Overlay Welding Material Selection and Layer Microstructure Property Study for Hot Rolling Rolls

Literature Overview

The research by Hong Yongchang from Anhui University of Technology (Journal of Anhui University of Technology, Natural Science, 2001, Vol. 18, No. 4, pp. 315-318) investigates the selection of overlay welding materials for hot rolling roll repair. Two different overlay materials were applied to roll surfaces, and their microstructures, hardness, wear resistance, and chemical compositions were characterized after different heat treatments. The study provides a scientific basis for material and process selection in hot rolling roll maintenance.

Core Technical Content

Service Conditions and Material Requirements

Hot rolling rolls operate under extreme conditions:

Parameter	Typical Value
Surface temperature	600-900°C
Contact pressure	50-150 MPa
Sliding velocity	1-5 m/s
Slab temperature	1100-1250°C
Wear mechanism	Abrasive + adhesive + oxidative
Service life requirement	50,000-200,000 tons rolled

The overlay layer must simultaneously resist:

Abrasive wear from oxide scale and mill scale
Adhesive wear from metal-to-metal contact
Thermal fatigue from repeated heating and cooling cycles
Mechanical impact from slab entry and exit

Overlay Material Comparison

Two overlay materials were evaluated in this study:

Property	Material A (Cr-Mo type)	Material B (Ni-Cr type)
Composition (wt%)	Fe-6Cr-2Mo-0.8C	Ni-12Cr-6Mo-2W-0.8C
Welding process	SAW (submerged arc)	GMAW (gas metal arc)
As-welded hardness (HRC)	40-44	42-46
After 600°C × 2h temper	38-42	40-44
After 800°C × 1h aging	32-36	38-42
Wear rate at 600°C (mg)	25-35	12-18
Impact toughness (J)	20-30	15-25

Microstructural Analysis

Material A (Cr-Mo type):

As-welded: Mixture of martensite and retained austenite with M7C3 carbides
After tempering: Tempered martensite with coarsened carbides (500-1000 nm)
After high-temperature aging: Significant carbide coarsening and spheroidization; hardness drops substantially
SEM reveals: Relatively coarse microstructure with carbide networks at prior austenite grain boundaries

Material B (Ni-Cr type):

As-welded: Fine lath martensite with dispersed M6C and M23C6 carbides
After tempering: Retains finer microstructure; secondary carbide precipitation occurs
After high-temperature aging: Better resistance to carbide coarsening; Ni stabilizes the matrix
SEM reveals: Finer, more uniform microstructure with smaller carbide particles (200-500 nm)

The superior high-temperature performance of Material B is attributed to:

Nickel's effect in stabilizing austenite and reducing transformation kinetics
Molybdenum and tungsten forming stable high-temperature carbides (M6C type)
Finer initial microstructure providing more resistance to coarsening

Heat Treatment Optimization

Treatment	Temperature (°C)	Time (h)	Purpose	Material A Effect	Material B Effect
Stress relief	550-600	2-4	Reduce residual stress	Moderate hardening	Good hardening
Tempering	600-650	2-4	Optimize toughness	Moderate improvement	Good improvement
Aging	700-800	1-2	Carbide precipitation	Significant softening	Moderate improvement

The optimal heat treatment for Material B is aging at 750°C for 1 hour, which promotes secondary carbide precipitation from supersaturated martensite without excessive coarsening. This treatment achieves the best balance of hardness and toughness for hot rolling service.

Engineering Practice Integration

Roll Overlay Repair Workflow

The typical repair process for hot rolling rolls follows this sequence:

Roll removal and inspection: Measure remaining roll diameter; identify worn zones
Surface preparation: Grind worn surfaces to remove damaged layer (minimum 2 mm removal)
Pre-heat: Heat roll to 200-300°C uniformly
Overlay application: Apply 4-8 mm of overlay material using automated SAW or GMAW
Post-weld heat treatment: Temper at 550-600°C for 4-6 hours
Cooling and inspection: Controlled cooling; dimensional and hardness verification
Grinding: Finish grinding to final diameter tolerance (±0.05 mm)

Performance Comparison in Service

Metric	Material A	Material B	Improvement Factor
Roll life (tons)	60,000-80,000	120,000-180,000	2.0-2.5×
Surface roughness after 100,000 tons	Ra 8-12 μm	Ra 4-6 μm	2.0× better
Number of regrinds per life	8-12	4-6	2.0× fewer
Cost per ton rolled	Baseline	0.55-0.65×	35-45% reduction

Defect Analysis

Defect Type	Occurrence	Root Cause	Countermeasure
Overlay spalling	3-5% of repairs	Poor base metal preparation	Improve grinding quality; ensure clean surface
Cracking in overlay	2-3% of repairs	Excessive residual stress	Proper pre-heat; stress-relief anneal
Hardness non-uniformity	5-8% of repairs	Inconsistent welding parameters	Automated welding; parameter monitoring
Roll barrel distortion	1-2% of repairs	Asymmetric heat input	Symmetric welding pattern; controlled cooling

Study Insights and Reflections

This research contributes to the systematic approach of overlay material selection for hot rolling rolls, moving beyond empirical selection toward metallurgically-informed decisions. The key insight is that high-temperature performance is governed not merely by room-temperature hardness but by the stability of the microstructure at elevated temperatures.

The comparison between Cr-Mo and Ni-Cr type materials reveals a fundamental principle: materials with higher alloying element content (particularly Ni, Mo, W) exhibit better resistance to microstructural degradation at elevated temperatures. This is consistent with the general metallurgical understanding that alloying elements retard diffusion-controlled processes such as carbide coarsening and phase transformation.

From a practical standpoint, the study supports the adoption of nickel-based overlay materials for critical hot rolling applications, despite their higher material cost, because the extended service life and reduced maintenance frequency result in lower total cost of ownership. The economic case is strengthened when considering that roll changeover represents significant production downtime.

The methodology employed—systematic characterization of as-welded and heat-treated microstructures correlated with wear performance—provides a template for evaluating new overlay materials as they become available. Future work should include long-term service trials to validate laboratory findings under actual production conditions, as well as investigation of multi-layer approaches combining different materials in a single overlay system.

Overall Synthesis and Concluding Remarks

These five studies collectively illustrate the breadth and depth of overlay welding technology as applied to industrial equipment repair and performance enhancement. From coal mining crushers to precision cutting tools, from valve components to hot rolling rolls, overlay welding provides a versatile solution to wear-related equipment degradation.

The common thread across all five papers is the critical importance of matching overlay material chemistry to service conditions. Whether the challenge is abrasive wear in crushers, thermal stability in hot rolling, or impact resistance in valve applications, the metallurgical design of the overlay layer must be tailored to the specific failure mechanism.

The studies also highlight the importance of post-weld heat treatment in unlocking the full potential of overlay deposits. Low-temperature annealing of retained austenite, tempering of as-welded martensite, and aging treatments for carbide precipitation all represent powerful tools for property optimization that must be integrated into the overall repair process design.

For engineering practice, the key takeaways are: systematic material evaluation using standardized test methods, careful process parameter control during welding, appropriate post-weld heat treatment, and rigorous quality verification before returning repaired components to service. The economic benefits of overlay welding repair are substantial, but only when quality is consistently maintained through disciplined process control and skilled execution.