Tempering Media

[dickb]

I am using a gas oven to temper fully hardened steel. The oven is about forty yeas old. 

I am using a type K thermocouple  meter to display the oven temperature. 

From a cold start I bring the oven slowly up to the temperature I am trying to get. It takes 10 or 15 minutes. 

The  type K thermocouple meter show the  oven temperature going through wide swings of about plus and minus 30 degrees. 

I'm thinking of burying the workpiece and the sensing element of the meter in a bed of sand to even out the wide temperature fluctuations.   

Would welcome any comments. 

[Frosty]

The problem with gas ovens and even temperature is how fast gas produces heat. The thermostat has a high and a low limit for any given temperature, +/- 15 deg sounds about right.

The oven temp falls till it hits the lower limit and fires off rising rapidly till it reaches the high limit and shuts off. Being a gas burning appliance it must be vented to allow combustion products to escape so it cools faster. Up and down. 

Not so good for tempering but there are ways around it. #1 and the reason as much as I love the rapid response of a gas fire I have an electric oven. The limit switch runs about +/- 2f. and because the elements are rather thick they heat more slowly and the oven is less well ventilated. Still not great for tempering but better.

If you want even temperature in a household oven use a mufffle. A piece of reasonably heavy walled pipe with end caps to prevent convection.  Give it a try before risking a blade, one of those laser temperature reader things will let you see how much more even a piece of pipe will remain while the sheet metal oven walls bounce up and down.

Buried in a deep pan of sand will work too, it's the greater thermal mass (specific heat) that evens the internal temperature for you. 

Frosty The Lucky.

[Dennis T.]

I have had the same thing happen to me. I have tried the sand method ... it works but the sand takes forever to heat up. While waiting for the sand to warm up, I watched the temp readings. What I found was the longer the oven was on, the more stable the temp got. The result is, instead of using sand, I just let the oven pre heat for  15 minutes before putting blades in there. I've had good results.