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 consider- able 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 af- ter 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 pre- form 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 ex- ceeding 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-harden- ing 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

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