Mechanical Properties of Electron Beam Welded Spheroidal Graphite Cast Iron and Mild Steel Welded Joints*

Transcription

1 Materials Transactions, Vol. 52, No. 10 (2011) pp to 1925 #2011 Japan Foundary Engineering Society Mechanical Properties of Electron Beam Welded Spheroidal Graphite Cast Iron and Mild Steel Welded Joints* Shinichi Sekiguchi 1 and Fumio Shibata 2 1 Precision Machinery Company, Ebara Corporation, Fujisawa , Japan 2 College of Science and Technology, Nihon University, Funabashi , Japan Direct between spheroidal graphite cast iron () and mild steel () was conducted using electron beam to study the microstructure and the mechanical properties of the welds. Results showed that one-pass yielded an over-hardened fusion zone exhibiting acicular martensite structure (815 HV) with cracks and porosities. Two-pass contained fewer porosities. Both one-pass and two-pass displayed acicular martensite and ledeburite within the microstructure of spheroidal graphite cast iron in heat-affected zone. The tensile strength of both one-pass and two-pass welded joints was lower than that of mild steel base metals. These joints ruptured at the fusion zone or the mild steel bond. However, two-pass welded joints demonstrated tensile yield stress values greater than or equal to those of mild steel base metals. Hardening of the fusion zone and the heataffected zone of spheroidal graphite cast iron made the impact strength values of two-pass welded joints conspicuously lower than those of the spheroidal graphite cast iron base metal. The fatigue strength of two-pass welded joints almost equaled that of mild steel base metal, with a fatigue limit of 209 MPa. [doi: /matertrans.f-m ] (Received February 14, 2011; Accepted June 2, 2011; Published September 25, 2011) Keywords: spheroidal graphite cast iron, mild steel, electron beam, number of passes, microstructure, mechanical properties 1. Introduction Compared to other cast irons, spheroidal graphite cast iron has superior mechanical properties, and, in recent years, has been widely applied in the fabrication of mechanical structural components and cast iron pipe. In order to achieve high functionality and to improve cost efficiency, components made by of spheroidal graphite iron casting to steel parts has been examined. 1) For example, industrial pump casings of spheroidal graphite cast iron typically are attached to steel pipes with mechanical fasteners (bolts). Welding of these casings to the pipes may reduce fluid leakage and reduce costs by simplifying the assembly. However, spheroidal graphite cast iron is difficult to weld, 2) and fusion spheroidal graphite cast iron to steel is even more challenging. Therefore, the application of these two materials to each other is limited. Welding is difficult because the process involves the rapid fusion and solidification of high-carbon spheroidal graphite cast iron, which results in the formation of ledeburite in the weld zone. The ledeburite in the weld can cause weld defects, such as cracks, due to embrittlement and significantly reduce the mechanical properties of the welded joint. Various authors have reported on fusion between spheroidal graphite cast iron and steel and, examined the microstructure and joint performance obtained by shielded metal arc, 3,4) tungsten inert gas, 5 7) metal active gas, 4) and gas. 4) In addition, some studies, 8 20) including one by A. Matting, et al., have reported on electron beam, which can provide higher energy density than other methods and minimize the thermal effect on base metals, to between gray cast irons or spheroidal graphite cast irons. For the details of *This Paper was Originally Published in Japanese in J. JFS 82 (2010) electron beam between spheroidal graphite cast iron and mild steel, only one report (Hatate, et al. 21) ) is available, stating that mild steel was butt welded to a 15 mm thick, spheroidal graphite cast iron plate using insert metal comprising spheroidal graphite cast iron alloyed with 35% nickel. Therefore, this present study examines the microstructure and joint strength obtained by direct, square butt, electron beam of spheroidal graphite cast iron to mild steel. 2. Experimental Procedures Table 1 shows the chemical compositions and mechanical properties of the base metals. Spheroidal graphite cast iron (equivalent to, JIS G5502) and mild steel (equivalent to, JIS G3101) were used as base metals. The microstructure and hardness of each base metal are shown in Fig. 1. The microstructure of the spheroidal graphite cast iron is composed mainly of pearlite but also contains a slight amount of ferrite around the graphite; its hardness is 246HV0.3. The microstructure of the mild steel is composed of pearlite and ferrite; its hardness is 137 HV0.3. Welding tool was a 6 kw high-voltage, high-vacuum electron beam machine (all vacuum type). Immediately before, the base metals were demagnetized, and the butt surface was degreased with methyl ethyl ketone. Then, the base metals were square butt welded by irradiating an electron beam onto the butt surface from above (downward penetration ), as shown in Fig. 2. The dimensions of the base metals were t mm. Test specimens were joined with one-pass welds and two-pass welds. Table 2 shows the conditions. In Table 2, the a b value 13) refers to an active beam parameter (a b ¼ D 0 =D F, D 0 : Objective Distance and D F : Focal Distance).

2 Mechanical Properties of Electron Beam Welded Spheroidal Graphite Cast Iron and Mild Steel Welded Joints 1921 Table 1 Fig Chemical compositions and mechanical properties of base metals. Chemical composition (mass%) Materials C Si Mn P S Mg Cu Cr C.E. Base metal Materials Tensile Impact Fatigue Elongation strength value limit (%) (MPa) (J/cm 2 ) (MPa) Base metal (a) 246HV (b) 137HV 100µm 100µm Microstructure and Vickers hardness of base metals. Fig. 2 Table 2 Electron beam Welding direction The microstructure was microscopically observed by cutting the bead transversely, polishing the cut surface, and then etching the surface with 4% nital. Furthermore, the chemical analysis of the fusion zone and the EPMA line analysis of the were conducted. For material tests, a Micro Vickers hardness tester (with a HV of 0.3) was used to measure the hardness at 0.1 mm pitch in the weld cross section. Tensile tests (using the test specimens No. 4, defined by JIS Z2201 Test pieces for 200 Joint configuration of butt. Welding conditions by butt. (unit:mm) Number of passes One-pass and Two-pass Vacuum 1: Pa Acceleration voltage 150 kv a b value 13Þ 0.9 Beam current 30 ma Welding speed 600 mm/min Welding heat input 4500 J/cm tensile test for metallic materials ), impact tests (Non-notch, mm, at test temperatures of 77 to 373 K), and rotary bending fatigue tests (at a rotational speed of 3000 min 1 ) were carried out to determine the mechanical properties of the welded joints. The surface of the welded joints fracture in tension was observed with a scanning electron microscope (SEM). 3. Results and Discussion 3.1 Microstructure and hardness distribution of the welds Figure 3 shows the microstructures of the one-pass and two-pass welds. The microstructure in the one-pass fusion zone exhibits acicular martensite and fine cracks. Compared to the one-pass, the microstructure in the two-pass fusion zone exhibits coarse acicular martensite, but no cracks. Slower cooling of two-pass weld due to the slightly wider fusion zone of two-pass (one-pass : 0.62 mm and two-pass : 0.78 mm) may be partly responsible for this difference. In the fusion zone near the bond of spheroidal graphite cast iron, the twopass shows a coarser acicular microstructure than the one-pass. In the fusion zone near the bond of mild steel, the appearance of acicular martensite in the one-pass is similar to that in the two-pass. The microstructure of spheroidal graphite cast iron in the heat-affected zone exhibits martensite and ledeburite in the vicinity of the graphite near the bond. For both the one-pass and the two-pass, the microstructure of mild steel in the heat-affected zone exhibits pearlite. Table 3 shows the results of the chemical analysis of the fusion zone. The carbon content in the fusion zone is nearly identical 0.85% for the one-pass and 0.86% for the two-pass. The silicon content is slightly higher in the two-pass 0.57% for one-pass vs. 0.83% for the twopass. The contents of manganese, phosphorous, sulfur and magnesium are almost the same regardless of the number of passes. These similar chemical compositions support the theory that the slower cooling of two-pass weld, rather than chemistry, is responsible for the coarser microstructure in the two-pass fusion zone. Figure 4 shows the results of the EPMA line analysis of the two-pass. In the fusion zone, there are concentration gradients of iron, carbon, silicon and manganese within approximately 200 mm of the bond in the mild steel. At the bond of spheroidal graphite cast iron, small concentration gradients of silicon and manganese are observed, but the magnitude and width of variation are smaller than the gradients at the bond of mild steel. In the two-pass fusion zone, the microstructure near the bond of spheroidal graphite cast iron is different from that near the bond of mild steel, possibly because of the difference in fusion zone composition. Figure 5 shows the hardness distributions of the one-pass and the two-pass. The hardness of the onepass fusion zone is 775 to 849 HV, averaging 815 HV, and is significantly higher than that of the spheroidal graphite cast iron base metal (approximately 240 HV). The hardness of the two-pass fusion zone is 387 to

3 1922 S. Sekiguchi and F. Shibata Table 3 Result of chemical compositions of fusion zones by butt (mass%). Fusion zone Butt Position of observation C Si Mn P S Mg One-pass <0: Two-pass <0: Crack 200 µ m (a) one-pass 200 µ m (b) two-pass Fig. 4 Bond of by one-pass Bond of by one-pass Bond of by two-pass Fig. 3 Result of line analysis by butt. Note: 1) Two-pass. Bond of by two-pass Microstructure of butt welds. Fig. 5 Vickers hardness distribution of butt welds. 820 HV, averaging 566 HV, and is lower than that of the onepass fusion zone. The coarser microstructure in the two-pass fusion zone may underlie this difference in hardness. The maximum hardness of the two-pass fusion zone is 820 HV, is near the bond of mild steel and is almost equal to the average hardness of the one-pass fusion zone (815 HV). This similarity in hardness may result from the consistent, acicular martensite microstructure in the fusion zone near the bond of mild steel is regardless of the number of weld passes. The maximum hardness of spheroidal graphite cast iron in the heat-affected zone is 869 HV for the one-pass and 823 HV for the two-pass, representing a significant increase in the hardness for both weld types. The maximum hardness of mild steel in the heat-affected zone is 465 HV for the one-pass and 362 HV for the two-pass, indicating a decrease in hardness as the number of passes increases.

4 Mechanical Properties of Electron Beam Welded Spheroidal Graphite Cast Iron and Mild Steel Welded Joints 1923 Position of SEM observation (a) Macrostructure Fig. 6 Comparison tensile strength and elongation of base metals and welded joints. (b) SEM photograph Fig. 8 Macrostructure and SEM photograph of tensile fractured surface. Notes: 1) Two-pass. 2) Tensile fractured surface: side. Fig. 7 Test specimens after tensile test, fractured surface and breaking position of welded joints. Note: N.P., Number of passes. 3.2 Mechanical properties of the welded joints Figure 6 compares the tensile strength and elongation of the base metals, the one-pass welded joint, and the two-pass welded joint. The welded joint tensile test was made on four test specimens for each weld type. The tensile strength of the one-pass welded joints ranged from 199 to 256 MPa, averaging 240 MPa. The tensile strength of the two-pass welded joints varied from 297 to 329 MPa, averaging 311 MPa. Thus, the average strength of the two-pass welded joint is 71 MPa higher than that of the one-pass welded joint; for all four test specimens of the two-pass welded joint, the tensile strength is equal to or greater than the yield stress of the mild steel base metal. The joint elongation of the twopass is also greater than that of the one-pass : 0.20 to 0.61% for the one-pass and 1.60 to 1.97% for the two-pass, averaging 0.40% and 1.74%, respectively. Figure 7 shows the tensile fracture surfaces and breaking positions of the one-pass and two-pass welded joints. The tensile breaking position of the one-pass welded joint is located in the fusion zone with coarse porosities observed on the fracture surface. The average porosity area ratio of the four test specimens on the tensile fracture surface of the onepass welded joint 10) is about 12%. The porosities probably are formed by the generation of CO gas through chemical reactions in the molten metal. 13) The tensile breaking position of the two-pass welded joint is located in the fusion zone and the bond of mild steel. A small number of fine porosities are observed on the fractured surface, and the size of porosities is significantly smaller than that in the one-pass. The area ratio of porosity on the tensile fracture surface of the two-pass welded joint is about 0.5%. For the two-pass, this fact suggests that the number of porosities in the fusion zone decreases with the less rapid cooling of the fusion zone and the facilitation of gas release through the stirring of the molten metal. 13) Figure 8 shows (a) the macrostructure and (b) the SEM photographs of the mild steel side tensile fracture surface of the two-pass welded joint. The tensile fracture surface of the joint exhibits concavities at the mild steel bond fracture surface and convexities at the fusion zone fracture surface. Fine porosities are also observed. A large part of the microfracture surface of the fusion zone (convex) has a quasicleavage fracture appearance. As is evident from these findings, the static tensile strength of the two-pass welded joint is higher. For all the joints, however, the tensile strength is lower than that of the mild steel base metal. For the one-pass, the authors attribute this result mainly to the fine weld cracks and porosities in the fusion zone. With two-pass, fewer cracks and porosities mean hither-strength joints. For both type of weld, significant hardness increases in the fusion zone near the bond of mild steel create an over-hardened area, which provides a fracture path, resulting in decreased tensile strength. Figure 9 shows the relation between the impact value and the test temperature for the spheroidal graphite cast iron base metal and the two-pass welded joint. The impact value of the spheroidal graphite cast iron base metal tends to decrease abruptly in a test temperature range of 348 to 248 K. At a test temperature of 223 K or less, the impact value is nearly constant at or just below 6 J/cm 2, suggesting that the transition temperature of the spheroidal graphite cast iron base metal is around 300 K. The impact value of the two-pass welded joint is 6 J/cm 2 or less in the entire test temperature range (373 to 77 K), and no transition behavior is observed. At a test temperature of 293 K, the impact value of the twopass welded joint is 2.6 J/cm 2, significantly lower than that of the spheroidal graphite cast iron base metal (19.6 J/cm 2 ). The two-pass welded joint fractures due to brittleness at a test temperature of 293 K or more and has an impact value significantly lower than that of the spheroidal graphite

5 1924 S. Sekiguchi and F. Shibata Fig. 9 Relation between impact value and testing temperature of base metal and welded joints. Fig. 11 S-N curves of base metals and welded joints. the mild steel base metal have the fatigue strength at 6: cycles, while the test specimens with porosities present in the fusion zone have the fatigue strength at 1: cycles. For the two-pass welded joint, the fatigue strength of the test specimens with no porosities present in the fusion zone is almost equal to that of the mild steel base metal. Thus, the over-hardened portion of the fusion zone or spheroidal graphite cast iron in the heat-affected zone had little negative effect on the fatigue strength. However, the porosities in the fusion zone cause stress concentrations there, resulting in development of crack, and materially decreasing fatigue strength. 16) 4. Conclusions Fig. 10 Test specimens after fatigue test and breaking position of welded joints by butt. Note: 1) Two-pass. cast iron base metal. Thus, it fails to achieve good joint performance. Figure 10 shows the fatigue breaking position of the twopass welded joint. Except for one specimen that did not fracture due to fatigue, the fatigue breaking position of the two-pass welded joint is located in the mild steel base metal for five test specimens and in the fusion zone for two test specimens. The test specimens fractured in the fusion zone exhibit porosities on the fracture surface. Figure 11 shows the S-N curve of the two-pass welded joint. The rotating bending fatigue limit of the two-pass welded joint is 209 MPa and almost equal to that of the mild steel base metal. Since the two-pass welded joint has an average tensile strength of 310 MPa, the fatigue limit/tensile strength ratio of the joint is approximately 0.67, which is greater than that of the mild steel base metal (approximately 0.48). Except for the test specimens with porosities present in the fusion zone, the fracture occurred mainly in the mild steel base metal at cyclic stresses of 215 to 273 MPa. At a cyclic stress of 232 MPa, for example, the specimens fractured in The microstructure and strength of electron beam welded joints between spheroidal graphite cast iron and mild steel were examined. The conclusions are summarized below. (1) In all cases, a large part of the microstructure in the fusion zone exhibited acicular martensite. The acicular martensite of the two-pass welds was coarser than that of the one-pass welds. In the two-pass fusion zone, the microstructure near the bond of spheroidal graphite cast iron exhibits coarse acicular martensite and that near the bond of mild steel has cementite. These microstructures differed in both. (2) A significant increase in hardness was apparent in both the fusion zone (one-pass : 815 HV on average and two-pass : 566 HV on average) and the heat-affected zone of spheroidal graphite cast iron (onepass : 869 HV at the maximum and two-pass : 823 HV at the maximum). (3) For all welded joints, the tensile strength was lower than that of the mild steel base metal. In addition, the breaking positions were located in the fusion zone and at the bond with mild steel. The two-pass welded joint exhibited a tensile strength equal to or greater than the yield stress of the mild steel base metal; its average tensile strength and average elongation were 311 MPa and 1.74%, respectively. (4) The impact value of the two-pass welded joint was 2.6 J/cm 2 at a test temperature of 293 K and significantly lower than that of the spheroidal graphite cast iron base metal (19.6 J/cm 2 ). (5) The fatigue strength of the two-pass welded joint was almost equal to that of the mild steel base metal. The

6 Mechanical Properties of Electron Beam Welded Spheroidal Graphite Cast Iron and Mild Steel Welded Joints 1925 rotating bending fatigue limit of the two-pass welded joint was 209 MPa, meaning that the fatigue limit/ tensile strength ratio was approximately 0.67 and greater than that of the mild steel base metal. Acknowledgments The authors are grateful to the research students from the Department of Mechanical Engineering and the Department of Precision Machinery Engineering, College of Science and Technology, Nihon University, for their kind cooperation in the experiments of this study. REFERENCES 1) K. Asano and T. Noguchi: J. JFS 78 (2006) 98. 2) Jpn. Weld. Soc.: YousetsuSetsugou binran (Maruzen, 1990) p ) T. E. Kihlgren and H. C. Waugh: Welding J. 32 (1953) ) N. Fujii, J. Takahashi, H. Suzuki and K. Yasuda: Q. J. Jpn. Weld. Soc. 23 (2005) ) S. Hiratuka, H. Horie, M. Nakamura, T. Konishiki, M. Aonuma and T. Kobayashi: J. JFS 70 (1998) ) M. Aonuma, S. Hiratuka, H. Horie, M. Nakamura and T. Konishiki: J. JFS 72 (2000) ) S. Hiratuka, H. Horie, T. Konishiki and M. Nakamura: J. JFS 78 (2006) ) A. Matting and K. Seifert: Schweiben und Schneiden 20 (1968) ) F. Shibata: J. JFS 69 (1997) ) F. Shibata, S. Ando and N. Fujisaki: J. Jpn. Weld. Soc. 51 (1982) ) F. Shibata and S. Ando: Trans. JWS 14 (1983) ) S. Ando and M. Ookubo: Q. J. Jpn. Weld. Soc. 2 (1984) ) F. Shibata: IMONO 56 (1984) ) F. Shibata: IMONO 59 (1987) ) K. Tagashira, S. Kamoda and T. Hashimoto: J. S. Precision Eng. 53 (1987) ) F. Shibata and Y. Uchida: IMONO 60 (1988) ) F. Shibata: Trans. JWS 22 (1991) ) F. Shibata: Trans. JWS 22 (1991) ) F. Shibata: Trans. JWS 22 (1991) ) F. Shibata: IMONO 64 (1992) 9. 21) M. Hatate, T. Shiota, Y. Nagasaki, N. Abe, M. Amano and T. Tanaka: J. High Temp. Soc. 33 (2007) 313.