1. Introduction Along with carbon, silicon, manganese and sulfur, phosphorus is traditionally regarded as one of the five elements normally present in gray cast iron [1, 2]. Phosphorus has limited solubility in austenite, which decreases by increasing the carbon content. Therefore during solidification of cast iron, phosphorus segregates into the melt. Eventually, in the absence of carbide-forming components such as Cr, V and with low cooling rates (cast in sand mould), phosphorus forms a eutectic of ferrite(containing some phosphorus in solid solution)and iron phosphide(Fe 3 P). Called steadite, the complex contains as a whole 10.2% of phosphorus and 89.8% of iron [3–5]. This eutecticstructure solidifies at 952ºC.Solidification temperature of the melt gradually lowers when the amount of phosphorus is increased.Because of the pronounced segregation, steadite particles may become visible at room temp- -erature and at phosphorus content as low as 0.05%P. These particles are Located in areas where solidification occurs last. During solidification of gray iron with a phosphorus content of more than 0.2%, steadite solidifies as sepa- -rate particles at the junction of three cells to form concave triangular-shaped constituents.Atabove0.4%a cellular phosphide network is formed surrounding eutectic cells and dendrites [1,6].
Nowadays, phosphorus is used as an alloying element for special applications. The classical example is brake shoes for trains with phosphorus contents of 2.5–3.5%P.Due to its high wear resistance and lack of sparking,high phosphorus gray Cast iron is the best material selection for brake shoes [7,8]. For production of radiators and cylinder liners medium phosphorus(0.45–0.65%P) for thin wall castings1.2–1.6%P and for art castings 1%P gray iron are employed. The average hardness of binary phosphide eutectic is 570HV which, in Compar- -ison with hardness of normal pearlite (252 HB), raises the hardness of cast iron. Phosphorus iron, having an adequately high and stable friction coefficient, best satisfies friction requirements [10,11]. The intensity of wear of high Phosphorus iron is lower by a factor of two to three compared with conventional iron that is not alloyed with phosphorus [1, 12 ]. Excessive phosphorus content raises the brittleness of gray iron because of the brittle and intergranular steadite and reduces tensile strength [9,13,14].
2. Experimental procedure Table 1 shows the chemical composition of iron ingot and steel scrap used to prepare phosphorus gray cast iron. In order to adjust the alloying elements and to keep them at a constant level, ferroalloys of Table 2 were used.
Melting of iron ingot and steel scrap was performed in an induction furnace with medium frequency. After melting, manganese, silicon and phosphorus ferroalloys were Added respectively to adjust the cast iron composition according to Table 3. For inoculation, ferrosilicon in the size of 1–4mm and in amount of 0.3 wt% of the melt was added to the melt at 1380 ºC and immediately poured into the sand mould.
To prepare samples for microstructure study, sand papers up to No. 2500 were used and polishing was carried out by 0.3 m alumina. To reveal the Overall microstructure, 2% nital solution and for eutectic phosphide detection, specific color etchant of Murakami’s reagent with composition of 10 g of pota- -ssium ferricyanide K 3 Fe(CN) 6, 10g of potassium hydroxide KOH and 75 ml distilled water were used. Optical microscope was used to study microstruct- -ures. 1:The tensile strength samples were prepared according toA536-84 standard and tested in an MTS universal instrument at the rate of 1 mm/min. 2:Impact test was performed according to ASTM subsize E23 standard by a VELPERT machine. 3:Brinell hardness testing was carried out using 187.5 kg force and a balld- -iameter of 2.5mm by an ESEWAY machine.
3. Results and discussion 3.1. Microstructure The phosphide eutectic in the microstructure is shown in Fig. 1 using five different phosphorus percentages.
As can be seen from Fig.2, by increasing phosphorus percent, the amount of steadite increases. By increasing phosphorus from 0.45 to 2.58 wt%, the phosphide eutectic structure increases from 4.7 to 17.81% in gray cast iron.
The eutectic phosphide is shown in high magnification in Fig. 3. This eutectic structure is shaped like the herringbone pattern, which is a characteristic of the binary eutectic of ferrite-iron phosphide.
3.2. Cooling curve Fig. 4 shows the cooling curve of phosphorus gray cast iron. In addition to ordinary points (liquidus, eutectic and eutectoid), the phosphide eutectic point is also discernible. When the phosphorus content is higher than 0.45%, the phosphide eutectic arrest appears on the cooling curve. The temperature of steadite formation in 1.57%P is 935 ºC, which is lower than the ledeburite eutectic temperature (about 1121 ºC). This indicates that the steadite is the last part of melt to solidify.
The effect of phosphorus on ledeburite eutectic temperature is shown in Fig. 5. Increasing phosphorus decreases the eutectic temperature. This may be attributed to an increase in low-temperature phosphide eutectic,which decreases eutectic temperature of cast iron.This Is in good agreement with other authors’ results . This decrease is useful in increasing fluidity and in casting thin walls.
3.3. Hardness and mechanical properties 3.3.1. Hardness Effect of phosphorus on hardness of gray cast iron is shown in Fig. 6. Increasing phosphorus from 0.45 to 2.58 wt% in gray cast iron, hardness increases from 215.43 to 249.38 HB.This is due to the hard phosphide eutectic. The microhardness of the phosphide eutectic is 486HV and matrix microstructure is completely pearlitic because of the high amount of manganese (about 0.6%). Therefore increasing steadite increases the hardness of cast iron overall.
3.3.2. Mechanical properties Brittle and intergranular, steadite has harmful effects on mechanical prop- -erties of gray cast iron as shown in Fig. 7. By increasing phosphorus from 0.45 to 2.58 wt% in gray cast iron the tensile strength decreased from 297.5 to 184.1MPa and impact strength changed from 4.3 to 2.7 J. The phosphide eutectic solidifiesafter the ledeburite eutectic transformation. On The other hand,steadite is the last Structure to solidify in inclusionprone grain boundaries. This leads to nucle- -ation of cracks and helps decrease tensile strength and impact energy in the presence of steadite.
4. Conclusions 1. Eutectic phosphorus solidifies as the final part of the melt in grain boundaries. 2. Increasing phosphorus from 0.45 to 2.58% increases the eutectic phosphide phase in cast iron from 4.7 to 17.81%. 3. Increasing phosphorus from 0.45 to 2.58% decreases the ledeburite eutectic from 1139.7 to 1102.5 ºC temperature of cast iron. 4. Increasing phosphorus from 0.45 to 2.58% increases hardness from 215.43 to 249.38 HB. 5. Increasing phosphorus from 0.45to2.58% decreases tensile strength from 297.5 to 184.1MPa. 6. Increasing phosphorus from 0.45 to 2.58% decreases impact energy from 4.3 to 2.7 J.