Predicting Toughness with Steel Composition

[Larrin]

One difficulty with selecting between the myriad of options when it comes to steel figuring out how they all compare to each other. Each steel company provides some information in data sheets but usually toughness numbers from one can not be compared to another company's data, and often the information that is available is incomplete. Sometimes, it almost seems as if the steel companies are intentionally hiding data so that we don't know that their product may not have very good toughness, for example. On top of this, we have information from various knifemakers and knife users based on their own experience, which is often not very useful. "I have a knife in S30V at 62 Rc and I can't even get it to chip!" How to know what is true and what is false? Even if we take the reported information at face value, it's not as if all of these makers and users can provide us quantitative comparisons between all of the various grades.

There are many factors which affect toughness in steels, including hardness, grain size, composition of the matrix, phase fractions (such as retained austenite), carbide size, and carbide volume. However, one factor which has a strong effect on toughness is carbide volume, as most knife steels have a significant amount of carbide, and carbides are significantly harder and more brittle than the steel. This was identified by Crucible in a patent on 3V where they identified carbide volume as a significant parameter in steel design for high toughness:

They heat treated all of the steels to around 59-61 Rc so that hardness was not a convoluting factor and got a very good correlation between carbide volume and toughness. However, it can be expensive and time consuming to use metallography (polishing, etching, and microscopy) to determine the carbide volume of all of our favorite steels. So I used Thermodynamic software to calculate a predicted carbide volume for all of the steels with reported toughness numbers in Crucible patents, and compared it with another plot where I used the Crucible reported carbide volume numbers:

The R2 value is virtually identical which is a measure of how good the fit is with the trend line. This shows that the predicted carbide volume numbers are correlating well with the experimentally measured values and that both are a relatively good predictor of toughness. In general, the calculated values for carbide volume are lower than for the experimentally measured, which makes sense because the calculated values assume an infinite hold time at the austenitizing temperature. Surprisingly, the PM grades do not seem to have significantly better toughness than the conventional grades, which explains why Crucible and Bohler-Uddeholm have often touted their transverse numbers rather than longitudinal (these are all longitudinal measurements). The S30V datasheet, for example, claims that its longitudinal toughness isn't any better than conventional steels.

With this as our starting point, relative toughness can be estimated for virtually any steel, though it works best for tool steels and martensitic stainless steels. Simple carbon steels often have high carbon in solution which lowers their toughness and the heat treatments usually rely on very short hold times so the predicted values for carbide volume are not as close. These predictions can be used to answer questions like: Do stainless steels have lower toughness than non-stainless for a given carbide volume? Is Elmax predicted to be tougher than S30V? and other exciting possibilities.

I have done a similar analysis to predict CATRA edge retention but that will be for another day.

 

[DevinT]

Good stuff Larrin.

 

[jdm61]

Good info. So which type or mode of "toughness" or strength is L6 so well known for? Tensile strength? if so how does that apply to tools of the type we might want to make, be it a knifelike object or say a tomahawk/breaching axe, etc.

 

[i4Marc]

I love posts like this. I can't say I understand it like some of you but I'm following along. Thanks for sharing. I look forward to the ensuing discussion.

 

[DevinT]

Good info. So which type or mode of "toughness" or strength is L6 so well known for? Tensile strength? if so how does that apply to tools of the type we might want to make, be it a knifelike object or say a tomahawk/breaching axe, etc.

Impact toughness, measured with charpy.

Hoss

 

[jdm61]

Impact toughness, measured with charpy.

Hoss

I asked because I have seen charpy notched figures where it is lower than some other steels, but charpy unnotched where the number is well over 200

 

[DevinT]

I asked because I have seen charpy notched figures where it is lower than some other steels, but charpy unnotched where the number is well over 200

Sometimes unnotched goes beyond the upper limit of testing so they have to notch them. Doesn’t matter if they are notched or not as long as the samples are all tested and compared the same.

Hoss

 

[Larrin]

Good info. So which type or mode of "toughness" or strength is L6 so well known for? Tensile strength? if so how does that apply to tools of the type we might want to make, be it a knifelike object or say a tomahawk/breaching axe, etc.

These are all charpy c-notch impact toughness tests. C-notch, u-notch, and unnotched numbers are relatively similar. V-notch is a more severe notch that has much lower values; it is required for high toughness materials and steels. Almost no knife steels are tested with v-notch because of their relatively low toughness. Another major type of toughness testing is called fracture toughness but isn't used often enough with knife steels to be useful for this discussion.

Tensile strength is not a measure of toughness and actually correlates very closely with hardness. There are conversion charts online for converting Rc to tensile strength.

Impact toughness is the generally accepted test for predicting resistance to chipping or fracture in tool steels, so I would expect it to be useful for predicting chipping and fracture in knives and knifelike objects as well.

 

[Larrin]

L6 has good toughness for the same reasons as other simple medium carbon steels like 1075 or 5160. It has a low volume of carbides, and better avoids excess carbon in solution so that we don’t have brittle plate martensite. It also has the nickel addition, of course, which helps improve toughness.

 

[jdm61]

My problem is that see number all over the place. For L6, I have seen one as low as 40 for 61Rc, and in the next breath, 72 for about the same hardness. Also, have you ever seen a test for 5160 because I have not been able to find those numbers?

L6 has good toughness for the same reasons as other simple medium carbon steels like 1075 or 5160. It has a low volume of carbides, and better avoids excess carbon in solution so that we don’t have brittle plate martensite. It also has the nickel addition, of course, which helps improve toughness.

 

[Larrin]

My problem is that see number all over the place. For L6, I have seen one as low as 40 for 61Rc, and in the next breath, 72 for about the same hardness. Also, have you ever seen a test for 5160 because I have not been able to find those numbers?

If you have some specific examples we can discuss what may be causing the differences. If I have seen 5160 tests I don’t remember where.

 

[DevinT]

My problem is that see number all over the place. For L6, I have seen one as low as 40 for 61Rc, and in the next breath, 72 for about the same hardness. Also, have you ever seen a test for 5160 because I have not been able to find those numbers?

This is not about specific numbers, it is about predicting toughness based on carbide volume.

Hoss

 

[Tin.Man]

Thanks for putting in all the work man. Love reading this stuff!!

 

[Justin Schmidt]

Wow I didn't know A2 was that tough. Wonder how this list would compare to other materials like 52100, w2, AEBL etc

 

[Willie71]

Wow I didn't know A2 was that tough. Wonder how this list would compare to other materials like 52100, w2, AEBL etc

A2 is tougher than most simple carbon steels. A2 is overlooked way too much, and I am guilty of that myself. You need to get into shock resistant steels to get tougher, with a few exceptions like 5160, or 15n20.

 

[jdm61]

Warren, some folks in the specialty cutlery biz have recognized the great "bang for the buck" potential of A2 in hard use knives for quite some time. There seems to be a cluster of them up in the UP of Michigan.

 

[Justin Schmidt]

But if you get a2 to the higher hardness for a more keen edge does it loose that toughness?

 

[Willie71]

But if you get a2 to the higher hardness for a more keen edge does it loose that toughness?

With any steel, you lose toughness as you increase hardness. There’s a range of a few Rc points with any steel that the best compromise between toughness and hardness for that steel and going harder or softer gives up too much of the balance. For example, running W2 under Rc 61 gives you a little more toughness while giving up a disproportionate amount of wear resistance. Inversely, running 5160 at Rc63 won’t give you a proportional increase in wear resistance for how much toughness you lose. Most of our blade steels have their balance somewhere between Rc58-63.

 

[jdm61]

There are some steels that act a little funny if you try to "toughen" the up by leaving them soft. I have heard such things about leaving CruForgeV below say 59. Likewise, I have heard that the practical toughness "peak" of L6 is like 57-58 and f you try to go for 60, it loses much of its advantage to A2. 60 seem to be the common hardness for A2.

 

[Lapedog]

I have heard that it is often oversimplified that more hard = less toughness and vice versa.

I heard it is not that simple and there are things you can do to a steel that will make it tougher and harder. For example if you botch the heat treat it can make the steel both softer and less tough.