Microstructure and High-Temperature Wear Resistance of Nitrogen-Alloyed Hardfacing Alloys

Literature Overview

The research by Yang Ke and colleagues, published in Welding Journal in 2011 (Vol. 32, No. 1, pp. 5-8), investigates the effects of nitrogen alloying on martensitic stainless steel hardfacing alloys, with particular focus on microstructure evolution and high-temperature wear performance. The study was conducted through a collaboration between Hehai University, Jiangsu University, Huazhong University of Science and Technology, and Shanghai Jihao Wang Shipbuilding Technology Development Co., Ltd., supported by university research funding and provincial key laboratory open research funds.

Technical Innovation: Nitrogen Alloying

Nitrogen is an interstitial alloying element that occupies octahedral interstices in the crystal lattice of steel. Unlike substitutional alloying elements, nitrogen has a very strong effect on the microstructure and properties of steels even at relatively low concentrations. In hardfacing alloys, nitrogen alloying introduces several beneficial effects:

1. Solid solution strengthening: Dissolved nitrogen atoms create lattice distortions that impede dislocation motion, increasing the yield strength and hardness of the matrix.
2. Precipitation strengthening: Nitrogen combines with carbide-forming elements such as Nb and Ti to form composite carbonitrides (NbN, TiN, Nb(C,N), Ti(C,N)) that provide exceptional dispersion strengthening.
3. Carbide modification: Nitrogen alters the morphology and stability of carbides, promoting the formation of finer, more uniformly distributed particles.
4. Austenite stabilization: Nitrogen stabilizes the austenite phase, increasing the amount of retained austenite in the microstructure.

Microstructural Characteristics of Nitrogen-Alloyed Deposits

The researchers found that the nitrogen-alloyed martensitic stainless steel hardfacing deposit consists primarily of martensite and composite carbonitrides of niobium and titanium. The carbonitrides are finely dispersed throughout the martensitic matrix, creating a microstructure that is fundamentally different from conventional carbon-only hardfacing deposits.

High-Temperature Wear Mechanism Analysis

A significant contribution of this research is the detailed analysis of the high-temperature wear mechanism between the hardfacing alloy and carbon steel counterfaces. The researchers identified a specific wear mechanism that is particularly relevant to industrial applications involving metal-to-metal contact at elevated temperatures:

Wear Mechanism Description

During high-temperature sliding contact between the hardfacing alloy and carbon steel, the following sequence of events occurs:

1. Oxide scale formation: At elevated temperatures, the carbon steel counterface forms an oxide scale (primarily Fe2O3 and Fe3O4) on its surface.
2. Oxide transfer: The oxide scale on the carbon steel surface adheres to the hardfacing alloy surface during sliding contact.
3. Abrasive action: The transferred oxide particles act as third-body abrasives between the two surfaces, causing abrasive wear of the hardfacing alloy.
4. Material removal: The abrasive action removes material from the hardfacing surface, leading to progressive wear.

This mechanism is classified as a form of abrasive wear induced by transferred oxide particles, which is distinct from direct metal-to-metal adhesive wear. Understanding this mechanism is critical for selecting appropriate hardfacing materials for high-temperature applications.

Effect of Nitrogen Alloying on Wear Resistance

The nitrogen-alloyed hardfacing alloy demonstrates superior high-temperature wear resistance compared to the conventional alloy due to:

• Higher hardness at elevated temperature: The Nb-Ti composite carbonitrides maintain their stability and strengthening effect at high temperatures, preserving the hardness of the deposit.
• Improved oxide resistance: The alloying elements in the deposit form more protective oxide films that reduce the rate of oxide scale formation and transfer.
• Enhanced resistance to third-body abrasion: The harder, more wear-resistant surface of the nitrogen-alloyed deposit is less susceptible to damage from transferred oxide particles.

Alloy Design Considerations

The selection of alloying elements for nitrogen-alloyed hardfacing deposits requires careful consideration of several factors:

1. Nitrogen content: The nitrogen content must be optimized to maximize the beneficial effects without causing excessive porosity or nitride formation that could degrade toughness. Typical nitrogen contents in hardfacing alloys range from 0.5 to 1.5 wt%.
2. Carbide-forming elements: The addition of Nb and Ti provides the carbonitride-forming elements that create the beneficial dispersed particles. The ratio of Nb to Ti should be optimized to achieve the desired particle size, distribution, and stability.
3. Stainless elements: Chromium and molybdenum provide the corrosion resistance and red hardness necessary for high-temperature applications. The chromium content must be sufficient to form a protective chromium oxide film while maintaining adequate toughness.
4. Nickel: Nickel stabilizes retained austenite and improves toughness, which is important for deposits that experience thermal cycling and mechanical impact.

Engineering Application Scenarios

The nitrogen-alloyed hardfacing technology described in this study is applicable to several industrial scenarios:

• Cement kiln components: Rotary kilns, preheater tubes, and burner components in cement plants operate at elevated temperatures with abrasive wear from cement clinker and dust.
• Power plant components: Boiler tubes, furnace linings, and heat exchanger components face high-temperature abrasive wear from fly ash and slag.
• Mining equipment: Crusher components, conveyor rollers, and grinding media in mining operations experience abrasive wear at moderate to elevated temperatures.
• Steel mill components: Hot rolling mill guides, transfer rollers, and furnace components face extreme thermal and abrasive conditions.

Study Insights and Implications

This research makes a significant contribution to the understanding of high-temperature wear mechanisms in hardfacing alloys. The identification of oxide-transfer-induced abrasive wear as the dominant mechanism provides engineers with a clear target for material optimization. By designing alloys that resist oxide formation and transfer, or that are inherently resistant to abrasive wear from oxide particles, the service life of hardfaced components can be substantially extended.

The use of nitrogen as an alloying element in hardfacing alloys represents an underexploited technology that offers significant potential for performance improvement. The formation of Nb-Ti composite carbonitrides provides a level of dispersion strengthening that is difficult to achieve through carbon-only alloying, and the stability of these carbonitrides at elevated temperatures is a critical advantage for high-temperature applications.

For engineers developing hardfacing solutions for high-temperature wear applications, this research provides both fundamental understanding and practical guidance. The wear mechanism analysis enables rational material selection, while the alloy design principles offer a framework for developing new hardfacing compositions tailored to specific service conditions. The collaborative approach involving academic institutions and industry partners demonstrated in this study is a model for effective technology development that bridges the gap between fundamental research and industrial application.