CONTENTS INTRODUCTION

ADI WITH ENHANCED ABRASION RESISTANCE

Carbidic ADI (CADI) Cold-rolled ADI Bainitic-martensitic ADI

ADI WITH ENHANCED STRENGTH & TOUGHNESS PROPERTIES

Ausformed ADI (AADI) Two-step ADI ADI with mixed structure Thin wall ADI

FUTURE TRENDS

– Squeeze casting of ADI

Austempered ductile iron ADI is a relatively new engineering material with exceptional combination of mechanical properties and marked potential for numerous applications.

The attractive properties of ADI return to its distinct and unique microstructure, which consists of fine acicular ferrite within C-enriched stabilized austenite (ausferrite)

The austempering transformation in ADI can be described as two stage reaction:

Stage I Reaction: o + HC (toughening)Stage II Reaction: HC + (embrittlement)

INTRODUCTION

ADI can be twice as strong as commercial SG iron at the same level of elongation.

ADI approaches the properties of a structural low alloy steels

ADI exhibits excellent fatigue strength and wear resistance.

Processing of the two austempering reactions can be followed by the change of mechanical properties vs. austempering time. Point A: completion of the first reaction. Point B: onset of the second reaction.

Time interval between A and B “the process window” represents the allowable austempering time for processing to obtain optimum mechanical properties.

Process Window:

Best buy: When comparing relative cost per unit of yield strength, in most instances ADI is the best buy.

100% recyclable.

10% less dense than steel. The relative weight per unit of strength of ADI compared to other materials allows economies of design without loss of performance. Thus for a given shape, an ADI component will be 10% lighter. Further, ADI is as strong or stronger than microalloyed forged steel with comparable ductility and fracture toughness, and has better low temperature impact strength.

ADI WITH ENHANCED ABRASION RESISTANCE

Carbidic ADI (CADI) Cold-rolled ADI Bainitic-martensitic ADI

Carbidic Austempered Ductile Iron

What is carbidic ADI (CADI)?

CADI is a ductile iron containing carbides, that is subsequently austempered to produce an ausferritic matrix with an engineered amount of carbides

CADI exhibits adequate toughness and excellent wear resistance. The abrasion resistance of this new material is improved over that of ADI and increases with increasing carbide content.

In a number of wear applications, it can compete favorably with high-Cr abrasion resistant (AR) irons in addition it providing improved toughness.

0.5% Cr

1% Cr

Methods of carbide introduction include:

(a) As-Cast Carbides: Alloying with carbide stabilizers, e.g., Cr, Mo, Ti, etc Controlling the cooling rate during solidification or during shakeout Adjusting the carbon equivalent to produce hypo-eutectic composition Surface chilling which may give either clear or definite chill or indefinite

directional chilling

(b) Carbides Precipitated During Austempering: Extending the second stage austempering to the extent where fine

carbides start to precipitate from the high carbon austenite HC +

Carbides produced from techniques can be “dissolved” to a controlled extent by austemper heat treatment

Methods of carbide introduction include:

(c) Mechanically introduced carbides: Crushed MxCy carbides are strategically placed in the mold cavity at the

desired location. The metal then fills in around the carbides resulting in a continuous iron matrix with discrete carbides mechanically trapped.

This method allows the engineer the option of placing carbides only where needed resulting in conventional ductile iron matrix throughout the rest of the casting

This process is currently only practised by license to Sadvik Corporation and the specific method used to contain the carbides “in place” during mold flitting needs further investigation

These particular carbides are essentially unaffected by subsequent austempering process

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0% Cr

0.5% Cr

0.25% Cr

1% Cr

Effect of Cr-addition and austempering time on wear property of CADI

Prospects and Potentials of CADI

Advantages Risks/Disadvantages Market Opportunities

• CADI is more wear resistant than grade 5 ADI with acceptable toughness

• CADI is less expensive and tougher than 18% Cr-white irons

• No capital investment is required for the foundry to add this new product

• CADI exhibit only limited machinability – possibly grinding only

• If alloying is used, the returns must be segregated

• Additional operation and costs may be incurred if carbides are cast-in

• Replaces Mn-steel at equal or lower cost

• Replaces 18% Cr white iron at lower cost

Potential ApplicationsAutomotive Camshafts and can followersAgricultural Rippers, teeth, plow points, wear plates and harvesterRailroad Contact suspension components and railcar/hopper car wear platesConstruction and mining Digger teeth and scarifiers, cutters, mill hammers, guards, covers,

chutes, plates, housings, transport tubes and elbows, rollers and crusher rollers

General industrial Pump components, wear housings and plates, conveyor wear parts, skids and skid rails, rollers and blast parts

Cold Rolling of ADI

The change in slope is associated with transformation induced plasticity (TRIP) Initial segment is characterized by the plastic deformation of r, at higher strains the deformation

process is modified by the formation of strain-induced martensite, which takes place when the deformation of austenite is exhausted.

In the course of fracture toughness tests of ADI in the upper bainite region, containing high volume fraction of retained austenite, r (martensite) transformation induced plasticity TRIP occurs, leading to superior toughness compared to cast iron

ln

ln

ln True Stress Versus lnTrue Strain

Variation of volume

fractions of retained

austenite and mechanically

formed martensite with cold reduction

percent

Strain-hardening of ADI

ADI components such as transmission gears, crankshafts, train car wheels are subjected to extensive machining during manufacturing and the strain-hardening behavior of ADI has profound influence on machining tool life and part surface finish.

In many applications, ADI components undergo substantial plastic strains (e.g., fatigue, wear). The total life cycles of these components are, therefore, influenced by the strain-hardening characteristics of the material

Strain-hardening of the ADI matrix causes strain-induced martensite formation and this contributes to the high wear resistance of ADI.

With increasing applications of ADI as a substitute for forged steels in manufacturing industries, the understanding of strain-hardening behavior of ADI is crucial

Because

The structural refinement is the main factor controlling the strength.

The increase in strength and hardness values with simultaneous decrease in ductility and impact is attributed to the increase of the hardening of ADI with cold deformation by both:

Deformation process (deformation bands and twins) Deformation induced martensite.

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UTSElongation

Variation of Elongation and Ultimate Tensile Strength with

Cold Reduction Percent

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impactToughness

Variation of Vickers Hardness and Impact Toughness with Cold

Reduction Percent

Bainitic / Martensitic ADI (B/M ADI)

A new grade of high wear resistant ADI is produced by adding less expensive alloying elements such as Si and Mn in the range of 2.5-3.0% (Si/ Mn ratio is 1.5) with minor additions of B, Ti.

The structure contains mainly acicular structure; bainite and martensite with some retained austenite. Castings are quenched and then tempered at a temperature range as for austempering (250-400 oC).

B/M ADI dual-phase structure has heigher toughness than single bainite or martensite. Developed mainly for mill balls with good combination of toughness (15 J), high hardness (55 HRC) and fatigue properties that surpasses that of high Cr-cast iron.

B/M ADI has good work-hardening capacity - the wear resistance is comparable to high Cr-white cast ion and twice of Mn-steel.

Abrasion Resistance of ADI

The good abrasive wear resistance of ADI can be attributed to:

1. Work-hardening of the retained austenite

2. Retained austenite transformation to martensite due to induced stresses and surface deformation during service

3. Presence of as-cast carbides with the addition of Mn, Cr, V and B

4. Hardness improvement by subsequent heat treatment of austenitization and austempering

ADI WITH ENHANCED STRENGTH AND TOUGHNESS

PROPERTIES

Ausformed ADI (AADI) Two-Step ADI Thin Wall ADI

ADI Processing Variables

The rate of ferrite formation during stage I austempering may be controlled by the following processing variables:

Chemical: including alloy content selection, which may be necessary for hardenability, together with the austenitization temperature selection which controls the matrix carbon content.

Thermal: including austempering temperature and time Mechanical: including mechanical deformation before austempering (ausforming) or

after austempering (cold rolling).

Ausforming of ADI (AADI)

Ausforming is introduced into the austempering schedule just after the quench but before any substantial transformation of austenite.

The mechanical deformation step could provide a mechanical driving force in the form of strain defects in addition to the chemical thermo-dynamic driving force in order to accelerate the rate of stage I of austempering.

The finer scale and more homogeneous ausferrite results in a dramatic increase in strength, hardness and wear resistance.

Effect of Ausforming on Microstructure

a

(a) Conventionally Processed

b

TR~80%

(b) Ausformed to 12.5% Reduction

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TR~85%

(c) Ausformed to 25% Reduction

Conventionally Processed Ausformed to 25% Reduction

• Ausforming was found to have a significant refinement effect on the microstructure of 2.0% Ni alloyed iron austempered for 10 minutes at 395°C.

• Structural refinement increases with the degree of ausforming reduction.

• Ausforming to 25% reduction had a significant structural refinement effect even after one minute austempering.

Transformation Kinetics

$ The rate of stage I transformation was higher in the unalloyed ADI compared to the Ni-alloyed irons.

$ For a given short austempering time, the ausferrite transformation was markedly accelerated due to the driving force introduced by deformation.

$ Ausforming to 25% reduction followed by austempering for one minute resulted in extremely high volume fraction of ausferrite of more than 80%.

Transformation Kinetics

In the conventionally processed irons, the saturated austenite total carbon content C.X remarkably increases only after 10 minutes austempering.

Ausforming results in faster progress of stage I reaction at short austempering times.

Total C-content of saturated austenite C.X undergoes a slight decrease after about 100 minutes austempering which may indicate the onset of stage II austempering transformation.

2% Ni

Strength

Both the yield and ultimate tensile strength values of AADI were superior to those of conventionally processed irons.

Dramatic increase in yield strength values (70%) and ultimate strength values (50%) of 2% Ni alloyed irons austempered for 10 minutes has been noticed.

Two different mechanisms may attribute to the increased yield and ultimate strengths of AADI.

The microstructure refinement can contribute to the higher yield strength,

Warm working of austenite increases the dislocation density in the bainitic ferrite, resulting in elevated yield and ultimate strength values.

Ductility

Low ductility of 2.0% Ni-ADI at short austempering times is due to the high amounts of martensite generated during cooling. Prolonged holding improves ductility as c and hence -stability decrease leading to lower amounts of martensite on cooling.

Ausforming to 12.5% reduction promotes the formation of more uniform ausferrite and results in an increase of -stability within ausferrite leading to improved ductility.

Increased ausforming to 25% reduction results in slight decrease in ductility. At such rather high degree of warm deformation, the ferrite nucleation in austenite is markedly enhanced. A large number of ferrite platelets separated by very thin films of austenite is formed and such matrix would limit the high plasticity of the retained austenite to be manifested, leading to some decrease in ductility.

Ultimate tensile strength is plotted against ductility and a curve generated from the minimum specifications for ADI (ASTM Standard A897-90) is superimposed on the plot for comparison.

Both UTS and ductilities of AADIs are superior to those of conventionally processed ones, regardless of the austempering times.

Elongation, %

The austempering time inminutes is indicated to each

data point

Applications of Ausformed ADI

It is more practical that the advantage of ausforming would be taken by forging rather than by rolling. The forging process may be performed on cast preforms, following the following steps:

1. Austenitized2. Quenched to the austempering

temperature3. Inserted into a die4. Pressed or forged to the final

shape5. Returned back into the

austempering bath to complete the accelerated transformation

In situations, where very severe deformation occurs, the work-piece may not need to be returned to the austempering bath to complete the transformation to ausferrite – transformation will have been completed by the time the work-piece is extracted from the die

Schematic of Ausforming Cast Preform

Applications of Ausformed ADI (Cont’d.)

The idea of creating preforms in ductile iron and then ausforming them to final shape could be quite effective for relatively simple shaped castings that must meet high demanding strength and ductility requirements, e.g. connoting rods for automotive applications.

This concept has been utilized to produce tank track center-guides, using a finite element simulation technique to match both the preform design and the die design so that a uniform equivalent strain throughout the casting averaged ~20% - No fracture or cracking tendencies have been reported.

Conclusions of Ausforming

Ausforming refined the ausferritic microstructure which is consistent with an increased ferrite nucleation rate associated with structural defects such as dislocations introduced by ausforming.

Both metallographic and XRD techniques indicate an enhanced stage I kinetics of the ausferritic formation and the ausferrite was more uniformly formed throughout the structure.

Alloying with 2.0% Ni decreased the rate of ausferrite transformation, particularly in the conventional undeformed ADI. The unreacted retained austenite after short austempering times transformed to martensite on cooling to room temperature.

The influence of ausforming to 12.5 and 25.0% thickness reduction on both the ultimate and yield strength was quite dramatic, strengthening mechanism involving both a refined microstructural scale and an elevated dislocation density in both phases of ausferrite was suggested.

Ausforming decreased the ductility of the unalloyed ADI, whereas it significantly improved the ductility of Ni-alloyed iron. Higher degrees of deformation are necessary to alleviate the deleterious effect of alloy segregation on ductility.

Two-Step Austempering of ADI

The mechanical properties of ADI are dependent on:

• The fineness of ferrite and austenite in ausferrite• The austenitic carbon (X C) where:

X is the volume fraction of austeniteC is the austenite C-content

Nucleation of C during austempering (and thus fineness of ausferrite) depends of supercooling, whereas higher C in austenite (C) is possible at higher austempering temperature.

Combining large supercooling and higher austempering temperature, will increase the austenitic carbon (XC) and in the same time refines the ausferrite structure.

Two-step austempering involves:

First quenching to a lower temperature from austenitizing temperature – thus increasing supercooling and nucleation.

Raising the temperature of the quenching media to facilitate faster diffusion of carbon and thereby increasing C-content of austenite and austenitic carbon in the matrix.

A-B: heal up to the austenitizing temperatureB-C: hold at the auslenitizing temperature (2 hours)C-D: quench to the firs! austempering temperatureD-E: hold at the first austempering temperature (for a few minutes until nucleation is completedE-F: raise temperature immediately to second austempering F-G: hold at the second austempering temperature (usually 2 hours)

Fracture Toughness of Two-Step ADI

The influence of applying different austempering treatment techniques (conventional and two-step) on fracture toughness of unalloyed and alloyed ductile iron (0.4% Mo and 1.5% Ni) at different austempering temperatures (270, 300, 330, 360oC) has been studied.

Two-step austempering process increases the abrasion wear resistance of ADI. The increase is related to the microstructural parameters, especially :

X C / √ d (where : d is the ferritic cell size

Strain hardening exponent (n-value)

The major wear resistant mechanism in ADI is:

The microstructural refinement in ausferrite

The solution strengthening effect (high c-content in austenite)

Strain hardening effect of the austenite phase

A new ADI was suggested with optimal ductility through the development of dual-phase microstructure (ferrite-ausferrite), by:

Intercritical annealing (partial austenitization) in the ( + + graphite) region,

Austempering at 250 - 400C

Colonies of proeutectoid ferrite are introduced within the ausferrite matrix.

Superior strength-ductility combination was achieved in ADI with duplex microstructure.

ADI with mixed structure provides a satisfactory solution: where FDI does not have the necessary impact resistance and yield strength.or:where ADI does not provide the required deformation level or the required machinbability

Higher deformations are required for automotive suspension parts, in particular for legal purposes in context of accident and hence, there has been a real interest to produce ADI with enhanced ductility properties.

ADI with Mixed (Ferrite-Ausferrite) Structure

High Strength Thin Wall ADI Castings

Limitations related to casting modulus are dictated by the capability of the quenching medium to extract heat from the part at a rate that avoids the austenite to pearlite transformation.

In heavy sections, segregation to cell boundaries of alloying elements (e.g., Mn and Mo) needed to achieve the required hardenability levels embrittles the austempered structure.

In thin wall components, the formation of as-cast carbides, if not totally dissolved during austenitization, by high Si-content or due to an excessive nodule count can limit the ductility.

It is then possible to further improve the properties of such castings with the austempering process. As a result, the thin wall ADI becomes a logical choice for the production of small, light weight and cost effective automotive components, e.g., hollow connecting rods.

Justification

Thin-Wall Ductile Iron Castings

Thin-Wall Ductile Iron Castings

Thin-wall ductile iron castings produced by General Motors Research Laboratories, exhaust manifold, differential carrier and power steering gear housing.

Examples of Lightweight, Ductile Iron, Mounts and Brackets Made for Ford by Intermet Wagner Foundry

Thin-Wall Ductile Iron Castings

Increased undercooling of thin wall ductile iron castings results in:

(a) Nodule count increase + refinement of austenite dendrites

Increased G/ surface area enhances transformation kinetics, alleviates segregation and promotes austemperability.

2mm – 20% insulating sandNodule count 600/mm2 (As-Cast)

2mm – nodule count 300/mm2

Thin-Wall Ductile Iron Castings

(b) Higher tendency for carbide formation – could be avoided by:

Adjusting chemical composition and increasing carbon equivalent

Addition of insulating sand to the mold material

Dissolution of carbides during austenitization

2mm – nodule count 740

Effect of Casting Wall Thickness and Austempering Temperature on Mechanical

Properties

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FURTHER DEVELOPMENT

Squeeze Casting of ADI

Over the past few years, a novel technique is under development simultaneously at TU Aachen-Foundry Institute (Prof Sahm) and at Component CPC, Finland (Johansson) to produce superior quality ADI castings using squeeze casting of molten metal in permanent mold followed by in-situ heat treatment of the hot knock-out castings in the austenite.

Process Steps and Temperature Regime of the SQ Process and the In-situ Heat Treatment

Some Tensile Test Results of SQ Cast Samples Compared to EN Standard

Comparison of Different Suspension Materials

Results of Tensile Tests and Hardness of SQ ADI

Unique Advantages of SQ ADI

Sound castings can be produced without feeders or gating system as the solidification expansion was used to counteract solidification shrinkage.

Increased heat transfer avoids formation of macro- and micro-segregation, which decrease the mechanical properties of ADI.

Chemical composition of ductile iron can be selected to avoid any metastable solidification in spite of the extremely fast solidification.

The production process is shorter and less energy consuming as the elimination of sand from the process would allow the hot castings coming out from the permanent mold to be directly introduced to the heat treatment furnace.

The structure of the SQ ADI is much finer (the graphite as well as the ausferrite), which means better mechanical properties (ultimate tensile strength, elongation and fatigue strength).

The casting surface is entirely free from any surface defects, which again means higher fatigue strength.

The machinability is better More environmentally friendly.

CONCLUDING REMARKS

1. Extensive research work over the past decade has helped to develop the property combination of ADI in three directions:

Increased strength, ductility and toughness Enhanced wear resistance/toughness combination Improved machinability

2. ADI offers high levels of mechanical properties at a competitive cost. When the high strength of ADI is taken into account, it could successfully compete with lightweight alloys, as the additive weight required to give unit strength is lower. Moreover, when the relative cost of ADI required to give unit strength is considered, ADI seems to be one of the cheapest alloys. These points have yet to be fully appreciated by many design engineers. Currently, novel processing techniques adopted in achieve better strength and toughness properties include ausforming, cold-rolling, two-step austempering, squeeze casting and others.

3. The current research work aiming at improving machinability of ADI looks rather vital for the future of this material. The available machining techniques required for forging steel are not always suitable for ADI components, particularly on a high volume machining line dedicated to the production of one specific product. This problem can be minimized with the development of ferritic or ferritic + ausferritic ADI structures.

4. Carbidic as well bainitic/martensitic ADI offer opportunities for superior wear resistance, combined with reasonable toughness, which may open new applications for ADI.