Microstructure and Properties of Cold Shear Blade Hardfacing Metal A Study Note

Literature Overview

This paper by Li Da, Liu Ligang, Yang Yulin, and Yang Qingxiang from Yanshan University, published in the Transactions of the China Welding Institution (Vol. 28, No. 2, 2007, pp. 25–28), investigates the influence of alloy composition in hardfacing electrode flux coatings on the microstructure and mechanical properties of cold shear blade hardfacing deposits. The research was supported by the Hebei Provincial Science and Technology Key Project (04212201D) and the Hebei Provincial Doctoral Fund (B2002222). Cold shear blades are critical cutting tools used extensively in steel processing lines for cutting hot-rolled strip and plate at ambient temperature, and their service life is directly governed by the wear resistance and toughness of the hardfacing overlay.

Core Technical Findings

The authors employed metallographic examination, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and micro-hardness testing to characterize the hardfacing deposits produced with electrodes of varying alloy compositions. The key findings are summarized below.

Carbon Content Effect on Hardness

The hardness of the hardfacing deposit increases monotonically with increasing carbon content in the electrode flux coating. This is consistent with the well-established metallurgical principle that carbon promotes the formation of hard carbide phases (such as Fe₃C, Cr₇C₃, and Cr₂₃C₆) and stabilizes high-hardness martensitic structures during rapid solidification. The carbon enrichment in the flux ensures that the molten weld pool achieves a sufficient carbon activity, preventing dilution-induced softening from the low-carbon base metal.

Molybdenum Addition and Tempering Resistance

A critical finding is the recommendation to add an appropriate amount of metallic molybdenum to the flux coating to improve the temper resistance of the hardfacing deposit. Molybdenum is a potent secondary hardening element that delays the decomposition of martensite and suppresses the precipitation of low-hardness carbides during tempering. The study demonstrates that when metallic molybdenum is present in the deposit, a secondary tempering treatment at approximately 550 °C reduces the residual austenite content to approximately 1%, while simultaneously achieving the maximum hardness of 58.6 HRC.

Residual Austenite Control

Residual austenite in hardfacing deposits is a double-edged sword. While a small amount (typically 5–15%) contributes to toughness and reduces cracking susceptibility, excessive residual austenite (>20%) significantly degrades hardness and wear resistance. The authors demonstrate that the molybdenum-containing deposit, after secondary tempering at 550 °C, achieves a remarkably low residual austenite fraction of approximately 1%, indicating that the retained austenite has been effectively stabilized or transformed. This is attributed to the combined effect of molybdenum promoting bainite transformation and the thermal cycling of the two-stage tempering process.

Technical Parameters Summary

Engineering Practice Implications

From a practical standpoint, the findings have direct relevance to the repair and maintenance of cold shear blades in steel processing plants. Cold shear blades typically experience severe abrasive wear from cutting high-strength steel strips, and the overlay must maintain hardness under repeated thermal cycling during operation. The recommendation for a two-stage tempering process at 550 °C is particularly valuable because it provides a controlled post-weld heat treatment that minimizes cracking while maximizing hardness retention.

In my experience with shear blade maintenance programs, the selection of hardfacing electrode composition is often overlooked in favor of simply maximizing as-welded hardness. This study reinforces the importance of considering temper stability, especially when the blade is exposed to temperatures exceeding 400 °C during operation or during subsequent hot-rolling passes. The molybdenum addition strategy aligns with the design philosophy used in high-speed steel and wear-resistant tool steels, where Mo₂C and Mo₆C carbides provide exceptional thermal stability.

Process Considerations for Hardfacing Electrode Selection

1. Flux carbon activity management: The flux must contain sufficient carbon donors (such as Fe₃C, graphite, or carbonite) to compensate for carbon burn-off during arc welding.
2. Molybdenum form: Metallic molybdenum particles in the flux ensure direct alloying rather than relying on dissolution from complex carbides, which may not release Mo efficiently at welding temperatures.
3. Two-stage tempering: The first temper relieves residual stresses and partially transforms retained austenite; the second temper at 550 °C completes the transformation and promotes secondary hardening via fine carbide precipitation.

Key Questions and Reflections

One question that arises from this study is whether the 550 °C tempering temperature is optimal for all cold shear blade applications. Blades used for cutting low-carbon steel may operate at lower temperatures, in which case a lower tempering temperature (300–400 °C) might preserve higher hardness without sacrificing toughness. Conversely, blades used in hot-strip mills may experience temperatures exceeding 600 °C, requiring even greater temper resistance.

Another consideration is the dilution effect. The study does not explicitly quantify the dilution rate between the hardfacing deposit and the base metal. In practice, dilution of 10–30% is typical for single-pass hardfacing, and this can significantly reduce the effective carbon and alloy content in the deposit. Multi-pass hardfacing or the use of high-alloy filler metals may be necessary to achieve the reported hardness values in industrial settings.

Study Insights and Implications

This paper provides a clear demonstration that hardfacing electrode design must be approached as a holistic metallurgical problem rather than a simple hardness optimization exercise. The interplay between carbon content, alloying elements (particularly molybdenum), residual austenite fraction, and post-weld heat treatment creates a multi-variable optimization landscape. The 58.6 HRC hardness achieved after secondary tempering is remarkable for an electrode-based hardfacing process, and it suggests that with proper alloy design and thermal processing, electrode hardfacing can rival certain powder-based or metal-ceramic hardfacing systems in terms of achievable hardness.

For engineers involved in cutting tool maintenance and repair, the key takeaway is that the choice of hardfacing electrode should be driven by the service temperature and wear mechanism, not merely by as-deposited hardness specifications. The molybdenum-enhanced, two-stage tempered approach offers a practical pathway to extending cold shear blade life, particularly in applications where thermal cycling is significant. Future work should explore the combination of molybdenum with other secondary hardening elements such as tungsten and vanadium to further enhance thermal stability and wear resistance.