Knife Steels-Material Science
 

I wanted to start a thread about knife steels, kind of a materials engineering topic that would talk about steels used to make knives. This will range from car springs to particle metal steels. We can see if I've learned anything teachable in the last twenty five years of working this stuff.

My hope is to dispel some myths and aid in the maintenance of whatever knife you have or might have to make useful.

The biggest difference between knives is any knife over no knife at all. I'm not going to judge or promote, just trying to help clear up some common misconceptions, if there are any.

More to follow and starting questions welcomed.


In a recent conversation with a QP I learned that field welding can be important. When we get to carbon contents in various steels, I have some useful stuff there too.

There will be as much Redneck Engineering as possible here.

Finally, something I know a little about.

If we have not beaten it to death elsewhere, for a starter, I would like to see a little about the older carbon steels, tool steels, stainless and near stainless steels, heat treating, hardening, blade profiles, thicknesses, grinds, bevels, etc.

Just a few thoughts.

TR

Mr. Harsey,
When you temper your blades, do you use oil or nitrate salt baths?

Easy question first, All the steels I use here are air hardening and no oil or salt baths are used or needed.

Reapers question might take volumes but I will use redneck engineering to narrow down the answers.

That was not a single question, but was a variety of topics that I would like to know more about.

Hope that at least some of it is in the direction you were looking.

TR

Yes, you asked a very good series of questions and my response referring to that as a single question was an attempt at understatement.
Yes this is the direction I hoped to go. Well done.

First basic, steel in it's most basic form is iron with carbon. The higher the carbon content, the more the steel can be hardened by heat treating which involves heating a given steel to it's "transformation point" and a rapid, controlled cooling to lock the carbides into a particular type of structure. The more carbon a steel has, the more potential it has to be hardened.
An even simpler definition of heat treating steel is "changing the physical characteristics without changing the chemical composition".

The three most basic categories of steel are Mild Steel, Medium Carbon Steel and high carbon steel. There are many alloys of steels within each of these groups.
Here are some simple examples of the carbon steels:
Low carbon steels: structural steels, car bodies, common steel purchased for welding AKA "mild steel", etc.
Medium carbon steels: Firearm barrels, hammers, gears, axles, spring steels, etc.
High carbon steels: Knives, machine tooling, punches, forming dies, drill bits, milling cutters, saws, spring steels etc.

more to follow.

note: I find metals fascinating, and as a mechanical engineer I need to know all about their charactersitics, hence the questions, but I have little working experience with high carbon steels like Mr. Harsey uses every day. So...

What is the typical AISI number of steel you use Mr. Harsey?

Maytime,
I have just reviewed my material data sheets for the steels I use here and cannot find any AISI designation and to the best of memory, I've never seen one for either the 154CM I've used so much of in the past or the CPM S-30V we use now.
Both steels are made by Crucible Materials Corporation in Syracuse New York.

Edited to add: Maytime, so you can jump ahead: www.crucibleservice.com

The topic of alloys is very interesting. Historically some steels were much better than others because the naturally occurring ore dug from the ground contained alloys unknown to the steel makers but beneficial to the performance of the finished blade.
The Indian Wootz steel (think fine Persian blades) of old is a good example of this. When the ancient mine, the original source, of this iron ore played out, the quality of the swords and daggers diminished but no one knew why.

The first alloy of Iron to make steel is carbon. Both high carbon steels and tool steels may contain similar amounts of carbon but not used for the same jobs.
The difference is in the degree of refinement during steel making.

Many truck springs contain .90 carbon content but would not be acceptable for some tool and die jobs needing a tool steel with the same carbon content. This is because the truck springs contain some impurities and inclusions like slag that still make great springs but do not have the grain refinement and purity of manufacturing to hold an extremely fine edge in a hard use production tool.

I have seen some decent knives made from truck leaf springs.

Is this an alloy which has adequate potential to be a good (economical) knife steel, if hardened and heat treated properly?

TR

Good question and the answer is YES, spring steels, like 1095 can make good knives. They are very forgable and with just a little care can be field hardened and tempered using only the simplest of tools and fire.

I've seen video of full sized jeep springs made into big blades by heating the entire spring, hanger ends and all, over an open fire to orange heat and hammered over a set chisel make the rough cuts then forged to a finished blade. All this with a fire, hammer and crude anvil.

Be careful where you park your jeeps.

If your trying this yourself and in doubt of exactly what steel you have, make a test piece from the steel, heat it up so a magnet no longer sticks to the surface (good indicator of transformation point in simple hardenable steels) and try quenching in warm (100-120 F) 20 weight oil rather than water to minimize the chance of cracking. If it gets hard enough to become difficult to file then the oil quench works. If not try water quench next. If neither of these work, do not proceed with this steel.
Always temper simple tool steels for at least an hour or two at 300-400 F after hardening.

A knife blade can be forged to usable net shape without grinding and made excellent by some simple draw filing.
Yes I've done this before with good results. No real shop needed only some simple tools.

I just received a hard use brush and wood cutting knife from Taiwan, made by an indigenous traditional knifemaker there from leaf spring steel. The knife is very well made and a nice example of direct craft with the file marks part of the design.

My dad had a book, Knights Modern Seamanship. It was written in the 30's. It was a great resource book. It had directions on making a shief knife using a worn file.

Bill do you think old files would be a good source for a knife blank?

Hollis, A file will make a knife but it's a tough way to go.
Most files are designed to cut steel and have a very high carbon content for maximum hardness. This makes them hard to work (make into a knife) and they tend to be brittle in hard use.

I'd rather go with the spring.

Edited to add: Hollis, by a good source of knife steel, around here that means a steel I can work with in a timely and resource effective manner. Sometimes "free' can cost you a lot of time and money.

A better answer is yes they will make a good knife for some applications but the difficulty of working the file into a usable knife blade would put it down on my list of materials to use.

And then there is passivation in the Stainless Steels!!!:D

Bill, thank you for the reply.

H.

I was gonna get here later but since you brought it up...

The trait exhibited by stainless steels not rusting is known as "passivity" and this refers to the material corroding to a point that a film is formed which acts as a barrier to further corrosion.

"Passivation" refers to a process where the stainless steel is, for example, placed in hot nitric acid to remove all surface contamination so nothing is left over from the manufacturing process that will act like a battery on the surface of the steel to set up corrosion. This is not a good process to do on the kitchen stove top.

The term stainless steel covers many types of alloys, the ones we are interested in are the hardenable or austenitizing steels.


Yes there are true tool steels that have good stainless abilities and these are also referred to as "stainless" even though they have little in common with non-hardening stainless steels.

Edited to add a footnote on stainless steels:
High temperature tempering of stainless tool steels can result in a loss of corrosion resistance because chromium carbides continue to form robbing the matrix of usable chromium. See next post.

tool steel basics
 

We all use tools made of one type of tool steel or another.

Entire libraries are dedicated to this topic and I'm not going to try and fill one up.

The tool steels we use are iron plus alloys carefully chosen by steel makers for certain performance characteristics combined with the ability to manufacture and cost considerations.

The metallurgists I work with call this "tool steel alloy design". They break this down into two basic parts, the matrix and the carbides.

Think of the matrix as the mortar and the carbides as the cobble stones.
Some alloys become the matrix, others produce carbides and some do either depending on the amount used or the type of heat treat.

This is the stuff that determines the traits we choose knives for, like edge holding (abrasion resistance), toughness (strength and ductility)and stain resistance

How a tool steel is made and heat treated determines the size of the individual carbides.

Anyone care to guess how this impacts knife blades for good or bad?


Not a test, just a conversation.

OH..OH.. I know!!!
Sorry, I took the class, so I will wait for other answers:D
JJ

John, Thanks for holding on.
Let's do this different to make it easier, maybe.

Please start naming the knife steels you've heard of or used.
Everyone is welcome to jump in.

Ok, I 'll jump.. My thoughts would be finer is better. Logic would be a more homogenous steel. But I am pretty much guessing. Isn't the difference being disccussed here, pretty small?

To name just a few I have seen, A2, O1, O2, D2, L6, W2, M2, 420, 440A, 440B, 440C, 154CM, ATS-34, 1045-1095, 5160, S-30V, and Damascus. Not steels, but also used for blades, Stellite, Talonite, Titanium.

TR

Hollis,
Yes usually finer grain is better and your logic is correct but not always easy to do in tool steels.

I think the differences between tool steels are surprisingly large and are quite noticable to the user in terms of overall performance and sharpenability.

The performance thing gets important when the knife becomes a life critical tool in an emergency.

I have a book some where that is on heat treating metal. I have a decent size furnace, Nicked named "hades". I think I understand what you are saying. The difference of a small %, or how quickly the quench is, or heated too has a tremedous effect from brittle to too soft (as in not holding a edge).

Ok.. here is a list of familiar knife steels. The asterik next to some refer to those that are multi task steels, both carbon, alloy and stainless that are used if other applications than just knife steel.

O1*, O2*, A2*, D2*, S7*, W1*, W2*, 440series stainless' A,B,C *, AUS-8, CPM-S30V, CPM S90V, 154CM, ATS34, BG42, 4140*, 4142*, 4145*, 1095*, 1018*, 300 series stainless (303,304,305,316 are used in knife parts, but not blades because they do not heat treat but will work harden when you least expect it. Just ask Bill) A36 is a good carbon mild steel for making other parts, High Speed Steel used in tooling is sometimes used by some makers for it's hardness. Sandvik series 12C27 is an example of a designer steel that is easy to machine in the annealed state, but is tough as hell when hardened which explains the manufacture of machining tools using this material.
Those that like a challange there is also Inconel. This is one of those steels that will belittle the best machine.

Good list of various steels, I'll begin to categorize soon.


Gunnerjohn,
This is complicated enough without mentioning other materials not used for knives. My poor little knifemaking brain is going to get a stress fracture if you keep this up.

The Inconel type metals (high nickel, chromium, molybdenum and sometimes niobium alloy with very little iron) have no knife blade application that I've ever heard of.

These alloys are designed for being resistant to specific types of corrosion, like in nuclear power generating plants.

Back to knife steels.

Mr. H, how about laminates? Say, as are used in traditional puukkos, with the outer layers soft to allow for a more flexible blade, but the inner layer (the one with the edge profiled on it) being harder to maintain its edge? How does one manufacture them without running into delamination challenges during use?

Good question.

Laminated steels are among the oldest known existing artifacts of iron work ever found in the greater region we now call Syria. (IIRC, from Scientific American article, unkown date) . The oldest known smelted iron ore is said to come from the Sinai Peninsula found in and among ancient copper smelting sites and preceding all this is the near pre-historic use of found meteoric iron by ancient peoples to fashion into tools. This meteoric iron is distinguished by it's nickel content.

High carbon steel was very precious when hand smelting Iron because it either had to be very carefully sorted from the original cooled melt or forged in in a reducing fire of charcoal (source of carbon) to gain enough carbon from "carbon migration" while the steel was at heat. This was some very resource and labor intense work.

I also think that the technologies of making ceramic wares was the source of the technology to smelt the coppers, bronzes and then steels.

To use this hardenable carbon steel frugally, it was laminated or forge welded to another lower carbon steel body to make the entire tool.

The actual "weld" involves heating up both pieces of steel together to just under melting temperature and quickly hammering them together before they cool to the point of not sticking. Fluxes of glassy material are used to float the oxidized steel scale off the surfaces and get a good weld. Steel reacts very quickly with oxygen at high temps and this will prevent a clean strong weld.

When done well, the two types of steel become one piece and can take some very hard use without cracking because the softer outside iron supports the hardened high carbon core that makes up the edge.

The same principles apply to Japanese sword making as well as many carpenters and timber frame tools like old chisels and slicks that can still be found in second hand stores and occasionally estate sales.
Look for the two different colors of steel in the tool body.

If memory serves, we can still buy new laminated wood working tools from some sources, especially fine Japanese woodworking tools.

The traditional Puukkos are a good example of this being done well on a production basis. I think the Puukkos steel is roll forged together in the mill then manufactured into knives. They are famous for toughness.

We also have some custom makers of modern laminated steels that are doing amazing work. My friend www.devinthomas.com comes to mind, he makes some exotic stuff in the high order stainless tool steels.

All Those Steels
 

Some overview before we go into details later.
You kids see there are a lot of different types of tool steels used for knives.

Many of these tool steels were developed for some other hardened tool steel application than knives, some steels are further developed specifically for blade steels and not just for hand held knives but also stuff like wood chippers, wood milling, steel, plastic, paper and other material cutting blades and industrial scale food machinery as well as the entire range of tooling like drill bits and milling cutters and saws used to machine steel.

The reason we see so many different types of tools steels is that alloys are chosen for particular traits in specific applications or modified as some alloys become scarce or difficult to acquire because of world politics.

The first thing a knife blade has to do is be hard enough so the cutting edge doesn't roll over or break off when we try and cut something. Tool steels need enough carbon to form carbides when hardened.

Other alloys form different kinds of carbides and add to the strength of the matrix but the first and most important alloy of "high carbon" tool steel is carbon in carefully controlled amounts varying from .75% to well over 1% and in the class called "ultra high carbon" a steels carbon amounts can be from 1.25% to 2%.

(darn phone, have business call from Italy, be back later)

Edited to add, It's later:
The main areas of performance knifemakers and users are concerned with are:

Hardness
Toughness
abrasion resistance
stain resistance

These are all mechanical traits of a given tool steel that are both related to each other and quite seperate.

Anyone know the difference between hardness and toughness?

IIRC, hardness is the resistance to penetration of the surface of a material by a hard object. Toughness I believe is ductile or yield strength. Could be wrong.

Correctomundo Sir Maytime.


Hey, I didn't know you guys had computers up in Alaska!

OK, back to work.
The significance of the hardness and toughness thing goes to not just what alloys are in a tool steel but how the steel is made because this is how carbide grain size is controlled. Carbide size has much to do with toughness, edgeholding and toughness. You will see this material again.

Here is something very important to know about knife steels. The hardness, usually stated in a Rockwell "C" Scale number, is not a direct indicator of blade quality or edge holding. In other words, the higher the indicated Rockwell hardness does NOT mean the knife holds a better edge

The first step would seem to be selecting an alloy that has the correct elements with the properties to produce the characteristics we are looking for. Beyond the iron and carbon that define steel, we could add varying quantities of chromium, nickel, cobalt, tungsten, vanadium, molybdenum, niobium, titanium, zirconium, manganese, silicon, or even copper. Each of these, in varying quantities, can give steel specific characteristics. The best known is the addition of chromium to make "stainless steel". Too much or too little, and you get something else.

Once we have the properties we are looking for in the chemical composition, the next step would be to properly harden it to develop the crystalline structure or grain that we are looking for in the various portions of the blade. Steel, even the same alloy, can be heated and cooled to produce everything from a useless paperweight to a superb cutting instrument.

I would think that the grain would be affected by the austenitizing or martensitizing. As noted, steel changes structure and therefore properties during heating and cooling. Some characteristics would be more desirable than others, but you might find both helpful in different areas of the knife. Look at a single-bit axe head with both a cutting edge and a hammering edge on opposite ends, each needs specific properties, yet both are part of the same piece of steel alloy. On a knife, you might want a tougher spine, and a harder edge.

Steel exists in a variety of crystallive forms, Austenite, Bainite, Martensite, Cementite, Ferrite, and Pearlite. Isn't the goal to try and produce more Bainite and Pearlite in the back of the blade and Martensite along the cutting edge? That would seem to be the solution to the toughness vs. hardness quandary. You could ruin a well-selected steel alloy for your knife by improperly heat treating it, or optimize a lesser steel for your purpose with a good heat treat.

Obviously, the knife cuts on its edge. You can use a knife for a pry bar, but it is not optimized for that. The edge of a well honed knife is not smooth, as most people think, but is very jagged. That is how it cuts. When the carbides and crystals at the edge are very hard, but the steel matrix is not, you get an optimized edge for cutting, yet is easily sharpened. The Crucible S30-V has the right carbides to work for this process. It is up to the knifemaker to expose them at the edge, and bring the hardness and toughness to the different parts of the blade, as required.

IIRC, the Japanese bladesmiths produced some very fine blades with little knowledge of metallurgy or chemistry by proper heat treating. They kept the spine softer and tougher, while creating the harder, better curtting Martensite along the edge by an advanced heat treat. As I understand it, the blades were generally straight prior to heat treating, and the curve of the Japanese sword occurred due to the properties and structure of the steel changing along the different parts of the blade durting their heat treat.

Just a few thoughts, probably in over my head, but I got that way by listening to smart people. Back to my cave.

TR

This question will likely show my ignorance but.....are there any blade makers doing the old Japanese techniques that we've heard about that give 'legendary' strength and edge to a blade...or is it all myth.

ss

Reaper,
Highly simplified response to your comments and question.
Each type of tool steel has a range of hardnesses it can be hardened and tempered to and as you indicate with the example of the Axe, you can have two (or more) different hardnesses in the same finished tool.

Differential hardening is easier to do with water and oil quenching hardenable tool steels but does happen to some smaller degree with the air hardening tools steels which tend to be much more homogeneous in the heat treat because the quench time isn't quite so critical.

While on the topic of hardnesses, they are many wrong ways to get the "correct" Rockwell hardness scale reading that result in a steel not being as good as it could be. Rockwell hardness is a test to be used in the shop under controlled heat treat conditions in order to be understood and used well.

The Japanese sword smiths, who mastered the craft well over 700 years ago, may not have understood the science of what they did but they fully understood the process that produced the best results and did it very well.

Remember, this is when all steel was "hand made" with charcoal fired melts and forges way before big power sources and mills.

Swatsurgeon, Yes we have modern makers of traditional sword making techniques that are doing very good work.

Yoshindo Yoshihara (check this next link out!) http://www.legacyswords.com/Yoshiharanihonto.htm from Japan comes to mind. My friend Bob Lum (knifemaker) met with him last week in New York City at the big knife show there.
Note the well defined differential hardening in the images.
Note the price the set of swords sold for at the end.

Bill:

Can you comment on forged blades, versus stock removal, and the effect of the forging on the steel grain and crystalline structure?

TR

Good question.
Historically the only way to refine a steel was by hand hammer forging (drawing out), re-stacking the steel and forge welding solid then forging it again, repeat until good results which if you made swords meant making swords that didn't break or bend easily.

This concept for steel refining was done on a much larger scale in the big steel mills into the early 1900's. English Shear Steel was a good example we know about in Oregon because this was how the plate for really big circular saw blades used in the sawmills was made.

Forging breaks up the large crystal/grain structures that resulted during the original melt of steel cooling into an ingot. Alloys tend to gather together during the initial cooling and form large crystals that would be too large and brittle to make into strong steel.
Now we are using steels that are highly refined during the steel making and forging (rolling into usable bar stock or sheet) processes. I seriously doubt any positive change in the steels grain structure can be accomplished by forging many modern tool steels.
Many of the top metallurgists in the country will back me up on this.

With some of the steels we use forging can easily cause more damage than good and the best the bladesmith can hope for is not ruin the steel when they forge the exotic (highly alloyed) steels. This is because the temperature range at which the steel can be forged is both high and narrow. Forging the steel outside these heat ranges can/will cause damage like the grain tearing apart resulting in cracks that will propagate during heat treat or use.

We are lucky to have many accomplished bladesmiths working in the world today and I don't want them hunting me down for any perceived slight to their craft. Keep working guys!
Industry will never duplicate the fine craft of forge welded patterns and making steel into blades like these guys do using the straight high carbon steels and I have the highest respect for the work being done.
This is what Yoshindo Yoshihara and his brother are doing, keeping a fine craft alive. They do amazing work at the forge.

Edited to continue:
Stock removal is the shop practice of knifemakers (myself included) of purchasing a particular type of steel, chosen for a range of best possible performance characteristics, which is roll forged into a specific shape and thickness of either bar or sheet.

Upon arrival in the knife shop the steel is then formed into a knife blade by cutting, abrasive grinding, machining, heat treating and finishing.

These steels have undergone considerable forging processes under tightly controlled temperature and atmosphere conditions in the mills before they arrive in the shop. Optimum heat treat and control of manufacturing process is the key to good results with these steels, not forging in the knife shop.

Before this rodeo gets out of control with too much detail and I lose all interested readers, I'll try to explain why we are going into this detail.

A knife cuts well because of how the cross section of the blade is shaped or what knifemakers call blade and edge geometry.

The thinner the blade and acutely sharper the angle of the edge, the better it will cut.

The limiting factor on blade geometry is the strength of the steel and even the size of the carbides.

When we have a given blade steel with optimum heat treat and we need it to stand up to increasingly tougher jobs, the only way to make it stand up to the demands is to increase the thickness of the blade and the edge. Think of how swords are different than paring knives.

About carbide size
I've worked with the tool steel called D-2 for many years. It's a common planer blade steel used in my regions sawmills. People keep bringing me handfuls of used planers blades made of this stuff. I keep thinking it might be useful for something someday. The pile keeps growing.

Much of the D-2 I've actually made into knives and master drill/machining patterns was purchased new. Here is the thing with D-2, it has the largest carbide size of any tool steel i know about. This is a problem because you CANNOT place an extremely fine or acute edge on a blade made from this without having carbide chip-out on that edge.
Edges on D-2 have to be less acute of an angle.
The macro grain on this steel can be seen thru the grinding and buffing stages of finishing the blade.

There can be very large and discernible differences in how tool steels perform when made into knives.

Edited to add: D-2 is a legitimate knife blade steel. Some good knifemakers use it very successfully.
Here is the difference, you cannot grind D-2 into a super thin edge (like .015 to .007 thousandths of an inch thick before first sharpening) and expect it to hold up. It has to be left a bit thicker and I'd be comfortable with an edge thickness more like .035 thousandths of an inch thick for a folding knife or fixed blade hunter.
D-2, because of it's big carbide/grain structure does not have the transverse bend fracture strength of other tool steels so the edge will not stand up as well to prying or side load.

Carbide grain size can be measured and documented with a scanning electron micrscope. I do not have one of these in my shop but the metallurgists whose phone no.s I keep handy do.

One of the finest grain tool steels available (not stainless) is called 52100 and is made by Timken-Latrobe here in the USA. This is a bearing and bearing race steel and be gotten very hard by heat treat. This steel has gained favor among many bladesmiths.

Anyone care to guess how a steel testing softer on the Rockwell hardness scale can out perform the 52100 in edge holding type cutting tests?

Does this acount for the joke about D2 taking a crappy edge and holding it forever?

I have also heard it has a rep for not polishing well due to the orange peel surface.

The D2 I have carried is superb.

BTW, I had a scholarship from Latrobe Steel, they were good people.

Timken is famous for their fine bearings. I had no idea they had merged.

TR