After the workpiece is heated to austenitize, it is first immersed in a salt bath or alkali bath near the M point temperature (150℃~260℃), maintaining a保定 time until the entire workpiece reaches the medium temperature. Then, it is removed and air-cooled. The quenching process for ductile iron pipe fittings to obtain martensitic structure is called martensitic grading quenching. This method is easier to operate than double medium quenching, significantly reducing internal stress in the workpiece, minimizing deformation and cracking, and achieving more uniform hardness. It is mainly suitable for small-sized, complex-shaped, or uneven cross-section carbon steel and alloy steel workpieces. The quenching process called bainite isothermal quenching is the one where the workpiece is rapidly cooled to the bainite transformation temperature range (260℃~240℃) and held isothermally to transform austenite into lower bainite. This method significantly reduces quenching stress and deformation, and the workpiece exhibits good strength, toughness, and wear resistance after bainite isothermal quenching. However, the production cycle is long, and the efficiency of ductile iron pipe fittings is low.