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The influence of atmospheric humidity and grain size on the friction and wear of AISI 304 austenitic stainless steel
Materials Letters 57 (2003) 4505 – 4508 www.elsevier.com/locate/matlet
The influence of atmospheric humidity and grain size on the friction and wear of AISI 304 austenitic stainless steel G. Bregliozzi a, A. Di Schino b, J.M. Kenny b,*, H. Haefke a b
a CSEM Swiss Center for Electronics and Microtechnology, Inc., Rue Jaquet-Droz 1, CH-2007, Neuchaˆtel, Switzerland Materials Science and Technology, Materials Engineering Center, University of Perugia, Loc. Pentima Bassa 21, 05100 Terni, Italy
Received 27 March 2003; accepted 10 April 2003
Abstract The tribological properties of a ultra-fine AISI 304 austenitic stainless steels obtained by means of a martensitic transformation and subsequent austenite reversion are reported. The effects of the grain size on the wear resistance of such material is, for the first time, investigated as a function of the atmospheric humidity. Decrease of relative humidity in wear tests of AISI 304 steel produces an increase in weight loss and in the friction coefficient. A beneficial effect of grain refining is also shown with respect to large grain steel in that the finer grain steel produces less initial weight loss and the weight loss with an increase in the humidity is also less pronounced. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Atmospheric humidity; Grain size; AISI 304 steel
Austenitic stainless steels possess both good corrosion resistance and formability, but they also have a relatively low yield strength. It is well known that the mechanical properties of austenitic stainless steels are sensitive to the chemical composition (which can induce hardening by both substitutional and interstitial solid solutions) as well as to the grain size [1]. Recently, there have been developments in this direction, where changes in chemical composition induced by nitrogen addition have been taken advantage of [2]. Another effective way to increase the yield strength without impairing ductility is by grain refine-
* Corresponding author. Tel.: +39-744-492939; fax: +39-744492924. E-mail address: [email protected] (J.M. Kenny).
ment. Since austenitic stainless steels do not undergo a phase transformation at typical annealing temperatures, the only way to refine the grain is by recrystallization after cold rolling. However, strengthening by grain refinement is limited due to the high recrystallization temperature of this stainless steel grade. The recrystallization temperature of AISI 304 is above 900 jC and the minimum grain size obtained is in the range of 10 – 30 Am [3,4]. An alternative way to obtain fine grains is by a martensite –austenite reversion after annealing at low temperatures, as reported in investigations carried out to analyse the effects of martensitic transformation and subsequent austenitic reversion on the grain refining of AISI 304 stainless steel and its influence on the tensile properties of the steel [5]. In previous publications, we examined the influence of grain size and chemical composition on
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00351-3
4506
G. Bregliozzi et al. / Materials Letters 57 (2003) 4505–4508
Table 1 Chemical composition (mass%) of the steel C
Si
Cr
Ni
Mo N
S
Mn P
Table 2 Grain size and mechanical properties of the AISI 304 studied samples
AISI 304 0.037 0.26 17.97 8.55 0.22 0.046 0.003 1.04 0.025
the mechanical and corrosion resistance of this type of steel [6,7]. In this paper, the wear resistance of an austenitic stainless steel is studied as a function of grain size and atmospheric relative humidity. The chemical composition of the austenitic stainless steel under consideration is shown in Table 1. In order to favour an austenite – martensite transformation, samples of hotrolled and annealed steel were cold-rolled down to 90% deformation after having been quenched in liquid nitrogen. The details related thereto are reported in Ref. [8]. The martensitic structure obtained in this manner was annealed at 780 jC for
Fig. 1. Microstructure of AISI 304 samples: (a) annealed at 780 jC for 5 min; (b) annealed at 1100 jC for 15 min.
AISI 304
Grain size (Am)
Hardness (HV)
Tensile strength (N/mm2)
0.2% Yield strength (N/mm2)
2.5 40
242.5 163.5
790 650
480 240
5 min and at 1100 jC for 15 min in order to obtain the two different microstructures, reported in Fig. 1, resulting in an average grain size of 2.5 Am (FG) and 40 Am (LG), respectively. The sample surfaces were then polished by using increasingly finer abrasive papers, down to a 1-Am grit. The grain size of the two samples, determined using an automatic image analyser, and the mechanical properties are reported in Table 2. The friction coefficient was measured using a tribometer in a ball-on-disk (BoD) configuration, whereby a steel ball (AISI 52100) slides over the
Fig. 2. (a) Friction coefficient dependence on relative humidity and grain size for AISI 304 samples. (b) Weight loss dependence on relative humidity and grain size for AISI 304 samples.
G. Bregliozzi et al. / Materials Letters 57 (2003) 4505–4508
4507
austenitic stainless steel with a constant linear speed of 10 cm/s and under a load of 2 N. During the test, the temperature of the air was kept constant at 20 F 1 jC. The stop criterion of the tests was 400 m, which corresponds to 21,000 laps. In order to evaluate the weight loss and the friction coefficient with a change
Fig. 4. Micrographs of severely damaged worn surfaces perpendicular to the wear track. Note the crack beneath the surface.
Fig. 3. Micrographs of severely damaged worn surfaces and roughness value: (a) fine grain steel at a relative humidity of 20%; (b) fine grain steel at a relative humidity of 80%.
Fig. 5. Micrographs of the counterbody: (a) formation of transferred layer on the surface after the running-in stage; (b) wear of the surface after a fourth of the test has been completed.
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G. Bregliozzi et al. / Materials Letters 57 (2003) 4505–4508
in the relative humidity and grain size of the austenitic stainless steel, a humidifier was placed in the ball-ondisk chamber and the relative humidity was kept constant at 20%, 50% and 80%. To ensure reproducibility, every test was repeated two or three times. At the end of each test, the ball and sample were disassembled from the tester and the debris were removed using an ultrasonic bath. Wear losses were measured in grams, with precision to the fifth decimal place, with an analytical balance. After wear, the structure of the worn surface of the steel samples (parallel to the sliding direction and normal to the worn surface) and the worn surface of the counterbody (parallel to the sliding direction) were studied using a scanning electron microscope. The effect of relative humidity and the grain size on the friction coefficient and on the wear of AISI 304 are shown in Fig. 2a and b, respectively. Finely grained steel shows itself to be more wear resistant than the large grained steel. The hardness of the steel is the predominant factor: the finely grained steel, (with a higher hardness) shows a weight loss and a friction coefficient lower than the relatively softer large grain steel [9]. In Fig. 2, the effect of relative humidity on the friction coefficient and wear of AISI 304 is also evident. An increase in relative humidity results in a decrease in the friction coefficient and weight loss. This effect suggests that a decreased interaction between the counterbody and the steel samples leading to less adhesion and, therefore, to lower friction and less damage to the steel sample surface. Scanning electron micrographs from worn tracks demonstrate severe wear damage on the surface area, resulting from adhesion and abrasion mechanisms. Upon analysis of Fig. 3, where the wear track of the samples tested at a relative humidity of 20% and 80% is shown, some material transfer from the counterbody to the steel surface is evident. An increase in surface damage when the relative humidity is decreased is also evident and this is confirmed by roughness analysis of the two samples. The sample tested at 20% of relative humidity present an average roughness of about 0.15 Am (Ra mean roughness) respect of 0.06 Am of the samples tested at 80%. A cross-section of the wear track shows the formation of a crack beneath the sub-surface for all the grain sizes,
due to a fatigue process (Fig. 4) [10]. In Fig. 5, the worn surfaces of the counterbody at the beginning and at the end of the tests are shown. Fig. 5a clearly shows the formation of a transfer layer from the steel sample to the counterbody which may be a possible reason for the extremely high and variable friction coefficient in the first stage of the test. After about a fourth of the test, the friction becomes more stable without any high variations and after this value start the damage of the counterbody surface and at the end of the tests, it shows many scars (Fig. 5b). Decreasing the grain size of the steel sample produces a decrease in the wear of the counterbody (with respect to the large grained sample), whereas an increase in relative humidity produces an increase in the wear of the counterbody for both grain dimensions. To conclude, the effect of relative humidity on the behaviour of wear is important in predicting the seasonal wear of the steels. Decreases of relative humidity in wear tests of AISI 304 steel produces an increase in weight loss and in the friction coefficient. A beneficial effect of grain refining has also been demonstrated with respect to large grain steel in that the finer grain steel produces less initial weight loss and the weight loss with an increase in the humidity is also less pronounced.
References [1] A. Di Schino, J.M. Kenny, M. Barteri, Mater. Eng. 11 (2000) 141. [2] A. Di Schino, J.M. Kenny, M.G. Mecozzi, M. Barteri, J. Mater. Sci. 35 (2000) 4803. [3] P. Marshal, Austenitic Stainless Steels, Elsevier, Amsterdam, 1984. [4] P. Humprehys, Recrystallization and Related Phenomena, Pergamon, London, 1999. [5] A. Di Schino, I. Salvatori, J.M. Kenny, Proc. of the First ICASS Conference, Tsukuba, vol. 327, 2002. [6] A. Di Schino, J.M. Kenny, J. Mater. Sci. Lett. 21 (2002) 1631. [7] A. Di Schino, L. Valentini, J.M. Kenny, Y. Gerbig, I. Ahmed, H. Haefke, Surf. Coat. Technol. 161 (2002) 224. [8] A. Di Schino, I. Salvatori, J.M. Kenny, J. Mater. Sci. 37 (2002) 4561. [9] A. Borrutto, I. Taraschi, Wear 184 (1995) 119. [10] H.-K. Oh, K.-H. Yeon, H.Y. Kim, J. Mater. Process. Technol. 95 (1999) 10.
The influence of atmospheric humidity and grain size on the friction and wear of AISI 304 austenitic stainless steel G. Bregliozzi a, A. Di Schino b, J.M. Kenny b,*, H. Haefke a b
a CSEM Swiss Center for Electronics and Microtechnology, Inc., Rue Jaquet-Droz 1, CH-2007, Neuchaˆtel, Switzerland Materials Science and Technology, Materials Engineering Center, University of Perugia, Loc. Pentima Bassa 21, 05100 Terni, Italy
Received 27 March 2003; accepted 10 April 2003
Abstract The tribological properties of a ultra-fine AISI 304 austenitic stainless steels obtained by means of a martensitic transformation and subsequent austenite reversion are reported. The effects of the grain size on the wear resistance of such material is, for the first time, investigated as a function of the atmospheric humidity. Decrease of relative humidity in wear tests of AISI 304 steel produces an increase in weight loss and in the friction coefficient. A beneficial effect of grain refining is also shown with respect to large grain steel in that the finer grain steel produces less initial weight loss and the weight loss with an increase in the humidity is also less pronounced. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Atmospheric humidity; Grain size; AISI 304 steel
Austenitic stainless steels possess both good corrosion resistance and formability, but they also have a relatively low yield strength. It is well known that the mechanical properties of austenitic stainless steels are sensitive to the chemical composition (which can induce hardening by both substitutional and interstitial solid solutions) as well as to the grain size [1]. Recently, there have been developments in this direction, where changes in chemical composition induced by nitrogen addition have been taken advantage of [2]. Another effective way to increase the yield strength without impairing ductility is by grain refine-
* Corresponding author. Tel.: +39-744-492939; fax: +39-744492924. E-mail address: [email protected] (J.M. Kenny).
ment. Since austenitic stainless steels do not undergo a phase transformation at typical annealing temperatures, the only way to refine the grain is by recrystallization after cold rolling. However, strengthening by grain refinement is limited due to the high recrystallization temperature of this stainless steel grade. The recrystallization temperature of AISI 304 is above 900 jC and the minimum grain size obtained is in the range of 10 – 30 Am [3,4]. An alternative way to obtain fine grains is by a martensite –austenite reversion after annealing at low temperatures, as reported in investigations carried out to analyse the effects of martensitic transformation and subsequent austenitic reversion on the grain refining of AISI 304 stainless steel and its influence on the tensile properties of the steel [5]. In previous publications, we examined the influence of grain size and chemical composition on
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00351-3
4506
G. Bregliozzi et al. / Materials Letters 57 (2003) 4505–4508
Table 1 Chemical composition (mass%) of the steel C
Si
Cr
Ni
Mo N
S
Mn P
Table 2 Grain size and mechanical properties of the AISI 304 studied samples
AISI 304 0.037 0.26 17.97 8.55 0.22 0.046 0.003 1.04 0.025
the mechanical and corrosion resistance of this type of steel [6,7]. In this paper, the wear resistance of an austenitic stainless steel is studied as a function of grain size and atmospheric relative humidity. The chemical composition of the austenitic stainless steel under consideration is shown in Table 1. In order to favour an austenite – martensite transformation, samples of hotrolled and annealed steel were cold-rolled down to 90% deformation after having been quenched in liquid nitrogen. The details related thereto are reported in Ref. [8]. The martensitic structure obtained in this manner was annealed at 780 jC for
Fig. 1. Microstructure of AISI 304 samples: (a) annealed at 780 jC for 5 min; (b) annealed at 1100 jC for 15 min.
AISI 304
Grain size (Am)
Hardness (HV)
Tensile strength (N/mm2)
0.2% Yield strength (N/mm2)
2.5 40
242.5 163.5
790 650
480 240
5 min and at 1100 jC for 15 min in order to obtain the two different microstructures, reported in Fig. 1, resulting in an average grain size of 2.5 Am (FG) and 40 Am (LG), respectively. The sample surfaces were then polished by using increasingly finer abrasive papers, down to a 1-Am grit. The grain size of the two samples, determined using an automatic image analyser, and the mechanical properties are reported in Table 2. The friction coefficient was measured using a tribometer in a ball-on-disk (BoD) configuration, whereby a steel ball (AISI 52100) slides over the
Fig. 2. (a) Friction coefficient dependence on relative humidity and grain size for AISI 304 samples. (b) Weight loss dependence on relative humidity and grain size for AISI 304 samples.
G. Bregliozzi et al. / Materials Letters 57 (2003) 4505–4508
4507
austenitic stainless steel with a constant linear speed of 10 cm/s and under a load of 2 N. During the test, the temperature of the air was kept constant at 20 F 1 jC. The stop criterion of the tests was 400 m, which corresponds to 21,000 laps. In order to evaluate the weight loss and the friction coefficient with a change
Fig. 4. Micrographs of severely damaged worn surfaces perpendicular to the wear track. Note the crack beneath the surface.
Fig. 3. Micrographs of severely damaged worn surfaces and roughness value: (a) fine grain steel at a relative humidity of 20%; (b) fine grain steel at a relative humidity of 80%.
Fig. 5. Micrographs of the counterbody: (a) formation of transferred layer on the surface after the running-in stage; (b) wear of the surface after a fourth of the test has been completed.
4508
G. Bregliozzi et al. / Materials Letters 57 (2003) 4505–4508
in the relative humidity and grain size of the austenitic stainless steel, a humidifier was placed in the ball-ondisk chamber and the relative humidity was kept constant at 20%, 50% and 80%. To ensure reproducibility, every test was repeated two or three times. At the end of each test, the ball and sample were disassembled from the tester and the debris were removed using an ultrasonic bath. Wear losses were measured in grams, with precision to the fifth decimal place, with an analytical balance. After wear, the structure of the worn surface of the steel samples (parallel to the sliding direction and normal to the worn surface) and the worn surface of the counterbody (parallel to the sliding direction) were studied using a scanning electron microscope. The effect of relative humidity and the grain size on the friction coefficient and on the wear of AISI 304 are shown in Fig. 2a and b, respectively. Finely grained steel shows itself to be more wear resistant than the large grained steel. The hardness of the steel is the predominant factor: the finely grained steel, (with a higher hardness) shows a weight loss and a friction coefficient lower than the relatively softer large grain steel [9]. In Fig. 2, the effect of relative humidity on the friction coefficient and wear of AISI 304 is also evident. An increase in relative humidity results in a decrease in the friction coefficient and weight loss. This effect suggests that a decreased interaction between the counterbody and the steel samples leading to less adhesion and, therefore, to lower friction and less damage to the steel sample surface. Scanning electron micrographs from worn tracks demonstrate severe wear damage on the surface area, resulting from adhesion and abrasion mechanisms. Upon analysis of Fig. 3, where the wear track of the samples tested at a relative humidity of 20% and 80% is shown, some material transfer from the counterbody to the steel surface is evident. An increase in surface damage when the relative humidity is decreased is also evident and this is confirmed by roughness analysis of the two samples. The sample tested at 20% of relative humidity present an average roughness of about 0.15 Am (Ra mean roughness) respect of 0.06 Am of the samples tested at 80%. A cross-section of the wear track shows the formation of a crack beneath the sub-surface for all the grain sizes,
due to a fatigue process (Fig. 4) [10]. In Fig. 5, the worn surfaces of the counterbody at the beginning and at the end of the tests are shown. Fig. 5a clearly shows the formation of a transfer layer from the steel sample to the counterbody which may be a possible reason for the extremely high and variable friction coefficient in the first stage of the test. After about a fourth of the test, the friction becomes more stable without any high variations and after this value start the damage of the counterbody surface and at the end of the tests, it shows many scars (Fig. 5b). Decreasing the grain size of the steel sample produces a decrease in the wear of the counterbody (with respect to the large grained sample), whereas an increase in relative humidity produces an increase in the wear of the counterbody for both grain dimensions. To conclude, the effect of relative humidity on the behaviour of wear is important in predicting the seasonal wear of the steels. Decreases of relative humidity in wear tests of AISI 304 steel produces an increase in weight loss and in the friction coefficient. A beneficial effect of grain refining has also been demonstrated with respect to large grain steel in that the finer grain steel produces less initial weight loss and the weight loss with an increase in the humidity is also less pronounced.
References [1] A. Di Schino, J.M. Kenny, M. Barteri, Mater. Eng. 11 (2000) 141. [2] A. Di Schino, J.M. Kenny, M.G. Mecozzi, M. Barteri, J. Mater. Sci. 35 (2000) 4803. [3] P. Marshal, Austenitic Stainless Steels, Elsevier, Amsterdam, 1984. [4] P. Humprehys, Recrystallization and Related Phenomena, Pergamon, London, 1999. [5] A. Di Schino, I. Salvatori, J.M. Kenny, Proc. of the First ICASS Conference, Tsukuba, vol. 327, 2002. [6] A. Di Schino, J.M. Kenny, J. Mater. Sci. Lett. 21 (2002) 1631. [7] A. Di Schino, L. Valentini, J.M. Kenny, Y. Gerbig, I. Ahmed, H. Haefke, Surf. Coat. Technol. 161 (2002) 224. [8] A. Di Schino, I. Salvatori, J.M. Kenny, J. Mater. Sci. 37 (2002) 4561. [9] A. Borrutto, I. Taraschi, Wear 184 (1995) 119. [10] H.-K. Oh, K.-H. Yeon, H.Y. Kim, J. Mater. Process. Technol. 95 (1999) 10.