Effect of Titanium Addition on Microstructure and Properties of Plasma Surfaced Ni-Based Cr3C2 Coatings

Literature Overview

The paper by Si Songhua, Jiang Shanshan, and Wang Yanyan from Anhui University of Technology, published in Materials Science and Process in 2019, investigates the influence of titanium addition on Ni-based Cr3C2 composite coatings produced by plasma arc surfacing on Q235 carbon steel substrates. Three coating configurations were fabricated: a baseline Ni50 alloy coating, a Cr3C2/Ni composite coating with 40 wt% Cr3C2, and a Ti/Cr3C2/Ni composite coating with 40 wt% Cr3C2 and 1 wt% Ti. The study systematically examines how the combined addition of Cr3C2 and Ti modifies the microstructure, hardness, and tribological behavior of the surfacing deposits.

Core Technical Findings

Microstructural Evolution

The Ni50 baseline coating exhibits a typical dendritic gamma-Ni structure interspersed with eutectic phases between the dendrite arms. This microstructure is characteristic of Ni-based alloy deposits cooled from the plasma arc pool, where the high cooling rate promotes rapid solidification of the Ni-rich matrix. The Cr3C2/Ni composite coating shows a significant transformation: a large volume fraction of primary Cr3C2 carbide particles is dispersed within a fine dendritic matrix. The Cr3C2 particles act as nucleation sites and physical barriers during solidification, interrupting the dendrite growth and refining the overall microstructure.

The most remarkable finding concerns the Ti/Cr3C2/Ni composite coating. The introduction of 1 wt% Ti causes the carbide particles to decrease in size and distribute more uniformly throughout the deposit. This microstructural refinement is attributed to multiple mechanisms: Ti acts as a strong carbide former that competes with Cr for available carbon, leading to the formation of smaller, more numerous TiC or mixed Ti-Cr carbide particles; Ti also modifies the melt pool fluidity and solidification front morphology, promoting a more homogeneous nucleation pattern. The resulting microstructure is substantially finer than the Cr3C2/Ni coating without Ti addition.

Hardness and Wear Performance

The hardness increase exceeding 26% for both composite coatings is directly attributable to the dispersion strengthening effect of the hard carbide phase. The Cr3C2 carbide has a Vickers hardness of approximately 2500 HV, which far exceeds the ~300 HV of the Ni50 matrix. The wear resistance improvement follows a similar trend but with a more pronounced effect from Ti addition: the Ti-containing coating achieves a 2.1x improvement compared to the 1.4x improvement of the Cr3C2-only coating. This indicates that Ti does not merely add another carbide phase but fundamentally improves the effectiveness of the existing carbide dispersion.

Mechanistic Interpretation

The enhanced wear resistance of the Ti/Cr3C2/Ni coating can be understood through the following mechanisms:

1. Dispersion strengthening: The finer and more uniformly distributed carbide particles provide more frequent obstruction to dislocation motion and abrasive particle penetration.
2. Load-bearing capacity: Smaller, more uniformly spaced carbides distribute the contact stresses more evenly across the coating surface, reducing localized stress concentrations that initiate microcracking.
3. Matrix refinement: The refined dendritic matrix surrounding the carbides has improved toughness, which helps prevent intergranular cracking during sliding contact.
4. Tribological film formation: Ti-containing carbides may form a more stable and protective tribofilm during sliding, reducing adhesive wear components.

Engineering Practice Implications

From a practical standpoint, this work has significant relevance to the repair and protection of carbon steel components in aggressive wear environments. Q235 carbon steel is widely used in structural applications, and plasma surfacing provides a cost-effective method to extend component life without replacement. The plasma arc surfacing process offers several advantages over alternative thermal spray methods:

• Excellent metallurgical bonding between the coating and substrate
• Dilution rates typically controllable within 10-20% depending on powder feed rate and arc parameters
• Suitable for in-situ repair of large components that cannot be removed for machining

The optimal Ti content of 1 wt% identified in this study is practical and achievable with commercially available powder blends. However, engineers should note that the study does not address the effect of multiple surfacing passes, interpass temperature control, or post-weld heat treatment on the final properties. In production applications, multi-pass surfacing is often required to achieve adequate coating thickness, and the microstructure of the final pass may differ from single-pass specimens.

Key Questions and Reflections

The study raises several important questions for further investigation. First, the tribological testing appears to have been conducted under dry sliding conditions, which may not represent real service environments where lubrication, corrosion, or thermal cycling are present. Second, the bond strength between the coating and substrate is not reported, which is critical for assessing whether the coating will delaminate under impact or thermal shock loading. Third, the role of Ti in modifying the dilution zone microstructure and the potential for Ti-induced cracking sensitivity deserves attention, particularly given that Q235 steel has relatively low carbon content and limited hardenability.

This work demonstrates that a small addition of Ti (1 wt%) can substantially enhance the effectiveness of a Cr3C2-reinforced Ni-based surfacing coating. The synergistic interaction between Ti and Cr3C2 in promoting microstructural refinement represents a valuable alloy design strategy for wear-resistant surfacing applications. For pipeline and pressure vessel repair operations, where carbon steel components are frequently exposed to erosive or abrasive service, this coating system offers a promising solution with quantifiable performance improvements over conventional Ni50 surfacing.

Study Insights and Outlook

The systematic comparison of three coating configurations in this study provides clear evidence that the combined addition of Cr3C2 and Ti is more effective than Cr3C2 alone in improving wear resistance. The 50% improvement in relative wear resistance from adding 1 wt% Ti to an already Cr3C2-reinforced coating demonstrates the significant leverage that microstructural refinement provides over simple volume fraction increases. Future research should explore the effects of higher Ti contents, different Ti delivery methods (Ti powder versus TiC or TiB2 additions), and the interaction between Ti and other potential alloying elements such as Mo, W, or Nb. The findings are particularly relevant for surface engineering of piping systems in mining, cement, and power generation industries where carbon steel components suffer from abrasive wear and require periodic repair.