Effect on Quench Oil Temp of Multiple Blades in a Short Time?

[Col Defender]

Hi all - now in the process of grinding three 16" long Kukris. Soon I will be ready to heat treat them. (Two burner Majestic Forge with quench in canola oil.). If I quench the first blade at 110 degrees F it will heat up the oil considerably and the second blade more so. How hot can I let the oil get before I have to stop and let it cool? I'd like to do all three blades in one fell swoop but don't know if I can do that safely.

Thoughts?

 

[Stacy E. Apelt - Bladesmith]

The oil should be no more than 140, with 130 a better limit. If doing multiple quenches with large blades, 5 gallons of oil is a good idea. Three gallons is what I would call a minimum. The fast way to cool oil is to freeze a few bottles of water and dunk one in the oil and run it up and down the tank for a minute. It will drop the oil very fast.

 

[timgunn1962]

It seems like an obvious thing to do but I've never seen it done, so presumably I'm missing something?

If you put your quench oil tank in a bigger tank of water, it should greatly increase the heat capacity of the quench system and reduce the temperature rise.

Doing some massively oversimplified math, assuming that everything has the same specific heat capacity:

If we start with a 1lb blade at 1550 degF and quench it into 5 gall of oil (about 35 lb; oil is lighter than water) at 110 degF, we'll have dumped the heat corresponding to 1440 lb-deg into 36 lb of "stuff", giving a temperature rise in the oil of 40 deg and a quenched temperature of 150 deg for the blade.

If we do it again, but this time put the 5 gall quench tank into a bigger container with 10 gall of water in contact with the outside of the oil container, we'll have the original 35 lb of oil plus 80 lb of water, making 136 lb of "stuff" once we include the blade. We'll need to get the whole lot to 110 deg to start, which may be a minor pain but not overly difficult.

The same 1440 lb-deg into 136 lb of "stuff" would give a temperature rise of only 10.6 deg and a quenched blade temperature of 121 deg.

In reality, the benefit should be greater, since water has a higher heat capacity than just about any other common substance. However, the heat transfer from the oil container to the water would not be instant, so the quenched temperature of the first blade would end up somewhere between 121 and 150 deg, with subsequent blades getting about 11 deg higher each time.

Overall, it would probably not be as good as increasing the quench oil volume by 10 gallons if you are looking to quench 3 blades together, but if you can quench the 3 blades in succession with a minute or two between them to let the heat transfer from the oil to the water, it should get a lot of the effect without the cost of the extra oil.

 

[mete]

Don't play with numbers like that ,just get a bigger tank. As the oil gets hotter you are in effect lowering the hardenability

 

[Col Defender]

The oil should be no more than 140, with 130 a better limit. If doing multiple quenches with large blades, 5 gallons of oil is a good idea. Three gallons is what I would call a minimum. The fast way to cool oil is to freeze a few bottles of water and dunk one in the oil and run it up and down the tank for a minute. It will drop the oil very fast.

Thanks, I like that idea!

 

[P.Brewster]

If you put your quench oil tank in a bigger tank of water, it should greatly increase the heat capacity of the quench system and reduce the temperature rise.

Yes, I've done it and it works. However to work well you need agitation in the water and ideally in the oil too.

 

[Drew Riley]

It seems like an obvious thing to do but I've never seen it done, so presumably I'm missing something?

If you put your quench oil tank in a bigger tank of water, it should greatly increase the heat capacity of the quench system and reduce the temperature rise.

Doing some massively oversimplified math, assuming that everything has the same specific heat capacity:

If we start with a 1lb blade at 1550 degF and quench it into 5 gall of oil (about 35 lb; oil is lighter than water) at 110 degF, we'll have dumped the heat corresponding to 1440 lb-deg into 36 lb of "stuff", giving a temperature rise in the oil of 40 deg and a quenched temperature of 150 deg for the blade.

If we do it again, but this time put the 5 gall quench tank into a bigger container with 10 gall of water in contact with the outside of the oil container, we'll have the original 35 lb of oil plus 80 lb of water, making 136 lb of "stuff" once we include the blade. We'll need to get the whole lot to 110 deg to start, which may be a minor pain but not overly difficult.

The same 1440 lb-deg into 136 lb of "stuff" would give a temperature rise of only 10.6 deg and a quenched blade temperature of 121 deg.

In reality, the benefit should be greater, since water has a higher heat capacity than just about any other common substance. However, the heat transfer from the oil container to the water would not be instant, so the quenched temperature of the first blade would end up somewhere between 121 and 150 deg, with subsequent blades getting about 11 deg higher each time.

Overall, it would probably not be as good as increasing the quench oil volume by 10 gallons if you are looking to quench 3 blades together, but if you can quench the 3 blades in succession with a minute or two between them to let the heat transfer from the oil to the water, it should get a lot of the effect without the cost of the extra oil.

I don't know enough to agree or disagree with your numbers, but I imagine heat transfer would play a significant factor one way or the other. At room temperature, I question the affect that standing water would have a relatively thick walled, low volume quench tank.


Personally, I try not to quench a long of large blades in rapid succession, but if I'm doing a "handful", I'll normally stir my oil up to help equalize it a bit more, and bring down the temp a few degrees. On rare occasion, I've dumped out some of the warm oil and added cool back into the mix, or even used a separate quench tank with oil already at the temperature I need it.

 

[Stacy E. Apelt - Bladesmith]

Tim, I am in the Virgin Islands, and don't have my books here, but it looks like you left the factor of five ( gallons) out of the formula. IIRC, the temperature rise for one pound of hot steel in 5 gallons of oil is around 10 degrees. That also agrees with my direct experience in quenching blades. With most large blades it takes three to five blades to raise my 4 gallon quench tank enough to need cooling.

Another method of keeping the temperature down in a large quench tank is to use a cooling pump and radiator. The pump and radiator are both found online or at any auto parts shop. Use a transmission fluid auxiliary pump and a transmission cooler ( auxiliary radiator). An automotive cooling system thermostat switch also can be used to turn the pump on at a pre-set temp ( or use a cheap PID). An added plus is that if you switch the pump on just before the quench, you can get added circulation to speed cooling. Place the return tube from the pump in the center of the tank bottom with the end of the tube facing up. This makes a rising column of oil that greatly reduces the vapor jacket phase and speeds cooling and evenness of cooling rate. If you are really going for a commercial setup. add a cooling fan to the radiator. The auto parts store is the place again, with the standard radiator fan being perfect. Another plus is that the fan, pump, and switch are all made to run on 12VDC.

 

[timgunn1962]

Thanks Stacy, having real-world numbers makes a bit more sense. I tend to think mostly in metric and then have to convert.

I'd initially assumed that everything has similar specific heat capacity: it was obviously wrong, but was somewhere to start from and it kept the math simple. I did point out that it was a massive oversimplification, to be fair.

In fact, steel seems to have a Specific Heat Capacity of around 450 J/kgK, a typical quench oil an SHC of around 1950 J/kgK and water an SHC of around 4200 J/kgK. That 4:1 ratio of SHCs between steel and oil accounts for the 4:1 difference between my first approximation and your real numbers.

I'd assumed 5 US gallons of oil would weigh about 35 lb and it seems it's actually around 37 lb; a tiny error compared to the SHC assumption.

Running the numbers again:

1 lb of steel at 1550 degF being cooled to 110 degF will release 163440 J

1550 degF -110 degF = 1440 degF = 800K

0.454 kg * 800K * 450 J/kgK = 163,440 J

5 gallons of oil weighs about 37 lb, say 17 kg, so has a heat capacity of 17 kg * 1950 J/kgK = 33150 J/K

The pound of steel has a heat capacity of 0.454 kg * 450 J/kgK = 204 J/K

Combined, they have a heat capacity of 33354 J/K

The total temperature rise (from the initial quenchant temperature of 110 degF) will be 163440 J/ 33354 J/K = 4.90 K = 8.82 degF call it 9 deg.

This neglects the thermal mass of the quench tank and several other real-world factors, but it gives a ballpark figure.

If we now calculate with an extra 10 gallons of water in the system:

We are putting the same 163440 J into the system.

The system comprises the oil and steel with a heat capacity of 33354 J/K, plus 35 kg of water with a Specific Heat Capacity of 4180 J/kgK for a heat capacity of 146,300 J/K (10 gallons is roughly 35 litres at 1 kg/litre for water)

The combined heat capacity is 33354 J/K + 146300 J/K =179,654 J/K

163440 J / 179654 J/K = 0.91 K = 1.64 degF call it 2 deg


It's clearly not going to be the best way to do things. I'm not sure it would even be a good way to do things and for the guys who HT lots of big stuff on a regular basis, there are obviously better approaches.

I brought it up because a big (cheap) bucket and (free) water seems like it might offer a cheap way of extending the capability of the kit someone already has, in order to occasionally HT larger work than they would normally consider practicable.

Also because I'm curious whether anyone has tried it and found it works, or doesn't.