Bainitic cast iron with spheroidal graphite

Area	Foundry
title	Ausferritic cast iron with spheroidal graphite
Latest edition	2012-01
ISO	-

Bainitic cast iron with spheroidal graphite, also Austempered Ductile Iron (ADI), according to the translation, is an austenitic - ferritic cast iron with spheroidal graphite (but not bainite ; the name is historically determined, but incorrect).

definition

EN-GJS-800-8, V = 500: 1

ADI is a low-warpage isothermally tempered cast iron with spheroidal graphite. It is characterized by a very attractive combination of strength and elongation at break as well as high fatigue strength and good wear behavior . The basic mass of the ADI apparently has a bainite-like structure , consisting of needle-like carbide-free ferrite and carbon-enriched stabilized retained austenite without carbides. The retained austenite should be relatively stable (1.8–2.2% C) and not convert to martensite at low pressure or if the temperature falls below RT . C-contents of 1.2–1.6 in the retained austenite only make it metastable (easy to transform).

properties

The mechanical properties of the ADI are described in the European standard EN 1564 .

Short name	Relevant wall thickness t in mm	Tensile strength R _m in N / mm²	_Yield strength R _p0.2 in N / mm²	Elongation at break A in%
EN-GJS-800-10 EN-GJS-800-10-RT	t ≤ 30 30 ‹t ≤ 60 60 ‹t ≤ 100	800 750 720	500	10 6th 5
EN-GJS-900-8	t ≤ 30 30 ‹t ≤ 60 60 ‹t ≤ 100	900 850 820	600	8th 5 4th
EN-GJS-1050-6	t ≤ 30 30 ‹t ≤ 60 60 ‹t ≤ 100	1050 1000 970	700	6th 4th 3
EN-GJS-1200-3	t ≤ 30 30 ‹t ≤ 60 60 ‹t ≤ 100	1200 1170 1140	850	3 2 1
EN-GJS-1400-1	t ≤ 30 30 ‹t ≤ 60 60 ‹t ≤ 100	1400 Between manufacturers and customers to agree	1100	1 Between manufacturers and customers to agree

The machinability of the castings after the ADI heat treatment is very difficult and in most cases only possible by grinding (especially with metastable residual austenite, due to martensite formation). For this reason it is necessary that the castings are machined to size before tempering . The change in volume due to the structural transformation must be taken into account.

Manufacturing

Cast iron with spheroidal graphite (GJS - formerly GGG) serves as the basis for ADI. In order to influence the position of the so-called pearlite nose and the bainite nose (see Fig. 1) in the ZTU diagram and thereby achieve the isothermal transformation without pearlite or bainite formation , the melts intended for ADI are usually made with small amounts of copper, Alloyed molybdenum, manganese and nickel. Because of the necessary macro and micro homogeneity (even distribution of graphite spheres, minimum of 100 graphite spheres per mm², maximum of 0.5% non-metallic inclusions, maximum porosity of 1%, etc.) high quality requirements are placed on the ADI castings.

Fig. 1 ZTU diagram

The workpieces, which are usually machined to size, are heated to an austenitizing temperature of 850–950 ° C in a furnace under a protective atmosphere. The holding time is determined by the wall thickness and the chemical composition of the castings.

Fig. 2 ADI heat treatment

After complete austenitization, the casting is quickly transferred to the quenching bath and cooled (quenched) to the desired temperature for isothermal transformation. A salt bath is very often used as a quenching medium. The transition temperature is between 220 and 450 ° C (see Fig. 2). The conversion time is several hours and is determined according to the isothermal ZTU diagram. The conversion is carried out either in a salt bath or an oven. Too little holding leads to metastable residual austenite, too long holding leads to conversion into (real) bainite (ferrite with integrated carbide precipitation); both unfavorable for the ADI properties.

The temperature of the isothermal transformation influences the resulting structure and thus the mechanical properties of the castings. The lower the temperature, the higher the resulting hardness and strength of the material (or the lower the amount of retained austenite). The higher-strength ADI grades (> EN-GJS-1200-2) sometimes contain free carbide components.

application

Because of the mechanical properties already mentioned - high toughness, tensile strength, wear resistance, hardness and good elongation properties - the ADI is used more and more frequently in the mechanical engineering and vehicle industries. It is typically used in the manufacture of gears, crankshafts for heavy diesel engines , locomotive wheels , pressing tools, rollers , planetary gears and the like. Ä.

Web links

Table of contents of DIN EN 1564: 2012-01 at Beuth-Verlag