Effect of spheroidization of cementite in ductile cast iron

Cast iron is an important class of structural materials that find wide applications for various industries, particularly in bulk component forms including automotive, machine tools, piping, and electrical machinery. In terms of tonnage use, cast irons may be more like that of structural steels. Typical components made from cast irons include engine blocks, valves, pistons, clutch plates, liners, brake drums, motors, and generators housings [1]. Characteristic advantages of cast iron are castability, machinability, wear/abrasion resistance, damping capacity, compressive strength, corrosion resistance, and low cost over steels. Low cost and castability of the cast iron are extremely significant, so it may replace certain steels especially for making bulky and intricate engineering components [2].

From the microstructural point of view, typical cast iron should have graphite embedded in 100% pearlitic matrix in different morphologies [3]. The impact resistance or toughness of the cast iron is substantially lower compared to structural steels, thus limiting their use for fatigue and impact loading conditions. To put it simply, cast iron is significantly brittle and lacks the toughness needed for use in special applications [1,4–5]. Throughout the years, the toughness of cast iron improved in progression through gray cast iron, malleable cast iron, ductile cast iron and austempered ductile iron (ADI), the last being the toughest among the family of cast irons [5]. Over the last two decades, a significant effort has been made to adjust the ductile cast iron matrix for improved mechanical properties by applying austempering heat treatment correctly and thereby producing ADI. However, it is important to note that the limitations of ADI are cracking problems during austempering treatment, poor weldability, additional costs, and complex heat-treatment sequence, so the commercial usage of ADI has not been fully materialized [6–9]. In 1948, ductile iron was introduced as a revolutionary new material that combined excellent castability of gray cast iron with superior mechanical properties especially the toughness of malleable cast iron, in as-cast condition [4]. It is now a well-known industrial practice that ductile irons possess spheroidized free carbon in the form of graphite nodules usually embedded in the pearlitic matrix in as-cast condition, and the matrix in the microstructure can be modified appropriately by applying various heat treatments [10]. By applying spheroidizing heat treatment, spheroids embedded in the ferritic matrix can be formed both in graphite and cementite [11]. There are numerous publications on spheroidizing heat treatment applied to various grades of steel. Four main methods are used for spheroidizing cementite in steels, viz very slow cooling below the lower critical temperature (A1), repeated heating above and cooling below A1, heating quenched steel just below A1, and extended holding just below A1 [12–13]. Therefore, cementite spheroidizing methods in steels are well known and widely used in the steel industry. It is important to note here, though, that these efforts in the cast iron industry are so minuscule. Published literatures, directly, or even indirectly, maybe one or two on this topic. There are two such main publications worthy of mention here. The latest analysis of high V–Cr–Ni cast irons fatigue behavior reported that carbides spheroidization may enhance cast iron fatigue behavior. Interestingly, however, during the fatigue test, coarse spheroidized carbides served as crack initiation sites and significantly reduced the fatigue limit of the material. It was suggested that shape fineness and uniform carbide distribution are critical for enhancing cast iron toughness and fatigue resistance [14]. Another study reported the spheroidization of ductile iron and can directly link to this work. They successfully applied spheroidizing heat treatments to both hardened and normalized ductile cast irons to produce spheroidized cementite with major improvements in both strength and toughness. However, there are two stages to the actual heat-treatment cycle: hardening/normalizing and spheroidizing [15].

Hence, an alternative process that is simple and cost effective must be developed. The objective of this work is to implement one such process for the ductile iron as a cost-effective and simple alternative to ADI. The research aims to achieve the objective by applying spheroidization heat treatment appropriately to a pearlitic/ferrite–pearlite ductile cast iron to obtain the bimodal distribution of graphite and cementite spheroids within the ferritic matrix. This work also explores the possibility of applying spheroidizing heat treatment directly to the as-cast ductile iron to disintegrate and spheroidize the continuous cementite network in the pearlite for enhanced toughness.

A ductile iron sample received in the form of rectangular slabs was sectioned to obtain a coupon size specimen of 2.540 cm $ \times $ 2.540 cm $ \times $ 1.905 cm. The chemical composition of the ductile iron is shown in Table 1 as confirmed by spark emission spectroscopy. Also, ductile iron specimens were sectioned and machined to produce tensile specimens according to ASTM standard E8.

Spheroidization heat treatment for coupon specimens as well as tensile samples is conducted using a muffle furnace in still air. The specimens were preserved for spheroidization at 670°C for 5, 10, and 15 h, followed by slow furnace cooling. The specimens were removed from the furnace and subjected to metallography and mechanical testing once they reached room temperature. Before spheroidization treatment, it was ensured that all the samples were in the annealed condition.

For a few selected specimens including the as-received ductile iron sample and specimens spheroidized at 670°C, tensile tests were carried out as per the ASTM standard. The tests were carried out at the strain rate of 1 mm/min using a Shimadzu AG-X plus^TM 100 kN universal testing machine. The data processed was then presented as the stress–strain curves and used to obtain tensile properties including strength and ductility of as-received and spheroidized specimens. During the tensile test, signs of necking were visually observed in the ruptured tensile specimens.

Using a Brinell hardness tester with 29.41 kN load and a 10-mm diameter hardened steel ball indenter, hardness test was carried out. For all coupon specimens, hardness data were obtained under both as-received and heat-treated conditions.

All coupon specimens and tensile specimens were subjected to standard metallographic specimen preparation procedures involving sectioning, belt grinding, fine grinding, and disk polishing accompanied by chemical etching using 2vol% nital. The Zeiss AXIO A1TM optical microscope and JEOL JSM-6380LA scanning electron microscope (SEM) were used to carry out microstructural characterization. Fracture surfaces of the tensile pecimens were analyzed using SEM. Additionally, longitudinal sections of a few selected tensile-fractured specimens were also investigated using an optical microscope and SEM to study the fracture path.

From microstructural findings (Figs. 1 and 2), it is apparent that denuded cementite zone increases with the holding time, while continuous cementite from pearlite discretizes nonuniformly into clusters of spheroidized cementite. This means that in cast iron, unlike in steels, uniform distribution of spheroidized cementite is considered impossible. SEM micrographs shown in Fig. 3 indicates that the denuded cementite region enlarges with an increase in the holding time, leaving only graphite spheroids embedded in the ferritic matrix. The high-magnification SEM micrographs of ductile irons shown in Fig. 4 confirm the above fact and that the cementite plates in the pearlite gradually discretize with an increase in the holding time. It is important to note that as the holding time increases, the microstructure becomes increasingly non-uniform. Thus, the overall mechanical properties of the ductile iron can be expected to degrade during spheroidization with prolonged holding time.

Figure 1.  Optical images of the ductile irons (a) in the as-received condition and after spheroidization at 670°C for (b) 5 h, (c) 10 h, and (d) 15 h, showing the monotonous decrease in the amount of cementite as the holding time increases (1—Pearlite; 2—Graphite; 3—Ferrite; 4—Disintegrated cementite from pearlite colonies; 5—Cementite spheroids; 6—Cluster of cementite spheroids).

Figure 2.  Magnified optical images of the ductile irons (a) in the as-received condition and after spheroidization at 670°C for (b) 5 h, (c) 10 h, and (d) 15 h, showing that the continuous cementite in pearlite is gradually discretized (1—Pearlite; 2—Graphite; 3—Ferrite; 4—Disintegrated cementite from pearlite colonies; 5—Cementite spheroids; 6—Cluster of cementite spheroids).

Figure 3.  SEM images of the ductile irons (a) in the as-received condition and after spheroidization at 670°C for (b) 5 h, (c) 10 h, and (d) 15 h, confirming that the amount of cementite decreases with increase in the holding time (1—Pearlite; 2—Graphite; 3—Ferrite; 4—Disintegrated cementite from pearlite colonies; 5—Cementite spheroids; 6—Cluster of cementite spheroids).

Figure 4.  Magnified SEM images of the ductile irons (a) in the as-received condition and after spheroidization at 670°C for (b) 5 h, (c) 10 h, and (d) 15 h, confirming that the continuous cementite discretizes increasingly with an increase in the holding time (1—Pearlite; 2—Graphite; 3—Ferrite; 4—Disintegrated cementite from pearlite colonies; 5—Cementite spheroids; 6—Cluster of cementite spheroids).

The tensile strength of ductile iron decreases as predicted, with an increase in the holding time as shown in Fig. 5. However, the ductility of the material shows an increasing and then decreasing trend with the holding time. Once toughness is measured by the multiplication of tensile strength and ductility as an estimate of the area under the stress–strain curve, it is obvious that the toughness shows an increasing and then decreasing trend with an increase in the isothermal holding time at 670°C (Table 2). This suggests that for the best combination of toughness and other mechanical properties, the spheroidization temperature and time should be optimized for each cast iron grade. Since the objective of this work is to increase and optimize the toughness of the ductile iron, spheroidization at 670°C for 5 h provides the best result. Measurements of hardness shown in Table 2 show a decreasing trend with an increase in the holding time, as the amount of cementite decreases.

Figure 5.  Stress vs. strain curves of the ductile irons in the as-received condition and after spheroidization at 670°C for 5, 10, and 15 h.

In general, the fracture of cast iron including ductile iron is brittle in nature. However, the ruptured ductile iron tensile specimens (Fig. 6) indicate the ductility of cast iron can be significantly improved when spheroidized properly, as shown by the necking of the tensile specimen. This is confirmed by the magnified image shown in Fig. 6(e). Indeed, ductile iron spheroidization at 670°C for 5 h show a significant amount of necking before the fracture. This observation is unique as a necking phenomenon in the cast iron family which is not widely reported.

Figure 6.  Fractured specimens after tensile tests for the ductile irons (a) in the as-received condition, after spheroidization at 670°C for (b) 5 h, (c) 10 h, and (d) 15 h, and (e) showing the necking before fracture in (b).

Figs. 7 and 8 show longitudinal sections of the tensile specimens taken far from and close to the fracture surfaces, respectively. Graphite spheroids coarsened with an increase in the spheroidization holding time suggest increased decomposition of cementite. It is important to note that specimens spheroidized for 5 and 10 h show significant plastic deformation of the matrix, as shown by the elongated spheroids in the necked region (Figs. 8(b) and 8(c)). No such elongation of spheroids observed in the as-received and spheroidized for 15 h specimens (Figs. 8(a) and 8(d)) suggests minimal plastic deformation.

Figure 7.  Optical images of longitudinal sections away from the fractured surfaces of tensile specimens (a) in the as-received condition and after spheroidization at 670°C for (b) 5 h, (c) 10 h, and (d) 15 h

Figure 8.  Optical images of the longitudinal sections close to the fractured surfaces of tensile specimens (a) in the as-received condition and after spheroidization at 670°C for (b) 5 h, (c) 10 h, and (d) 15 h

Fig. 9 shows the SEM fractography of tensile specimens. It is natural that the as-received specimen with a continuous network of cementite fractured in a brittle manner, leaving a significant amount of cleavages in the fracture surface (Fig. 9(a)). Fig. 9(b) exhibits perfectly formed dimples in specimen spheroidized for 5 h, with a very small amount of cleavages. Cleavages are likely discrete in specimens spheroidized for 15 h due to the non-uniform distribution of spheroid colonies (Fig. 9(d)).

Figure 9.  Fractography images showing (a) fine cleavages in the as-received ductile iron, (b) fine dimples in the specimen spheroidized for 5 h, (c) both cleavages and dimples in the specimen spheroidized for 10 h, and (d) coarse cleavages in the specimen spheroidized for 15 h

Based on the spheroidization heat treatments followed by microstructural and mechanical characterization conducted for the experimental ductile cast iron in this work, the following conclusions can be drawn:

(1) The toughness of ductile cast iron can be improved by applying spheroidization heat treatment in an as-received annealed condition without a hardening step. For the experimental ductile iron used in this study, it is confirmed that if the spheroidization heat treatment is optimized, ductility and toughness can be increased respectively by 90% and 40% at the expense of an 8% decrease in the tensile strength.

(2) For the as-received/annealed ductile iron, spheroidization at 670°C for 5 h yields the optimal mechanical properties. Spheroidization treatment with prolonged holding time gives rise to adverse effects on both strength and ductility, likely due to the microstructural inhomogeneity and cementite decomposition in the matrix.

Authors thank the Department of Metallurgical and Materials Engineering, NITK, India, for providing experimental facilities to complete this work.