Because nodular cast iron contains graphite phase, which is two-phase solidification, the carbon content of its matrix can be changed by heat treatment process. ADI isothermally quenched ductile iron is a kind of casting alloy with high strength and good plasticity and toughness, which is obtained from spheroidal graphite iron through isothermal quenching heat treatment, and is mainly based on austenite. It is also known as austenitic ductile iron. Because there is no brittle cementite in the austenite structure, ADI has excellent toughness and can effectively slow down crack expansion. The mechanical properties are shown in the table.
| Brand | Tensile strength Rm/MPa | Yield strength Rp0.2/MPa | Elongation A/% |
| QTD800-10 | 800 | 500 | 10 |
| QTD900-8 | 900 | 600 | 8 |
| QTD1050-6 | 1050 | 700 | 6 |
GB/T24733-2009 “Isothermally quenched ductile iron castings” has made corresponding provisions on the grade, technical requirements, sampling, etc. of isothermal quenched ductile iron. Its general properties include elastic modulus of 17000 MPa, Poisson’s ratio of 0.28 and thermal diffusion coefficient of 12 × 106 m2/s, specific heat 2.5 J/(kg · K), thermal conductivity 1.08 W/(m · K), density 7.7 g/cm3. At present, QT800 is mainly used to replace ordinary nodular cast iron and low alloy steel parts, and QT900 is used to replace 40Cr spherical core.
The main manufacturers of ADI parts include Henan Oudiai, Hebei Qinghe Hengji Machinery Casting Co., Ltd., Dalian Sanming Factory, etc. Its product categories have also developed from several parts such as leaf spring support, bracket, towing hook, etc. to dozens of parts. In the trial production of 5t high mobility off-road vehicle, Dongfeng Motor Co., Ltd. adopted ADI to replace traditional steel castings, and optimized the structure based on the excellent casting performance and mechanical properties of nodular cast iron. Under the CAE simulation analysis, the weight of the key stress parts is reduced as much as possible, and the total weight of the 14 parts is reduced from 630.62 kg to 380.66 kg after the weight reduction, The weight reduction rate reached 39.6%, and the number of ADI parts was further increased in the subsequent improvement, so that the weight of ADI parts of the whole vehicle reached 550.4 kg; FAW Group actively carried out the research and development work of ADI on the chassis parts of heavy-duty trucks and mining vehicles. The front beam, steering knuckle, rear axle, wheel hub, etc., improved the design by using the excellent mechanical properties of ADI to reduce costs on the basis of lightweight.
A variety of ADI materials with various structures and processes for preparing ADI have been developed worldwide: austenitic ADI, non-austenitic ADI, duplex ADI, carbide-containing ADI, low-carbon equivalent ADI, two-step ADI, deformation AOI, extrusion ADI, as-cast alloy ADI, continuous cooling quenching ADI and isothermal quenching ADI, but at present, the conventional isothermal quenching ADI for obtaining austenitic ductile iron is still the main method.
Isothermal quenching treatment method: heat the nodular cast iron to the austenitizing temperature zone, hold it for a certain time, make its matrix fully austenitized, and then rapidly quench the casting into the salt bath furnace, oil tank or flowing bed furnace, so that it can be rapidly cooled to the austenite transformation temperature zone, hold it isothermal for about 1 to 4 hours, and then air cooled to room temperature. After isothermal quenching, the matrix structure is composed of high carbon austenite, acicular ferrite and graphite ball. The shape, size and quantity of graphite ball have no obvious change.
Advantages of isothermal quenching treatment: less alloying elements are added than as-cast alloy method, lower production cost, easy control of cooling rate, and stable structure. Although more alloying elements are added than continuous cooling and quenching process, the structure is stable, it is not easy to form internal stress, and tempering treatment is not necessary.