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This simple but informative guide to the basics of steel is limited to the discussion of useful bits which a knife collector could find informative and which would expand in a beneficial way the knowledge of someone in the early stages of knife collecting. I am not a Metallurgist, nor do I pretend to be a steel “Expert”. When it comes to knife collecting, such is not necessary. However a working knowledge of the basics will assist in making your collecting decisions and will go a long way toward filling your knife stable with thoroughbreds. |
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What is steel?
At its most basic, steel is essentially Iron combined with Carbon. Other alloys can also be added and the presence of these alloys will add characteristics to the material which are useful according to the purpose for which it is being created. One alloy may add a degree of corrosion resistance, while another increases toughness. This recipe has been changed and modified endlessly throughout the centuries as the quest for the "Ultimate Steel" continues. While no such thing may actually exist, this process of discovery has left us with a variety of steels that are well suited to different tasks and are chosen according to the anticipated use.
What is Stainless Steel?
Stainless steel is steel which contains the element Chromium in sufficient quantity that the steel takes on a rust resistant property. This quantity has not been firmly agreed upon, however is generally accepted for cutlery purposes to be within the 13% range. Stainless steel is known to "Stain Less", but can still show some evidence of corrosion if exposed to particularly acidic or corrosive environments. In other words it is corrosion resistant as opposed to corrosion proof.
What is Alloyed Steel?
Here we begin to get into the nuts and bolts of cutlery steel development. Alloyed steel is a general term indicating a substance in which additional elements have been added beyond those necessary to elevate the material from iron to steel. In other words, Carbon may be present (which creates steel from iron) with Chromium added (to increase corrosion resistance) and Vanadium added (to increase hardness). Further additions will change the material correspondingly and in this way the characteristic of the steel is engineered to match its intended use.
As one element is added to positively enhance a certain property, another is often negatively affected. For example, adding more Carbon will increase the hardness and thus edge retention of a knife, however as this property goes up, the property of toughness goes down. Toughness is the ability of the blade to withstand shock and impact without cracking, chipping or snapping. So while the knife may have been made to take and keep a very sharp edge (desirable), those properties have also created a very brittle blade (undesirable). To counter this another element may be added, but while helping, it too also presents other issues and thus the dance continues.
Again, achieving just the right balance of trade offs between these different characteristics is the aim of the steelmaker and the "right" balance depends on the intended use of the steel.
What is a "Rockwell Hardness?"
This measurement, often seen in the technical spec. write up of knives, is a measure of the hardness of the steel used on the knife. The number displayed is a result of a test in which a machine known as an indenter applies a load to the steel and the depth of the indentation made is measured. How deep or shallow the indenter has been able to penetrate into the steel under a predetermined load will reflect how hard that steel is.
Most knives will have a Rockwell Hardness (or HRC) displayed as a range in the upper 50 or lower 60 range. It is typical to see a knife spec. stating HRC = 55 - 57. Ranges in the mid 50’s would represent a softer steel than ones in the upper 50's and some knife steels are hardened into the 60’s. The harder the steel is the longer the knife will hold its edge. However steels at this level of hardness will also be more difficult to sharpen and will be more brittle and susceptible to chipping.
Laminated Steel
Laminated steels attempt to solve the tradeoff between toughness and hardness not through recipe ingredients but by combining steels that have different strengths into a single functional unit. The typical laminated steel has a core of very hard (and brittle) steel which provides excellent edge holding ability. It is then wrapped with a softer steel which provides toughness and flexibility. The spine and both sides of the blade are the soft steel with the harder steel peeking out along the edge. A laminate line may be seen along the edge where the two steels meet.
How Does Heat Treatment work?
No discussion of blade steel could be had without addressing the heat treatment process. Every bit as important as the elements of the steel recipe is the heat treatment the steel receives. While much could be said on this science, my goal is to provide a basic understanding and thus will be a brief overview.
The purpose for heat treating steel is to modify the mechanical properties of the steel. When steel is heated to a high temperature (about 1350 degrees) it begins to change its molecular structure from a ferrite state to an austenite state (and thus losing its magnetic properties). This state is desirable and is the first step in altering the steel’s properties to perform as needed. Depending on the amount of Carbon in the steel, the temperature may have to be higher or lower to achieve this state. Once the steel is in an austenite state, the performance properties may be determined by the rate of cooling and the medium used to accomplish the cooling.
Cooling, or quenching may be accomplished quickly or slowly and each way will affect the properties of the steel. The quench medium may also vary from simply air cooling to water quenching, oil quenching or other means. In a very general sense, the faster the steel is cooled, the harder it will be and the slower it is cooled the softer it will be. A rapid cool down process results in a martensite structure in which the molecular structure of the steel is very tight and fine, but with high internal stress. A slow cool will produce a molecular structure that is coarser and without the high internal stress.
The manufacture of knife steel involves the process of normalizing, annealing, quenching and tempering. Prior to working the steel it is desirable for the steel to be in a stress free state and this can be done by normalizing which is the repeated slow, even, heating of the steel to non-magnetic and then allowing to air cool. This should be done several times (taking care not to over-heat the steel) and will result in the even dispersion of the iron and carbon, with a uniform microstructure inside the steel.
Once this is accomplished the blade may be annealed to soften the steel and prepare it for shaping and working. Annealing is the heating and very slow cooling of the steel which results in a soft, ductile steel that may be easily filed, stamped, etc... To do this the steel is heated to non-magnetic and then buried in a media such as hardwood ash or vermiculite which acts as an insulator and will permit the steel to cool only very slowly, much slower than air cooling. After cooling in the ash (24 hrs), the blade may be worked to its final shape.
The next step is quenching in which the blade is re-heated to non-magnetic and then rapidly cooled, typically in oil heated to around 150 degrees. The purpose of this is to re-harden the blade to a useful knife hardness. The trick is to cool the blade rapidly enough that martensite is achieved and not something short of that, such as pearlite. The quenching process is the only one in which a drastic change of temperature is desirable. In the other steps it is important to maintain the temperature evenly, within specific ranges and without large shifts.
Assuming the quenching has been properly done, the blade is now martensetic, but with high internal stress that can leave the blade prone to breakage. To resolve that problem and create a tough yet still hard blade, the steel is subjected to tempering. Tempering is the re-heating of the steel, but to a much lower temperature, perhaps around 400 degrees for one hour. This will permit the martensite to form without cracks and become a stable internal structure.
There are many variations to this process and different opinions as to what procedure, material and temperature range is best. Additionally, depending on the type of steel being worked, with the different combination of alloys, one process may be preferable to another. Finally, the purpose for which the steel will be used is another variable, further increasing the possible permutations.
What is "Powdered Steel"?
This is an expensive but high quality process of combining precise amounts of pre-selected alloys that have been physically powdered, for the purpose of engineering a steel with the exact quantities of each alloy in an exactly even dispersion, which results in a very stable and uniform structure. Once the alloys have been combined they are subjected to heat (Sintering) where the steels self weld together and form a highly pure and uniform product.
What are the elements of steel?
If you have ever looked into what steels are made of you may have seen a bewildering set of elements named as the primary components. Without some knowledge of what good each of these does, it can feel like trying to read Greek. Each element can be added in precise quantities to optimize its beneficial properties. These below are some, but certainly not all, of the more common elements used in steel and the reasons why.
Carbon:
This is the primary element which is added to iron to create steel. Carbon produces a hardening affect in the alloy and the higher the content of carbon in the steel, the greater the hardness will be. Getting too hard though is not a good thing as this can result in a brittle blade which chips easily on impact. Carbon in the right amount allows a blade to be sharpened to a very keen edge and hold it well.
Chromium
This is the element which when added creates a stainless steel and is generally required in an amount of at least 13% to be considered true stainless steel. Chromium added to steel increases the blades hardness and ability to withstand staining and corrosion. Adding too much chromium can contribute to a brittle blade, so a balance must be struck. A "stainless" steel blade can still stain and even rust, although not at the rate a pure carbon steel blade would as the chromium aids in corrosion resistance.
Manganese
This element also contributes to hardness as well as strength and wear resistance. Manganese improves a steel during the manufacture process and most knives have some fraction of manganese in them.
Vanadium
Like Chromium, Vanadium will also increase hardness but also will improve a blades ability to take a very sharp edge. Vanadium improves stability in the steel at high temperatures and is found in hard tool steels. Vanadium can result in Rockwell hardness levels above 60.
Nickel
This increases the toughness of a steel and may play a role in corrosion resistance.
Silicon
Adds to the strength of a steel and assists in the manufacturing process.
Boron
Added in small amounts to increase hardenability. Boron steel is often added to steel for hard use implements such as spades or other tools.
Definitions:
Edge Retention
Simple to define, edge retention is the degree to which a knife will maintain a sharp edge when used. Not so simple is achieving such. The edge retention of a pocket knife will be achieved through a different steel than that of a machete and the medium in which the cutting will be done plays an important role. If cutting through tough, fibrous material is to be done, then wear resistance will be critical to edge retention. If the cutting is to involve dense materials such as wood, then strength will be important to edge retention. It’s about more than just making a sharp edge.
Toughness
The ability to withstand impact and sudden shock forces that may be involved in activities such as hacking, chopping, throwing or shearing forces encountered through prying and twisting are what is in view here. A very hard, brittle blade would chip or snap under these conditions and would not be considered "tough". Although perhaps counter-intuitive, the toughest blades are those made from a softer steel which can absorb these forces without damage.
Wear Resistance
Closely related to toughness, wear resistance is a measure of the amount of abrasion a blade steel can withstand without degrading effect. A rolled edge or scratching of the steel is in view here. Adding Carbon will increase a steel’s wear resistance.
Corrosion Resistance
The ability to resist rust and pitting. Higher degrees of Chromium added to the mix will enhance this ability. Even full stainless steel (at least 13% chromium) can still stain and corrode, although at a much slower rate than a pure carbon steel. H1 is a non-carbon steel developed to be essentially rust proof.
Tensile Strength
This is a measure of the ability of a steel to resist a force attempting to bend or break it.
Steels
420/420J2
A soft steel and therefore quite tough but not very wear resistant. This steel will suffer when used on abrasive materials and is not known for holding an edge well. Inexpensive to manufacture and thus popular on low end knives, this steel is useful and has it place, but not as a top end product. Sometimes used as the softer outer steel on a laminated blade (SOG mini-Vulcan).
440A/440B/440C
Better than their 420 cousin, the 440’s are a series of steels with increasing Carbon content from A to C. 440A is recognized as a good but minimum quality general use steel (better than 420). The 440B is better and thought quite well of, and 440C is excellent. Each is known for good toughness and edge retention and has been used to good effect in knives of all sorts for many years.
AUS6/AUS8/AUS 10
Similar to the 440 series but slightly better in the sense these steels do have a Vanadium content which the 440’s lack and therefore can acquire a keener edge. The AUS series falls into the same categories as 440 with the good/better/best progression. AUS8 is good, mid-grade steel and AUS10 competes with steels like ATS-34.
1055/1095 Carbon
The "10" series steels have an increasing amount of carbon in them as the number increases and a corresponding degree of wear resistance as well. 1095 is the default knife steel out of this series (Tops Tom Brown Trapper). Items such as swords or hatchets are often made out of the lower carbon 1055. The Cold Steel Vietnam Tomahawk is made from 1055.
ATS-34 & 154CM
These two steels are quite similar to each other. Both considered premium, high quality steel for a knife. These steels will hold an edge very well yet still retain a good degree of toughness. Not as corrosion resistant as the 400 series, but still considered an upgrade in comparison.
VG-10
A stainless steel with a high Vanadium content which can be sharpened to an extremely keen edge. Good toughness, wear resistance and edge holding as well.
S30V
Specifically designed to be a cutlery steel, this steel has excellent toughness as well as wear resistance, hardness and edge holding. An all around excellent performer and is found on the best production knives.
O1
A good steel that is easy to work with. O1 takes an edge well and holds it. It is also quite tough but can succumb to corrosion more easily than others.
D2
Although not a true stainless, D2 has enough Chromium to thwart corrosion to a good extent. This steel excels in toughness and wear resistance and with a good ability to resist corrosion it makes for a fine choice.
H1
A precipitation hardened steel, H1 is made with nitrogen instead of carbon and thus is impervious to rust. It is the choice for diving knives which will come in contact with salt water or other highly corrosive environments.
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