Low-alloy steel castings Heat Treatment
Low-alloy steel castings are those containing alloy elements such as chromium, molybdenum, and nickel, in a lower concentration than those used in stainless steel. As a result, low-alloy steel castings have excellent mechanical, physical and chemical properties, but are usually not corrosion resistant and are hydroscopic. As such, low-alloy steel castings have to be heat treated to enhance their properties.
Most low-alloy steel castings are used in applications that require strength and toughness. As such, they are typically heat treated to achieve the required combination of properties. Heat treatment is the process of heating and cooling a material at specific temperature ranges with the intention of changing or modifying the properties of the material. Heat treatment options for low-alloy steel castings may include strain hardening, stress relief, annealing, tempering and quenching.
Strain hardening is used to increase the strength and hardness of the casting by introducing strain deformations. This is done by heating the casting to a temperature above the limit of the material and then subjecting it to rolling, drawing, and forging operations. Stress relief is used to reduce the stresses in a casting to prevent cracking or failure due to thermal shock. This is done by heating the casting to a temperature well below its recrystallization temperature.
Annealing is used to relieve stresses, improve ductility, and ensure machinability. This process is done by heating to an austenitizing temperature, cooling slowly to a point below the ferrite-transformation temperature, and then cooling at a moderate rate. Tempering is used to improve toughness, ductility, and strength by introducing metastable compounds into the casting. This is done by reheating the part to a temperature that is below the austenitizing temperature and then cooling slowly. Quenching is used to again increase the strength and hardness of the casting. This is done by heating the casting to a pre-determined austenitizing temperature and then cooling rapidly.
Heat treatment yields different results depending on the combination of temperature and the amount of time the metal is exposed to each temperature. It is important to choose the proper heat treating parameters to achieve the desired mechanical properties in the casting. The common parameters used in heat treating low-alloy steel castings are austenitizing temperature, cooling rate, tempering temperature, and quenching medium. Depending upon the design and application of the part, each of these parameters will vary.
Heat treatment of low-alloy steel castings is essential to ensure the castings have the desired strength and ductility. Strain hardening, stress relief, annealing, tempering, and quenching are the five most common methods used to heat treat low-alloy steel castings. It is important to select the proper parameters of each of these processes to ensure the castings have the desired properties.
