Application of Austenitic Ductile Iron in Automotive Exhaust Manifold

During the extensive use of Ferritic Ductile iron, nickel is often added to improve its heat resistance. When the content reaches 1wt%, austenite begins to appear in the structure. With the increase of nickel content, the austenite area gradually increases.

In order to meet the requirements of different parts, Austenitic Ductile Iron with different compositions, mechanical properties, physical properties and good economic and forming properties has been widely used.Austenite is obtained by alloying on the one hand and heat treatment on the other.Material transformation temperature between austenite and ferrite during heat treatment is affected by many factors such as chemical elements. Austenite obtained by heat treatment process is initially considered to be a very concise method. Some achievements have been made by many scholars.

This process is often referred to as tempered austenite (ADI).Scholars represented by D.Mandal have found that the austenitizing temperature has a great influence on the final structure and mechanical properties, and the main materials of which are high silicon steel, Cu-Ni-Mo and aluminium-rich carbon steel.

Although it can also improve the content of C, Cu, Ni, Mo and other elements, in order to obtain uniform austenitized ductile iron, certain casting control and special heat treatment equipment are required. As a result, the additional cost is very high and has not been further promoted.

Overaddition is also known as high alloy cast iron.Austenitic ductile iron of nickel, nickel-manganese, manganese-copper and nickel-manganese-copper series has been formed.

According to research reports, the shape and quantity of graphite in ductile iron and its structure have a great influence on its properties, so many manufacturers pay great attention to the selection of raw materials and the control of liquid phase composition in order to produce qualified Austenitic Ductile iron. For example, Changchun FAW Group uses PRO-CAST software to design and control the casting, such as Changchun FAW Group uses PRO-CAST.Castings are designed by software.Different kinds of spheroidizing agents have been developed to obtain more spherical graphite, such as FeSiMg7RE3 alloy spheroidizing agent for QTRSi4Mo1 exhaust manifold casting.

Before and after 1990s, according to the requirements of Euro I and Euro II standards, the working efficiency of some high-grade automotive engines has been improved, resulting in exhaust temperature exceeding 900 degrees Celsius.In view of the excellent high-temperature mechanical properties of high-Ni Austenitic Ductile Iron (commonly referred to as “Ni-resistant cast iron” in the United States), some foreign automotive manufacturers have begun to try to use it in the manufacture of exhaust manifolds for high-grade engines.At present, there are 9 types of Austenitic Ductile Iron in American ASTM standard, such as D-2, D-2B, D-2C, D-3, D-3A, D-4, D-5, D-5B and D-5S, with nickel content of 18%-38%.

In the same period, due to the limited use of nickel in China, the research on high-nickel ductile iron was relatively late. High-nickel Austenitic Ductile iron was listed as corrosion-resistant cast iron in the Manual of Foundry Technical Standards (May 2004 edition).The research institutes represented by Huazhong University of Science and Technology (formerly Huazhong Institute of Technology) and Zhejiang University started to study the properties of high-nickel Austenitic Ductile iron. With the continuous emergence of achievements, the enterprise gradually attracted attention and application.

With the deepening of research field and continuous improvement of its performance, new materials emerge in an endless stream, such as Ni20, Ni20Cr2 and Ni25, the emergence of the automotive industry changes with each passing day in order to increase driving power, improve economy and reduce environmental pollution. Turbocharging technology is widely used in the engine of cars, such as Audi, Passat, FAW-VW Audi B7, Jetta.As a result of A4, exhaust temperature becomes higher and higher. Common ductile iron and medium silicon-molybdenum cast iron can no longer meet the requirements. High nickel austenitic ductile iron will be replaced.Nowadays, the commonly used high nickel ductile iron mainly contains 18-36% nickel and about 6% chromium. Its tensile strength is 330-550 MPa and elongation is 4-40%, such as high nickel D5B ductile iron, NiSiCr3552 and NiSiCr3532 of Xixia Internal Combustion Engine Inlet and Exhaust Pipe Co., Ltd.However, due to the economic cost, sometimes nickel coating or surface coating is used, sometimes nickel coating or surface coating is used instead of part of nickel by manganese, and sometimes manganese is used instead of part of nickel.