Why Is A Multiple High-Temperature Tempering Process Necessary For High-Cobalt Steels? Achieve Peak Hardness & Stability

Multiple high-temperature tempering cycles are essential for high-cobalt steels like Vanadis 60 to eliminate unstable microstructures and achieve maximum durability. Because cobalt acts as a strong stabilizer, a single thermal cycle is insufficient to fully transform the steel; you must employ repeated heating between 500°C and 560°C to force the material into its hardest, most stable state.

High cobalt content prevents Vanadis 60 from fully transforming into hard martensite during the initial quench, leaving behind soft, unstable sections. Multiple tempering cycles are the specific mechanism used to convert this remaining "retained austenite" into hardened martensite and precipitate carbides for superior wear resistance.

The Challenge of Retained Austenite

Cobalt’s Stabilizing Effect

The defining characteristic of Vanadis 60 is its high cobalt content. While cobalt improves heat resistance, it chemically stabilizes austenite, the phase of iron that exists at high temperatures.

The Quenching Aftermath

When you quench standard steel, austenite transforms rapidly into hard martensite. However, due to the stabilizing nature of cobalt, Vanadis 60 retains high levels of retained austenite even after the initial quench. This leaves the material softer than intended and structurally unstable.

The Mechanics of Multiple Tempering

Triggering Decomposition

To correct the microstructure, the steel must be subjected to high temperatures, typically in the 500°C to 560°C range. These temperatures provide the thermal energy required to destabilize the retained austenite, triggering its decomposition.

Transformation During Cooling

Crucially, the transformation into new, hard martensite does not happen while the steel is hot, but during the cooling phase following the temper. Because this new martensite is untempered (brittle) immediately after forming, subsequent cycles are required to temper this new layer and ensure uniformity.

Achieving Secondary Hardening

Precipitating Carbides

Beyond converting austenite, these cycles induce a dispersed secondary hardening effect. The heat causes alloy carbides to precipitate out of the matrix, providing a significant boost to the material's overall hardness.

Dimensional Stability

Retained austenite is unstable and can transform over time during service, causing the part to change size or warp. By forcing this transformation during the tempering process, you ensure dimensional stability, meaning the tool or component will maintain its precise shape during use.

Understanding the Trade-offs

Process Intensity vs. Performance

The primary trade-off with Vanadis 60 is time. Standard steels may only require a single or double temper at lower temperatures. Vanadis 60 demands a rigorous, multi-step regimen (2-3 cycles), increasing energy consumption and processing time to guarantee the material performs as specified.

Temperature Sensitivity

Precision is non-negotiable. If the tempering temperature falls below the 500°C threshold, the retained austenite may not decompose effectively. Conversely, exceeding the optimal range can lead to over-tempering, reducing the hardness gained from the secondary hardening effect.

Making the Right Choice for Your Goal

To maximize the utility of Vanadis 60, align your heat treatment strategy with your specific performance requirements:

If your primary focus is Maximum Wear Resistance: Adhere strictly to the 500°C–560°C range to maximize carbide precipitation and the secondary hardening effect.
If your primary focus is Dimensional Precision: Prioritize completing all three tempering cycles to eliminate virtually all retained austenite, preventing future warping.

Properly executed multiple tempering turns the chemical stubbornness of cobalt into a structural advantage, yielding a steel that is both exceptionally hard and dimensionally reliable.

Summary Table:

Feature	Single Tempering Cycle	Multiple Tempering Cycles (2-3)
Microstructure	High Retained Austenite (Unstable)	Transformed Hardened Martensite
Dimensional Stability	Poor (Risk of warping/growth)	Excellent (Fully stabilized)
Hardness Level	Lower (Soft sections remain)	Maximum (Secondary hardening effect)
Wear Resistance	Standard	Superior (Carbide precipitation)
Cooling Phase	Partial transformation	Complete conversion and stress relief

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References

Florentino Álvarez-Antolín, Alejandro González-Pociño. Effect of Heat Treatment on the Microstructure and Mechanical Properties of Vanadis 60 Steel: A Statistical Design Approach. DOI: 10.3390/solids6030046

This article is also based on technical information from Kintek Furnace Knowledge Base .