Differential hardening is a method used in forging

Forging

Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons...

 sword

Sword

A sword is a bladed weapon used primarily for cutting or thrusting. The precise definition of the term varies with the historical epoch or the geographical region under consideration...

s and knives

Knife

A knife is a cutting tool with an exposed cutting edge or blade, hand-held or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools...

 to increase the hardness of the edge without making the whole blade brittle

Brittle

A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...

. To achieve this, the edge is cooled more rapidly than the spine by adding a heat insulator to the spine before quenching. Clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 or another material is used for insulation. It can also be achieved by carefully pouring water (perhaps already heated) onto the edge of a blade as is the case with the manufacture of some kukri

Kukri

The kukri is a curved Nepalese Knife, similar to the machete, used as both a tool and as a weapon...

.

This technique is mainly used in the Chinese jian

Jian

The jian is a double-edged straight sword used during the last 2,500 years in China. The first Chinese sources that mention the jian date to the 7th century BCE during the Spring and Autumn Period; one of the earliest specimens being the Sword of Goujian.Historical one-handed versions have blades...

 and the katana

Katana

A Japanese sword, or , is one of the traditional bladed weapons of Japan. There are several types of Japanese swords, according to size, field of application and method of manufacture.-Description:...

, the traditional Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

ese sword, and the khukuri, the traditional Nepal

Nepal

Nepal , officially the Federal Democratic Republic of Nepal, is a landlocked sovereign state located in South Asia. It is located in the Himalayas and bordered to the north by the People's Republic of China, and to the south, east, and west by the Republic of India...

ese knife. Most blades made with this technique have visible temper lines.

Another process, often referred to as differential hardening, but in reality differential tempering can also be obtained by quenching the object uniformly, then differentially tempering

Tempering

Tempering is a heat treatment technique for metals, alloys and glass. In steels, tempering is done to "toughen" the metal by transforming brittle martensite or bainite into a combination of ferrite and cementite or sometimes Tempered martensite...

one part of it with a torch or some other directed heat source. The heated portion of the metal is softened by this process.http://www.primitiveways.com/pt-knives-1.html

External links

• Claying blades -- Differential hardening with clay