The previous article in this series dealt with a little about why blade steel matters and a little with metallurgy with a good rant thrown in for measure. It can be found here. This article is going to lay out a little about traits steel possesses and how they work in tension with each other, a bit about heat treating and blade geometry, and then a word on the Rockwell scale and whether or not it matters.
The Power of Three
All of the elements and processes discussed in the previous article combine to make steels with different attributes. In cutlery steels there are three broad traits that exist in equipoise with each other--usually increasing performance in one will decrease performance in another or both. Additionally, within these three traits there are sub-traits that again exist in a balance with other sub-traits. The elements used and the methods of heat treating determine how well a steel performs, both on the big trait level and in each sub-traits.
The big three are:
1. Corrosion resistance
Corrosion resistance generally means resistance to rust, though other things can and do cause corrosion. Generally speaking lower carbon content equals less rust and higher carbon content equals more rust. Other things make a difference too. Chromium is the usual answer to rust problems, but recently some makers have taken a different approach to the rust problem. The idea is interesting, instead of increasing the chromium content, these makers have REMOVED the carbon and added nitrogen. This has traditionally been a huge no-no because the thing that makes (or made) steel better than straight iron was the hardness carbon lent the mixture. Nitrogen can be used to harden iron, though until recently it was difficult to do. Now that we can do it we are getting very corrosion resistance blade steels, like H1, X15TN, and Elmax that can be hardened quite high as well.
Hardness is usually represented by a Rockwell score, more on that later as well. Generally it related to how resistant a steel is to wear damage and how long, in knife steels, it can retain an edge. Carbon in large amounts typically adds to hardness. ZDP-189, for example, has a HUGE amount of carbon proportionally speaking at 3.00%. Many non-stainless steels don't even have that much carbon. But with lots of carbon comes lots of potential for rust (hence the 20% chromium content in ZDP-189). Many people have found that ZDP-189 can tarnish or get off colored even without actual rusting because of the high carbon content. I am lucky in that I have not had that happen, but lots of folks have. One caution about hardness--the harder something is the more brittle it is. Diamonds for example are VERY hard and can shatter. Ceramic is likewise very hard but can can shatter if impacted even moderately.
Toughness is a steel's ability to absorb shock and force and still retain its shape. If hardness is best seen in something like ceramic, toughness is best seen in something like plastic or taffy. You can smash a ceramic cup with a baseball bat, but you can't smash a piece of taffy (unless you freeze it, which is really cool to do by the way). Iron is naturally tough and toughness is seen in steels with low Rockwell scores. But sometimes toughness is really important. In a survival blade that has to withstand tremendous forces, such as those seen in batonning wood, a hard steel would just shatter.
The trick with steels is that if hardness is increased, usually toughness and corrosion resistance suffer, and so on with each attribute. It is not possible for a steel to be everything for everyone. The question is how do you balance these traits with your intended use.
Finally a word about other traits of steel. You might see someone say that they want a steel with good wear resistance, chip resistance, edge retention, or ease of sharpening. Most of these different, more specific traits are related to one of the three major traits. Wear resistance is highly correlated, but not the exact same as hardness. Chip resistance is highly correlated but not the exact same as toughness. So once you find a specific task and you have figured out how your steel to performance in the three major areas, further research might help you figure out which sub-traits to emphasize.
Just about any steel chart worth its salt will tell you what elements are included and at what percent. They will also tell you about which elements promote one of the sub traits over another.
Heat Treat and Blade Geometry
If there is any part of metallurgy that is still shrouded in mystery, at least for the general knife-buying public, it is heat treating. There are all sorts of methods and myths. For example a lot of folks believe that hammer blows near the edge of a blade can "pack" the edge. This is 100% horseshit. The density of steel is controlled by the chemical structures and unlike sand, steel is not meaningfully compressible by mere hammer blows. If anyone tells you that they can pack an edge or wants to sell you a knife with a packed edge walk away. They are simply a more sophisticated pick pocket. It IS possible to differentially harden steel, which is what proponents of packed edges seem to claim happens, but hammer blows from a human being will not do it. There is an old fashioned method of differential hardening used in Far East bladesmithing involving clay and selective quenching and there is a vastly more high tech method, seen in these blades, but Thor himself could not do what the charlatans claim they can do when making a packed edge.
But there are folks that can get better performance out of steel through finishing properly. Paul Bos, a long time metallurgist associated with Buck Knives, has an amazing reputation for his Bos heat treat. It is proprietary and a well guarded trade secret, and unlike the packed edge bullshit, it really is better. A Buck Bos-treated blade will perform substantially better than the same steel treated by someone else. My experience has proven this to be true as has the experience of millions of Buck knife owners. Bos is for real. It makes a difference and a substantial one.
Let's cut through all of the murky claims and reputations and look at what heat treating really does. Heat treating steel is design to do one thing--control the chemical (crystal or grain) structure of the steel alloy. When steel is made, the microscopic crystal structures are set into the strongest configuration. As the steel cools, things shift around. Think of the steel like instant pudding--as it cools things shift around and the pudding develops different textures in different places. Steel acts the same way if it is not cooled in a very controlled way--the helpful properties of the steel are distributed unevenly.
This is because steel is an allotrope. That is, it can have the SAME EXACT ingredients but produce very different materials. Diamonds and graphite are good examples of carbon allotropes and a perfect example of why heat treating is important. Diamonds contain a vastly more sturdy configuration of carbon atoms than graphite does and that is why, even though they are made of the same thing, carbon, diamonds are harder than graphite. As the steel cools the chemical composition remains the same, but the chemical structure does not. As the temperature changes, a different allotrope of the steel emerges, one with inferior properties.
Typically heat treating heats the steel up again, redoing the structure as this happens, and then cools in a very, very controlled and precise way. This precision cooling limits the restructuring of the chemical components of the steel and forces them to remain in a closer to ideal configuration. Conversely, some heat treat with cold temperatures (SOG does this). In essence they flash freeze the steel, sealing the more idealized structures in their original positions through SUPER cold temperatures.
Heat treating is important, but it is really hard to judge the effectiveness of one heat treat over another. I don't really like the SOG cryo treating, but that comes from lots of experience. In contrast, I really like the Buck heat treating, but again this is from a lot of experience. Because testing a heat treat requires multiple samples and lots of time, it is hard to convey one method's effectiveness in a two week long testing period (this is one reason I like to update the reviews a year later). Good heat treat is really helpful, a cheap and easy way of improving lesser steels, but it is hard to figure out which ones work and which don't.
Blade geometry, however, can be readily subjected to immediate analysis. Blade geometry is really, really important. How important? I think it is more important than the steel choice itself. In fact, what knife makers call blade geometry I split into two categories in my knife scoring system--blade shape and grind. So a knife with great blade geometry can max out a score of 4, while a knife with great steel can only get a 2. In fact, I think if you know what you are doing you can make up for a subpar steel with good blade geometry. Additionally, if you want to really take advantage of high hardness steels, you can play a little with the blade geometry and get truly spectacular results. Blade geometry is a general category for two aspects of a blade, as I mentioned above--grind and blade shape, and it also interplays with how you sharpen the blade.
Grind is the cutting profile of a knife. There are a ton of different names and grind types, but they all break down into three main groups--concave, convex, and flat grinds. Concave grinds are called hollow grinds. Convex grinds, which typically have no secondary grind or bevel (the cutting edge itself), are known by many names--appleseed grind, Moran grind (after Bill Moran). There are some differences between appleseed grinds and Moran grinds, but in general they are all slightly bowed out if you looked at them from the tip. These grinds are the most difficult to do because they essentially require the grinder to work the blade at all points, never simply resting the steel on the belt. They are also very strong and generally more expensive (because of all the labor involved). Flat grinds are just like they sound--flat. Typically they have a secondary grind as well. Finally, some companies, like Emerson, sharpen their knives only on one side. They may have a typical grind and a secondary bevel on one side or they may grind only one side and leave the other completely flat. This makes for easier field sharpening and a thinner profile, but it is a different experience using these "chisel" ground blades to cut with. Additionally because of the thinner profile they can chip more often than traditional V grinds.
There is a fourth grind, one that takes some three dimensional visualization to understand, called a Besh Wedge. Here is a video of the Besh Wedge in action, as I have found it impossible to describe adequately in words:
It is like no other grind out there and has ENORMOUS tip strength comparatively speaking. I could see this grind becoming more popular in the future, but for now it is relegated to a few Buck blades, a few Boker knives, Meyerco blades, Blackhawk knives and Beshera's own custom knives. It is pretty freaking amazing though, and proof that even after 10,000 years of working with edged tools human beings are still smart enough to innovate.
In terms of grind, I like simple grinds. The multifaceted grinds or the complex grinds can work in specific applications, but for EDC/utility tasks simpler is better. I like Spyderco's full flat grind. I also like SOG's full flat grind. I think the high hollow grind on the Sebenza is great for slicing, but when you have to do heavy duty cutting it can bind. This has not happened to be, but I imagine if you were cutting material that tended to bunch, like lots of cardboard, it could happen. As with most things, grinds follow the simpler the better rule.
In addition to the grind, there is also the blade shape itself. There are literally dozens of blade shapes out there, but for me there are a few that stand out. The classic SOG drop point blade is probably my favorite overall shape, seen in my Flash I review. The long primary edge coupled with a nice belly provides a lot of cutting power. I also like the leaf shape blade from Spyderco as well as their saber type grind seen on the Delica, Endura, Military, and Paramilitary. All are excellent cutters and easy to maintain.
I dislike recurves of all kinds. They are unnecessarily hard to sharpen and though they obviously give you a longer cutting edge in a shorter distance, it is not worth the hassle in my opinion. I also dislike tanto points as they do not typically perform roll cuts well. A reverse tanto, seen in the Benchmade 940, for example, is a very good design, incorporating the tip strength of a tanto and the belly of a drop point.
Again, as with most things, the simpler the better.
The Rockwell Scale is not so much a scale as it is a method and a set of different scales. There are four primary Rockwell Scales and the one used in the knife industry, the Rockwell Hardness C scale, is for medium hardness materials, such as hard steel. Rockwell A is used for tungsten based materials, i.e. VERY hard stuff. Rockwell B is for softer stuff, like brass. All of the scales work the same way. They are measurement of a material's resistance to deformation. A tip or cone of a known hardness material, usually steel, is attached to a device that looks like a drill press. The material being tested is laid in a bed beneath the tip and the tip is pressed down into the test material. The depth of penetration into the test material is compared to other known hardness materials and the test material is then given a score.
Most cutlery steels score somewhere between 45-70. A 45 would be very soft steel, used in very high impact operations like a ax or a maul. Some of the hardest utility steels score closer to the 70, with ZDP-189 hitting 66 on a regular basis. Typically the scores are given as a range, usually two points, so ZDP-189 will be listed as a HRC of 64-66. This range reflects the variation between steels of the same make and it also reflects variations in test results.
Steels of a particular type have a "recommended" hardness. That is, the maker of the steel has a target range in mind for the hardness of their steels and they believe that this range gets the best performance from the steel. Crucible, for example, usually recommends between 58-62 HRC on S30V. Knife makers can increase or decrease the hardness of a steel based on their heat treat method. Bob Dozier, for example, gets very high hardness out of D2, more so than others, based on his high treat method, which is, of course secret.
I think that utility blades benefit from a high hardness because it means less maintenance. I like ZDP-189, but I have seen some blades with micro chipping on the edge. It can be fixed, but if you prefer no edge chipping, S30V or S35VN might be the way to go. They are softer but still capable of getting hard enough for really steep cutting angles. I also like D2, but it requires anti rusting maintenance, a tradeoff for the harder steel. In a survival blade, I like 1095 from ESEE which hits in the mid 50s on the HRC.
Hopefully, this second piece has been interesting and useful. Next up, a list of steels and explanations of their properties plus a peek at my personal favorites.