Zhuojin Pipe Fitting Co., LtdEffect of Titanium Addition on Microstructure and Wear Resistance of High-Chromium Alloy Surfacing Layer
Literature Overview
This paper by Liu Yue and colleagues from Henan University of Science and Technology investigates the influence of titanium (Ti) addition on the microstructure and wear resistance of high-chromium alloy surfacing layers deposited on Q235 carbon steel. Published in Chinese Journal of Materials Research (Vol. 27, No. 6, 2013, pp. 641-646), the study was supported by the Henan Provincial Science and Technology Key Project (112102213117). The research employs a combined wire-and-alloy-powder surfacing method to deposit high-chromium alloy layers with varying Ti content, and characterizes the resulting microstructure and tribological performance using a comprehensive suite of analytical techniques.
Experimental Design and Materials
The surfacing method used in this study combines welding wire with alloy powder blocks, which is an effective approach for achieving higher alloy content in the deposit than would be possible with wire alone. This technique is particularly useful for depositing high-alloy materials where the dilution from the base metal would otherwise reduce the alloy content below the level required for optimal properties. The base material Q235 is a common carbon structural steel, which serves as a practical substrate for evaluating the surfacing technology's effectiveness.
The characterization methods employed include metallographic microscopy for microstructure observation, X-ray diffraction (XRD) for phase identification, optical emission spectrometry for chemical composition analysis, Auger electron spectroscopy for surface composition analysis, scanning electron microscopy (SEM) for detailed microstructural examination, a pin-on-disc or similar abrasion tester for wear testing, and a Rockwell hardness tester for hardness measurement. This multi-technique approach provides a comprehensive understanding of the relationship between composition, microstructure, and wear performance.
Microstructure Evolution with Ti Addition
The study reveals that the high-chromium alloy surfacing layer consists primarily of α-Fe (ferrite), γ-Fe (austenite), carbides, and carbonitrides. A key finding is the formation of fine Ti_x(N,C)_y carbonitride particles dispersed within the α-Fe matrix. These Ti carbonitrides are significant because they serve as hard reinforcement particles that contribute to wear resistance through a composite mechanism.
The effect of Ti content on microstructure is summarized as follows:
| Ti Content | Hardness (HRC) | Wear Loss (g) | Microstructural Feature |
|---|---|---|---|
| ~3% (optimal) | 60 | 0.2066 | Fine Ti_x(N,C)_y particles in α-Fe matrix |
| Lower Ti | Lower than 60 | Higher than 0.2066 | Fewer hard particles |
| Higher Ti | Variable | Variable | Increased carbide/carbonitride quantity |
As the Ti content increases, the quantity of carbides and carbonitrides in the surfacing layer increases. This is consistent with the known thermodynamic stability of titanium carbides and carbonitrides, which have very negative formation enthalpies and are therefore thermodynamically favored to form during solidification. The increased hard particle volume fraction directly contributes to improved wear resistance by providing more obstacle sites for abrasive wear mechanisms.
Wear Mechanism Analysis
The wear resistance improvement achieved through Ti addition can be explained through the concept of composite wear resistance, where the hard particles act as a wear-resistant skeleton within the matrix. During abrasive wear, the hard Ti carbonitride particles resist micro-cutting and ploughing by abrasive particles, while the surrounding matrix provides the necessary toughness to prevent particle pull-out and matrix fracture. The effectiveness of this mechanism depends on the optimal balance between hard particle volume fraction, particle size, and particle-matrix bonding strength.
The optimal Ti content of approximately 3% represents a balance between maximizing hard particle formation and avoiding excessive brittleness. At lower Ti contents, there are insufficient hard particles to provide significant wear resistance improvement. At higher Ti contents, while more hard particles form, the increased brittleness of the overall microstructure can lead to matrix cracking and particle pull-out under wear loading, potentially reducing wear resistance. This is a classic example of the composition-property optimization problem encountered in surface engineering.
Phase Composition and Thermodynamic Considerations
The presence of both α-Fe and γ-Fe phases in the surfacing layer indicates a complex solidification behavior. In high-chromium alloys, the formation of austenite is influenced by the Cr content, which is a ferrite-forming element, and the presence of austenite-stabilizing elements such as Ni and Mn. The Ti addition primarily forms Ti carbonitrides rather than significantly altering the Fe phase balance, but the removal of carbon from the matrix by Ti carbonitride formation can indirectly affect the phase fractions.
The Ti_x(N,C)_y carbonitrides are particularly interesting from a metallurgical perspective because they incorporate both carbon and nitrogen. The presence of nitrogen in the deposit, likely introduced through the flux or shielding gas in the surfacing process, is critical for forming these carbonitrides rather than pure carbides. The nitrogen content in the molten pool and its interaction with Ti during solidification are important process parameters that influence the type and distribution of Ti compounds formed.
Engineering Practice Considerations
From a practical standpoint, the use of combined wire-and-powder surfacing for high-chromium alloy deposition offers several advantages. The powder component allows for precise control of alloy addition, enabling the optimization of Ti content to achieve the desired wear resistance. The method is also compatible with standard surfacing equipment and can be adapted for on-site repair applications. However, the process requires careful control of powder feeding rate and wire feed rate to maintain consistent deposit composition and microstructure.
The optimal Ti content of 3% with resulting HRC 60 hardness and 0.2066 g wear loss represents a benchmark for high-chromium alloy surfacing design. Engineers targeting specific wear resistance requirements can use this data as a starting point for composition optimization, adjusting the Ti content and other alloying elements to achieve the desired performance in specific service conditions.
Key Questions and Reflections
Several important questions remain unanswered by this study. First, the effect of Ti addition on the toughness and fatigue resistance of the surfacing layer is not addressed. High hardness achieved through hard particle reinforcement often comes at the expense of toughness, and the balance between wear resistance and fracture resistance is critical for many engineering applications. Second, the study does not examine the effect of post-weld heat treatment on the Ti carbonitride distribution and wear properties, which could potentially optimize the particle size and morphology for improved performance.
Additionally, the long-term wear behavior under different wear modes (abrasive, adhesive, erosive, and corrosive-abrasive) is not investigated. The wear test results reported likely represent a specific wear condition, and the performance under other wear modes may differ significantly. For example, in erosive wear conditions, the hard particle reinforcement mechanism may be less effective if the particles are pulled out from the matrix under impact loading.
Study Insights and Implications
This research demonstrates that titanium addition is an effective strategy for improving the wear resistance of high-chromium alloy surfacing layers through the formation of hard Ti carbonitride particles. The optimal Ti content of approximately 3% provides a good balance between hard particle volume fraction and matrix toughness, resulting in HRC 60 hardness and significantly reduced wear loss. The combined wire-and-powder surfacing method offers practical flexibility for achieving targeted alloy compositions. For engineers developing wear-resistant surfacing solutions for mining, cement, and material handling equipment, this study provides valuable guidance on the role of Ti as a microalloying element for hard particle reinforcement, and underscores the importance of understanding the wear mechanism to select the appropriate alloy composition for specific service conditions.