Effect of Microstructure on Properties in Roll Surfacing

Literature Overview

This paper by Liu Lu, Guo Jian, Zhao Guang, Li Bo, Zhao Jianglin, and Han Peng from the Technical Center of Tangshan Steel Group, published in Physical Testing (2023, Vol. 41, No. 4, pp. 1-4), investigates the relationship between microstructure and surface hardness in roll surfacing alloys. Two different flux-cored welding wires (420L and YD255Mo-S) were used for submerged arc surfacing on 45# steel substrate. The study examines the microstructural characteristics, hardness distribution, and hardness uniformity of the surfacing layers.

Core Technical Content

The surfacing process employs submerged arc welding (SAW) with flux-cored wires, which is a common industrial method for roll surfacing due to its high deposition rate and good productivity. The two consumables studied represent different metallurgical approaches to achieving wear resistance:

The key finding is that the YD255Mo-S wire produces a surfacing layer with higher average hardness and better hardness uniformity. The maximum error of 9.1% compared to 11.9% indicates more consistent microstructure and hardness distribution across the surfacing layer.

Microstructural Analysis

The microstructural differences between the two surfacing layers are fundamental to understanding their property differences. The 420L wire produces a dendritic microstructure consisting of Fe-Cr solid solution and dispersed carbides. This structure is characteristic of a hypoeutectic composition where the solid solution matrix forms a dendritic skeleton with carbides precipitating in the interdendritic regions.

The YD255Mo-S wire produces a microstructure dominated by needle-like martensite with retained austenite. The needle-like morphology of martensite provides high hardness through solid solution strengthening and dislocation density, while the retained austenite contributes to toughness and can transform to martensite during use, providing additional hardening.

The hardness uniformity of the YD255Mo-S surfacing layer is superior because the martensitic microstructure is more homogeneous than the dendritic structure. In the dendritic microstructure, hardness varies significantly between the dendrite cores (lower hardness, more solid solution) and the interdendritic regions (higher hardness, more carbides). In the martensitic structure, the hardness is more uniform because the martensite needles are fine and uniformly distributed.

Hardness Measurement and Characterization

The hardness measurements using HV0.2 (Vickers hardness with 0.2 kgf load) are appropriate for thin surfacing layers where deeper penetration tests would include base metal influence. The small indent size ensures that the measurement reflects the true properties of the surfacing layer without significant dilution from the substrate.

The maximum error values (11.9% for 420L and 9.1% for YD255Mo-S) represent the standard deviation of hardness measurements across the surfacing layer, normalized by the average hardness. Lower error indicates better uniformity, which is critical for applications where consistent wear resistance is required across the entire surfacing surface.

The average hardness values of 620 HV0.2 and 634.6 HV0.2 are both in the range suitable for roll surfacing applications, where hardness of 500-700 HV is typically required for adequate wear resistance without excessive brittleness.

Engineering Practice and Consumable Selection

For engineers selecting surfacing consumables for rolling rolls, this study provides several important insights:

• The YD255Mo-S wire offers better hardness uniformity, which is critical for maintaining consistent roll surface quality during the entire service life.
• The martensitic microstructure provides good combination of hardness and toughness, reducing the risk of spalling or chipping during operation.
• The retained austenite in the YD255Mo-S surfacing layer provides a strain-hardening reserve that can extend service life under cyclic loading.
• Submerged arc surfacing is the appropriate process for roll applications due to its high deposition rate and ability to build thick surfacing layers efficiently.

The 45# steel substrate is a common choice for roll cores because of its good weldability and mechanical properties. The surfacing layer must provide adequate metallurgical bonding with the substrate while delivering the required surface properties. The dilution rate in submerged arc surfacing typically ranges from 20-40%, which can affect the final composition and microstructure of the surfacing layer.

Key Questions and Reflections

Several aspects of this study warrant further consideration:

1. How does the hardness and microstructure evolve after heat treatment? The as-welded martensitic structure may require tempering to optimize toughness.
2. What is the dilution rate from the 45# steel substrate, and how does it affect the final composition and microstructure of each surfacing layer?
3. How do the two surfacing layers perform under actual rolling conditions, including thermal cycling and mechanical loading?
4. What is the effect of surfacing layer thickness on the hardness uniformity and microstructure?

The finding that microstructure type (dendritic vs. martensitic) significantly affects hardness uniformity is a valuable design principle. For applications requiring consistent surface properties, martensitic surfacing layers may be preferred over carbide-dendritic structures, even if the average hardness values are similar.

Summary

This paper provides a comparative analysis of two submerged arc surfacing consumables for rolling roll applications, demonstrating that the YD255Mo-S wire produces a surfacing layer with higher average hardness and superior hardness uniformity due to its martensitic microstructure. The study highlights the importance of microstructure type in determining not only the average hardness but also the consistency of properties across the surfacing layer. Engineers selecting surfacing consumables for roll applications should consider hardness uniformity as a critical parameter alongside average hardness, as consistent properties are essential for maintaining roll surface quality throughout the service life.