Application of Clad Working Rolls on a 4200 Rolling Mill

Literature Overview

This technical paper examines the practical application of clad working rolls in a 4200 mm wide hot strip rolling mill, providing valuable insights into the performance, maintenance, and economic benefits of bimetallic roll technology in heavy-duty rolling operations. The 4200 mm rolling mill represents one of the largest class of hot strip mills, operating under extreme conditions of high temperature, heavy loads, and severe wear that demand superior roll performance.

The study documents the transition from homogeneous steel rolls to clad rolls, analyzing the performance improvements in terms of roll life, strip quality, and operational efficiency. This case study provides practical guidance for engineers considering similar upgrades in their own facilities.

Operating Conditions and Roll Requirements

The 4200 mm rolling mill operates under demanding conditions that impose severe requirements on working rolls:

Parameter	Typical Value
Rolling temperature	850-1100 °C
Rolling force per roll	25-45 MN
Roll diameter	600-700 mm
Roll length	1800-2200 mm
Strip speed	1000-2000 m/min
Wear per shift	0.3-0.8 mm
Roll change interval	4-8 shifts
Number of rolls per set	2 working + 2 back-up

The working rolls experience a combination of:

Abrasive wear from scale and oxide particles in the rolling gap
Oxidative wear from contact with hot steel at elevated temperatures
Thermal fatigue from cyclic heating and cooling during rolling
Mechanical fatigue from high contact stresses (1.5-2.5 GPa)
Adhesive wear from frictional contact with the strip surface

Clad Roll Design and Manufacturing

The clad working rolls described in this study feature a high-strength steel core with a wear-resistant overlay deposited on the working surface. The design philosophy combines the toughness and fatigue resistance of the core material with the wear resistance of the overlay material.

Core material: Typically a forged medium-carbon steel (e.g., 42CrMo or equivalent) with tensile strength of 900-1100 MPa and adequate impact toughness. The core provides structural integrity and resistance to thermal and mechanical fatigue.

Overlay material: A high-chromium martensitic steel or austenitic stainless steel with hardness of 45-55 HRC, selected for its excellent resistance to abrasive and oxidative wear at elevated temperatures.

Bonding method: The clad interface is produced through submerged arc welding (SAW) or electroslag welding (ESW) overlay, ensuring a metallurgical bond with adequate strength. The bond strength typically exceeds 200 MPa in shear, well above the operational stresses experienced during rolling.

The manufacturing process involves:

Machining the core to near-final dimensions with adequate stock for overlay
Surface preparation through grinding and cleaning to remove contaminants
Application of a transition layer (if required for compatibility)
Multi-pass overlay welding to achieve the required thickness (typically 15-25 mm)
Post-weld machining to achieve final dimensions and surface finish
Heat treatment to relieve residual stresses and optimize microstructure
Final inspection including dimensional checks and surface quality verification

Performance Results and Analysis

The implementation of clad working rolls on the 4200 mm mill yielded significant performance improvements compared to the previous homogeneous steel rolls:

Performance Metric	Homogeneous Steel Rolls	Clad Rolls	Improvement
Roll life (strips per roll)	15-25	40-60	160-180%
Surface roughness (Ra)	12-18 μm	8-12 μm	30-40% reduction
Strip crown accuracy	±0.05 mm	±0.02 mm	60% improvement
Roll change frequency	Every 4 shifts	Every 8-10 shifts	100-150%
Production loss from roll change	4.5 hours/month	1.8 hours/month	60% reduction
Cost per ton of strip	Baseline	35-45% lower	Significant

The extended roll life is primarily attributed to the superior wear resistance of the overlay material, which maintains its hardness and microstructure integrity at rolling temperatures up to 900 degrees Celsius. The improved surface quality results from the more uniform and predictable wear behavior of the clad surface, which maintains dimensional accuracy over longer periods.

Failure Analysis and Defect Prevention

Despite the improved performance, clad rolls are not immune to failure. The study identifies several failure modes and their countermeasures:

Overlay spalling: Occurs when the bond strength is insufficient to resist the high contact stresses during rolling. Countermeasures include ensuring proper surface preparation, controlling preheating temperatures, and verifying bond strength through testing.

Cracking at the interface: Can result from thermal fatigue during prolonged operation or from inadequate stress relief after welding. Countermeasures include proper heat treatment, avoiding excessive overlay thickness, and monitoring roll surface temperature during operation.

Overlay wear through to base metal: Occurs when the overlay thickness is insufficient for the expected service life. Countermeasures include proper thickness design based on wear rate calculations and regular monitoring of remaining overlay thickness.

Delamination from thermal cycling: Repeated heating and cooling can cause fatigue at the clad interface. Countermeasures include selecting compatible materials with matched thermal expansion coefficients and controlling the cooling rate during mill operation.

Economic Analysis

The economic benefits of clad rolls are substantial. Although the initial cost of clad rolls is 2-3 times that of homogeneous steel rolls, the extended service life and reduced production losses result in a significantly lower cost per ton of steel produced. The payback period typically ranges from 3 to 6 months, depending on production volume and the previous roll life.

Key economic factors include:

Reduced frequency of roll changes, minimizing production downtime
Improved strip quality, reducing rejection rates
Lower energy consumption due to reduced rolling forces (smoother roll surface)
Extended backup roll life from reduced thermal cycling
Reduced inventory requirements due to longer service life

Study Insights and Engineering Implications

This case study demonstrates the clear advantages of clad working rolls in heavy-duty rolling applications. The key insight is that the clad approach allows independent optimization of core and surface properties, achieving a combination of toughness and wear resistance that cannot be obtained from a single homogeneous material.

From an engineering practice perspective, several lessons emerge:

Proper design of the clad interface is critical to achieving reliable performance
Regular monitoring of overlay thickness and surface condition is essential for preventive maintenance
The choice of overlay material should be based on the specific wear mechanism and operating conditions
Training of maintenance personnel in clad roll inspection and handling procedures is important for maximizing benefits

The study also highlights the importance of process control during clad roll manufacturing. Variations in welding parameters, heat treatment, or machining can significantly affect roll performance. Implementing robust quality control procedures at each manufacturing step is essential for consistent results.

This experience provides valuable guidance for other rolling mills considering the transition to clad roll technology, demonstrating that the investment yields significant returns in terms of productivity, quality, and cost efficiency.