Metallurgy of backsprings

[Tyson A Wright]

There have been a number of really valuable threads on blade metallurgy here, but are there any on metallurgy for slipjoint backsprings? I haven't been able to find them. I'm thinking of this because of a thread on the Traditionals subforum about Queen knives, and how's it's not uncommon for their backspring to break, even when the knife is sitting at rest. What is the best steel for this knife application? What is the best heat treatment? How can this part of a knife be optimized?

Here's the first post on the thread on broken Queen springs:

Sorting through the collection today. Opened a box just to check. Here is what I found. I think this is the fourth Queen knife with a broken spring. Yikes! It was a beautiful knife.

 

[Signalprick]

If springs are snapping its because they were simply tempered too hard. I shoot for a temper in the 45 - 48 HRC range. Some probably go even lower I'm sure.

 

[Tyson A Wright]

If springs are snapping its because they were simply tempered too hard. I shoot for a temper in the 45 - 48 HRC range. Some probably go even lower I'm sure.

That makes sense, but what I'm curious about is more details about the metallurgy of springs. We talk about hardness, toughness, corrosion resistance, and wear resistance for blades, and how those interact. For springs, I think at least wear resistance (for the interface between the spring and the tang) and corrosion resistance might be important. What other categories? How can this element of knife design be optimized?

My impression is that, with a few exceptions, most of the time makers just use the same steel as they used for the blade, just tempered a little softer. (One exception I can think of is that I had a Camillus-made Buck 303 with stainless blades and carbon springs.) Are there other combinations that might be better?

 

[Signalprick]

That makes sense, but what I'm curious about is more details about the metallurgy of springs. We talk about hardness, toughness, corrosion resistance, and wear resistance for blades, and how those interact. For springs, I think at least wear resistance (for the interface between the spring and the tang) and corrosion resistance might be important. What other categories? How can this element of knife design be optimized?

My impression is that, with a few exceptions, most of the time makers just use the same steel as they used for the blade, just tempered a little softer. (One exception I can think of is that I had a Camillus-made Buck 303 with stainless blades and carbon springs.) Are there other combinations that might be better?

Hard to say what all metals are good for slip joints. A lot! AEB-L, CTS-XHP, Magnacut, 154CM, CPM 154, CPMS30V are a few I'm aware of that make good stainless slippies. There is a ton of carbon and tool steels that are used also. For wear a lot of it depends on the shape of the tang and also the surface finish of your spring and tang. You don't want a 60 HRC tang with square edges digging into a 45 HRC spring. You need to round your backsquare edges so they ride smooth. You also don't want to finish your spring and tang to the same grits either or they could gall.

 

[DevinT]

There’s a good chance that the manufacturer was blanking springs in the transverse direction instead of the longitudinal direction causing weakness.

The same metallurgy applies to springs as blades except you’re looking for toughness mostly.

Hoss

 

[P.Brewster]

I recently did some research into industrial/general-purpose springs. Here are some links:

Heat Treatment of Spring Steels | Metallurgy

In this article we will discuss about the heat treatment of spring steels. Any metal, or alloy which can be hard drawn, or rolled to fairly high strength and retains sufficient ductility to form, may be used for springs, or any alloy which can be heat treated to high strength and good ductility...

www.engineeringenotes.com

^ Shows a chart of 1065-ish steel that is tempered at 400-500°C

Why Tensile Strength Matters in Spring Manufacturing - Yost Superior

One of the most fundamental factors of spring design is the tensile strength — resistance to breaking down under tension — of the material.

www.yostsuperior.com

What Makes Spring Materials “Springy” - Yost Superior

One of the innate characteristics of most metals is the ability to “spring” back to its original state when bent of contorted.

www.yostsuperior.com

https://www.youtube.com/c/TheBomPE/search?query=spring

^ good engineering YouTube channel

 

[Bill DeShivs]

I have made literally thousands of knife springs from 1095.

 

[Tyson A Wright]

I have made literally thousands of knife springs from 1095.

Three of the four knives I'm most likely to have in my pocket have 1095 blades, but that doesn't mean that all the work on knife blade metallurgy isn't interesting. All three of those knives have 1095 backsprings, and they work great. But in the same way that people are trying to optimize blade steel, I'm just wondering what optimizing backspring steel would even look like. Obviously, not breaking (as seems common from Queen slipjoints from late in the company's life) would be a minimum. But as I said in the first post, I'm curious how someone would even optimize for this, if that was a goal.

 

[Londinium Armoury]

I'm actually in the middle of making a backspring for a slipjoint right now. I'm making it out of 26C3 white steel, probably temper it to around 45-50 nice and blue.

 

[000Robert]

I don't remember ever having a backspring break on me. But Buck slipjoints are pretty much all I have ever used.