Most elements in the periodic table are metals. They share some important characteristics. Generally, metals are solid at room temperature, good conductors of heat, good conductors of electricity, shiny and become highly reflective when smooth, and malleable and ductile. When a pure metal is hot enough to be in liquid form, the arrangement of the atoms is constantly changing. As the metal cools, the atoms take on a very orderly, three-dimensional, geometric arrangement. This arrangement is called a crystalline structure. The phase diagram is a tool for understanding the phase changes for a metal. Pure metals have a clearly defined melting point. Solidification or freezing occurs at a constant temperature. As a pure liquid metal is cooled, its temperature drops to the solidification temperature and remains at that temperature until all of the liquid has solidified. There are three solid phases of the iron-carbon mixture that are important in understanding the metallurgy of iron and steel. They are ferrite, austenite, and cementite. One of the most important phase diagrams in engineering applications is the iron-carbon system. Heat treatment may be employed to improve tensile strength, ductility, toughness, wear resistance, machinability, formability, bending quality, corrosion resistance, magnetic properties, and other properties.