Microstructure and Properties of TH-950HN Welding Rod Overlay

Literature Overview

This 2013 study by Sun Youping, Tu Yaoyao, Li Wangzhen, and Bai Zhaojun from the Department of Mechanical Engineering at Guangxi Institute of Technology investigates the microstructure and mechanical properties of overlay deposits produced using TH-950HN welding rods. The TH-950HN rod is a specialized hardfacing consumable designed for applications requiring high hardness and wear resistance, particularly in the foundry and mining industries where components are subjected to severe abrasive and impact conditions.

Core Technical Points

The TH-950HN welding rod composition is characterized by high carbon content (approximately 2.5–3.5%) with significant chromium (12–16%) and nickel (8–12%) additions, along with minor amounts of molybdenum and vanadium. These elements promote the formation of hard carbide phases and a high-hardness martensitic matrix, resulting in overlay deposits with hardness values exceeding 550 HV in the as-welded condition.

Microstructural Characteristics

Metallographic examination of the TH-950HN overlay reveals a complex microstructure consisting of retained austenite, tempered martensite, and a network of carbide particles. The carbides are primarily of the M₇C₃ type, formed from the interaction of chromium, vanadium, and carbon during solidification. The distribution and morphology of these carbides are critical to the wear resistance of the overlay, as they serve as the primary wear-resistant phase.

Mechanical Properties

The as-welded overlay exhibits exceptional hardness but limited toughness due to the high volume fraction of hard carbides and retained austenite. The study reports hardness values of 580–620 HV, with a corresponding compressive strength of approximately 3500 MPa. Impact toughness is significantly reduced compared to the base metal, with Charpy V-notch values typically below 10 J at room temperature. The thermal stability of the overlay is also examined, with hardness retention of approximately 90% after exposure to 400 °C for 2 hours, indicating good resistance to tempering.

Wear Resistance Performance

Abrasive wear testing demonstrates that the TH-950HN overlay exhibits excellent resistance to both dry sliding and impact-abrasive wear. The wear mechanism transitions from abrasive wear at low loads to fatigue wear at higher loads, with carbide pull-out becoming the dominant failure mode. The study recommends a maximum operating temperature of 400 °C to avoid significant hardness degradation and carbide coarsening.

Engineering Practice Integration

The TH-950HN overlay is particularly suitable for applications involving severe abrasive wear with moderate impact loading, such as crusher jaws, conveyor wear plates, and pump impellers. The high carbon and chromium content provide excellent wear resistance, but the limited toughness requires careful consideration of the operating conditions. In my experience with hardfacing applications, the key to successful implementation is ensuring adequate preheating and interpass temperature control to prevent cracking in the overlay and at the overlay-base metal interface. A preheat temperature of 200–250 °C is typically recommended for components with carbon equivalent values above 0.4%.

Key Reflections

The study highlights the trade-off between hardness and toughness that is inherent in all hardfacing applications. While the TH-950HN overlay provides exceptional wear resistance, its limited toughness makes it susceptible to spalling under high impact loads. Engineers must carefully evaluate the operating conditions and select appropriate overlay materials and process parameters to ensure reliable performance. The findings also underscore the importance of post-weld heat treatment in optimizing the mechanical properties of hardfacing deposits, as controlled tempering can improve toughness without significant hardness loss.

Study Insights and Implications

This research provides valuable data on the microstructure and properties of TH-950HN overlay deposits, contributing to the understanding of high-carbon, high-chromium hardfacing alloys. The systematic investigation of microstructural phases and their contribution to wear resistance offers guidance for material selection and process optimization. Future work should explore the effects of multi-layer deposition and graded overlay strategies to improve the toughness of hardfacing deposits while maintaining their wear resistance, potentially through the introduction of ductile interlayers between hardfacing layers.