The full name of isothermal quenching is austenitic isothermal quenching. Austenitic isothermal quenched ductile iron (add) is obtained from nodular cast iron after austenitic isothermal quenching. Its microstructure is related to the isothermal quenching temperature. It is generally believed that 350 ~ 450 ℃ is the upper bainite transformation zone and 230 ~ 350 ℃ is the lower bainite transformation zone. Isothermal quenching is carried out in the upper bainite transformation zone (generally 350-380 ℃) in order to obtain upper bainite and a certain amount of high carbon stable austenite, Its mechanical properties can reach: tensile strength ob ≥ 1000MPa, elongation after fracture 6 ≥ 10%, notchless impact toughness AK > 80J / cm2, hardness not less than 30hrc, with good impact toughness and fatigue strength.
Isothermal quenching in the lower bainite transformation zone (generally 270 ~ 330 ℃) is used to obtain the lower bainite structure (often accompanied by a small amount of austenite and martensite). Its mechanical properties can reach: the tensile strength is still ≥ 1200Mpa, the elongation after fracture is ≥ 2%, the impact toughness without notch AK >, 30J / cm2, and the hardness is not less than 38hrc. It has good wear resistance and high fatigue strength. At lower temperatures (< 250 “C) After isothermal quenching, martensitic matrix structure with a small amount of austenite is obtained, which has high hardness and wear resistance. Austenitic austempered ductile iron material replaces forged steel to make crankshaft. Compared with static strength and bending fatigue strength, austenitic austempered ductile iron crankshaft can be competent for the service conditions of high-power engine. The development of ADI materials improves the mechanical properties of ductile iron materials and expands the application space of ductile iron materials. At the same time, due to the significant cost advantage, austenitic austempered ductile iron will be a very promising material.
At present, the main problems existing in ADI production in China are: the basis of ADI production, the quality of nodular cast iron crankshaft blank is unstable, and few manufacturers can stably mass produce castings that meet the quality requirements of ADI crankshaft blank: the heat treatment equipment is backward and can not meet the requirements of isothermal quenching process; Processing problems have not been well solved, etc. High quality ductile iron blank is the basis of ADI production. The quality of ductile iron blank will directly affect the performance and production cost of ADI. Stable chemical composition, high spheroidization rate, many and evenly distributed graphite balls, constant matrix structure, dense structure and no casting defects are the requirements of ADI for ductile iron blanks.
The matrix contains a certain amount of high carbon retained austenite, which greatly improves the properties of the material. High carbon retained austenite has good toughness and exists discontinuously in the matrix structure. On the one hand, for the fatigue cracks originating from or near the surface of the crankshaft, the existence of residual austenite in the microstructure will make the material strain strengthened, at the same time, the plastic deformation of austenite will hinder the crack propagation, reduce the crack propagation rate, and improve the fracture toughness; On the other hand, Austenite Deformation induces martensite transformation in the plastic zone at the fatigue crack tip, which alleviates the stress concentration there.