Effect of Surfacing Welding Current on Microstructure and Properties of 16Mn Steel

Literature Overview

This paper by Wang Chunhua and He Xinghua, published in the Journal of Hot Working Technology in 2018 (Vol. 47, No. 5, pp. 173-176), investigates how manual arc welding (SMAW) current intensity affects the microstructure and mechanical properties of a hardfacing overlay deposited on 16Mn steel plates. The research was supported by the National Natural Science Foundation of China (Project No. 51374120) and was motivated by the practical need to improve the wear resistance of the central trough in scraper conveyors used in mining and bulk material handling applications.

Core Technical Findings

The authors conducted systematic hardfacing experiments on 16Mn steel plates that matched the material composition and heat treatment process of the actual conveyor trough. Different welding currents were applied using SMAW technique, and the resulting overlay layers were examined through metallographic observation and mechanical property testing.

The key findings can be summarized as follows:

Microstructural Analysis and Mechanism Interpretation

The overlay microstructure consists of three primary phases: martensite (the dominant hardening phase), retained austenite (which provides a supportive matrix for wear resistance), and carbide precipitates (which contribute to hardness but may reduce toughness). The interplay between these phases is critical for balancing hardness and wear resistance against toughness.

As the welding current increases, the heat input rises, which promotes the decomposition of retained austenite into martensite and carbides. This explains the observed decrease in retained austenite content and the coarsening of martensite. The higher heat input also accelerates grain growth in the weld metal, leading to coarser martensitic laths and plates. While this results in higher hardness due to the increased volume fraction of hard martensite and carbides, it simultaneously degrades toughness because coarse martensite is inherently more brittle.

The river pattern fracture morphology observed at all current levels is a classic indicator of cleavage fracture in martensitic structures. This confirms that the overlay layers, while providing excellent wear resistance, suffer from inherently low fracture toughness. This is a fundamental trade-off in hardfacing applications where the primary goal is wear resistance rather than impact resistance.

Engineering Practice Implications

The finding that 150 A provides the optimal wear resistance is particularly valuable for field applications. At this current level, the retained austenite content is sufficient to act as a supportive phase that absorbs deformation energy during sliding contact, thereby enhancing wear resistance beyond what pure martensite could achieve. The retained austenite undergoes strain-induced martensitic transformation during wear, generating new hard martensite in the wear zone, which continuously replenishes the hard phase in the contact area.

For practical implementation in scraper conveyor repair and maintenance, the following process recommendations can be derived:

1. Current Control: Maintain welding current at approximately 150 A to optimize the retained austenite-to-martensite ratio.
2. Preheating: Given the low toughness of the overlay, preheating the base material to 150-200°C is advisable to reduce thermal stresses and minimize cracking risk.
3. Interpass Temperature: Keep interpass temperature below 150°C to avoid excessive grain coarsening and to maintain the desired microstructure.
4. Post-Weld Treatment: A low-temperature tempering treatment (200-300°C) may be applied to relieve residual stresses without significantly reducing hardness.

Key Questions and Reflections

One notable limitation of this study is that the fracture toughness values at all current levels are low, as evidenced by the river pattern fracture morphology. This raises an important engineering question: how can the toughness of hardfacing overlays be improved without sacrificing wear resistance? Possible approaches include multi-layer hardfacing with alternating layers of different compositions, or the use of post-weld thermal treatments that selectively transform retained austenite to improve toughness.

Another consideration is the scalability of this research from laboratory conditions to field conditions. The experiments were conducted on flat steel plates under controlled conditions, whereas actual conveyor troughs have complex geometries with varying thicknesses and orientations. The thermal behavior and resulting microstructure may differ significantly in these practical scenarios.

Summary

This study provides valuable quantitative insights into the current-dependent behavior of SMAW hardfacing overlays on 16Mn steel. The identification of 150 A as the optimal current for wear resistance, coupled with the mechanistic explanation involving retained austenite support, offers a clear process window for field application. However, the persistently low toughness across all current levels remains a challenge that requires further investigation through microstructure optimization and process parameter refinement. The findings are directly applicable to the repair and maintenance of mining conveyor equipment where wear resistance is the primary performance requirement.