CPM-20CV: A Great Choice for a Knife Blade
CPM-20CV: A Great Choice for a Knife Blade
When a knife maker plans a new design, the selection of blade steel makes a critical contribution to the success or failure of the product. Blade size, blade deployment, handle materials, aesthetic considerations, the ergonomics of handle shape and balance: All these choices affect the outcome of the design process, but perhaps no single decision carries more weight than the selection of steel alloy to form the blade itself.
Four reasonably objective criteria and one somewhat subjective measure of performance make the list of properties that knife makers evaluate when they choose a steel for a new product. Some of these properties trade off with one another on opposing ends of a single continuum, making it impossible to optimize all of the qualities in any single steel alloy.
1. Hardness measures how well a steel resists forces that seek to deform it. Since the early 20th century, scientific measurements of material hardness have relied on a set of laboratory tests called Rockwell scales, named after Hugh Rockwell, co-inventor of the test apparatus. Among those scales, the C scale is used to represent hardness values for knife steels, with numeric values followed by the abbreviation “HRC.” In alloy hardness testing, a specified force drives a diamond-tipped implement into a sample of the material, and the depth of the resulting dent reflects the material’s ability to withstand the blow. The Rockwell scales use arbitrary numbers to represent their scores.
Although you can use Rockwell scores to compare one steel’s hardness testing results to another’s, you can’t correlate the scores with any specific observation from or outcome of the test itself because the Rockwell scores are abstractions. Most knife steels score between 58 and 62 HRC, with the majority between 50 and 60.
A knife blade that bends permanently while it’s in service proves that its steel lacks enough hardness for the task. In some cases, a designer can compensate for this deficiency by making all or part of the blade thicker, or by changing the way the steel is manufactured to increase its intrinsic hardness. In other cases, the only solution is to choose a harder steel alloy.
In Benchmade knives that use Crucible Industries’ CPM 20CV, hardness measures 59 to 61 HRC. Because heat treatment “recipes” affect the final hardness of a blade steel, another manufacturer may quote different HRC values for the same steel. This hardness is slightly greater than that of Benchmade’s implementation of 440C, a longstandingly popular alloy for knife making, which measures 50 to 60 HRC.
2. Toughness represents how well a steel withstands impact or stress without displaying breakage, chips, or cracking. A toughness failure can cause a catastrophic outcome if a knife breaks in use, resulting in potential injury to the user as well as the unexpected outcome of the task at hand. For a knife blade, it’s easier to predict the gradual course of wear than the sudden occurrence of a toughness failure, and better to wear out than to fail.
Inadequate toughness can result from an alloy formula itself, from the way the steel was heat treated, from inappropriate grinding during fabrication, or from many combinations of other factors.
Toughness is more difficult to quantify than hardness because no single test provides a standardized measure of the attribute. Typically, toughness testing relies on measures of how far a steel can bend before it breaks, or of how well the material survives impact. In many cases, toughness and hardness compete for supremacy, in that the rigidity that equals toughness can come at the expense of the ability to bend without deformation.
Crucible Industries’ CPM 20CV offers toughness that’s essentially equal to that of 440C, a time-tested stainless steel often chosen for blade making.
3. Wear resistance measures a steel’s ability to come into contact with a wide variety of substances and resist either losing part of its volume when it rubs against rough material or picking up bits of matter from other items. The causes of wear can come from the very items that the user attempts to cut, as well as from other influences in the working environment. Wear testing simulates real-world contact between a knife blade and a source of destructive interaction.
When a rough surface rubs away at the blade of a knife and removes part of the metal, that form of wear constitutes abrasion. Abrasive wear can turn the sharp edge of a blade into a dull and rounded surface, even without applying high pressure to the knife. Conversely, when the smooth surface of a knife blade comes into contact with smooth but hard surfaces such as other steel objects, the result can transfer material onto the knife blade. Unlike abrasion, adhesion requires high enough pressure to create the amount of friction necessary to tear the surface of the blade.
Crucible Industries’ CPM 20CV offers essentially five times the wear resistance of 440C.
4. Corrosion resistance reflects how well a steel avoids developing oxidation in response to exposure to humidity, moisture, or salt in its environment. The most common way to increase corrosion resistance lies in the introduction of chromium into the chemistry of an alloy. Although the familiar term “stainless steel” actually represents an overly optimistic misnomer, chromium content levels do correlate with the ability to delay or reduce oxidation, and to enable preventive maintenance to forestall it.
Crucible Industries’ CPM 20CV displays greater corrosion resistance than 440C, with six-sevenths the resistance for 440C than for CPM 20CV.
5. Edge retention attempts to quantify how well and how long a blade stays sharp through usage. The Cutlery and Allied Trade Research Association, or CATRA, has developed a test machine that attempts to assess relative edge retention. In the test apparatus, the knife under examination mounts with the cutting edge pointing up, and a stack of special synthetic paper impregnated with 5% silica lowers onto the blade. The blade moves back and forth, cutting into the paper, and the silica content introduces a slight amount of wear onto the edge. The test measures the blade’s performance in its factory-new condition, as well as the number and depth of cuts that the blade can complete. This combination of Initial Cutting Performance, or ICP, with Total Card Cut, or TCC, makes up the evaluation parameters. The same testing apparatus can evaluate sharpening equipment as well. The CATRA test instrument forms part of the evaluation equipment in use at many leading knife manufacturers, including Benchmade, Buck Knives, and Spyderco.
Crucible Industries’ CPM 20CV achieves a CATRA test score of 180, compared to 440C’s score of 100.
The full and exact alloy chemistry of Crucible Industries’ CPM 20CV remains a proprietary secret. However, the nominal content of the steel is well documented at 1.9% carbon, 20.0% chromium, 1.0% molybdenum, 4.0% vanadium, 0.3% silicon, 0.6% tungsten, and 0.3% manganese. The high carbon content yields considerable hardness. The substantial amount of chromium points to corrosion resistance. In fact, on the market today, CPM 20CV contains the greatest amount of chromium of any stainless steel with a high level of vanadium. That vanadium content yields toughness, wear resistance, and edge retention. The alloy’s manganese boosts hardness, tensile strength, and wear resistance. Its molybdenum enhances edge retention. Tungsten boosts wear resistance, and silicon both increases hardness and prevents pitting.
The CPM Process and Its Advantages
In 1970, Crucible Industries of Solvay, New York, introduced the patented steel-making process it designed, called Crucible Particle Metallurgy. This innovative manufacturing method overcomes specific critical disadvantages of conventional steel making, yielding a product that improves on the in-use performance of traditionally produced alloys.
In the old-fashioned method of making steel, the ingredients of an alloy melt in an electric arc furnace. After the molten metal undergoes a secondary refining step, it transfers to a ladle that pours it into molds. The problem with this sequence of steps occurs as the alloy cools into ingots.
Mixed together in a molten state in a furnace, the elements that make up a steel alloy mix together thoroughly. Solidifying and cooling in ingot molds, the elements separate into a coarse microstructure that isn’t uniform throughout. Additional processing can break up the segregated elements and attempt to restore the homogeneous mixture that formed in the furnace, but the elements never fully recombine. In a steel with as substantial a percentage of alloying elements and as large an amount of carbon as the formula for CPM 20CV incorporates, the effects of elemental segregation persist even more, and the negative effects on the capabilities of the finished metal are more pronounced, than they would be with a simpler alloy combination.
Crucible’s patented CPM process alleviates these problems before they can start. Other than the initial need to melt the alloying elements together in a furnace, CPM shares few procedural steps with conventional steel making. Once the CPM alloy melts and mixes, the molten material moves through a small nozzle, atomizing into a spray of fine particles propelled by high-pressure gas. The steel particles cool into tiny spheres, forming a powder that retains the original homogeneous makeup of the molten metal. Each particle of metal constitutes a miniature ingot.
The powdered steel loads into a sealed canister from which the air is removed. The container undergoes Hot Isostatic Pressing, or HIP, a process that uses heat and pressure to combine the steel particles into a solid called a compact. The compact can be milled into forms that suit specific product manufacturing processes, and can undergo the heat treatment necessary to finalize and optimize the specific performance of the steel for a desired application.
CPM 20CV is a martensitic steel, which refers to its hard crystalline structure with lens-shaped or lenticular grains. In a carbon-rich alloy like CPM 20CV, the process of austenitization heats the steel until its crystalline structure changes to become austenite instead of ferrite. Martensite forms when austenite cools so quickly that the carbon in the steel supersaturates a special form of ferrite. The atoms in the metal actually rearrange into a form that demonstrates a change in density and, therefore, in volume. Martensite exhibits a significant increase in toughness over its precursors.
The process of heat treating begins with a slow warmup, preheating to a uniform temperature before increasing to austenizing temperature, which is determined by the steel’s alloy chemistry. The quenching phase drops the temperature by 1,000 degrees F or more, enabling the metal to become martensitic. Tempering completes the hardening process and counteracts the brittle nature of martensite.
Comparison to Other Knife Steels
When you examine the performance of CPM 20CV relative to other leading steel alloys, you quickly discover that it offers numerous properties that are ideal for knife making.
At 1.9% carbon, 20.0% chromium, 1.0% molybdenum, 4.0% vanadium, 0.3% silicon, 0.6% tungsten, and 0.3% manganese, CPM 20CV excels at wear resistance and corrosion resistance. It also offers good toughness. Its edge retention capabilities make it easy to keep sharp but potentially difficult to sharpen.
CPM M4, also a Crucible Industries product, incorporates 1.42% carbon, 4.0% chromium, 5.25% molybdenum, 4.0% vanadium, 0.06% silicon, 5.5% tungsten, and 0.3% manganese. Although it’s a high-carbon steel, it incorporates less carbon than CPM 20CV, as well as less chromium. In conjunction with carbon, CPM M4’s combination of molybdenum, vanadium, and tungsten produces excellent toughness, high hardness at 62 to 64 HRC, and equally high levels of wear resistance. Unlike CPM 20CV, however CPM M4 is not a stainless steel. Preventing the development of oxidation on this alloy requires special effort. CPM M4 is at least a third tougher than CPM 20CV, with equal wear resistance and perhaps one-seventh the corrosion resistance.
440C is a martensitic stainless steel with excellent corrosion resistance and a high degree of hardness at HRC 58 to 60. A mainstay among knife makers, it contains the most carbon of all three of the steels in the 440 group. In comparison to CPM 20CV, it offers equal toughness, approximately one-fifth the wear resistance, and approximately six-sevenths the corrosion resistance. Its edge retention capabilities also fall behind those of CPM 20CV.
The 2016 Benchmade product lineup features two knives that incorporate blades made from Crucible Industries’ CPM 20CV. These include the Benchmade 928 Proxy and the Benchmade 698 Foray.
The Benchmade 928 Proxy features Blue Class reliability and durability for everyday use. Designed by Warren Osborne, it includes a drop-point blade with either a plain or a partially serrated edge in an uncoated satin finish. The handle scales combine 6AL-4V titanium on the left to support the knife’s monolock design and desert tan G10 laminate on the right. The reversible tip-up stainless steel split arrow pocket clip attaches with three Torx screws. The Benchmade 928 Proxy measures 8.85 inches overall and 5.09 inches closed, with a 3.87 inch blade that measures 0.15 inches thick. The handle measures 0.5 inches thick. This tactical, outdoor, and survival-oriented knife weighs 4.86 ounces. Its manufacturer’s suggested retail price is $295.
Also a Blue Class knife, the Benchmade 698 Foray represents the design work of Allen Elishewitz, and updates the Benchmade 690 that became Shooting Industry magazine’s 2001 Knife of the Year. This ambidextrous AXIS Lock design uses a drop-point blade, available either with a plain edge or with serrations, and only in satin finish. CPM 20CV represents an upgrade for this new knife over its predecessor, which used 154 CM. The Benchmade 698 Foray features contoured black G10 fiberglass composite handle scales with 410SS stainless steel liners. The reversible tip-up pocket clip attaches to either handle scale. The knife measures 7.32 inches long overall and 4.14 inches closed, with a 3.24 inch blade that measures 0.137 inches thick. The handle measures 0.56 inches thick. This everyday carry and outdoor knife weighs 3.58 ounces. The manufacturer’s suggested retail price is $225.
Crucible Industries created CPM 20CV for use in fabricating injection molding equipment parts, pelletizing and granulating knives, food processing equipment, and knives for individual use. All these applications demand the ability to withstand wear under demanding circumstances. CPM 20CV’s properties enable it to offer the hardness, toughness, wear and corrosion resistance, and edge retention that yield great performance for the demanding knife owner.