Influence of Spheroidizing Treatment on the Graphite and Mechanical Properties of Cast High-nickel Austenitic Ductile Iron

Abstract

Cast high-nickel austenitic ductile iron (CHNADI) is widely used in high temperature, low temperature and strong corrosive environments due to its good comprehensive mechanical properties. However, its low yield strength limits its application in nuclear power equipment. Herein, the influence of nodularizer and processing temperature on the graphite and mechanical properties of CHNADI was investigated to significantly promote its properties. With the increase of nodularizer dosage, the mechanical properties of tensile strength, yield strength, elongation, and impact toughness first increase and then decrease. When the dosage of nodularizer is 0.9 mass% and the processing temperature is 1480°C, CHNADI exhibits excellent mechanical properties with the hardness of 132 HB, tensile strength of 450 MPa, yield strength of 224 MPa, elongation rate of 46% and impact toughness of 91 J. In this work, a new strategy of preparing CHNADI without heat treatment is proposed to meet the strict requirements of this material, which is significant for the development of CHNADI.

1. Introduction

Cast high-nickel austenitic ductile iron (CHNADI) is a series of ductile iron with a nickel content of 13.5 mass% or above. CHNADI is made in several different compositions to produce the required properties.^1,2,3,4) It has many excellent characteristics due to its austenite matrix and the high alloy content.^5,6,7) CHNADI has good thermal shock resistance, corrosion resistance, high-temperature oxidation resistance, low-temperature impact toughness, so it is widely used to manufacture fan covers for nuclear power plants, seawater pumps, valves, supercharger housings, and exhaust components with heat and corrosion resistance.^8,9,10)

Some scholars have found that the matrix structure of CHNADI is composed of austenite dendrites, graphite nodules and a small amount of intergranular carbides.^{11,12,13,14,15)} The mechanical properties can be modified by changing the amount and distribution of matrix phases or microstructures.^{16,17,18,19,20)} Chemical elements and inoculation process have a significant influence on the mechanical properties of CHNADI.^{21,22,23,24,25,26,27,28)} Farjad Alabbasian et al.²⁹⁾ observed the effect of inoculation and casting modulus on the microstructure and mechanical properties of CHNADI, and its tensile property was improved by increasing graphite nodularity and nodule quantity and decreasing the carbide formation tendency in the inoculation process. Choi et al.³⁰⁾ found that adding 0.02%–0.04% rare earth elements could improve the nodularity of graphite nodules and mechanical properties of thin-walled ductile iron castings. K. M. Ahmad et al.³¹⁾ reported that there was an increase in hardness and a decrease in tensile strength with the increase of manganese content in cast iron. G. Rivera et al.³²⁾ studied the influence of inoculation process, chemical composition and cooling rate on the solidification macrostructure and microstructure of ductile iron. The result show that the carbon equivalent affected the grain size, while cooling rate had little effect on the grain size and morphology, but had a significant effect on the dendrite arm spacing of austenite.

Numerous researches on the production, microstructure, and mechanical properties of CHNADI have been published, among which the research on the new-style CHNADI has certain reference value.^{33,34,35,36,37,38,39,40)} However, some aspects that affect the graphite and mechanical of new-style CHNADI remain unclear. There are fewer researches on the influence of spheroidizing treatment on the graphite and mechanical properties of ductile iron. This study focused on the effect of the amount of nodularizer and processing temperature on the graphite and mechanical properties of CHNADI. Samples with different nodularizer and processing temperatures were characterized by the scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) to determine the microstructural characteristics, such as graphite nodule quantity and graphite nodularity in the structure. The hardness, tensile strength, elongation, and impact toughness of the samples measured to determine the mechanical properties of CHNADI.

2. Materials and Methods

2.1. Materials

In this experiment, cast pig iron, industrial pure iron (99 mass%), nickel plate (99.5 mass%), and manganese metal (99.5 mass%) were used to prepare CHNADI. Ni–Mg alloy and Si–Ba–Fe alloy were used as the nodularizer and the inoculant, respectively. The above raw materials used in the study were all industrial products, the chemical compositions of which were shown in Table 1.

Table 1. Chemical compositions of the raw materials (mass%).

Raw materials	C	Si	Mn	P	S	Ni	Mg	Ba	Fe
Cast pig iron	4.0	0.1	0.1	0.03	0.02	–	–	–	Balance
Industrial pure iron	0.02	0.2	0.2	0.02	0.02	–	–	–	Balance
Manganese	–	–	99.5	–	–	–	–	–	Balance
Nickel	–	–	–	–	–	99.5	–	–	Balance
Ferrosilicon	–	65	–	–	–	–	–	–	Balance
Nodularizer	–	–	–	0.01	0.01	82	16	–	Balance
Si–Ba–Fe	–	70	–	–	–	–	–	1.5	Balance

The requirements for chemical compositions and mechanical properties of CHNADI are shown in Tables 2 and 3, respectively.

Table 2. Chemical compositions of CHNADI (mass%).

Element	C	Si	Mn	P	Ni	Cr
Content	≤2.9	2.0–3.0	1.8–2.4	≤0.08	21–24	≤0.5

Table 3. Mechanical properties of CHNADI.

Mechanical properties	Rp0.2 (MPa)	Rm (MPa)	A%	HB
Performance requirements	≥210	≥390	≥20	≤175

2.2. Experimental Procedures

The material studied was prepared in a 50 kg medium frequency induction furnace. The mould was made by using Y-block shaped pattern with minimum thickness of 30 mm, the sand mould of which was shown in Fig. 1.²⁶⁾ The “sandwich technique” was used for nodularization and inoculation treatments. The inoculant composed of 50 mass% Si–Fe and 50 mass% Si–Ba–Fe was used before baking, and its amount is 1.5 mass% of the weight of molten iron. Different levels of processing temperature and the amount of nodularizer were selected to prepare samples for experimental research. The amount of nodularizer ranged from 0.3 to 1.2 mass%, and the processing temperature of CHNADI ranged from 1440°C to 1500°C. The experimental conditions and composition requirements of the samples in this study are shown in Table 4.

Fig. 1.

Schematic diagram and dimensions of Y-block (mm).

Table 4. The experimental conditions and chemical composition of CHNADI produced in the present study.

NO.	Nodularizer (mass%)	T/°C	Inoculant (mass%)	Composition (mass%)
				C	Si	Mn	Ni	P	S	Mg
A1	0.9	1440	1.5	2.82	2.49	2.20	22.1	0.018	0.022	0.092
A2		1460		2.80	2.51	2.18	22.0	0.019	0.020	0.090
A3		1480		2.76	2.50	2.22	21.9	0.016	0.018	0.089
A4		1500		2.81	2.48	2.19	22.2	0.018	0.023	0.082
B1	0.3	1480	1.5	2.82	2.52	2.19	22.2	0.016	0.020	0.028
B2	0.6			2.83	2.53	2.22	22.1	0.018	0.019	0.062
B3	0.9			2.79	2.51	2.21	22.0	0.016	0.017	0.089
B4	1.2			2.78	2.49	2.20	21.9	0.017	0.021	0.110

2.3. Characterization

In this experiment, the DNS100 electronic universal testing machine was used to test the tensile strength, yield strength and elongation of CHNADI. The tensile specimens were round bars, and the tensile speed was 0.5 mm/min. The impact test was conducted by a Charpy impact tester. The dimensions of tensile test and impact test specimens are shown in Fig. 2 according to ASTM standard E 8M and E 327.^41,42) FM-ARS900 hardness tester was used to test hardness of CHNADI. The graphite morphology was rated for the nodularity and nodule count in accordance with ASTM standard A 247.⁴³⁾ The microstructure was characterized by X-ray diffraction (XRD; D8 Advance, Bruker, Germany) with CuKα radiation (λ = 1.5406 Å), and SEM (SU-8010, Hitachi, Japan), with an accelerating voltage of 15 kV, equipped an EDS spectrometer.

Fig. 2.

Dimensions of (a) tensile test specimen and (b) impact test specimen (mm).

3. Results and Discussion

3.1. Graphite of CHNADI

3.1.1. Effect of Processing Temperature on Graphite

Under the condition of fixed inoculant, nodularizer and chemical elements, 1440°C, 1460°C, 1480°C, and 1500°C were selected as the processing temperatures of this experiment. The graphite and mechanical properties of CHNADI were investigated at room temperature. Figure 3 shows that the microstructure consists of graphite nodules and defective graphite embedded in austenitic matrix. As shown in Fig. 3, the nodularity gradually becomes better with the increase of processing temperature before 1480°C and decreases at above 1480°C, which indicates that too low or too high processing temperature will reduce the nodularity of graphite. More defective graphite will be formed at lower temperature. Figure 3(a) shows that a large number of defective graphite are formed on the grain boundary, which is the result of low-temperature treatment. The reason is that the low temperature of molten iron is not conducive to the floating of inclusions and the diffusion of carbon atoms into the graphite nodules, resulting in the formation of more defective graphite. Defective graphite structure is harmful to the mechanical properties of CHNADI.^44,45,46) The nodularizer will be severely oxidized under too high processing temperature. Moreover, prolonging the solidification time will lead to the decline in nodularity. Table 5 list the analysis results of graphite nodules in CHNADI at different temperatures, and it shows that the nodularity ranges from 82% to 92%, the nodule count ranges from 222 to 252, and the nodule size ranges from 18.0 μm to 30.2 μm. It can be seen from Fig. 3 that an ideal matrix structure can be obtained at 1480°C.

Fig. 3.

Graphite of CHNADI with 0.9 mass% nodularizer at different processing temperatures: (a) 1440°C, (b) 1460°C, (c) 1480°C, (d) 1500°C; (e) and (f) magnified image of defective graphite. (Online version in color.)

Table 5. Analysis of graphite with 0.9 mass% nodularizer and different processing temperatures.

No	Average nodule size (μm)	Nodule count (No/mm2)	Nodularity (%)	T/°C
1	18.0	222	83	1440
2	18.4	244	88	1460
3	20.6	252	92	1480
4	30.2	228	82	1500

The effect of processing temperature on the structure of CHNADI are mainly as follows. The processing temperature lower than 1450°C will lead to the incomplete melting and agglomeration of nodularizer at the bottom of the ladle. As a result, the nodularity of CHNADI is reduced. The rapid cooling and slow diffusion of carbon atoms lead to a low nodularity at the low temperature, and it is beneficial to the formation and growth of graphite nodules with the increase of temperature due to the accelerated diffusion of carbon atoms. When the processing temperature is too high, the molten iron starts to react violently after being injected into the ladle. A large amount of magnesium vapor escapes, which reduces the magnesium absorption rate. The graphite nodules will decay at too high temperatures, resulting in a decrease in nodularity. The processing temperature has an effect on the solidification process of CHNADI. Studies have shown that the prolongation of the solidification time caused by excessive overheating will significantly reduce the nodularity of ductile iron.^47,48) Reasonable processing temperature should ensure that the nodularizer reacts slowly and stably for a relatively long time, so the absorption rate of magnesium will be high.

Figure 4 shows the XRD pattern of CHNADI from different processing temperatures. It is known that the nickel content of CHNADI is over 22 mass%, and its phase at room temperature is austenite from different processing temperatures. It has been reported that the as-cast structure is a mixture of two phases, and the fine particles embedded in the austenitic matrix are the Fe–Ni intermetallic compound in the form of FeNi₃ phase.²⁹⁾ The three peaks in Fig. 4 represent the typical diffraction pattern of the austenitic γ-Fe. With the increase of processing temperatures, the characteristic peaks become sharper and tend to shift to the left, which indicates that the grain size increases and the crystallinity becomes better.

Fig. 4.

XRD patterns of CHNADI with 0.9 mass% nodularizer at different processing temperatures: (a) 1440°C, (b) 1460°C, (c) 1480°C, (d) 1500°C. (Online version in color.)

3.1.2. Effect of Nodularizer Dosage on Graphite

Under the condition of fixed inoculant, processing temperature, and chemical elements, the dosage of nodularizer was selected to be 0.3, 0.6, 0.9, and 1.2 mass% of the weight of molten iron, respectively. After nodularization at 1480°C, the residual content of Mg in the CHNADI was determined to be 0.028, 0.062, 0.089 and 0.11 mass%, respectively. The microstructures of the sample with different dosages of nodularizer are shown in Fig. 5, indicating that the matrix structure is composed of austenite, graphite nodules and defective graphite. As shown in Fig. 5(a), when the dosage of nodularizer is small, a large number of defective graphite are formed at the grain boundary, which is harmful to the mechanical properties.⁴⁹⁾ The analysis results of graphite nodules in CHNADI with different nodularizer dosages are listed in Table 6. As the nodularizer dosage increases, the nodularity first increases and then decreases. It can be seen from Table 6 that when the nodularizer dosage in CHNADI is in the range of 0.6–0.9 mass%, the nodularity is more than 86% with the nodule quantity of 232–256 and nodule size of 18.6–20.8 μm, which meets the requirements of castings on nodularity. These results are considered to depend on the variation of the nodularizer dosage. When the nodularizer dosage is small, the content of magnesium residual is less than 0.07 mass%, and the nodularity is poor. When the nodularizer dosage is too large, the content of residual magnesium exceeds 0.1 mass%, and the nodularity decreases.⁵⁰⁾

Fig. 5.

Graphite of CHNADI with the processing temperature of 1480°C and different nodularizer dosages: (a) 0.3%, (b) 0.6%, (c) 0.9%, (d) 1.2%; (e) and (f) magnified image of defective graphite. (Online version in color.)

Table 6. Analysis of graphite with different amount of nodularizer at 1480°C.

No	Average nodule size (μm)	Nodule count (No/mm2)	Nodularity (%)	Nodularizer (%)
1	17.4	208	76	0.3
2	18.6	232	86	0.6
3	20.8	256	93	0.9
4	19.6	206	79	1.2

The effect of the nodularizer dosage on the structure of CHNADI are mainly as follows. As magnesium is the most important element in producing graphite nodules, the amount of residual magnesium determines the quantity, size and roundness of graphite nodules in CHNADI. Magnesium will react with oxygen and sulfur before it becomes available to spheroidize the graphite phase. The formation of graphite nodules is closely related to residual magnesium. A small amount of residual magnesium means that the added nodulizer is insufficient, which can not completely eliminate the negative influence of interfering elements, promote multi-directional production, form graphite nodule, and thus produce more defective graphite.⁵¹⁾ High content of residual magnesium is not conducive to the formation of regular graphite nodule, which leads to cracking of the graphite nodule. During the process of graphite growth, the residual magnesium will gather on the faster-growing crystal planes, which hinders the preferred growth of graphite in one direction and promotes the multi-directional production. If the content of residual magnesium is high, magnesium will remain in graphite, which can reduce the interaction between carbon atoms in the graphite nodules, split the graphite nodules and generate more defective graphite.^52,53)

3.1.3. Nodularization Mechanism

Graphite nodules in CHNADI are crystals superimposed by many regularly arranged carbon atoms, and the hexagonal lattice structure of graphite crystal is shown in Fig. 6. The basal plane is the dense plane of atoms in graphite crystal. Carbon atoms are bonded together by a strong covalent bond to form a firmly bonded atomic layer. The growth of graphite crystal always starts from the crystallization center, which comes from some impurity particles in the liquid phase of CHNADI. When the surface structure and lattice constant of these particles are close to a certain crystal plane of graphite crystal, carbon atoms can be deposited on the particles to form a nucleation matrix. When the size and surface energy of the nucleation matrix reaches a certain level, it will become the graphite crystal nucleus. Figure 6(c) shows that there are layered flakes in graphite, which are angularly oriented between the crystal planes and form many steps. The formed steps continue to accumulate carbon atoms to achieve the growth of graphite.^54,55,56) Stefanescu et al.⁵⁷⁾ demonstrated that the magnesium can promote the nodularization of graphite in ductile iron. When the concentration of residual magnesium at the growth front is sufficient, the growth of graphite on the a axis is limited and forms graphite nodules. When the concentration of residual magnesium is not enough to limit the growth of graphite in a direction, the defective graphite will form.

Fig. 6.

Hexagonal lattice structure of graphite crystals, (a) graphite lattice model; (b) crystal plane and crystal orientation; (c) crystal defects that provide graphite growth steps. (Online version in color.)

3.2. Mechanical Properties of CHNADI

3.2.1. Hardness

Figure 7 shows the influence of varying nodularizer and processing temperature on the hardness in all samples. The results reveal that the hardness first decreases and then increases with the increase of the nodularizer dosage at the same temperature. When the processing temperature is 1480°C and the dosage of nodularizer is 0.8 mass%, the hardness of CHNADI decreases to the minimum value of 132 HB. CHNADI with higher nodularity exhibits lower hardness, as shown in Fig. 7. Fine defective graphite can increase the hardness of ductile iron by causing more severe segregation in the matrix.

Fig. 7.

Influence of nodularizer on hardness (HB) of CHNADI with different processing temperatures. (Online version in color.)

3.2.2. Tensile Strength and Yield Strength

Figure 8 reveals the influence of nodularizer and processing temperatures on the tensile strength and yield strength of CHNADI. Compared with tensile strength, yield strength is more difficult to meet the requirements of the material properties. Figure 8 shows that with the increase of the nodularizer dosage at the same temperature, the tensile strength and yield strength first increase and then decrease, which is consistent with the change of nodularity analyzed above. When the processing temperature is 1480°C and the nodularizer dosage increases from 0.3 to 0.9 mass%, the tensile strength and yield strength of CHNADI increases from 420 MPa and 197 MPa to the maximum value of 450 MPa and 224 MPa, respectively. With the nodularizer dosage increasing from 0.9 to 1.2 mass%, the tensile strength and yield strength of CHNADI decrease from 450 MPa and 224 MPa to 428 MPa and 201 MPa, respectively. The amount of graphite nodules and nodularity play a significant role in improving the tensile strength and yield strength of ductile irons. The results show that the increase of graphite nodularity leads to the increase of both tensile and yield strength.^15,30,36) Since graphite nodules can change the stress distribution of CHNADI, if there are more flake graphite, the stress of CHNADI castings will be concentrated, which will be more easily damaged.

Fig. 8.

Influence of nodularizer on (a) tensile strength and (b) yield strength of CHNADI with different processing temperatures. (Online version in color.)

3.2.3. Elongation

Figure 9 reveals the influence of varying nodularizer and processing temperatures on the elongation of samples. The results show that the elongation first increases and then decreases with the increase of nodularizer dosage at the same temperature. When the treatment temperature is 1480°C and the dosage of nodularizer ranges from 0.3 to 0.9 mass%, the elongation of CHNADI increases from 30% to the maximum value of 46%. With the further increase of the nodularizer dosage from 0.9 to 1.2 mass%, the elongation rate decreases from 46% to 33%. When the nodularizer dosage is the same, the elongation of ductile iron increases first and then decreases, which is consistent with the influence of temperature on nodularity analyzed above. When the nodularizer dosage is 0.9 mass%, the elongation increases from 36% to 46% with the temperature increasing from 1440°C to 1480°C, and decreases to 38% with the further increase of temperature. The graphite morphology plays an important role. The more the graphite shape deviates from the ideal spherical shape, the lower the elongation.⁵⁸⁾ By selecting reasonable nodulizer dosage and processing temperature, the spheroidizing treatment can increase the nucleation position for graphite growth of and reduce the tendency of defective graphite. As shown in Figs. 3, 5 and 9, the samples with higher nodularity and ideal graphite shape have better elongation than other samples.

Fig. 9.

Influence of nodularizer on the elongation of CHNADI with different processing temperatures. (Online version in color.)

3.2.4. Impact Toughness

Figure 10 shows the influence of the nodularizer dosage at different temperatures on the impact toughness of CHNADI without defects test at room temperature. With the increase of the nodularizer dosage, the impact toughness increases first and then decreases. When the nodularizer dosage ranges from 0.3 to 0.9 mass% and the processing temperature is 1480°C, the impact toughness ranges from 78 J to 91 J. With the further increase of nodularizer dosage, the impact toughness will decrease. When the temperature increases from 1440°C to 1480°C with the nodularizer dosage of 0.9 mass%, the impact toughness increases from 76 J to 91 J. With the further increase of temperature, the impact toughness will decrease. The nodularity, graphite morphology and graphite size all affect the impact toughness of CHNADI. High nodularity and regular morphology can significantly improve the impact toughness of CHNADI, and absorb more energy under the action of external force. Regular graphite nodules can reduce the shearing effect on the matrix of CHNADI, and avoid stress concentration and damage to the body under the action of external force.

Fig. 10.

Influence of nodularizer on the impact toughness of CHNADI without defect test at room temperature with different processing temperatures. (Online version in color.)

4. Conclusions

The effects of nodularizer dosage and processing temperature on the graphite and mechanical properties of CHNADI have been investigated. The conclusions are summarized as follows.

(1) The microstructure of CHNADI is composed of graphite nodules embedded in austenite matrix. The formation of graphite nodules is closely related to the residual magnesium. Appropriate residual magnesium content can ensure the excellent matrix structure of ductile iron and avoid the formation of irregular graphite nodules and defective graphite.

(2) With the increase of nodularizer dosage, the mechanical properties such as tensile strength, yield strength, elongation, and impact toughness first increase and then decrease, while the hardness first decreases slightly and then increases.

(3) The mechanical properties of CHNADI improve with the increase of processing temperature before 1480°C, but they will be reduced with continuous increase of the processing temperature.

(4) When the nodularizer dosage is 0.9 mass% and the processing temperature is 1480°C, CHNADI has excellent mechanical properties, with the hardness of 135 HB, the tensile strength of 450 MPa, the yield strength of 224 MPa, the elongation of 46% and the impact toughness of 91 J, which successfully solves the problem that the yield strength of CHNADI can not meet the application requirements.

Acknowledgment

The authors acknowledge the financial support from the National Natural Science Foundation of China (51834004, 51774076 and 51704063).

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