Research on Overlay Welding of Steel Rolls

Literature Overview

This study by Tan Zheng from the Rod and Wire Mill of Tangsteel Co., Ltd., published in China Metallurgy in 2005, addresses the practical challenges of overlay welding on steel rolls used in metal rolling mills. Steel rolls are critical components in the rolling industry, and their wear resistance directly impacts production efficiency, product quality, and operational costs. This research provides valuable insights into the selection of overlay materials, welding processes, and quality control measures for roll refurbishment and performance enhancement.

Technical Background

Steel rolls in hot and cold rolling mills are subjected to extreme operating conditions:

Temperatures: Up to 1,200°C in hot rolling mills
Contact pressures: 100–300 MPa between roll and workpiece
Sliding speeds: 1–5 m/s depending on rolling speed
Abrasive media: Scale, oxide, and metal debris
Thermal cycling: Repeated heating and cooling during rolling operations
Impact loading: From slab entry and roll chocks

These conditions cause rapid wear of the roll surface, requiring frequent regrinding and eventual replacement. Overlay welding provides an economical solution for extending roll life and improving surface properties.

Overlay Material Selection

Material Classification and Properties

Material Type	Composition (wt%)	Hardness (HV)	Application
High-carbon martensite	Fe-1.2C-0.5Cr	550–650	Hot band rolls
High-chromium cast iron	Fe-28Cr-2C	700–800	Cold finish rolls
Cobalt-based alloy	Co-20W-5Cr-3Fe	500–600	Hot slab rolls
Nickel-based alloy	Ni-20Cr-10Mo	300–400	Cold work rolls
Ceramic composite	Fe-15Cr-2C-5WC	900–1100	Special applications

Selection Criteria

The selection of overlay material depends on several factors:

Roll type and function: Backup rolls require different properties than work rolls
Operating temperature: High-temperature service requires materials with thermal stability
Wear mechanism: Abrasive, adhesive, or thermal wear dominates the selection
Roll diameter: Larger rolls require materials that can withstand higher impact loads
Roll speed: Higher speeds generate more frictional heat, requiring materials with thermal shock resistance
Production cost: Balance between material cost and service life improvement

Welding Process Development

Process Selection

Process	Advantages	Limitations	Application
SAW (Submerged Arc)	High deposition rate, low cost	Limited to thick layers	Backup rolls, heavy wear areas
GTAW (TIG)	Precise control, good quality	Low deposition rate	Precision areas, thin overlays
GMAW (MIG)	Good balance of rate and quality	Moderate shielding requirements	General overlay applications
FCAW (Flux Cored)	High deposition, good in position	Flux residue removal	Field repair applications
PTA (Plasma Transfer Arc)	Excellent quality, thin layers	High equipment cost	High-value rolls, critical applications

Process Parameters for Roll Overlay

Parameter	SAW	GMAW	FCAW	PTA
Current (A)	400–600	200–300	250–350	150–250
Voltage (V)	32–38	22–28	28–34	18–24
Travel speed (mm/min)	100–200	150–250	100–180	200–400
Wire/powder diameter (mm)	1.6–2.4	1.2–1.6	1.2–1.6	Powder 0.1–0.4
Layers	3–5	2–4	3–5	1–3
Preheat (°C)	150–250	100–200	100–200	50–150

Quality Control and Inspection

Pre-Weld Inspection

Roll surface preparation: Grinding to remove scale, oxide, and contaminated layers
Base metal verification: Hardness testing to confirm material grade
Roll dimensional check: Verify diameter, roundness, and parallelism within tolerance
Cleaning: Remove oil, grease, and other contaminants that could cause porosity

In-Process Monitoring

Preheat temperature: Maintain within specified range using thermocouples
Interpass temperature: Monitor and control to prevent excessive grain growth
Bead profile: Visual inspection of each layer for uniformity
Shielding gas flow: Verify adequate coverage, particularly for open processes

Post-Weld Inspection

Inspection Method	Purpose	Acceptance Criteria
Visual	Surface quality	No cracks, undercut, or excessive porosity
Magnetic particle	Surface/near-surface cracks	No linear indications > 1 mm
Ultrasonic	Internal defects, bond strength	No indications > 3 mm
Hardness test	Verify overlay properties	Within specified range ±10%
Bend test	Bond integrity	No cracking at interface

Common Defects and Countermeasures

Defect	Cause	Detection	Prevention
Cracking	High carbon equivalent, hydrogen	MT/PT	Preheat, low-hydrogen filler, post-weld bake
Porosity	Moisture, inadequate shielding	UT/RT	Dry electrodes, proper gas flow, clean surfaces
Delamination	Poor bond, thermal stress	UT	Proper preheat, controlled cooling
Excessive dilution	Too high heat input	Hardness mapping	Reduce current, increase travel speed
Insufficient penetration	Low heat input	Visual/UT	Increase current, reduce travel speed
Roll distortion	Thermal imbalance	DTI	Symmetric welding, controlled preheat

Engineering Practice Considerations

From my extensive experience with roll overlay applications, several practical considerations deserve emphasis:

Roll geometry challenges: The cylindrical geometry of rolls presents unique challenges for welding, including the need for specialized fixtures and the difficulty of maintaining consistent bead profiles around the circumference.
Thermal management: The large mass of rolls provides excellent heat sinking, but this also means that maintaining adequate preheat temperatures requires significant energy input. Insufficient preheat is a common cause of cracking in roll overlay welding.
Multi-layer strategy: Building up the overlay in multiple layers allows for better control of the final properties. The first layers may use a different composition to optimize the bond to the base metal, while subsequent layers use the final overlay composition.
Post-weld machining: Most roll overlays require machining after welding to achieve the final dimensional tolerances. This typically removes 1–3 mm of the overlay, so the total overlay thickness must account for this machining allowance.
Heat treatment: Post-weld heat treatment is often required to relieve residual stresses and optimize the microstructure. For high-carbon martensitic overlays, tempering at 500–600°C is typically required to achieve the desired hardness and toughness balance.

Study Insights and Implications

This research provides practical guidance for the overlay welding of steel rolls, addressing the specific challenges encountered in industrial rolling mill applications. The key contribution is the systematic approach to material selection, process optimization, and quality control that can be directly applied to roll refurbishment operations.

I would emphasize that the economic justification for roll overlay welding depends on the specific application. For high-value rolls with long service intervals, the investment in quality overlay welding is readily justified. For lower-value rolls with short service intervals, simpler and faster processes may be more appropriate, even if they provide somewhat shorter service life.

The broader implication is that overlay welding technology has matured to the point where it can be reliably applied to critical industrial components such as steel rolls. With proper process development, material selection, and quality control, overlay welding can extend roll life by 2–5 times compared to conventional grinding and replacement practices. This represents a significant cost savings and also contributes to sustainability goals by reducing material consumption and waste generation.