Nitinol or Nickel Titanium: The Hottest New (Apparently Not New:1962) Steel Used in Tactical Knife Making
Nitinol or Nickel Titanium: The Hottest New (Apparently not New:1962) Steel Used in Tactical Knife Making.
Strider Knives 60 NiTiNOL Blades
For those that missed it, Strider Knives debuted knives at the 2008 Blade Show with blades made of a metal named 60 NiTiNOL. NiTiNOL is short for Nickel Titanium Naval Ordinance Laboratory, indicating its’ composition and where it was originally developed. As the number 60 implies, NiTiNOL contains 60% nickel with titanium making up the remaining material. Since its’ invention, processes have been developed to make NiTiNOL a high strength, wear-resistant metal alloy.
In the last few months, Crucible Research was able to develop methods of production for NiTiNOL. Property sheets indicate that 60 NiTiNOL has a hardness of 62 Rockwell C or more. Strider material states that the 60 NiTiNOL used in their knives is 65 Rockwell C.
Some of the other desirable traits of the metal are non-corrosiveness and approximately 25% less weight than steel.
Scales and values
There are several alternative scales, the most commonly used being the “B” and “C” scales. Both express hardness as an arbitrary dimensionless number.
|A||HRA||60 kgf||120° diamond cone†||Tungsten carbide|
|B||HRB||100 kgf||1⁄16-inch-diameter (1.588 mm) steel sphere||Aluminium, brass, and soft steels|
|C||HRC||150 kgf||120° diamond cone||Harder steels|
|D||HRD||100 kgf||120° diamond cone|
|E||HRE||100 kgf||1⁄8-inch-diameter (3.175 mm) steel sphere|
|F||HRF||60 kgf||1⁄16-inch-diameter (1.588 mm) steel sphere|
|G||HRG||150 kgf||1⁄16-inch-diameter (1.588 mm) steel sphere|
|†Also called a brale indenter|
- Except for one very limited exception,[clarification needed] the steel indenter balls have been replaced by tungsten carbide balls of the varying diameters. Scales using the ball indenter have a “W” suffix added to the scale name to indicate usage of the carbide ball, for example “HR30T” is now “HR30TW”.
The superficial Rockwell scales use lower loads and shallower impressions on brittle and very thin materials. The 45N scale employs a 45-kgf load on a diamond cone-shaped Brale indenter, and can be used on dense ceramics. The 15T scale employs a 15-kgf load on a 1⁄16-inch-diameter (1.588 mm) hardened steel ball, and can be used on sheet metal.
Readings below HRC 20 are generally considered unreliable, as are readings much above HRB 100.
 Typical values
- Very hard steel (e.g. a good knife blade): HRC 55–62 (Hardened tool steels such as D2)
- Axes, chisels, etc.: HRC 40–45 (about 1045 carbon steel)
- Brass: HRB 55 (Low brass, UNS C24000, H01 Temper) to HRB 93 (Cartridge Brass, UNS C26000 (260 Brass), H10 Temper)
Several other scales, including the extensive A-scale, are used for specialized applications. There are special scales for measuring case-hardened specimens.
Nitinol alloys exhibit two closely related and unique properties: shape memory and superelasticity (also called pseudoelasticity). Shape memory refers to the ability of nitinol to undergo deformation at one temperature, then recover its original, undeformed shape upon heating above its “transformation temperature”. Superelasticity occurs at a narrow temperature range just above its transformation temperature; in this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity, some 10-30 times that of ordinary metal.
The term nitinol is derived from its composition and its place of discovery: (Nickel Titanium Naval Ordnance Laboratory). William J. Buehler along with Frederick Wang, discovered its properties during research at the Naval Ordnance Laboratory in 1962.
While the potential applications for nitinol were realized immediately, practical efforts to commercialize the alloy didn’t take place until a decade later. This delay was largely because of the extraordinary difficulty of melting, processing and machining the alloy. Even these efforts encountered financial challenges that weren’t really overcome until the 1990s, when these practical difficulties finally began to be resolved.
The discovery of the shape-memory effect in general dates back to 1932 when Swedish researcher Arne Olander  first observed the property in gold-cadmium alloys. The same effect was observed in Cu-Zn in the early 1950s.
How it works
Nitinol’s unusual properties are derived from a reversible, solid state phase transformation known as a martensitic transformation.
At high temperatures, nitinol assumes an interpenetrating simple cubic crystal structure referred to as austenite (also known as the parent phase). At low temperatures, nitinol spontaneously transforms to a more complicated “monoclinic” crystal structure known as martensite. The temperature at which austenite transforms to martensite is generally referred to as the transformation temperature. More specifically, there are four transition temperatures. When the alloy is fully austenite, martensite begins to form as the alloy cools at the so-called martensite start, or Ms temperature, and the temperature at which the transformation is complete is called the martensite finish, or Mf temperature. When the alloy is fully martensite and is subjected to heating, austenite starts to form at the As temperature, and finishes at the Af temperature.
Crucial to nitinol’s properties are two key aspects of this phase transformation. First is that the transformation is “reversible,” meaning that heating above the transformation temperature will revert the crystal structure to the simpler austenite phase. Upon heating, however, there is a slight upward shift in the temperatures, now beginning at the As temperature, and finishing at the Af temperature. The second key point is that the transformation in both directions is instantaneous.
Martensite’s crystal structure (known as a monoclinic, or B19′ structure) has the unique ability to undergo limited deformation in some ways without breaking atomic bonds. This type of deformation is known as twinning, which consists of the rearrangement of atomic planes without causing slip, or permanent deformation. It is able to undergo about 6-8% strain in this manner. When martensite is reverted to Austenite by heating, the original austenitic structure is restored, regardless of whether the martensite phase was deformed. Thus the name “shape memory” refers to the fact that the shape of the high temperature austenite phase is “remembered,” even though the alloy is severely deformed at a lower temperature.
A great deal of force can be produced by preventing the reversion of deformed martensite to austenite – in many cases, more than 100,000 psi. One of the reasons that nitinol works so hard to return its original shape is that it is not just an ordinary metal alloy, but what is known as an intermetallic compound. In an ordinary alloy, the constituents are randomly positioned on the crystal lattice; in an ordered intermetallic compound, the atoms (in this case, nickel and titanium) have very specific locations in the lattice. The fact that nitinol is an intermetallic is largely responsible for the difficulty in fabricating devices made from the alloy.
The scenario described above (cooling austenite to form martensite, deforming the martensite, then heating to revert to austenite, thus returning the original, undeformed shape) is known as the thermal shape memory effect. A second effect, called superelasticity or pseudoelasticity is also observed in nitinol. This effect is the direct result of the fact that martensite can be formed by applying a stress as well as by cooling. Thus in a certain temperature range, one can apply a stress to austenite, causing martensite to form while at the same time changing shape. In this case, as soon as the stress is removed, the nitinol will spontaneously return to its original shape. In this mode of use, nitinol behaves like a super spring, possessing an elastic range some 10 to 30 times greater than that of a normal spring material. There are, however, constraints: the effect is only observed some 0-40 degrees C above the Af temperature.
Nitinol is typically composed of approximately 50 to 51% nickel by atomic percent (55 to 56% weight percent). Making small changes in the composition can change the transition temperature of the alloy significantly. One can control the Af temperature in nitinol to some extent, but convenient superelastic temperature ranges are from about -20 degrees to +60 degrees C.
One often-encountered complication regarding nitinol is the so-called R-Phase. The R-Phase is another martensitic phase that competes with the martensite phase mentioned above. Because it does not offer the large memory effects of the martensite phase, it is, more often than not, an annoyance.
Making nitinol and nitinol devices
Nitinol is exceedingly difficult to make due to the exceptionally tight compositional control required, and the tremendous reactivity of titanium. Every atom of titanium that combines with oxygen or carbon is an atom that is robbed from the NiTi lattice, thus shifting the composition and making the transformation temperature that much colder. There are two primary melting methods used today:
- Vacuum Arc Remelting: This is done by striking an electrical arc between the raw material and a water-cooled copper strike plate. Melting is done in a high vacuum, and the mold itself is water cooled copper, so no carbon is introduced during melting.
- Vacuum Induction Melting: This is done by using alternating magnetic fields to heat the raw materials in a crucible (generally carbon). This is also done in a high vacuum, but carbon is introduced during the process.
While both methods have advantages, there are no substantive data showing that material from one process is better than the other. Other methods are also used on a boutique scale, including plasma arc melting, induction skull melting, and e-beam melting. Physical vapor deposition is also used on a laboratory scale.
Hot working of nitinol is relatively easy, but cold working is difficult because the enormous elasticity of the alloy increases die or roll contact, leading to tremendous frictional resistance and tool wear. For similar reasons, machining is extremely difficult—to make things worse, the thermal conductivity of nitinol is poor, so heat is difficult to remove. Grinding (abrasive cutting), Electrical discharge machining (EDM) and laser cutting are all relatively easy.
Heat treating nitinol is delicate and critical. It is the essential tool in fine-tuning the transformation temperature. Aging time and temperature controls the precipitation of various Ni-rich phases, and thus controls how much nickel resides on the NiTi lattice; by depleting the matrix of nickel, aging increases the transformation temperature. The combination of heat treatment and cold working is essential in controlling the properties of nitinol.
- Is Nitinol the new super steel?
- Is it cost effective?
- Or is it just a gimmick? used to create super expensive products
- Non corrosive
- Lighter than conventional steel
- As you can see on the Rockwell Scale 62 C is the highest number in hardened knife steels. “Very hard steel (e.g. a good knife blade): HRC 55–62”
I would like to see more from this material. Is it better than S30V or does the complexity to make Nitinol put it behind in desirability.
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