Development of Wear-Resistant Surfacing Electrodes for Hot Rolling Rolls

Literature Overview

The paper by Ying Pengzhan, Ge Changlu, and Cai Yingjun, published in Transactions of the China Welding Institution (1997, Vol. 18, Issue 2, pp. 6–10), reports on the development of a wear-resistant surfacing electrode specifically designed for hot rolling mill rolls. The research was conducted at China University of Mining and Technology, Xuzhou, with funding from the Ministry of Coal Industry. The work addresses a practical and economically significant problem in the steel and mining industries: the rapid wear of hot rolling rolls during operation and the need for reliable, cost-effective repair through surfacing.

Technical Background

Hot rolling rolls are subjected to extreme conditions during operation, including high temperatures (up to 1200°C at the roll surface), high contact stresses, and abrasive wear from scale and oxide particles. The roll surface must maintain dimensional accuracy and surface finish over extended periods, and wear leads to increased scale adhesion, surface roughness, and ultimately roll failure. Surfacing with a wear-resistant alloy provides an effective repair method, but the surfacing alloy must meet several demanding requirements: high hardness, good wear resistance at elevated temperatures, thermal stability, and adequate toughness to resist spalling and cracking under impact loading.

Electrode Development and Metallurgical Design

The authors systematically adjusted the flux composition of the surfacing electrode to optimise the alloy system deposited on the hot rolling roll substrate. The key design philosophy was to achieve a microstructure consisting of martensite, residual austenite, and hard carbides, which provides a combination of hardness, toughness, and wear resistance.

The flux composition was iteratively refined to control the carbon and alloy content of the deposit. The basic flux was selected for its low hydrogen content and its ability to produce a clean, slag-free weld surface. The addition of CaF₂ helped to stabilise the arc and reduce spatter, while CaCO₃ provided additional carbon to the weld metal.

Microstructural Analysis

Metallographic examination and X-ray diffraction analysis confirmed that the surfacing deposit consisted of martensite, residual austenite, and carbides. The carbide phase was identified as a mixture of Cr₇C₃ and M₇C₃ (where M represents Cr, Mo, and Fe), which are hard, wear-resistant phases that provide abrasion resistance. The residual austenite, stabilised by the alloying elements, contributes to toughness and reduces the susceptibility to cracking during service.

The microstructure is characterised by fine, needle-like martensite with dispersed carbide particles. The grain size is refined by the rapid cooling rate of the surfacing deposit, which contributes to the high hardness and wear resistance.

Wear Performance

The authors conducted both laboratory wear tests and industrial trials to evaluate the performance of the developed surfacing electrode. In the laboratory, high-stress abrasion wear testing was performed against a standard 45 steel reference material. The results showed that the surfacing alloy exhibited a wear resistance 6.1 times that of 45 steel, a significant improvement that translates directly into extended roll life and reduced downtime.

The thermal stability test at 580°C for 1000 minutes is particularly significant, as it simulates the conditions experienced by the roll surface during hot rolling. The fact that the hardness remained at HRC 58–59 after prolonged exposure at elevated temperature indicates that the microstructure is stable and that the carbides are resistant to coarsening and dissolution. This thermal stability is a critical requirement for hot rolling roll surfacing alloys.

Engineering Practice Considerations

In practice, the surfacing of hot rolling rolls requires careful attention to several factors:

1. Preparation of the roll surface: The existing worn surface must be ground or machined to remove scale and to provide a clean, flat surface for the surfacing deposit. The surface roughness should be controlled to ensure good fusion between the deposit and the base metal.
2. Interpass temperature control: The surfacing deposit is susceptible to cracking if the interpass temperature is too high, as this promotes grain growth and reduces the toughness of the martensitic matrix. An interpass temperature of 150–250°C is typically recommended.
3. Post-weld heat treatment: In some cases, a stress relief treatment is required after surfacing to reduce residual stresses and to stabilise the microstructure. However, the heat treatment must not exceed the temperature at which the hardness begins to degrade, which in this case is well above 580°C.
4. Number of passes: The surfacing layer thickness should be optimised to provide adequate wear resistance without excessive build-up that would require subsequent machining. Typically, a single pass with a 3.2–4.0 mm electrode provides a deposit thickness of 2–4 mm, which is adequate for most hot rolling roll applications.

Study Insights

This work demonstrates the power of systematic electrode design in solving practical engineering problems. The iterative approach to flux composition adjustment, combined with rigorous metallographic and wear testing, is a model of applied metallurgical research. The 6.1× improvement in wear resistance over 45 steel is a substantial result that would have had immediate economic impact in the steel and mining industries. The thermal stability of the surfacing alloy at 580°C is particularly noteworthy, as it indicates that the alloy is suitable for the most demanding hot rolling applications. In my own experience with roll surfacing, the challenge is always to balance hardness with toughness; the residual austenite in this alloy provides a useful buffer against cracking, which is a common failure mode in high-hardness surfacing deposits.