Joseph, with the Japanese style of quenching, the edge is hard, but the spine is pretty soft. A blade done with Japanese style heat treating will take a bend and not spring back. I have bent one almost 180 degrees and it stays bent, with edge cracks of course. There is an area just above the hamon that is like a transition zone between hard martensite edge and softer pearlitic spine which I believe has some of both. So say your edge below the hamon is 60 rockwell, the transition zone will be 40 to 50 rockwell, and the spine area in the 30's. Also 5160 is not a good candidate for a clay quench. It is a deep hardening steel and there will not be much of a hamon, plus the hamon will be fairly wide. A better idea is to full quench the blade, then temper it, then draw the spine with a torch in a water bath. A blade correctly done this way can bend 90 degrees and return to about only a 20 degree bend.

As for the tempering. Leaf springs can be a number of steels, from 5160, 9260, 1060, etc. You are not exactly sure what you got, so you have to do testing with different heat treatments to see what works best. I use known steel. In other words new steel. I know what the chemistry is and what the heat treat sequence is. Also it is not that expensive. One other big thing with used leaf springs is you do not know what type of use or abuse they have been through. You may have micro cracks in the steel structure due to excessive bending or flexing. Hard to know and I do not like to take chances, so new steel for me.

Assuming the steel is 5160. I heat to non magnetic and hold at 1500 degrees for a couple of minutes, then quench in 100 to 120 degree canola oil or parks 50 oil. Full quench. I then clean the blade and into the tempering oven at 400 degrees for two hours. Let the blade cool and repeat the two hours at 400. I will then clean the blade to 120 grit and draw the spine of the blade with a torch, with the edge in a water bath, at least twice. I also draw the tang soft. Then I will finish grind the blade and sharpen and test.

Hope this helps.

Brion

Joseph,

Leaf springs are great mystery steels to work with when you are learning hammer control, however a good steel of known origin is much better when you are heat treating for performance. I know that statement will be backed up by anyone on this forum. I started out with them too but after cracking 2 or 3 when you have put so much work into one it will become frustrating. They are not all 5160. With the car industry as well as any other. A company may use one thing or another differently according to supplier pricing and other factors that have nothing to do with how good a knife someone can make with one of their springs.

A spring can contain many different combinations of iron, chromium, and all kinds of other things.

Have you broken any of your blades on purpose to see what the grain structure looks like? If not I would suggest you do (as an experiment). Maybe a couple or 3. Do a different heat treat on each one and take notes on each one. Look at the grain at the edge of the break and closer to the spine.

Perhaps the softness in the steel is due to not a high enough temperature on the steel.

Checking with a magnet is a good foolproof way to find the critical temp. Though I did have one knife to date that did not get hard at that temp. My uncle had made it from a file and couldn't get it hard. I was tasked with doing it in my propane forge. The knife was normalized 3 times and brought to nonmagnetic and then edge quenched several different times. I noticed the tip of the knife had gotten hard and nothing else...The tip had gotten hotter than the rest of the blade. I ended up having to bring the whole blade up hotter than critical to get it to harden in oil.

My point after all that rambling is go ahead and use leaf springs for now. But do your own testing and be sure you aren't (unintentionally blowing smoke) when you tell a customer how well your blades will perform. Just because someone else is getting super performance from leaf springs doesn't mean you will have the same luck.

Awesome answers, both of you!! The thing with new steel is that one piece that is 1/4" thick, and over 2 or 3 feet long costs a fortune for me...in comparison to getting leaf springs free from somebody, and a whole stack at that... Maybe if I start making money off this business, I'll get new steel.

So, it sounds like you are both saying that if they work, then they are ok to use, right? Jared- I did break a blade by accident. I didn't temper it enough, (it was a leaf spring, with clay quench) Anyway,, the grain was super fine. As far as hamons go... Brion, you're right...they are pretty difficult to get out :-S They don't show up as pronounced. I'll have to try that thing with the torch..thanks for the advice there!!

Jared..my bowies from springs really do have superior performance compared with mass produced knives on the market..don't worry, I'm not basing it off of someone elses stuff <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//wink.gi f' class='bbc_emoticon' alt=';)' /> Maybe not as good as something from bar stock...I'll have to get some and do some vs. testing between the two.

Thanks for the responses...both were well thought out. I appreciate all the help!!

Joseph, I would use the found steel for practicing forging or making test knives. And yes as Jared states, definitely do test blades. Test them to destruction as a matter of fact. This will tell you a lot about your heat treating. Also keep notes on your procedures so you can refer back to them. For customers I would use a new steel. You are selling your work to a person, it should be the best work you can produce. With your found spring steel, you may have an idea it is 5160, but you do not know for sure. Plus again you do not know what stresses the steel has been put through. There could be stress fractures within the steel which could cause problems. Why take a chance? A four foot bar of new 5160, 1/4x 1 1/2, will run you about twenty dollars from Aldo Bruno, the New Jersey Steel Baron. Again, why take a chance, and your customers are worth it.

Brion

when I was starting out I would get drops from a local spring shop. They were the ends of bars 4-10 long thickness ranging from 3/16-1/2" and width from 1 1/4-4". all new steel just cut from the bar, most of the time I got it for free or $5-10 a bucket(I think I made the guy a knife at some point)most of the time I could deturman what the alloy was by the paint on the end of the bar (from his supplier, purple was 9260 green 6150 and yellow 5160 I asked they spring guy when picking up and he gave me the color code) something to look into if you don't want to spend money on know steel

MP

Jared... Just for the record, for those reading this, I called up a spring shop, and asked what Chevy springs are made of. They replied that practically all springs are 5160... So, I guess that's pretty good!! You are right though...I should get into more of bar stock.

Thanks for all the help!! I appreciate it.

Joseph

beckerforged.com

Hello Joseph,

I apologize for coming into this so late but I just got back from teaching my dagger class in ME. Joseph, to be honest I am with you, while I have successfully edge quenched I found the results not up to my preferences like a clayed blade, if I had to do a differential hardening.

One thing that is always worth revisiting on this topic is the concept of flexibility. The actual “stiffness” of a piece of steel is not a function of heat treatment at all but solely based upon its thickness. Along with this the amount that the steel can flex without taking a set or breaking is also in the realm of cross sectional thickness- e.g. a fillet knife will pass the ABS 90 degree test every time with flying colors, regardless of the heat treatment it just won’t due too well on the 2x4, unless you have a lot of time to waste.

If you want “spring” and not “bendy” then differential hardening is not the way to go at all, instead differential tempering is how to approach the situation. Springs that bend are not springs, springs will break if pushed too far but if they are ductile they will just bend and not function as a spring. So for a blade that will spring back to straight, make the blade thin and nix the edge quench or clay and fully harden it before drawing the spine with a spring temper.

Even if we stick with the differential hardening the next challenge will be that 5160 has alloying that is going to fight you every step of the way in doing that. It was created specifically to avoid making soft parts so that springs could have consistent hardness. You can indeed get a hamon on this steel, I did often many years ago when I played with it, but the approach is different than other hamon. In traditional hamon the clay retards the cooling of the quenchant regardless of how hot the steel under is, with 5160 the clay cannot do this well enough, so the idea is to retard the heating with a clay thick enough to insulate it in all operations. So if you are going to do it- cake it on there (1/4” or better).

On the tempering I can usually tell how a person heated the blade for quenching and what their soaking procedures consist of for hardening just by hearing the temperatures they use for tempering. There are several distinct phases of tempering, based upon what occurs in the steel in a given tempering range. 320F is actually below the first phases that bring about noticeable change. Below 350F there is but a slight relaxation from the body centered tetragonal (alpha martensite configuration) and very little precipitation of tempering carbides. If maximum hardness was obtained this temperature my work with 5160 due to the fact that its martensite (the hard stuff) is lath in nature, but higher carbon steel will probably fail quite dramatically with this little tempering treatment. You may find this temperature range works for you now but if you strive for greater temperature controls (soak and quenching) in hardening you will find the benefits of greater toughness while maintaining higher edge hardness with more thorough tempering.

It is worth noting that the manufacturer of any item can use whatever steel they find most economical for the purpose, and a spring does not meet more than a couple of the requirements for a knife, so while 5160 may be the most common on the U.S. for this application, there are other alloys used. One of the main reasons I would shun many used steels is that even if you have the chemistry, you cannot guarantee the physical ant thermal history of the item. Prior thermal treatments can have just as much effect on our heat treatments as the chemistry itself, and stress/strain issues all the way up to fractures are just part of the life of many high carbon items.