Effect of Microstructure on Properties in Rolling Roll Overlay Welding

Literature Overview

This paper by Liu Lu, Guo Jian, Zhao Guang, Li Bo, Zhao Jianglin, and Han Peng from the Technical Center of Tangsteel Company (Hesteel Group), published in Physical Testing (2023, Vol. 41, No. 4, pp. 1–4), investigates the relationship between microstructure and surface hardness in rolling roll overlay welds produced with two different flux-cored wire consumables. The study employs submerged arc overlay welding on 45# steel substrates and characterizes the resulting microstructures and hardness distributions. The findings provide direct guidance for consumable selection in rolling roll repair applications, where overlay layer hardness and uniformity are critical for service life.

Core Technical Content

Consumable Comparison

Two flux-cored wires were evaluated:

Microstructural Analysis

420L Wire Overlay Layer: The microstructure consists of a dendritic pattern with Fe-Cr solid solution matrix and carbide precipitates. The dendritic morphology is characteristic of directional solidification under the thermal gradient established by the base material. Carbides precipitate at dendrite boundaries and within the interdendritic regions, contributing to hardness through particle dispersion strengthening.

YD255Mo-S Wire Overlay Layer: The microstructure is dominated by black needle-like martensite with retained austenite. The martensitic transformation occurs during cooling of the weld metal, driven by the rapid cooling rate inherent to overlay welding on a thermally massive base. The needle morphology indicates lenticular or acicular martensite, which is typical of high-carbon, high-alloy steels with high hardenability. The retained austenite fraction is stabilized by the high alloy content (Cr, Mo) and contributes to the overall toughness of the overlay layer.

Hardness Distribution and Uniformity

The YD255Mo-S wire produced a higher average hardness (634.6 HV0.2 vs. 620 HV0.2) with lower deviation (9.1% vs. 11.9%). This indicates that the martensitic microstructure provides more uniform hardness distribution across the overlay layer compared to the carbide-reinforced dendritic structure.

The superior uniformity of the YD255Mo-S overlay can be attributed to:

1. Homogeneous martensitic transformation: Martensite forms throughout the weld metal with relatively uniform composition, producing consistent hardness.
2. Carbide-free or low-carbide structure: Without coarse carbide particles, there is no localized hardness variation between carbide-rich and carbide-poor regions.
3. Solid solution strengthening: The high Cr and Mo content provides uniform solid solution strengthening throughout the matrix.

In contrast, the 420L overlay layer exhibits hardness variation because:

1. Carbide distribution non-uniformity: Carbides are not uniformly distributed, leading to local hardness variations.
2. Dendrite arm spacing variation: The dendritic structure has varying arm spacing, affecting local microstructure and hardness.
3. Solid solution composition variation: Microsegregation during dendritic solidification creates local composition variations in the Fe-Cr solid solution.

Engineering Practice Integration

Rolling Roll Service Requirements

Rolling rolls in steel mills experience:

• Compressive contact stress: From the rolling force transmitted through the rolled material.
• Thermal cycling: From the hot rolled stock, causing thermal expansion and contraction.
• Wear: From friction with the rolled material and scale.
• Rolling fatigue: From cyclic contact stress.

The overlay layer must provide:

• Sufficient hardness to resist wear and deformation.
• Uniform hardness to prevent localized failure initiation.
• Adequate toughness to resist rolling fatigue and thermal cracking.
• Good adhesion to the base material to prevent spalling.

Consumable Selection Guidelines

Based on the findings:

• YD255Mo-S wire is preferred when hardness uniformity is critical, such as in precision rolling applications where consistent deformation behavior is required.
• 420L wire may be acceptable when slightly lower average hardness is tolerable and cost is a consideration.
• The retained austenite in the YD255Mo-S overlay may provide additional toughness, which is beneficial for fatigue resistance.

Study Insights

This paper highlights a fundamental principle in overlay welding: microstructure governs properties, and different consumables produce fundamentally different microstructures with distinct property profiles. The choice between carbide-reinforced and martensite-based overlay microstructures is not merely a matter of hardness but involves trade-offs in uniformity, toughness, and fatigue resistance. Engineers should not select overlay consumables based solely on published hardness values but should understand the microstructural mechanism responsible for the hardness and evaluate whether that mechanism aligns with the service requirements. The lower hardness deviation of the YD255Mo-S overlay is particularly significant for rolling applications, where uniform hardness prevents localized deformation and ensures consistent product quality.