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Tempering folder springs?


Carlos Lara

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So, I've been working on getting my WWII folder back together. The cleaning/polishing/sharpening is going well, and is almost ready. However, it has one broken spring that I need to machine/heat treat/temper. I ordered some 1070 instead of 1095, as I'd rather the spring be too soft than too hard! I think I know what I need to do to machine and heat treat it, for machining I'm planning to cut it out with my diamond cut off blade, rough shape it with a grinder and fine shaping with files. To heat treat, I'm just going to use a propane torch to get it to bright red/non magnetic and quench in oil or water. What I'm not sure about is the temper. Clearly, since the last one broke, it wasn't tempered properly. My thought is that I can just heat it in my oven at 300F for 2 hours, but I'm not sure that's going to be enough. Reading online, there's all kinds of opinions as to how to do it! Other options I've read is just to heat it until it blues, or just heat it to 675F (but that's for 1095 springs). I have an oven, but no other equipment for tempering, though I can maybe put something together if necessary! What do you guys recommend? Also, do you think I should temper the spring that isn't broken at the same time, to avoid any future breakage? Thanks!

 

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Use your forge to heat it up before quenching, that'll give a much more even heat.  Oil quench. And you will need a full spring temper of 675-750 F evenly applied to the whole thing.  Since you don't have an HT oven, you can try to do this by very carefully heating it with a torch to a uniform dark blue.  The is not ideal, but it's better than nothing.  After you harden it go ahead and temper immediately at 450 F in your oven.  This will remove stresses and give you a good starting point.  Once that's been done twice, polish and fully degrease the spring and hang it on a wire through one of the holes.  Carefully wave it through a propane torch flame, not stopping in the flame, to slowly heat it more.  The very tips of the spring can stay harder, but you need the bendy parts to go to a full dark blue.  Polish, degrease, and repeat. 

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Ok, thanks Alan! Just so I get this straight, heat treat in the forge to bright orange, oil quench, then straight into the oven at 450 after the quench? say for 2 hours? Then heat treat, quench and into the oven for 2 hours again? Let it cool to room temp, polish, degrease, then hang it on the wire, blue with the torch slowly and evenly until the important parts are dark blue, then let it cool slowly to room temp. Polish, degrease, and temper again. 

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1 hour ago, Carlos Lara said:

heat treat in the forge to bright orange

Learn to look for decalescence and recalescence.  That is the visual cue for the proper phase change.  

1 hour ago, Carlos Lara said:

oil quench, then straight into the oven at 450 after the quench? say for 2 hours? Then heat treat, quench and into the oven for 2 hours again?

No, just do the high heat and quench once.  You want to do just the temper cycle twice, one hour each time.  The second probably isn't necessary since you will be doing more temper cycles later, but is easy and doesn't hurt.  

1 hour ago, Carlos Lara said:

Let it cool to room temp, polish, degrease, then hang it on the wire, blue with the torch slowly and evenly until the important parts are dark blue, then let it cool slowly to room temp.

Yes, but it doesn't need to cool slowly here.  Any speed is fine.  And yes, you want a second temper cycle here, too.  

 

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If the torch temper is a little scary, you could try the following; assuming it sounds less scary and you have a thermometer that goes hot enough (a thermocouple probe would be ideal).  

 

Option 1:  

Heat up some steel chunks to as close to 725-750F as possible.  You'll want as much mass as, so bigger chunks or more smaller ones.  pre-heat your spring as much as you are comfortable without going over 700F.  Put your pre-tempered (that 450F temper described earlier) spring in the middle of all your big chunks of hotter steel and put them in something very insulating.  This could be wrapping it up in refractory blanket, bury it all in ash, etc.  If you have a temperature probe make sure that is in there near your spring as well.  If you don't have a probe, but instead have an IR thermometer, you'll have to do some experiments where you open up your heat pack and check the temperature.  If your insulation and thermal mass are adequate, you can still be at 675-700F after one hour.  If not, you may need to have multiple set-ups running at once so you can pull out of the first and directly into the second, and if need be start re-heating the first setup for round 3.  

 

Option 2: 

Use your forge to heat a bunch of sand.  Again, more thermal mass the better.  Thoroughly mix the sand and check the temp.  You can keep heating and stirring the sand while your part is in there, and with enough thermal mass the temp will not swing up or down quickly.  Take it off the forge heat when getting too hot, put it back on when getting too cold.  

 

Option 3:  

Find something that boils at the temperature you want and boil it with your forge or other heater.  It won't go above its boiling point.  Suspend your spring in the oil as it boils.  Mineral oil can be anywhere between 572-1112F (IARC 1984).  Paraffin wax is somewhere around 700F or more.  Feel free to mix these 2 to get the right temp (in the event you get mineral oil that boils too cold and wax that is too hot).  I think they will stay together enough over the course of a temper cycle or 2 that you should be good, but stirring and checking the temperature often would be wise.  

 

I've done things like 1 and 2 a few times (not for tempering steel, but heat is heat).  3 is definitely just a theory for me.  

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On 9/15/2023 at 12:15 AM, Jerrod Miller said:

Feel free to mix these 2 to get the right temp (in the event you get mineral oil that boils too cold and wax that is too hot).  I think they will stay together enough over the course of a temper cycle or 2 that you should be good, but stirring and checking the temperature often would be wise.  

Doesn't that just give two boiling temperatures? First of the oil boiling until the oil is boiled off, then the wax? 

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Jeroen Zuiderwijk

Facebook page: https://www.facebook.com/barbarianmetalworking

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Being similar hydrocarbon chains, I think there is a significant chance they will interact with each other to be more like one material.  But that is indeed why I recommended frequent stirring and temperature checks.  

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So, today I gave it the first try! Machining the spring went well, it's pretty close to the original. The hardening also went well, I warmed up some mineral oil until I just couldn't touch it, heated up the spring nice and even in the forge past decalescence, quenched nicely, and tempered at 450 in the oven for an hour. Next, I hung the spring on a wire, and suspended it with some fire bricks. I started heating it with the propane torch continuously moving up and down, then blew past the blue stage into grey! So I have to start over. The blue came pretty fast, and I sort of saw it run up and down the steel, immediately turning to grey as it went. I was heating continuously with the torch, so for try #2, I'm going to heat for a bit, let it cool, heat for a bit, let it cool, to see if I can approach the blue stage a bit slower. If that doesn't work, I'm going to start working through Jerrod's tips one by one! I have plenty of heavy wrought, and we have a big bag of ash from the barbecue, and I have a thermocouple that I still have to figure out how to use! If I can't get that working, I'll get an IR thermometer, I'll probably have plenty of uses for it!

 

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Edited by Carlos Lara
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The trick with using the torch is to hold it far away from the steel and use slow sweeping motions to slowly and evenly heat the spring.  It should take at least 30 seconds if you're doing it right, and up to a minute is even better.  But the heat gun may work fine, too.

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  • 2 weeks later...

Well, I had a bit more time to work on this today. I was casting copper alloys, so I decided to heat treat it again with my casting furnace. That all went well, and I'm happy to report the bluing with the heat gun also went well!

 

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Here's the heat gun I used:

 

https://www.homedepot.ca/product/wagner-wagner-furno-300-heat-gun/1001075715

 

I found I could actually use the highest heat setting, the lower heat setting was just taking too long (the temperature listed is probably the heat of the element, but the air coming off of it is probably a bit lower). I found that moving it along the length it just wouldn't blue, but if I held it in a certain spot for a bit, the spot would change colour slightly, and then blue as I moved the heat away. It changed to a sort of dull grey/red, then blue. I had the nozzle right up against the piece, almost touching it. It was much more controlled than the torch, but it did take quite a while (at least 20 minutes for each temper). Thanks for all the help!

 

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1 hour ago, Carlos Lara said:

I found that moving it along the length it just wouldn't blue, but if I held it in a certain spot for a bit, the spot would change colour slightly, and then blue as I moved the heat away. It changed to a sort of dull grey/red, then blue.

This is a good illustration of why going by color for tempering isn't a good idea for optimal results.  Clearly when you moved the heat source away, it wasn't getting hotter there, yet all of a sudden it started changing color.  That is because the conditions changed and the oxide layer started forming to get those colors.  And the colors kept changing as the part continued to cool off; so clearly we can see that color isn't locked into a specific temperature instantly.  

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