TH-950HN Electrode Overlay Welding Microstructure and Mechanical Properties

Literature Overview

This paper, published in Foundry Technology (2013, Vol. 34, No. 3, pp. 364-365) by Sun Youping, Tu Yaoyao, Li Wangzhen, and Bai Zhaojun from Guangxi Institute of Technology, investigates the microstructure and mechanical properties of TH-950HN electrode overlay welding on Q345B base metal. The study focuses on the carbide morphology, distribution, and hardness characteristics of the overlay layer, providing critical insights into the wear resistance mechanism of this chromium-based hard-facing alloy system. The research is funded by the Guangxi Institute of Technology Doctoral Fund and Natural Science Foundation, reflecting the academic interest in understanding the metallurgical behavior of high-chromium overlay alloys.

Microstructure Characterization and Carbide Morphology

The overlay layer microstructure is dominated by chromium carbide phases, specifically primary (Cr,Fe)₇C₃ carbides and eutectic (Cr,Fe)₇C₃ carbides distributed within a martensitic matrix. The morphology and size of these carbides vary significantly with depth from the weld surface, which has direct implications for wear resistance performance.

The transition from blocky carbides at the surface to skeletal and elongated carbides at depth reflects the changing solidification conditions during welding. At the weld surface, rapid cooling produces a fine, dispersed carbide structure that provides effective microstructural hardening through dispersion strengthening. As depth increases, slower cooling rates allow carbide growth and coarsening, leading to the skeletal morphology characteristic of slower solidification rates.

The blocky carbides at the surface, approximately 20 μm in size, represent an optimal balance between carbide size and dispersion density for abrasion resistance. Larger carbides provide greater resistance to micro-ploughing and micro-cutting mechanisms, while the dispersed distribution ensures that the matrix remains capable of supporting the carbide particles without premature debonding.

Hardness Distribution and Wear Resistance Mechanism

The hardness distribution of the TH-950HN overlay layer demonstrates uniformity both in the width direction and depth direction, with values ranging from 56 to 65 HRC throughout the 0-10 mm depth range. This uniformity is a critical advantage for wear-resistant applications, as it ensures consistent performance regardless of the wear depth or contact location on the overlay surface.

The high hardness achieved through dispersion strengthening of chromium carbides in a martensitic matrix provides excellent resistance to abrasive wear. The (Cr,Fe)₇C₃ carbides, with their high intrinsic hardness and moderate toughness, are particularly effective against sliding and rolling abrasion mechanisms common in mining, construction, and material handling applications.

The uniformity of hardness across the overlay layer is attributed to the consistent chemical composition and solidification conditions maintained throughout the multi-pass overlay welding process. Unlike some overlay alloys that exhibit significant hardness gradients due to varying dilution or cooling rates, the TH-950HN system demonstrates robust microstructural stability that translates to reliable field performance.

Engineering Application Considerations

For engineering applications involving wear-resistant overlay welding on Q345B structural steel, several practical considerations emerge from this study. The compatibility between the TH-950HN overlay alloy and Q345B base metal, both being iron-based alloys with similar thermal expansion coefficients, minimizes the risk of thermal stress cracking at the overlay-base metal interface. This compatibility is essential for ensuring long-term service life in cyclic loading or thermal cycling environments.

The 10 mm minimum depth of uniform hardness provides sufficient wear allowance for most industrial applications, including conveyor rollers, crusher liners, and earthmoving equipment components. However, for applications requiring deeper wear resistance, such as large mining equipment or heavy-duty grinding mills, the overlay thickness must be carefully designed to accommodate expected wear rates over the component's service life.

The electrode welding process used for TH-950HN application offers flexibility for field repair and retrofit applications where automated welding equipment is unavailable. This manual welding capability is particularly valuable for on-site maintenance of mining and construction equipment, where downtime minimization is critical to operational economics.

Study Insights and Metallurgical Reflections

The microstructural evolution observed in this study—blocky carbides at the surface transitioning to skeletal carbides at depth—provides valuable insights into the solidification metallurgy of high-chromium overlay alloys. The 20 μm carbide size at the surface suggests that the cooling rate at the weld surface is sufficient to produce a fine, well-dispersed carbide structure, while the deeper layers experience slower cooling that allows carbide coarsening and network formation.

From a metallurgical perspective, the (Cr,Fe)₇C₃ carbide phase is particularly advantageous for wear resistance applications because it combines high hardness with adequate toughness, unlike the extremely hard but brittle Cr₇C₃ carbides found in higher chromium alloys. This balanced combination of hardness and toughness makes the TH-950HN alloy suitable for applications involving both abrasive and impact wear, such as crusher jaws and conveyor components that experience both sliding contact and occasional impact loading.

The study's focus on microstructure-property relationships provides a foundation for optimizing the overlay welding process parameters to control carbide morphology and size distribution. By manipulating welding heat input, interpass temperature, and cooling rate, it may be possible to further refine the carbide structure and enhance wear resistance performance.

Summary and Outlook

The TH-950HN electrode overlay welding system on Q345B base metal demonstrates excellent wear resistance characteristics through a well-dispersed chromium carbide microstructure with uniform hardness of 56-65 HRC across the overlay depth. The blocky carbide morphology at the surface, transitioning to skeletal carbides at depth, reflects the solidification conditions during multi-pass overlay welding and provides effective dispersion strengthening throughout the overlay layer. This alloy system is particularly suitable for applications requiring both abrasion resistance and impact toughness, with the 10 mm uniform hardness depth providing adequate wear allowance for most industrial applications. Future optimization efforts should focus on controlling carbide size and distribution through welding parameter refinement to further enhance wear resistance performance while maintaining the alloy's inherent toughness characteristics.