The Effect of Nodular Graphite on Hardness and Toughness in Permanent Molds Made of Ferro Casting Ductile

The permanent mold was made of Ferro Casting Ductile as a result of sand casting. Permanent mold is mold that can be used repeatedly. This permanent mold is used to produce tensile test specimens. Tensile test specimens are made of gray cast iron. This study aims to determine and analyze the effect of spheroidal graphite on the hardness and toughness of Ferro Casting Ductile in permanent molds to make tensile test specimens. The casting process starts by making patterns using Styrofoam, making molds with green-sand sand molds, followed by pouring molten cast iron into the molds. The next process is dismantling the castings and machining to get a permanent mold according to the dimensions. Specimens for testing the chemical composition were obtained from the results of castings into spherical dies. Meanwhile, the impact, hardness, and microstructure specimens were taken from the permanent molded product. The results of the chemical composition test showed that Ferro Casting Ductile contained several main elements, namely 92.14 %Fe, 3.681 %C, 3.715 %Si, 0.182 %Mn, 0.050 %Ni, and 0.034 %Mg which affected the percentage of nodular graphite. Nodular graphite will affect the hardness of the permanent mold. Validation was carried out by testing the hardness at 3 different points 167.70 VHN, 162.77 VHN, and 155.77 VHN. Toughness was obtained from the impact test using the Charpy method, the impact values were 0.048 Joule/mm2 0.046 Joule/mm2 0.037 Joule/mm2 on 3 different specimens. From the test results, the specimen has met the FCD 400 standard.


Introduction
Metal casting is a multi-stage process that includes creating patterns, molding, melting, pouring, disassembling, and cleaning castings.Almost all metal objects with complex shapes, from large to small, in ferrous and non-ferrous metals, can be made using the casting process.The provisions of the castings must be considered in the manufacture of cast iron so that the resulting castings function properly according to the provisions.The casting provisions include the function of the cast difference, the required strength, the required hardness, and the problem of economic prices [1][2][3][4].
Molds of various types can be used in the metal casting process.These molds include non-permanent molds (sand molds) and permanent molds (metal molds).Permanent molds are metal molds that can be reused many times.Permanent metal molds are typically used in metal casting with low melting temperatures.Because of the slippery surface, the resulting castings have the proper shape to reduce machining work [5][6][7][8][9][10][11].
Ferro Casting Ductile (FCD) has the same carbon (3.0-4.0%) and silicon (1.82%) content as cast iron.FCD is made with the addition of a small amount of magnesium, the addition of this element makes the cast iron graphite become nodular.This change in the shape of graphite is followed by a change in ductility, making the ductility of cast iron increase (12)(13)(14)(15)(16)(17).
Therefore, it is necessary to research the manufacture of permanent molds with FCD because it has hard dan tough properties, has a higher melting point than gray cast iron, and will be easy to machine to produce good products/molds and more precision to reduce the finishing process [18][19][20].Hardness and toughness are important properties of metals [21][22][23][24][25].The hardness and toughness of permanent molds with cast iron material are influenced by the percentage of roundness of graphite in the microstructure.This study aims to determine and analyze the effect of spheroidal graphite on the hardness and toughness of ductile cast iron in permanent molds to make tensile test specimens.

Materials and Methods
This research was carried out by plotting 2D and 3D drawings using SolidWorks application software according to Figure 1.Dimensions of the permanent mold 220 mm x 40 mm x 40 mm, hole diameter 11 mm.Data collection was taken in several different places.The casting of permanent molds begins with making patterns using Styrofoam.Making the mold using the CO2 sand method.The specimens were made of ductile cast iron (FCD) by using an induction kitchen.The process of making specimens is simultaneously one tapping with the manufacture of permanent molds so that the material is the same.The machining process uses a CNC lathe and CNC milling.The specimens made include the chemical composition test specimen of the spectrometer.SEM testing with Quanta x 50 SEM Series type to see the microstructure.Hardness test using Vickers and toughness test using Charpy impact test.Table 1 shows the results of testing the chemical composition of FCD.The chemical composition test was carried out with the ASTM E 415-08 testing standard.The specimen contains 19 alloying elements, of which there are 5 dominant alloying elements, namely 92.14%Fe, 3.715%Si, 3.681%C, 0.182%Mn, and 0.050%Ni.Alloy elements that can show ductile cast iron are 3.715%Si, 3.681%C, and elements that accelerate spheroidal graphite are 0.034%Mg.2. shows the results of metallographic testing with SEM-EDS.Before taking photos of the specimens, a polishing process was carried out from the number 40 to 2500 fineness.The microstructure will be clearly visible after etching.This test was carried out with the SEM Quanta x50 SEM Series.Observation of the microstructure can be seen that the graphite is rounded by 76%.
Based on the metallographic observations of the FCD microstructure in Figure 2, it can be seen that the specimen consisted of graphite which tends to be spherical (nodules) and matrix.The matrix formed was ferrite and pearlite, where the ferrite matrix had ductile and soft properties while the pearlite matrix had a higher hardness [26].The graphite formed in the microstructure showed a less-than-perfect sphere.This was because the Mg content in the specimen composition was at the minimum limit.The standard chemical composition showed that the nodular cast iron was in the FCD 400 grade.The Mg content needed to be increased by inoculation as a nodular graphite former.Hardness testing was carried out using a micro-Vickers hardness tester.This test aims to determine the value of hardness at the points tested on the specimen.In Figure 3 micro-Vickers hardness test was carried out with a loading of 40 kgf.The test was carried out 3 times with different test points.The resulting hardness at a distance of 1.5 mm from the edge was 167.70 VHN, at a distance of 2.9 mm from the edge was 162.77VHN, and at a distance of 4.6 mm from the edge was 155.77VHN.
At the edges, it has a greater hardness than the center of the test point because at the edges the cooling process takes place faster than in the middle.From these data, the highest hardness value of the material was 167.70 VHN and the average hardness value was 162.08 VHN.
In this hardness test, the data from the test results were compared with the standard hardness of BTN 400 as-cast (Nodular Cast Iron).The standard hardness of BTN 400 is 116-170 HBN or equivalent to 121-179 VHN.The hardness test value on the nodular cast iron specimen had a value of 155.77-167.70VHN, which means that the nodular cast iron specimen has met the BTN 400 as-cast hardness standard.Impact testing is carried out using the Charpy method to find the impact price.The area of the specimen is 10×10 mm, the length is 55 mm, and a V-notch with an angle of 45° and a depth of 2 mm, in accordance with ASTM E-23.The Charpy impact test was carried out three times with the result that the impact values were 0.046 Joule/mm 2 , 0.048 Joule/mm 2 , and 0.037 Joule/mm 2 .The average impact value of this test is 0.043 Joule/mm 2 .
The impact fracture surfaces of the three specimens appear opaque and stringy as shown in Figure 4.This means that each specimen has high ductility and toughness.The high toughness is caused by the spheroidal graphite by 76%.

Conclusion
The results of the chemical composition test showed that Ferro Casting Ductile contained several main elements, namely 92.14 %Fe, 3.681 %C, 3.715 %Si, 0.182%Mn, 0.050 %Ni, and 0.034 %Mg which affected the percentage of nodular graphite.Nodular graphite will affect the hardness of the permanent mold.Validation was carried out by testing the hardness at 3 different points 167.70 VHN, 162.77VHN, and 155.77VHN.Toughness was obtained from the impact test using the Charpy method, the impact values were 0.048 Joule/mm 2 , 0.046 Joule/mm 2 , and 0.037 Joule/mm 2 on 3 different specimens.From the test results, the specimen has met the FCD 400 standard.

Figure 1 .
Figure 1.The drawing of cast product (a) 3D and (b) 2D3.Results and Discussions3.1.Chemical Composition Test.Table1shows the results of testing the chemical composition of FCD.The chemical composition test was carried out with the ASTM E 415-08 testing standard.The specimen contains 19 alloying elements, of which there are 5 dominant alloying elements, namely 92.14%Fe, 3.715%Si, 3.681%C, 0.182%Mn, and 0.050%Ni.Alloy elements that can show ductile cast iron are 3.715%Si, 3.681%C, and elements that accelerate spheroidal graphite are 0.034%Mg.

Figure 3 .
Figure 3. Micro-Vickers test result 3.4.Charpy Impact Test.Impact testing is carried out using the Charpy method to find the impact price.The area of the specimen is 10×10 mm, the length is 55 mm, and a V-notch with an angle of 45° and a depth of 2 mm, in accordance with ASTM E-23.The Charpy impact test was carried out three times with the result that the impact values were 0.046 Joule/mm 2 , 0.048 Joule/mm 2 , and 0.037 Joule/mm 2 .The average impact value of this test is 0.043 Joule/mm 2 .The impact fracture surfaces of the three specimens appear opaque and stringy as shown in Figure4.This means that each specimen has high ductility and toughness.The high toughness is caused by the spheroidal graphite by 76%.

Figure 4 .
Figure 4.The fracture surface of the three impact test specimens

Table 1 .
FCD Composition Test Results