Microstructure and Properties of K360 Wear-Resistant Steel Hardfacing Alloy Layer

Literature Overview and Background

K360 is a commercial designation for a high-carbon, high-chromium wear-resistant steel used as a base material in various industrial applications requiring exceptional abrasion resistance. The hardfacing alloy layer deposited on K360 steel is designed to further enhance the surface wear resistance while maintaining adequate toughness to prevent catastrophic failure under impact or thermal cycling conditions. The referenced study provides a comprehensive investigation of the microstructure evolution and mechanical properties of the hardfacing overlay deposited on K360 wear-resistant steel, offering valuable insights for optimizing the hardfacing process and material selection.

K360 steel itself contains approximately 0.45% C, 1.0% Mn, 0.4% Si, 0.2% Cr, and 0.1% Mo, with a typical hardness of 350-400 HV in the quenched and tempered condition. The hardfacing alloy is designed to achieve a hardness exceeding 600 HV while maintaining a Charpy impact energy above 10 J at room temperature.

Microstructural Characterization

The hardfacing overlay microstructure consists of a complex mixture of phases that contribute to the overall wear resistance:

The microstructure exhibits a clear columnar grain structure perpendicular to the substrate surface, with the columnar grains growing from the fusion boundary into the overlay. The grain size decreases with increasing distance from the fusion boundary due to the increasing cooling rate, resulting in a finer microstructure in the upper portion of the overlay.

The carbide morphology is particularly important for wear resistance. The primary M7C3 carbides appear as blocky particles with a size of 5-15 μm, while the secondary M23C6 carbides form a network along the prior austenite grain boundaries. The distribution of these carbides is relatively uniform, which is critical for consistent wear performance across the overlay surface.

Mechanical Properties and Performance

The mechanical properties of the K360 hardfacing overlay are summarized as follows:

The hardness of the overlay layer is primarily attributed to the high volume fraction of hard carbides and the hardened martensitic matrix. The impact energy, while lower than the base metal, is still adequate for most industrial applications involving moderate impact loading. The bond strength exceeds 400 MPa, indicating excellent metallurgical bonding between the overlay and the K360 base metal.

Wear Testing and Mechanism Analysis

Abrasion testing was conducted using the ASTM G65 pin-on-disk method with alumina (Al2O3) and silicon carbide (SiC) abrasive disks. The wear rate was measured as the mass loss per unit sliding distance, and the wear mechanisms were analyzed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS).

The wear mechanisms identified include:

1. Abrasive wear: The dominant mechanism, characterized by material removal through ploughing and cutting by hard abrasive particles. The hard carbides in the overlay matrix resist cutting by abrasive particles, significantly reducing the wear rate.
2. Adhesive wear: Observed as small nodules on the wear surface, resulting from localized bonding between the overlay and the counterface. This mechanism is more pronounced in the softer matrix regions between carbides.
3. Fatigue wear: Identified by micro-cracks and spalling on the wear surface, resulting from cyclic contact stress. The fatigue cracks initiate at carbide-matrix interfaces and propagate through the matrix.

The overlay layer demonstrated a wear rate of 0.8-1.2 mg/N·m against Al2O3 abrasive, compared to 2.5-3.5 mg/N·m for the K360 base metal, representing a 2.5-3.0x improvement in wear resistance.

Engineering Practice and Process Optimization

The hardfacing process parameters for K360 steel are optimized as follows:

• Preheating temperature: 200-250°C to prevent base metal cracking and reduce residual stress.
• Interpass temperature: Maintained below 250°C to control dilution and minimize H pickup.
• Number of passes: 2-3 passes for a total overlay thickness of 6-10 mm.
• Post-weld heat treatment: Tempering at 400°C for 2 hours to relieve residual stress and improve toughness.

The study emphasizes the importance of controlling the dilution ratio between the overlay and the base metal. Excessive dilution leads to a reduction in overlay hardness and wear resistance, while insufficient dilution can result in poor bonding and increased susceptibility to cracking. The optimal dilution ratio is approximately 15-20%, which can be achieved by using a multi-pass technique with the first pass deposited at a slightly reduced current.

Study Insights and Reflections

The comprehensive investigation of the K360 hardfacing overlay provides valuable data for material selection and process optimization in industrial applications. The clear correlation between carbide morphology, volume fraction, and wear resistance underscores the importance of microstructural control in hardfacing alloy design. The finding that the overlay achieves a 2.5-3.0x improvement in wear resistance over the base metal, while maintaining adequate toughness, demonstrates the effectiveness of the hardfacing approach for enhancing the service life of K360 components.

The study also highlights the need for careful process control, particularly regarding preheating and interpass temperature, to ensure consistent overlay quality. The recommended tempering treatment at 400°C is a practical and effective approach to balancing hardness and toughness, and should be considered as a standard practice for K360 hardfacing applications.

Overall, this work contributes to the body of knowledge on hardfacing alloys for wear-resistant steels and provides practical guidance for engineers seeking to extend the service life of K360 components through surface engineering. The detailed microstructural analysis and mechanical property data will be useful for benchmarking and comparison in future research and development efforts.