Reason for pre-heating AAA quench oil

[S obsessed]

What is the exact reason for pre-heating the AAA quench oil from Dubois. The P50 or parks 50 is fine to be run at room temp. The AAA oil lists ideal temp range to be 130-160f. It also lists it to be used from 77-180f. Warming canola from what I gather was always to make it thinner and speed up its cooling ability. Test coupons for medium hardenability steels like 5160, 6150, 4140, 80crv2 have fully hardened and test the same rockwell in room temperature AAA and P50. So is the warming it up to 130f to get it to match it's medium to fast quench speed or is it to possibly slow the lower end temperature as the steel goes through the martensite finishing temperature to negate added stress kind of like marquenching? Or is it to homogenize the oil somehow and make it more consistent? Recently I hardened a whole bunch of 6150 punches and chisels and some larger items and the oil got to the upper limit of it's temperature and instead of letting the tools cool fully down in the oil I was placing them in a bath of soapy water. The last couple were a bit too hot and the stress from that last bit of cooling in the water cracked them up pretty good. So the last bit of cooling is obviously very important as p50 didn't do this to them and I know that getting the curve in a clay backed blade is dependent on that fast drop at the end that water can bring. I have 5 gallons and to quench small things I don't want to heat the whole thing up if it doesn't need it but if there is a good reason to then I will. Perhaps running some steels in it that balance on the edge of fast or medium would need it to be warmed up but it seems like p50 works fine for those anyway.

 

[A.McPherson]

If it says that the ideal temp is 130-160 the it has been designed to deliver the advertised performance at that temp.

 

[Stacy E. Apelt - Bladesmith]

The difference is the quench speed.

At room temp, Parks #50 works. It is a fast quench. This mix of oils is designed to simulate water speed (almost) but slow the final speed at the bottom of the CCT curve. Heating it makes it less effective. This is the only room temp blade quenchant I am aware of.

At 130°F, AAA works because it is a little slower and less violent. The drop from 400°F (Ms) to 130°F (80% martensite) stops when the blade reaches the tank temperature, and then you remove it and let it cool in the air. Sticking the blade in a bucket of water isn't wise in most cases. That would speed up the cooling from 130° to ambient, which may lead to cracking in some steels.
If quenching multiple blades, starting at 120°F is what most folks do to allow for the tank temperature rise. Using a large volume quench tank (5 gallons or more) is also a good practice. I consider 3 gallons the minimum for quenching knife blades properly.

Brine quench is heated to around 120°F for the same reason. It slows the CCT to allow the final stages of the martensitic transformation to occur without cracking the blade (most of the time).

Theoretically, leaving the blade in the quench tank for a few minutes is the best procedure. This assures the blade only drops to 130°F and at a slow pace. From there it can cool to ambient and reach Mf in still air. Most folks pull it after about 10 seconds, and that usually is fine.

When a carbon steel blade is taken out of the quench it should be wiped off and hung in still air. Cooling in water or even laying it on the anvil may cause cracks or a warp to form.

The cause of cracking occurs during the transformation from austenite to martensite between 400°F and ambient. Most of the transformation is done by 130°F. The last 50° to room temp should be slow as well. This is a very violent transformation and exerts great stress on the blade. The higher the carbon content, and the lower the alloying, the worse the risk of cracking by too rapid cooling.

One thing that confuses many people is that drop from Ac1 to 900°F needs to be very fast for low alloy high carbon steels. This drop needs to happen in less than 1 second! BUT, from there it needs to slow down to avoid cracking or warping the blade. This is why different steels need different quenchants. Parks#50 is close to a universal quenchant, and if it is all you have will work.
AAA is a good second quenchant to have on hand. If it is all you have, you will be OK for most carbon steels.

 

[S obsessed]

Thanks for the thorough answer Stacy. That is exactly what I was looking for.

 

[daily_grind]

Ok - I'm guessing Ms = upper limit of Martensite transformation temperature, What is CCT, Ac1?
Maybe I should buy Larrin's book already ...

 

[Stacy E. Apelt - Bladesmith]

YES, you should buy Larrin's book!

Ms is Martensitic transformation Start, This is around 400°F.
Mf is Martensite transformation Finish. For simple carbon steels, this is around 100°F, but most folks consider ambient as the Mf for simple carbon steel. High alloy and stainless steels have a much lower Mf - some down as low as -100°F.
CCT is a diagram of the Continuous Cooling Transformation
TTT is a diagram of the Time Temperature Transformation curve. It is also called an isothermal diagram.
Ac1 is the temperature on a TTT diagram where austenite starts forming ferrite when cooling. It is where the clock starts ticking as steel cools from austenite as the quench starts. Ar1 is where the ferrite becomes austenite when heating. They are not the same temperature. Think of c= cooling and r= rising.) CCT and TTT are very similar and can be considered the same by anyone who isn't a metallurgist.
The Eutectic is the point where there is exactly the necessary amount of carbon to form steel with no excess or lack. Technically, that is .76% carbon for a no-alloy steel. Steels that fit this range are called Eutectoid steels.

These charts show how fast the quench speed needs to be to pass te pearlite nose ... and when it can be slower beyond that point ... for each steel type.
The charts below are for eutectoid steels (between .75 and .85% carbon). That is why the Ac1 is called Ae1 on this chart.