Surface Hardfacing of Aluminum Billet Continuous Casting and Rolling Mill Roll Cores

Literature Overview

This study note examines the surface hardfacing of roll cores for aluminum billet continuous casting and rolling mills, reported by Wang Xiangdong from Bohai Aluminum Industry Equipment Factory in 2000. Aluminum rolling presents unique challenges compared to steel rolling, including low rolling temperatures, high ductility of the workpiece, and sensitivity to surface defects. The roll core, typically made of a ductile cast iron or low-alloy steel body, requires a hardfaced surface to resist galling, wear, and thermal fatigue during continuous rolling operations.

Core Technical Challenges

Aluminum Rolling Conditions

Aluminum billet rolling differs fundamentally from steel rolling in several respects:

Rolling temperatures are relatively low (typically 200–500 °C for hot rolling, ambient for cold rolling).
Aluminum is highly reactive and tends to adhere to roll surfaces, causing galling and material transfer.
The low melting point of aluminum (660 °C) means that even moderate heat input during hardfacing can cause unwanted intermetallic formation if incompatible materials are used.
Surface quality requirements are stringent, as aluminum products are often used in consumer and aerospace applications.

Roll Core Material and Overlay Requirements

The roll core for aluminum mills is typically made of a ductile cast iron (e.g., QT500-7) or a low-alloy steel body to provide sufficient toughness and resistance to rolling loads. The hardfacing layer must provide:

Requirement	Specification
Hardness	40–55 HRC
Wear resistance	High resistance to galling and adhesive wear
Thermal fatigue resistance	Adequate for rolling temperature cycles
Surface smoothness	Ra ≤ 1.6 μm after grinding
Bond strength	≥ 200 MPa (shear)
Compatibility with aluminum	No intermetallic formation at service temperatures

Overlay Process Selection

For aluminum mill rolls, the preferred overlay methods include:

Submerged Arc Welding (SAW): Suitable for build-up of thick layers with good penetration and low spatter. However, the high heat input may cause excessive dilution.
Gas Metal Arc Welding (GMAW): Provides good control over heat input and dilution, suitable for thinner overlay layers.
Plasma Transferred Arc (PTA) Surfacing: Offers excellent control over composition and dilution, ideal for precise overlay of wear-resistant alloys.
Flame Spraying: Used for the final surface layer, providing a controlled microstructure with low residual stress.

The selection of the overlay process depends on the required layer thickness, composition control needs, and production volume. For aluminum mill rolls, PTA surfacing is increasingly favored due to its ability to produce layers with precise composition and minimal dilution.

Defect Analysis and Quality Control

Common Defects in Aluminum Mill Roll Hardfacing

Defect	Cause	Impact	Countermeasure
Cracking	High residual stress, incompatible materials	Roll failure, surface defects	Preheat, use compatible filler, stress relief
Spalling	Poor bond strength, thermal fatigue	Surface roughness, product defects	Optimize microstructure, control cooling rate
Galling	Incompatible overlay composition	Material transfer, surface damage	Select non-stick alloy, surface treatment
Excessive dilution	High heat input, thin layer	Reduced hardness, poor wear resistance	Control heat input, use PTA or low-heat processes
Porosity	Moisture, poor gas protection	Reduced strength, surface defects	Dry materials, ensure proper shielding

Metallurgical Bonding and Interfacial Reactions

The metallurgical bond between the roll core and the hardfacing layer is critical. In aluminum rolling applications, the overlay must not react with aluminum during service. This requires careful selection of overlay composition to avoid elements that form intermetallic compounds with aluminum (e.g., titanium, zirconium, and certain rare earth elements). The transition zone should exhibit a gradual compositional gradient to minimize residual stresses and improve bond strength.

Engineering Practice and Process Optimization

Process Parameter Optimization

The hardfacing process for aluminum mill rolls requires careful optimization of the following parameters:

Preheating temperature: 150–250 °C to reduce residual stresses without causing base steel softening.
Heat input: Low to moderate (1.5–3.0 kJ/mm) to minimize dilution and control microstructure.
Interpass temperature: Controlled to maintain adequate toughness in the overlay.
Cooling rate: Gradual cooling to prevent cracking and stabilize the microstructure.
Post-weld heat treatment: Stress relief at 550–650 °C to reduce residual stresses.

Performance Evaluation

The performance of hardfaced aluminum mill rolls is evaluated through:

Hardness mapping: Ensuring uniform hardness across the roll surface.
Wear testing: Measuring wear rate under simulated rolling conditions.
Thermal fatigue testing: Evaluating resistance to thermal cycling.
Service monitoring: Tracking roll life, surface quality, and product quality in production.

Key Reflections

The hardfacing of aluminum mill roll cores represents a specialized application that requires careful consideration of material compatibility, process parameters, and service conditions. The relatively low rolling temperatures and the reactive nature of aluminum impose unique constraints on overlay material selection. Engineers must balance hardness and wear resistance with toughness and thermal fatigue resistance to achieve optimal roll performance. The evolution from empirical approaches to systematic metallurgical analysis and process optimization has significantly improved the reliability and performance of hardfaced aluminum mill rolls. Future developments should focus on advanced overlay materials with improved thermal fatigue resistance and surface properties tailored to specific aluminum alloy products.