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The SAE 8620 alloy steel used as the material of Grade 80 lifting

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The SAE 8620 alloy steel used as the material of Grade 80 lifting Powered By Docstoc
					The SAE 8620 alloy steel used as the material of Grade

                   80 lifting chain and fittings in Yoke
             Steven Honga, T.S. Liana, L.Y. Tsenga H.C. Linb, K.M. Linc and Y. Lib


                      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


Keywords: Grade 80 lifting chain and fittings, SAE 8620, heat treatment, mechanical property,

corrosion.


Abstract. 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 material properties of SAE 8620 steel, including
the microstructure, mechanical strength, impact toughness and corrosion resistance are
clarified in this study. 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.


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 -1 . 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 α 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 α 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 α ferrite occurs. This feature is reasonable because the α 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 α 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 α ferrite and Fe3C cementite during the tempering process.
     Table 2 shows the cross-sectional hardness of normalized and tempered SAE 8620
steels (Φ 30 mm). In Table 2, the normalized specimen has a low hardness due to its
microstructure of coarse pearlite and α ferrite. All the 300℃~500℃tempered 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℃) 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℃and 200℃ 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℃, -40℃ and
200℃, 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℃~200℃.
   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.
Table 1 The standard criteria and measured chemical compositions of SAE 8620 steel by

      Optical Emission Spectrometer



                         SAE 862
                                      Standard criteria          Measured compositions
           Elements (wt%)

                    Fe                   balanced                      balanced

                    C                    0.17~0.23                   0.206~0.225

                    Si                   0.15~0.35                   0.223~0.240

                   Mn                    0.60~0.90                   0.831~0.846

                    Cr                   0.40~0.65                   0.415~0.447

                    Ni                   0.40~0.70                   0.461~0.546

                   Mo                    0.15~0.30                   0.168~0.179

                    P                      ≤ 0.03                    0.011~0.013

                    S                      ≤ 0.03                    0.006~0.007

                    Al                       --                      0.042~0.043

                   Cu                      ≤ 0.03                    0.026~0.029

                    V                        --                      0.002~0.005

                   Zn                        --                      0.001~0.004




     Table 2 The cross-sectional hardness of normalized and tempered SAE 8620 steels.



                                                    Hardness (HRC)

                         SAE 8620        specimen center → specimen edge

                                                   (distance     6 mm)

                         normalized          <5           <5             <5

                    300℃ tempered           39.8          40.8           46.2

                    400℃ tempered           36.2          38.9           43.3

                    500℃ tempered           30.6          32.4           38.5
       (a)                 (d)
                                                     (b)

                           (c)


                           (b)




                      30 mm
        (c)                                          (d)




Fig.1. The cross-sectional optical microstructures of normalized SAE 8620 steel. (b), (c) and (d)

      exhibit the microstructures at the corresponding positions in (a), respectively.




                                 Decarburization layer




   Fig.2. The optical microstructures around the outer surface of normalized SAE 8620 steel.
           (a)                 (d)                     (b)

                                (c)


                               (b)




                           30 mm
             (c)                                       (d)




Fig.3. The optical microstructures of SAE 8620 steel after quenching and 300°C tempering. (b),

      (c) and (d) exhibit the microstructures at the corresponding positions in (a), respectively.
        (a)                 (d)                        (b)

                            (c)


                            (b)




                        30 mm

              (c)                                      (d)




Fig.4. The optical microstructures of SAE 8620 steel after quenching and 400°C tempering. (b),

      (c) and (d) exhibit the microstructures at the corresponding positions in (a), respectively.
        (a)                  (d)                      (b)

                             (c)


                             (b)




                        30 mm
          (c)                                         (d)




Fig.5. The optical microstructures of SAE 8620 steel after quenching and 500°C tempering. (b),

      (c) and (d) exhibit the microstructures at the corresponding positions in (a), respectively.
  (a)                          o
                                                                                               (b)                                   o
                   1400       25 C               o                                                                  1400       - 40 C               300 C
                                                                                                                                                         o
                                             300 C                                                                                                        o
                                                      o                                                                                                400 C
                   1200                          400 C                                                              1200

                   1000                                                                                             1000




                                                                                                     stress (MPa)
    stress (MPa)




                                                                                                                                                                       o
                                                                           o
                                                                         500 C                                                                                    500 C
                   800                                                                                              800                                                     normalized
                                                                           normalized
                   600                                                                                              600

                   400                                                                                              400

                   200                                                                                              200

                     0                                                                                                0
                          0        5   10   15       20           25           30       35     40                          0         5        10    15       20   25       30   35   40
                                            strain (%)                                                                                               strain (%)
                                                 (c)                                     o
                                                                                       200 C
                                                                          1400                                        o
                                                                                                                    300 C
                                                                          1200                                                   o
                                                                                                                            400 C
                                                                          1000
                                                          stress (MPa)




                                                                                                                                     o
                                                                                                                                500 C
                                                                           800
                                                                                                                                     normalized
                                                                           600

                                                                           400

                                                                           200

                                                                               0
                                                                                   0     5     10      15             20       25        30    35     40
                                                                                                            strain (%)



Fig. 6 The engineering stress-strain curves of the normalized and tempered specimens of
               SAE 8620 steel, tested at (a) 25℃, (b) -40℃ and (c) 200℃.
                                    240
                                                                                o
                                    220                                     500 C
                                    200                                     normalized
             impact value (J/cm )


                                    180
            2




                                                                                o
                                    160                                     400 C
                                    140
                                    120
                                    100                                         o
                                                                            300 C
                                     80
                                     60
                                     40
                                     20
                                      0
                                      -200 -150 -100 -50   0   50 100 150 200
                                                                     o
                                             impact temperature ( C)



Fig. 7 The impact values tested at various temperatures for the normalized and tempered

       specimens of SAE 8620 steel.
                                    1.8
                                                                   normalized
                                    1.6                               o
                                                                   300 C
             weight loss (mg/cm )


                                    1.4
            2




                                                                      o
                                                                   400 C
                                                                      o
                                    1.2                            500 C

                                    1.0
                                    0.8
                                    0.6
                                    0.4
                                    0.2
                                    0.0
                                          0   1   2    3   4   5     6    7     8   9   10
                                                      immersion time (days)

Fig. 8 The weight loss vs. immersion time for the normalized and tempered specimens of SAE

        8620 steels statically immerged in a 3.5% NaCl solution.
     (a)                                     (b)




     (c)                                     (d)




Fig. 9 Surface morphologies of the normalized and tempered SAE 8620 specimens after

      immersion of 10 hours. (a) normalized, (b) 300°C tempered, (c) 400°C tempered, (d)

      500°C tempered.
                              0.5



                              0.0
              Potential (V)




                                          o
                                     400 C
                              -0.5



                              -1.0        o
                                     300 C
                                                o
                                              500 C
                                                      normalized
                              -1.5



                              -2.0
                                     -7         -6         -5        -4         -3   -2   -1
                                                                           2
                                                                Log I (A/cm )




Fig. 10. The polarization curves of the normalized and tempered SAE 8620 steels.
(a)                                       (b)




                     (c)




Fig. 11 Surface morphologies of the tempered SAE 8620 specimens after polarization test. (a)

300°C tempered, (b) 400°C tempered, (c) 500°C tempered.

				
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