Optimization of austenitizing parameters and Austempering transformation of nodular cast iron

In the 1930s, the isothermal quenching heat treatment technology was put forward in the United States through the isothermal transformation of steel. In the 1940s, the technology was applied to gray cast iron. In the 1950s, International Harvester company tested it on Nodular Iron and obtained higher performance ductile iron, which attracted people’s attention, but it has not been formally developed and applied. By the end of 1960s, austempered ductile iron was successfully used in gear parts, which greatly promoted the development of austempering process. Until the end of 1970s, austempered ductile iron (ADI) was developed and applied in many countries.

In foreign countries, austempered ductile iron is called ADI because of its special heat treatment process. Before 2000, it was widely called Austempered Ductile Iron in China. However, it has been found that due to the high content of Si in nodular iron, when the undercooled austenite is cooled to the γ – α transformation temperature zone, more Si elements have been dissolved in the transformed ferrite α, which inhibits the formation of carbides between α grains. This kind of carbide free α is called bainitic ferrite (α b), which is different from bainite B (containing Fe3C) in steel Although the α) of or ε carbides are similar in morphology, they are essentially different, and their mechanical properties are also very different. At the same time, with the progress of γ – α transformation, the untransformed γ on both sides of the α strip will be continuously enriched with carbon to a certain extent and stabilized. At this time, the growth of α will also stop, so that the transformation reaction of γ – α will not be completed 100%, and finally a bunch of needle like ferrite and high carbon austenite will be formed in ADI matrix. Therefore, ADI is generally called ductile iron in China. It is also because of this unique structure that the properties of ADI are equivalent to those of cast steel and forged steel. At the same time, the martensitic transition point of untransformed γ in the interlaced α B matrix can be reduced to about – 100 ℃ due to the supersaturated C element in solid solution, which effectively improves the thermodynamic stability of untransformed γ in ADI matrix.

So far, ADI Because of its high strength, high toughness, high bending fatigue strength, high fracture toughness, high specific strength and good wear resistance and shock absorption, it is known as a new generation of engineering structural materials, mechanical equipment lightweight materials and the most promising material to “replace steel with iron”. Now it has been widely used in transportation, agricultural machinery, military equipment and mechanical equipment and other engineering fields. According to incomplete statistics, by the end of 2019, the annual output of ADI materials in China has reached nearly 200000 tons, including a variety of engineering parts, wear-resistant parts, gears and crankshafts.

However, as a big foundry country, China’s annual ADI production only accounts for 1.1% of the annual output of ductile iron, which is far behind that of the United States, which accounts for 10% of the annual output of ductile iron. This is mainly due to the lack of due R & D investment, resulting in the inability to provide stable and sound ductile iron billets for ADI preparation. In addition, there are also problems such as lack of basic data under heat treatment process, unclear internal fine structure of ADI, and central hardenability of thick and large parts, which greatly hindered the development and application of ADI in the domestic market [19]. Therefore, it is necessary to strictly control the composition design, melting and spheroidizing inoculation process of nodular iron to prepare high-quality nodular iron blank, and then systematically and deeply study the distribution of carbon element in the process of nodular iron austenitizing, the fine structure of matrix structure after isothermal transformation and the correlation between process factors and Adi mechanical properties, which will help to obtain high-quality ADI castings.