JTknives
Blade Heat Treating www.jarodtodd.com
- Joined
- Jun 11, 2006
- Messages
- 8,630
So my quench plate set up is my big bottle neck now so I’m looking at adding at least 3 more sets. I would like a total of 6 sets of plates. The hurdle quickly becomes apparent when you start adding up cost for 6 sets. I’m also thinking of ditching the woodworkers vise and going Pneumatic.
But the Issue at hand is the plates themselves. That much aluminum is a small fortune. This got me thinking why aluminum. I know it’s the “industry” standard so to speak, But why? The obvious answer is fast thermal conductivity but was it ever questioned? Or was it just assumed it was the best material because it’s whats used for heat sinks. But my question is how fast do you really need to be. We know from liquid quenching that faster is not always better. So I started digging into the numbers to see what’s going on.
Let’s set the stage with the thermal conductivity of the standard. 6061 Aluminum has the thermal conductivity of 167W/(m·K). Now let’s look at something much cheeper like 1018 steel. 1018 has a thermal conductivity of 52W/(m·K). That’s quite a bit slower but does it matter.
If we look at the steels we plate quench thy are designed to be air cooled in still air for the most part. I think the industry does s 3-5 bar nitrogen quench. But that is huge ovens with a ton of heat to remove. So let’s look at the thermal conductivity of air. Air actually has a better thermal conductivity then pure nitrogen. But it’s still between 0 to 100mW/(m·K), That’s milliwatts not watts. So at 1600°C air has a thermal conductivity of .1W/(m·K). That’s 1,670 times slower then aluminum and 520 times slower then steel. So it would seam that steel would be plenty fast enough to quench air gardening steels. But if you think about it another way your quenching a steel blade and it will only transfer heat up to a max rate of its self.
But wait I know I can here people in the background yelling that steel holds is heat longer then aluminum so the quench plates will get hotter and take longer to cool off. That’s not necessarily true if you do the math. We will just average the math as it makes things simpler. So aluminum is half the weight of steel, It’s also half as ridged. Aluminum has a specific heat of .220Btu/(lb-°F). This means it takes .220 btu’s to increase 1lb of aluminum 1°f. Mild Steel on the other hand has a specific heat of .120Btu/(lb-°F). So as you can see aluminum looks much better as it seams like it can hold 2 the btu’s per deg then steel. But remember that’s per pound. So one pound of steel is half the size of the same weight of aluminum. So In fact thy hold the same btu’s per volume. So a 1”x4”x12” plate of steel and aluminum would infact be the same temp with x btu input from a blade. And about aluminum cooling off faster is also false. The plates will only cool as fast as the thermal conductivity of the air that’s around them.
But this brings up another thing to think about when comparing steel to aluminum as quench plates. I had not thought about this till I started researching. Aluminum oxide is what aluminum creates on it’s outside skin. It’s not very thick but it’s still there. This could be the thing that tilts the scale one way or the other on weather one metal is better or not then the other. So I dug around and found a paper talking about thermal the conductivity of aluminum and aluminum oxide. I was shocked to see that aluminum oxide has a thermal conductivity of less then 35 w/mk. And as it gets hotter it gets less. At 250°C it’s sitting at under 20 w/mk. And if your aluminum purity goes down to 90% your sitting at just over 10 w/mk. This is crazy but makes sense if you think about it. One of the best refractory we use in forges is bubble alumina which is made out of aluminum oxide. It’s just a thin layer but it’s always there.
So it looks like steel is not a bad option for quench plates and aluminum was chosen because it is assumed to be the better conductor. Sorry for the loooooog rambling but I figured I would document my thought process here.
But the Issue at hand is the plates themselves. That much aluminum is a small fortune. This got me thinking why aluminum. I know it’s the “industry” standard so to speak, But why? The obvious answer is fast thermal conductivity but was it ever questioned? Or was it just assumed it was the best material because it’s whats used for heat sinks. But my question is how fast do you really need to be. We know from liquid quenching that faster is not always better. So I started digging into the numbers to see what’s going on.
Let’s set the stage with the thermal conductivity of the standard. 6061 Aluminum has the thermal conductivity of 167W/(m·K). Now let’s look at something much cheeper like 1018 steel. 1018 has a thermal conductivity of 52W/(m·K). That’s quite a bit slower but does it matter.
If we look at the steels we plate quench thy are designed to be air cooled in still air for the most part. I think the industry does s 3-5 bar nitrogen quench. But that is huge ovens with a ton of heat to remove. So let’s look at the thermal conductivity of air. Air actually has a better thermal conductivity then pure nitrogen. But it’s still between 0 to 100mW/(m·K), That’s milliwatts not watts. So at 1600°C air has a thermal conductivity of .1W/(m·K). That’s 1,670 times slower then aluminum and 520 times slower then steel. So it would seam that steel would be plenty fast enough to quench air gardening steels. But if you think about it another way your quenching a steel blade and it will only transfer heat up to a max rate of its self.
But wait I know I can here people in the background yelling that steel holds is heat longer then aluminum so the quench plates will get hotter and take longer to cool off. That’s not necessarily true if you do the math. We will just average the math as it makes things simpler. So aluminum is half the weight of steel, It’s also half as ridged. Aluminum has a specific heat of .220Btu/(lb-°F). This means it takes .220 btu’s to increase 1lb of aluminum 1°f. Mild Steel on the other hand has a specific heat of .120Btu/(lb-°F). So as you can see aluminum looks much better as it seams like it can hold 2 the btu’s per deg then steel. But remember that’s per pound. So one pound of steel is half the size of the same weight of aluminum. So In fact thy hold the same btu’s per volume. So a 1”x4”x12” plate of steel and aluminum would infact be the same temp with x btu input from a blade. And about aluminum cooling off faster is also false. The plates will only cool as fast as the thermal conductivity of the air that’s around them.
But this brings up another thing to think about when comparing steel to aluminum as quench plates. I had not thought about this till I started researching. Aluminum oxide is what aluminum creates on it’s outside skin. It’s not very thick but it’s still there. This could be the thing that tilts the scale one way or the other on weather one metal is better or not then the other. So I dug around and found a paper talking about thermal the conductivity of aluminum and aluminum oxide. I was shocked to see that aluminum oxide has a thermal conductivity of less then 35 w/mk. And as it gets hotter it gets less. At 250°C it’s sitting at under 20 w/mk. And if your aluminum purity goes down to 90% your sitting at just over 10 w/mk. This is crazy but makes sense if you think about it. One of the best refractory we use in forges is bubble alumina which is made out of aluminum oxide. It’s just a thin layer but it’s always there.
So it looks like steel is not a bad option for quench plates and aluminum was chosen because it is assumed to be the better conductor. Sorry for the loooooog rambling but I figured I would document my thought process here.
