Novel Processing Techniques and Applications of Austempered Ductile Iron (Review)

Journal of the University of ChemicalA.A. Nofal,Technology L. Jekova and Metallurgy, 44, 3, 2009, 213-228

NOVEL PROCESSING TECHNIQUES AND APPLICATIONS OF AUSTEMPERED DUCTILE IRON (REVIEW)

A.A. Nofal, L. Jekova1

Central Metallurgical R&D Institute (CMRDI) Received 08 May 2009 P.O. Box 87 Helwan, Cairo, Egypt Accepted 13 July 2009 E-mail: [email protected]

1Institute of Metal Science, Bulgarian Academy of Science 67, Shipchenski prohod blvd., Sofia, Bulgaria

ABSTRACT

The as-cast mechanical properties of ductile iron can be significantly improved through an austempering heat treatment. This has led to the birth of a new member of the cast iron family, the austempered ductile iron (ADI) with its unique microstructure; spheroidal graphite in an ausferritic matrix. The excellent property combination of ADI has opened new horizons for cast iron to replace steel castings and forgings in many engineering applications with considerable cost benefits. Thanks to the extensive research efforts made all over the world over the past few years, new processing techniques have opened even more opportunities for this very prospective material acquire better combinations of strength, ductility, toughness, wear properties as well as machinability. This review describes the key features of those normal processing techniques and the resulted new applications for ADI. The new developed techniques include ausforming, cold rolling, two-step austempering, ADI with mixed (ferritic-ausferritic) structures, ferritic (austenite-free) ADI, carbidic ADI (CADI), squeeze-cast ADI, bainitic/martensitic ((B)M), dual-phase ADI and finally thin-wall ADI castings. Through- out the review, special focus will be made on the research work done at CMRDI over the past decade. Keywords: cast iron, austempered ductile iron, microstructure, spheroidal graphite, ausforming, cold rolling, two- step austempering.

INTRODUCTION The morphology of the final two-phase matrix microstructure is determined by the number, shape and Austempered ductile iron (ADI) is a relatively new size of the initially formed ferrite platelets in the first engineering material with exceptional combination of stage austempering reactions. The control of this stage mechanical properties and marked potential for numer- of transformation will, therefore, ultimately control the ous applications [1-4]. The attractive properties of ADI final microstructure and mechanical properties. The rate return to its distinct and unique microstructure, which of ferrite formation during stage I austempering may be consists of fine acicular ferrite with carbon enriched sta- controlled by chemical, thermal or mechanical process- bilized austenite (ausferrite). The austempering transfor- ing variables [5]. mation in ADI can be described as two-stage reaction: Since the announcement of the first production Stage I Reaction: γ → α + γ (toughening) of ADI in the late decades of the last century, a world- c HC Stage I Reaction: γ → α + ε−carbides (embrittlement) wide explosion in research started, which provided a HC

213 Journal of the University of Chemical Technology and Metallurgy, 44, 3, 2009 sound foundation for expanding the production of this Naturally, an optimum final microstructure could prospective material in many industrialized countries be produced by including elements of all three process- during the 1990s and beyond. By the turn of the coun- ing variables. It has been shown [6-9] that mechanical try, the ADI market had begun to rapidly accelerate processing of ADI can act as a control valve for the from a modest beginning in the early 1970s to an esti- stage I austempering reaction. In ausformed austempered mated worldwide production level of 150,000 tons in ductile iron (AADI), mechanical deformation is uti- 2005. This growth is expected to continue with the an- lized to affect the microstructure and, consequently, the nual world production of ADI reaching 300,000 tons mechanical properties of ductile iron due to accelera- by the end of next year 2010. This article is a trial to tion of ausferrite reaction, refining the microstructure report the extensive efforts made over the last few years and increase of the structural homogeneity. to optimize the property combination of this very pro- A recent work [10] has shown that ausforming spective material. The novel applications associated with up to 25 % reduction in height during a rolling opera- these developments will be shortly discussed. This re- tion contributed to add a mechanical processing com- view indicates that extensive work has been conducted ponent to the conventional ADI heat treatment thus over the past years by CMRDI staff. increasing the rate of ausferrite formation and leading to a much finer and more homogeneous ausferrite prod- AUSFORMED AUSTEMPERED DUCTILE IRON uct (Fig. 2). The effect of ausforming on the strength (AADI) values was quite dramatic (Fig. 3) (up to 70 and 50% It has been shown [5,6] that the rate of ferrite increase in the yield and ultimate strength respectively) formation during stage I austempering may be controlled [10]. A mechanism involving both a refined microstruc- by the following processing variables: tural scale as a result of enhanced ferrite nucleation • Chemical - including alloy content selection for a hardenability purposes together with the austenitization temperature selection which controls the matrix carbon content. • Thermal - including austempering temperature and time. • Mechanical - including mechanical deformation introduced into the austempering schedule just after quenching, but before any substantial transformation of austenite (ausforming) (Fig. 1). b

Fig. 2. SEM Micrographs of ADI alloyed with 2% Ni austempered at 375°C for 1 min. (a) Conventionally processed; (b) ausformed to 25 % reduction. Arrows indicate the Brittle Martensite formed in many zones in the Fig. 1. Schematic representation of the ausforming process. conventionally processed ADI.

214 A.A. Nofal, L. Jekova

lized to produce tank track center guides [13] using a finite element simulation technique to match both the preform design and the die design so that a uniform equiva- lent stain throughout the casting averaged : 20 %. No inclination - to fracture or cracking has been reported.

COLD ROLLING OF ADI When the ausferrite is subjected to stresses exceeding its yield strength, both retained austenite and ferrite undergo plastic deformation, resulting in a partial transformation of the residual austenite into martensite, with its very hard-and-brittle tetragonally-dis- Fig. 3. Yield strength vs austempering time and ausforming tributed body-centered structure. The ADI is thus reduction for adis alloyed with 2 % Ni. strengthened by the combined effect of strain-hardening and the presence of martensite. The strain-induced martensite transformation was reported [14] to result together with an elevated dislocation density was sug- in martensite contents of up to 25 % in microstructures gested. Hardenability elements such as Ni and Mo are subjected to 25 % cold-work, which is the practical usually added to increase hardenability of thick section threshold of severe crack formation. Such cold-work castings, and ausforming to higher degrees of deforma- reduction is considerably higher than the strain the as- tion was found necessary to alleviate the deleterious cast ductile iron can support before cracking. effects of alloy segregation on ductility [11]. Transformation of austenite to martensite by de- It is more practical that the advantage of formation has been extensively studied in austenitic stain- ausforming would be taken by forging rather than by less steel [15,16]. For austempered ductile iron (ADI), rolling. The forging process may be performed on cast M. Johansson [17] reported that under wear conditions, preforms, austenitized and quenched to the austempering it work hardened rapidly due to the partial transforma- temperature, inserted into a die, pressed or forged to tion of retained austenite to martensite, which improved the final shape and then returned back into the the fatigue properties. The optimum benefits were ob- austempering bath to complete the accelerated trans- tained when volume fraction of retained austenite was formation. Minimal deformation degrees by conventional higher than 30 %. Very little has been reported [6,18] forging standards, i.e. an average strain of 25 % would on the martensite transformation induced by cold roll- be sufficient for the forming part of the processing se- ing and its effect on microstructure and hardness of low quence. It has been reported [12] that in situations where alloy ADI. Aranzabal [19] showed that, in the course of very severe deformation occurs, the work-piece may not fracture toughness tests, ADI in upper bainite region, need to be returned to the austempering bath to com- containing high volume fraction of retained austenite plete the transformation to ausferrite, as the latter will >30%, γα→ (martensite) transformation induced plas- have been completed by the time the work-piece is ex- ticity occurs, leading to superior toughness compared tracted from the die. with conventional cast iron. The idea of creating preforms in ductile iron and In recent work [20,21], cold rolled ADI flat ten- then ausforming then to final shape could be quite ef- sile specimens were prepared along the rolling direc- fective for relatively simple shaped castings that must tion and the specimens were subjected to uniaxial ten- meet high demanding strength and ductility require- sile test. From the ln true stress vs ln true strain curve ments, e.g. connecting rods for automotive applications. shown in Fig. 1, it is evident that the data are fitted by It is understood that certain deviations in design ele- two interesting straight lines over the entire range of ments of both preform as well as the die set should be strain. At plastic strain ε = 0.0094 there is an obvious involved compared to the design of conventional ADI increase in slope of the straight line fitting the data cor- process. The above mentioned concept has been uti-

215 Journal of the University of Chemical Technology and Metallurgy, 44, 3, 2009

is believed that the initial segment of ln stress versus ln strain corresponding to the lower plastic strain (Fig. 4) is characterized by the plastic deformation of the retained austenite. At higher strains as previously reported the deformation process is modified by the formation of strain induced martensite which takes place when the deformation of austenite has been exhausted [20].

Effect on Mechanical Properties Increasing the cold rolling (CR) reductions, the amount of retained austenite (γ ) was decreased due to r partial transformation of γ to martensite, Figs. 6 and 7 r Fig. 4. ln true stress versus ln true strain. indicate that the amount of mechanically generated martensite increases with increasing the CR reduction, responding to the strain hardening exponent n. This Fig. 7. increase was previously observed [19,22] in the alloyed As can be seen from Figs. 8 and 9, the elonga- ADI, which means that the Holloman equations, con- tion and impact toughness decrease while the ultimate structing the relationship between true stress and true tensile strength, and hardness increase with increasing strain (σ = κ ε) is not followed. The change in slope of CR reduction. This is attributed to increase of the hard- the ln σ - ln ε representation can be associated with ening of the investigated ADI with cold deformation TRIP effect. Fig. 4 shows the change in the instanta- processes (deformation bands and twins) and deforma- neous n during the tensile testing. The instantaneous n tion - induced martensite. It must be mentioned that (n*) is the n-value at a given strain based on the the observed changes in the mechanical properties at Holloman equation where n*=d ln σ /d ln ε. This value light cold deformation (7 % reduction) are mainly at- is determined from the true stress - true strain curve. tributed to the hardening of this alloy by plastic defor- As shown from Fig. 5, n* increases with the tensile strain. mation concentrated in γ . At this light deformation the r This increase is associated with the strain-induced mar- amount of mechanically formed martensite is very small tensitic transformation during the tensile test. The gen- (Fig. 6) [20]. erated strain by the volume expansion accompanying With increasing applications of ADI as a substi- the martensitic transformation, stimulates new marten- tute for forged steels in manufacturing industries, strain sitic transformation resulting in the increase in the in- hardening of ADI is attracting more attention and more stantaneous n-value strain. As a consequence of changes research is required for better understanding of this phe- in the structure in the course of tensile deformation, it nomena. This may be attributed to the following factors:

40 18 38 16 36 14 34 12 32 10 30 Austenite 8 28 Martensite Martensite Instantaneous n (n*) Retained Austenite 26 6 24 4 22 2 0 5 10 15 20 25 30 Cold Deformation % Fig. 5. Variation of instantaneous value of n with plastic Fig. 6. Variation of volume fractions of retained austenite strain. and mechanically formed martensite with cold reduction pct.

216 A.A. Nofal, L. Jekova

19%Red Xã =28.7% 7%Red. Xã =36%

γ Fig. 7. Microstructures of ADI after two different reductions showing marked decrease of r at 19 % reduction.

• ADI components such as transmission gears, 1350 8 crackshafts, train car wheels are subjected to extensive machining during manufacturing and the strain-hard- 1300 ening behavior of ADI has profound influence on ma- 1250 7 chining tool life and part surface finish. 1200 • In many applications, ADI components undergo UTS substantial plastic strains (e.g. fatigue, wear). The total 1150 6 Elongation life cycle of those components are, therefore, influenced 1100 UTS by the strain-hardening characteristics of the material. Elongation 1050 5 • Strain-hardening of the ADI matrix causes 0 5 10 15 20 25 30 strain-induced martensite formation and this contrib- Cold Deformation% utes to the high wear resistance of ADI. Effect of Cold Rolling on the Structure Fig. 8. Variation of elongation and ultimate tensile strength Characteristics with cold reduction pct. In the same work carried out at CMRDI [21, 22], for X-ray diffraction (XRD) analysis, the XRD patterns of the prepared sample were collected by Diano 460 44 diffractometer. Ni-filtered Co-Kα radiation at 50 kV 440 42 40 and 30 mA was used. The peak intensities of the hole 420 38 patterns were collected by the step scanning technique 400 36 380 with small step size (∆2θ = 0.05°). Hole-pattern refine- 34 360 ment of Rietveld method was applied using FullProf Hardness 32

340 Toughness Impact [23] and BGMN [24] computer programs. Quantitative Hardness 30 320 analysis [25] of the present austenite γ and ferrite + impact 28 martensite (α + α ) phases was used to determine the 300 26 ’ 0 5 10 15 20 25 30 weight fraction of these phases. The structure charac- Cold Deformation % teristics (lattice parameters, crystallite size and internal Fig. 9. Variation of vickers hardness and impact toughness residual micro-strain) of existing phases were estimated. with cold reduction pct.

217 Journal of the University of Chemical Technology and Metallurgy, 44, 3, 2009

0% Ni 0.95% Ni 1.9% Ni Internal Strain (a.u) Strain Internal (ã)

á Ü

( / ) Fig. 10. Full width at half-maximum intensity of α- and γ- phases versus cold reduction pct.

Peak shape was fitted using Thompson-Cox-Hast- ing-Pseudo-Voigt function. In the present work the full (a.u) Strain Internal width at half maximum (FWHM) was determined for selected (220) and (200) diffraction peaks from γ- and α + α’ phases, respectively. The peak positions (2θ) of diffraction lines were determined at maximum intensities. From Fig. 10, it is evident that there is an increase Fig. 11. Variation of the internal strain arbitrary unit in (a) in peak broadening due to cold deformation for both austenite (b) (ferrite + Martensite)-phases with CR austenite (γ) and ferrite + martensite (α + α’) phases, reduction. but with small variation in case of α + α’. The observed increase in FWHM due to CR represents the contribution of internal strain (Gaussian part) and crystallite size (Lorentzian part), Figs. 11 and 12 0% Ni show the variation of strain and size as a function of 0.95% Ni

CR reduction. It is clear that the internal strain increases 1.9% Ni in case of γ- and decreases in α-phase while the crystallite size decreases in both cases as the deformation increases. The decrease of internal strain in α-phase with SizeGrain (a.u) deformation may be attributed to some strain relief (ã) mechanism.

(á / Ü ) TWO-STEP AUSTEMPERING OF ADI The mechanical properties of ADI are mainly dependent on: • The fineness of ferrite and austenite in ausferrite • The austenite carbon (X ,C ) where: γ γ

X is the volume fraction of austenite; SizeGrain (a.u) γ C is the austenite C-content. γ Both these factors depend on the austempering temperature. Higher undercoolings enhances the nucleation of ferrite from the parent austenite and hence promotes finer ausferrite structure with higher yield and Fig. 12. Variation of the crystallite size in arbitrary unit in tensile strength but lower ductility. On the other side, austenite and (ferrite + martensite)-phases with CR reduction.

218 A.A. Nofal, L. Jekova

a-b he ating to austenitiza tio n temperature a c b-c austenitizing c-d q uenching to the sup ercoole d temperature d-e few minutes holding a t the first au st empe ring temperature e-f hea ting immed iately to seco nd au st empe ring temperature f-g ho lding at secon d austempering temperature (~2 hr) g-h a ir cooling to room temperature

f g Temperature Temperature

d e

a h Time

Fig. 13. Scheme of the two step austempering process.

higher austempering temperatures result in coarser feathery ferrite and austenite with lower strength but higher ductility properties. Moreover, higher austempering temperatures lead to higher (X ,C ) pa- γ γ rameter which, in turn, increases fracture toughness and fatigue strength of ADI. It is possible, therefore, to optimize the mechanical properties of ADI by ausferrite refinement as well Fig. 14. The influence of applying different austempering as increasing the austenite carbon. Hence, the novel treatment techniques (conventional and two-step) on concept of two-step austempering was conceived [26], fracture toughness of unalloyed and alloyed ductile iron (0.4% Mo and 1.5% Ni) at different austempering which involves first quenching the alloy to a lower tem- temperatures (270, 300, 330, 360oC) has been studied [30]. perature (250-270°C) after austenitization and thus increasing the undercooling, and then, once the nucleation of ferrite is complete, immediately raising the tem- crostructural refinement in ausferrite and solution perature of the quenching media to a higher tempera- strengthening effect (high C-content in austenite) along ture to enhance faster diffusion of carbon and increase with strain hardening effect of the austenite phase [27,28]. austenite carbon (X ,C ) in the matrix. A scheme of this γ γ Meanwhile, the two-step austempering process process is shown in Fig. 13. resulted in higher crack growth rate and lower fatigue The two-step austempering process has resulted threshold than the single step ADI process. The crack in higher wear resistance in ADI compared to the con- growth rate increment due to the two-step austempering ventional single-step austempering process. An analyti- process increases with the austempering temperature cal model for the abrasion wear behavior of ADI re- [29]. Moreover, it has been shown in a recent publica- vealed the dependence of wear behavior of ADI on the tion carried out at (CMRDI) [30] that the two-step microstructural parameters, especially the parameter austempering process increases the fracture toughness X C /√d where d is the ferritic cell size as well as the γ γ of ADI, where the increment in the fracture toughness strain hardening exponent (n-value). The major wear by the two-step process is more pronounced in the un- resistant mechanism in ADI was shown to be the mi- alloyed irons [30] as shown in Fig. 14.

219 Journal of the University of Chemical Technology and Metallurgy, 44, 3, 2009

ADI WITH MIXED (FERRITE-AUSFERRITIC) plex ADI [33]. Such irons were also found to exhibit STRUCTURE superior mechanical properties to those exhibited by ADI offers an excellent compromise between the ductile irons with ferrite-martensite matrices [34,35]. values of proof stress and elongation; both are very highly Recently, an attempt was made to investigate the effect appreciated in the field of suspension parts in the auto- of alloying elements Ni, Mo and Cu on the properties of motive industry. Many potential uses of ADI in compe- ADI produced by intercritical annealing and highest duc- tition with forged steel and aluminum alloys can be tility (16 %) and impact strength (145 J) were achieved opened due to strong possibilities of weight and vol- in ADI alloyed with 1 % Ni and 0.25 % Mo [36]. The ume reductions. wear resistance of ADIs with dual matrix structures was A car steering knuckle, which has high safety re- found to decrease with the increase of proeutectoid fer- quirement is a new potential application for ADI, where rite and decrease of ausferrite content [37]. the impact resistance is the principle criterion determin- The properties of the new mixed structure coming part size and design. The main advantage in using pared to both ADI and ferrite ductile iron (FDI) is shown ADI is a question of weight and cost. Ferritic ductile in Table 1 [31], which emphasizes the main points of iron (FDI) is less expensive than forged steel but requires improvement: heavier sections due to its lower strength. Replacement • Replacement of FDI with ADI structure re- of FDI with ADI results in 30 % improvement of energy sults in an improvement of tensile strength, yield strength absorbed, but the plastic deformation becomes lower. and impact energy. Higher deformations are required for suspension parts, • Replacement of conventional ADI with that of in particular for legal purposes in contex of accident and a mixed structure leads, to hardness reduction (and hence there has been a real interest to produce ADI with hence better machinability) together with an increase enhanced ductility properties. in impact deformation. For an identical heat treatment Recently [31, 32], a new ADI was suggested with cycle, the properties of ADI with mixed structure seem optimal ductility through the development of dual-phase to effectively depend on the Si-content. The low-Si grade microstructures (ferrite-ausferrite or ferrite-martensite) seems more attractive as it has tensile strength, proof by the proper heat treatment. Such dual phase (duplex) strength and hardness comparable with those of pearl- microstructures are obtained by intercritical annealing itic structures with elongation and impact strength close (partial austenitization) in the (α + γ + graphite) region to those of FDI. The higher strength values may be at- followed by austempering at 250-400°C, and hence, colo- tributed to the more complete austempering transfor- nies of proeutectoid ferrite are introduced within an mation together with the absence of any pearlitic struc- ausferrite matrix. Superior strength-ductility combina- ture in the last to freeze zones. Moreover, the low Si- tion was achieved in ADI with duplex microstructure content reduces the hardening effect of Si on ferrite compared with conventional ductile irons [32]. More- which leads to significant deterioration of impact over, tensile strength and elongation of ferritic ductile strength. The low Si type has higher impact energy than iron (FDI) could be doubled in a ferrite-ausferrite du- FDI and higher impact deformation than ADI [31].

Table 1. Points of improvement [31].

FDN ADI Mixed Structure

Tensile strength MPa 510 1060 700

Proof stress (0.2%) MPa 380 725 550

Elongation % 14.5 14.5 14.5

Hardness HB 207 321 250

Machinability +++ + ++

Impact (charpy) J/cm2 140 180 160

Deflection (drop) mm/kJ 38 21 30

220 A.A. Nofal, L. Jekova

• The ADI with the mixed (ferrite-ausferrite) could be achieved through the selection of Cr-content/ structure provides a satisfactory solution where FDI does austempering temperature combination [38]. not have the necessary impact resistance or ADI does (b) Carbides precipitated during austempering: not provide the required deformation level or the re- extending the second stage austempering will result in quired machinability. the precipitation of fine carbides from the high carbon austenite: γ →α+ ε. ΗC CARBIDIC ADI (CADI) (c) Mechanically introduced carbides; crushed CADI is ductile iron containing carbides that is M C carbides are strategically placed in the mold cav- x y subsequently austempered to produce an ausferritic ma- ity at the desired location. The metal then fills in around trix with an engineered amount of carbides. Volume frac- the carbides resulting in a continuous iron matrix with tion of carbides may be controlled by partial dissolution discrete carbides mechanically trapped. This method during the subsequent austenitization process and hence allows the engineer the option of placing carbides only the proper abrasion resistance/toughness combination may where needed resulting in conventional ductile iron be reached. CADI exhibits excellent wear resistance and matrix throughout the rest of the casting. These par- adequate toughness. The abrasion resistance of this new ticular carbides are essentially affected by subsequent material is superior to that of the ADI and increases with austempering process. This technique is currently only increased carbide content. In a number of wear applica- practiced by license to Sadvik corporation and the spe- tions, it can compete favorably with high Cr-abrasion cific method used to contain the carbides in place resistant irons with improved toughness. during mold filling needs further investigation. Several methods have been suggested to intro- (d) Fully or mostly ferritic matrix is hard face duce carbides to the structure of ADI [38-40]. welded in the area of greatest wear, which results in a (a) As cast carbides can be introduced to the struc- carbidic weld and a heat affected zone at the weld/cast- ture of ADI through alloying with carbide promoting ing interface. Subsequent austemper heat treatment has elements such as Cr, Mo, Ti, etc., controlling the cool- little or no effect on the weld structure, depending on ing rate during solidification, adjusting the carbon equiva- the chemical composition of the weld material chosen, lent to produce hypoteutectic composition or through whereas the heat affected zone is eliminated and a fully surface chilling. Recently, it has been reported that good ausferritic matrix results in all areas except the weld balance between wear resistance and impact toughness area itself. In some weld applications, powdered metal

Table 2. Prospects and potentials of CADI.

221 Journal of the University of Chemical Technology and Metallurgy, 44, 3, 2009

• 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 Fig. 15. Process steps and temperature regime of the SQ process surface defects, which again means higher fatigue and the in-situ heat treatment [42]. strength. • The machinability is better. carbides can be purged into the molten weld to provide • More environmentally friendly. additional wear resistance [38]. Table 3 [43] shows some tensile test results of a The advantages, disadvantages, market opportu- squeeze casted ring gear test samples compared to EN nities as well as potential applications of CADI are il- standard. Fiat suspension fork was tested for fatigue lustrated in Table 2 [38]. strength and amazing results were achieved. This component had to pass the test without cracks loaded with SQUEEZE CASTING OF ADI 250 KN after lifetime of 300,000 cycles. The squeeze ADI was produced without an austenitizing step cast forks could pass the test with 5000 KN without based on a patent published in 1985 by P.B. Magalhas failure up to 3-10 million cycles. The tensile properties [41]. In this process, casting in a permanent mold al- were much higher than pearlitic ductile iron or even lowed the ejection of the part at a temperature level better than microalloyed steels as shown in Table 4 [43], above 850°C where a completely austenitic range could yield strength was much higher than steel and elonga- be guaranteed. Subsequent quenching in a salt bath tion about the same. would lead to the ADI ausferritic microstructure. Based on this process, a novel technique has been BAINITIC/MARTENSITE (B/M) DUAL- simultaneously developed at TU-Aachen Foundry In- PHASE ADI stitute and component CPC - Finland [42, 43] to pro- A new grade of wear resistant ductile iron, with duce superior quality ADI castings, using squeeze cast- properties similar to those of ADI was recently devel- ing of molten metal in permanent mold, followed by oped by combining less expensive alloying with a con- in-situ heat treatment of the hot knock-out casting in trolled heat treatment to produce a bainitic-martensitic the austenite range followed by normal austempering in dual-phase structure. Alloying elements such as Si and a salt bath. Fig. 15 shows a schematic representation of Mn promote bainitic transformation were added in the the process [42]. range of 2.5-3.0 % each. Moreover, such alloying facili- This technique seems to have some unique ad- tates the separation of bainitic transformation from mar- vantages, such as: tensitic one. Manganese significantly reduces the Ms tem- • Sound castings can be produced without feed- perature, dissolves unlimitedly into austenite and im- ers or gating system as the solidification expansion was proves hardenability. At Mn-contents higher than 3.0 %, used to counteract solidification shrinkage. austenite increases on the account of bainite and car- • Increased heat transfer avoids formation of bides starts to precipitate. Manganese carbides forma- macro- and micro-segregation, which decrease the me- tion is restrained effectively by silicon, which promotes chanical properties of ADI.

222 A.A. Nofal, L. Jekova

the formation of bainitic ferrite and the enrichment of chromium cast iron and twice that of manganese steel. asutenite with carbon. The resulting increase in austen- Under conditions of low impact load, such as that in ite stability will reduce the possibility of manganese the case of grinding balls and liners in the small and carbides formation and manganese dissolves in austen- medium diameter ball mills, the B/M ductile iron can ite and ferrite. The pearlite formation is avoided by replace the manganese steel as a wear resistant mate- controlled cooling heat treatment; consisting of three rial. In such application, the B/M ductile iron shows stages: good work-hardening effect due to the presence of the (1) Water spraying quenching is applied for rapid retained austenite. The surface work-hardening effect cooling from austenitization temperature to about 300°C can considerably improve the wear resistance of the in a few minutes which suppress any pearlite transfor- hardened surfaces, while the core of the balls remains mation. tough. (2) Soaking in a heat preservation setting for Currently, there is an increasing interest [46] in bainitic transformation over a range of temperature form the as-cast austenite-bainite ductile iron, produced by the spraying end temperature to 200°C for 2 hrs. alloying with >3.0 % Ni, and up to 0.8 % Mo and up to (3) Air cooling to room temperature for marten- 1.0 % Cu. The tensile strength and elongation of the as- sitic transformation. cast alloyed ductile iron were shown to be higher than The resulting microstructure containing bainite, those of irons subjected to hot-shake-out from sand martensite and 8-10 vol. % retained austenite along with molds at 250°C and held for two hours. The inferior the graphite spheroids will give an excellent combina- properties in the second case were attributed to the in- tion of hardness and toughness which reach 51.5 HRC creased levels of retained austenite. and 21.7 J/cm2, respectively, and this could be attributed to the following factors [44, 45]: AUSTENITE FREE ADI i) The bainite needles split the undecompensed Recently [47], it has been reported that fully fer- austenite and effectively decrease the size of martensite ritic (austenite free) ADI could be produced by leading to improved strength and toughness. austempering at 260°C and then tempered at 484°C for ii) High toughness of bainite restricts the propa- 2 hrs, without compromising the mechanical proper- gation of cracks originating mainly at the graphite/ma- ties. The process was rather sensitive to the austempering trix interface and hence toughens the iron. temperature, initial austempering at 385°C and tem- iii) The presence of retained austenite (8-10 vol. pering at the same temperature of 484°C resulted in a %) contributes to the toughening effect. drastic reduction in the ductility and fracture toughness The impact wear resistance of the B/M ductile of the material. The machinability, however, was im- iron was found to be comparable with that of the high proved in both cases. Further development in this di-

Table 3. Some tensile test results of squeeze cast test samples compared to EN standard [43].

Test 1 EN Test 2 EN Tensile strength, MPa 1238 1200 1115 1000 Yield strength, MPa 968 850 839 700 Elongation, % 13.4 2 15.3 5 Hardness, HB 388 340/440 363 300/360

Table 4. Comparison of different suspension fork materials [43].

GJS-600-5 SQ ADI (Tested) Microalloyed Steel Tensile strength, MPa 600 950 1000 Yield strength, MPa 370 750 550 Elongation, % 10 11 12

223 Journal of the University of Chemical Technology and Metallurgy, 44, 3, 2009 rection may lead to considerable enhancement of the materials, type and amount of inoculating material in machinability properties of ADI. combination with the spheroidizing method adopted, casting design and other foundry basic practices [53,54]. HIGH-NITROGEN ADI When the commercial introduction of ADI in Recently high nitrogen stainless steels were de- 1972, consistent efforts have been made to identify new veloped to take advantage of the austenite stabilizing applications of this new emerging material, however, effect of nitrogen to replace nickel, with the apparent difficulties have been encountered in producing ADI economic advantages [48, 49]. Meanwhile, thick wall thicker than 100 mm due to the segregation of ADI castings are usually alloyed with nickel and/or hardenability elements added to prevent pearlite for- molybdenum for hardenability purposes to avoid the mation. Such difficulty in obtaining the required formation of pearlite in the centre of the slowly cooled austemperability and the heterogeneous microstructures castings. do not represent a real problem when producing thin Currently attempts are being carried out at wall ADI castings due to the insignificant segregation CMRDI to alloy ADI with nitrogen for the same pur- tendency associated with rapid solidification of those pose. Nitrogen is added to the molten iron either by thin wall castings. The use of ADI in thin-wall and high melting under high pressure or by adding nitrogen con- strength parts has, however, been mentioned in a very taining solid materials. limited number of reference [55, 56]. Successful case was recently reported [57], where a hollow connecting THIN-WALL ADI CASTINGS rod for a two-cylinder car engine and a front upright To achieve fuel economy in automotive indus- for a racing car were successfully made of thin wall try, reducing the vehicle weight has been a major re- ADI, which confirms the capability of ADI to build search area of interest over the last few decades. Al- complex thin walled parts of high strength. With recent though the general trend has been to use low density development in inoculation theory and practice, it be- materials (aluminum, magnesium and composites) in- came possible to cast thin-wall ductile iron parts com- stead of cast iron and steel in the automotive industry, pletely free from carbides. Consequently, further im- numerous examples have been recently noted in the lit- provements in the properties of thin wall ADI castings erature where iron castings started again to replace alu- could be achieved with the austempering process. In a minum in the industry. This comparison is encouraged recent study [58], the results of a R&D program on the by the increased strength, ductility, stiffness, vibration effect of wall thickness (3-10 mm) and silicon content damping capacity, as well as reduced cost [50]. If the (2.4-2.7 %) on the properties of ADI has been reported. yield stress/cost ratio of the various materials is com- It has been shown that thin-wall ADI castings pared, the new member of the ductile iron family, the austempered at 360°C and containing low silicon can ADI, is most of the time the winner. When mechanical exhibit ultimate strength exceeding 1100 MPa with more properties, density and cost are included in material than 10 % elongation. This is an indication that evaluation, ductile iron may offer more advantages than austempered thin-wall ductile iron is becoming a logi- aluminum, particularly if thin wall ductile iron parts cal choice for the production of small, light weight and could be produced without further heat treatment pro- cost effective automotive components. However, more cesses. The potentials for ductile iron applications for data about the metallurgy of thin-wall ADI castings lightweight automotive components have been limited seems to be of practical interest. by the capability to produce as-cast free thin wall parts Recently [59], 2 mm ADI plates with a homoge- (2-3 mm) [51, 52]. Production of thin-wall ductile iron neous ausferritic and nodule count of 300 nodules/mm2 castings still represents a daily challenge in modern were produced at CMRDI. It was found that decreasing foundries. Review of the recent literature shows that the wall thickness leads to reduced amounts of retained thin-wall ductile iron has been successfully produced austenite and structure refinement, which in turn in- for many years, thanks to the optimization of some criti- crease the hardness. Increasing the austempering tem- cal production parameters: pouring temperature, chemi- perature from 350°C to 400°C resulted in reduced ten- cal composition, thermal conductivity of the molding sile strength values (950 and 1000 MPa for 8 and 2 mm

224 A.A. Nofal, L. Jekova

wall thickness, respectively) to (775 and 875 MPa for Indonesia, 23-25 Nov., 2005. the same wall thickness), increased impact strength (from 2. J.R. Keough, K.L. Hayrynen, Developments in the 40 to 80) and from 100 to 125 J at wall thicknesses of Technology and Engineering Application of 2 and 8 mm, respectively, apparently due to increased Austempered Ductile Iron (ADI), Proceedings of the amounts of retained austenite at higher austempering 8th Int. Symposium on Science and Processing of Cast temperatures. Iron, Beijing, China, Oct. 16-19, 2006, 474-479. 3. K.L. Hayrynen, J.R. Keough, Austempered Ductile CONCLUSIONS Iron - the State of the Industry in 2003, 2003 Keith Mills Symposium on Ductile Cast Iron, 2003. • Extensive research work over the past decade 4. J.R. Keough, K.L. Hayrynen, Is It Time for a Grade has helped to develop the property combination of ADI 800 ADI in North America?, AFS Trans., v 113, in three directions: paper 03-089, 2005, 851-875. - increased strength, ductility and toughness 5. S.M. Dettloff, C.M. Burke, R.A. Johnson, B.N. Olson, - enhanced wear resistance/toughness combination D.J. Moore, K.B. Rundman, Ausforming Austempered - improved machinability Ductile Iron, International Scientific Conference on • ADI offers high levels of mechanical proper- ADI-Foundrys Offer for Designs and Users of Cast- ties at a competitive cost. When the high strength of ings, Cracow, II/1-II/8, 23-24 Nov., 2000. ADI is taken into account, it could successfully com- 6. D.J. Moore, K.B. Rundman, T.N. Rouns, The Effect pete with lightweight alloys, as the additive weight re- of Thermomechanical Processing on Bainite For- quired to give unit strength is lower. Moreover, when mation in Several Austempered Ductile Irons, First the relative cost of ADI required to give unit strength is International Conference in ADI, ASM, 1985, 13- considered, ADI seems to be one of the cheapest alloys. 31. These points have yet to be fully appreciated by many 7. J.D. DelaO, C.M. Burke, B. Lagather, D.J. Moore, design engineers. Currently, novel processing techniques K.B. Rundman, Thermomechanical Processing of adopted in achieve better strength and toughness prop- Austempered Ductile Iron: Thermomechanical Pro- erties include ausforming, cold-rolling, two-step cessing and Mechanical Properties of Hypereutec- austempering, square casting and others. toid Steels and Cast Irons, The Minerals, Metals and • The current research work aiming at improv- Materials Society, Sept. 14-18, 1997, 79-100. ing machinability of ADI looks rather vital for the fu- 8. J. Aachary, D. Venugopalan, Microstructural Devel- ture of this material. The available machining techniques opment and Austempering Kinetics of Ductile Iron required for forging steel are not always suitable for during Thermomechanical Processing, Metallurgi- ADI components, particularly on a high volume ma- cal and Materials Trans. A, 31-A, 2000, 2575-2585. chining line dedicated to the production of one specific 9. B.N. Olson, Ch. Brucke, J. Parolini, D.J. Moore and product. This problem can be minimized with the de- K.B. Rundman, Ausformed-Austempered Ductile Iron velopment of ferritic or ferritic + ausferritic ADI struc- (AADI), International Conference on ADI; DIS and tures. AFS, Louisville, KY, USA, 29-60, 2002, 25-27. • Carbidic as well bainitic/martensitic ADI offer 10. H. Nasr El-din, A. Nofal, M. Ibrahim, Ausforming opportunities for superior wear resistance, combined of Austempered Ductile Iron Alloyed with Nickel, with reasonable toughness, which may open new appli- International J. of Cast Metals Research, 19, 3, cations for ADI. 2006, 137-150. 11. A.A. Nofal, H. Nasr El-din, and M.M. Ibrahim, REFERENCES Thermomechanical Treatment of Austempered Ductile Iron, Proceedings of the 8th Symposium on 1. R.A. Harding, The Production, Properties and Auto- Science and Processing of Cast Iron SPCI-8, Beijing, motive Applications for Austempered Ductile Iron, China, Oct., 16-19, 2006, 397-402. Asia-Europe Environment Forum Conf., Jakarta, 12. B.N. Olson, D.J. Moore, K.B. Rundmanb, G.R.

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58. M.Gagné, C. Labrecque, M. Popescu, M. Sahoo, 59. M.M. Mourad, K.M. Ibrahim, M.M. Ibrahim, A.A. Effect of Silicon Content and Wall Thickness on the Nofal, Optimizing the Properties of Thin Wall Properties of Austempered Ductile Irons, AFS Trans., Austempered Ductile Iron, 68th World Foundry Con- 110, 2006. gress (WFC), Chennai, India, 7-10 Feb., 2008, 161-166.

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