Carpenter CTS-XHP Steel

The process of creating a new steel alloy, or repositioning an existing one, with the knife making industry in mind requires precise consideration of the demands of blade users and the ways in which alloy chemistry can help meet them. Metallurgists will tell you that however hard they try, they can’t formulate a “perfect” steel for any application, including knife making. Every alloy represents some form of compromise among attributes. That’s because adding or boosting the amount of one element to heighten one aspect of a blade steel’s performance can have an adverse effect on another critical element of the metal’s ability to withstand the stresses applied to a cutting edge in the real world.

With those considerations in mind, Carpenter created its CTS family of alloys for use in cutting blade applications, including implements for use in surgery, food processing and recycling equipment, ice skates, scissors, and hand tools, as well as bladed implements for sporting, hunting, self-defense, and military applications. This family of specialty alloys offers outstanding edge retention in steels that machine to a fine edge and display consistent response to heat treatment. Of course, virtually all of these steels originated for use in other applications before they gained the CTS designation. For example, CTS XHP started out as a formulation targeted toward the aerospace industry’s need for durable bearings.

The CTS alloys fall into three categories, including low alloy martensitic stainless steels, bearing steels, and powder metals. Low alloy martensitic stainless steels incorporate only about 0.6 percent carbon, along with between 12 percent and 18 percent chromium. Their simple basic chemistry lends itself to modifications through the additions of small amounts of other elements, including molybdenum, vanadium, tungsten, and niobium, to enhance specific performance characteristics such as edge retention or corrosion resistance. Bearing steels bump up both the carbon content and the amounts of additional elements, forming more-complex chemistry than low alloy martensitic stainless steels. Finally, powder metal alloys use new-era steel making techniques to produce high levels of wear resistance. CTS XHP fits into the powder metal alloy category.

Among the members of the CTS alloy family, CTS XHP offers a high degree of corrosion resistance as well as excellent edge retention. With an ability to resist humidity and other elements of a domestic environment, as well as the elements present in a very mildly challenging industrial environment, CTS XHP’s corrosion resistance equals that of 440C, a high-carbon stainless steel with good wear resistance and hardness. CTS XHP also demonstrates hardness that nearly matches what metallurgists expect from the tool steel D2. Depending on its heat treatment and other factors, CTS XHP can reach hardness levels of 60 to 64 HRC as measured on the Rockwell C scale. The combination of corrosion resistance and hardness means that CTS XHP compares either as a high-hardness version of 440C or as a D2 replacement with greater corrosion resistance.

Micro-Melt Steel Production

In October 2009, Carpenter launched the CTS family as a product line of 14 alloys specifically targeting various needs of the knife making industry. Some of the CTS alloys, including CTS XHP, are fabricated through Carpenter’s proprietary Micro-Melt powder metallurgy. This technological breakthrough overcomes a major limitation of traditionally produced steel alloys.

One of the biggest challenges facing blade steel producers lies in formulating the right combination of elements in the right proportions to produce alloys that offer the desired mix of hardness, toughness, corrosion resistance, edge retention, and wear resistance necessary for use in knives that can hold up to the demands of daily use. Beyond the alloy chemistry itself, however, the production process through which the steel takes shape also plays a definitive role in determining how the alloy performs.

In traditional steel making, the elemental components of an alloy mix and melt together in an electric arc furnace. When the mixture reaches the readiness point, it pours into molds that produce individual ingots. As the metal cools, however, the individual elements that make up the alloy separate from one another, producing a result that offers limited consistency throughout each ingot because pockets of specific elements can form as inclusions that disrupt the uniformity of the metal. This process of separation, called segregation, curtails the performance capabilities of the steel as well as its batch-to-batch reliability. Segregation’s effects become more pronounced as alloy formula complexity increases, making the problem an even greater concern when it comes to the steels best suited to blade production, most of which feature complex chemistries. Corrosion resistance and wear resistance especially suffer when elemental segregation occurs during steel production. Some forms of post production processing can overcome segregation at least partially, but the effects linger at least to some extent regardless of the steps taken to neutralize or offset them.

Solving the problem of segregation required an enormous amount of ingenuity. Crucible Industries became the first to do so when it pioneered the process known as Crucible Powder Metallurgy, or CPM. Other steel producers, including Carpenter Technology, have formulated their own versions of this process. Carpenter’s Micro-Melt alloys demonstrate the homogeneously refined microstructure typical of powder metallurgy.

The process of powder metallurgy miniaturizes the production of ingots down to particle size. Instead of large molded steel products, the outcome of the Micro-Melt process is a fine powder. The elemental components of an alloy mix and melt together at extremely high temperatures in a vacuum inside an electric arc furnace. The vacuum melting process helps raise the uniformity of the alloy mixture at the same time that it eliminates inclusions or irregularities. Once molten and mixed, the alloy pours into an electroslag heated tundish, or trough. In the tundish, a plasma torch heats the alloy to purify it. The molten result streams through a nozzle in the form of a mist, where it encounters high-pressure liquid nitrogen. The interaction of molten alloy and inert gas produces atomized droplets of steel that quickly form into powdered particles approximately 150 micrometers in size. Because the molten metal forms and cools so rapidly into tiny pieces, the forces that introduce segregation have neither the time nor the space to exert a negative influence on the resulting powder as it forms. Instead of a large blob of steel cooling in a mold, the outcome of powder metallurgy turns each tiny particle into a cleanly homogeneous miniature ingot.

Various processes can turn this metal powder into its intermediate or final forms. In some cases, the steel producer is charged with crafting short-run or experimental products through on-site prototyping processes. For customers such as knife makers, who buy their metals in strip or billeted forms, the particulate outcome of powder metallurgy becomes the input into a process known as hot isostatic pressing. Once the steel maker cleans the particles and sorts them by size, the alloy powder loads into an autoclaved chamber or enclave, in which heat and pressure form the particulate steel into a billet or roll, condensing it into a solid mass. This heat-and-pressure process, called sintering, takes place just below the temperature at which the metal would melt. The combination of heat and pressure alters the fundamental chemistry of the steel, combining elements together through processes of interaction and inclusion. For example, carbon interacts with vanadium and also becomes incorporated into iron molecules. When heat treatment forces carbon to saturate into iron, the resulting chemical structure takes on the name austenite, or austenitic steel.

Once the sintering process alters the alloy’s chemical processes, the steel is quenched in air or liquid, rapidly lowering its temperature. At this point, the carbon that had entered iron molecules becomes a permanent part of them. This further alteration of austenitic steel produces martensitic steel, which exhibits a lenticular, or lens shaped, grain structure. Further heat treatment at low temperatures raises the toughness of martensitic steel at the expense of some hardness and strength, but it also neutralizes the brittle nature of the untempered alloy.

The combination of powder metallurgy and hot isostatic pressing adds complexity to the alloy manufacturing process and can raise the price of the steel, but the homogeneous, unsegregated result can be worth the additional cost. Powdered processes can produce steels with greater edge retention than those that emerge from traditional processes. Powdered alloys also polish well. The down side of their ability to accept and hold a fine, sharp edge appears when they must be sharpened. Powdered steels typically require more effort and patience to sharpen than lesser alloys do.

The final hardness of a powder metallurgical steel alloy results partially from its chemical composition and partially from the way it’s heat treated in post-production steps. For knife makers, too much hardness can yield too little resistance to damage. As with many aspects of steel performance, the outcome of heat treatment represents a compromise between two competing but desirable attributes.

Alloy Formula and Elemental Benefits

Carpenter creates CTS XHP with a nominal alloy formulation that includes 1.6 percent carbon, 16 percent chromium, 0.5 percent manganese, 0.8 percent molybdenum, 0.4 percent silicon, 0.4 percent vanadium, and 0.3 percent nickel.

Carbon increases hardness and wear resistance at the expense of reduced toughness and potential brittleness. CTS XHP guards against brittleness in the heat treatment stage that follows sintering and quenching during the initial production of the alloy.

Chromium gives stainless steel both its categorization and its corrosion resistance. An alloy requires a minimum of 12 percent chromium to qualify as a stainless steel. The designation “stainless” represents an oversimplification or overstatement, of course, as any stainless steel reacts at least to some extent to the enduring presence of substances that cause oxidation. The term “corrosion resistance” more accurately states the performance of stainless steel. Chromium also contributes to increased hardness and greater tensile strength. Like too much carbon, too much chromium can decrease toughness.

Manganese raises hardness, wear resistance, and tensile strength, and helps reduce brittleness. Like too much carbon, too much manganese can lead to brittleness. During steel production, manganese aids in the process of removing oxygen and therefore staving off the prospect of pitting.

Molybdenum fosters edge retention and increases high-temperature strength.

Silicon increases hardness. Like manganese, silicon helps deoxidize steel during molten phases of production, reducing the likelihood of pitting in the finished product.

Vanadium helps yield a finer-grained steel that demonstrates increased toughness after heat treatment. Vanadium also increases edge retention and wear resistance.

Nickel boosts toughness and low-temperature strength.

CTS XHP in Knife Production

When CTS XHP entered the Carpenter Technology lineup as a blade steel in October 2009, knife makers explored it as a substitute for familiar alloys such as 440C and D2. In addition, they looked at it on its own merits as a new option on which they might come to rely. Because custom knife makers create their products in smaller quantities than production manufacturers do, they have less economic ability and incentive to take risks on unfamiliar steels. As a result, new steels tend to materialize in production knives before they show up in custom products. In fact, some large companies go to great lengths to feature the “new steel on the block” as a means of attracting and retaining customers who buy on the basis of seeking out new materials.

CTS XHP offers features that the knife making industry immediately found attractive. Its wear resistance, the promise of high performance reliability from batch to batch of the alloy, its edge retention, its corrosion resistance: These qualities offer obvious benefits in knife production. At the same time, its high performance parameters come at a reasonable price.

Carpenter courted leading knife designers, including Gayle Bradley, Tom Overeynder, and Warren Osborne, inviting them to experiment with CTS XHP and other steels in the CTS family. The fine grain structure, machinability, heat treatability, corrosion resistance, and edge retention of the alloy attracted their attention, as did CTS XHP’s ability to hold a mirror finish without extensive polishing.

Carpenter also dedicated itself to building Spyderco’s interest in its CTS family. Beginning with the inception of the CTS suite as a product line, Spyderco has used as many as nine of the CTS family of blade steel alloys in its products. Along with CTS XHP, these include CTS 204P, CTS B70P, CTS B75P, CTS BD1, CTS BD30P, CTS 204P, CTS 20CP, and CTS 40CP.

Spyderco planned its initial use of CTS XHP in the creation of a 5.3-ounce knife with a 3.13-inch skinner blade. The remainder of the knife’s materials also consisted of Carpenter steels, including a jewelry-grade alloy called Bio-Blue for one of the handle scales. The knife went into pre-production in 2012, slated for distribution to every one of Carpenter’s employees, who numbered nearly 5,000 at the time.

In the 2016 Spyderco product lineup, eight knives feature blades manufactured from Carpenter’s CTS XHP. These include two flippers, the C172CFTIP Domino and the C182CFTIP Dice; the Chaparral Series of gentlemen’s folding knives, including the C152CFP Chaparral Carbon Fiber/G-10 Laminate, the C152TIP Chaparral Titanium, the C152STIBLP Chaparral Blue Stepped Titanium, and the C152STIP Chaparral Stepped Titanium; the C160P Foundry gentlemen’s folding knife; the C158TIP Techno specialty knife; the C185TIP Slysz Bowie specialty knife; and the Ethnic Series C173GP Hungarian Folder.

Elemental Alloy Formulation Comparisons: Carpenter CTS XHP vs. 440C and D2