Influence of alloying on the uniformity of strength and structure of gray iron

Influence of alloying on the uniformity of strength and structure of grayironLili Xiao1, Fengzhang Ren1,a, Mengqi Liu2,Yufei Wang1, Nawen Zhang3,andRui Wu31.School of Materials Science and Engineering, Henan University of Science and Technology,Luoyang 471003, Henan, China2 Wuhan Iron and Steel Group Echeng Iron and Steel Co., LTD. Wuhan,436000, Hubei, China3 Department of Mechanical and Electrical Engineering, Luoyang Institute of Science andTechnology, Luoyang 471023, Henan, Chinaa corresponding author: renfz@Keywords: Gray cast iron, Hardness, Section sensitivity, Machinability.Abstract. Two gray cast irons with equal tensile strength were prepared to investigate the influence of alloying on the machinability of gray iron. After 75SiFe modifying treatment, they were alloyed by the mixture additive containing RE, Cr, Mn, Si and Fe with a certain proportion and pure copper, respectively. The hardness, section sensitivity, structuralhomogenity and machinability were tested in this experiment. The results show that both of the gray irons have the same brinell hardness and the micro-hardness. The section sensitivity of the gray iron alloyed by the mixture additive is smaller than the gray iron alloyed by copper. The main cutting force of the cast iron alloyed by the mixture additive is lower than that of the cast iron alloyed by copper, therefore, the gray cast iron alloyed by the mixture additive has a better machinability than the cast iron alloyed by copper.IntroductionIn advanced countries, researches related to the machinability of gray cast iron had been carried out early, especially on the effect of alloying on machinability of gray cast iron. In 1954, Loria had been reported that microstructure of cast iron has a great influence on the machinability of cast iron [1]. Ryntz et al. found that the machinability of gray cast iron can be also affected by the hardness of gray iron cast, and a suitable hardness can provide an excellent machinability [2, 3]. Janowak et al. carried out machinability tests using two Class 45C high strength gray irons containing a nominal 0.1% and 3% intercellular carbides. The results showed that the service life of cutting tool decreases with increasing the content of intercellular carbide [4]. The influence of the mass fraction of microcarbides on the machinability of cast iron was investigated by Bates [5]. In his experiment, the dimensions of microcarbides during casting were controlled by controlling the cooling rate from eutectic temperature to eutectoid temperature and the concentration of pearlite stabilizing elements. It indicated that the mass fraction of microcarbides is a main factor to influence the machinability of the gray cast iron.In China, few reports on the machinability of gray cast irons are available. It could be found that the wearing of cutting tools for the homemade castings is much more serious than that for the imported castings. For example, a Chinese foundry found that the wearing for cutting the domestic castings is 10 times more than that for cutting the imported castings under the identical processing and machining conditions [7]. The alloying technique is an effective method to improve mechanical properties of gray cast iron parts [8]. Alloying can not only improve the machinability at thin section by minimizing the chilling tendency, but also enhance properties at thin and thick-walled section by grain refinement [9]. Therefore, alloying is a feasible technology to improve the machinability of gray cast iron [10].In this paper, the gray cast irons were inoculated by SiFe 75, and then alloyed by the pure copper and the mixture additive of RE, Cr, Mn, Si and Fe, respectively. Moreover, their tensile strength, hardness, microstructure and machinability were tested.Experimental producesTwo kinds of the experimental gray cast irons were prepared in a 100 kg induction furnace, using the pig iron, return scrap and steel scrap with the mass ratio of 50:10:40. The inoculation treatment was carried out in casting ladle at 1495°C. The experimental gray cast irons were alloyed using the copper and the mixture additive of 5.3%RE, 38.7%Cr, 8.5%Mn, 13.5%Si and Fe respectively. FeSi75 was used as the inoculant in this experiment, and the chemical compositions of the two gray cast irons were given in Table 1. The pure copper, the mixture additive and the FeSi75 inoculant were added into molten iron with a mass fraction of 0.4%. Four standard tensile specimens adopting GB/T 228-2002, a cylindrical sample with the dimensions of Φ100 mm×350 mm for evaluating the machinability and a step sample, as shown in Fig.1 for evaluating Brinell hardness and the section sensitivity were machined for each gray cast iron.Table 1 Chemical composition of the experimental gray cast ironsFig.1 A sketch map of step sampleResults and AnalysisThe tensile strength, Brinell hardness, micro-hardness and the cutting force of both gray cast irons were tested. The metallographic specimens were cut from the tensile samples nearby the fracture, which was analyzed to evaluate graphite morphology. The Brinell hardness on each step of the step sample were tested using HB 3000 hardometer, and then the hardness of each step were used toevaluate the section sensitivity of the gray cast irons. The maximum difference of the Brinell hardness between the thick and thin section (max ΔHB=HBmax –HBmin) was used to denote the sectionsensitivity. The homogeneity of matrix structure was evaluated by the difference in micro-hardness of metallographic specimens.Influences of Alloying Treatments on Tensile Strength The influence of the alloy elements on tensile strength are shown in Table 2. It can be found from Table.2 that the gray cast irons alloyed by copper and the mixture additive have the same tensile strength, closing 300 MPa, and the graphite grade and the pearlite content are identical.Table.2 Influence of alloy elements on tensile strengthC Si Mn PSCu Cr Mo CE Molten iron 3.47 1.7-1.8 0.7~0.9 ~0.05 0.00 ~0.15 0.15-0.2Cu3.131.89 0.75 0.036 0.13 0.37 0.33 0.027 3.35The mixture 3.121.900.910.037 0.044-0.30-3.35Sample Alloyelement Inoculant Tensile strength(MPa)Graphite Pearlitecontent1 Cu 75SiFe 297 A 952 The mixture 75SiFe 306 A 95The Influences of Alloying Treatments on the Section Sensitivity The Brinell hardness and sectio n sensitivity are shown in Table. 3. Because of the copper addition, the hardness of the cross-section of the step sample increases with the decreasing specimen thickness, and the hardness increases dramatically at the thickness of 15 mm, moreover, the copper addition make the value ofΔHB increase, which means the section sensitivity is undesirable. Conversely, the hardness of the every cross-section on the step sample tends towards consistent due to the addition of the mixture additive. Thus the mixture additive leads a decreasing section sensitivity of gray cast iron.We can also found that both of the gray cast irons have a similar average Brinell hardness HB, but the maxΔHB of the gray cast iron alloyed by copper is 19, meanwhile the gray cast iron alloyed by the mixture additive is 9. Based on test results, the gray cast iron alloyed by the mixture additive has a better section sensitivity.Table 3 Hardness in different sections of step specimenSample Alloy element andinoculantHB90mm 60mm 30mm 15mm HB MaxΔHB1 Cu+75SiFe 198 197 199 216 202 192 The mixture +75SiFe 208 199 201 207 203 9 Influences of Alloying Treatments on Microstructure Uniformity Literature [11-12] had been demonstrated that the uneven microstructure, especially the hard precipitate existing in microstructure, leads a poor processing performance of cast iron.In this experiment, Specimens of 20 mm ×20 mm×20 mm were cut from each gray cast iron, and then were polished by the traditional metallographic process. The micro-hardness uniformity was tested to represent the microstructure uniformity, and the results were listed in Table. 4. It can be found from Table. 4 that the average micro-hardness of the gray cast iron alloyed by copper is slightly higher than the gray cast iron alloyed by the mixture additive but the micro-hardness of a specific point of the gray cast iron alloyed by copper may be lower than that of the gray cast iron alloyed by the mixture additive. The maximum deviation of the gray cast iron alloyed by copper is 34 whereas the minimum is 22, and the maximum deviation of the gray cast iron alloyed by the mixture additive is 14 whereas the minimum is 22. So the micro-hardness uniformity of pearlite in gray cast iron could be significantly improved by the alloying treatment of the mixture additive.Table.4 The micro-hardness depression values of pearlit martixSample Alloy element andinoculantHVValue of each point Average anddeviation1 Cu+75SiFe 248, 255, 264, 248, 250, 270, 263,223, 264, 2783422-256+2 The mixture+75SiFe249, 234, 250, 267, 233, 234, 261,234, 259, 2311422-245+Influences of Alloying Treatments on the Machinability of Gray Cast Iron The revolving speed of 160 r·min-1 and the feed rate of 0.294 mm·r-1 remained constants, but the cutting depth was changeable. The main cutting force is shown in Fig. 2. As shown in Fig. 2, the main cutting forces ofthe gray iron alloyed by the mixture additive reduce 23%, 55% and 55.2% at the corresponding cutting depth 1.5 mm, 1.75 mm and 2.0 mm compared with the gray iron alloyed by coppe. So the cutting force of the gray cast iron alloyed by copper is higher than that of the gray cast iron alloyed by the mixture additive. In a cutting process, the magnitude of the cutting force reflects the difficulty degree of gray cast iron to be cut. The higher cutting force, the more difficult the material to be cut. Therefore, the machinability of the gray cast iron alloyed by the mixture additive is better than that of the gray cast iron alloyed by copper.In this paper, it can be found that the tensile strength of gray cast iron is not associated with the machinability, therefore the gray cast irons with the same tensile strength don't necessarily have the same machinability.500600700800900100011001200C u t t i n g f o r c e / NSpecimenCumixture3004005006007008009001000C u t t i n g f o r c e / NSpecimenmixtureCu3004005006007008009001000C u t t i n g f o r c e / NSpecimenmixtureCua ）b ）c ）Fig.2 Main cutting force of test specimensa-the cutting depth 2mm, b-the cutting depth 1.75mm, c-the cutting depth 1.5mmConclusions1. After the inoculation of 75SiFe, the gray cast irons alloyed the mixture additive and copper almost have the same tensile strength, brinell hardness and micro-hardness.2. The gray cast irons alloyed by the mixture additive have a better section sensitivity.3. The micro-hardness homogeneity of gray cast irons can be improved by the mixture additive.4. The gray cast iron alloyed by the mixture additive has a better machinability than the gray cast irons alloyed by copper. AcknowledgementThis study was supported by the Henan Province Program for Science and Technology Innovation Talents (104100510005 and 2009HASTIT023), by the Henan Province Key Problem Project (092102210012) and by the Luoyang Applied Technology Study and Development Foundation (0901011A and 1101093A). References[1] Loria E A. Machinability and microstructure of cast ion. AFS Transactions, 1954, 62: 163. [2] Ryntz E D Jr, Arnson H L. Influence of mold inoculation with ferrosilicon inserts on hardnesscontrol in alloy gray iron castings.AFS Transactions, 1975, 83: 211. [3] Ryntz E D Jr, Arnson H L. Influence of mold inoculation withferrosilicon inserts on hardnesscontrol in alloy gray iron castings.Modern Casting, 1976, 66(1): 53. [4] Janowak J F, Gundlach R B. 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