Research on Thermal Fatigue Resistance of Ductile Iron

Ductile iron is commonly used to make structural parts on mechanical parts and equipment, mainly due to its relatively low cost, controllability of graphite spheroidizing process and matrix, good mechanical properties and excellent stability.The automobile exhaust manifold is one of the most important parts in the automobile exhaust system and withstands mechanical and thermal loads.

Thermal loads during acceleration and deceleration, or during start-up and stop, can cause thermal fatigue in the vehicle exhaust manifold.Thermal fatigue of exhaust manifold is mainly caused by internal stress caused by temperature gradient inside and outside the workpiece during its working process and is an important factor affecting the life of exhaust manifold.Due to this hot and cold cycle, thermal fatigue often changes the shape of the exhaust manifold, causing cracks and breakage in severe cases.As a result, the exhaust manifold of the vehicle may fail earlier than expected, affecting the safety and reliability of the engine.

Ferritic ductile iron represented by high silicon-molybdenum ductile iron has been considered as an ideal material for manufacturing automotive exhaust manifolds due to its good high temperature strength, oxidation resistance, low growth characteristics and low production costs.About 70% of the exhaust manifolds of Ford and Dongfeng automobiles in the United States use this material.The design of the vehicle structure and engine will affect the operating temperature of the exhaust manifold and the temperature difference between the parts.At present, the exhaust temperature is generally 870-900 C. This kind of automobile exhaust manifold made of high silicon-molybdenum ductile iron is no longer suitable for such high operating temperature. Therefore, it is necessary to develop high temperature materials with higher heat resistance.

High-nickel ductile iron belongs to high-alloy cast iron. The content of nickel, the main alloy element, generally varies from 18% to 22%. It has austenitic structure and good high-temperature mechanical properties and stability. In particular, the addition of rare-earth elements can significantly improve its thermal fatigue properties.

FAW Volkswagen has been using German standards for the production of automotive exhaust manifolds for a long time, resulting in high costs.With the rapid development of automobile industry, manufacturing cost has become the main consideration. Under this circumstance, the goal of localization of automobile exhaust manifold material is put forward.In order to achieve this goal, Jilin University and FAW Foundry Co., Ltd. jointly developed new high silicon-molybdenum ductile iron and high nickel austenitic ductile iron to meet the different vehicle types of FAW Group.In order to guarantee the performance and safety of automobile exhaust manifold manufactured with it, it is necessary to carry out thermal fatigue test.

The thermal fatigue behavior of high silicon molybdenum nodular cast iron and high nickel nodular cast iron under different temperature cycles is mainly studied, and the test data are obtained in order to find a kind of nodular cast iron material which can meet the exhaust manifold requirements of FAW new vehicle, and to provide basis for other automobile companies to select the reasonable exhaust manifold material.