Steven Hong (a), T.S. Lian (a), L.Y. Tseng (a) H.C. Lin(b), K.M. Lin (c) and Y. Li (b) (a) Yoke Industrial Corporation, Taichung, TAIWAN (b) Department of Materials Science and Engineering, National Taiwan University, Taipei, TAIWAN (c) Department of Materials Science and Engineering, Feng Chia University, Taichung, TAIWAN
1. Introduction
The lifting chain and fittings have been used widely in the fields of architecture, construction, transportation, petrochemistry, mining and fishing et al. in the past century. The raw materials for lifting chain and fittings are almost the high strength low alloy (HSLA) steels, especially the Nickel-Chromium-Molybdenum steels. These HSLA steels can exhibit not only the ultra-high mechanical strength, but also the excellent performances of toughness, hardenability, dynamic fatigue, notch-impact sensitivity, machining, welding, and workability [1-3]. SAE 8620 alloy steels, belonging to HSLA-80 grade steels, are often used as the raw materials for carburization to raise their surface hardness. These carburized SAE 8620 steels are widely used as the wear-resistant parts of bearing and gears [4]. Meanwhile, SAE 8620 steels have been developed to be an excellent material for the Grade 80 lifting chain and fittings in Yoke Industrial Corporation, Taiwan, because they can satisfy well the requirement for these components. The surface hardness of the direct-quenched SAE 8620 steels can reach 37˜43 HRC. Even after tempering at temperatures higher than 400°C, the SAE 8620 steels still exhibit well-satisfied strength for Grade 80 lifting chain and fittings [5,6]. The SAE 8620 steels also exhibit excellent performances of forging, welding, and machining. However, to upgrade the product’s performance, safety and application fields, it is important to more understand the properties of SAE 8620 steels, including the mechanical strength, impact toughness and corrosion behavior. Besides, the technique of heat treatment will also significantly affect the performance of SAE 8620 steels. To the author’s knowledge, however, there is no systematic investigation on these subjects. Therefore, the present study aims to investigate the material properties of SAE 8620 steels, including the microstructure, mechanical strength, impact toughness and corrosion resistance. Meanwhile, the effects of heat treatment on SAE 8620 steels are also discussed in this study.
2. Experimental Procedure
The SAE 8620 steel bars with diameter of 30mm were provided by China Steel Corporation, Taiwan. The chemical compositions of SAE 8620 steel bars were measured by using the Optical Emission Spectrometer. The specimens for various testing were mechanically machined from these bars and then heat treated according to the standard processes. They were normalized at 900°C for 1 hour in a salty bath and then air cooled. Some normalized specimens were heated up to 900°C again, maintained for 20 min. and then quenched into brine water. The quenched specimens were tempered at 300˜500°C for 1 hour. The specimen hardness was measured in a Rockwell hardness tester. For each specimen, the average hardness value was obtained from at least five test readings. Tensile testing was carried out in an Instron tensile testing machine with a strain rate of (5×10^-3)/s . The tensile specimens were prepared according to the ASTM E 8M-01 Standard [7]. Standard impact specimens (ASTM-E 23-02a) [8] were prepared and Charpy impact testing was carried out at -196°C ˜ +200°C. Electrochemical potentiodynamic measurement was carried out by an EG&G Model 273 potentiostat. The experiment was conducted at 25°C in a 3.5wt% NaCl solution and the scanning rate was 2mVs^-1. Immersion tests of corrosion were conducted at 25°C in a 3.5wt% NaCl solution for 1 to 10 days. Following this, the specimens were removed and cleaned in ethyl alcohol using an ultrasonic cleaner. The weight loss of the specimen during immersion test was calculated by subtracting the weight of the specimen after immersion test from that before test. The specimen weight was measured by a precise electronic balance to 0.01mg accuracy. The specimen microstructure and corroded morphology were observed by optical microscope (OM) and scanning electron microscope (SEM).
3. Results and Discussion
3.1 Chemical composition
It is well known that the HSLA steels can improve their various properties by addition of different alloy elements. Ni element can increase the impact toughness and Mn, Cr, Mo elements can raise the hardenability. The carbides of these alloy elements are quite stable and will inhibit the softening phenomenon of martensite during the tempering process. According to the Standards of ASTM A-952-02 [5] and EN1677 [6], the HSLA steels for the Grade 80 lifting chain and fittings must comprise at least 2 elements among Ni, Cr and Mo elements. Their composition criteria are Ni = 0.4%, Cr = 0.4%, Mo = 0.15%, P = 0.030%, S =0.030% and Al =0.025%. The chemical compositions of SAE 8620 steel bars used in this study are presented in Table 1. As shown in Table 1, the chemical compositions of SAE 8620 steel bars fall in the region of standard criteria for the Grade 80 lifting chain and fittings.
3.2 Microstructure and hardness
Fig. 1 shows the cross-sectional optical microstructures of normalized SAE 8620 steel with diameter of 30 mm. In Fig. 1, it can be clearly seen that the normalized SAE 8620 steel exhibits a typical mixed microstructures of pearlite (dark area) and alpha ferrite (white area), no matter at the center or edge of the cross section. Meanwhile, there appears an obvious decarburization layer with about 0.4 mm at the outer surface of normalized SAE 8620 steel, as shown in Fig. 2. Figs. 3-5 show the cross-sectional optical microstructures of SAE 8620 steel after quenching and then tempering at 300°C, 400°C and 500°C, respectively. In Fig. 3, the 300°C tempered specimen exhibits a typical microstructure of martensite within all the cross section. This indicates that the SAE 8620 steel has sufficient hardenability and hence all cross section of the steel bar with diameter of 30 mm can transform to martensite from austenite during the quenching process. Meanwhile, as mentioned in Section 3.1, the addition of Ni, Mn, Cr, Mo elements can raise the steel’s hardenability. The carbides of these alloy elements are quite stable and will inhibit the softening phenomenon of martensite during the tempering process. Hence, the 300°C tempered specimen still exhibits the needle-type martensite structure. However, in Figs. 4 and 5, the 400°C and 500°C tempered specimens show the microstructures of tempered martensite with some obvious ferrite phase, especially at the specimen’s central part. This indicates that the quenching martensite can transform into fine alpha ferrite and Fe3C cementite, namely a typical tempered martensite during the 400°C and 500°C tempering process. As compared in Figs. 4 and 5, the higher the tempering temperature is, the more quantity of alpha ferrite occurs. This feature is reasonable because the alpha ferrite and Fe3C cementite will grow quickly at higher tempering temperatures. Carefully examining Figs. 4 and 5, one can also find that there is even no obvious alpha ferrite at the outer parts, as shown in Figs. 4(d) and 5(d). This phenomenon demonstrate that a rapider quenching rate will more inhibit the transformation of quenching martensite into alpha ferrite and Fe3C cementite during the tempering process. Table 2 shows the cross-sectional hardness of normalized and tempered SAE 8620 steels (phi 30 mm). In Table 2, the normalized specimen has a low hardness due to its microstructure of coarse pearlite and alpha ferrite. All the 300°C˜500°Ctempered specimens exhibit a quite high hardness, HRC = 30.6, because of their hard microstructures of needle-type martensite and/or tempered martensite. In Table 2, one can also find that the specimen hardness decreases with increasing tempering temperature, and there appears a lower hardness at the central part for each tempered specimen. These features are ascribed to the lower quenching rate at the central part and the more obvious softening phenomenon at higher tempering temperature. These results are consistent with the observed microstructures in Figs. 3˜5.
3.3 Tensile and impact tests
Fig. 6(a) shows the engineering stress-strain curves tested at ambient temperature (25°C) for the normalized and tempered SAE 8620 steels. As can be seen in Fig. 6(a), the normalized specimen exhibits a lower ultimate tensile stress, say 500 MPa, and a higher fracture strain of 37%. For the tempered specimens, the ultimate tensile stresses increase significantly although their tensile strains decrease at the same time. The ultimate tensile stresses of 300°C tempered specimen can even reach 1300 MPa, which is much higher than that of the normalized specimen. With increasing the tempering temperatures, the ultimate tensile stress decreases a little and the elongation slightly increases. To understand the tensile property at low and moderate temperatures, the normalized and tempered SAE 8620 steels have also been conducted the tensile tests at -40°Cand 200°C and their engineering stress-strain curves are presented in Figs. 6(b) and (c), respectively. As compared in Figs. 6(a-c), the SAE 8620 steels exhibit similar values of ultimate tensile stress and elongation when they are tested at 25°C, -40°C and 200°C, no matter in the normalized or tempered states. This indicates that the Grade 80 lifting chain and fittings made of SAE 8620 steels can be used safely at a wide temperature range, say -40°C~200°C. The impact values tested at various temperatures for the normalized and tempered specimens of SAE 8620 steels are presented in Fig. 7. As can be seen in Fig. 7, the normalized and 400~500°C tempered specimens can have high impact values, but the 300°C tempered specimen exhibits a much lower impact value than the other specimens. Besides, the brittle-ductile transient temperatures of the tempered specimens are found to be lower than that of the normalized specimen. The higher the tempering temperature, the lower the transient temperature is.
3.4 Corrosion behaviors
Fig. 8 illustrates the weight loss of the normalized and tempered specimens of SAE 8620 steels as a function of immersion time. The weight losses of all these specimens progressively increase with immersion time. Moreover, the weight losses of the 400˜500°C tempered specimens are less than those of the normalized and 300°C tempered specimens, regardless of the immersion time. This feature indicates that the 400˜500°C tempered specimens have better corrosion resistance than the normalized and 300°C tempered specimens. This phenomenon is consistent with the SEM observation of corroded morphologies shown in Fig. 9. As can be seen in Fig. 9(a-b), the whole specimen surfaces have been severely corroded for the normalized and 300°C tempered specimens. However, for the 400˜500°C tempered specimens, the corroded surfaces only exhibit some pitting holes, as shown in Fig. 9(c-d). This indicates that the 400˜500°C tempered specimens are susceptible to local attack and thereby forming many corrosion pits. These pits would only produce a slighter corrosive damage and hence their weight loss is smaller. Fig. 10 shows the polarization curves of the normalized and tempered SAE 8620 steels, conducted in a 3.5% NaCl solution at room temperature. As can be seen in Fig. 10, the sequence of the corrosion potential of SAE 8620 steels is 400°C tempered > 300°C tempered > 500°C tempered > normalized. This indicates that the tempered SAE 8620 specimens are more passive in the 3.5% NaCl solution than the normalized one. Among these specimens, the 400°C tempered one exhibits the best corrosion resistance. In Figure 10, some inflective points in the polarization curves are clearly observed. This feature is ascribed to the phenomenon of local pitting corrosion, which often occurs for the Cr alloy steels. The cathode ions in the 3.5% NaCl solution will penetrate and destroy the metal-oxide films and hence produce the pitting corrosion. After that, the voltage continuously increases but the current doesn’t have obvious change. This phenomenon is the as-called passivation. The passive films will form on the alloy surface during the passivation process. Until the voltage reaches a critical value, the passive films break down, and the current increases with the voltage again. All these phenomena discussed above are consistent with the SEM observation of corroded morphologies shown in Fig. 11. As can be seen in Fig. 11(a, c), the specimen surfaces have been more corroded for the 300°C and 500°C tempered specimens. However, for the 400°C tempered specimen, the corroded surfaces only exhibit some pitting holes after the potentiodynamic test, as shown in Fig. 11(b).
4. Conclusion
The SAE 8620 alloy steel can satisfy well the requirement of EN1677 Standard, and is selected as the material of Grade 80 lifting chain and fittings in Yoke Industrial Corporation, Taiwan. The 400°C tempering after brine-water quenching is found to be an optimal process for the SAE 8620 steel to exhibit an excellent overall performance. These 400°C tempered specimens can exhibit an ultimate tensile strength of 1100 MPa and a brittle-ductile transient temperature of -40°C. Besides, the 400°C tempered SAE 8620 steel is only susceptible to local attack of pitting corrosion and can exhibit an excellent corrosion resistance.
References
[1] Ej. Czyryca: Key Eng. Mater. Vol. 84-85 (1993), pp. 491-520. [2] A. Ghosh, S. Das, S. Chatterjee and P. Ramachandra Rao: Materials Characterization Vol. 56 (2006), pp. 59-65. [3] K. Sampath: J. Materials Engineering and Performance Vol. 15(1) (2006), pp. 32-40. [4] A. Sagin and A. Topuz: Material Prufung Vol. 47(9) (2005), pp. 523-528. [5] Standard Specification for Forged Grade 80 and Grade 100 Steel Lifting Components and Welded Attachment Links, Designation: A 952/A 952M-02. [6] Components for Slings-Safety, Grade 8, SVENSK STANDARD SS-EN 1677. [7] Standard Test Methods for Tension Testing of Metallic Materials [Metric], Designation: E 8M-01. [8] Standard Test Methods for Notched Bar Impact Testing of Metallic Materials, Designation: E 23-02a.
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