Austempered Ductile Cast Iron

1- Introduction:

ADI was discovered almost 50 years ago and found successful commercial application in 1972. Today

ADI has become the material of choice for the designer, as it offers the best design combinations of

low cost, design flexibility, machinability, high strength to weight ratio, good toughness, wear

resistance & fatigue strength. ADI is more environment friendly than many competing materials. See

figure 1

Fig. 1. A comparison of the yield (proof) strength of ADI to that of several engineering materials [1].

ADI is obtained by heat treatment. The latter modifies the microstructure of the basic cast iron to obtain

a bainitic type matrix. This cast iron treatment enables reaching mechanical characteristics comparable with

those of certain steels. The ADI castings manufactured to high specifications require a control of the cast

iron GGG (Ductile Iron) metallurgic quality, follow-up the bainitic treatment with austenitizing, followed by

a stage chill (Austempered) in order to guarantee an ADI stable microstructure [2].

2- Background of ADI [3]:

The austempering process was first developed in the early 1930’s as a result of work that Bain, et al, was

conducting on the isothermal transformation of steel. In the early 1940’s Flinn applied this heat treatment to

cast iron, namely gray iron. In 1948 the invention of ductile iron was announced jointly by the British Cast

Iron Research Association (BCIRA) and the International Nickel Company (INCO).

By the 1950’s, both the material, ductile iron, and the austempering process had been developed.

However, the technology to produced ADI on an industrial scale lagged behind. The 1970’s would arrive

before highly efficient semi-continuous and batch austempering systems were developed and the process

was commercially applied to ductile iron.

By the 1990’s, ASTM A897-90 and ASTM A897M-90 Specifications for Austempered Ductile Iron

Castings were published in the US while other specifications were developed worldwide. In addition, a new

term to describe the matrix microstructure of ADI as “ausferrite” was introduced.

The five Grades of ADI according to ASTM A897/897M are listed in Table 1. Figures 2 (a) and (b)

show the ausferrite microstructure for Grades 1 and 5 ADI, respectively.

Table 1 ASTM A897/897M -02 Minimum Property Specifications for ADI Castings

Figure 2a: Photomicrograph of Grade 1 ADI.

Specimen was etched with 5% Nital.

Figure 2b: Photomicrograph of Grade 5 ADI.

Specimen was etched with 5% Nital.

3- Characteristics of ADI [2]:

toughness, ductility and high ultimate tensile strength,

high fatigue strength,

high wear resistance,

high shock resistance.

excellent flowability and low production costs,

possibility to make complex forms with a minimum of machining,

high amortization capacity,

characteristics not sensitive to the massivity.

4- Applications of ADI:

The development and commercialization of Austempered Ductile Iron (ADI) has provided the design

engineer with a new group of cast ferrous materials which offer the exceptional combination of mechanical

properties equivalent to cast and forged steels and production costs similar to those of conventional Ductile

Iron. In addition to this attractive performance: cost ratio, ADI also provides the designer with a wide range

of properties, all produced by varying the heat treatment of the same castings [4].

Table 2 Typical applications of Ductile Iron for Austempered Ductile Iron

Light Auto and Truck Components:

Light vehicle ADI applications include

suspension components, knuckles, spindles,

hubs, tow hooks, hitch components, differential

gears and cases, engine and accessory brackets,

camshafts, engine mounts, crankshafts and

control arms. Constant velocity joints for four

wheel drive GM vehicles have been produced in

ADI since 1978 and currently run at volumes of

over 5,000 per day.

Construction and Mining Components:

This segment includes all manner of collets, ring

carriers, wear plates, sprockets, covers, arms,

knuckles, shafts, rollers, track components, tool

holders, digger teeth, cutters, mill hammers,

cams, sway bars, sleeves, pavement breaker

bodies and heads, clevises, and conveyor

components.

Railroad

Gears:

Gears represent some of the best known, most

widely publicized and high potential uses of

ADI. During the early 1970’s the Finnish

company Kymi Kymmene Metall began to

replace forged steel with ADI in a in a wide

range of gears, with highly satisfactory results.

In North America, ADI achieved a major

breakthrough in 1977, when General Motors

converted a forged and case hardened steel ring

gear and pinion to ADI for Pontiac rear drive

cars and station wagons. The decision came after

nine years of development work and six years of

field testing. The automaker was able to gain

both significant cost savings and product

improvement by changing to ADI.

Agricultural :

Farming and agricultural applications for ADI include plow points, till points, trash cutters, seed boots, ammonia

knives, gears, sprockets, knotter gears, ripper points, tractor wheel hubs, rasp bars, disk parts, bell cranks, lifting

arms, and a great variety of parts for planters, plows, sprayers and harvesters.

5- Composition of ADI:

In many cases, the composition of an ADI casting differs a little from that of a conventional ductile iron

casting. When selecting the composition, consideration should be given to the elements that adversely affect

casting quality e.g. formation of carbides and inclusions. A typical composition of ductile iron casting used

for making Austempered ductile iron is given in the table [4].

Table 3 Typical Composition of Ductile Iron for Austempered Ductile Iron

There are three important points to consider when selecting the chemical composition of ADI.

The iron should be sufficiently alloyed to avoid transformation of pearlite but not over alloyed.

The micro structure should be free from intercellular carbides and phosphides.

The tendency for chemical segregation should be minimized for the sake of uniformity in the cast

component.

6- Heat treatment of ADI [5]:

ADI requires a two-stage heat treatment. The first stage, austenitizing, requires heating to and holding at

about 900 °C (1650 °F). This is followed by the second stage, which requires quenching and isothermally

holding at the required austempering temperature, usually in a salt bath. Typically, Austempered ductile iron

is produced by heating the castings in a controlled atmosphere to an austenitizing temperature between 815

to 925 °C (1500 to 1700 °F). The castings are held at temperature for a long enough time to saturate the

austenite with carbon in solution. The castings are then cooled at a rate sufficiently fast enough to avoid the

formation of pearlite and other high-temperature transformation products to the appropriate transformation

temperature (this may vary from 230 to 400°C, or 450 to 750 °F, depending on the hardness and strength

required). The castings are held at the selected transformation temperature for a long enough time to

produce the desired properties. Austempered ductile iron transformed in the 370 °C (700°F) range exhibits

high ductility and impact resistance at a tensile strength of about 1035 MPa (150 ksi). When transformed at

260 °C (500 °F), it exhibits wear resistance comparable to case hardened steel and tensile strength in excess

of 1380 MPa (200 ksi).

Figure 3 Sequence of the heat treatment cycle [6].

Thus, the selection of the austempering temperature and time of holding is critical. In general,

austempering in the 240 to 270°C (465 to 520 °F) range provides a component that has maximum strength

but limited ductility, while austempering in the 360 to 380 °C (680 to 715 °F) range yields a component that

exhibits maximum ductility and toughness, in combination with relatively high strength, albeit lower than

that obtained at the lower austempering temperature (see Fig. 4). Figure 5 compares the strength and

ductility of as-cast nodular irons and of nodular irons subjected to the heat treatments discussed in this

section.

Figure 4 Relationship between austempering

temperature and the strength and ductility of a 1.5

Ni – 0.3 Mo alloyed ductile iron. Austenizing

temperature was 900 oC.

Figure 5 Strength and ductility ranges of as-cast

and heat-treated nodular irons

7- Mechanical properties of ADI [6]:

The combination of high strength, toughness, excellent machinability and wear resistance is the primary

appeal of Austempered ductile iron (ADI). Designers have exploited these properties to produce many

engineering parts such as gears, automobile crankshaft and axle box. Furthermore, Austempered ductile iron

has replaced forged steel in many of its applications. ADI provides the designer with a family of materials

with attractive properties usually at lower cost and weight than the competing materials. Therefore, the

material has been recommended for high duty applications. See table 1 that shows the mechanical properties

of ASTM grades of Austempered Ductile Cast Iron.

8- Weldability of ADI :

The Weldability term relates to all aspects of welding that it must have the metal to produce a good weld

and do not affect the mechanical properties of heat affected zone. The materials may have good Weldability,

under a set of specific conditions, but it can lower Weldability if these conditions change. To produce a good

weld should be improved Weldability, given a good design of the joint, a good selection of filler metal,

preheating and post heating, having a good control of heat input and its dissipation during the welding

process, because otherwise have a good handle on the above parameters can be found welding defects [7].

The weldability of the cast iron is determined by the metallurgical properties of the base metal, chemical

composition and heat treatments, as well as the size and shape of the piece, the filler metal and welds metal

sensitivity to dilution with heterogeneous base metals. The dilution is carried out by mixing the fille metal

and base metal. If the weld metal cannot tolerate the mixture of base metal, there is little weldability, where

the percentage of dilution of the weld metal depends on the heat input, welding process and the

configuration of the joint [7].

When the base metal cannot withstand the heating and cooling cycles imposed by the welding, the

cracks appear an says that the material has low weldability. Moreover when the metallurgical characteristics

of the weld metal and base material are undesirable with respect to weldability, these can be corrected using

adequate protection with shielding gases, fluxes specific for filler metal, welding process and in some cases

with heat treatments of preheating and post-heating [7].

For welding of ADI, two problems have to be considered. First, the formation of massive cementite in

as-welded ductile iron weld and partial fusion zone because of fast cooling rate not only increases cooling-

cracking susceptibility of the welded joint but also impairs its machinability. Secondly, after austempering

the microstructure and properties of ADI, weld should match those of ADI [8]. Finally, we will try more

welding processes with different variables in order to encounter the previous problems.

9- References :

1- ADI, K.A. Smith, L.Z. Jones

2- REPAIRING TECHNOLOGY OF HIGH-STRENGTH CAST IRONS by Jozef Meško, Anton

Hopko, Peter Fabian – May 2011

3- The Production of Austempered Ductile Iron (ADI), Kathy L. Hayrynen - Applied Process

Technologies Division, Livonia, MI - 2002 World Conference on ADI

4- http://www.ductile.org

5- ASM Handbook, Volume 1, Properties and Selection: Irons, Steels, and High Performance Alloys,

September 2005

6- Improvement in strength and toughness of austempered ductile cast iron by a novel two-step

austempering process, Jianghuai Yang, Susil K. Putatunda

7- WELDABILITY OF CAST IRON WITH THREE ELECTRODES OF HIGH NICKEL, Jose

Luddey Marulanda Arevalo, Mauricio Escobar, Gabriel Castaño.

8- Welding consumable research for austempered ductile iron (ADI), D.Q. Sun∗, X.Y. Gu, W.H. Liu,

Z.Z. Xuan