What is Secondary Hardening?

Description

Secondary hardening is an unusual property where steel increases in hardness during high-temperature tempering, typically above 400°C. VG-10 exhibits this behavior, making it suitable for scissor blades that receive titanium or ceramic coatings applied at elevated temperatures.

What is Secondary Hardening?

Secondary hardening is a phenomenon where steel increases in hardness during tempering at elevated temperatures (typically 400-550°C), contrary to the normal expectation that tempering reduces hardness. This occurs because alloying elements like molybdenum, vanadium, and tungsten form extremely fine carbide precipitates that strengthen the martensite matrix more than the concurrent softening mechanisms weaken it.

Why It Matters for Scissors

Secondary hardening is directly relevant to coated scissors — a growing category in the professional market. Titanium coatings, ceramic coatings, and other surface treatments are applied at temperatures ranging from 200-500°C depending on the process. For most scissor steels, these temperatures cause significant softening — a blade tempered for optimal hardness at 150-200°C will lose 3-5 HRC if reheated to 450°C for coating.

VG-10 is specifically noted by Takefu Special Steel as exhibiting secondary hardening behavior: it “manifests secondary hardening suitable for coated blades up to ~450°C.” This means a VG-10 blade can undergo a high-temperature coating process without losing hardness — in fact, the blade may emerge harder than before coating.

This property gives VG-10 a significant advantage over steels like GIN-3 or AUS-8 for coated scissor applications. Manufacturers choosing to offer titanium-coated scissors often specify VG-10 precisely because of this secondary hardening capability, ensuring the coating process does not compromise blade performance.

Technical Detail
Secondary hardening in VG-10 is driven by the precipitation of nano-scale alloy carbides during high-temperature tempering. The mechanism proceeds as follows: **Stage 1 (150-300°C):** Normal tempering occurs. Transition carbides precipitate from the supersaturated martensite, causing the expected slight decrease in hardness. This is the standard tempering range for uncoated scissors. **Stage 2 (300-400°C):** Continued softening as transition carbides coarsen and cementite begins to form. Hardness continues to decrease. **Stage 3 (400-550°C):** Secondary hardening peak. Molybdenum-rich carbides (Mo2C) and vanadium-rich carbides (VC) nucleate as extremely fine precipitates — often only 2-5 nanometers in diameter. These coherent or semi-coherent precipitates create strong strain fields in the surrounding martensite lattice, impeding dislocation movement and increasing hardness. The net effect is a hardness increase that peaks around 450-500°C for VG-10, sometimes exceeding the original as-quenched hardness. The VG-10 composition that enables this includes: - **Molybdenum (0.9-1.2%):** Primary contributor to secondary hardening via Mo2C precipitation - **Vanadium (0.1-0.3%):** Secondary contributor via VC precipitation and grain boundary pinning - **Carbon (0.95-1.05%):** Provides the carbon for carbide formation By comparison, steels without significant Mo/V additions (such as GIN-3 with only 0.3% Mo and no vanadium) show only monotonic softening with increasing tempering temperature — they have no secondary hardening response. The practical implication for scissor coating processes is significant. Physical Vapor Deposition (PVD) titanium coatings are typically applied at 200-400°C — within the secondary hardening range for VG-10. Chemical Vapor Deposition (CVD) processes run even hotter. The ability of VG-10 to maintain or increase hardness at these temperatures means that manufacturers can apply durable surface coatings without the separate re-hardening step that would be needed with other steels. It is worth noting that secondary hardening is a double-edged property. If a VG-10 blade is inadvertently overheated during grinding or sharpening (generating localized temperatures above 400°C), it can develop hard spots that respond differently to subsequent sharpening. This is one reason why professional scissor sharpeners must control grinding temperatures carefully, especially on VG-10 blades.

Sources

Frequently Asked Questions

VG-10's molybdenum and vanadium content causes fine alloy carbides (Mo2C, VC) to precipitate within the martensite during tempering at 400-500°C. These nano-scale carbide particles increase hardness beyond the as-quenched level, counteracting the normal softening from tempering.

Scissors with titanium or ceramic blade coatings benefit most. These coatings are applied at temperatures that would soften most steels, but VG-10's secondary hardening response means the blade actually maintains or increases hardness during the coating process.

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