Microstructure and Mechanical Properties of TH-950HN Electrode Overlay Welding

Literature Overview

This paper by Sun Youping, Tu Yaoyao, Li Wangzhen, and Bai Zhaojun, published in Foundry Technology (Vol. 34, No. 3, 2013, pp. 364-365), examines the microstructure and mechanical properties of overlay deposits produced with TH-950HN welding electrodes on Q345B structural steel. The study focuses on the morphology and distribution of carbide phases and their relationship to hardness performance.

Core Technical Content

TH-950HN is a high-carbon, high-chromium martensitic stainless steel welding electrode designed for hardfacing applications. The deposit is characterized by a high volume fraction of hard carbide phases that provide wear resistance. The study reveals that the primary strengthening phases are primary (Cr,Fe)7C3 carbides and eutectic (Cr,Fe)7C3 phases, with their morphology varying significantly with depth from the weld surface.

Microstructural Characteristics

The hardness profile is remarkably uniform, ranging from 56 to 65 HRC within the first 10 mm of depth. This uniformity is a critical engineering advantage, as it ensures consistent wear performance across the entire deposit thickness rather than only at the surface.

Interpretation of Technical Points

The morphological evolution of (Cr,Fe)7C3 carbides with depth is a direct consequence of solidification conditions. At the weld surface, rapid cooling rates promote the formation of small, equiaxed primary carbides. As the depth increases, the cooling rate decreases, allowing more time for carbide growth and the development of skeletal and dendritic morphologies. The transition from blocky to skeletal to long strip-shaped carbides reflects the progressive decrease in solidification rate from the surface to the root of the deposit.

The (Cr,Fe)7C3 phase is a hexagonal close-packed carbide that forms preferentially in high-carbon, high-chromium alloys. Its formation is governed by the chromium-to-carbon ratio and the cooling rate. In TH-950HN deposits, the chromium content is sufficient to stabilize the M7C3 phase as the primary carbide, rather than the M23C6 or M6C phases that might form in lower-chromium alloys. The iron substitution in the carbide lattice is a result of the dilution from the Q345B base metal, which introduces additional iron into the molten pool.

The uniform hardness distribution from 56 to 65 HRC across 10 mm depth is metallurgically significant. In many hardfacing alloys, there is a pronounced hardness gradient from the surface to the root, with the surface being harder due to faster cooling and finer microstructure. The relative uniformity observed in TH-950HN deposits suggests that the alloy composition is well-balanced, with sufficient carbon and chromium to maintain hard carbide formation even at lower cooling rates near the root.

Engineering Practice Implications

For overlay welding applications on wear-critical components such as grinding mill liners, crusher hammers, and conveyor rollers, the uniform hardness profile of TH-950HN deposits is a significant advantage. Engineers can specify deposit thickness based on expected wear depth without worrying about a dramatic drop in hardness near the root.

However, the morphology of the carbides at depth—skeletal and long strip-shaped—raises concerns about toughness and spalling resistance. While blocky carbides are generally considered beneficial for wear resistance, elongated carbides can act as crack initiation sites and reduce the fracture toughness of the deposit. In applications where impact loading is significant, such as mining equipment, this trade-off between hardness and toughness must be carefully evaluated.

The study does not address the intermetallic compound formation at the dilution zone between the deposit and the Q345B base metal. In practice, this interface is critical for bond strength and resistance to spalling. The high chromium and carbon content of the TH-950HN deposit may promote the formation of brittle intermetallic phases at the interface, particularly under high-temperature service conditions.

Key Questions and Reflections

The study provides valuable microstructural data but lacks several important characterizations that would enhance its practical utility. Mechanical property testing beyond hardness—such as impact toughness, fatigue strength, and spalling resistance—would provide a more complete picture of the deposit's performance under actual service conditions. Additionally, the study does not report on the welding parameters used, which makes it difficult to correlate the observed microstructure with specific process conditions.

Another important consideration is the effect of dilution on the deposit composition. The Q345B base metal introduces additional iron, manganese, and silicon into the molten pool, which can alter the phase balance and carbide morphology. A systematic dilution study, varying the number of passes and interpass temperature, would provide valuable guidance for optimizing deposit composition in multi-pass applications.

The hardness range of 56-65 HRC places the TH-950HN deposit in the medium-hardness category for hardfacing alloys. While this is adequate for many abrasive wear applications, it may be insufficient for severe sliding wear or high-temperature wear conditions. Engineers should consider this when selecting the alloy for specific service environments.

Study Insights and Implications

This research confirms that TH-950HN welding electrodes produce deposits with consistent hardness and effective carbide-based strengthening across a practical deposit thickness. The morphological evolution of (Cr,Fe)7C3 carbides with depth is a well-documented phenomenon in high-carbon, high-chromium hardfacing alloys, and the findings are consistent with established solidification metallurgy principles.

For engineers selecting hardfacing alloys for pipeline components, valve seats, or pump impellers, the key takeaway is that TH-950HN provides reliable hardness uniformity but may require careful evaluation of toughness requirements. The deposit is well-suited for abrasive wear applications where impact loading is minimal, but alternative alloys with better toughness may be preferable for impact-abrasion environments.