deep bending rollers (used in the final bending pass of strip production). These rollers operate under extreme conditions involving high contact stress, abrasive wear from the strip surface, and cyclic thermal loading. The paper presents cladding technology solutions to extend roller service life and restore worn dimensions.

Operational Challenges and Material Requirements

Assistant rollers and deep bending rollers in hot strip mills face the following demanding service conditions:

Condition	Typical Value	Consequence
Contact stress	1.5–2.5 GPa	Surface plastic deformation, spalling
Surface temperature	300–600 °C (hot strip)	Oxidation, thermal fatigue
Sliding speed	10–20 m/s	Abrasive wear, adhesive wear
Strip surface contaminants	Scale, oxide, lubricant residues	Accelerated wear
Cyclic loading frequency	10–50 cycles/hour	Fatigue crack initiation

The base material of these rollers is typically a high-chromium cast iron (e.g., 40CrMnMo or similar) or a forged alloy steel. The cladding layer must provide superior wear resistance and hardness while maintaining adequate toughness to resist spalling under high contact stress.

Cladding Process Selection and Parameters

The study evaluates multiple cladding approaches for these roller applications:

Process	Advantages	Limitations	Typical Clad Composition
GMAW (MIG)	High deposition rate, good for large areas	Higher dilution, coarser microstructure	Cr13 / Cr26 cast iron
GTAW (TIG)	Low dilution, fine control	Low deposition rate, labor-intensive	Hardfacing alloy
SAW	Very high deposition rate, deep penetration	Requires flux, limited position flexibility	Cr-Mo alloy
PTA (Plasma Transfer Arc)	Excellent control, low dilution	High equipment cost, powder handling	Cr-based hardfacing
Laser cladding	Very low dilution, precise geometry	Limited to small areas, high cost	Ni-Cr or Cr-based

For assistant rollers, the primary requirement is abrasion resistance against strip scale, and the cladding layer typically contains 12–14% Cr with carbide-forming elements (Mo, V) to produce hard M7C3 carbides. For deep bending rollers, which experience higher contact stress, the cladding must balance hardness (50–55 HRC) with fracture toughness to prevent spalling.

Microstructural Considerations

The microstructure of the cladding layer is critical for performance. In Cr-based hardfacing alloys, the distribution of carbides (M7C3, M23C6, or MC) determines wear resistance. A well-designed cladding composition produces a fine, evenly distributed carbide network in a martensitic or martensite-plus-austenite matrix. The following microstructural features are particularly important:

Carbide morphology: Spherical or short-rod carbides provide better toughness than long, interconnected carbide networks.
Matrix composition: A mixed martensite-austenite matrix offers better resistance to thermal fatigue than fully martensitic structures.
Interface integrity: The clad-base interface must be free of cracks, porosity, and unmelted inclusions. A gradual transition zone with controlled dilution is desirable.
Residual stress: The cladding process introduces residual compressive stress in the surface layer, which is beneficial for fatigue resistance. However, excessive tensile residual stress at the interface can promote spalling.

Engineering Implementation and Quality Control

The cladding process for large-diameter rollers requires careful planning of the welding sequence to minimize thermal distortion and ensure uniform coverage. The following quality control measures are recommended:

Surface preparation: Thorough cleaning and roughening of the roller surface to ensure mechanical and metallurgical bonding.
Preheating: 200–300 °C to reduce thermal gradient and prevent cracking in the base material.
Welding sequence: Multi-pass, overlapping beads with controlled interpass temperature (150–250 °C).
Post-weld treatment: Stress relief at 600–650 °C to reduce residual stress without softening the cladding layer excessively.
NDT: Magnetic particle testing (MT) of the cladding surface and ultrasonic testing (UT) of the interface.
Hardness verification: Surface hardness measurement at multiple points to confirm uniform cladding quality.

Key Questions and Reflections

A significant engineering question is the long-term performance of cladded rollers under cyclic thermal and mechanical loading. While laboratory tests confirm high hardness and wear resistance, field performance depends on the integrity of the clad-base interface under repeated stress cycles. Engineers should monitor roller condition during service, including periodic NDT and dimensional measurements, to detect early signs of spalling or delamination.

Another reflection concerns the economic balance between cladding frequency and roller replacement. Cladding can extend roller life by 2–3 times, but the cost of cladding, downtime, and quality control must be weighed against the cost of new rollers. A life-cycle cost analysis is essential for optimal maintenance planning.

Summary

The cladding technology for assistant rollers and deep bending rollers represents a practical solution to the challenge of extending the service life of high-wear rolling mill components. The study demonstrates that appropriate cladding composition, process selection, and quality control can achieve significant improvements in wear resistance and operational reliability. Engineers involved in rolling mill maintenance should consider cladding as a cost-effective alternative to complete roller replacement, provided that the process is properly qualified and monitored throughout the service life of the cladded component.