2021 Volume 62 Issue 10 Pages 1556-1561
The purpose of this study is to investigate the influence of chunky graphite on the characteristics of fatigue crack propagation in heavy section spheroidal graphite cast iron. Samples containing and not containing chunky graphite were cut from a large ingot of spheroidal graphite cast iron. The fatigue crack propagation test conformed to ASTM. Stress ratio R was 0.1, and the specimen used was of the 1CT type with a thickness of 12.5 mm, and humidity RH = 0%.
In the samples containing chunky graphite, the slope of the linear approximation m at the IIb stage of fatigue crack propagation becomes larger and fatigue crack growth rate becomes faster than the spheroidal graphite samples. The reason seems to be that crack propagates along the graphite connected to the chunky graphite. The decrease in the threshold stress intensity factor range ΔKth of the chunky graphite samples was smaller than the decrease in tensile strength. In the chunky graphite samples, the fracture surface roughness became large and the crack closure induced by fracture roughness occurred conspicuously. The threshold stress intensity factor range decreased due to the influence of the chunky graphite. However, it was found that threshold stress intensity factor range does not decrease as much as the tensile strength due to the effect of the crack closure induced by fractured surface roughness.
This Paper was Originally Published in Japanese in J. JFS 91 (2019) 844–849.
Spheroidal graphite cast iron is a material that has toughness comparable to that of steels, is excellent in castability and machinability, and has a lower melting temperature than steel. Due to these characteristics, it is used as a substitute for steels in a wide range of areas such as automotive parts and water and sewer pipes.1) However, such components for mechanical structures are subjected to cyclic loads, so it is necessary to give due consideration to fatigue failure when designing. It is known that spheroidal graphite cast iron has a lower fatigue limit and more variability than steels because the included casting defects and graphite become the origin of fatigue cracks, and the effective area decreases.2) Also, it is known that the fatigue crack generated from the stress concentration part does not propagate when the stress amplitude near the fatigue limit is applied, and it has been reported to relate the state of the crack to the threshold stress intensity factor range ΔKth.3) The threshold stress intensity factor range ΔKth, which indicates the non-propagation limit of fatigue cracks, is an important parameter for evaluating fatigue crack propagation test, and many reports have been made for spheroidal graphite cast iron.4–9)
It has been reported that abnormal graphite called chunky graphite occurs inside of the ingot when casting large parts using spheroidal graphite cast iron.10–17) It is known that chunky graphite crystallizes in the central part of heavy section spheroidal graphite cast iron, but the crystallization process is unknown in the observation of the structure of chunky graphite, the structures that are not spheroidal graphite aggregate and crystallize on the material surface. However, it is connected inside the material and crystallizes into large graphite.11,12) As a result, the amount of iron in the matrix at the chunky graphite decreases, and mechanical properties are decreasing.18,19) However, few reports have investigated the relationship between chunky graphite and fatigue crack propagation. In this study, samples containing spheroidal graphite and chunky graphite were taken from one block of large ingot, and the effect of chunky graphite on fatigue crack propagation was investigated.
Table 1 shows the chemical composition of the spheroidal graphite cast iron used as the test material. The casting conditions were as follows: Ductile cast iron, scrap steel, ferrosilicon, ferromanganese, and carburizer were mixed, and the original molten iron was melted in a 1.5t high-frequency induction furnace. Fe–Si inoculation was performed in a ladle, and spheroidizing treatment was performed with Fe–Si–Mg–RE spheroidizing agent. After that, it was poured into a self-hardening sand mold placed in a 700 × 700 × 700 mm metal frame at a pouring temperature of 1550 K to produce an ingot of h200 × w200 × t500 mm. The ingot was cut, and the cut surface was polished to find a portion containing a large amount of chunky graphite. Then, CT specimen and tensile test specimen were taken from the positions shown in Fig. 1(a) and (b). The shape and size of the tensile test specimen conformed to JIS No. 14A,20) and the diameter of the parallel portion was 5.5 mm, and the length of the parallel portion was 30 mm. The tensile test was carried out at room temperature, and conducted by crosshead speed of 0.5 mm/min. Also, two strain gauges were attached facing each other in the center of the parallel portion of the tensile test specimen to measure 0.2% proof stress and Young’s modulus. Of these specimens, the specimens without chunky graphite were called as “Spheroidal”, and the specimens containing chunky graphite were called as “Chunky”, and two specimens were called as A and B, respectively. Microstructures of each test material, mean diameter of graphite nodule, area ratio of graphite nodule, nodularity, area ratio of pearlite, and Brinell hardness are shown in Table 2. Fine graphite, which is considered to be chunky graphite, was observed in Chunky A and Chunky B. As a result of measuring the proportion of chunky graphite in the total graphite area in the structure observation sample, 58% was contained in Chunky A, and 88% was contained in Chunky B. Chunky B contained more chunky graphite than Chunky A.
Shape and dimensions of ingot, and specimens cutting position in ingot.
Although this fine graphite is internally connected, it was described as a reference value in Table 2 because the fine graphite was individually recognized and calculated. In the sample where chunky graphite is crystallized, the mean diameter of graphite nodule is reduced remarkably, and the area ratio of graphite nodule is increased remarkably. Furthermore, nodularity decreased conspicuously, and area ratio of pearlite is increased slightly, but the Brinell hardness was almost the same. Table 3 shows the mechanical properties of Spheroidal and Chunky. Spheroidal and Chunky have 0.2% proof stress, and Young’s modulus is almost the same. However, the tensile strength, elongation, and reduction of area are reduced significantly. There was a portion where graphite was uniformly attached in addition to the dimple fracture surface with the spheroidal graphite as the nucleus, and this is presumed to be chunky graphite. It was confirmed that the content of chunky graphite in the tensile specimen was about the same as in the microstructure observation sample.
The fatigue crack propagation test was carried out using an electro-hydraulic fatigue tester (made by Shimadzu Co. Ltd.) with a capacity of ±19.6 kN. The shape of the specimens was the same as those employed for CT-type fatigue tester21) compliant with ASTM standards, and the sheet thickness of the specimens was 12.5 mm (Fig. 2). To facilitate the propagation of pre-cracks, chevron notched was machined at the tip of the notch. The test was conducted using a sinusoidal load control system with a load repetition frequency of 20 Hz, and a gradually decreasing load system with C = −0.08 in the K-gradient at the stress ratio, R = 0.1. The test environments were controlled so that the humidity was 0 to 5% in dry air. For the humidity control method, dry air (relative humidity RH = 0) was obtained from the air compressed by the compressor through an air filter and a heatless air dryer. The air obtained by this method was sent to a vinyl chamber that covers the circumference of the specimen, and the specimen was tested at constant humidity.
Shape and dimensions of CT specimen.
The length of the cracks on both sides were measured at a resolution of 0.01 mm using a movable-type reading microscope, and the average values were assumed to be the crack lengths a. The crack growth rate da/dN was calculated using the secant method and was obtained using the load repetition number N that required for a crack to propagate by 0.5 mm. A da/dN and stress intensity factor range ΔK was used for sorting and a comparative study of crack propagation characteristics. To calculate ΔK, use eq. (1), which is applicable to a wide range (0.2 ≤ a ≤ 1) of the ratio α (= a/W) of the crack length a at the distance W between the load line of the specimen and the edge.22,23)
| \begin{equation} \Delta K = \frac{\Delta P}{B\sqrt{W}}\left\{ \frac{(2 + \alpha)(0.886 + 4.64\alpha - 13.32\alpha^{2} + 14.72\alpha^{3} - 5.6\alpha^{4})}{\sqrt{(1 - \alpha)^{3}}} \right\} \end{equation} | (1) |
Where, ΔP is the load range, and B is the thickness of specimen (12.5 mm).
Measurements for the crack closure were conducted using the unloading elastic compliance method. A load cell was used to detect the applied load P, and a strain gauge attached to the specimen surface was used to detect the strain ε. The ideal crack has a proportional relationship between P and ε. However, when the crack closure occurs, the relationship between P and ε deviates from the proportional relationship. Therefore, the strain signal (V) during proportional relationship amplified to 0, and the closure diagram is obtained. The crack opening ratio U and effective stress intensity factor range ΔKeff were calculated from Pmax, Pmin, and Pop using eq. (2).
| \begin{equation} U = \frac{P_{\text{max}} - P_{\text{op}}}{P_{\text{max}} - P_{\text{min}}} = \frac{K_{\text{max}} - K_{\text{op}}}{K_{\text{max}} - K_{\text{min}}} = \frac{\Delta K_{\text{eff}}}{\Delta K} \end{equation} | (2) |
Where, ΔKeff is the stress intensity factor range where the load in the crack closure region is subtracted from ΔP, and it truly contributes to the crack propagation.
After the fatigue crack propagation test, the CT specimen was forcibly fractured and the fracture surface was observed by a scanning electron microscope (SEM).
3.2 Fracture surface roughness measurementThe crack closure is divided into three types. First, roughness-induced crack closure caused by the contact of irregularities on the fracture surface. Second, oxide-induced crack closure caused by the fretting phenomenon due to the fracture surface contact. Third, the plasticity-induced crack closure caused by propagating the plastic region at the crack tip.24) In this study, the test environment was dry air, and there was no difference in the 0.2% proof stress of “Spheroidal” and “Chunky”. As a result, the difference in the crack closure was assumed to be mainly due to the crack closure induced by fracture roughness. Consequently, the fracture surface roughness after the test was measured. The fracture surface roughness was measured by measuring the line roughness on the fracture surface after the fatigue crack propagation test using a 3D shape measuring machine VR3100 manufactured by KEYENCE. The measurement range was IIb and IIa region of crack propagation, and the arithmetical mean deviation of roughness Ra was measured at 0.5 mm intervals in the direction perpendicular to the crack propagation direction. In the vicinity of the crack stop, the forced break and the part where the measurement range of line roughness overlaps was excluded.
Figure 3(a), (b) shows the da/dN-ΔK curves of the obtained test materials. The crystallization of chunky graphite caused a difference in the crack growth rate. At IIb stage (ΔK = 20.0∼12.5 MPa$\sqrt{m} $), the slopes of Chunky A and B are larger than those of Spheroidal A and B. The IIb stage of this da/dN-ΔK curve was approximated as a straight line, and the Paris’s law exponent m was obtained from eq. (3).
| \begin{equation} \frac{\text{d}a}{\text{d}N} = c (\Delta K)^{m} \end{equation} | (3) |
Relationship between da/dN and ΔK in materials containing spheroidal and chunky graphite.
It is known that the value of m ranges from 2 to 7 for a wide range of materials. The m values were 2.8 and 3.6 for Spheroidal A and B, respectively, while those for Chunky A and B were 4.5 and 7.3, respectively, and the m value of the sample in which chunky graphite was crystallized was large, and the crack growth rate was accelerated. Also, the Chunky B sample containing a large amount of chunky graphite has a higher m value, and the acceleration effect is greater than Chunky A.
In addition, we focused on the decrease of threshold stress intensity factor range ΔKth. The ΔKth values for each test material are 10.0 MPa$\sqrt{m} $ for both Spheroidal A and B, 9.5 MPa$\sqrt{m} $, and 8.9 MPa$\sqrt{m} $ for Chunky A and B, respectively, as shown by the arrows in the figure for each sample. The ΔKth of the sample in which chunky graphite crystallized is slightly lower than that in the sample containing spheroidal graphite. Moreover, ΔKth of Chunky B sample containing much chunky graphite was slightly lower. The reason for this will be described later in the section on fracture surface observation results.
In addition, Fig. 4(a) and (b) show the relationship between the crack opening ratio U and ΔK of each test material. In Spheroidal A and B, the U value decreases uniformly with decreasing ΔK from about U = 0.8. In Chunky A, the U value decreases to around 0.5 even if ΔK decreases but increases again from ΔK = 17 MPa$\sqrt{m} $ to about 0.6. After that, it decreased significantly from ΔK = 11 MPa$\sqrt{m} $, and became about 0.4 at ΔKth. On the other hand, in Chunky B, the U value uniformly decreases with decreasing ΔK from about U = 0.7. Although there is a difference in the behavior of the U value of the two samples in which chunky graphite is crystallized, it is smaller than the two materials of spheroidal graphite in the entire ΔK region. From U value the sample in which chunky graphite is smaller than the two materials of spheroidal graphite in the entire ΔK region, it is considered that the crack closure occurs more markedly in the sample containing chunky graphite than in the sample containing spheroidal graphite.
Relationship between U and ΔK in materials containing spheroidal and chunky graphite.
The relationship between the crack growth rate da/dN and the effective stress intensity factor range ΔKeff excluding the effect of crack closure is shown in Fig. 5(a) and 5(b). Table 4 shows the values of the lower limit effective stress intensity factor range ΔKeff-th, excluding the effects of m, ΔKth, and crack closure of each test material. As shown by the arrows in Fig. 5 for each sample, Spheroidal A and B are 4.3 MPa$\sqrt{m} $ and 4.6 MPa$\sqrt{m} $, and Chunky A and B values are 3.6 MPa$\sqrt{m} $ and 3.7 MPa$\sqrt{m} $. The samples with the same shape of graphite showed almost the same value. However, even if the effect of crack closure was excluded, the samples containing chunky graphite had lower ΔKeff-th.
Relationship between da/dN and ΔKeff materials containing spheroidal and chunky graphite.
Figure 6 shows the observation results of the macroscopic fracture surface of each test material. In Chunky A and B, a black spot pattern with a diameter of about 3 mm was observed on the fracture surface. Figure 7 is an enlargement of this part. From the result of the fracture surface observation of the CT specimen, the black spot pattern on the fracture surface is different from the place where the spheroidal graphite is distributed, and the chunky observed in the structure observation sample and the fracture surface observation after the tensile test. Since it was graphite very similar to graphite, it was decided to be chunky graphite. Figure 8 shows the results of SEM observation of chunky graphite and spheroidal graphite on the fracture surface of Chunky A near ΔK = 11∼12 MPa$\sqrt{m} $. Chunky graphite, which looks like spots on the macroscopic fracture surface, is formed by dense flake graphite. The decrease in ΔKth of the samples containing chunky graphite described above is considered to be because the fatigue cracks propagate mainly in the matrix structure in the case of spheroidal graphite, but in the chunky graphite in the case of chunky graphite. Also, comparing the fracture surfaces in the spherical graphite and chunky graphite regions, it appears that the fracture surface roughness is different. Therefore, the fracture surface roughness of each test material was measured.
Observation of macroscopic fracture surface in CT specimen containing spheroidal and chunky graphite.
Observation of microscopic fracture surface of CT specimen containing chunky graphite.
Observation of microscopic fracture surface of spheroidal and chunky graphite area in chunky A specimen (ΔK = 11∼12 (MPa$\sqrt{\text{m}} $)).
Figure 9(a), (b) show the measurement results of the line roughness in the thickness direction of the specimen on the fracture surface of each test material. The figure shows the arithmetical mean deviation of roughness Ra on the vertical axis and the stress intensity factor range ΔK on the horizontal axis. Ra of Spheroidal A and B is almost constant regardless of the decrease of ΔK. Ra of Chunky A and B is larger than that of Spheroidal in all ΔK values. In Chunky A and B, there was a difference in the behavior of the fracture surface roughness with the change of ΔK, but no correlation with the behavior of the U value was found. Table 5 shows the average Ra of each test material. Chunky A and B are rougher than Spheroidal A and B, and there is a significant difference, especially in Chunky B, which contains much chunky graphite. Generally, the crack propagation near the lower limit is affected by the microstructure, and the crack propagate in a zigzag. Therefore, the fracture surface becomes rough. At this region, crack propagation involves shear mode II mixed with open mode I. For this reason, it is said that the unevenness of the fracture surface causes misalignment and the fracture surface comes into contact before it is completely unloaded, resulting in crack closure induced by fracture roughness.25,26) In this study, the fracture surface roughness of the sample containing chunky graphite was larger than that of the sample containing spherical graphite. Therefore, it is considered that crack closure induced by fracture roughness was prominent at the location where chunky graphite crystallized.
Relationship between line roughness and ΔK containing spheroidal and chunky graphite.
From the above results, characteristics of fatigue crack propagation deteriorate when chunky graphite crystallizes. The decrease in ΔKth depends on the amount of crystallization of chunky graphite. However, the degree of decrease in ΔKth due to chunky graphite is smaller than the decrease in static strength. The reason is that the fracture surface roughness of the crack surface that propagated inside the chunky graphite becomes large due to the crystallization of the chunky graphite, and as a result, the crack closure induced by fracture roughness occurs significantly.
By carrying at the fatigue crack propagation test for spheroidal graphite cast iron in which chunky graphite was crystallized, the following conclusion were obtained.
