High-Temperature Wear Behavior of Overlay Hardfacing Layer on 42CrMo Continuous Casting Rolls

Literature Overview

This paper by Cheng Jianguo, Pan Longbo, Zhang Shuo, Jiang Ji, Cheng Caijin, Xie Qingzhong, Xing Xueqiang, and Si Tingzhi, published in Materials Protection (2021, Vol. 54, No. 7, pp. 37-43), investigates the high-temperature wear behavior of a 414N hardfacing layer applied to 42CrMo continuous casting rolls. The research was funded by the Anhui Provincial Key Research and Development Program (Grant No. 1804a09020067) and conducted at Baowu Heavy Industry Co., Ltd. The study provides valuable insights into the wear mechanisms operating at elevated temperatures relevant to continuous casting operations.

Background and Application Context

Continuous casting rolls are subjected to severe thermal and mechanical loading during the solidification of steel slabs, blooms, or billets. The roll surface is exposed to:

• Molten and semi-solid steel — Temperatures up to 1400–1500°C at the pour point
• Thermal cycling — Repeated heating and cooling as the strand passes through the mold and secondary cooling zones
• Mechanical contact — Contact pressure from the solidifying strand
• Oxidative environment — Scale formation on the strand surface creates abrasive particles
• Cooling water — Direct water impingement in secondary cooling zones

The 42CrMo base material provides good strength and toughness but lacks the surface hardness and wear resistance required for extended service life. The 414N hardfacing overlay is applied to enhance surface properties while maintaining the structural integrity of the roll body.

Experimental Methodology

Wear Testing Conditions

The high-temperature wear tests were conducted at three temperatures:

Characterization Techniques

• SEM (Scanning Electron Microscopy) — Surface morphology and wear mechanism identification
• EDX (Energy Dispersive X-ray Spectroscopy) — Elemental composition of worn surfaces and oxide layers
• XRD (X-Ray Diffraction) — Phase identification in oxide layers and worn surfaces

Key Findings

Wear Performance Degradation with Temperature

The study reveals a clear and significant degradation of wear performance with increasing temperature:

This rapid degradation between 400°C and 500°C indicates a critical transition temperature range where the wear mechanism fundamentally changes.

Wear Mechanism Evolution

At 400°C:

• Oxidative wear — Formation of protective oxide layer that partially resists further oxidation
• Abrasive wear from wear debris — Hard debris particles embedded in the surface create abrasive action
• Minor oxide spallation — Some oxide layers detach but not extensively

At 500°C and 600°C:

• Oxidative wear — Accelerated oxidation due to higher temperature
• Oxide spallation wear — Dominant mechanism where oxide layers crack and detach, exposing fresh surface to further oxidation
• Reduced abrasive debris contribution — White-bright oxide debris significantly decreases

Oxide Layer Phase Composition

The oxide layers formed on the hardfacing surface consist of three phases:

1. α'-Fe — Metastable iron phase, likely formed from rapid oxidation
2. Cubic structure (Fe,Cr)2O3 — Chromite-type spinel oxide
3. Hexagonal structure (Fe,Cr)2O3 — Corundum-type oxide

The phase composition and chromium content in these oxides are identified as the key factors determining wear mechanism and performance.

Chromium Depletion Effect

At temperatures ≥500°C, the study observes:

• Significant reduction in white-bright oxide debris in wear tracks
• Notable decrease in chromium content in dark-colored oxide layers

This chromium depletion is critical because chromium is the primary alloying element responsible for forming protective chromium-rich oxides. When chromium is depleted from the oxide layer, the protective function is compromised, leading to accelerated oxidation and spallation.

Technical Analysis and Mechanism Discussion

Oxidation Kinetics and Protective Oxide Formation

The effectiveness of the 414N hardfacing layer at elevated temperatures depends on its ability to form a protective, adherent oxide layer. At 400°C, the oxide layer retains sufficient chromium content to provide protective behavior. However, at higher temperatures, several factors contribute to chromium depletion:

1. Increased diffusion rates — Higher temperatures accelerate the diffusion of chromium from the substrate to the oxide layer, but also increase the rate at which chromium is consumed in oxide formation.
2. Oxide layer instability — The (Fe,Cr)2O3 phases may become thermodynamically unstable at higher temperatures, leading to cracking and spallation.
3. Preferential oxidation — Iron oxidizes preferentially to chromium at certain temperature ranges, depleting chromium from the near-surface region.

Wear Mechanism Transition

The transition from abrasive/oxidative wear at 400°C to predominantly oxidative/spallation wear at 500–600°C represents a fundamental change in the wear process. At lower temperatures, the oxide layer is stable enough to remain adherent, and wear is dominated by mechanical abrasion from debris particles. At higher temperatures, the oxide layer becomes unstable and spalls off, exposing fresh metal to rapid re-oxidation, creating a cycle of oxidation-spallation-re-oxidation that accelerates material loss.

Engineering Implications for Continuous Casting Roll Design

Temperature Management

The findings suggest that maintaining roll surface temperatures below 500°C is critical for optimal wear performance. This has direct implications for:

• Cooling water flow rates — Adequate secondary cooling to maintain surface temperatures in the favorable range
• Roll design — Thermal mass and geometry should be optimized to manage heat flux from the strand
• Operating procedures — Casting speed and temperature control should consider roll thermal conditions

Material Selection Considerations

The chromium depletion phenomenon suggests that hardfacing materials with higher chromium content or additional refractory alloying elements (such as aluminum, silicon, or rare earth elements) may offer improved high-temperature wear resistance. The 414N material, while effective at 400°C, shows significant performance degradation at higher temperatures.

Service Life Prediction

The quantitative wear data (2.1× and 2.5× degradation at 500°C and 600°C respectively) can be used to develop service life models for continuous casting rolls. These models should incorporate the actual temperature profile experienced by the roll surface during operation.

Comparison with Related Literature

Study Insights and Reflections

This paper provides valuable quantitative data on high-temperature wear behavior that is directly applicable to continuous casting roll design and maintenance. The identification of the critical temperature transition at approximately 500°C is particularly significant for engineers responsible for roll cooling system design and operating procedure development. The detailed characterization of oxide layer phases and the identification of chromium depletion as a key degradation mechanism offer clear directions for material improvement. For engineers working on hardfacing material development, the findings suggest that future materials should focus on maintaining chromium availability in the oxide layer at elevated temperatures, possibly through the addition of diffusion barrier elements or the use of multi-layer overlay designs. The practical significance of this work lies in its direct applicability to improving the service life and reliability of continuous casting rolls, which are critical components in steel production.