2023 Volume 64 Issue 4 Pages 920-924
In recent years, with the active use of Mn-containing high-tensile strength steels as an automotive body material, it has become increasingly difficult to recycle Mn-containing high tensile strength steel scrap as raw material to produce cast iron, due to the fact that Mn is known as an element promoting the chilling tendency of cast iron melt. Therefore, in this study, the effect of Mn on the fatigue limit of flake graphite cast iron was investigated for the purpose of increasing the strength of cast iron and recycling high tensile strength steel. Flake graphite cast irons with different Mn contents (0.5 mass% and 2.0 mass%, the carbon equivalent corresponding to FC200, FC300 and FC400 of JIS specification (G 5501)) were prepared for plane bending fatigue test.
For melting, pig iron and electrolytic iron were used as iron sources, Fe–Si, Fe–Mn, and Fe–S were used as component adjusting materials, and Ca–Si was used as an inoculant. The melting temperature was 1803 K, the inoculation temperature was 1753 K, and the mold was poured at 1723 K.
Due to the addition of Mn, the Mn/S ratio increases, resulting in the crystallization of A-Type graphite in high-manganese flake graphite cast iron. With the increase of Mn content, the graphite fraction decreases, the lamellar layer becomes denser and homogeneous, the matrix structure is strengthened, and the tensile strength and fatigue limit of high-manganese flake graphite cast iron are improved. Therefore, it was suggested that high-manganese flake graphite cast iron can be used for thin-walled parts as a cast iron with excellent fatigue strength. In addition, in this thin-walled flake graphite cast iron, Mn acts as a matrix structure strengthening element rather than a chilling promoting element.
This Paper was Originally Published in Japanese in J. JFS 94 (2022) 601–605.
By adding alloying elements and microstructure control, high-tensile strength steel (HTSS) is steel material with higher strength than rolled steel for general structures. Manganese (Mn) is currently used with an addition of more than 2.0 mass% (hereafter, as %). HTSS has been chosen as an automotive body material because its properties are ideally suited for automotive body materials in perspectives of increasing strength and reducing weight recently.1–3) The scraps products from the pressing process, such as HTSS, are recycled as an iron source for cast iron.
Conversely, Mn inhibits the activity of carbon in the molten iron and encourages pearlitization in cast iron, which strengthens the matrix.4) However, it also suppresses graphite crystallization and stabilizes cementite, which supports the tendency of the matrix structure to chilling.5) Additionally, A. Roula and G.A. Kosnikov investigated the effect of Mn on the graphite crystallization process.6) According to these reports, recycling HTSS is a problem when Mn-containing HTSS scrap is used as an iron source for cast iron, as the chilling-promoting effect of Mn reduces the cast iron’s toughness and machinability. Although Mn removal and dilution with high-purity pig iron are considered solutions, they increase costs and cause slag disposal. Therefore, it is expected that the recycling rate will increase and costs will reduce if HTSS scraps can be recycled as an iron source for JIS-standard (G 5501) cast iron without removing Mn.
The flake graphite cast iron (FGI) with 2.0% Mn exhibits the best mechanical properties due to the change in graphite type to A-type, the reduction in graphite size, and the increase in the pearlite area fraction with increasing Mn content, according to a study7) on the effects of carbon equivalent (CE) value and cooling rate on the mechanical properties and the chilling tendency of FGIs with different Mn contents shows. However, the fatigue limit must be improved to introduce this technology to actual products. High strength in thin-walled cast iron is also in higher demand to lower the weight of automobiles and other vehicles. FGI has superior castability and machinability compared to spheroidal graphite cast iron, but its strength is significantly low. However, the high-manganese flake graphite cast iron (Hi-Mn FGI) has a tensile strength significantly higher than the JIS standard; therefore, the fatigue limit of Hi-Mn FGI is expected to improve.
Based on these considerations, to increase the recycling of HTSS and strengthen cast iron, two types of FGIs with 0.5% Mn and 2.0% Mn (which is similar to the composition of recycled HTSS) were prepared, and the effect of Mn on the fatigue limit was examined in this study.
Table 1 shows the chemical composition of the specimens. To prepare hypoeutectic FC200, FC300 near the eutectic point, and hypereutectic FC400 containing 0.5%Mn and 2.0%Mn (6 types in total), the CE value is modified by changing the content of carbon (C) and silicon (Si) (CE value = C% + 0.33Si%). The samples prepared in this experiment are thin and plate-shaped, measuring 120 mm in length, 40 mm in width, and 5 mm in thickness.
Pig iron and electrolytic iron were used as the iron sources, Ferrum (Fe)–Si, Fe–Mn, and Fe–Sulphur (S) was used as component adjusting materials, while calcium (Ca)–Si was used as an inoculant with the Si content adjusted to 0.3% by ladle inoculation. Pouring conditions are the maximum melting temperature of 1803 K, the inoculation temperature of 1753 K, and the pouring temperature of 1723 K.
2.2 Experimental method and observationThe tensile specimens were cast from the same molten metal in a shell type (φ30 mm) for tensile testing, processed into JIS-8C (based on ISO 6892 “Metallic materials - Tensile testing”) specimens, and subjected to tensile testing.
The fatigue tests were conducted using a plane bending fatigue testing machine in accordance with JIS Z 2273 (PBF-30X, Tokyo Koki Co., Ltd., Japan). The plane bending fatigue tests were conducted at room temperature under the following test conditions: a 20-Hz sinusoidal load with a stress ratio of −1. The test stop condition is that the loading stress decreased to 70% of the starting, or the number of cycles reached 1 × 107 cycles, and the fatigue limit was defined as the maximum loading stress that did not lead to failure until 1 × 107 cycles. Figure 1 depicts the dimensions of the plane bending fatigue test specimen.
Dimension of plane bending fatigue test specimen.
The specimens were mirror-finished by polishing before being etched with 3% nitric alcohol (Nital) for microstructure analysis. Sumitomo Metal Mining Co., Ltd. used the KOKUEN KYUJYOKARITSU SOKUTEI Ver. 2.20 (based on JIS G5502) to measure the ferrite and pearlite fractions, among other things. Image-J8) was used to determine the fraction of graphite (fg). A scanning electron microscope (Analytical Scanning Electron Microscope, JSM-6510A, JEOL, Japan) was used to examine the perlite microstructure.
Since Brinell hardness tests could not be done on the plane bending fatigue test specimen due to its 3 mm thickness, samples were obtained from near the fatigue fracture surface, and Vickers hardness tests were performed. The test load was 294 N using a diamond square-weight indenter with a facing angle of 136°. And a load time of 15 s. The average value of 10 points, excluding the maximum and minimum, was taken as the Vickers hardness and converted to Brinell hardness based on ASTM-E140.
Figure 2 shows the specimens’ microstructure image, graphite morphology, pearlite, and ferrite fractions. Table 2 shows the results for the diagonal measurement of Vickers indentation, Vickers hardness, and Brinell hardness. A trace of ferrite was found around the graphite in the matrix structure of 0.5% Mn-FGI materials, whereas the 2.0% Mn-FGI materials were all pearlite. In other words, as the Mn content increases, the pearlite fraction is close to 100%, indicating the pearlite-promoting effect of Mn as described in the introduction.4) Therefore, Mn acted as a strengthening element in the matrix structure and increased the Brinell hardness.4) Furthermore, the graphite types are C-type for 0.5Mn-FC200, A+C-type for 0.5Mn-FC300, and A+E-type for 0.5Mn-FC400, with C- and E-type graphite decreasing and A-type graphite increasing as the Mn content increases to 2% in each FGI. This agrees with a study,9) which reported that as the Mn/S ratio increases from 5 to around 20, the graphite type is more likely to crystallize as A-type graphite.
Microstructure, graphite shape, fraction of pearlite and ferrite of specimens.
Figure 3 shows the fg and tensile strength relationship specimens with various Mn contents. For each material, the specimens with a 2.0% Mn content have a lower fg, and higher tensile strength. Mn is an element that decreases the activity of carbon in molten metal,4) which is why the fg decreases in specimens with 2.0% Mn content. Therefore, from the viewpoint of graphite structure, adding Mn to such FGI is effective in improving strength because the graphite type will only be disorderly and uniformly crystallized into A-type graphite,10) and the fg decreases.
Relationship between fraction of graphite and tensile strength in specimens with different Mn contents.
The plane bending fatigue test results are shown in Fig. 4. Increasing the Mn content from 0.5% to 2.0% increased the fatigue limit by 21 MPa for FC200, 10 MPa for FC300, and 59 MPa for FC400, respectively. For FC400 in particular, a 38% increase in the fatigue limit was observed, with the highest increase in tensile strength.
S-N diagram of specimens.
Since the fatigue strength of FGI depends on the graphite and the matrix structure,11) the fatigue limit can be improved by increasing the Mn content as considered in this study.
The graphite Feret-diameter of each specimen was measured to identify the effect of Mn on graphite. The average graphite length of the random 20 graphites measured using SEM images and fg of each specimen are shown in Fig. 5. A difference of about 5 µm was observed for FC200. In contrast, similar values were observed for FC300 and FC400, and the change in fg due to Mn content was only up to 1.6% for each FGI material. Although these small graphite changes (decrease in graphite area fraction and graphite length, increased crystallization of A-type graphite, which has the best fatigue resistance compared to other types12)) are also effective in improving fatigue limits, it is unlikely that this is the main cause of 38% increase in fatigue limit.
Average graphite length and fraction of graphite of specimens.
As it has been reported that the lamellar layer spacing significantly influences the mechanical properties of pearlitic steels,13,14) the pearlite structure was examined accordingly, and the lamellar layer spacing was determined using SEM at a magnification of 20000 times. To determine the lamellar layer spacing, the cementite layer perpendicular to the polished surface was chosen, a perpendicular line was drawn from the center of two adjacent parallel cementite layers, and measurements were obtained at five random positions along the line. The average lamellar layer spacing for each specimen is shown in Fig. 6. The lamellar layer spacing of the 2.0Mn-FGI materials decreased by 83 nm in FC200, 59 nm in FC300, and 56 nm in FC400 compared to that of the 0.5Mn-FGI materials. This is due to an increase in the Mn/S ratio by adding Mn,9) which reduces the lamellar layer spacing and suppresses the coarsening of cementite.15) Figure 7 shows the relationship between fatigue limit and lamellar layer spacing of specimens. In the 93 nm to 190 nm range, a clear trend is observed that the fatigue limit increases with smaller lamellar layer spacing. Studies14,16,17) on the effect of lamellar layer spacing on crack propagation have shown that dislocation motion is prevented at the ferrite-cementite lamellar interface, which inhibits crack initiation and propagation in pearlite structure. Therefore, it is reported that pearlite with smaller lamellar layer spacing has superior resistance to fatigue crack propagation.
Lamellar layer spacing of specimens.
Relationship between fatigue limit and lamellar layer spacing of specimens.
However, 0.5Mn-FC300 with larger lamellar layer spacing shows a higher fatigue limit than 0.5Mn-FC400 with smaller lamellar layer spacing. The 0.5Mn-FC300 showed fewer differences in graphite structure and the fraction of pearlite than 0.5Mn-FC400, so the variation in lamellar layer spacing was observed. The variation of lamellar layer spacing was ±19 nm for 0.5Mn-FC400, which is larger than that of 0.5Mn-FC300 (±9 nm). Therefore, it is considered that 0.5Mn-FC300 shows a higher fatigue limit than 0.5Mn-FC400 from the viewpoint of the fatigue crack propagation mechanism in pearlite. Moreover, it was confirmed that the increase in Mn content inhibited each specimen’s variation in lamellar layer spacing. It can be inferred that the homogenizing of lamellar layer spacing contributed to the improvement of the fatigue limit. Therefore, in FC300, the decrease in lamellar layer spacing due to the increase of Mn content is small (±9 to ±4 µm), and the improvement in fatigue limit due to the increase of Mn content is about 6% only. Meanwhile, in FC400, the variation in lamellar layer spacing was significantly decreased (±19 to ±7 µm), and FC400 also showed a minimum value in lamellar spacing and is therefore considered to show as much as 38% improvement in fatigue limit.
As a result, the increase in Mn content results in a decrease in the lamella layer spacing, a decrease in the graphite area fraction and length, an increase in A-type graphite, and an improvement in the fatigue strength of flake graphite cast iron. In addition, it was confirmed that Mn acts as a matrix strengthening element rather than a chilling-promoting element in thin-walled flake graphite cast irons. Therefore, Mn-containing HTSS can be used as an iron source for thin-walled high manganese flake graphite cast iron without de-Mn treatment, which is expected to improve recycling and reduce cost. Meanwhile, it is suggested that high manganese flake graphite cast iron, as a cast iron with excellent fatigue strength, can be used for thin-walled parts.
In this study, various FGI materials with varying Mn contents were prepared, and the effect of Mn on the fatigue limit was investigated. The results are summarized as follows.
