Types of spring materials used in spring fabrication vary depending on the design and application requirements. Principally, these materials consist of spring steels, a group of medium-to-high carbon, low alloy steels characterized by their excellent yield strength—an important material property that allows a spring to bend, twist, compress or extend and return to its original shape without deforming. In other words, yield strength is the maximum amount of stress a material can handle before it starts to permanently deform, which is an essential characteristic of every type of spring.