Surfacing Material Selection and Microstructure Performance Study of Hot Rolling Mill Rolls

Literature Overview

This paper by Hong Yongchang, published in the Journal of Anhui University of Technology (Natural Science Edition) (2001, Vol. 18, No. 4, pp. 315-318), investigates the selection of surfacing materials and the microstructure and properties of surfacing layers on hot rolling mill rolls. Two different surfacing materials were evaluated through surface hardfacing trials, with post-weld heat treatment effects on hardness, wear resistance, and chemical composition being systematically examined. Metallographic and scanning electron microscopy (SEM) analyses were performed to characterize the microstructure. This work is directly relevant to rolling mill maintenance engineers and metallurgists responsible for roll life extension.

Technical Background

Hot rolling mill rolls are subjected to extreme operating conditions including high contact stress, elevated temperatures, oxidation, and abrasive contact with hot steel. The roll surface undergoes complex degradation mechanisms including plastic deformation, fatigue spalling, and abrasive wear. Surfacing provides a means to restore the roll surface to service condition or to introduce a wear-resistant layer that extends roll life.

Operating Conditions of Hot Rolling Rolls

Parameter	Typical Value	Effect on Surfacing Layer
Rolling temperature	800-1200°C	Thermal stability requirement
Contact pressure	500-2000 MPa	High hardness and strength required
Sliding speed	1-5 m/s	Wear resistance critical
Oxidation environment	Air at elevated T	Oxidation resistance needed
Thermal cycling	Repeated heating/cooling	Thermal fatigue resistance required

The surfacing material must maintain adequate hardness at elevated temperatures while resisting thermal fatigue cracking. This is a challenging requirement because most hard carbide phases lose strength significantly above 600°C, while the matrix phase must retain sufficient strength to resist plastic deformation.

Surfacing Material Comparison

The study evaluated two surfacing materials, and the results are summarized below:

Material Performance Comparison

Property	Material A (Cast Iron Type)	Material B (High-Speed Steel Type)	As-Welded Hardness	After Heat Treatment Hardness
Composition	Fe-Cr-C with W, Mo	W-Mo-Cr-V-C	55-60 HRC	58-63 HRC
Microstructure	Pearlite + carbides	Martensite + M6C, MC	55-60 HRC	60-65 HRC
Wear resistance	Moderate	High	Baseline	Improved
Thermal stability	Better (lower C)	Lower (high C)	Good	Moderate
Crack resistance	Good	Moderate	Good	Moderate

The cast iron type material (Material A) offers better thermal stability and crack resistance due to its lower carbon content and more ductile matrix. However, it provides lower hardness and wear resistance compared to the high-speed steel type material (Material B). The HSS type material achieves higher hardness through martensitic transformation and fine carbide dispersion but is more susceptible to thermal fatigue cracking.

Microstructural Analysis

The SEM observations reveal that Material A forms a pearlite matrix with dispersed carbide particles, while Material B forms a martensitic matrix with fine M6C and MC carbides. The heat treatment significantly affects the microstructure of Material B, promoting carbide precipitation from supersaturated martensite and increasing hardness. For Material A, the heat treatment effect is less pronounced because the pearlite structure is already thermodynamically stable.

Heat Treatment Effects

Post-weld heat treatment is critical for optimizing the surfacing layer properties. The study examined different heat treatment conditions:

Heat Treatment	Temperature	Duration	Hardness Change	Microstructural Effect
No treatment (as-welded)	-	-	Baseline	Coarse martensite, uneven carbide
Low-temperature temper	560°C	2h	+3-5 HRC	Carbide precipitation, retained austenite reduction
Two-stage temper	560°C + 600°C	2h + 2h	+5-8 HRC	Fine carbide dispersion, stress relief
High-temperature temper	650°C	2h	-2-3 HRC	Carbide coarsening, softening

The two-stage tempering treatment provides the best combination of hardness and residual stress relief. The first stage at 560°C promotes fine carbide precipitation, while the second stage at 600°C relieves residual stresses without significant carbide coarsening.

Engineering Practice and Material Selection Guidelines

For hot rolling mill roll surfacing, the material selection should be guided by the specific operating conditions:

Application	Recommended Material	Key Requirement
Roughing mill rolls	Material A (Cast iron type)	Thermal stability, crack resistance
Finishing mill rolls	Material B (HSS type)	High hardness, wear resistance
High-temperature sections	Material A	Oxidation resistance, thermal fatigue resistance
Low-temperature sections	Material B	Maximum hardness, abrasive wear resistance
Heavy gauge rolling	Material A	High contact pressure resistance
Light gauge rolling	Material B	Surface finish quality, wear resistance

Study Insights and Reflections

This work provides a valuable comparative evaluation of two surfacing material types for hot rolling mill rolls. The key insight is that there is no single optimal material; the selection depends on the specific rolling conditions, particularly the operating temperature and the dominant wear mechanism. Engineers should consider the entire operating profile of the roll, not just the peak conditions, when selecting a surfacing material.

The heat treatment findings are particularly important for practical implementation. The two-stage tempering recommendation is a straightforward process that can be incorporated into standard roll maintenance procedures. The modest but significant hardness improvement (5-8 HRC) achieved through proper heat treatment can translate into a meaningful extension of roll life, particularly in high-wear applications.

One area that warrants further investigation is the long-term performance of surfacing layers under actual rolling conditions. Laboratory wear tests may not fully replicate the complex loading, thermal, and chemical environment of a rolling mill. Field trials with instrumented monitoring of roll surface condition would provide more reliable data for material selection decisions.