How to minimise Retained Austenite in Knife Steel

I’ve written several heat-treating guides and articles on my blog/website, and while I mention “Retained Austinite” quite a bit, it is time that I covered the topic more in-depth and answer what it is and why we as knifemakers should be mindful of it and what we can do to minimise RA in our knife blades.

What is Retained Austinite?

Retained austenite is the austenite phase that remains untransformed in steel after quenching, rather than fully transforming into martensite. Austenite is a high-temperature face-centered cubic (FCC) crystal structure that can dissolve more carbon compared to the body-centered cubic (BCC) or body-centered tetragonal (BCT) structures of ferrite and martensite.

This retained austenite occurs when the steel is not quenched to a temperature low enough to form 100% martensite. However, the transformation is rarely complete, and some austenite remains in the final microstructure. The amount of retained austenite depends on factors like stabilising alloying elements such as Nickel, Manganese, Carbon, quenchant temperature, austenite grain size, cooling rate during the quenching and subsequent thermal treatments [4].

Pros and Cons of Retained Austenite in Knife Blades

Typical retained austenite (RA) values in common blade steels range from about 5% to 25%, depending on the type of steel, carbon content and its specific heat treatment.

• 1095: Retained austenite levels can range from 5% to 15%
• 440C: Typically retains about 10% to 20% (read my guide)
• D2: Retained austenite can be around 5% to 10% (read my guide)
• M390: Retained austenite levels can be higher, often in the range of 15% to 25% (read my guide)

Pros:

• Small amounts of retained austenite can increase toughness and ductility compared to a fully martensitic structure. This can help prevent chipping and cracking of the blade edge.
• The austenite phase has higher strength than tempered martensite. This contributes to the blade’s overall hardness and wear resistance.

Cons:

• Excessive retained austenite (above 12-15%) is generally considered detrimental as it can lead to reduced toughness, dimensional instability, chipping, and premature failure.
• Even if the final hardness is acceptable (ie 60 HRC), it will lead to poor performance, and result in deformation at lower stresses than would typically be seen at the same hardness with lower retained austenite.
• Retained austenite is a softer phase compared to martensite. Too much of it reduces the blade’s wear resistance and edge retention.
• Excess retained austenite makes blades difficult to sharpen and extremely hard to deburr (“feathery” burr).

Controlling Retained Austenite

To optimize the amount of retained austenite in a knife blade, the following methods can be employed (different steels may respond differently):

1. Adjust the steel’s carbon content: Steels with higher amounts of carbon (>0.8%) tend to have more retained austenite after quenching. The austenitization temperature significantly influences the amount and stability of retained austenite.
   • A higher austenitising temperature generally leads to increased amounts of retained austenite after quenching and can lower the Mf below room temperature. This will require careful post-quench processes to ensure effective conversion to martensite.
   • Lower temperatures produce lower amounts of retained austenite that can transform effectively during tempering cycles. Recommended if you don’t have/use post-quench cold treatments like “Cyro”.
   • Ensuring that you soak the steel at the austenitizing temperature for a sufficient amount of time (15-30 minutes) will help achieve a more homogenous austenite phase.
2. Optimize the quenching process: The transformation to martensite is primarily controlled by temperature, not time. Faster quenching rates promote martensite formation and reduces retained austenite[3]. Use the recommended quenching quenchant to rapidly cool the blade below the martensite finish (Mf) temperature. Agitate the knife in the quenchant to maximize the cooling rate and avoid high oil temperatures. For Air-hardening steels, a plate quench together with compressed air will achieve faster cooling rates.
3. Limit stabilisation: If the steel is left at room temperature (>1 hour), the retained austenite can become stabilized (Room temperature ageing effect), making it more resistant to transformation during subsequent treatments. Therefore, the longer you wait before cooling/tempering the steel, the less retained austenite will transform. Experimental data show that if a sample with retained austenite is subjected to cold treatment shortly after quenching, a significant percentage (up to 70%) of the retained austenite can be transformed. However, if the blade has been allowed to sit for a longer period (e.g. 50 hours), only about 30% of the retained austenite may transform during the same treatment [1].
4. Employ cryogenic treatment: Cooling the blade below room temperature using liquid nitrogen or other cryogenic media (the coldest you have available) can convert most of the retained austenite to martensite, especially in high-alloy steels. Think of cryo as an extension of the quench down to lower temperatures. This is more important for high alloy / stainless steels (but not limited to) as their martensite finish (Mf) temperature can be below room temperature. It is also not recommended to do a snap temper before cyro.
5. Temper at higher temperatures: The stability of retained austenite can vary based on the steel’s composition and the tempering conditions. Therefore, the effectiveness of multiple tempering cycles can depend on the specific characteristics of the steel being used. So follow the recommended specifications for your chosen steel. Multiple tempering cycles can further reduce retained austenite levels, further improving dimensional stability and mechanical properties. Avoid very high temperatures or over-tempering as this can cause other unwanted issues like embrittlement, carbon diffusion, formation of softer phases and a loss of stain resistance. Any tempering process must be balanced against the need to maintain sufficient hardness in the final steel.

Key Take Aways

• Retained Austinite is in every knife blade but care should be taken to manage the levels of RA for optimal performance.
• High carbon-containing steels (>0.8% C) will be more prone to retaining more austenite at room temperature due to higher martensite start temperatures (Ms).
• Lower-range austenitization temperatures should be used if retained austenite needs to be minimized [2]
• Minimise the time between quenching and cold treatments to reduce stabilisation.
• Multiple tempering cycles at a sufficiently high temperature, further reduce retained austenite levels by providing additional opportunities for transformation[3].

In summary, retained austenite plays a dual role in knife blade performance. A small amount of retained austenite can enhance toughness and prevent chipping, but excessive amounts can reduce hardness and edge retention. Use lower austenitization temperatures, fast quenching, multiple high-temperature tempering cycles, and cryogenic treatment to reduce retained austenite levels in knife steels effectively.

Citations:

1. https://gearsolutions.com/departments/hot-seat/transformation-of-retained-austenite/
2. https://knifesteelnerds.com/2018/03/01/austenitizing-part-2-effects-on-properties/
3. https://www.researchgate.net/publication/362069334_Reduction_of_Retained_Austenite_in_Tool_Steels
4. https://www.researchgate.net/publication/352246309_THE_POSSIBILITIES_OF_THE_RETAINED_AUSTENITE_REDUCTION_ON_TOOL_STEELS