Heat Treating for most spring?

[Mrhappybottms]

First off I will say that this has nothing to do with knives, but it does have to do with heat treating. I am planning a project where I need to forge my own leaf spring. I'm thinking of forging it out of steel(i got it at homedepot so i don't know what kind it is) into the shape I need(which is tapering both ends of the steel then putting a bow into it). My question would be how can I get the most spring into it? I want it to be able to bend and return to the original shape with out it breaking or loosing the shape I forged it into. Feel free to ask any questions if it would help be able to answer my question.

 

 

Since this is not about blades,  I will relocate it to the proper area.

[dablacksmith]

well the first problem is your steel ...home depo does not sell spring steel ... at least the one here does not... you will want to get a known spring steel . Then you will want a way to get the entire thing up to around 1500 degrees . then you need to quench it in a bath (i prefer oil quench) then you need a oven to temper it . it is not going to be easy to make .... Good luck!

[Steve Sells]

home depot does not sell any heat treatable steels around here.  what grade of steel to you have ?

[Charles R. Stevens]

Most large cities have a spring shop that makes custom automotive, truck and trailer springs. You can get a drop from them, and even have them heat treat it. You can also use used automotive, truck or trailer spring. But it may have flaws due to fatigue.

[BobL]

RE" "how can I get the most spring into it? I want it to be able to bend and return to the original shape with out it breaking or loosing the shape I forged it into"

 

 

What your specification lacks is

1) how far you expect it to move and

2) what sort of force you are going to apply (or) the return force you are looking for. 

The ratio of these two are related in a quantity called Elasticity or Young's Modulus and is the standard measure of the "springiness "of a material.

 

All solid materials have "spring" over a certain distance range but beyond that distance (called the elastic limit) they deform and remain deformed, more bending and they break.

 

For mild steel the distance range over which it remains elastic is limited, and as others have said heat treatment changes nothing.

Perhaps counter counter intuitively, spring steel has an almost identical elasticity to mild steel, but spring steel be heat treated to increase the distance range over which it remains elastic.

If spring steel is bent beyond its elastic limit it too will stay bent.

 

In other words, heat treatment of a specific piece of spring will not change the return force available for a given deformation but it will change the distance over which it can be bent and still return to its original shape. Obviously if the object can be deformed over a greater distance it can return a greater force, otherwise the way to change the return force is to change the shape of the object. The other way of course is to change the material.

 

There is a good discussion here about this. http://bbs.homeshopmachinist.net/threads/29279-Modulus-of-elasticity-changes-due-to-heat-treatment-of-metals

[Mrhappybottms]

It may help to say that the project is a crossbow, so I'm going to forge the bow part of it, so it isn't going to be to big and isn't going to flex a ton. It will need to be as powerful as normal crossbows.

[swedefiddle]

Good Morning,

 

You can learn to forge to the shape that you want, with the piece of material that you have purchased. Then you can put it down and copy what you have done, to a piece of spring steel. Then you can talk to your local spring business, where you bought the spring, about doing a heat treat for you.

 

This is like wanting to learn how to build a car, but you want to start with a sports car. Not impossible but a great way to lose interest in a real hurry. Most new people just want to start out with a sword, not just a measly lesson in how to forge and maybe make a letter opener first.

 

I don't want to pee on your parade. Look for a Blacksmith Group in your area and learn how to forge and why we do certain things. Maybe in a couple years you can graduate to a crossed-Bow (yes I know the spelling).

 

Neil

[Charles R. Stevens]

A one peice prod is not impossible, by it consider a laminated prod (like a automotive spring with the shorter leaves inside the arc) lubrication of the leaves with a graffite compound will increase efficiancy. Besides the thinner leaves are much easyer to work with.

[Nobody Special]

Much as it hurts me to use stock removal alone, why not just cut down a leaf spring? You should be able to easily get at least 150-200 lbs of pull depending on what you're using and the length of your draw, which'll probably be 8 inches or so. (Note the crossbow in my avatar pic, I think there's some more pics in my gallery). That should be sufficient to the point you'll darn near nead a goat's foot lever or cranequin to pull it.

 

If you're just getting to spring tempering, this is NOT what you want to start on. Imagine what could happen if it cracked and split while you're tensioning it.....or worse yet, while under tension with a bodkin headed bolt in it ready to fire. They don't always just bend. Trust me on this one. The multi-part prod suggested by Charles works well too.

 

Additionally, you might want a weaker bow while you're sorting out the technical bits. The really heavy ones can cause all kinds of additional problems, such as.....say, the nut splitting in two and flying apart, whilst and at the same time even, the string releases throwing your bolt forward when you're not ready for it.........not that I've experienced such a thing........... :)

[thingmaker3]

What you want is called "yield strength." 5160 quenched in oil and tempered at 500F for an hour or two will greatly exceed your requirements.

 

You can usually get 5160 remnants from places that replace springs on vehicles.

[SagebrushBlades]

Unfortunately, Home Depot and other box hardware stores only sell mild "structural" or "weld" steel barstock, which is not hardenable. 

 

Buying some 5160 or similar bar stock from a knife supply or steel supply company is the only way to go if life or limb are at stake on the finished product. but then again, I wouldn't advise doing your own amature forge work if that is the case.

 

 

Your best bet for free or cheap spring is going to a spring manufacturer or junkyard and asking for scrap leafspring. You may luck into one that is already in the rough shape you need, but it can also be forged as needed.

 

If you forge it remember to normalize and then aneal at least once before moving on to final heat treating.

 

Heat treating scrap steel will depend on what alloy the original spring was made of, (5160 in old trucks, but any number of alloy spring steels these days) but in very general terms it requires heating to cherry red, quench in oil and then temper between 500 - 700 deg F. Lower temperature end gives more spring but may crack under heavy loads, higher end will give more flexability so it whont be as prone to cracking under a heavy load. Either way it should return to shape when the load is removed unless overstressed.

[SagebrushBlades]

Oh just saw the crossbow part!!!

If you have to ask about heat treating, you are not ready to forge your own crossbow.

 

If you decide to go stock removal as Nobodyspecial suggested, use only NEW spring steel of a known type and have a professional do the heat treating! Scrap from a shop that makes springs is fine, as long as it is NOT post consumer (recycled) scrap.

 

I have one homemade crossbow I made a long time ago, but the bow arm for that was purchased as a kit. I junked the rest of the pieces when the nut blew off the front and other such nasties and built a new stock and mounts myself.

As the former 3D Archery director for an IWLA club in northern VA I have seen bow arms break on both traditional and modern (compound) bows and crossbows. Anyone who walks away unhurt after that is incredibly LUCKY. Most catch either the string or a chunk of the bow arm across their skull. Stronger the bow, the more catastrophic the failure...