Spyderco Edge-U-Cation® - SPYDERCO SALT SERIES
STILL SETTING THE STANDARD IN ULTRA-CORROSION-RESISTANT CUTTING TOOLS
When it comes to evaluating blade materials, there are three primary qualities that must be considered: wear resistance, toughness, and corrosion resistance. While not mutually exclusive, these qualities definitely do compete with each other in defining the properties of a steel. Some blade steels, by design, emphasize one or two qualities at the expense of a third. Others use a finely balanced blend of alloys and specialized manufacturing methods to offer a calculated compromise of all three.
No matter which approach you take, the definition of the “perfect” blade steel will always be subjective; however, some materials clearly achieve that “calculated compromise” of properties far better than others. Two steels that do it exceptionally well are the secret of Spyderco’s remarkable Salt Series.
Blade Steel Basics
To understand what makes our Salt Series so special, it helps to have a basic understanding of how steel works and how it is manipulated to create knife blades. Traditional steels are produced by adding carbon to iron. In its annealed, or soft, state, simple steel consists of small carbon atoms that exist in the spaces between the material’s larger iron atoms. In this condition, the atoms are naturally arranged in a cubic structure called ferrite.
When that steel is heated to a sufficiently high temperature, its atomic structure changes. The high heat allows the carbon in the steel to become soluble and combine with the ferrite to create a different structure called austenite. Also known as gamma-phase iron (γ-Fe), austenite is a metallic, non-magnetic form of iron, or a solid solution of iron, with an alloying element.
In the austenite “phase,” the carbon atoms occupy the spaces between the iron atoms in the cubic ferrite arrangement. If, while in this condition, the steel is rapidly cooled or “quenched,” this atomic structure is “locked in.” Rather than the original cubic arrangement, the atomic structure is distorted to create an extended tetragon (quadrilateral) shape. This distortion makes the steel much harder and stronger and yields a new phase of the material called martensite. The more carbon present in the martensite structure, the higher its strength.
If the transition from austenite to martensite is incomplete, the steel is said to have “retained austenite.” While generally not desirable in most applications, the increased toughness of retained austenite and its unique ability to transform to martensite at lower temperatures give it remarkable potential in specialized applications. More on that later…
In traditional carbon steels, the quenching process that creates martensite makes the material extremely hard, but brittle and vulnerable to fracture. To make the steel tougher, while still retaining adequate hardness to take and keep a good edge, the steel is reheated to a lower temperature and allowed to cool slowly. This process, called tempering, alters the size and distribution of the carbides in the martensite to form a microstructure called "tempered martensite". Typically, this process is applied evenly to produce uniform hardness through the piece and is known as “through tempering.” Some tools, however, like cold chisels, are purposely heated unevenly to produce variations in hardness throughout the piece. This is known as "differential tempering."
Austenitic Stainless Steels and Precipitation Hardening
One of the major drawbacks of carbon steel is its vulnerability to corrosion. For applications that require the strength of steel in environments where rust is a concern, the steel industry developed stainless steel. Stainless steels are iron-based alloys that contain a minimum of about 11% chromium and often additional elements, including carbon, nitrogen, nickel, copper, niobium, and others. The chromium added to the material allows parts made from stainless steel to form a passive film of chromium oxide on their surface. This film is what protects them from corrosion.
The most common forms of stainless steel are 200 and 300-series austenitic stainless steels, which contain 16% to 30% chromium and 2% to 20% nickel. Non-hardenable by heat treating and non-magnetic in their annealed (soft) state, these popular steels do become somewhat magnetic when cold worked.
Another popular category of stainless steel is precipitation-hardened stainless. Precipitation hardening, also called age hardening, is a heat-based process that increases the tensile and yield strength of a material and can be used on many malleable metals. Precipitates are small impurity regions that form in a material when it is no longer able to dissolve the impurity. For example, aluminum can dissolve some copper when heated to high temperature. When the temperature is decreased, however, the extra copper “precipitates” by forming small copper-rich regions in the aluminum. These precipitates impede dislocation motion within the material, affecting its internal stresses and hardening it.
In many stainless steels, the large amounts of nickel and chromium used to achieve corrosion resistance make traditional hardening and tempering methods ineffective. However, by applying this same process, precipitates of chromium, copper, and other elements, along with carefully controlled heating cycles, can actually strengthen the steel to nearly the same degree as hardening and tempering.