Heat treat dilema/question(s).

The only thing I would add to Page's explanation is about soak times per half inch of thickness; to achieve best results, that should be your minimum soak for any thickness under 1/2". If you make blades thicker then soak longer, but it doesn't work going the other way. As in it says to soak for 20 min per half inch of thickness, so my blade is 1/8" so I soak for 5 min. TOTALLY WRONG. Thats another common misconception.
If you don't believe me, prove me wrong, I have done the tests and so have others. (I was using O-1)
Del
 
Page, Del, Thank you very much. I played around with it for quite a bit today and got good and bad pieces out. Your posts have helped me to come to an understanding why the ones that worked worked, and the ones that didn't, didn't.
 
12345678910 said:
You could take that one step further and add more thermal mass.

A cake pan full of sand allowed to heat up and stabilize, will even out temp variations with the element cycling on and off.
Big steel plates in the oven will also have a similar effect.
Click to expand...

I think alton brown on good eats mentioned or hinted at this. A large heat holding object will help a lot at stabilizing temperatures in cooking ovens since they ahve to cycle on then off then back on etc. The stone keeps the oven at a more consistent temperature.
Thats why i leave my pizza stone in the oven all the time. I think it takes longer to heat up though.
 
Thanks Del, it was late and I forgot to put that in.

If you decide to try the extra thermal mass thing (which I do not recommend for that kiln) that extra thermal mass will add significantly to your preheat time

I make very thin blades, in my kiln I typically use a half hour as my base time for HT, my blades are typically flat ground to the spine and typically 5/32 or thinner at the spine, they typically take about 10 minutes to come up to temperature with the kiln preheated for an hour, and I use the half hour mark from when I put the blade in as a minimum, if my blade sits longer, it doesn't hurt

-Page
 
When heating up an oven, the heat source radiates heat in all directions inside the oven. The first thing to absorb the energy is the air in the oven. The gaseous air quickly heats up ,due to its low mass. This starts to circulate in an upward direction due to convection.Thus, the top of the oven gets hotter first. If we place a blade in the oven at this point, it will heat up poorly, because the air is a poor emitter of heat ( having little mass). Heating of the blades will be very inconsistent and unreliable compared to the thermocouple reading.

The second thing to heat up is the sides of the oven. These are solid matter, and have to absorb a lot more heat energy to increase in temperature. The more mass in the sides, the slower the heat up rate. However, the mass in the sides, ( which include the top and bottom), will become radiant surfaces once fully heated, and thus even out the temperature.

The reason a poured refractory lined forge or oven works better than one with plain K-wool is that there is far more thermal mass. Placing dense objects in a HT oven or forge will do the same thing to a lesser degree. The key thing to remember is that it will take time for all that mass to absorb the heat energy. Allowing the oven or forge to run for 15 to 30 minutes will assure a fully heat soaked liner. This will give you much more even heating of the blade, and more consistent readings of temperature. When fully soaked, the bottom and top of an oven will read virtually the same temp in a closed oven.

In the steel industry, the inside of huge ovens and smelters are many feet of refractory bricks,with many more feet of insulation around that. It may take days to come up to temperature. Once all that mass is fully soaked it may stay hot for a week after the burners are shut off.
In the vacuum of space, an object that is hot will remain hot for a very long time, maybe millions of years.

These issues should all be considered when designing and building .....and using....a forge or HT oven. I know all thus science isn't everyones thing, but there is a technical reason behind why some things work and some don't. Understanding the "WHY" will greatly aid the "HOW".
 
In addition to understanding the why, I also like to develop a feel. To get the feel, I decided to use heat coloring of some titanium.

For an interesting experiment, try throwing in a piece of Titanium (I used 6Al4V) for heat-coloring. Titanium develops color due to an oxidization layer that is temperature dependent.

It doesn't have to be at temperature for very long to achieve the color for a particular temperature, but boy does it take a long time to heat up to the ambient temperature in a hot kiln!

Titanium isn't all that great a conductor of heat, so this is somewhat expected. So, I figure, I can start the soak timer after I give the foil wrapped blade as much time to come up to heat as that titanium.

Since grain growth appears to occur only if over temperature, I'm pretty sure I can let it soak an extra 1/2 hour without hurting anything.
 
If decarb protected 01, you could probably hold it at 1475° for most of the day with no grain growth.
 
that is for straight from the hardening without tempering? did you grind below the decarb layer?
also what are you using for oil, and what temperature is your oil at?

-Page
 
Straight out of the quench. I am using hardness testing files for testing.

I did.

mcmaster carr 11 second oil. Quenchfast.

It is at 130F.

I have our oven guy coming in to see if the oven is calibrated. Everything in there is glowing Orange at 1600 so I am pretty sure it is.
 
IMHO, the testing files are a waste of cash. Their use and results are a bit arbitrary.

Clean up a coupon or two and take it to a good machine shop and tell them you are trying to diagnose a problem with your HT oven. Have them Rockwell the piece, and then compare that to your files. I'm willing to bet they do it for free.

Are you sure about the steel?
 
Yup. Out of the brown Starrett wrapper with the 01 ink sown the side and into the oven. I'm going to try the 5/32" tomorrow. I was testing with 1/4"

As for the files they seem to come out correct on some known hardness steel I have.
 
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also did the piece get warm when you were grinding it? (you probably know better than that but I have to ask)

Ditto what Stacy said

-Page
 
I'd wonder more about the thermocouple accuracy, but would also do as Stacy suggested and get your test pieces read with a real hardness tester rather than the hardness files

_page
 
Carl, I'm sorry I never answered your question on emissivity, hard to post from the phone. Anyways, its the ratio of energy radiated from an object at a given temperature vs a perfect radiator (flat black object) at the same temperature. Emissivity is also temperature dependant, so polished 321 stainless steel might be .18 at 300° but .49 at 1500°, whereas polished 17-7ph stainless steel is .09 and .16 respectively. However, the 321 with black oxide finish would be .76 @ 800°. You'll notice that the steels I listed had an increase in emissivity as temperature increases - however, ceramics will often have a drop in emissity as temp increases. So you measure a ceramic shelf in your kiln at 600° and the polished blade on it - wth they won't read the same. Now crank it up to 1500° and they still don't read equal temps, but the percentage of offset is less- wth?! This is all a long winded way to say that IR thermometers are not the set it, forget it, one size fits all-point and shoot measurement devices folks often think.
 
stevomiller said:
Carl, I'm sorry I never answered your question on emissivity, hard to post from the phone. Anyways, its the ratio of energy radiated from an object at a given temperature vs a perfect radiator (flat black object) at the same temperature. Emissivity is also temperature dependant, so polished 321 stainless steel might be .18 at 300° but .49 at 1500°, whereas polished 17-7ph stainless steel is .09 and .16 respectively. However, the 321 with black oxide finish would be .76 @ 800°. You'll notice that the steels I listed had an increase in emissivity as temperature increases - however, ceramics will often have a drop in emissity as temp increases. So you measure a ceramic shelf in your kiln at 600° and the polished blade on it - wth they won't read the same. Now crank it up to 1500° and they still don't read equal temps, but the percentage of offset is less- wth?! This is all a long winded way to say that IR thermometers are not the set it, forget it, one size fits all-point and shoot measurement devices folks often think.
Click to expand...

That, sir, is awesome.

Thank you.
 
I had my oven guy come in and we are dead on the money there. We also took two of the coupons and sandwiched the thermocouple wire between them. We were at temp for 30 mins inside, and brought it out and quenched. The temps dropped to 650F in around 7 seconds before the thermocouple fell out from between the two pieces. The files still lead me to believe 55RC. I'm going to try and get under a actual hardness tester still. Would it seem that my oil is too slow? (3 gallons of Quenchfast at 130F)

So I decided to do an oil "interrupted" water quench. I pulled the coupons out and quenched in oil for 10 seconds. Wiped and went into water. Now the pieces test out above 60 RC. Has anyone done this before? Is it be terrible to do this?
 
Carl_First_Timer said:
I had my oven guy come in and we are dead on the money there.The temps dropped to 650F in around 7 seconds before the thermocouple fell out from between the two pieces. The files still lead me to believe 55RC. I'm going to try and get under a actual hardness tester still. Would it seem that my oil is too slow? (3 gallons of Quenchfast at 130F)

So I decided to do an oil "interrupted" water quench. I pulled the coupons out and quenched in oil for 10 seconds. Wiped and went into water. Now the pieces test out above 60 RC. Has anyone done this before? Is it be terrible to do this?
Click to expand...

Carl,

I have sort of skimmed over the responses here, and I'm sure some what I am about to say may have already been covered.

Just a couple of things to think about: If your oven is OK, that is a plus. If you went down to 650 in seven seconds then you have probably already passed the "Nose Curve", therefore, pulling a blade and quenching in oil for ten seconds, THEN going into water is most probably not gaining you anything.

I am not familiar with Quenchfast, so I can't comment on the speed, but you need to match the quench oil with the steel.

Those hardness files are only a guide. Your best bet is to have them tested on a calibrated Rockwell type tester. Then, they will only give a good reading if they are clean and any de-carb is removed prior to testing.

I'll try to go back and re-read some of the comments and if I can think of anything else (with my feeble mind) to add, I will help if I can.

Think "Time and Temp". This thought process is needed not only for "hardening" a blade, but also for "Tempering" it back to where it needs to be.

Good luck to you my friend, and if I can help, just let me know.

Robert
 
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