Editing Differential heat treatment (section)

====Metallurgy====
When a sword, knife or tool is evenly quenched, the entire object turns into martensite, which is extremely hard, without the formation of soft pearlite. Tempering reduces the hardness in the steel by gradually changing the martensite into a microstructure of various [[carbide]]s, such as [[cementite]], and softer [[Allotropes of iron|ferrite (iron)]], forming a microstructure called "[[Tempering (metallurgy)#Physical processes|tempered martensite]]". When tempering high-carbon steel in the blacksmith method, the color provides a general indication of the final hardness, although some trial-and-error is usually required to match the right color to the type of steel to achieve the exact hardness, because the carbon content, the heating speed, and even the type of heat source will affect the outcome. Without the formation of pearlite, the steel can be incrementally tempered to achieve the proper hardness in each area, ensuring that no area is too soft.<ref>''Advanced materials & processes, Volume 149'' By the American Society of Metals – ASM International p. 114</ref> In arming swords, for instance, because the blade is typically rather wide and thin, a blade can be prone to bending during combat. If the center of the blade is too soft, this bending may likely be permanent. However, if the sword is tempered to a springy hardness, it will be more likely to return to its original shape.<ref name="autogenerated2007"/>