Where do knife steels come from?

[me2]

Toughness is generally defined in one of 3 ways, depending on context.  First is impact toughness.  Pretty self explanatory.  How much energy does it take to break a standard size piece of material.  Second is fracture toughness, a measure of a materials reaction to ever present flaws.  Third is more general; how much energy can a material absorb before breaking, even when loaded slowly, as opposed to impact.  

Springiness is a function of edge angle and yield strength.  Lower angles flex with less force, but a higher yield strength will resist permanent bending.  When the yield strength is exceeded, you get edge rolling, or bending.

An overall higher hardness might conceal fairly diverse microstructures.  An edge at 60 HRc might have very high hardness martensite mixed with a fair bit of very soft retained austenite.  This edge could bend more easily than a HRc 56 edge that is universally martensite with little to no retained austenite.  Its like a chain where every 10th link isn't welded shut vs a smaller chain with all of the links welded.

[EOU]

Thank you me2 for the explanation and definition of toughness. We just don't know. It simply perplexes us that so little force is required to permanently roll the edge. Keep in mind that the force we are applying is considerably less than the weight of the knife itself. We are applying that force in a very localized manner (the radius of a .75 inch diameter linear ball bearing) but still, nothing remotely close to cutting a frozen hot dog into two pieces. One possible explanation is that we are just crappy sharpeners and we advance that theory not just as some self effacing comment but as a genuine possibility. Another possibility is that edge apexes, via the sharpening process,  undergo  structural transformations that we don't yet understand. 

As an aside to this conversation, we tested a different Henkel knife. One with a much thinner blade than our chef's knife. We look at several different things but one of the things we are beginning to look at is the relationship between initial sharpness level and the amount of roll. So far, duller edges definitely roll less...and significantly less. This is not to say, however, that duller edges test sharper than their sharper counterparts after rolling. They do begin to test very similarly though post rolling. 

The method that we currently use to roll edges produces very undramatic microscopic pictures from a side perspective. There is little to see. We have begun to look at these edges "edge on" though with some success. Here's a couple of examples:

                                                      
     Beginning BESS Score 152                                    Beginning BESS Score 360

This is the same knife edge at two different points on the edge. In the 152 picture we think that we can see the light reflected off the rolled edge and in the 360 picture we couldn't. This makes sense to us because the 152 edge rolled about 200 points while the 360 edge rolled only 60 points.

Well we've managed to move this conversation off topic so we're going to go ahead and post here but also pick up the last two or three posts and post them in the Rolling Edges thread as well to preserve continuity.

[grepper]

Mr. Mike uttered, "For example, we reported that our HRC50 1095 spring steel knife edge rolled almost twice that of our HRC56 Henkel edge.  Our edge rolling element impinges on the edge at a 10 degree angle and moves linearly down the edge with only 150 grams of force applied. Why didn't the edge just "pop" back to, at or near, it's original position?"

You ask a very good question Mr. Mike.  Spring steel is exactly that; springy. Does that mean that the spring characteristic in the metal at the edge has been removed? 

So I'll ask the question again.  Why wouldn't the the spring steel edge spring back after only being bent 10°?

[Mark Reich]

Well, "spring steel" isn't necessarily any more accurately described than "surgical steel". In reality, surgical steel is garbage, and 1095 is not used for springs.

Virtually all steel gets "springy" simply from being heavily tempered martensite. A better description of 1095 in spring condition is just "1095- XXtra soft", which, at HRC 50, is exactly what you have.

5160 (actual spring steel) at RHC 50 would work a lot better.

What this new implement is actually going to do is determine edge stability. 

If I haven't said that, I surely should have. I know I've been thinking it.

[grepper]

So, stuff like

   

5160?  What makes is so springy?  I wonder what RHC those springs are.

If you were to put an edge on those springs, would you expect the edge to roll or be more roll resistant than regular steel used in knives?

[Mark Reich]

In 5160, you have medium-high carbon, so it doesn't over-harden, or work harden, which is key. It has a little chromium, and a little manganese, which I'm sure helps, but I think it's just a tough steel that has a simple HT to make it springy and stable.

I doubt if those springs are hard enough to have edge stability, but I don't have first hand knowledge. Springs are pretty specialized, especially in heat treating. I could probably find out a lot about Wolff springs, which would be a great starting spot for investigation.

I think you'll find that HT has as large a role as steel. I think it's important to know about edge stability, and the optimum harness for each steel's edge stability. I'm sure you'll find tool (knife) steel isn't the best for springy steel. Their optimum hardness is too hard. Very springy steel will probably be highly alloyed,

I would expect soft steel to roll more in general, but straighten out easier.

Sorry, I don't have an abundance of knowledge right off the top of my head

[me2]

Spring steel is a vague term, perhaps even more so than surgical stainless as Mark mentioned.   There are a staggering variety of springs and materials used to make them.  The main requirement is sufficient strength to prevent bending in the chosen application.  A secondary consideration is the fatigue life of the material.  I suspect this is the reason for choosing medium carbon steels for many applications, as the carbides found in higher carbon steels might act as initiation points for fatigue cracks.  Or it could just be these are often more expensive.  

5160 is often used in automotive support applications.  1095 is used in tape measures.  301 stainless is used in some applications, and makes a fine spring once cold rolled or drawn into wire.  

Higher hardness makes a "better" spring, but there is a point off diminishing returns.  A spring with too high a hardness will break if pushed beyond its limit, where a proper spring will bend.  Try to roll a high hardness edge and it will deflect further and still return to straight.  However, push it to hard and it will break/chip.  A softer edge will just roll over and can possibly be straightened.

Edges behave oddly to most of us because they are one of the few things we deal with on such a small scale, as in the edge thickness of a sharp edge is less than 1 micron.