Toughness of High Temperature Steels
High temperature steels have been in use for many years due to their excellentability to withstand extreme temperatures. There are three basic types of high temperature steels: hot-rolled low-alloy, quenched and tempered low-alloy, and ferritic steels. Each type has specific characteristics that can be tailored to a particular application due to their toughness and strength at high temperatures, making them desirable in a wide range of industries.
The toughness of high temperature steels is a measure of their resistance to brittle fractures without yielding any plastic deformations. This is determined by the steels ability to resist loads that would cause it to tear or de-laminate. To measure the toughness of a steel, a Charpy impact test is performed. A pendulum is used to determine the amount of energy needed to break a sample of the steel and the results are measured in Joules per square millimeter of material. The impact test can lead to failure in two ways: brittle fracture or hardening and softening. If the material has a brittle fracture, it indicates that the steel is not as tough as desired.
One of the main factors contributing to the toughness of high temperature steels is their ability to resist creep. Creep is the steady deformation and strain of a material at elevated temperatures caused by the force of external loads. This can be the result of long term thermal cycling, welding, or other high temperature stresses. The rate at which creep occurs significantly increases as temperature increases, so it is important to use a steel that can resist this type of damage at higher temperatures. The creep strength of a steel is usually characterized by its rupture stress in the form of a curve. This usually involves plotting the strain rate versus the applied load with a decrease in the strain rate indicating that the material can resist creeping better.
High temperature steels also differ in their high-temperature ductility. Ductility is a measure of the ability of a material to deform plastically under a load without fracturing and is important for forming and machining operations. High temperature steels have a high-temperature ductility that increases with increasing temperature. This property makes them particularly well suited for use in high temperature applications. The ductility of a material is characterized by a stress-strain curve which involves plotting the stress versus the strain of the material. As the strain increases, the stress increases until it reaches the yield point then it begins to decrease as the material starts to deform plastically.
Finally, high temperature steels can be characterized by their high temperature resistance to oxidation and corrosion. Oxidation and corrosion can occur in high temperature environments due to the presence of high concentrations of oxygen, moisture, and other corrosive gases. In order to prevent these two mutually exclusive phenomena, the surface of high temperature steels must be oxidation and corrosion resistant. This is usually achieved through anodization, which is a process where the surface of a material is converted into an oxide layer in order to increase its resistance to corrosion.
High temperature steels are essential components in many applications that require extreme temperatures. Their versatility, strength, and resistance to damage ensure that they will remain in use for many years. The toughness, creep strength, high-temperature ductility, and resistance to oxidation and corrosion of high temperature steels are all important factors to consider when selecting a steel for a specific application.
