# Blade is blue after tempering



## da_mich* (May 16, 2021)

Hello,

i made my first chinese style knife and after tempering it's blue. I used Uddeholm 1770 (C70 / 1.1620) steel and tempered it with my kitchen oven at 250°C / 482 °F for 1h like in the datasheet. To achieve such a color, the blade would have to have been ~ 320°C / 608 °F? The max temp setting of my oven is 250°C / 482 °F. So i set it to max power.
Do you have an idea why the blade is blue now? Can I still use the blade?

extra information:
- I never used Uddeholm 1770 before
- The blade was in the oven while he was heating up from 20°C -> 250°C
- Maybe there was oil residue on the blade
- Maybe the temperature control of the oven is very imprecise. But i think its impossible to reach 320°C with a old kitchen oven





Thanks for help,
Michael


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## daveb (May 17, 2021)

Can you check oven temp with an oven thermometer? Oven temps / dials are notoriously inaccurate.


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## da_mich* (May 17, 2021)

Unfortunately, I currently don't have a thermometer that is suitable for this temperature range


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## Kippington (May 17, 2021)

It's probably your oven. Sometimes the elements are exposed with direct line of sight to the blade. You gotta remember that they work by going either 100% or 0% - nothing in-between, so a full blast of radiating heat to the steel isn't so nice.

Also as daveb said, you cant rely on the oven thermostat. It's probably a super cheap component, most likely a bi-metallic strip.


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## da_mich* (May 17, 2021)

Thanks, I made only a handfull of knives and never had this problem before. Is it possible to re-harden the steel and then carry out a new heat treatment?


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## Kippington (May 17, 2021)

da_mich* said:


> Is it possible to re-harden the steel and then carry out a new heat treatment?


Yep, you can probably do that... just have to be careful of decarb.


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## da_mich* (May 17, 2021)

Thanks, I'll try again. Next time I'll protect the blade from direct line of sight to the heating elements with a baking sheet. Maybe a friend has a thermometer too.


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## M1k3 (May 17, 2021)

Let it heat up first also.


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## RDalman (May 17, 2021)

da_mich* said:


> Thanks, I'll try again. Next time I'll protect the blade from direct line of sight to the heating elements with a baking sheet. Maybe a friend has a thermometer too.


Yes these kindof ovens can both vary in spots and in time, typically ime they run a little hotter. 
Better if you put something thicker on it, like steel. Or set the blade in sand. Btw, why such a high tempering temp? I wouldnt go above 200c


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## da_mich* (May 17, 2021)

The sellers information says 250°C / 482°F. Maybe it´s a wrong information?


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## MarcelNL (May 17, 2021)

the heating elements are usually exposed on the top side. Buy an infrared heat gun with sufficient range to check with accuracy?


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## RDalman (May 17, 2021)

da_mich* said:


> The sellers information says 250°C / 482°F. Maybe it´s a wrong information?


You can temper in range of 150-300 c for this steel depending on desired hardness, ofc nothing wrong with a 56 hrc blade. Just my preference is 63+ even for "heavy use" knives for better edge holding. This steel is very tough also even on higher hardness.


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## VICTOR J CREAZZI (May 17, 2021)

I put my blades inside a preheated heavy wall tube to block any radiated heat from the elements and help even out the temps.

Some people claim that you can't rely on the temper colors because of the long periods of time that the steel is at the tempering temperature, or that the oil on the blade changes things, but I always sand some shiny areas of the blade before tempering and only get a light straw at 400º F


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## inferno (May 17, 2021)

MarcelNL said:


> the heating elements are usually exposed on the top side. Buy an infrared heat gun with sufficient range to check with accuracy?



wont work. the infrared from the heating elelment will get reflected by stuff in the oven and you get a very false reading. ir thermometers are also only calibrated for a certain emmisivity and this needs to be known.

one thing that works is putting some charcoal in the oven. then measure that. that is quite accurate.

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i usually use an amprobe meter with a real k type probes in my oven. my oven fluctuates -+2C after its stabilized for 30 minutes or so.

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listen to robin, temper lower.


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## da_mich* (May 17, 2021)

I bought a oven thermometer(50°C->300°C) for 10€ on Amazon. I will place it next to the knife. I will also let the oven preheat for 1 hour.


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## Bobby2shots (May 17, 2021)

daveb said:


> Can you check oven temp with an oven thermometer? Oven temps / dials are notoriously inaccurate.



That's often an easy "fix". On many stoves you can simply pull/remove the temperature control knob, and you'll see a shaft/tube with a tiny screw inside. Using a small screwdriver, just turn the screw left or right to lower or raise the oven's temperature.

Example; ,,, you've set the oven temperature to 350, but your oven thermometer reads 320. Without changing the oven temperature knob setting, simply remove the temperature control knob by pulling it gently, exposing the shaft onto which the knob is mounted, and using a small screwdriver, turn the screw inside the shaft "up" (usually clockwise). Turning that screw will only raise or lower the oven temperature, without affecting the knob's original reading. Keep heating the oven until the internal temperature of the oven (according to your oven thermometer) matches the oven's original temperature control knob setting of 350 degrees. Replace the knob by sliding it back on the shaft,,,and voila,,, a precisely set temperature control knob. You'll be amazed at how much of an impact this can have,,, especially when following a recipe that recommends a specific temperature.

MANY stoves leave the factory without the oven temperature being properly calibrated, and can be "off" by 50 degrees or more. It's often a 5-minute job to correct and re-calibrate,,,,,if you have a screwdriver that fits. (flat blade jewellers' screwdriver often does the job).


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## Kippington (May 18, 2021)

Bobby2shots said:


> That's often an easy "fix". On many stoves you can simply pull/remove the temperature control knob, and you'll see a shaft/tube with a tiny screw inside. Using a small screwdriver, just turn the screw left or right to lower or raise the oven's temperature.
> 
> Example; ,,, you've set the oven temperature to 350, but your oven thermometer reads 320. Without changing the oven temperature knob setting, simply remove the temperature control knob by pulling it gently, exposing the shaft onto which the knob is mounted, and using a small screwdriver, turn the screw inside the shaft "up" (usually clockwise). Turning that screw will only raise or lower the oven temperature, without affecting the knob's original reading. Keep heating the oven until the internal temperature of the oven (according to your oven thermometer) matches the oven's original temperature control knob setting of 350 degrees. Replace the knob by sliding it back on the shaft,,,and voila,,, a precisely set temperature control knob. You'll be amazed at how much of an impact this can have,,, especially when following a recipe that recommends a specific temperature.
> 
> MANY stoves leave the factory without the oven temperature being properly calibrated, and can be "off" by 50 degrees or more. It's often a 5-minute job to correct and re-calibrate,,,,,if you have a screwdriver that fits. (flat blade jewellers' screwdriver often does the job).


That's cool to know... but a lot of us are going for more accuracy than a bi-metallic strip can give us. It's more a problem with the thermostat accuracy, rather than the calibration. My standard house oven has an error margin of about 80 °C, regardless of what I set it to.

Hell, I also don't trust my expensive infrared thermometer either (for reasons inferno mentioned above), so I use a K type thermocouple which can still get things wrong, especially just above the temps I temper at


> A characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its Curie point, which occurs for type K thermocouples at around 185 °C.


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## RDalman (May 18, 2021)

Lets just say. A hardness test or two on finished blade is not bad


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## da_mich* (May 18, 2021)

Today i received the thermometer and testet it. I have set the oven to max. power (250°C). Thermometer accuracy is +-10°C.


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## MarcelNL (May 18, 2021)

whether the IR gun works or not is questionable but IMO mainly depending on the type of material the backplate or whatever you point at is made of (as already eluded to). Our previous oven (Siemens Steam oven that could do sous vide) had a black backplate and the set temperature correlated within very few degrees between IR gun and the temp probe that came with the oven. Pointing the IR gun at reflective surfaces is definitely not going to work, but using something like the charcoal as mentioned will work.

Thermocouples that do ~1000C do exist, all a matter of selecting the right probe (casing and thickness) for the job at hand. In my coffee roaster I upgraded to RTDs to minimize noise and increase response speed.


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## natto (May 19, 2021)

da_mich* said:


> Today i received the thermometer and testet it. I have set the oven to max. power (250°C). Thermometer accuracy is +-10°C.


HT temperature is more effective than time. Simple ovens exceed the temperature most when heating up. Did you watch the upcycle?


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## da_mich* (May 19, 2021)

Yes I watched it and there was no overshoot. It was difficult to get to the specified temperature. The oven is old, cheap and underpowered. I don't know where the blue color comes from.


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## inferno (May 21, 2021)

MarcelNL said:


> whether the IR gun works or not is questionable but IMO mainly depending on the type of material the backplate or whatever you point at is made of (as already eluded to). Our previous oven (Siemens Steam oven that could do sous vide) had a black backplate and the set temperature correlated within very few degrees between IR gun and the temp probe that came with the oven. Pointing the IR gun at reflective surfaces is definitely not going to work, but using something like the charcoal as mentioned will work.
> 
> Thermocouples that do ~1000C do exist, all a matter of selecting the right probe (casing and thickness) for the job at hand. In my coffee roaster I upgraded to RTDs to minimize noise and increase response speed.



i have thermocouples that goes up to 1100C or so. but the ones i put in my oven are the thin wire ones you get with any industrial meter (no casing at all), because they can handle like 250C or so and you can bend them. and then i got another cheap wire one that can handle 400C. its easy to close the oven with the wire probes. 

then i have some high temp ones with inconel outer casing. i get them from here. good search filters on the site.








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made in germany. i think i had my probes in a few days. quite cheap too.


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## VICTOR J CREAZZI (May 21, 2021)

Just thinking out loud here. 

If your oven is rated at 250º C at max power. Does that mean that the element at max power is held fully on and an expected temp of 250º C will be reached accounting for insolation losses? If that is the case, I would expect very poor regulation and the max temperature would be dependent on the ambient temperature and other factors rather than a bimetal regulator.


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## da_mich* (May 21, 2021)

If you set the oven to 250° C max power, the heat element is not 100% on. I hope the rain stops this weekend, then I can try again to harden the blade. I will report back to you after quenching/tempering.


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## inferno (May 22, 2021)

if i set my plates to max its off and on its like 90/10%, pwm pulse width modulation. its never 100/0 like old style plates.old style plates are 100% until its "6" then its 90 or 85% on "5", thats how it works.


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## MarcelNL (May 22, 2021)

nice to see some alternatives for Thermcouples, I got mine from TC Direct, including a custom made one for the coffee roaster. In the end I changed to RTD's (phidget sells a large number of TC and RTDs and works seemless with Artisan the roasting software), for the purpose of roasting where you need to manage heat using delta Bean temp they are best, my roasting results improved a lot due to the upgrade.


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## da_mich* (May 22, 2021)

Now it's better:


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## tostadas (May 26, 2021)

da_mich* said:


> Now it's better:


Nice hunk of steel you got there


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