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Environmental embrittlement of automobile spring steels caused by wet–dry cyclic corrosion in sodium chloride solution
Corrosion Science 47 (2005) 2450–2460 www.elsevier.com/locate/corsci
Environmental embrittlement of automobile spring steels caused by wet–dry cyclic corrosion in sodium chloride solution Shin-ichi Komazaki a,*, Kazuya Kobayashi a, Toshihei Misawa a, Tatsuo Fukuzumi b a
Department of Materials Science and Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050-8585, Japan b Mitsubishi Steel Mfg. Co., Ltd., 3-2-22 Harumi, Chuo-ku, Tokyo 104-8550, Japan Received 29 August 2004; accepted 24 October 2004 Available online 23 May 2005
Abstract The susceptibility to environmental embrittlement (EE) of automobile spring steels was investigated using six different steels. Slow strain rate tensile test and thermal desorption spectroscopic analysis were applied to specimens subjected to wet–dry cyclic corrosion tests in a NaCl solution. Experimental results revealed that the reduction in ductility after the corrosion tests was pronounced with increasing strength level. This degradation was closely associated with the resistance to pitting corrosion. Consequently, the hydrogen absorbed in steel and the corrosion pit as a geometric damage were responsible for the EE of spring steels. The hydrogen in rust layer had no significant influence on the EE. 2005 Elsevier Ltd. All rights reserved. Keywords: Low alloy steel; A. Hydrogen absorption; C. Hydrogen embrittlement; C. Pitting corrosion; C. Rust
*
Corresponding author. Tel.: +81 143 46 5668; fax: +81 143 46 5601. E-mail address: [email protected] (S.-i. Komazaki).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.10.008
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
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1. Introduction From the viewpoints of environmental conservation and energy saving, a mass reduction of automobile has been taken up as one of worldwide important issues. However, suspension spring steels for automobile suffer possible degradation of mechanical properties attributable to wet–dry cyclic corrosion in a solution containing chloride ions. These ions result from de-icing salts (sodium chloride, calcium chloride, magnesium chloride, etc.), which are scattered on road for melting ice and/or snow in winter. This degradation is considered to be caused by both of corrosion pits as a geometric damage and hydrogen generated due to pitting corrosion [1–4]. In general, the susceptibility to this environmental embrittlement (EE) increases with increasing strength level of steel because of decrease in notch toughness and/or increase in susceptibility to hydrogen embrittlement. This increase in susceptibility to EE hinders the practical application of high strength spring steels to automobile suspension, which is essential for reducing a weight of automobile. In this study, the dependence of tensile strength on the susceptibility to EE was investigated using six different spring steels with a wide variety of strength levels. A slow strain rate tensile (SSRT) test and thermal desorption spectroscopic (TDS) analysis were applied to the specimens subjected to wet–dry cyclic corrosion tests in a sodium chloride solution to examine the susceptibility to EE and the distribution of hydrogen in the specimens, respectively. Additionally, the specimens, from which diffusible hydrogen or all hydrogen had been removed by two different heat treatments after the corrosion tests, were also prepared and submitted to the SSRT tests to separate the effects of corrosion pits and hydrogen on the EE.
2. Materials and experimental procedures The materials used in this study were six different automobile spring steels with various tensile strengths. The chemical compositions of the steels are summarized in Table 1. The SUP12 is an automobile spring steel standardized by Japanese Industry Standard (JIS). Other five steels were developed by Mitsubishi Steel Mfg. Co., Ltd. and have excellent corrosion resistance attributed to the addition of Ni, Cr, Mo, Cu and B [1]. Their heat treatments and tensile strengths are shown in Table 2. Wet–dry cyclic corrosion tests were carried out using a specimen (/ 1.8 mm, GL 15 mm) for the SSRT test. The surface of the specimen was covered with epoxy region except for the gauge portion. One cycle of the corrosion test consisted of immersion of the specimen in a 5 mass% sodium chloride solution (35 ± 0.5 C) [5] for 3 h and subsequent air-drying at room temperature for 21 h. After 20 cycles, size of each corrosion pit was measured by a scanning electron microscope (SEM). Then, the corroded specimen was submitted to the SSRT test at a strain rate of 6.7 · 10 6 s 1 at room temperature in the atmosphere. In addition, the thermal desorption spectroscopic (TDS) analysis was applied to the corroded specimens for investigating the amount and distribution of hydrogen, which was generated with corrosion and absorbed into the specimens.
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SUP12 HDS13 HDS13M-1 HDS13M-2 HDS13M-3 HDS13M-4
C
Si
Mn
P
S
Ni
Cr
Mo
Cu
Al
Nb
V
Ti
B
N
0.59 0.49 0.50 0.48 0.46 0.44
1.44 1.47 1.57 1.58 1.57 1.46
0.75 0.61 0.40 0.40 0.41 0.40
0.017 0.013 0.007 0.009 0.009 0.009
0.004 0.003 0.004 0.008 0.006 0.003
0.04 1.46 0.49 0.52 0.53 0.50
0.75 0.98 1.49 1.55 1.61 1.51
0.01 0.77 0.35 0.34 0.36 0.37
0.09 0.10 0.26 0.23 0.22 0.27
0.028 0.028 0.020 0.070 – –
– 0.025 0.020 0.015 0.022 0.024
– 0.11 0.08 0.09 0.10 0.08
– – 0.027 0.035 0.036 0.028
– – 0.0027 0.0022 0.0029 0.0025
– – – 0.0058 – –
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
Table 1 Chemical compositions (mass%) of steels used in this study
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
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Table 2 Heat treatments and mechanical properties of steels
SUP12 HDS13 HDS13M-1 HDS13M-2 HDS13M-3 HDS13M-4
Quenching
Tempering
Tensile strength, MPa
1173 K–15 min/OQ 1173 K–15 min/OQ 1173 K–30 min/OQ 1173 K–30 min/OQ 1173 K–30 min/OQ 1173 K–30 min/OQ
648 K–30 min/AC 623 K–30 min/AC 623 K–30 min/AC 723 K–1 h/AC 673 K–1 h/AC 673 K–1 h/AC
2269 2173 2219 1871 1985 2020
3. Results and discussion 3.1. Resistance to pitting corrosion After 20 cycles of corrosion tests, thick rust layer was removed from the surface of the corroded specimen in an alkaline solution. Next, the specimen was cut longitudinally into two halves to observe corrosion pits formed on the specimen surface. Fig. 1 shows examples of corrosion pits observed on the specimen surfaces and the cross-sectional surfaces of the SUP12, HDS13 and HDS13M-3. Larger corrosion pits are formed on the SUP12. Fig. 2 shows the relationship between the depth and radius of each corrosion pit. The corrosion pits are almost hemisphere, but the radius tends to be slightly larger than the depth as the pit size increases. The SUP12 has many corrosion pits of which size is larger than 50 lm, while there is no such a large pit in the HDS13M-3 to the contrary. Consequently, the resistance
Fig. 1. Corrosion pits observed on specimen surfaces and cross-sectional surfaces.
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S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
Radius, c/µm
150
100
50 HDS13
SUP12 0
0
50 100 Depth, a/µm
0
50 100 Depth, a/µm
HDS13M-3 0
50 100 Depth, a/µm
150
Fig. 2. Relationship between depth and radius of each corrosion pit.
of the SUP12 to pitting corrosion was inferior to those of other steels, and the maximum or average size of corrosion pit had a tendency to decrease with decreasing tensile strength. 3.2. Tensile properties after corrosion test After 20 cycles of corrosion tests, the corroded specimens were subjected to a slow strain rate tensile (SSRT) test without removing the rust layer. Fig. 3 shows examples of changes in load–displacement curves of the SUP12 and HDS13. The maximum loads and the corresponding displacements significantly decrease with the corrosion tests, resulting in sever embrittlement. In the case of the SUP12, the fractures of the corroded specimens apparently occur at the elastic region. Similar degradation of tensile properties was also observed in other steels, although the degree of degradation was different depending on the steel.
Load, P/kN
10
Intact specimen (0 cycle)
Corroded specimen (20 cycles)
SUP12
5 1 mm
0
Load, P/kN
10
HDS13 Intact specimen (0 cycle)
Corroded specimen (20 cycles)
5 1 mm
0 Displacement, d/mm Fig. 3. Examples of changes in load–displacement curves due to wet–dry cyclic corrosion tests.
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Figs. 4 and 5 show the fracture surfaces of the SSRT test specimens of the SUP12 and HDS13, respectively. The central areas of all fracture surfaces corresponded to brittle fractures, such as quasi-cleavage and intergranular fractures. The other circumferential areas were ductile fractures, i.e., shear lips. The intergranular fracture was partially observed both in the corroded and in the intact (as-received) specimens
Fig. 4. Fracture surfaces of the SSRT test specimens of the SUP12.
Fig. 5. Fracture surfaces of the SSRT test specimens of the HDS13.
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
Normalized displacement to failure
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1 0.8 0.6 0.4 0.2 0 1800
1900
2000
2100
2200
2300
2400
Tensile strength, σB /MPa Fig. 6. Normalized displacement to failure plotted as a function of tensile strength.
of the SUP12, particularly around the crack initiation sites (Fig. 4(b) and (d)). The areas of brittle fracture surfaces of the corroded specimens were larger than those of the intact specimens in all steels. This increase in brittle fracture surface was more pronounced as the degradation of tensile properties was distinct. It is worthy of note that most of crack initiation occurred at the bottom of corrosion pit in the corroded specimens, as can be clearly seen in Fig. 5(d). The normalized displacement to failure was defined as a ratio of the displacement for the corroded specimen to that for the intact one to evaluate and compare the susceptibility to environmental embrittlement (EE) of six spring steels. The normalized displacement estimated is plotted as a function of tensile strength in Fig. 6. It decreases monotonously with increasing tensile strength, and this decrease reflects the increase in susceptibility to EE. This variation in EE is likely to be closely associated with the corrosion resistance, because, as mentioned above, the resistance to pitting corrosion decreased with increasing tensile strength as well as the susceptibility to EE. The effects of corrosion pits and hydrogen, the latter of which is generated by corrosion and absorbed in the specimen, on the EE are individually examined in the next section. 3.3. Effects of heat treatments on EE Two different heat treatments were applied to the corroded specimens after the wet–dry cyclic corrosion tests to vary the distribution of absorbed hydrogen and to understand the role of hydrogen. One is the heat treatment of 300 C for 3 h and termed the ‘‘low-temperature treatment’’. The other is the ‘‘high-temperature treatment’’ which consists of another quenching and subsequent tempering suitable for each spring steel (Table 2). The purpose of the former is to remove only diffusible hydrogen, which is well known to cause hydrogen embrittlement of high strength steels [6–8]. The later is the heat treatment for releasing all absorbed hydrogen. It
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
Load, P/kN
10
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SUP12 Intact (0 cycle)
Corroded (20 cycles)
Low-temp. T. (300 °C×3 h)
High-temp. T. (Quenching + Tempering)
5 1 mm
0
Load, P/kN
10
HDS13 Intact (0 cycle)
Corroded (20 cycles)
Low-temp. T. (300 °C×3 h)
High-temp. T. (Quenching + Tempering)
5 1 mm
0 Displacement, d/mm Fig. 7. Effects of heat treatments on load–displacement curve of SUP12 and HDS13.
had been confirmed beforehand that the strengths of the steels showed no significant change by these heat treatments. Fig. 7 shows the effects of the treatments on the load–displacement curve of the SUP12 and HDS13. The curves in both steels show almost no variation even if the low-temperature treatment is applied. On the other hand, the steels recover their strength and ductility as a result of the high-temperature treatment, although their ductility is slightly lower than that of the intact specimen. These results indicate that diffusible hydrogen has no influence on the degradation of tensile properties in contrast to the results reported in earlier studies [6–8]. On the contrary, the hydrogen, which is desorbed above 300 C and is believed to harmless to mechanical properties, seems to be concerned in the present EE. The normalized displacements of the heattreated specimens are given in Fig. 8 along with the results of the corroded specimens. As mentioned above, there is almost no difference between the corroded specimen and the specimen submitted to the low-temperature treatment. The ductility is not completely restored to the original levels by the high-temperature treatment in spite of the removal of all absorbed hydrogen. The degradation remained after this treatment is likely to result from the corrosion pit as a geometric damage. 3.4. Hydrogen desorption characteristics The thermal desorption spectroscopic (TDS) analysis was applied to the corroded and de-rusted specimens for a better understanding of the above-mentioned phenomena. Fig. 9 shows examples of hydrogen evolution profiles obtained from the HDS13M-1. Hydrogen is not detected below 350 C at all for the intact specimen and there is a small broad peak at around 550 C in its profile. It is apparent from the hydrogen profile of the corroded specimen that two large peaks appear at about
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
Normalized displacement to failure
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1 0.8 0.6 0.4 Corroded specimen Low-temp.T (300°C× 3 h) High-temp.T (quenching + tempering)
0.2 0 1800
1900
2000
2100
2200
2300
2400
Tensile strength, σB /MPa Fig. 8. Normalized displacement to failure plotted as a function of tensile strength.
Hydrogen evolution rate, R/mass ppm min-1
0.04 Corroded specimen De-rusted specimen
0.03 Intact specimen
0.02
0.01
0 0
600 200 400 Temperature, T/ °C
800
Fig. 9. Examples of hydrogen evolution profiles observed on HDS13M-1.
250 and 400 C due to the cyclic corrosion test in addition to the increase in height of the peak at 550 C. The heights of these three peaks are almost the same although some serration can be seen on the profile. It is very important to emphasize that the hydrogen evolved below 300 C, which is generally known as diffusible hydrogen, disappears completely and that the hydrogen at around 400 C significantly decreases as a result of the removal of the rust layer. In this case, the rust layer was carefully removed from the specimen surface by mechanical polish using an emery paper of #600. From the above results, the hydrogen absorbed in the specimen can be divided into the following two types. (1) The hydrogen, which is evolved below around 300 C, corresponds mainly to the hydrogen released from the rust layer, because
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
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this hydrogen disappears by removing the rust layer. The desorption of this hydrogen results in the hydrogen profile with an evolution peak at around 250 C. (2) Most of the hydrogen evolved above 300 C exists in the steel and forms the profile with two large peaks at around 400 and 550 C. 3.5. Effects of hydrogen and corrosion pits on EE
Normalized displacement to failure
Based on both results of the SSRT tests using the heat-treated specimens and the TDS analyses, the effects of hydrogen and corrosion pits on the EE can be separated as follows. The EE of the corroded specimens (m in Fig. 10), from which all hydrogen was removed by the high-temperature treatment, results from only the corrosion pit as a geometric damage. On the other hand, the EE of the corroded specimen (d in Fig. 10) and the specimen subjected to the low-temperature treatment (h in Fig. 10) is caused by the hydrogen in the steel in addition to the corrosion pit. The hydrogen in the rust has no influence on the EE, because the susceptibility to EE shows almost no variation even though the corroded specimen was heat-treated at 300 C where hydrogen in the rust is removed from the specimen. In this way, the difference in the susceptibility to EE between the six automobile spring steels was closely associated with the resistance to wet–dry cyclic corrosion. The reduction in corrosion resistance facilitates the formation of large corrosion pits which is responsible for geometric damage and generation of hydrogen. As can be seen in Fig. 10, the difference in normalized displacement between the intact specimen and the high-temperature treated specimen is almost the same as that between the specimens subjected to the low and high-temperature treatments. Therefore, the contribution of the absorbed hydrogen in the steel to the EE can be roughly estimated to be almost equal to that of the corrosion pits.
1 Corrosion pit
0.8 0.6
Hydrogen in steel 0.4 0.2 0 1800
Corroded specimen Low-temp.T (300 °C×3 h) High-temp.T (quenching tempering) 1900
2000
2100
2200
2300
2400
Tensile strength, σB/MPa Fig. 10. Effects of hydrogen and corrosion pits on environmental embrittlement.
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4. Conclusions The dependence of tensile strength on the susceptibility to environmental embrittlement (EE) was investigated using six different automobile spring steels. A slow strain rate tensile (SSRT) test and thermal desorption spectroscopic (TDS) analysis were applied to specimens subjected to the wet–dry cyclic corrosion tests in a 5 mass% NaCl solution. Additionally, the specimens, from which diffusible hydrogen or all hydrogen had been removed after the corrosion tests, were also prepared and submitted to the SSRT tests to separate the effects of corrosion pits and hydrogen on the EE. From the present investigation, the following conclusions can be drawn: 1. The resistance of the present automobile spring steels to the wet–dry cyclic corrosion in the NaCl solution has a tendency to decrease with increasing tensile strength. The susceptibility to EE increases with tensile strength and is closely associated with the resistance to pitting corrosion. 2. The hydrogen absorbed in the specimen by the cyclic corrosion can be divided into the following two types: (a) The hydrogen evolved below 300 C by TDS corresponds to hydrogen released from the rust layer. The desorption of this hydrogen results in the hydrogen profile with an evolution peak at around 250 C. (b) Most of the hydrogen evolved above 300 C exists in the steel and forms two large peaks at around 400 and 550 C on TDS. 3. The effects of the corrosion pit as a geometric damage and the hydrogen in the steel on the EE are roughly estimated to be almost the same. The hydrogen in the rust has almost no influence on the EE.
References [1] [2] [3] [4] [5]
T. Fukuzumi, T. Misawa, Tetsu-to-Hagane 88 (2002) 73. T. Fukuzumi, S. Komazaki, T. Misawa, Tetsu-to-Hagane 88 (2002) 81. T. Nakayama, A. Inada, M. Shimotsusa, N. Ibaraki, K. Kawada, Materia Japan 41 (2002) 230. T. Tsubota, K. Kawada, T. Nakayama, N. Ibaraki, CAMP-ISIJ 16 (2003) 564. Committee for Investigation of Corrosion on Suspension Springs, Transactions of the JSSR 40 (1995) 103. [6] M. Nagumo, ISIJ International 41 (2001) 590. [7] K. Takai, R. Watanuki, ISIJ International 43 (2003) 520. [8] S. Komazaki, R. Maruyama, T. Misawa, ISIJ International 43 (2003) 475.
Environmental embrittlement of automobile spring steels caused by wet–dry cyclic corrosion in sodium chloride solution Shin-ichi Komazaki a,*, Kazuya Kobayashi a, Toshihei Misawa a, Tatsuo Fukuzumi b a
Department of Materials Science and Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050-8585, Japan b Mitsubishi Steel Mfg. Co., Ltd., 3-2-22 Harumi, Chuo-ku, Tokyo 104-8550, Japan Received 29 August 2004; accepted 24 October 2004 Available online 23 May 2005
Abstract The susceptibility to environmental embrittlement (EE) of automobile spring steels was investigated using six different steels. Slow strain rate tensile test and thermal desorption spectroscopic analysis were applied to specimens subjected to wet–dry cyclic corrosion tests in a NaCl solution. Experimental results revealed that the reduction in ductility after the corrosion tests was pronounced with increasing strength level. This degradation was closely associated with the resistance to pitting corrosion. Consequently, the hydrogen absorbed in steel and the corrosion pit as a geometric damage were responsible for the EE of spring steels. The hydrogen in rust layer had no significant influence on the EE. 2005 Elsevier Ltd. All rights reserved. Keywords: Low alloy steel; A. Hydrogen absorption; C. Hydrogen embrittlement; C. Pitting corrosion; C. Rust
*
Corresponding author. Tel.: +81 143 46 5668; fax: +81 143 46 5601. E-mail address: [email protected] (S.-i. Komazaki).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.10.008
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
2451
1. Introduction From the viewpoints of environmental conservation and energy saving, a mass reduction of automobile has been taken up as one of worldwide important issues. However, suspension spring steels for automobile suffer possible degradation of mechanical properties attributable to wet–dry cyclic corrosion in a solution containing chloride ions. These ions result from de-icing salts (sodium chloride, calcium chloride, magnesium chloride, etc.), which are scattered on road for melting ice and/or snow in winter. This degradation is considered to be caused by both of corrosion pits as a geometric damage and hydrogen generated due to pitting corrosion [1–4]. In general, the susceptibility to this environmental embrittlement (EE) increases with increasing strength level of steel because of decrease in notch toughness and/or increase in susceptibility to hydrogen embrittlement. This increase in susceptibility to EE hinders the practical application of high strength spring steels to automobile suspension, which is essential for reducing a weight of automobile. In this study, the dependence of tensile strength on the susceptibility to EE was investigated using six different spring steels with a wide variety of strength levels. A slow strain rate tensile (SSRT) test and thermal desorption spectroscopic (TDS) analysis were applied to the specimens subjected to wet–dry cyclic corrosion tests in a sodium chloride solution to examine the susceptibility to EE and the distribution of hydrogen in the specimens, respectively. Additionally, the specimens, from which diffusible hydrogen or all hydrogen had been removed by two different heat treatments after the corrosion tests, were also prepared and submitted to the SSRT tests to separate the effects of corrosion pits and hydrogen on the EE.
2. Materials and experimental procedures The materials used in this study were six different automobile spring steels with various tensile strengths. The chemical compositions of the steels are summarized in Table 1. The SUP12 is an automobile spring steel standardized by Japanese Industry Standard (JIS). Other five steels were developed by Mitsubishi Steel Mfg. Co., Ltd. and have excellent corrosion resistance attributed to the addition of Ni, Cr, Mo, Cu and B [1]. Their heat treatments and tensile strengths are shown in Table 2. Wet–dry cyclic corrosion tests were carried out using a specimen (/ 1.8 mm, GL 15 mm) for the SSRT test. The surface of the specimen was covered with epoxy region except for the gauge portion. One cycle of the corrosion test consisted of immersion of the specimen in a 5 mass% sodium chloride solution (35 ± 0.5 C) [5] for 3 h and subsequent air-drying at room temperature for 21 h. After 20 cycles, size of each corrosion pit was measured by a scanning electron microscope (SEM). Then, the corroded specimen was submitted to the SSRT test at a strain rate of 6.7 · 10 6 s 1 at room temperature in the atmosphere. In addition, the thermal desorption spectroscopic (TDS) analysis was applied to the corroded specimens for investigating the amount and distribution of hydrogen, which was generated with corrosion and absorbed into the specimens.
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SUP12 HDS13 HDS13M-1 HDS13M-2 HDS13M-3 HDS13M-4
C
Si
Mn
P
S
Ni
Cr
Mo
Cu
Al
Nb
V
Ti
B
N
0.59 0.49 0.50 0.48 0.46 0.44
1.44 1.47 1.57 1.58 1.57 1.46
0.75 0.61 0.40 0.40 0.41 0.40
0.017 0.013 0.007 0.009 0.009 0.009
0.004 0.003 0.004 0.008 0.006 0.003
0.04 1.46 0.49 0.52 0.53 0.50
0.75 0.98 1.49 1.55 1.61 1.51
0.01 0.77 0.35 0.34 0.36 0.37
0.09 0.10 0.26 0.23 0.22 0.27
0.028 0.028 0.020 0.070 – –
– 0.025 0.020 0.015 0.022 0.024
– 0.11 0.08 0.09 0.10 0.08
– – 0.027 0.035 0.036 0.028
– – 0.0027 0.0022 0.0029 0.0025
– – – 0.0058 – –
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
Table 1 Chemical compositions (mass%) of steels used in this study
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
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Table 2 Heat treatments and mechanical properties of steels
SUP12 HDS13 HDS13M-1 HDS13M-2 HDS13M-3 HDS13M-4
Quenching
Tempering
Tensile strength, MPa
1173 K–15 min/OQ 1173 K–15 min/OQ 1173 K–30 min/OQ 1173 K–30 min/OQ 1173 K–30 min/OQ 1173 K–30 min/OQ
648 K–30 min/AC 623 K–30 min/AC 623 K–30 min/AC 723 K–1 h/AC 673 K–1 h/AC 673 K–1 h/AC
2269 2173 2219 1871 1985 2020
3. Results and discussion 3.1. Resistance to pitting corrosion After 20 cycles of corrosion tests, thick rust layer was removed from the surface of the corroded specimen in an alkaline solution. Next, the specimen was cut longitudinally into two halves to observe corrosion pits formed on the specimen surface. Fig. 1 shows examples of corrosion pits observed on the specimen surfaces and the cross-sectional surfaces of the SUP12, HDS13 and HDS13M-3. Larger corrosion pits are formed on the SUP12. Fig. 2 shows the relationship between the depth and radius of each corrosion pit. The corrosion pits are almost hemisphere, but the radius tends to be slightly larger than the depth as the pit size increases. The SUP12 has many corrosion pits of which size is larger than 50 lm, while there is no such a large pit in the HDS13M-3 to the contrary. Consequently, the resistance
Fig. 1. Corrosion pits observed on specimen surfaces and cross-sectional surfaces.
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S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
Radius, c/µm
150
100
50 HDS13
SUP12 0
0
50 100 Depth, a/µm
0
50 100 Depth, a/µm
HDS13M-3 0
50 100 Depth, a/µm
150
Fig. 2. Relationship between depth and radius of each corrosion pit.
of the SUP12 to pitting corrosion was inferior to those of other steels, and the maximum or average size of corrosion pit had a tendency to decrease with decreasing tensile strength. 3.2. Tensile properties after corrosion test After 20 cycles of corrosion tests, the corroded specimens were subjected to a slow strain rate tensile (SSRT) test without removing the rust layer. Fig. 3 shows examples of changes in load–displacement curves of the SUP12 and HDS13. The maximum loads and the corresponding displacements significantly decrease with the corrosion tests, resulting in sever embrittlement. In the case of the SUP12, the fractures of the corroded specimens apparently occur at the elastic region. Similar degradation of tensile properties was also observed in other steels, although the degree of degradation was different depending on the steel.
Load, P/kN
10
Intact specimen (0 cycle)
Corroded specimen (20 cycles)
SUP12
5 1 mm
0
Load, P/kN
10
HDS13 Intact specimen (0 cycle)
Corroded specimen (20 cycles)
5 1 mm
0 Displacement, d/mm Fig. 3. Examples of changes in load–displacement curves due to wet–dry cyclic corrosion tests.
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
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Figs. 4 and 5 show the fracture surfaces of the SSRT test specimens of the SUP12 and HDS13, respectively. The central areas of all fracture surfaces corresponded to brittle fractures, such as quasi-cleavage and intergranular fractures. The other circumferential areas were ductile fractures, i.e., shear lips. The intergranular fracture was partially observed both in the corroded and in the intact (as-received) specimens
Fig. 4. Fracture surfaces of the SSRT test specimens of the SUP12.
Fig. 5. Fracture surfaces of the SSRT test specimens of the HDS13.
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
Normalized displacement to failure
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1 0.8 0.6 0.4 0.2 0 1800
1900
2000
2100
2200
2300
2400
Tensile strength, σB /MPa Fig. 6. Normalized displacement to failure plotted as a function of tensile strength.
of the SUP12, particularly around the crack initiation sites (Fig. 4(b) and (d)). The areas of brittle fracture surfaces of the corroded specimens were larger than those of the intact specimens in all steels. This increase in brittle fracture surface was more pronounced as the degradation of tensile properties was distinct. It is worthy of note that most of crack initiation occurred at the bottom of corrosion pit in the corroded specimens, as can be clearly seen in Fig. 5(d). The normalized displacement to failure was defined as a ratio of the displacement for the corroded specimen to that for the intact one to evaluate and compare the susceptibility to environmental embrittlement (EE) of six spring steels. The normalized displacement estimated is plotted as a function of tensile strength in Fig. 6. It decreases monotonously with increasing tensile strength, and this decrease reflects the increase in susceptibility to EE. This variation in EE is likely to be closely associated with the corrosion resistance, because, as mentioned above, the resistance to pitting corrosion decreased with increasing tensile strength as well as the susceptibility to EE. The effects of corrosion pits and hydrogen, the latter of which is generated by corrosion and absorbed in the specimen, on the EE are individually examined in the next section. 3.3. Effects of heat treatments on EE Two different heat treatments were applied to the corroded specimens after the wet–dry cyclic corrosion tests to vary the distribution of absorbed hydrogen and to understand the role of hydrogen. One is the heat treatment of 300 C for 3 h and termed the ‘‘low-temperature treatment’’. The other is the ‘‘high-temperature treatment’’ which consists of another quenching and subsequent tempering suitable for each spring steel (Table 2). The purpose of the former is to remove only diffusible hydrogen, which is well known to cause hydrogen embrittlement of high strength steels [6–8]. The later is the heat treatment for releasing all absorbed hydrogen. It
S.-i. Komazaki et al. / Corrosion Science 47 (2005) 2450–2460
Load, P/kN
10
2457
SUP12 Intact (0 cycle)
Corroded (20 cycles)
Low-temp. T. (300 °C×3 h)
High-temp. T. (Quenching + Tempering)
5 1 mm
0
Load, P/kN
10
HDS13 Intact (0 cycle)
Corroded (20 cycles)
Low-temp. T. (300 °C×3 h)
High-temp. T. (Quenching + Tempering)
5 1 mm
0 Displacement, d/mm Fig. 7. Effects of heat treatments on load–displacement curve of SUP12 and HDS13.
had been confirmed beforehand that the strengths of the steels showed no significant change by these heat treatments. Fig. 7 shows the effects of the treatments on the load–displacement curve of the SUP12 and HDS13. The curves in both steels show almost no variation even if the low-temperature treatment is applied. On the other hand, the steels recover their strength and ductility as a result of the high-temperature treatment, although their ductility is slightly lower than that of the intact specimen. These results indicate that diffusible hydrogen has no influence on the degradation of tensile properties in contrast to the results reported in earlier studies [6–8]. On the contrary, the hydrogen, which is desorbed above 300 C and is believed to harmless to mechanical properties, seems to be concerned in the present EE. The normalized displacements of the heattreated specimens are given in Fig. 8 along with the results of the corroded specimens. As mentioned above, there is almost no difference between the corroded specimen and the specimen submitted to the low-temperature treatment. The ductility is not completely restored to the original levels by the high-temperature treatment in spite of the removal of all absorbed hydrogen. The degradation remained after this treatment is likely to result from the corrosion pit as a geometric damage. 3.4. Hydrogen desorption characteristics The thermal desorption spectroscopic (TDS) analysis was applied to the corroded and de-rusted specimens for a better understanding of the above-mentioned phenomena. Fig. 9 shows examples of hydrogen evolution profiles obtained from the HDS13M-1. Hydrogen is not detected below 350 C at all for the intact specimen and there is a small broad peak at around 550 C in its profile. It is apparent from the hydrogen profile of the corroded specimen that two large peaks appear at about
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Normalized displacement to failure
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1 0.8 0.6 0.4 Corroded specimen Low-temp.T (300°C× 3 h) High-temp.T (quenching + tempering)
0.2 0 1800
1900
2000
2100
2200
2300
2400
Tensile strength, σB /MPa Fig. 8. Normalized displacement to failure plotted as a function of tensile strength.
Hydrogen evolution rate, R/mass ppm min-1
0.04 Corroded specimen De-rusted specimen
0.03 Intact specimen
0.02
0.01
0 0
600 200 400 Temperature, T/ °C
800
Fig. 9. Examples of hydrogen evolution profiles observed on HDS13M-1.
250 and 400 C due to the cyclic corrosion test in addition to the increase in height of the peak at 550 C. The heights of these three peaks are almost the same although some serration can be seen on the profile. It is very important to emphasize that the hydrogen evolved below 300 C, which is generally known as diffusible hydrogen, disappears completely and that the hydrogen at around 400 C significantly decreases as a result of the removal of the rust layer. In this case, the rust layer was carefully removed from the specimen surface by mechanical polish using an emery paper of #600. From the above results, the hydrogen absorbed in the specimen can be divided into the following two types. (1) The hydrogen, which is evolved below around 300 C, corresponds mainly to the hydrogen released from the rust layer, because
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this hydrogen disappears by removing the rust layer. The desorption of this hydrogen results in the hydrogen profile with an evolution peak at around 250 C. (2) Most of the hydrogen evolved above 300 C exists in the steel and forms the profile with two large peaks at around 400 and 550 C. 3.5. Effects of hydrogen and corrosion pits on EE
Normalized displacement to failure
Based on both results of the SSRT tests using the heat-treated specimens and the TDS analyses, the effects of hydrogen and corrosion pits on the EE can be separated as follows. The EE of the corroded specimens (m in Fig. 10), from which all hydrogen was removed by the high-temperature treatment, results from only the corrosion pit as a geometric damage. On the other hand, the EE of the corroded specimen (d in Fig. 10) and the specimen subjected to the low-temperature treatment (h in Fig. 10) is caused by the hydrogen in the steel in addition to the corrosion pit. The hydrogen in the rust has no influence on the EE, because the susceptibility to EE shows almost no variation even though the corroded specimen was heat-treated at 300 C where hydrogen in the rust is removed from the specimen. In this way, the difference in the susceptibility to EE between the six automobile spring steels was closely associated with the resistance to wet–dry cyclic corrosion. The reduction in corrosion resistance facilitates the formation of large corrosion pits which is responsible for geometric damage and generation of hydrogen. As can be seen in Fig. 10, the difference in normalized displacement between the intact specimen and the high-temperature treated specimen is almost the same as that between the specimens subjected to the low and high-temperature treatments. Therefore, the contribution of the absorbed hydrogen in the steel to the EE can be roughly estimated to be almost equal to that of the corrosion pits.
1 Corrosion pit
0.8 0.6
Hydrogen in steel 0.4 0.2 0 1800
Corroded specimen Low-temp.T (300 °C×3 h) High-temp.T (quenching tempering) 1900
2000
2100
2200
2300
2400
Tensile strength, σB/MPa Fig. 10. Effects of hydrogen and corrosion pits on environmental embrittlement.
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4. Conclusions The dependence of tensile strength on the susceptibility to environmental embrittlement (EE) was investigated using six different automobile spring steels. A slow strain rate tensile (SSRT) test and thermal desorption spectroscopic (TDS) analysis were applied to specimens subjected to the wet–dry cyclic corrosion tests in a 5 mass% NaCl solution. Additionally, the specimens, from which diffusible hydrogen or all hydrogen had been removed after the corrosion tests, were also prepared and submitted to the SSRT tests to separate the effects of corrosion pits and hydrogen on the EE. From the present investigation, the following conclusions can be drawn: 1. The resistance of the present automobile spring steels to the wet–dry cyclic corrosion in the NaCl solution has a tendency to decrease with increasing tensile strength. The susceptibility to EE increases with tensile strength and is closely associated with the resistance to pitting corrosion. 2. The hydrogen absorbed in the specimen by the cyclic corrosion can be divided into the following two types: (a) The hydrogen evolved below 300 C by TDS corresponds to hydrogen released from the rust layer. The desorption of this hydrogen results in the hydrogen profile with an evolution peak at around 250 C. (b) Most of the hydrogen evolved above 300 C exists in the steel and forms two large peaks at around 400 and 550 C on TDS. 3. The effects of the corrosion pit as a geometric damage and the hydrogen in the steel on the EE are roughly estimated to be almost the same. The hydrogen in the rust has almost no influence on the EE.
References [1] [2] [3] [4] [5]
T. Fukuzumi, T. Misawa, Tetsu-to-Hagane 88 (2002) 73. T. Fukuzumi, S. Komazaki, T. Misawa, Tetsu-to-Hagane 88 (2002) 81. T. Nakayama, A. Inada, M. Shimotsusa, N. Ibaraki, K. Kawada, Materia Japan 41 (2002) 230. T. Tsubota, K. Kawada, T. Nakayama, N. Ibaraki, CAMP-ISIJ 16 (2003) 564. Committee for Investigation of Corrosion on Suspension Springs, Transactions of the JSSR 40 (1995) 103. [6] M. Nagumo, ISIJ International 41 (2001) 590. [7] K. Takai, R. Watanuki, ISIJ International 43 (2003) 520. [8] S. Komazaki, R. Maruyama, T. Misawa, ISIJ International 43 (2003) 475.