Effect of Surfacing Current on Microstructure and Properties of 16Mn Steel Plate Surfacing Layer

Literature Overview

This paper by Wang Chunhua and He Xinghua, published in the journal "Hot Working Technology" in 2018 (Vol. 47, No. 5, pp. 173-176), investigates the influence of surfacing welding current on the microstructure and mechanical properties of a wear-resistant overlay deposited on 16Mn steel plate using manual arc welding (SMAW). The study was supported by the National Natural Science Foundation of China (Grant No. 51374120) and was motivated by the practical need to extend the service life of middle trough components in scraper conveyors used in coal mining operations. The authors selected 16Mn steel plate with the same material grade and heat treatment condition as the actual middle trough to ensure that the experimental results could be directly transferred to engineering applications.

Core Technical Findings

The surfacing layer microstructure consists of three principal phases: martensite, retained austenite, and carbides. The key finding is that the welding current acts as the dominant variable controlling the phase composition and grain morphology. As the current increases, the retained austenite content decreases, the martensite grains become coarser, the overlay hardness increases, but the toughness decreases correspondingly. All impact fracture surfaces exhibited river patterns regardless of the current level, indicating that the toughness of the surfacing layer was inherently limited across all tested conditions. The optimal wear resistance was achieved at a welding current of 150 A, which the authors attributed to the supportive role of retained austenite in the microstructure.

The retained austenite plays a critical dual role: at moderate levels it acts as a support phase that accommodates plastic deformation through transformation-induced plasticity (TRIP), thereby improving wear resistance without sacrificing all toughness. However, excessive current reduces the retained austenite fraction and coarsens the martensite, leading to a brittle microstructure prone to cracking under impact loading.

Process Analysis and Engineering Implications

From a process metallurgy perspective, the welding current controls the heat input and cooling rate of the surfacing layer. A higher current produces a larger molten pool and slower cooling, which promotes the decomposition of retained austenite into martensite and carbides during solidification and subsequent cooling. The coarsening of martensite at high currents is consistent with classical solidification theory, where slower cooling allows more time for carbon partitioning and grain growth. The river pattern fracture morphology observed across all current levels suggests that the surfacing layer is inherently a high-carbon martensitic system with limited ductility, which is typical of hardfacing alloys designed for abrasion resistance.

In engineering practice, this finding has direct relevance to the repair and maintenance of scraper conveyor troughs in mining environments. The 150 A recommendation provides a practical process window for field welders performing overlay repairs. However, the inherently low toughness raises concerns about impact loading during material handling operations. A practical mitigation strategy would involve multi-pass surfacing with alternating current levels, or post-weld tempering to partially transform retained austenite and refine the carbide distribution. The study also highlights the trade-off between hardness and toughness that is fundamental to all hardfacing applications: maximizing wear resistance inevitably compromises impact resistance, and the optimal balance must be determined by the specific service conditions.

Study Insights and Reflections

This paper demonstrates a clear and well-documented approach to surfacing parameter optimization. The selection of 16Mn steel as the base material is representative of low-alloy structural steels widely used in mining equipment, making the results broadly applicable. The observation that all fracture surfaces showed river patterns is an important caution: engineers should not assume that a hardfacing overlay provides adequate impact resistance simply because it resists abrasion. In conveyor trough applications, the combination of abrasive wear from coal and particulate matter with occasional impact from dropped material demands a carefully balanced microstructure. The 150 A optimum identified here should be validated through field trials under actual operating conditions, as laboratory wear tests may not fully capture the complex loading sequences encountered in service. Future work should explore the effect of multi-layer surfacing, interpass temperature control, and post-weld heat treatment on achieving a more balanced combination of hardness and toughness.