Microstructure and Properties of Surfacing Metals for Cold Shear Blades

Literature Overview

This study by Li Da, Liu Ligang, Yang Yulin, and Yang Qingxiang from Yanshan University investigates how the alloy composition of surfacing electrodes affects the microstructure and mechanical properties of cold shear blade overlay deposits. Published in the Journal of Welding (Vol. 28, No. 2, 2007, pp. 25-28), this paper addresses a practical industrial challenge: extending the service life of cold shear blades used in metal forming operations where high hardness, wear resistance, and toughness are simultaneously required.

Core Technical Findings

The authors employed metallographic examination, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and hardness testing to characterize surfacing deposits produced with different electrode compositions. The key finding is that the hardness of the surfacing metal increases with the carbon content in the electrode flux coating. However, carbon alone is insufficient for achieving optimal performance because the resulting microstructure is prone to temper embrittlement at elevated service temperatures.

The critical discovery is the role of molybdenum addition. When metallic molybdenum is incorporated into the flux coating in appropriate amounts, the surfacing metal exhibits significantly improved temper resistance. After a secondary tempering treatment at approximately 550°C, the residual austenite content in molybdenum-containing deposits drops to around 1%, and the hardness reaches a maximum of 58.6 HRC. This is a remarkable achievement because most high-carbon martensitic steels would soften considerably after tempering at this temperature.

Microstructural Interpretation

The presence of molybdenum plays a dual role in the surfacing microstructure. First, it stabilizes the martensitic phase against temper decomposition by forming fine, coherent precipitates within the martensite matrix. Second, it suppresses the formation of large grain-boundary carbides that would otherwise cause brittle fracture. The reduction of residual austenite from higher levels down to approximately 1% after tempering indicates that molybdenum shifts the austenite-to-martensite transformation equilibrium, allowing more complete phase transformation during the tempering cycle.

Engineering Practice Implications

For engineers working with cold shear blades in pipe cutting or plate shearing operations, this research provides actionable guidance. The blade must maintain high hardness at the cutting edge while resisting thermal degradation during repeated cutting cycles where frictional heating can raise local temperatures above 400°C. The recommended practice is to select surfacing electrodes containing both adequate carbon and metallic molybdenum in the flux coating, followed by a controlled secondary tempering at 550°C. This combination ensures that the overlay retains its hardness even under thermal cycling conditions typical in cold shear operations.

A practical consideration is the dilution effect. When surfacing cold shear blades, the base material (typically medium-carbon steel such as 45# or 50# steel) dilutes the overlay alloy. Engineers should account for this by selecting electrode compositions with slightly higher carbon and molybdenum content than the target deposit composition, or by using multiple surfacing passes to achieve the desired alloy concentration.

Key Reflections

This study demonstrates that alloy design for surfacing applications requires a holistic approach rather than simple carbon enrichment. The molybdenum addition is not merely a hardening agent but a microstructural stabilizer that fundamentally alters the temper response of the deposit. For production environments where blade reconditioning is performed in-house, the secondary tempering step at 550°C is essential and should not be omitted. The residual austenite content of approximately 1% after tempering represents an optimal balance between hardness and toughness, as higher residual austenite would compromise dimensional stability during service.

The practical lesson is clear: when designing surfacing consumables for cold shear applications, the temper resistance of the overlay is as important as its as-deposited hardness. Engineers who focus solely on achieving high initial hardness may find that their blades soften rapidly during service, negating the benefits of the surfacing process. This work provides a solid foundation for developing more advanced cold shear blade surfacing solutions that combine high hardness with thermal stability.