After vacuum gas quenching D2 tool steel, even if the surface hardness reaches HRC 58–60, the microstructure may still contain 15% — 25% Retained Austenite.
Since austenite is a soft and unstable phase, it leads to two major technical issues:
- Hardness Limitation:Prevents the material from reaching its maximum potential of 62+ HRC.
- Dimensional Instability:Over time, retained austenite slowly transforms into martensite, causing microscopic volume expansion — a fatal flaw for high-precision molds.
Timing and Methodology of Sub-zero Treatment
To ensure complete transformation, sub-zero treatment should be performed immediately after the quenching cycle when the parts reach room temperature:
Process Temperature:Typically ranging from -70°C to -190°C. A vacuum cryogenic chamber using liquid nitrogen circulation is the most effective equipment.
Operational Flow:
Post-Quench:Once the workpiece cools below 50°C (cool to the touch).
Cryogenic Soaking:Gradually decrease the temperature to the target and hold for 2–4 hours.
Immediate Tempering:After cryogenic treatment, the parts must immediately undergo vacuum tempering to relieve stresses and stabilize the newly formed martensite.
Effects:
Typically gains an additional 1–3 HRC, significantly increasing tool wear resistance.
Eliminates the risk of deformation during service, extending the lifespan of precision cold work dies.
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