Quenching (update)

Quenching heat transfer
Schematic showing stages of Quenching

Heat transfer during quenching into liquid is complicated when the quench medium forms a gas when heated to high temperatures.

The quenching process is usually split into 3 -5 stages.

Stage 1. Vapour blanket.

Depends upon surface roughness, quenchant temperature, quenchant type, oxidation. Heat transfer by radiation through vapour layer into quenchant.

Stage 1b. Partial film boiling.


Stage 2. Nucleate boiling.

Conduction and Convection.


Stage 3. Convection.

Dominated by heat transfer in the quenchant by convection which can be natural, forced with turbulence or forced with lamellar flow.

Hala Salman-Hasan has been investigating heat transfer problems, a series of experiments have been made for quenching using an experimental rig we built at Cambridge. Here we can see a video of a 2 mm probe made from steel which has a thermocouple embedded. Data logging is from a computer with data logger. I think data logging at these speeds could also be achieved using an Oscilloscope, that would probably need an electrical engineer, and more importantly a trigger to start logging at the correct time.


Quenching into water

Typical Quenching Curve
Quenching curve for water at 42 Celcius, Heat Transfer as a function of temperature.

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L6 Steel Kinetics

This steel is popular for use in knives and swords like the Samurai sword “Bainite Katana”.

Custom Howard Clark L6 Katana
The Bainite Katana is made of a special purpose low-alloy steel. It is very resistant to bending, to the point of near unbreakability. These blades can be made lighter and thinner and still remain stronger than conventional steel or 1086. The blades are also springy rather than soft, they will flex more than a normal blade, but the shape will not be altered. These blades are excellent for tameshigiri as well as general sword work.

Composition; I found the composition of L6 is something like: Fe-0.7C-0.5Si-1.75Ni-0.5Mo-0.25V-0.25Cu.

I calculated the time-temperature-transformation diagram, i’m not too sure about the accuracy bainite start temperature using this program, I would like to be able to find the experimental results for this alloy.

Depending on the bainite start temperature, the kinetics are fast enough to allow isothermal transformation to bainite on a reasonable timescale, at temperature of around 300 C which would give a high strength bainite. Howard Clarke sells the swords with as a martensite/bainite sword. If the bainite start temperature is higher it would be easy to get a mixed microstructure.

I should try to make a prediction of bainite start temperature – I made a neural network model for this.

AISI L6 TTT diagram calculated with MAP program MUCG83

Even without carbide precipitation transformation results in a large volume fraction of bainite.
AISI L6 Volume fraction calculated with MAP program MUCG83

Presumably if the bainite Katana is used in marteniste/bainite condition it is produced by continuous cooling or by quenching. Quenching would be good if it can be done in at a rate which gives martensite on the outside of the blade for sharpness and hardness, and bainite in the centre to give toughness. The other possibility is that the continuous cooling gives a mixture of bainite and martensite at every location. This can have a higher strength or hardness than martensite alone because austenite will get enriched in carbon as the bainite transforms, increasing it’s contribution to the strength.

I’d really like to have a look at the microstructure of these swords to see what structure the bainite has. Also it would be interesting to measure the mechanical properties of the alloy in the same condition – I need to look for some literature on L6 I guess. It looks like they should be strengthened by carbides and by copper precipitation.

Howard Clark who makes these swords has a webpage at mvforge.com.

–edit 6 October 2007–
L6 TTT

Quenching

Quenching heat transfer

Heat transfer during quenching into liquid is complicated when the quench medium forms a gas when heated to high temperatures.

The quenching process is usually split into 3 -5 stages.

Stage 1. Vapour blanket.

Depends upon surface roughness, quenchant temperature, quenchant type, oxidation. Heat transfer by radiation through vapour layer into quenchant.

Stage 1b. Partial film boiling.


Stage 2. Nucleate boiling.

Conduction and Convection.


Stage 3. Convection.

Dominated by heat transfer in the quenchant by convection which can be natural, forced with turbulence or forced with lamellar flow.