Overlay Welding Technology for Rolls of Reversible Two-High Rolling Mill

Literature Overview

This 2001 paper by Gan Meijun from the Ningbo Institute of Metallurgy, published in Shanghai Metals (上海金属), describes the overlay welding technology applied to restore and enhance the wear resistance of rolls in a 360 mm × 760 mm reversible hot rolling mill. The paper covers the complete technical chain from material selection through welding parameter determination to quality verification, providing a practical reference for rolling mill maintenance engineers.

Technical Context

Rolls in reversible two-high hot rolling mills are subjected to extreme operating conditions:

Temperature: Surface temperatures of 800–1200°C during hot rolling operations.
Mechanical stress: Contact pressures of 100–300 MPa between the roll and the hot strip.
Wear mechanisms: Abrasive wear from scale, adhesive wear from metal-to-metal contact, and thermal fatigue from cyclic heating and cooling.
Service life requirement: Rolls must maintain dimensional accuracy and surface quality throughout their service life to ensure product quality.

When rolls reach the end of their service life, they can be refurbished by grinding down the worn surface and applying a new overlay layer, rather than being scrapped and replaced. This overlay welding refurbishment extends roll life significantly and reduces costs.

Overlay Welding Process Parameters

Parameter	Specification	Rationale
Roll dimensions	Φ360 mm × 760 mm	Standard hot rolling mill roll
Overlay material	Hardfacing electrode/wire with specific alloy composition	Provides wear and thermal resistance
Flux type	Rutile or basic flux (depending on electrode type)	Controls weld pool fluidity and slag properties
Welding process	SMAW (shielded metal arc welding)	Suitable for field conditions, flexible
Preheat temperature	200–300°C	Reduces cooling rate, prevents cracking
Interpass temperature	Maintained at 200–300°C	Consistent thermal cycle
Welding current	Process-specific (typically 200–300 A)	Controls deposition rate and penetration
Welding speed	Process-specific	Controls heat input and dilution
Overlay hardness	HRC 47	Balances wear resistance with impact toughness
Life improvement	1× (doubling of roll life)	Economic justification for refurbishment

Material Selection and Welding Material Specification

The selection of overlay welding material for hot rolling mill rolls must satisfy several competing requirements:

High hardness: To resist abrasive wear from scale and metal particles.
Good thermal shock resistance: To withstand repeated heating and cooling cycles without cracking.
Adequate toughness: To resist impact loading and prevent brittle fracture.
Low dilution sensitivity: The overlay properties must be maintained even with significant dilution from the roll base material.
Good bonding strength: The overlay must resist spalling from the roll surface under operating conditions.

The paper describes the selection of a specific electrode type and flux combination that achieves HRC 47 hardness in the overlay layer. This hardness level represents a deliberate balance — higher hardness would improve wear resistance but would reduce impact toughness and increase the risk of thermal fatigue cracking.

Microstructural Design of the Overlay Layer

The overlay layer microstructure typically consists of:

Matrix: Bainitic or martensitic iron-carbon matrix providing toughness.
Carbides: M₇C₃ or M₃C type carbides providing wear resistance.
Dilution zone: A transition region where the overlay alloy is mixed with the base material, creating a gradient in composition and properties.

The dilution zone is a critical area for engineering evaluation — it is often the weakest region of the overlay and the most likely location for spalling initiation.

Quality Control and Inspection

The paper emphasizes a comprehensive quality control program for roll overlay welding:

Pre-weld inspection: Verification of roll dimensional accuracy after grinding, surface cleanliness, and absence of cracks or other defects in the base material.
In-process monitoring: Control of preheat and interpass temperatures, welding parameter compliance, and visual inspection of each weld pass.
Post-weld inspection:

Visual examination (VE) of all overlay surfaces.
Magnetic particle testing (MT) for surface and near-surface cracks.
Hardness profiling across the overlay thickness to verify the hardness gradient.
Dimensional verification to confirm that overlay welding has not caused measurable distortion.

Functional testing: Trial rolling of a short section of material to verify that the refurbished roll performs satisfactorily in service.

Engineering Practice and Economic Analysis

The economic justification for roll refurbishment by overlay welding is straightforward:

A new roll of Φ360 mm × 760 mm for a hot rolling mill can cost significantly more than the cost of grinding and overlay welding a worn roll.
The overlay welding process restores the roll to its original dimensions and applies a wear-resistant surface that can extend the service life by approximately 100% (doubling the life).
The downtime for refurbishment is typically shorter than the lead time for a new roll, reducing production losses.

The paper's approach — systematic material selection, parameter optimization, and quality verification — represents a mature engineering methodology that can be applied to other heavy industrial component refurbishment applications.

Study Insights and Reflections

This paper, while focused on a specific industrial application, demonstrates several principles that are broadly applicable in welding engineering:

The importance of hardness-toughness balance: The selection of HRC 47 as the target overlay hardness reflects the engineering judgment required to balance competing mechanical properties. This is not a purely metallurgical decision but an economic one — higher hardness extends life but increases maintenance frequency if the overlay becomes too brittle.
The role of flux selection: In SMAW overlay welding, the flux type significantly influences weld pool fluidity, slag properties, and ultimately the surface quality of the overlay. This is often overlooked in favor of electrode selection, but the flux is equally important for achieving a smooth, defect-free overlay surface.
The value of systematic process documentation: The paper's structured approach to documenting material selection, welding parameters, and quality control criteria provides a model for welding procedure qualification in industrial maintenance applications.

For modern practice, the overlay welding of rolling mill rolls has evolved to include techniques such as submerged arc welding (SAW) for thicker deposits, plasma arc welding (PAW) for more precise control, and laser cladding for localized repair. However, the fundamental principles of material selection, thermal management, and quality verification remain unchanged from those described in this 2001 paper.