D2 Tool Steel
D2 Tool Steel -- A durable powerhouse
When tool and die makers specify a steel for their equipment, they often look to D2 as a first choice. Its hardness enables it to take a pounding without losing its shape. Its ample wear resistance suits the task of stamping and forming other metals, a task that manufacturing equipment must be able to repeat over and over again without the need for frequent replacement of parts. That mission calls for a steel that resists the twin forces of wear (abrasion and adhesion) and can withstand the constant loading forces that occur every time a die, punch, or chopping and shearing blade encounters the material it processes. Although other tool steels may offer better long-term, high-volume production capabilities without the need to regrind dies, D2 combines reasonable cost with broad availability.
For some of the same reasons that D2 makes for durable parts that can survive the compression and pressure in equipment on a factory floor, D2 also offers knife makers a desirable suite of performance factors that can lead it to the top of the list of blade materials. D2 owes its capabilities to its chemical makeup, as well as to the processing steps it undergoes during and after production.
Steel Production for Knife Making
Between production-quantity knife manufacturing and custom designers, the world of blade creation has grown to the point at which it has earned steel producers’ attention as a market segment with needs and priorities that drive its choices among available steels. Steel companies court knife makers’ business with analyses of their alloys designed to highlight performance parameters that favor the capabilities that make for great blades. Nonetheless, among the steels that have gained popularity among knife makers, including D2, almost every alloy on the list originated as a product targeted toward the needs of much bigger customers in much bigger fields.
Knife makers’ priorities overlap and intersect with those of other manufacturing processes and industries. As a result, they can find suitable materials among the lists of alloys that began as materials destined for use in automotive and aerospace construction, high-volume production equipment, and other industries that also depend on steel for their materials and workflows.
In the U.S., the classification of tool steels relies on a grading system that combines a letter with a number to produce a nomenclature that identifies how the metal is hardened and other characteristics of its use and performance. “W” denotes water hardened tool steel. “O” represents oil hardening. “A” and “D” indicate air hardening. “D” also points to tool steels classified as high carbon and high chromium because of their elemental composition.
Evaluating Steels and Their Performance
Every blade steel undergoes evaluation on the basis of five criteria, each one representing fundamentally desirable attributes that contribute to knife performance. The tests and measurements involved in these aspects of evaluation vary from longstanding laboratory methods to more nearly anecdotal observations based on real-world use.
Hardness—As it applies to steel, the standard laboratory measurement of hardness uses the Rockwell tester and its associated measurement scales to indicate resistance to changes in the shape of materials under impact. For knife steels, the Rockwell C scale provides these indentation resistance values, expressed as numbers followed by the designation “HRC” to indicate the specific Rockwell scale. Rockwell measurements do not correlate with any real-world dimensions. They simply provide indications of hardness that can be used to make comparisons among materials that have undergone comparable testing. Among knife blades, Rockwell measurements commonly range from 56 HRC to 62 HRC.
An alloy’s hardness can vary based on its elemental chemistry and as a result of the heat treatment it receives. Additionally, because many alloys, including D2, list a range of ingredient content for one or more of their component elements, the hardness of one manufacturer’s rendition of an alloy can show performance characteristics that differ from another manufacturer’s equivalent. Likewise, one batch of a steel may differ from another batch.
Toughness—A measure of a steel’s ability ability to bend without breaking and resume its shape without deformation, toughness also correlates with tensile strength. As toughness drops, brittleness rises.
D2 sometimes earns a reputation as a wear-resistant steel that lacks toughness, especially in comparison to other steels that also do not qualify to carry the designation “stainless.” Heat treatment temperatures, the number of tempering steps, and the way the steel is processed between steps can make or break D2’s toughness, potentially bringing it up so it balances with the alloy’s hardness. Blade thickness also plays a role in determining toughness. Thicker blades tend to show less flexibility than thinner blades, and some thin blades fail to return to their original conformation after exposures to bending forces.
To increase the toughness of D2, some steel manufacturers have created versions of the alloy produced through powder metallurgy rather than through conventional steel making procedures. The field of powder metallurgy arose as a response to the negative effects of alloy segregation that appear in traditional alloy manufacturing.
In the time-tested methods of steel production, the producer mixes iron, carbon, and other elements together in an electric arc furnace. This molten material pours into molds that form it into ingots. The act of cooling a large chunk of metal causes its alloying elements to separate, turning what emerged from the furnace as a cohesive, homogeneous mixture into an inconsistent blend. Slicing up an ingot and sampling the chemistry of various parts shows that some areas contain more of an element than others do, with pockets forming throughout the cooled metal. In alloys such as D2, which contain complex mixtures of numerous elements, the resulting steel offers inconsistent performance. Even through post production treatment can overcome some of the effects of segregation, it cannot restore full homogeneity to the alloy.
Powder metallurgy enables finished steel to retain its uniform chemistry and therefore its performance consistency. Instead of creating large ingots, the process miniaturizes the ingot down to the size of a grain of powder. To do so, this advanced production method melts the ingredients of an alloy recipe in a vacuum chamber, and then mists it through a nozzle into a high pressure blast of inert gas. The stream of molten steel turns into tiny droplets, which cool immediately into equally small individual ingots. The powder enters an autoclave in which it undergoes sintering, the combination of heat and pressure, which forms the particles into a solid mass. At the same time, sintering changes the chemistry of the steel, binding elements together at the molecular level. A quenching step rapidly drops the mixture’s temperature, again changing molecular structures to make the combination of elements a permanent state. Low-temperature heat treatment counters brittleness.
Wear resistance—Twin forces conspire to wear away a piece of steel during use. Abrasion removes bits of material from a blade that encounters rough surfaces. Through adhesion, particles detach from other surfaces and attach themselves to blade steel. D2’s performance in the area of wear resistance, its ability to avoid being worn away or falling prey to foreign matter dislodged from other materials, provides much of the basis for its rave reviews as a blade steel.
Remember that D2 was developed as an alloy for use in stamping dies, cutting blades in production equipment, and other metal parts that must repeat a single step under pressure countless times without changing shape. This constellation of attributes explains what attracted knife makers to D2 as a blade steel. Its original use defines and requires considerable wear resistance, an attribute that also contributes to superior blade performance. Even D2 can be outdone in terms of wear resistance, however, by alloys with more complex elemental chemistries and especially by steels produced through powder metallurgy.
Despite the huge advantages of D2’s wear resistance, these strengths also can serve as disadvantages. Specifically, D2 can be difficult to machine and sharpen. Although D2 blades hold their edges through long periods of wear, getting them sharp is a time-consuming task that requires expertise and the right tools.
Corrosion resistance—When a knife blade encounters salt water, acidic liquids such as citrus fruit juice, humid environments, and sustained exposure to moisture, the blade’s corrosion resistance describes its ability to avoid demonstrating pitting and rust. By their very nature, stainless steels are designed to display corrosion resistance, although even a stainless steel will begin to react to corrosive substances after prolonged exposure. Even though D2 does not qualify as a stainless steel, it nonetheless offers better corrosion resistance than many other non-stainless and high-carbon steels can muster.
When corrosion materializes, it begins with the appearance of a patina, a darkening of the surface of the metal. To prevent the development and progression of corrosive effects, a D2 blade should be kept clean and dry, maintained outside of its sheath to avoid exposure to the moisture that can build up in a form fitted leather or thermoplastic storage accessory. A suitable coating of wax also helps forestall corrosion’s ill effects on D2, which can include lowered edge retention along with compromised surface characteristics.
Edge retention—Measurements of edge retention may lack the long history of laboratory quantification and validation that characterize other steel performance criteria such as hardness, but the function of a cutting blade dictates that its ability to stay sharp serves as a basis for evaluating the choice of steel used in creating it. At the same time, a knife that holds an edge through long periods of wear may be the same knife that requires a skilled hand to sharpen effectively and correctly.
Unfortunately, some of these performance criteria compete with one another for supremacy. Hardness often trades off with toughness. As corrosion resistance rises, edge retention can drop, and vice versa. The alloy chemistry that fosters a dominant showing in one area of evaluation can stymie a steel’s ability to compete in another.
The nature of these unavoidable compromises underscores one of the reasons that no single steel presents a commandingly dominant argument to make it the primary, let alone the only, choice for all blades. Of course, some blades are designed to accommodate needs that draw less on some performance criteria than on others. For example, a blade that requires extreme hardness to withstand the pounding impact involved in batoning wood in the field may not require the toughness necessary to survive operations that can cause the blade to bend, or it may survive the bending without breakage but not without becoming permanently deformed.
The recipe of elements that makes up an alloy determines the steel’s performance in two ways. First, the specific elements that contribute to the mixture add their properties to the finished result. Second, the amounts of these elements also influence the behavior of the metal. The old caution that more does not always equal better holds true with steel alloys, given that too much of an element can lead to deficiencies in the metal’s performance instead of improvements.
D2’s formula can vary from manufacturer to manufacturer and on the basis of specific customers’ requests. As a rule, however, D2 includes between 1.4% and 1.7% carbon; 11% to 13% chromium; 0.07% to 1.2% molybdenum; maximum levels of four elements, including 0.6% manganese, 0.6% silicon, 1.1% vanadium, and 1.0% cobalt; 0.03% phosphorus; and 0.03% sulfur. These elements affect D2’s performance in varied ways.
The presence of carbon transforms iron into steel. At 1.4% to 1.7% carbon, D2 qualifies as a high-carbon steel. Carbon raises hardness and wear resistance, but in large amounts, it lowers toughness and introduces brittleness. Approximately 0.5% of the carbon in D2 enables it to reach desirable hardness levels. The rest of it, in conjunction with the less electronegative elements in the recipe, including chromium and vanadium, helps form hard particles called carbides. Carbides make up no more than 20% of the overall structure of a steel, but they hold the metal together and enhance its wear resistance.
When it comes to hardness, chromium ranks first on the Periodic Table of Elements. Chromium confers corrosion resistance, raises steel’s hardness, and adds tensile strength. Too much chromium reduces toughness, which is unsurprising given the element’s role in increasing hardness and the continuum between hardness and toughness.
Molybdenum boosts edge retention at the same time that it increases toughness and high-temperature strength. Molybdenum carbides contribute to the microstructure that resists wear.
Manganese contributes to hardness and wear resistance as well as to tensile strength. Too much manganese can result in cracking if a steel is quenched in water, but D2 is an air-cooled alloy. Manganese can combine with sulfur and phosphorus to banish brittleness and help deoxygenate steel during production.
Silicon boosts hardness. It also helps deoxygenate steel during production, reducing the prospect of pitting.
Vanadium helps produce fine-grained steel that exhibits marked toughness. It also enhances wear resistance and edge retention. Among all carbides, vanadium carbides make a dominant contribution to the wear resistance for which D2 earns praise.
Cobalt contributes to hardness, intensifies the performance characteristics of other elements in an alloy, and promotes rapid cooling during steel manufacturing.
Phosphorus boosts hardness. Too much phosphorus can result in brittleness.
Sulfur constitutes more of a nuisance element than a desirable component in an alloy, but most steels include at least a trace of it. In minute amounts, sulfur’s positive contribution consists of promoting machinability. Above that level, it reduces toughness.
D2 offers approximately the same level of toughness as other lastingly popular traditionally manufactured alloy steels, including 440C stainless steel, and at least a little less toughness than some other traditional choices such as 154 CM. Its wear resistance equals or exceeds these alternatives. In fact, the only alloy steels that exceed D2 on the criteria of toughness and wear resistance emerge from the more exotic production processes of powder metallurgy. This gives D2 the ability to produce knife blades that can withstand tough tasks in the field or on the job, including those that involve the risk of bending a blade under impact.
Stainless steels earn that designation because they incorporate between 12% and 14% chromium in their elemental makeup. Although D2 contains almost enough chromium to qualify as a stainless steel, much of that content binds with carbon during the production process to form chromium carbides. This leaves relatively little free chromium to provide the kind of corrosion resistance that typifies stainless steels. The net effect is that although D2’s elemental composition includes a high amount of chromium for a tool steel, it offers less corrosion resistance than its alloy chemistry would suggest.