1 step forwards, 2 steps back...

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JimDiGritz

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Oct 18, 2021
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So after making a dozen or so knives now, working up from hand files only and a blowtorch, through to a DIY fire-brick MAP gas forge through to finally buying a small grinder and a heat treat oven I'm suddenly encountering major HT issues..

My oven was supplied calibrated. It's rated up to 1200c.

Whilst I guess I have to trust the PID & Thermocouple I wanted to verify manually, and so bought a basic $80 Pyrometer.

Now I can't seem to get anything right..

This evening I brought the oven up to 800c. The Pyrometer reads 900+c off the inside (firebrick) wall, and well over 1000c+ on a test piece of steel. The Emissivity is set to .75 as per the instructions.

I put a small hand made 1095 broach inside to test and leave it to soak for 10 mins. The oven stabilized @ 805c. This broach is much smaller than my typical knife, maybe only 4 inches long and 1/8inch cross section...

After the 10 min soak, the steel looks a dull red, (by eye I would have left it at least another 5 mins in my MAP gas forge). However a quick test shows it's now non-magnetic and so after putting it back in for 5 mins I take out again (It's still only a dull orange) and quench it.

I quench in fast Rye Oil, however I barely get a hiss, let alone any flames/smoke.

Once cooled a sharp file seems to bite as per untreated 1095. My previous gas HT'ed steel felt more like oily glass under the file. This just feels like crud covered mild steel.

I don't have an RC tester or files, but to my amateur eye this steel isn't hardened.

How do I troubleshoot this?? The oven says 800c, my Pyrometer always reports much higher temperatures regardless of emissivity.

Any ideas appreciated. I bought the oven to get consistency, I'm feeling like selling it and shoving the steel into my BBQ charcoal!!!!!

Jim
 
I'm a big fan of doing test coupons.

Get some steel from a reputable supplier. Cut it up into similar-sized coupons.

Heat treat each of them at increasing temperatures and do a bend/break test on each. When you start to get too hot, you should start to notice grain growth. I think JT did a really good writeup on this. This should always be done with a new oven. It's much easier to verify results with a hardness tester though.

https://www.bladeforums.com/threads/aebl-heat-treat-test-data.1591028/

Also, try grinding a little into the steel before testing with a file. You may have some decarb that's causing the file to bit.
 
Hubert S. said:
To check whether the oven temperature is close to what it should be, try melting some salt. The melting temperature of salt is 1,474°F/801°C.
Click to expand...

I wondered about melting point depression from using iodized table salt instead of pure NaCl only to find that the iodization only causes only 1* of claimed depression to 800*C. I was surprised it was that small a change. That makes the salt test a lot easier. Good info! Thanks.
 
Hubert S. said:
To check whether the oven temperature is close to what it should be, try melting some salt. The melting temperature of salt is 1,474°F/801°C.
Click to expand...
That's an interesting idea - I guess a teaspoon of table salt on a small square of steel would work!?

If the melting salt proves that the oven is getting to 800, and assuming the Pyrometer was a waste of money (no idea how to use with different emissivities - I naively thought it would 'point and shoot' surface temperature, but apparently emissivity varies with all sorts of materials and conditions and so is either WAY off or my oven thermocouple is WAY off.. )

That said the steel WAS non-magnetic which shows the steel must be past 1420f/771c.

Maybe I really need to let the steel soak for 30+ mins????

I know they are experts but in every YouTube video the bladesmith puts the knife in the hot oven, leaves to soak for a few mins then pulls out glowing yellow hot and introduces to the quench oil with lashings of flame and smoke! Maybe it's CGI....
 
Just a couple of suggestions:
You can double check your pyrometer and oven probe/PID calibration with a glass of ice water. Just stir the ice water with your probe for a few seconds and see if it's reading 0C or 32F.

Also, are you letting your oven soak when it reaches setpoint? I normally wait anywhere from 30 minutes to an hour after my oven has reached my programmed setpoint to let everything equalize a bit more.
As most ovens initially reach set point, they do tend to overshoot just a bit. This is because your elements are basically all or nothing, or off or on. While the element is switched on, it may reach temperatures of 1300 or 1400C, even though you're only asking for 800C. The 800C that your PID is showing as the actual temp is simply what the thermocouple is reading in one small part of the oven. When the PID sees that it has reached setpoint, it will modulate your element on and off to try to maintain set point. The longer you let things soak, the shorter your element will stay energized to maintain set point, and the more even your temperatures throughout the inside of your oven should be. Most modern digital PIDs will have a built in algorithm to "learn" the most efficient way to reach this point, or in other words to hold a tighter tolerance for setpoint. You may notice that when you first fire your oven on, it may swing 10 or 20 degrees above or below setpoint, but after 45 minutes or so, it should only be swinging a degree or less.

Part of the "tolerance" you see will be due to how good your PID is, and the other part will be how thermally efficient your oven is.
 
Drew Riley said:
Just a couple of suggestions:
You can double check your pyrometer and oven probe/PID calibration with a glass of ice water. Just stir the ice water with your probe for a few seconds and see if it's reading 0C or 32F.

Also, are you letting your oven soak when it reaches setpoint? I normally wait anywhere from 30 minutes to an hour after my oven has reached my programmed setpoint to let everything equalize a bit more.
As most ovens initially reach set point, they do tend to overshoot just a bit. This is because your elements are basically all or nothing, or off or on. While the element is switched on, it may reach temperatures of 1300 or 1400C, even though you're only asking for 800C. The 800C that your PID is showing as the actual temp is simply what the thermocouple is reading in one small part of the oven. When the PID sees that it has reached setpoint, it will modulate your element on and off to try to maintain set point. The longer you let things soak, the shorter your element will stay energized to maintain set point, and the more even your temperatures throughout the inside of your oven should be. Most modern digital PIDs will have a built in algorithm to "learn" the most efficient way to reach this point, or in other words to hold a tighter tolerance for setpoint. You may notice that when you first fire your oven on, it may swing 10 or 20 degrees above or below setpoint, but after 45 minutes or so, it should only be swinging a degree or less.

Part of the "tolerance" you see will be due to how good your PID is, and the other part will be how thermally efficient your oven is.
Click to expand...
Thanks Drew, to be fair I didn't leave the oven soak for that long before I put the blade/broach in. Maybe 15 mins...

I'll try again tomorrow.

PS is there any downside to re Heat Treating and quenching my 1095 broaching tool? It's not a fine chefs knife but just wondered....
 
JimDiGritz said:
Thanks Drew, to be fair I didn't leave the oven soak for that long before I put the blade/broach in. Maybe 15 mins...

I'll try again tomorrow.

PS is there any downside to re Heat Treating and quenching my 1095 broaching tool? It's not a fine chefs knife but just wondered....
Click to expand...
re-heat treating and quenching again shouldn't be a problem.
 
An IR/laser (non-contact) pyrometer is pretty much useless above 500C. The emissivity will skew the reading. Laboratory pyrometers have adjustments for this, and still are only so-so at reading in the 800C range.
 
I believe the problem might be the oil for not being able to harden 1095.
Rye oil 50 is not a fast oil. Even though it is advertised as one.

This is the chart of Rye oil 50 for its speed:
View attachment 1696763

And this is the chart of a fast oil:
View attachment 1696764

Since you use rye oil, I assume you live in Europe. If you want Thermic INO 235, eurotechni from France carries it.
 
Tony Mont said:
I believe the problem might be the oil for not being able to harden 1095.
Rye oil 50 is not a fast oil. Even though it is advertised as one.

...
Click to expand...

Ugggh... but thanks for the insight Tony!

You are probably right (although my previous gas HTed knives seemed to quench just fine.. but who knows since there is no way to test hardness without a Rockwell machine)

The supplier states it's the correct oil for 1095.. apparently it's a direct alternative to Parks 50. although now I'm not so sure.....

They also sell a special "fast oil", however at £72/$100 plus shipping it looks pretty pricey....
 
Last edited:
I had the same impression but the temperature chart is taken from the official website of Rye oil. I'm not sure why Rye oil advertises this oil as fast since they data clearly shows that it's not.

I used to use this oil and more often than not I was having 1095 auto hamon even on thin knives (3mm-0.118" thickness). If the oil was indeed a fast oil, it shouldn't have a problem on such thickness (I was also grinding 90% prior the heat treatment).

As much as I know, Durixol V35 is a fast oil, I just don't know how fast (couldn't find a temperature chart online). You can contact this company here: https://bongroup.de/en/ , they will help you out on Durixol.

Personally, I chose Thermic INO235 from here https://www.eurotechni.com/en/thermic-ino235-accelerated-quenching-oil.html
The only reason I chose Thermic was due to the fact that after Brexit, Groundflatstock couldn't ship the oil in Europe. Also, Larrin Thomas confirmed the temperarute chart is pretty close to Parks50. Price wise, Thermic is almost 1/3 of the price on the litre. Eurotechni has more expensive shipping though.
 
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