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Most reliable method of measuring temps between toaster and nonmag?


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I'm in the process of building a Burke Bar.  If you're familiar with the YTer Essential Craftsman, you know he's a big fan of them.  Pic below of what one looks like if you haven't seen one before.

 

I'm making the business end out of a piece of Toyota 4runner truck spring leaf material, about 1/2" thick, thinning down at the point (see pic). 

 

My question is about tempering.  The temper on this tool needs to be in the spring range, not knife hardness.  Which is more than the 500-550 my toaster oven will deliver (I think).

 

In the video below, EC discusses the tempering of the BB end at about 14:40.  He says that bar needs to be in the "Royal Purple" stage so that it is tough and won't break.  This particular BB was sent to another YTer, Andrew Camarata.  If you know AC, you can probably guess where this is going:  AC pretty much immediately broke the business end of the bar.  In other words, the bar was not tempered enough and was too brittle.

 

So long winded way to ask:  What temps am I looking for for spring steel "springiness" for the business end and what's a reliable way to measure that temp.  Heat sources are either my toaster oven (550 max) or my wood fired forge.

 

BB pic:

 

burkebar.jpg

EC Burke bar build video: 

 

 

 

 

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to be kinda honest, when it come to a tool that expected to have a lot of abuse, and to have the best amount of spring quality, just normalize it as best you can. 

 

A spring steel in it's unhardened state is the toughest it will ever be, all be it - it is not going to hold an edge as well as something hardened. I don't know if you really need one with this type of tool. But I think you will find it really difficult to bend a piece of leaf spring at the end of a 5 foot bar that is normalized. (depending on the section your using) 

 

To hit the purple range, its not too difficult, I'd even temper something like this to blue if I thought a temper would be required. This does look to be something bigger than the toaster oven could handle.

 

I have a much smaller form of this tool, its only about 2 feet, made with an old farriers rasp.  Unhardened, over 60 years old and still going.

 

Also in watching the video, I think I would do a different order of operations.  I would probably weld the tool first, (with a preheat as I've recently learned how that can cause stresses to carbon steels) and then heat treat it.

Edited by Daniel W
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I was planning to harden only the spring end.  The handle is simple 1x2 tubing, not hardenable.

 

Good to know about normalizing it:  I figure welding the spring onto the handle would make that end the most toughest and wouldn't have to worry about it.

 

AC's tool broke right at the bend IRC; hence the bend to the edge concerns me most.

 

Thanks,

 

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I agree with Daniel about the order of operations.  As far as how to determine the temperature, oxide colors are notoriously inaccurate. Any bit of oil, even fingerprints, will throw off the colors.  The Tempil company makes crayons that burn off at very precise temperatures.  They're not that expensive. https://markal.com/products/tempilstik.  Just decide what temperature you want and buy the crayon that corresponds.  https://www.amazon.com/Tempil°-TS0650-Tempilstik-Temperature-Indicators/dp/B0013L76I4/ref=pd_day0_c_328_4/135-4027940-4541910?_encoding=UTF8&pd_rd_i=B0013L76I4&pd_rd_r=04e66810-aa94-4994-bbcc-fc99839da121&pd_rd_w=O8yNt&pd_rd_wg=atlM6&pf_rd_p=52c44813-ece5-43b5-85d3-aeb14689b184&pf_rd_r=213E3NX5DFK43F9SB9VX&psc=1&refRID=213E3NX5DFK43F9SB9VX, for example.  Buy one for 400 degrees as well, you'll want to preheat the spring steel to 400 when you weld it to the tubing.  They're easy to search, the name code is the temperature. That is, the one I linked, TS0650, burns off at 650 F.  TS0400 burns off at 400 F.

 

Finally, nonmagnetic is 1425 F.  Spring steel, assuming it's 5160 leaf spring, has to be about 125 degrees F hotter than nonmagnetic to fully harden.  Magnets are not your friend with 5160.  If you can't see decalescense, Tempilstik also makes one for 1550 degrees F.  TS1550, oddly enough.

 

Edited to add: If you do not preheat the spring and you weld it after HT, it will break at the weld.  

Edited by Alan Longmire
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Daniel and Alan nailed it pretty well.  The only other thing I will add is that, if you wanted to hardening it, you can temper it in complete darkness to the point it is just starting to show signs of glowing.  That will be about 900 F.  Do not let it slow cool from this temperature.  You want to heat and cool through the blue brittle range as quickly as possible when you have martensite to temper.  

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Yes rough abuse tools, if you choose to temper do it at pretty high temps.  Spring steel is more durable in its normal state than hardened, maybe or I should say there is argument there.  But this would fall under would you like a tool that bends and can be re-bent back to shape, or under tension that snaps when you need it. It may just be my personal preference that I would not harden this type of tool. I think If I found it necessary, I might just do the very tip of the tool, like the first 1in to 2 inches. And I would grind in the grove. 

 

If you really want to go above and beyond, instead of a spring, chose a different alloy or even a tool steel. I believe that Christ Centered Iron Works also has a video on making a burke bar that you can refer too. 

 

Leaf springs are just so easy to pick up and use though, no reason not to use one. Just remember that pre heat is essential before welding.  And I would say it's probably not a bad idea to attempt to normalize after. I recently did a project without a preheat - and I think that normalizing after might have helped it survive the abuse I put it though.  Just take note that welding carbon steel is the same as quenching it. I did not know this and have now changed my ways. Have fun in building your project and let us see the end results.

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