Microstructure and Growth Mechanism of Surfacing Layer Under Combined Action of B and Ti

Literature Overview

This paper by Wang Yu from Shenyang Special Equipment Inspection and Research Institute and Gou Jian and Liu Zhengjun from Shenyang University of Technology, published in the Journal of Shenyang University of Technology in 2017, investigates the microstructure and growth mechanism of surfacing layers containing boron and titanium under their combined action. Funded by the Liaoning Provincial Doctoral Research Start-up Fund (20131079), this research employs a self-shielded open-arc surfacing method using a proprietary iron-based wear-resistant flux-cored wire to prepare Fe-Cr-C-B-Ti surfacing alloys. The study provides valuable insights into the in-situ synthesis of hard phases and their growth mechanisms in multi-element surfacing alloys.

Core Technical Concepts

The research focuses on the synergistic effects of boron and titanium in iron-based surfacing alloys, with particular attention to the in-situ formation of TiC and M23(C,B)6 phases. The key composition ranges and their effects are summarized below:

The self-shielded open-arc surfacing method used in this study offers several practical advantages, including the elimination of external shielding gas, reduced equipment requirements, and suitability for field applications. The flux-cored wire design allows for precise control of alloy composition while maintaining the process flexibility of open-arc welding.

Microstructural Analysis and Phase Identification

The paper employs optical microscopy, scanning electron microscopy, and X-ray diffraction to characterize the microstructure and phase composition of the surfacing layers. The key findings include the in-situ synthesis of TiC and M23(C,B)6 phases within the surfacing deposit. These phases form during solidification through thermodynamic and kinetic mechanisms that are influenced by the B and Ti content.

The TiC phase forms preferentially at grain boundaries, with its quantity increasing as the titanium content increases. This trend is consistent with the thermodynamic stability of TiC, which has a very high melting point and forms readily in the presence of titanium and carbon. The grain boundary distribution of TiC is attributed to the preferential segregation of titanium to grain boundaries during solidification, where the local concentration of titanium reaches the threshold for TiC nucleation.

The M23(C,B)6 phase forms through epitaxial growth on the TiC particles, which serve as nucleation substrates. This epitaxial relationship is a critical finding, as it indicates that the TiC particles not only contribute directly to hardness but also facilitate the formation of the M23(C,B)6 phase through crystallographic matching. The combined presence of TiC and M23(C,B)6 phases creates a synergistic effect on the mechanical properties of the surfacing layer.

Growth Mechanism Analysis

The growth mechanism of the hard phases is analyzed in detail, with particular attention to the role of TiC as a nucleation substrate for M23(C,B)6 epitaxial growth. The crystallographic relationship between TiC and M23(C,B)6 is such that the lattice parameters are sufficiently matched to allow epitaxial nucleation, reducing the energy barrier for M23(C,B)6 formation. This mechanism is significant because it explains how the combined action of B and Ti leads to a more complex and effective hard phase structure than either element alone.

The paper also addresses the effect of boron content on the stability of TiC formation. As the boron content increases, the quantity of TiC phase becomes unstable, suggesting that boron competes with carbon for titanium atoms or alters the thermodynamic conditions for TiC formation. This competition between B and C for Ti is an important consideration in alloy design, as it affects the balance between TiC and M23(C,B)6 phase formation.

Effect on Mechanical Properties

The in-situ synthesis of TiC and M23(C,B)6 hard phases significantly improves the overall performance of the surfacing layer. The combined effect of these two hard phases provides superior wear resistance compared to surfacing layers containing only one type of hard phase. The TiC particles contribute to hardness through their intrinsic high hardness, while the M23(C,B)6 phase provides additional reinforcement through its complex carbide-boride structure.

The balance between Ti and B content is critical for achieving optimal properties. Excessive boron can destabilize TiC formation and lead to non-uniform phase distribution, while insufficient titanium limits the nucleation substrate available for M23(C,B)6 growth. The optimal composition window must be determined through systematic experimentation and microstructural characterization.

Engineering Practice Implications

For engineers in the steel pipe and fitting industry, this research has several important implications. The understanding of B and Ti combined action in surfacing alloys can be applied to the design of wear-resistant overlays for pipe fittings in abrasive service environments. The in-situ synthesis of hard phases during surfacing offers a practical approach to achieving high hardness and wear resistance without the need for post-weld heat treatment.

The self-shielded open-arc surfacing method used in this study is particularly relevant for field applications, where external shielding gas may not be available. This method can be used for repairing worn pipe fittings, applying wear-resistant overlays to critical components, and manufacturing new surfacing layers on existing pipe infrastructure. The flux-cored wire design allows for composition control and process flexibility, making it suitable for a wide range of industrial applications.

Key Questions and Reflections

Several important questions emerge from this research. First, how does the crystallographic matching between TiC and M23(C,B)6 affect the mechanical properties of the composite hard phase structure? Second, what is the optimal B/Ti ratio for achieving the best balance between TiC and M23(C,B)6 phase formation? Third, how do the growth mechanisms identified in this study change with different surfacing process parameters, such as heat input and cooling rate?

The research also raises questions about the long-term stability of the in-situ synthesized phases under service conditions. The TiC and M23(C,B)6 phases must maintain their integrity and distribution under thermal cycling, mechanical loading, and corrosive environments. Further investigation into the thermal stability and corrosion resistance of these phases is needed to ensure reliable performance in demanding service environments.

Summary

This paper provides valuable insights into the microstructure and growth mechanism of surfacing layers containing boron and titanium under their combined action. The in-situ synthesis of TiC and M23(C,B)6 phases through epitaxial growth on TiC nucleation substrates represents a significant advancement in understanding the hard phase formation mechanisms in multi-element surfacing alloys. For engineers in the steel pipe and fitting industry, this research offers practical guidance for designing wear-resistant surfacing alloys with optimized hard phase structures, and the self-shielded open-arc surfacing method provides a practical approach for field applications where traditional welding equipment may not be available.