Microstructure and Properties of Cold Shear Blade Overlay Weld Metal

Literature Overview

This paper published in the Transactions of the China Welding Institute (2007, Vol. 28, No. 2, pp. 25-28) by Li Da, Liu Ligang, Yang Yulin, and Yang Qingxiang from Yanshan University investigates the influence of alloy composition in overlay welding electrodes on the microstructure and mechanical performance of cold shear blade overlay deposits. The research was supported by the Hebei Provincial Science and Technology Program (04212201D) and the Hebei Provincial Doctoral Foundation (B2002222). The study employed metallographic examination, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and hardness testing to systematically characterize the overlay weld metal produced by different electrode formulations.

Core Technical Findings

The authors examined how variations in electrode flux composition—particularly carbon content and the addition of metallic molybdenum—affected the resulting overlay deposit properties. Two principal conclusions emerged from the experimental work:

1. The hardness of the overlay weld metal increases monotonically with increasing carbon content in the electrode flux coating.
2. The addition of an appropriate quantity of metallic molybdenum to the flux significantly improves temper resistance of the overlay deposit. When metallic molybdenum is present, the residual austenite content in the deposit after a secondary tempering treatment at approximately 550°C drops to around 1%, and the hardness reaches its maximum value of 58.6 HRC.

Interpretation of Technical Points

Role of Carbon in Hardness Enhancement

The relationship between carbon content and hardness in overlay weld metals follows the well-established principle that carbon promotes the formation of hard carbide phases and increases the carbon concentration in the retained martensite matrix. In cold shear blade applications, the overlay layer must withstand repeated cyclic cutting forces and abrasive contact with steel workpieces. Higher carbon content in the flux coating enriches the weld metal with carbon during solidification, promoting the formation of fine cementite (Fe₃C) particles and increasing the proportion of high-carbon martensite. This directly translates to higher as-welded hardness, which is critical for resisting plastic deformation during shearing operations.

Molybdenum and Temper Resistance

The finding that molybdenum addition reduces residual austenite after tempering at 550°C is particularly significant. Residual austenite in overlay deposits is generally undesirable in cold shear blade applications because it represents a soft, ductile phase that reduces the effective hardness and wear resistance of the cutting edge. Molybdenum acts as a potent austenite stabilizer during welding, but more importantly, it shifts the tempering behavior of the martensitic matrix. During tempering at 550°C, molybdenum-containing deposits exhibit a secondary hardening effect due to the precipitation of fine Mo₂C and MoC carbides, which compensates for the softening normally associated with high-temperature tempering. The reduction of residual austenite to approximately 1% after tempering indicates that the molybdenum addition modifies the TTT/CCT behavior sufficiently to allow complete transformation during the tempering cycle.

Process Metallurgy Considerations

The achievement of 58.6 HRC after a two-stage tempering treatment suggests a carefully designed thermal cycle. The primary purpose of the secondary tempering at 550°C is twofold: to relieve residual stresses developed during the rapid solidification of the overlay layer, and to induce secondary hardening through fine carbide precipitation. The fact that this treatment simultaneously reduces residual austenite indicates that the microstructural transformation kinetics in the molybdenum-containing deposit are such that the retained austenite becomes thermodynamically unstable at this temperature and transforms to tempered martensite plus fine carbides.

Engineering Practice Integration

In practical cold shear blade manufacturing, the overlay welding process must be carefully controlled to ensure proper dilution between the base steel and the overlay layer. Excessive dilution reduces the carbon and alloy content in the effective overlay layer, leading to lower hardness and reduced service life. The molybdenum addition finding is directly applicable to electrode design for industrial production: flux formulations should include metallic molybdenum powder at concentrations that optimize temper resistance without introducing excessive hot cracking susceptibility.

Key Questions and Reflections

A critical question arises regarding the long-term stability of the 58.6 HRC hardness in actual service conditions. Cold shear blades operate under cyclic loading and may experience temperatures well below 550°C during normal operation, meaning the tempering-induced secondary hardening should be stable. However, the residual stress field in the overlay layer after multi-pass welding remains a concern. High residual tensile stresses at the overlay interface could initiate fatigue cracking under cyclic loading, even if the hardness is optimal.

Another consideration is the toughness-hardness trade-off. Achieving 58.6 HRC in an overlay layer inherently limits the fracture toughness. For cold shear blades cutting high-strength steel, the overlay must also resist chipping and spalling under impact-type loading. The paper does not address impact or fracture toughness measurements, which represents a gap in the characterization.

The reduction of residual austenite to 1% after tempering is encouraging, but the question remains whether this transformation is complete and uniform throughout the overlay layer or whether pockets of retained austenite persist near the fusion boundary where dilution is highest.

Study Insights and Implications

This research demonstrates a clear pathway for optimizing cold shear blade overlay electrodes through strategic alloy design. The combination of increased carbon content for base hardness and molybdenum addition for temper resistance represents a synergistic approach to overlay electrode development. For engineers involved in cold shear blade manufacturing, the practical implication is straightforward: electrode formulations should be evaluated not only on as-welded hardness but also on post-tempering performance, with particular attention to residual austenite content after the intended tempering treatment. The molybdenum addition finding also opens the possibility of using overlay welding as a repair and refurbishment method for worn cold shear blades, provided the base metal is pre-treated to ensure proper fusion and minimize dilution effects.