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Properties of AustemperedDuctile Iron

ADIs microstructure consists of acicular fer- rite in a high carbon austenite matrix called ausferrite (Grade 1, 500X).

Table 1. The Five ASTM Standard ADI Grades (ASTM 897-90) Grade Tensile Yield Elongation Impact Typical

Strength Strength (%)* Energy Hardness(KSI)* (KSI)* (Ft-Lb)** (BHN)

1 125 80 10 75 269-3212 150 100 7 60 302-3633 175 125 4 45 341-4444 200 155 1 25 388-4775 230 185 N/A N/A 444-555

* Minimum Values **Un-notched Charpy Bars Tested at 72 7F

he development and commer-cialization of austempered duc-tile iron (ADI) has provided the de-sign engineer with a new group of cast ferrous materials that offer theexceptional combination of me-chanical properties equivalent tocast and forged steels and produc-tion costs similar to those of con-ventional ductile iron. In addition itsperformance-to-cost ratio, ADI alsoprovides a wide range of properties,all produced by varying the heattreatment of the same castings, rang-ing from 10-15% elongation with125 ksi (870 MPa) tensile strengthto 250 ksi (1750 MPa) tensilestrength with 1-3% elongation.

AustemperingDuctile iron foundries must produce

high-quality ductile iron to produce ADIcastings. Austempering cant cure poorquality iron. Rather, the effects of the slight-est defects (shrinkage, slag stringers, poormicrostructural features, etc. ) on the me-chanical properties of ductile iron become

magnified as a result of austempering.Austempering is a special heat treat-ment process that, when applied to ironsand steels, produces parts that are stron-ger and tougher than conventionally heattreated, as-cast or as-formed ferrous ma-terials. An austempering process is de-signed by the heat treat to produce thedesired mechanical properties specifiedby the customer.

The austemper heat treatment proce-dure for ductile iron castings in produc-ing ADI consists of three steps:

austeni t ize in the temperaturerange of 15501750F (840950C)for a time sufficient to produce afully austenitic matrix that is satu-rated with carbon;

rapidly cool the entire part to an

austempering temperature in therange of 450750F (230400C) with-out forming pearlite or allowing theformation of ausferrite to begin;

isothermally treat at theaustempering temperature toproduce ausferrite with an aus-tenite carbon content in therange of 1.82.2%.

ADI PropertiesA wide range of mechanical prop-

erties for ADI is available dependingupon the choice of heat treatment pa-rameters. Austempering at highertemperatures produces ADI withlower strength and hardness andhigher ductility and toughness com-

pared to lower austempering temperatures.The property combinations available forADI are documented in Table 1, which liststhe five ASTM standard grades of ADI.These specifications give the minimum ten-sile and impact levels along with typicalBrinell hardness values. Typical propertiesof ADI produced in industry as a functionof Brinell hardness are given in Fig. 1.

For a given level of ductility, ADI pro-vides twice the strength of conventionalductile iron. The strength of ADI is compa-rable to a variety of steels. The modulus of ADI, however, is about 20% less than thatof steel and must be accommodated for inthe early design stages.

Component weight reductions of greaterthan 10% can be realized by using ADI inplace of steel forgings. This often can be asignificant savings in terms of fuel con-sumption. ADI also has a relatively low weight per unit of yield strength, as shownin Fig. 2. (For this analysis, forged steel hasbeen normalized to 1.).

ADI rotating bending fatigue strengthsat 107 cycles for grades 1, 2 and 3 are 65, 70and 60 ksi, respectively. ADI has been docu-mented to exhibit fatigue strengths that areequal to or greater than forged steel. ADIalso responds favorably to surface treat-ments such as shot peening, which can re-sult in significant increases in fatiguestrength. ADI doesnt exhibit a true fatigue orendurance limit as a slight decrease in fatiguestrength at high cycles (>10 8) occurs. This de-crease in fatigue strength at high cycles, how-

ever, isnt as significant as that reported for fccmatrix materials such as aluminum and canbe accounted for in early design stages.

This Grade 1 ADI sway bar bushing for a Class 8 truck replaced forged and hardened steel with a resulting weight and cost reduction.

ADIs high strength-to-weight ratio has resulted in widespread applications that include auto and light/heavy trucks, construc- tion and mining equipment, railroad, agricultural, gears and crankshafts, among others.

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ADI offers excellent abrasion resis-tance. The austenite within theausferrite can undergo a strain-induced

transformation to martensite, resultingin an increase in flow stress and hard-ness. ADI can provide an equivalentlevel of abrasion resistance to bothaustempered and quench and temperedsteel at a lower hardness level.

Examination of the hardness levels of ADI (Table 1 and Fig. 1) might cause ma-chinability concerns. In practice, however,ADI can offer machining savings. Grades 1

1380

301 320 338 357 376 395 414 435 452 473 490 513 530

285 303 321 339 357 374 392 410 427 445 463 481 499

H V

BHN

E l o n ga t i o n

U n - n o t c h e d C h ar p y

T e n s i l e S t

r e n g t h

Y i e l d S t r e n

g t h

200

250

100

150

50

0

20

25

10

15

5

0

% ft. lb./Ksi MPa Joules

200

133

67

1035

1725

Fig. 1. This chart shows the typical properties of ADI as a function of Brinell hardness.

Fig. 2. ADIs relative weight per unit of yield strength per- forms well to other materials.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

CastAluminum

CastSteel

ForgedAluminum

DuctileIron

ForgedSteel

HeatTreated

Steel

ADIRatio: Unit of Weight/Unit of Yield Strength

and 2 are easier to machine than steels of anequivalent hardness due to the presence of graphite in the microstructure. Machining

requirements for the higher grades can becompleted prior to austempering becausethe part growth relationships during heattreatment are predictable (0.00050.003 in./in.), with more growth for ferritic thanpearlitic grades.

The damping capacity of ductile castiron is better than that of steel due to thepresence of graphite within the micro-structure. The unique microstructure of

ADI further enhances internal damping,especially in grades 4 and 5 with finerausferrite. Studies completed on

Kymenite ADI gears in Finland haveshown that vibrations are damped 40%faster in ADI than in steel components.This article was adapted from a chapter on Austempered Cast Irons from the Iron Casting Handbook and other materials prepared by John R. Keoughand Kathy L. Hayrynen, Applied Process,Inc., Livonia, Michigan.For more information, see Resources for Casting De- signers & Buyers, p. 67.

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