I made these hooks from a garage door spring, although far stronger than mild as is, I would to try heat treat them. 
 
my thoughts are veggie oil quench and temper in toaster oven. 
 

Any tips on hardening such thin stock? Thanks for looking

First, very nice.

Second, oil quench and temper to blue would probably work.

Third, why temper them at all? Normalized, they'd already be tougher than the equivalent hook in mild steel.

I think John means: why harden them and temper when just normalization will make a tough hook.  If they are hardened they should be tempered!

Yes, that's right; sorry. At least I didn't say "heat treat".

There's a youtube channel where the host makes clock parts, including small springs, screws, etc.  Many of the steel parts are tempered to blue in a bed of brass chips, set into an appropriate vessel over a burner.  The consistency and vibrancy of the blue coloring this method achieves is pretty remarkable.  

 

For gun springs, I temper them in molten lead.

Lead melts at 621.5°F / 327.5°C, but boils at 3,180°F / 1,749°C. Knowing which end of that range you're closer to is vital!

(Also, lead starts to give off fumes at 900°F / 482°C. Not only is that hazardous to your health, but it's also much too hot for proper tempering.)

Lol, first I'd make them from mild steel. Second, if I made them from a tool steel, I'd just normslize them. 

To answer your question, I would do a test piece and harden in water to see if it works.  Tom Bredlow turned me on to this. He tempers small springs with large cross section changes in corn oil. Put the corn oil in a large enough pan to cover your hooks. Then slowly raise the temp until the corn oil flashes. According to him, corn oil flashes at the temp he prefers for a spring temp. Works for me.

 

Corn oil has a smoke point of 457°F / 236°C, a flash point of 600°F / 315°C and a fire point of 700°F / 371°C. In the tempering color spectrum, 600°F is on the gray side of blue.

SAFETY NOTE: Whenever you're doing oil tempering, have a pan that's deep enough that the oil won't splash out, a tight-fitting lid to smother the flames, and a Class B fire extinguisher close at hand. The last thing you want is burning oil splashing about the place.

And NEVER throw water into an oil fire!

The local fire department did a demo in the parking lot of my elementary school when I was in third grade, to show exactly why one shouldn't do that. A firefighter in a full approach suit squirted water from an extinguisher into a metal bucket of burning oil, resulting in plumes of flame that were most impressive.

5 hours ago, JHCC said:

Knowing which end of that range you're closer to is vital!

You are absolutely correct, I should have included that in my post. After quenching them in warm oil, I have my lead pot set to 700°F and verified by a lead thermometer. For very thin springs I will back the heat to 650°F then test each V spring in my hand vise by compressing them several times.

I have used Bredlow's oil temper also and it does work, it's just handier for me to turn on the lead pot which always has lead in it. The spring stock I buy from Brownells says to temper to 650°-700°F if I remember correctly.

Edited August 11, 20214 yr by Irondragon ForgeClay Works
add oil temper

3 hours ago, JHCC said:

In the tempering color spectrum, 600°F is on the gray side of blue

And that's about right for a spring temper.

For those who don't know, Bredlow was the first president of ABANA, and at that time was the blacksmith for the National Cathedral. 

 

 

Just did a little poking around online and found the following information about the smoke, flash, and fire (autoignition) temperatures of some common oils. This is sorted by flash point and then by smoke point (ascending, in both cases):

Oil Type	Smoke Point		Flash Point		Fire Point
	(˚F)	(˚C)	(˚F)	(˚C)	(˚F)	(˚C)
Coconut Oil	385	196	563	295	626	330
Olive Oil - Extra Virgin	374	190	600	315	700	371
Olive Oil - Virgin	419	215	600	315	700	371
Olive Oil - Extra Light	468	242	600	315	700	371
Cottonseed Oil	450	232	606	319	680	360
Sunflower Oil - High Oleic	471	244	606	319	680	360
Sunflower Oil - Mid Oleic	412	211	607	319	678	359
Rice Bran Oil	444	229	615	324	695	368
Palm Olein	446	230	615	324	666	352
Palm Oil	489	254	615	324	666	352
Corn Oil	455	235	617	325	670	354
Canola Oil	457	236	619	326	662	350
Soybean Oil (hydrogenated)	446	230	626	330	680	360
Soybean Oil	464	240	626	330	680	360
Lard	464	240	626	330	680	360
Peanut Oil	446	230	633	334	680	360
High Oleic Canola Oil	464	240	644	340	680	360

(Sources: all these numbers come from the The Institute of Shortening and Edible Oils, with the exception of those for olive oils, which come from the website smartkitchen[dot]com.)

If you are going to do an oil temper, please remember to DRY YOUR WORKPIECE FIRST! You do not want moisture on your blade evaporating explosively and splattering hot oil all over the place. Naturally, the same applies to a lead temper.

JHCC,  Thank you for posting that.  I wonder if used cooking oils have different smoke, flash, and fire points. 

I'm also curious if people with peanut allergies would be affected by burnt peanut oil residue on ironwork.  

Finally, I wanted to mention something weird about lard.

Traditional Pork Carnita's recipies call for basically deep frying the meat in lard.  Once browned, many recipes call for adding a can or two of cola, orange juice, and other spices.  I was shocked the first time I watched a video of the preparation, because the boiling lard didn't react like boiling oil would when the cola was added.  The cook made a big point of saying "don't try this with anything other than lard".  I thought maybe this cook was unique, so I looked into it more, and discovered that it's very traditional.

 

I can't speak to that particular lard issue, but I do deep-fry my bacon in bacon fat. It doesn't make it any more fatty, and you can get perfectly even cooking to exactly the degree of crispness that you want. As a bonus, the fat that cooks out is already in your bacon fat supply when you're done.

 

lard makes better biscuits and bread. Also makes for very flaky pie crusts.

1 hour ago, rockstar.esq said:

I'm also curious if people with peanut allergies would be affected by burnt peanut oil residue on ironwork.

Peanut allergies are not a reaction to the oil as such, but to a protein in the solid part of the nut. Refined peanut oil that has had all the solids filtered out is not considered allergenic. Refined peanut oil is used as a frying oil in commercial applications, as it retains the high smoke, flash, and fire points of regular peanut oil but doesn't have much peanut flavor that might affect the taste of the food.

The allergenic proteins in the peanut solids are not affected by normal cooking temperatures (which can actually make them more allergenic), but I haven't been able to find anything about the temperature necessary to render them completely safe. 

Aye, yer Lardship!

JHCC, thanks for the answer about peanut allergies.

I was thinking more on the lard thing, and it seems likely to me that lard is mostly fat containing water which is why it doesn't explode when water is added.

 

 

Lard is 12-18% water at room temperature, but that will have cooked out at the temperatures necessary for browning.