Hardfacing Technology for Hot-Rolled Backup Rolls: Process Development and Industrial Performance

Literature Overview

The paper by Lin Jiandong from Panzhihua Iron and Steel Research Institute, published in the journal Electric Welder (2009, Vol. 39, No. 7, pp. 85-89), addresses a critical industrial problem in hot strip mills: the low service life of hardfaced backup rolls made from 70Cr3NiMo surface-quenched forged steel. The author identifies that China lacked a mature hardfacing material system for backup roll repair at that time, prompting the development of a proprietary flux-cored wire designated PRJ225, matched with flux 107. This study represents a systematic approach combining materials development, process optimization, and industrial validation—a model that remains relevant to modern repair engineering.

Core Technical Challenge

Hot-rolled backup rolls endure extreme conditions: high contact pressure (typically 100–300 MPa), cyclic thermal loading from hot strip (up to 900°C), and abrasive contact with work rolls. The base material, 70Cr3NiMo forged steel with surface induction hardening, provides adequate core toughness but the hardened surface layer is susceptible to spalling, plastic deformation, and abrasive wear during service. Conventional repair using H25Cr3Mo2MnVA flux-cored wire with SJ301 flux resulted in acceptable initial performance but insufficient long-term durability, with average tonnage throughput falling short of production requirements.

Materials Development: PRJ225 Flux-Cored Wire

The key innovation in this work is the development of PRJ225 flux-cored wire. While the paper does not disclose the complete chemical composition, the designation suggests a high-alloy composition with elevated carbon and chromium content designed to produce a high-volume-fraction of hard carbides (likely Cr7C3 and Fe3C) in a tough martensitic or bainitic matrix. The matching flux 107 was selected to provide adequate slag protection, appropriate dilution control, and favorable solidification characteristics.

Process Design Considerations

The hardfacing process for backup rolls requires careful attention to several factors. The preheating temperature must be sufficient to reduce thermal stress in the thick roll body (typically 200–300°C for forged steel of this class) while avoiding excessive softening of the induction-hardened layer. Interpass temperature control is critical to prevent cracking in the hardfacing layer, which typically contains high carbon equivalent. The paper emphasizes "appropriate hardfacing process" parameters, which in practice include:

• Welding current and voltage selection to achieve adequate penetration without excessive dilution from the base metal
• Travel speed optimization to balance deposition rate with cooling rate
• Layer-by-layer temperature management to control residual stress accumulation
• Post-weld stress relief treatment to reduce the risk of delayed cracking

Industrial Performance and Engineering Significance

The industrial trial results are impressive: average tonnage throughput exceeding 650,000 tonnes, with a maximum of 1,126,000 tonnes, representing a domestic leading level at the time. This performance surpasses that of other Chinese steel companies' hot-rolled backup roll hardfacing repairs. The improved contact fatigue resistance is particularly noteworthy, as backup roll failure is often initiated by subsurface fatigue cracking rather than surface wear alone.

Reflections and Engineering Implications

This study exemplifies the principle that material development must be coupled with process optimization and validated through industrial-scale trials. The approach of developing a proprietary consumable rather than relying on commercially available products reflects the reality that specialized applications often require tailored solutions. For modern engineers, this paper reinforces several lessons: the importance of understanding the specific failure mechanism (in this case, contact fatigue combined with wear), the value of matching consumable chemistry to service conditions, and the necessity of industrial validation before claiming technical superiority. The methodology—problem identification, materials development, laboratory characterization, and industrial trial—remains a sound engineering framework applicable to contemporary repair challenges.