Effect of Molybdenum and Nickel Content on Martensitic Stainless Steel Hardfacing Flux-Cored Wire Surfacing Layer

Literature Overview

This paper by Lü Yan, Yu Shengfu, Xing Shule, Liu Yulong, and Wang Lehu from Huazhong University of Science and Technology, published in Materials in Mechanical Engineering (2013, Vol. 37, No. 9, pp. 29-32), investigates the effect of molybdenum (Mo) and nickel (Ni) content on the microstructure, hardness, and high-temperature wear resistance of a martensitic stainless steel hardfacing flux-cored wire. The surfacing layers were deposited on Q235 steel substrates using submerged arc welding (SAW). This research is highly relevant to engineers working on hardfacing applications for mining equipment, cement industry components, and high-temperature wear-resistant surfaces.

Research Objectives and Methodology

The primary objective was to develop a martensitic stainless steel flux-cored wire with optimized Mo and Ni content for improved high-temperature wear resistance. The researchers systematically varied the Mo and Ni content in the flux-cored wire composition and deposited surfacing layers on Q235 steel substrates using SAW.

Wire Composition Variants

Testing Methods

The surfacing layers were characterized using the following methods:

• Optical microscopy - To examine the microstructure and identify phases present.
• X-ray diffraction (XRD) - To confirm phase identification.
• Hardness testing - Using Rockwell C hardness (HRC) to measure surface hardness.
• High-temperature wear testing - Using a pin-on-disc wear tester at elevated temperatures to measure mass loss.

Key Findings and Analysis

Microstructure Analysis

All surfacing layers exhibited a microstructure consisting of martensite, retained austenite, and carbides. The addition of Mo and Ni influenced the microstructure in the following ways:

1. Molybdenum effect - Mo promotes the formation of M6C and M23C6 carbides, which increase hardness but may reduce toughness if present in excessive amounts. Mo also stabilizes the martensitic phase and increases the hardenability of the weld metal.
2. Nickel effect - Ni stabilizes the austenitic phase, increasing the amount of retained austenite. This can improve toughness and reduce cracking susceptibility but may reduce hardness if the retained austenite fraction is too high.
3. Combined Mo-Ni effect - The combination of Mo and Ni at 2% each produced a balanced microstructure with adequate carbide content for hardness and sufficient retained austenite for toughness.

Hardness and Wear Resistance Results

The results clearly demonstrate that the addition of both Mo and Ni at 2% each provides the best combination of hardness and high-temperature wear resistance. The mass loss reduction of approximately 47% compared to the base composition represents a significant improvement in service life for high-temperature wear applications.

Mechanism of Improvement

The improved wear resistance can be attributed to several factors:

1. Carbide strengthening - Mo promotes the formation of fine, hard carbides (M6C, M23C6) that resist wear by ploughing and abrasion.
2. Solid solution strengthening - Both Mo and Ni dissolve in the martensitic matrix and increase its strength through solid solution effects.
3. Retained austenite transformation - The retained austenite stabilized by Ni can transform to martensite during wear, providing work hardening and increasing resistance to abrasive wear.
4. High-temperature stability - Mo increases the thermal stability of the martensitic phase, maintaining hardness at elevated temperatures where other alloys may soften.

Engineering Applications and Recommendations

This research has direct applications in several industrial sectors:

1. Mining equipment - Hardfacing of crusher jaws, conveyor rollers, and bucket teeth operating at elevated temperatures due to friction and impact.
2. Cement industry - Surfacing of kiln liners, mill liners, and preheater components subjected to abrasive wear at high temperatures.
3. Power generation - Hardfacing of turbine blades, fan blades, and wear plates in coal handling systems.
4. Steel mills - Surfacing of guide rollers, wear plates, and other components in hot rolling mills.

Process Recommendations

For optimal results when using this flux-cored wire composition:

• Use submerged arc welding (SAW) with a heat input of 20-35 kJ/cm.
• Maintain an interpass temperature below 200°C to avoid excessive grain growth.
• Apply multiple passes for thick surfacing layers, with each pass maintaining adequate dilution control.
• Preheat the base metal to 150-250°C for thick sections to reduce cracking susceptibility.
• Post-weld heat treatment (PWHT) at 550-600°C for 1-2 hours can improve toughness without significantly reducing hardness.

Study Insights and Reflections

This paper provides valuable insights into the role of Mo and Ni in martensitic stainless steel hardfacing alloys. The systematic approach of varying Mo and Ni content independently and in combination is a rigorous methodology that clearly demonstrates their synergistic effect.

One limitation of the study is the absence of toughness testing. While hardness and wear resistance are important, the toughness of the surfacing layer is critical for applications involving impact loading. The retained austenite fraction, which is increased by Ni, can be beneficial for toughness but may also be detrimental if it is too high, leading to dimensional instability during subsequent heat treatments or service.

Future research should investigate the effect of other alloying elements such as vanadium (V), tungsten (W), and cobalt (Co) on the high-temperature wear resistance of martensitic stainless steel hardfacing alloys. Additionally, the effect of welding parameters on the microstructure and properties of the surfacing layer should be studied, as variations in heat input can significantly influence the cooling rate and phase transformations.

The findings of this study provide a practical basis for the development of high-performance martensitic stainless steel hardfacing consumables for demanding high-temperature wear applications.