What is Retained Austenite?

Description

Retained austenite is untransformed austenite remaining in steel after quenching to room temperature. It is softer than martensite, causes dimensional instability over time, and reduces achievable hardness. It is eliminated by cryogenic treatment at -80°C to -196°C.

What is Retained Austenite?

Retained austenite is the portion of the high-temperature austenite phase that fails to transform into martensite during quenching. It remains trapped in the microstructure at room temperature as a soft, metastable phase. Because it is significantly softer than martensite and can transform unpredictably over time, it is considered a defect in hardened scissor blades that must be minimized.

Why It Matters for Scissors

Retained austenite creates two distinct problems for scissors. First, it directly reduces hardness — each 1% of retained austenite lowers the measured HRC by approximately 0.1 points. A blade with 20% retained austenite loses roughly 2 HRC compared to a fully transformed blade, representing a meaningful reduction in edge retention.

Second, retained austenite is metastable and can transform to martensite during service — triggered by mechanical stress from cutting, temperature changes in storage or shipping, or simply over time. This uncontrolled transformation causes a volume expansion of approximately 4%, creating dimensional changes in the blade. For scissors, where two blades must ride against each other with precise contact pressure, even micron-level changes alter the feel and cutting performance.

Sub-zero processing is specifically required for VG-10 and ZDP-189 to address retained austenite. VG-10 requires cryogenic treatment per Takefu Special Steel specifications, and ZDP-189’s extremely high carbon content (3%) pushes its martensite finish temperature so far below room temperature that standard quenching leaves 25-30% retained austenite without cryogenic treatment.

Technical Detail
The amount of retained austenite after quenching depends primarily on the martensite start (Ms) and martensite finish (Mf) temperatures of the steel, which are determined by its chemical composition. Carbon is the strongest depressor of Ms and Mf temperatures, followed by manganese, chromium, and nickel. The Koistinen-Marburger equation describes the fraction of martensite formed as a function of undercooling below Ms: f(martensite) = 1 - exp[-0.011 × (Ms - T)] Where T is the quenching temperature. This exponential relationship means that the last few percent of retained austenite are the hardest to convert — each additional degree of cooling converts less austenite than the previous degree. Estimated Ms and Mf temperatures for common scissor steels: | Steel | Ms (°C) | Mf (°C) | Retained Austenite at 20°C | |-------|---------|---------|---------------------------| | GIN-3 | ~220 | ~40 | 5-10% | | VG-10 | ~190 | ~-50 | 15-20% | | ZDP-189 | ~140 | ~-120 | 25-30% | The Mf temperatures above explain why different steels have different cryogenic requirements. GIN-3's Mf near 40°C means most transformation completes at room temperature. VG-10's Mf near -50°C means standard cryogenic treatment at -80°C is sufficient to complete the transformation. ZDP-189's Mf near -120°C means even standard cryo at -80°C leaves some retained austenite — deep cryogenic treatment at -196°C (liquid nitrogen) is needed for maximum conversion. Detection of retained austenite is typically performed by X-ray diffraction (XRD), which can quantify the phase percentage. Metallographic observation (etching and microscopy) can reveal retained austenite but is less precise for quantification. It is important to note that cryogenic treatment must be followed by tempering. The fresh martensite formed during cryo treatment is fully hard and untempered, making it extremely brittle. Without subsequent tempering, these freshly transformed regions become the weakest points in the microstructure, prone to cracking under the impact stresses of scissor cutting. The sequence for proper retained austenite elimination is: quench → cryogenic treatment → temper → (optional second cryo + temper cycle for maximum conversion).

Sources

Frequently Asked Questions

Without cryogenic treatment, VG-10 may retain 15-20% austenite, while high-carbon ZDP-189 can retain 25-30%. Even 10% retained austenite measurably reduces hardness and introduces dimensional instability that affects scissor blade alignment.

Scissors rely on precise blade alignment maintained over years of use. Retained austenite can spontaneously transform to martensite during service, causing microscopic dimensional changes that alter blade contact. This degrades cutting performance even though the blades appear undamaged.

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