- Email: [email protected]
Influence of chromium and vanadium in the mechanical resistance of steels
Available online at www.sciencedirect.com
Materials Chemistry and Physics 109 (2008) 212–216
Influence of chromium and vanadium in the mechanical resistance of steels L. Moro a,b,∗ , G. Gonzalez a,b , G. Brizuela b , A. Juan b , S. Simonetti a,b a
Departamento de Mec´anica, Universidad Tecnol´ogica Nacional, 11 de Abril 461, 8000 Bah´ıa Blanca, Argentina b Departamento de F´ısica, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bah´ıa Blanca, Argentina Received 7 June 2007; received in revised form 9 November 2007; accepted 14 November 2007
Abstract Steel exposed to high temperatures and stress in a corrosive environment suffers detriment in the mechanical properties and finally embrittlement. The objective of this work is to study the mechanical properties of steels of different chemical compositions under hydrogen attack (HA). The influence of chromium and vanadium in the mechanical resistance was analyzed. The 1.25Cr1Mo0.25V and 2.25Cr1Mo ferritic steels previously hydrogenated are studied at different temperatures and loads. The temperature, electric charge density and electrolyte concentration were modified during the electrochemical charge in order to find the optimum hydrogen income to the material. The metal test conditions were characterized by scanning electron microscopy. The material remains in stationary state for a period of time and the residual damage was evaluated. The steels were subject to torsion creep tests while the temperature and the load were maintained constant along the experience. We studied the relationship between the strain rate at the secondary stage and both the stress and temperature. The stress exponent and the activation energy were also determined. A comparison with the same steel samples without hydrogen charge was also performed. The microstructural mechanics during the tests and the embrittlement degree were also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrogen; Alloys; Creep; Corrosion
1. Introduction In petrochemical industry, energy generating plants and petrochemical refineries, service equipment such as pipes, heat exchangers and reactors are subject to high temperatures, mechanical stress and hydrogen environments that could promote the detriment of the mechanical properties. When the material is exposed to high temperatures and stress, a process of creep starts and it evidences both by microstructural changes and mechanical resistance decrease [1]. The study requires more attention if the material for service is under hydrogen atmosphere conditions. Hydrogen diffuses and could form a solid solution with many metals [2]. High temperature exposure of carbon and low-alloy steels to high-pressure hydrogen gas leads to a special form of degradation known as hydrogen attack ∗ Corresponding author at: Departamento de Mec´ anica, Universidad Tecnol´ogica Nacional, 11 de Abril 461, 8000 Bah´ıa Blanca, Argentina. Tel.: +54 291 4555220/95141; fax: +54 291 4595142. E-mail address: [email protected] (L. Moro).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.030
(HA). In contrast to hydrogen embrittlement, which degrades toughness at low temperatures, HA leads to a degradation of material properties at the operating temperature. Hydrogen attack is basically a decarburization reaction. If the attack is confined to the surface, it is known as surface attack. If the attack occurs at internal voids, the reported product is methane. These undesired loss of carbon as methane molecules could be trapped in cavities (voids) at the grain boundaries. A methane pressure in the cavities could be responsible for cavity growth. About two decades ago, Oriani observed that methane can be formed in steels that contain carbides or cementite, especially in grain boundaries [3]. Troiano et al. have experimentally determined the resistivity and measured the crack propagation speed. These authors found the process very discontinuous and explain that the propagation speed is controlled by hydrogen diffusion in the triaxial tensions at the extreme of the crack [4]. The HA is of high industrial and technological importance. The failure of steel-bared equipment involves high economical cost for the company [1].
L. Moro et al. / Materials Chemistry and Physics 109 (2008) 212–216
Damage severity depends on chemical composition, impurity contents and strain rate. HA in used ferritic steels can be detected in energy generating plant components. The presence of chromium increases the corrosion resistance due to a superficial oxide layer that inhibits the surface and prevents any further hydrogen diffusion in the bulk. Peddle and Pickles have reported that carbides and nitrides stables precipitates increase the mechanical resistance of ferritic steels. This stability is due to the high formation enthalpy of chromium, molybdenum and vanadium carbides [5]. Grabke et al. have observed that the fine precipitate of carbides, nitrides and carbon nitrides of Cr, Mo and V, especially in the grain boundaries of alloyed steels, prevents the dislocation mobility and improves the mechanical resistance [6]. Stone et al. have studied the behavior of different previously hydrogenated ferritic steels under stress and observed the separation between the grain boundaries due to intergranular fracture. This phenomenon is more important in steels with high density of carbides because the formation of methane could occur through several chemical reactions between carbides and hydrogen [7]. Viswanathan has analyzed the alloying element influence in the resistance to creep of previously hydrogenated ferritic steels. The author finds that the Cr, Mo, W, V, Ti, Nb, Zr, Mn and P increase the metal resistance while C, Ni, Cu and S favors the decohesion phenomenon [1]. Ishizuka et al. have subjected the 1.25Cr1Mo steel to different thermal treatments before being hydrogenated, observing that the quenched and annealing steels are very prone to weakening, while the cooled austenite steels increase the resistance to hydrogen due to the size and distribution of the carbides formed in the matrix [8]. Purmensky et al. have studied steels with Cr, Mo and V in their chemical composition. These steels exposed to high temperatures and stress, improve their mechanical resistance due the precipitation of Mo2 C carbides in 2.25Cr1Mo steels and V4 C3 carbides in V-steels [9]. In this paper, we compare the creep behavior of the 2.25Cr1Mo and 1.25Cr1Mo0.25V ferritic steels subject to torsion creep tests. We also studied used previously hydrogenated specimens to evaluate environmental influence in the mechanical resistance. We also analyzed the change in the characteristic parameters such as the stress exponent and the activation energy that is related with the strain rate, load and temperature applied. The torsion creep tests have the possibility to work to constant stress without the necessity of complex equipment [10].
2. Theoretical basis In torsion creep tests, the strain is calculated taking into account the angular movement of a determined point of the probe, maintaining the other end fixed. The torsional moment is obtained applying a torque and this is the unique cause of axis rotation. The shear stress τ, which acts over the outside diameter of cylinder specimen (with radius r), is calculated through the
213
applied couple (C) related to the machine applied load: τ=
2C πr 3
(1)
In order to compare the torsion creep test with the uniaxial tension creep test, the equivalent stress (σ) (2) and the shear strain are related following the von Mises creep criteria is applied [11] √ σ = 3τ (2) The relation between the rotation angle of the specimen (θ) and the γ shear strain in the L length of the calibrated zone is: γL = θr
(3)
The equivalent strain ε¯ due to the angular deformation is calculated by: 1 r ε¯ = √ θ 3L
(4)
The strain rate is determined using the creep power-law (which is related with the stress and temperature): ε˙ = Aσ n exp(−Q/RT )
(5)
where ε˙ is the strain rate, A the pre-exponential parts, n the stress exponent, Q the creep activation energy, R the universal gas constant and T the absolute temperature [12]. Using logarithms, Eq. (5) can be rewritten as Eqs. (6) and (7), obtaining the n and Q parameters for constant stress or temperature: ln ε˙ = ln (Aσ n ) −
Q RT
(6)
log ε˙ = log(A exp(−Q/RT )) + n log σ
(7)
The logarithmic graphic representations of these equations should result in straight lines, so the parameters can be easily obtained from the slopes. The strain rate corresponds to the secondary stage and both the time and the rupture strain are not taking into account [12]. 3. Experimental method The test was performed using 2.25Cr1Mo and 1.25Cr1Mo0.25V cylinder specimens. The chemical compositions of steels are summarized in Table 1. Table 1 Chemical composition of steels (weight %)
C Cr Mo V Ni Mn Si Cu Others Fe
2.25Cr1Mo
1.25Cr1Mo0.25V
0.16 2.23 0.96 – 0.07 0.40 0.03 0.09 S, Sn: <0.01 Balance
0.14 1.20 0.95 0.24 0.40 0.69 0.35 – S, P: <0.03 Balance
214
L. Moro et al. / Materials Chemistry and Physics 109 (2008) 212–216
Fig. 1. Schematic drawing of the steel specimen used for creep tests (mm). The steels sample under study was supplied as a cylinder specimen having a calibrated length of 7 mm and a radius of 3.5 mm in normalize conditions (Fig. 1) [10]. Steel 2(1/4)Cr1Mo having about 30 m average grain size, consisting of about 60% tempered bainite and 40% pro-eutectoid ferrite. Fig. 2 shows optical micrograph of the material in the as received condition. The specimen hydrogenation was performed with the cathodic charge method using NaOH 1N as basic electrolyte and As2 O3 as catalyst for electrochemical reaction. The process was carried out during 3 h with a current density of 100 mA cm−2 at 373 K. The charge (mA) and the time of hydrogenation were selected considering the minimum superficial damage and equilibrium absorption. The surfaces were examined by a JEOL scanning electron microscope (SEM) operating at 25 kV. Standard metallographic preparation techniques (mechanical grinding and polishing followed by etching in Nital) were applied prior to light microscope examination. After HA the material is left in stationary state for some weeks in order to eliminate the hydrogen and evaluate the residual damage. The creep test is performed using a torsion machine with an added electric oven. The hot end of a cromel–alumel thermocouple is located in contact with the surface of the calibrated zone of the specimen. The creep test is carried out at the equivalent stress (σ) of 168 MPa, and at temperatures of 843, 873 and 893 K, in probes of 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, with and without HA.
4. Results and discussion
Fig. 3. Creep curves of 2.25Cr1Mo steel at 168 MPa and different temperatures.
Figs. 3 and 4 show the strain curves for steels specimens as a function of time, at 168 MPa and for several temperatures with and without HA. Table 2 shows the strain rate values for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, with and without hydrogen attack at 168 MPa and different temperatures. We have calculated the strain rate for stationary state test conditions. Figs. 5 and 6 show the fitting lines using Eq. (6). In the same way, starting from Eq. (7), we have obtained the n stress
Fig. 2. Optical micrographs of the 2.25Cr1Mo steel in the as received condition (400×). Etching reagent: Nital 2%.
Fig. 4. Creep curves of 1.25Cr1Mo0.25V steel at 168 MPa and different temperatures.
L. Moro et al. / Materials Chemistry and Physics 109 (2008) 212–216
215
Table 2 Strain rate values (1/s) corresponding to 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, with and without hydrogen attack (HA), at 168 MPa T (K)
2.25Cr1Mo
843 873 893
1.25Cr1Mo0.25V
Without HA
With HA
Without HA
With HA
3.8778 × 10−8
4.8306 × 10−8
4.3101 × 10−8
1.9701 × 10−7 1.6545 × 10−6 7.7319 × 10−6
1.9255 × 10−7 3.3334 × 10−6
1.3734 × 10−6 8.6281 × 10−6
3.9701 × 10−7 1.0701 × 10−6
The activation energy values difference in the V-alloyed steels is related to the bigger precipitated carbide size that explains the higher mechanical resistance. The internal gas pressure generated in microcracks, especially in grain boundaries, produces the largest activation energy in the hydrogenated materials. In these voids, besides hydrogen, it is possible to form methane as a consequence of a chemical reaction between the carbon atoms from the steel matrix and hydrogen. The n stress exponent calculated at 873 K for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels are 2.4 and 8, respectively. This indicates that the dislocation movement dominates the creep mechanism. Maruyama et al. reported “n” values in 2.25Cr1Mo where all the values are greater than 4, supporting the occurrence of dislocation creep under service conditions of engineering plants [13]. During the motion, the dislocations increase the pileup when they find a
Fig. 5. Logarithmic plot to determine the activation energy (Q) for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels without hydrogen attack at 168 MPa. The straight line regression for the activation energy is shown.
exponent values at 873 K. Table 3 presents the values for the activation energy Q and the n stress exponent. The activation energy values at 168 MPa for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, without hydrogen attack are 536 and 406 kJ mol−1 , respectively. The energy values for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, charged with hydrogen are 652 and 457 kJ mol−1 , respectively. Table 3 Activation energy values Q at (kJ/mol) at 168 MPa and n stress exponent values at 873 K Steel
2.25Cr1Mo 1.25Cr1Mo0.25V
Q
n
Without HA
With HA
Without HA
With HA
536 406
652 457
2.4 8
5 11
Fig. 6. Logarithmic plot to determine the activation energy (Q) for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels with hydrogen attack at 168 MPa. The straight line regression for the activation energy is shown.
216
L. Moro et al. / Materials Chemistry and Physics 109 (2008) 212–216
precipitated carbides in the matrix formed because of the high temperature. The cross-slip mechanism increases with the consequence change in the slip plane [14,15]. The presence of Cr and V alloying increases the ferritc steels resistance. Purmensky et al. have obtained similar values in tensile test made in ferritic steels slightly alloyed [9]. The stress exponent for the steels charged with hydrogen suggests the detrimental effect of hydrogen and the corrosive action in the material.
Acknowledgements Our work was supported by Departamento de Ingenier´ıa Mec´anica UTN-FRBB, SEGECYT-UNS, PIP-CONICET, John Simon Guggenhein Memorial Foundation and Fulbright Commission. A. Juan, G. Brizuela and S. Simonetti are members of CONICET. References
5. Conclusions Hydrogen exerts deleterious effects in both steels. The severity of these effects depends on strength level, chemical composition, impurity content and microstructure. The results show the detrimental effect of hydrogen in steels that is noticed in the higher stress exponent and an increase in the activation energy. The resistance loss is indicated by the decrease test times. According to the literature, we can predict that the dominant mechanism is the creep by dislocations. The results presented in this paper show that the properties of Cr–Mo and Cr–Mo–V steels depend strongly on the carbide types, on the composition of these carbide, the temperature and the stress due to the applied loading. In solid solution the alloying elements from carbide precipitates which impede dislocation motion by a mechanism known as precipitation hardening. In addition, the vanadium carbide is more stable. In the studies performed in the V-alloyed steels, it can be seen both an increase in the n stress exponent and a simultaneous decrease in the Q activation energy that are related to the bigger particle size of the precipitated carbides at the grain boundaries. In consequence, more internal energy is necessary for the dislocation cross-slip movement.
[1] R. Viswanathan, Damage Mechanisms and Life Assessment of High Temperature Components, ASM International Metals, Park Ohio, USA, 1989 (Chapters 3 and 7). [2] Y. Fukai, The metal hydrogen system, in: U. Gonser (Ed.), Springer Series in Materials Sciences, Springer-Verlag, Berlin, 1993, p. 21. [3] R.A. Oriani, Metals Handbook, 9th ed., ASM International, Metal Park, OH, 1985, p. 28. [4] A.R. Troiano, Trans. Met. ASM 52 (1960) 54. [5] B.E. Peddle, C.A. Pickles, Can. Metall. Quart. 40 (2001) 105. [6] H.J. Grabke, F. Gehrmann, E. Riecke, Steel Res. 72 (2001) 225. [7] D. Stone, Hydrogen Attack in Cr–Mo Steels at Elevated Temperatures, Cornell University, Press, Ithaca, New York, 1983. [8] H. Ishizuka, R. Chiba, Corrosion 77 (1997) 14. [9] J. Purmensky, V. Foldyna, Z. Kubon, Key Eng. Mat. 171 (2000) 419. [10] A. Saenz, E. Obiol, L. Moro, M. Saggio, Informaci´on Tecnol´ogica, CIT, ISSN 0716-8756, 1998, p. 129. [11] G.E. Dieter, Mechanical Metallurgy, 3th ed., Mc Graw Hill, New York, 1986. [12] B. Wilshire, R.W. Evans, Introduction to Creep, Maney Publishing,London, UK, 1993. [13] K. Maruyama, K. Sawada, J. Koike, H. Sato, K. Yagi, Mat. Sci. Eng. A 224 (1997) 166. [14] J.P. Hirth, J. Lothe, Theory of Dislocations, Krieger Publishing Company, New York, 1992. [15] E.D. Hondros, M. Mc Lean, Structural Materials: Engineering Application Through Scientific Insight, Wood Head Publishing Limited, Cambridge, England, 1996.
Materials Chemistry and Physics 109 (2008) 212–216
Influence of chromium and vanadium in the mechanical resistance of steels L. Moro a,b,∗ , G. Gonzalez a,b , G. Brizuela b , A. Juan b , S. Simonetti a,b a
Departamento de Mec´anica, Universidad Tecnol´ogica Nacional, 11 de Abril 461, 8000 Bah´ıa Blanca, Argentina b Departamento de F´ısica, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bah´ıa Blanca, Argentina Received 7 June 2007; received in revised form 9 November 2007; accepted 14 November 2007
Abstract Steel exposed to high temperatures and stress in a corrosive environment suffers detriment in the mechanical properties and finally embrittlement. The objective of this work is to study the mechanical properties of steels of different chemical compositions under hydrogen attack (HA). The influence of chromium and vanadium in the mechanical resistance was analyzed. The 1.25Cr1Mo0.25V and 2.25Cr1Mo ferritic steels previously hydrogenated are studied at different temperatures and loads. The temperature, electric charge density and electrolyte concentration were modified during the electrochemical charge in order to find the optimum hydrogen income to the material. The metal test conditions were characterized by scanning electron microscopy. The material remains in stationary state for a period of time and the residual damage was evaluated. The steels were subject to torsion creep tests while the temperature and the load were maintained constant along the experience. We studied the relationship between the strain rate at the secondary stage and both the stress and temperature. The stress exponent and the activation energy were also determined. A comparison with the same steel samples without hydrogen charge was also performed. The microstructural mechanics during the tests and the embrittlement degree were also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrogen; Alloys; Creep; Corrosion
1. Introduction In petrochemical industry, energy generating plants and petrochemical refineries, service equipment such as pipes, heat exchangers and reactors are subject to high temperatures, mechanical stress and hydrogen environments that could promote the detriment of the mechanical properties. When the material is exposed to high temperatures and stress, a process of creep starts and it evidences both by microstructural changes and mechanical resistance decrease [1]. The study requires more attention if the material for service is under hydrogen atmosphere conditions. Hydrogen diffuses and could form a solid solution with many metals [2]. High temperature exposure of carbon and low-alloy steels to high-pressure hydrogen gas leads to a special form of degradation known as hydrogen attack ∗ Corresponding author at: Departamento de Mec´ anica, Universidad Tecnol´ogica Nacional, 11 de Abril 461, 8000 Bah´ıa Blanca, Argentina. Tel.: +54 291 4555220/95141; fax: +54 291 4595142. E-mail address: [email protected] (L. Moro).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.030
(HA). In contrast to hydrogen embrittlement, which degrades toughness at low temperatures, HA leads to a degradation of material properties at the operating temperature. Hydrogen attack is basically a decarburization reaction. If the attack is confined to the surface, it is known as surface attack. If the attack occurs at internal voids, the reported product is methane. These undesired loss of carbon as methane molecules could be trapped in cavities (voids) at the grain boundaries. A methane pressure in the cavities could be responsible for cavity growth. About two decades ago, Oriani observed that methane can be formed in steels that contain carbides or cementite, especially in grain boundaries [3]. Troiano et al. have experimentally determined the resistivity and measured the crack propagation speed. These authors found the process very discontinuous and explain that the propagation speed is controlled by hydrogen diffusion in the triaxial tensions at the extreme of the crack [4]. The HA is of high industrial and technological importance. The failure of steel-bared equipment involves high economical cost for the company [1].
L. Moro et al. / Materials Chemistry and Physics 109 (2008) 212–216
Damage severity depends on chemical composition, impurity contents and strain rate. HA in used ferritic steels can be detected in energy generating plant components. The presence of chromium increases the corrosion resistance due to a superficial oxide layer that inhibits the surface and prevents any further hydrogen diffusion in the bulk. Peddle and Pickles have reported that carbides and nitrides stables precipitates increase the mechanical resistance of ferritic steels. This stability is due to the high formation enthalpy of chromium, molybdenum and vanadium carbides [5]. Grabke et al. have observed that the fine precipitate of carbides, nitrides and carbon nitrides of Cr, Mo and V, especially in the grain boundaries of alloyed steels, prevents the dislocation mobility and improves the mechanical resistance [6]. Stone et al. have studied the behavior of different previously hydrogenated ferritic steels under stress and observed the separation between the grain boundaries due to intergranular fracture. This phenomenon is more important in steels with high density of carbides because the formation of methane could occur through several chemical reactions between carbides and hydrogen [7]. Viswanathan has analyzed the alloying element influence in the resistance to creep of previously hydrogenated ferritic steels. The author finds that the Cr, Mo, W, V, Ti, Nb, Zr, Mn and P increase the metal resistance while C, Ni, Cu and S favors the decohesion phenomenon [1]. Ishizuka et al. have subjected the 1.25Cr1Mo steel to different thermal treatments before being hydrogenated, observing that the quenched and annealing steels are very prone to weakening, while the cooled austenite steels increase the resistance to hydrogen due to the size and distribution of the carbides formed in the matrix [8]. Purmensky et al. have studied steels with Cr, Mo and V in their chemical composition. These steels exposed to high temperatures and stress, improve their mechanical resistance due the precipitation of Mo2 C carbides in 2.25Cr1Mo steels and V4 C3 carbides in V-steels [9]. In this paper, we compare the creep behavior of the 2.25Cr1Mo and 1.25Cr1Mo0.25V ferritic steels subject to torsion creep tests. We also studied used previously hydrogenated specimens to evaluate environmental influence in the mechanical resistance. We also analyzed the change in the characteristic parameters such as the stress exponent and the activation energy that is related with the strain rate, load and temperature applied. The torsion creep tests have the possibility to work to constant stress without the necessity of complex equipment [10].
2. Theoretical basis In torsion creep tests, the strain is calculated taking into account the angular movement of a determined point of the probe, maintaining the other end fixed. The torsional moment is obtained applying a torque and this is the unique cause of axis rotation. The shear stress τ, which acts over the outside diameter of cylinder specimen (with radius r), is calculated through the
213
applied couple (C) related to the machine applied load: τ=
2C πr 3
(1)
In order to compare the torsion creep test with the uniaxial tension creep test, the equivalent stress (σ) (2) and the shear strain are related following the von Mises creep criteria is applied [11] √ σ = 3τ (2) The relation between the rotation angle of the specimen (θ) and the γ shear strain in the L length of the calibrated zone is: γL = θr
(3)
The equivalent strain ε¯ due to the angular deformation is calculated by: 1 r ε¯ = √ θ 3L
(4)
The strain rate is determined using the creep power-law (which is related with the stress and temperature): ε˙ = Aσ n exp(−Q/RT )
(5)
where ε˙ is the strain rate, A the pre-exponential parts, n the stress exponent, Q the creep activation energy, R the universal gas constant and T the absolute temperature [12]. Using logarithms, Eq. (5) can be rewritten as Eqs. (6) and (7), obtaining the n and Q parameters for constant stress or temperature: ln ε˙ = ln (Aσ n ) −
Q RT
(6)
log ε˙ = log(A exp(−Q/RT )) + n log σ
(7)
The logarithmic graphic representations of these equations should result in straight lines, so the parameters can be easily obtained from the slopes. The strain rate corresponds to the secondary stage and both the time and the rupture strain are not taking into account [12]. 3. Experimental method The test was performed using 2.25Cr1Mo and 1.25Cr1Mo0.25V cylinder specimens. The chemical compositions of steels are summarized in Table 1. Table 1 Chemical composition of steels (weight %)
C Cr Mo V Ni Mn Si Cu Others Fe
2.25Cr1Mo
1.25Cr1Mo0.25V
0.16 2.23 0.96 – 0.07 0.40 0.03 0.09 S, Sn: <0.01 Balance
0.14 1.20 0.95 0.24 0.40 0.69 0.35 – S, P: <0.03 Balance
214
L. Moro et al. / Materials Chemistry and Physics 109 (2008) 212–216
Fig. 1. Schematic drawing of the steel specimen used for creep tests (mm). The steels sample under study was supplied as a cylinder specimen having a calibrated length of 7 mm and a radius of 3.5 mm in normalize conditions (Fig. 1) [10]. Steel 2(1/4)Cr1Mo having about 30 m average grain size, consisting of about 60% tempered bainite and 40% pro-eutectoid ferrite. Fig. 2 shows optical micrograph of the material in the as received condition. The specimen hydrogenation was performed with the cathodic charge method using NaOH 1N as basic electrolyte and As2 O3 as catalyst for electrochemical reaction. The process was carried out during 3 h with a current density of 100 mA cm−2 at 373 K. The charge (mA) and the time of hydrogenation were selected considering the minimum superficial damage and equilibrium absorption. The surfaces were examined by a JEOL scanning electron microscope (SEM) operating at 25 kV. Standard metallographic preparation techniques (mechanical grinding and polishing followed by etching in Nital) were applied prior to light microscope examination. After HA the material is left in stationary state for some weeks in order to eliminate the hydrogen and evaluate the residual damage. The creep test is performed using a torsion machine with an added electric oven. The hot end of a cromel–alumel thermocouple is located in contact with the surface of the calibrated zone of the specimen. The creep test is carried out at the equivalent stress (σ) of 168 MPa, and at temperatures of 843, 873 and 893 K, in probes of 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, with and without HA.
4. Results and discussion
Fig. 3. Creep curves of 2.25Cr1Mo steel at 168 MPa and different temperatures.
Figs. 3 and 4 show the strain curves for steels specimens as a function of time, at 168 MPa and for several temperatures with and without HA. Table 2 shows the strain rate values for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, with and without hydrogen attack at 168 MPa and different temperatures. We have calculated the strain rate for stationary state test conditions. Figs. 5 and 6 show the fitting lines using Eq. (6). In the same way, starting from Eq. (7), we have obtained the n stress
Fig. 2. Optical micrographs of the 2.25Cr1Mo steel in the as received condition (400×). Etching reagent: Nital 2%.
Fig. 4. Creep curves of 1.25Cr1Mo0.25V steel at 168 MPa and different temperatures.
L. Moro et al. / Materials Chemistry and Physics 109 (2008) 212–216
215
Table 2 Strain rate values (1/s) corresponding to 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, with and without hydrogen attack (HA), at 168 MPa T (K)
2.25Cr1Mo
843 873 893
1.25Cr1Mo0.25V
Without HA
With HA
Without HA
With HA
3.8778 × 10−8
4.8306 × 10−8
4.3101 × 10−8
1.9701 × 10−7 1.6545 × 10−6 7.7319 × 10−6
1.9255 × 10−7 3.3334 × 10−6
1.3734 × 10−6 8.6281 × 10−6
3.9701 × 10−7 1.0701 × 10−6
The activation energy values difference in the V-alloyed steels is related to the bigger precipitated carbide size that explains the higher mechanical resistance. The internal gas pressure generated in microcracks, especially in grain boundaries, produces the largest activation energy in the hydrogenated materials. In these voids, besides hydrogen, it is possible to form methane as a consequence of a chemical reaction between the carbon atoms from the steel matrix and hydrogen. The n stress exponent calculated at 873 K for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels are 2.4 and 8, respectively. This indicates that the dislocation movement dominates the creep mechanism. Maruyama et al. reported “n” values in 2.25Cr1Mo where all the values are greater than 4, supporting the occurrence of dislocation creep under service conditions of engineering plants [13]. During the motion, the dislocations increase the pileup when they find a
Fig. 5. Logarithmic plot to determine the activation energy (Q) for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels without hydrogen attack at 168 MPa. The straight line regression for the activation energy is shown.
exponent values at 873 K. Table 3 presents the values for the activation energy Q and the n stress exponent. The activation energy values at 168 MPa for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, without hydrogen attack are 536 and 406 kJ mol−1 , respectively. The energy values for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels, charged with hydrogen are 652 and 457 kJ mol−1 , respectively. Table 3 Activation energy values Q at (kJ/mol) at 168 MPa and n stress exponent values at 873 K Steel
2.25Cr1Mo 1.25Cr1Mo0.25V
Q
n
Without HA
With HA
Without HA
With HA
536 406
652 457
2.4 8
5 11
Fig. 6. Logarithmic plot to determine the activation energy (Q) for 2.25Cr1Mo and 1.25Cr1Mo0.25V steels with hydrogen attack at 168 MPa. The straight line regression for the activation energy is shown.
216
L. Moro et al. / Materials Chemistry and Physics 109 (2008) 212–216
precipitated carbides in the matrix formed because of the high temperature. The cross-slip mechanism increases with the consequence change in the slip plane [14,15]. The presence of Cr and V alloying increases the ferritc steels resistance. Purmensky et al. have obtained similar values in tensile test made in ferritic steels slightly alloyed [9]. The stress exponent for the steels charged with hydrogen suggests the detrimental effect of hydrogen and the corrosive action in the material.
Acknowledgements Our work was supported by Departamento de Ingenier´ıa Mec´anica UTN-FRBB, SEGECYT-UNS, PIP-CONICET, John Simon Guggenhein Memorial Foundation and Fulbright Commission. A. Juan, G. Brizuela and S. Simonetti are members of CONICET. References
5. Conclusions Hydrogen exerts deleterious effects in both steels. The severity of these effects depends on strength level, chemical composition, impurity content and microstructure. The results show the detrimental effect of hydrogen in steels that is noticed in the higher stress exponent and an increase in the activation energy. The resistance loss is indicated by the decrease test times. According to the literature, we can predict that the dominant mechanism is the creep by dislocations. The results presented in this paper show that the properties of Cr–Mo and Cr–Mo–V steels depend strongly on the carbide types, on the composition of these carbide, the temperature and the stress due to the applied loading. In solid solution the alloying elements from carbide precipitates which impede dislocation motion by a mechanism known as precipitation hardening. In addition, the vanadium carbide is more stable. In the studies performed in the V-alloyed steels, it can be seen both an increase in the n stress exponent and a simultaneous decrease in the Q activation energy that are related to the bigger particle size of the precipitated carbides at the grain boundaries. In consequence, more internal energy is necessary for the dislocation cross-slip movement.
[1] R. Viswanathan, Damage Mechanisms and Life Assessment of High Temperature Components, ASM International Metals, Park Ohio, USA, 1989 (Chapters 3 and 7). [2] Y. Fukai, The metal hydrogen system, in: U. Gonser (Ed.), Springer Series in Materials Sciences, Springer-Verlag, Berlin, 1993, p. 21. [3] R.A. Oriani, Metals Handbook, 9th ed., ASM International, Metal Park, OH, 1985, p. 28. [4] A.R. Troiano, Trans. Met. ASM 52 (1960) 54. [5] B.E. Peddle, C.A. Pickles, Can. Metall. Quart. 40 (2001) 105. [6] H.J. Grabke, F. Gehrmann, E. Riecke, Steel Res. 72 (2001) 225. [7] D. Stone, Hydrogen Attack in Cr–Mo Steels at Elevated Temperatures, Cornell University, Press, Ithaca, New York, 1983. [8] H. Ishizuka, R. Chiba, Corrosion 77 (1997) 14. [9] J. Purmensky, V. Foldyna, Z. Kubon, Key Eng. Mat. 171 (2000) 419. [10] A. Saenz, E. Obiol, L. Moro, M. Saggio, Informaci´on Tecnol´ogica, CIT, ISSN 0716-8756, 1998, p. 129. [11] G.E. Dieter, Mechanical Metallurgy, 3th ed., Mc Graw Hill, New York, 1986. [12] B. Wilshire, R.W. Evans, Introduction to Creep, Maney Publishing,London, UK, 1993. [13] K. Maruyama, K. Sawada, J. Koike, H. Sato, K. Yagi, Mat. Sci. Eng. A 224 (1997) 166. [14] J.P. Hirth, J. Lothe, Theory of Dislocations, Krieger Publishing Company, New York, 1992. [15] E.D. Hondros, M. Mc Lean, Structural Materials: Engineering Application Through Scientific Insight, Wood Head Publishing Limited, Cambridge, England, 1996.