Compensating for heat loss on the way to the quench

B

BKT

Joined
Feb 15, 2016
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Do you guys overshoot the necessary temperature in your oven to account for heat loss on the way to the quench? Especially for thin blades I can imagine it losing quite a number of degrees before actually hitting the oil.
 
...might I suggest not taking a leisurely stroll around the neighborhood in between removing your blades from the heat and quenching them? :D

No reason to overshoot the temp - it's not critical to quench exactly from the austemp you're sitting at, it's important that the blade be at austemp so that it reaches austemp, if you follow. As long as you get the blade below its target temp in time, and don't dawdle when getting from the oven, you'll be fine.
 
I wrote a simulation program to calculate the approximate rate of change of temperature based on heat loss via radiation and natural convection (basically still air) and for the first 3 seconds, a blade with a thickness of 1/10 inch would lose around 15°C per second. Afterwards, this rate of change slows down a bit to around 10°C/s and it continues down this trend.

What are the dimensions of your blade? I can run it through my program for you. I just need the length and width and the average thickness of the blade.
 
first i would check your shop layout. when heat treating, my quench is within arms reach of furnace, so blade goes from furnace to quench in a second or so. using Duurza's math, a blade at 820C(1510F) would be at 790C(1450F) when it hit the quench. i have practiced with cold blanks so it is one movement from furnace to quench.
scott
 
There are people here who can and I hope will explain this in great detail, but a simple version is this: the austenizing (soak) temperature is where the alloy needs to be to get the distribution and structure you want through a soak- the temperature that it has to be when it goes into the quench is a different, lower number.
Obviously if any part of the object you're quenching has a thin cross section, you need to work very quickly, but observing the color of the glowing steel in a dimly lit shop will tell you a lot about how much time you have- take a sample out of the oven and see how long you have before it goes black.
 
practiced with cold blanks so it is one movement from furnace to quench
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That is the trick to making a good quench - the heat loss in air from forge/oven to quench is just part of the quench process... just make it quick.
 
Thanks for all the responses. 1475* seems like a fairly specific number, so I figured maybe overshoot to 1500* to account for some heat loss on the way down. Doesnt sound like a huge deal so I will just proceed normally.
 
I have posted on this before, but will do it again.

The quench is a part of the hardening process. The name for the graph of this process is a CCC - Continuous Cooling Curve.

The key word is continuous.

The blade should be heated to the correct austenitization temperature and the alloying and carbon given sufficient time to become properly distributed. From that point on it should continuously cool.
If you take the blade above that point, you have changed the HT parameters ... which you do not want to do. If the blade cools a few degrees on the way to the quench tank, that is part of the continuous cooling, and is perfectly fine.

The major factors in the cooling are that the blade still be above 1350F before the actual quench, and that it cool from 1350F to below 900F in the needed time for the steel type
( from .7 seconds for 1095, to around 10 seconds for O-1, to over a minute for A2).
This temperature/time window is called the pearlite nose because it sticks out like a nose on the CCC chart.

The time from oven/forge to the quench tank is not part of this nose, and only requires that the steel stay above 1350F until it enters the quench tank. If the steel cools below 1350F before quench, or the quench medium does not cool the steel below 900F fast enough, the steel will convert into pearlite.

The pearlite nose time window is the time for the steel to pass from the critical point around 1350F and stay as austenite into super-cooled austenite as it makes it past the pearlite nose. Looking at a CCC (or TTT) chart will allow you to know the times for these things in the steel you are HTing. In general, the steel should pass the nose as smoothly as possible withing the time needed. However, the quench should not be more violent than needed. That is why there are several types and speeds of quanchants from water, to oils, to air.

Once past the 900F point the blade should continue to cool down to around 400F, where the conversion to martensite starts. From there it should continue to cool down to room temperature, and then be tempered immediately. Steels that need sub-zero/cryo should continue to cool down to -100/-350F, and then warm up to room temperature before the temper.

Because the structure is a super cooled fluid once below 900F, any long delay in the continuous cooling process can make it not fully convert into martensite.

During the time from 900F to 400F the blade is basically 100% austenite. Austenite is like rubber compared to martensite. You can straighten the blade easily during this 500 degree drop. You have nearly a minute when cooling the blade in air to do the straightening. If clamped between plates during this drop, the blade will cool straight.

Once the blade reaches 400F, all straightening must stop, as the martensite it converts to is very brittle and the blade will break. ( The blade can stay in the plates until room temp if desired).


At no time from the austenitization soak to room temperature should the blade have anything but a continuous drop in temperature. After the blade reaches Mf ( full martensite conversion), the heating and cooling in the two temper cycles should also be immediately and continuous.

You may have noticed that the words continuous and continue appear many times in the above .... that is because they are very important words in HT. The word fast or rapid is only used between 1350F and 900F. In all other parts of the CCC, speed is not a major factor. In most cases, rapid cooling in the other areas is not desirable.



What this all means:
Heat the blade to the HT target temperature as exactly as possible
Soak for the needed time at the exact target temperature
Remove the blade and transfer it to the quench tank smoothly and safely ( in a reasonably short time)
Quench the blade moving it up and down and/or in a slicing motion, but not from side to side
Continue to agitate as the blade cools to below 900F
Remove from the tank and let air cool or place between plates to avoid warp. Alternately, the blade may be left hanging in the quench tank for several minutes
Once past 200F, remove from plates/quench tank and let air cool to room temp *
Proceed with the temper cycles.
( * do any sub-zero or cryo directly after reaching room temp)
 
Stacy can you copy and paste that into the heat treat sticky? What an excellent explanation of process variables often left unsaid in heat treat discussions.
 
Like some have said, when you see that a steel has like 1095 or W2 a "window' of say .7-1 seconds, ti does not mean that you only have .7 seconds to get it out of the oven and into the oil or water. Going by the 1350F figure that Stacy quoted, and the 10- 15C per second someone else cited, that would mean that even at the lowest austenizing temp that you would typically used, say 1425 if you were trying to get a crazy hamon with W2, you would still have at least 3 seconds to get to the quench even with a fairly thin chefs knife blade and that assumes that the clay doesn't slow down the overall cooling,
 
I was thinking of buying a 55 gallon drum of Parks 50 so that I could just quench my entire oven, blades and all, without having to worry about removing them and potentially dropping below the nose before quench. Then it occurred to me that a thermal mass that large MIGHT not cool quickly enough, even in Parks, so I was thinking of maybe pre-loading my blades into a high temp slingshot of sorts, that trips when the oven door is opened. I just have to be standing in front of the door with a bucket of quench, ready to catch them when they come flying out.

Either that, or I'll just put my quench tank closer to the oven. :D
 
knife to a gunfight said:
I was thinking of buying a 55 gallon drum of Parks 50 so that I could just quench my entire oven, blades and all, without having to worry about removing them and potentially dropping below the nose before quench. Then it occurred to me that a thermal mass that large MIGHT not cool quickly enough, even in Parks, so I was thinking of maybe pre-loading my blades into a high temp slingshot of sorts, that trips when the oven door is opened. I just have to be standing in front of the door with a bucket of quench, ready to catch them when they come flying out.

Either that, or I'll just put my quench tank closer to the oven. :D
Click to expand...

The best way is to put quench tank IN the oven :D
 
knife to a gunfight said:
I was thinking of buying a 55 gallon drum of Parks 50 so that I could just quench my entire oven, blades and all, without having to worry about removing them and potentially dropping below the nose before quench. Then it occurred to me that a thermal mass that large MIGHT not cool quickly enough, even in Parks, so I was thinking of maybe pre-loading my blades into a high temp slingshot of sorts, that trips when the oven door is opened. I just have to be standing in front of the door with a bucket of quench, ready to catch them when they come flying out.

Either that, or I'll just put my quench tank closer to the oven. :D
Click to expand...

LOL

Dont laugh but I actually considered the idea of building a trap door on the bottom of my oven so i can just drop the knife into the quench.
 
BKT said:
LOL

Dont laugh but I actually considered the idea of building a trap door on the bottom of my oven so i can just drop the knife into the quench.
Click to expand...

The first question is "Why?", the second is - Have you considered what will happen when the hot plume of oil smoke hits the 1600F oven chamber???


The industrial HT ovens with the quench done in the chamber are usually very complex and the quench is under a vacuum atmosphere. Automation from oven to quench is NOT a home shop procedure.
 
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