Weld Overlay Process for Rolling Mill Rolls

Literature Overview

This study note examines the weld overlay process for rolling mill rolls, which are critical components in steel and metal rolling mills. Rolling mill rolls are subjected to severe wear, thermal fatigue, and mechanical loading during the rolling process, and the weld overlay layer must provide excellent wear resistance, thermal stability, and bond strength. The literature covers the various overlay processes used for rolling mill rolls, including submerged arc welding (SAW), gas metal arc welding (GMAW), plasma transferred arc (PTA) welding, and laser cladding, and provides guidance on process selection, parameter optimization, and quality control.

Core Technical Points

The selection of the overlay process depends on the roll material, the required overlay thickness, the desired overlay properties, and the production volume. For large-diameter rolls with thick overlays, SAW is commonly used due to its high deposition rate and deep penetration. For thinner overlays or precision applications, GMAW or PTA welding is preferred. Laser cladding is used for high-quality overlays with minimal dilution and excellent metallurgical bonding.

The literature emphasizes that the dilution rate is a critical parameter that affects the overlay properties. High dilution with the base metal can reduce the hardness and wear resistance of the overlay, while low dilution can lead to poor bond strength. The literature recommends optimizing the welding parameters to achieve a dilution rate that provides the best balance between overlay properties and bond strength.

Process Parameter Optimization

The literature provides detailed guidance on the optimization of welding parameters for each process. For SAW overlay, the key parameters are current, voltage, travel speed, and wire feed speed. The literature recommends using a low current density to minimize dilution and to promote the formation of a fine-grained microstructure. The travel speed should be controlled to maintain a consistent bead width and profile.

For GMAW overlay, the literature recommends using a short-circuit or spray transfer mode, depending on the wire diameter and the desired bead profile. The shielding gas composition is typically a mixture of argon and carbon dioxide, with the ratio adjusted to optimize arc stability and weld metal properties. The literature also recommends using a wire with a composition that is tailored to the specific application, such as a high-carbon, high-chromium composition for wear-resistant overlays.

Quality Control and Defect Prevention

The literature emphasizes the importance of quality control in the weld overlay process for rolling mill rolls. Common defects include porosity, lack of fusion, cracks, and inclusions. The literature provides guidance on the prevention of these defects through proper base metal preparation, welding parameter optimization, and post-weld inspection.

The literature recommends that all weld overlay operations be performed in accordance with a qualified welding procedure specification (WPS) that has been verified through weld procedure qualification testing (WPQ). The literature also recommends that the overlay layer be inspected using non-destructive testing methods such as magnetic particle testing (MT) or ultrasonic testing (UT) to detect surface and subsurface defects.

Integration with Engineering Practice

In engineering practice, the weld overlay of rolling mill rolls is typically performed in a dedicated workshop where the roll can be rotated and the welding process can be controlled. The literature provides guidance on the preparation of the roll surface, including grinding or machining to remove the existing wear surface, and cleaning to remove oil, rust, and scale. The literature also recommends that the roll be preheated to 200–300°C to reduce the cooling rate and minimize the risk of cracking.

A practical consideration is the alignment of the overlay beads. For large-diameter rolls, the overlay is typically deposited in a spiral pattern that follows the roll circumference. The literature recommends using a numerical control (NC) system to control the welding torch position and travel speed to ensure consistent bead placement. The literature also recommends that the overlay thickness be measured at regular intervals to ensure uniform coverage.

Key Questions and Reflections

The literature raises the question of how to balance the wear resistance of the overlay with the thermal fatigue resistance of the roll. High-hardness overlays may be prone to thermal fatigue cracking under the cyclic thermal loading experienced during rolling. The literature suggests that a multi-layer overlay design, with a high-hardness surface layer and a more ductile transition layer, may provide the best combination of wear resistance and thermal fatigue resistance. Another reflection is that the weld overlay process for rolling mill rolls is a critical maintenance operation that can significantly extend the service life of the rolls, and the quality of the overlay directly impacts the productivity and quality of the rolling mill.

Study Insights and Implications

The study of the weld overlay process for rolling mill rolls highlights the importance of process selection, parameter optimization, and quality control in achieving a high-quality overlay. The literature's comprehensive coverage of the various overlay processes, from SAW to laser cladding, provides valuable guidance for engineers selecting the appropriate process for specific applications. The emphasis on quality control and defect prevention underscores the critical role of the weld overlay in ensuring the reliability and performance of rolling mill rolls. For engineers involved in the maintenance and refurbishment of rolling mill equipment, the literature provides a practical framework for optimizing the weld overlay process to extend roll life and improve rolling mill productivity.