After the workpiece is heated and austenitized, it is first immersed in a salt bath or alkali bath with a temperature near the M point (150℃~260℃), holding for a certain period, until the entire workpiece reaches the medium temperature. Then, it is removed and air-cooled to obtain quenching process for spheroidal cast iron pipe fittings to achieve martensitic structure. The spheroidal cast iron pipe coatings come in several types and are referred to as martensite grading quenching. This method is easier to operate than double medium quenching, significantly reducing the internal stress of the workpiece, decreasing deformation and cracking, and resulting in more uniform hardness. It is mainly suitable for small-sized, complex-shaped, or uneven cross-section carbon and alloy steel workpieces. After the workpiece is heated and austenitized, it is rapidly cooled to the bainite transformation temperature range (260℃~240℃) and held isothermally, leading to the quenching process known as bainite isothermal quenching. This method significantly reduces quenching stress and deformation. The workpiece, after bainite isothermal quenching, exhibits good strength, toughness, and wear resistance, although the production cycle is long and efficiency is low. It is suitable for workpieces with complex shapes, high dimensional precision requirements, and high demands for both hardness and toughness, such as various small cold and hot forming dies, forming tools, and springs, etc. The hardenability of steel refers to its ability to achieve a certain depth of hardening layer during quenching under specified conditions (workpiece size, quenching medium), which is the depth of the martensitic structure obtained by the steel under specified conditions. Hardenability is a primary thermal treatment property of steel. The depth of the hardening layer is the vertical distance from the quenched surface of the workpiece to the specified hardness value (usually 550HV). The deeper the hardening layer, the better the hardenability. The main factors affecting the hardenability of steel are the chemical composition of the steel. Spheroidal cast iron pipe fittings with 0.77% carbon content, a eutectoid steel, have good hardenability in carbon steels. Most alloying elements (except for Co) can significantly improve the hardenability of steel. Additionally, the type of spheroidal cast iron pipe coating, the quenching heating temperature, and the original microstructure of the steel can also affect the hardenability. The hardenability of steel is a fundamental property and an important basis for rational material selection and correct formulation of thermal treatment processes.