Research on Overlay Materials and Processes for Hot Rolling Rolls

Literature Overview

This 1998 paper by Shen Fenggang and colleagues from Xi'an Jiaotong University represents a systematic approach to solving the complex tribological challenges faced by hot rolling mill rolls. The study investigates Cr-W-V and Cr-Mo-V overlay systems for backup rolls and edger rolls, addressing both thermal fatigue resistance and wear performance. The work is notable for its fundamental approach—starting from an analysis of the intrinsic factors governing thermal fatigue and wear, then rationalizing alloy design and microstructure targets before developing practical welding procedures.

Core Technical Analysis

Hot rolling rolls operate under extreme combined loading: cyclic thermal stress from contact with hot steel (typically 800-1200°C), mechanical contact stress from rolling loads (up to 2 GPa), and abrasive wear from scale and oxide particles. The overlay design must simultaneously address thermal fatigue cracking (initiated at the surface or subsurface) and abrasive wear (from scale removal and direct metal contact).

Alloy Design Philosophy

The researchers identified that the key metallurgical features for optimal roll overlay performance are:

1. A uniform, stable lath martensite matrix as the base microstructure
2. Dispersed, stable vanadium carbide (VC) particles for wear resistance
3. Avoidance of tungsten carbide (WC) as the primary reinforcement due to coarsening during thermal cycling

The Critical Role of Carbide Stability

The paper's most significant metallurgical insight is the observation that tungsten carbides (W₂C, W₆C₅) in Cr-W-V overlays tend to aggregate and coarsen during thermal fatigue cycling. This occurs because W-carbides have relatively low interfacial energy with the matrix and are susceptible to Ostwald ripening at elevated temperatures. In contrast, vanadium carbides (VC, V₄C₃) are thermodynamically more stable due to their higher lattice energy and stronger bonding with the matrix, making them resistant to coarsening even under repeated thermal cycling.

The Ni addition study reveals an important phase transformation consideration: while small amounts of Ni (<1%) can improve thermal fatigue by suppressing carbide coarsening and refining the martensite lath structure, excessive Ni significantly lowers the Ac1 temperature. This means that during normal rolling operation, the overlay may undergo unintended phase transformations (partial austenitization and softening) that compromise both hardness and thermal fatigue resistance.

Process Development

The researchers developed practical welding procedures for both backup rolls and edger rolls, recognizing that different roll types face different severity of service conditions:

• Backup rolls: Primarily subject to contact stress and thermal cycling; overlay thickness of 15-25 mm is typical
• Edger rolls: Subject to additional abrasive wear from scale; overlay thickness of 10-20 mm with emphasis on surface hardness

The welding process for roll overlays typically involves multi-pass arc welding (SMAW or SAW) with careful control of:

• Interpass temperature (maintained at 150-250°C to prevent cracking)
• Pass geometry (controlled bead width to height ratio of 2-3:1)
• Final heat treatment (tempering at 500-550°C to relieve residual stress while maintaining hardness)

Engineering Practice Integration

This paper's findings have direct relevance to modern hot strip mill operations where roll life directly impacts production efficiency and cost. The key engineering lessons include:

1. Carbide type selection is paramount: The choice between W-based and V-based carbide systems should be driven by thermal cycling severity rather than purely by initial hardness requirements.
2. Ni addition requires caution: While Ni can improve some properties, its effect on transformation temperatures makes it a double-edged sword in applications involving repeated thermal exposure.
3. Microstructure uniformity matters: The emphasis on uniform lath martensite with dispersed carbides highlights that overlay quality depends not just on composition but on process control to achieve the target microstructure consistently across the entire roll surface.

Key Questions and Reflections

The paper raises several important considerations for modern practice:

• The study was conducted in the late 1990s, and modern roll coatings increasingly employ hardfacing alloys with improved thermal stability. However, the fundamental metallurgical principles remain valid.
• The question of how overlay thickness affects thermal fatigue behavior is not fully addressed—thicker overlays may develop thermal gradients that create additional stress concentrations at the overlay-substrate interface.
• The paper focuses on thermal fatigue and wear separately, but in practice these mechanisms interact synergistically: thermal fatigue cracks can accelerate wear by exposing fresh surface material to abrasive attack.

Study Insights and Implications

This research exemplifies the systems engineering approach to overlay design: rather than simply selecting the hardest available alloy, the authors systematically analyzed the failure mechanisms (thermal fatigue cracking and abrasive wear), identified the metallurgical features that resist each mechanism, and then optimized the alloy composition and heat treatment to achieve the target microstructure. The finding that VC is superior to WC for thermal cycling applications is particularly valuable, as it contradicts the common assumption that harder carbides always provide better performance. For engineers specifying overlay materials for hot rolling applications, this work reinforces that the service environment—not just the material properties—must drive the alloy selection decision.