Knife Blade Hardness

Blade Hardness

Blade hardness is yet another issue. Steels with more carbon will get harder on the Rockwell scale and steels with less will harden to a lesser degree. Harder steel will hold an edge longer, naturally, but will be more brittle (less tough) and harder to sharpen. Softer steel will be tougher and easier to sharpen but won’t hold an edge as well. Most modern high tech stainless steels are hardened to RC 58 to 60 which is a pretty good range for most cutlery applications.

The Basic Processes of Hardening Steel

The basic process of hardening steel by heat treatment consists of heating the metal to a temperature at which austenite is formed, usually about 760° to 870° C (about 1,400° to 1,600° F) and then cooling, or quenching, it rapidly in water or oil. Such hardening treatments, which form martensite, set up large internal strains in the metal, and these are relieved by tempering, or annealing, which consists of reheating the steel to a lower temperature.

Tempering results in a decrease in hardness and strength and an increase in ductility and toughness. The primary purpose of the heat-treating process is to control the amount, size, shape, and distribution of the cementite particles in the ferrite, which in turn determines the physical properties of the steel. Many variations of the basic process are practiced.

Time-Quenching

In time-quenching the steel is withdrawn from the quenching bath when it has reached the temperature at which the martensite begins to form, and is then cooled slowly in air.

Martempering

In martempering the steel is withdrawn from the quench at the same point, and is then placed in a constant-temperature bath until it attains a uniform temperature throughout its cross section. The steel is then allowed to cool in air through the temperature range of martensite formation, which for most steels is the range from about 288° C (about 550° F) to room temperature.

Austempering

In austempering the steel is quenched in a bath of metal or salt maintained at the constant temperature at which the desired structural change occurs and is held in this bath until the change is complete before being subjected to the final cooling.

Case Hardening

Any of several processes for hardening the surfaces of steel products in order to make them more resistant to abrasion and wear, while leaving the interior soft and therefore tougher and more fracture-resistant. The hardening may be accomplished by dissolving carbon into the surface, called carburizing, or by adding nitrogen, called cyaniding or nitriding.

Carburizing

In Carburizing, the piece is heated in charcoal or coke, or in carbonaceous gases such as methane or carbon monoxide.

Cyaniding

Cyaniding consists of hardening in a bath of molten cyanide salt to form both carbides and nitrides.

Nitriding

In Nitriding, steels of special composition are hardened by heating them in ammonia gas to form alloy nitrides.