- Email: [email protected]
A miniaturized test method for the mechanical characterization of structural materials for fusion reactors
ELSEVIER
Journal of Nuclear Materials 233- 237 (1996) 1557-1560
journalof nuclear materials
A miniaturized test method for the mechanical characterization of structural materials for fusion reactors P. Gondi a, *, A. Donato b, R. Montanari a, A. Sill a a Mechanical Engineering Department, University ofRome-Tor Vergata, Rome, Italy b ENEA CRE, Fusion Sector, Frascati, Rome, Italy
Abstract This work deals with a non-destructive method for mechanical tests which is based on the indentation of materials at a constant rate by means of a cylinder with a small radius and penetrating flat surface. The load versus penetration depth curves obtained using this method have shown correspondences with those of tensile tests and have given indications about the mechanical properties on a reduced scale. In this work penetration tests have been carried out on various kinds of Cr martensitic steels (MANET-2, B A T M A N and modified F82H) which are of interest for first wall and structural applications in future fusion reactors. The load versus penetration depth curves have been examined with reference to data obtained in tensile tests and to microhardness measurements. Penetration tests have been performed at various temperature (from - 180 to 100°C). Conclusions, which can be drawn for the ductile to brittle transition, are discussed for MANET-2 steel. Preliminary results obtained on BATIVIAN and modified F82H steels are reported. The characteristics of the indenter imprints have been studied by scanning electron microscopy.
1. Introduction The possibility to have general information on mechanical properties by a non-destructive method working on a miniaturized scale appears suitable for local determination of these properties or for the characterization of materials directly in use. Small specimen techniques could be useful to test materials in particular after irradiation in the IFMIF (International Fusion Material Irradiated Facility), a source which will be built to irradiate materials with fusion-equivalent high energy (14 MeV) neutrons in a foreseen useful volume not exceeding 1 1. Miniaturized techniques for the evaluation of mechanical properties on samples after irradiation have been investigated considering microhardness measurements [1], disk bend [2] and shear punch tests [3] or Charpy tests performed with V-notched specimens smaller than the standard requirements [4,5]. Data of mechanical properties obtained with sub-sized specimens differ in general from data obtained in standard tests [6,7]. In
* Corresponding author. Tel.: +39-6-72594617; fax: +39-62021351; e-mail: [email protected].
order to assess bulk properties it is necessary to use empirical expressions or physical models. For instance, the radius of the plane stress state at the specimen surface is considered to be a material independent factor controlling size effects in Charpy tests (Kimura et al. [8]). Following a different approach, other methods are based on indentation tests performed on small areas of specimens. Yasuda et al. [9], for example developed an ultra microhardness technique for evaluating stress-strain properties of metals. The method developed by us [10,11] is based on the indentation of materials by means of a cylinder with a small radius and a flat penetrating surface. In this case miniaturization regards not the samples but only the tested zone: an advantage of this method is the possibility to test irradiated zones of particular interest such as welding joints. The load vs. penetration depth diagrams obtained with this test have shown correspondences with o--e curves obtained in tensile tests. Moreover, experiments carried out on some martensitic steels at different temperatures have given indications on the ductile to brittle transition temperature [12]. In this paper results of penetration tests regarding different Cr martensitic steels, candidates for structural appli-
0022-3115/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 I 15(96)003 15-7
1558
P. Gondi et al./Journal of Nuclear Materials 233-237 (1996) 1557-1560
Table l Steels compositions, Fe to balance (wt%)
cations in future fusion reactors, are presented and discussed.
MANET-2 C Cr Mo Ni Mn Nb V Ti Ta W B Nb Co Si AI Cu As Sb Sn N P S
2. E x p e r i m e n t a l Penetration tests were p e r f o r m e d by m e a n s o f cylindrical indenters with d i a m e t e r ~b = 1.0 m m and h e i g h t h = 1.5 ram. T h e indenters were m a d e o f W C , w h i c h g u a r a n t e e s g o o d rigidity and n o n - d e f o r m a b i l i t y also w h e n applied to s a m p l e s of hard materials. T h e e x p e r i m e n t a l apparatus allowed m e a s u r e m e n t s of the indenter a d v a n c e m e n t 6 in d e p e n d e n c e o f the applied force, P. U n d e r the a s s u m p t i o n that P is u n i f o r m l y distributed on the contact surface o f the s a m p l e s with constant pressure q = 4P/(Trq52) [10], q load vs. 6 penetration depth c u r v e s are obtained (in the following indicated as LP curves). T h i s work deals with tests carried out at c o n s t a n t penetration rate and at various temperatures (from - 180 to 100°C). T h e effect o f the penetration rate on the LP curves w a s also taken into a c c o u n t p e r f o r m i n g tests with different rates in the range from 0.1 to 2 r a m / r a i n . T h e c o m p o s i t i o n s o f the investigated Cr martensitic steels ( M A N E T - 2 , B A T M A N and modified F 8 2 H ) are g i v e n in Table 1. T h e preliminary thermal treatments c o n s i s t o f q u e n c h i n g and t e m p e r i n g for 2 h at 700°C for the M A N E T - 2 steel and o f n o r m a l i z i n g and t e m p e r i n g for 1 h at 730°C for the B A T M A N steel and for 1 h at 750°C for the m o d i f i e d F82H steel. T h e s p e c i m e n s were subjected to electrochemical polishing to avoid surface harde n i n g effects. Indenter and s p e c i m e n were dipped into chilled w a t e r ethanol solutions or into liquid nitrogen for tests p e r f o r m e d b e l o w r o o m temperature and into water baths for tests p e r f o r m e d above room temperature up to 100°C. O n the s a m e materials Vickers m i c r o h a r d n e s s m e a s u r e m e n t s , tensile tests with a strain rate o f ~ = 10 3 s 1 and C h a r p y impact tests h a v e been carried out.
6000qs
BATMAN
Mod. F82H
0.12 8.32 0.0154 0.0275 0.52 0.0021 0.20 0.09 0.00014 0.0062 0.0021
0.09 7.68 0.0030 0.020 0. I 1 0.0001 0.16 0.01 0.02 0.0002 0.0002 0.0001
0.0260 0.0085 0.0051
0.11 0.0030 0.01
0.0014 0.0072 0.0050 0.0026
0.0070 0.0020 0.0020
0.10 10.37 0.58 0.65 0.76 0.16 0.21
0.0075 0.005 0.18 0.007 0.010 0.010 0.0002 < 0.01 0.032 0.004 0.005
Characteristics o f the indentations have been studied by s c a n n i n g electron m i c r o s c o p y (SEM).
3. Results a n d discussion Previous e x p e r i m e n t s p e r f o r m e d on various metals and alloys s u c h as A1, Cu, Zn and plain carbon steel AISI 1040, have s h o w n that LP c u r v e s exhibit different stages [10]. T h e d e f o r m a t i o n is elastic for q values not e x c e e d i n g a critical limit qL w h e n q > qL a p e r m a n e n t indentation with sharp edge is observed. T h e first stage o f workh a r d e n i n g is linear (see also Fig. 1).
20000"
a)
5ooo-~,
~ " 150004000
13.
CL
~E
10000-
.2500
2000 5000-
qv
o~
- -&T
0,4
(mm)
HV 0
0
q
i
~
u
0 (~v
d(~Idt (mm/min)
Fig. 1. (a) LP curve for MANET-2 steel with indication of qy, qs limit loads by means of the arrows on the ordinate; (b) trends of the qr ~md qs limit loads vs. d ~ / d t penetration rate (indication of the yield stress o-r and ultimate tensile o-u from tensile tests are given by the arrows on the right ordinate; the HV microhardness value is given by the arrow on the left ordinate).
1.0~
P. Gondi et al. / Journal of Nuclear Materials 233-237 (1996) 1557-1560
1559
0.5
- 0"5t/z - 1.0 II 0
,
5
,
Z/O
10
15
Fig. 2. Trends o f axial stress (o-z), radial stress (o"x) and 45 ° shear stress ( r ) for a uniform q load applied by the cylindrical indenter.
With increasing penetration depth, starting from a load q r , LP curves show a second stage of non-linear work hardening which is followed by a third stage where they slowly tend towards saturation values qs, which are comparable to the hardness values for the smallest penetration rate. In the second and third stages large plastic deformations with protrusion of material around the indentation are observed. With a suitable choice of the coordinates correspondences exist between LP curves and o--E diagrams of tensile test. In particular, at small penetration rates (0.1 ram/rain), the ultimate tensile stress o"u equals about q s / 3 and the yield stress o-r about q y / 3 . The effects of penetration rates and test temperatures have been also considered. As discussed in detail in a previous work [11], by increasing the penetration rate a little increase of q r and a more evident increase of qs, that reaches values much higher than the hardness, have been found. As regards the test temperature, the transition for steels from ductile to brittle regime is accompanied by an increase of the work hardening rate during the third loading stage where the curves tend to the saturation values [ 12]. The above mentioned behaviour during penetration tests has been confirmed also for the materials examined in this
6000" 5000-
a)
Fig. 3. SEM images of the imprint for MANET-2 steel obtained with load q > qr (a) and of the zone around the imprint bottom (b).
work. Fig. la shows the LP curve obtained for MANET-2 steel at room temperature and a penetration rate of 0.3 m m / m i n : the loads q r and qs are indicated by arrows on the ordinate. The values of q r and qs vs. the penetration rate ( d 6 / d t ) are plotted in Fig. lb for MANET-2 steel. In this figure ~ry and crU obtained in tensile tests and the microhardness (HV) are also indicated by arrows on the ordinate. Comparable values of qs and HV are verified only at
b)
c)
4000"
~ 30002000 ~ 1000 0
0
0.1 6,2 013 0'.4
0
0.1 0.2 0.3 0.4 0
(mm)
0'.1 0'.2 013 0'.4 0.5
Fig. 4. LP curves for MANET-2 (a), BATMAN (b) and modified F82H (c) steels at T= - 180°C (~x), T= 20°C (/3) and T= + 100°C (y).
1560
P. Gondi et al./Journal of Nuclear Materials 233-237 (1996) 1557-1560
the smallest penetration rate, while the q r values show only small variations confirming the general correspondence with the yield strength. This behaviour can be interpreted considering the trends of stresses in radial (o R) and axial (o"z) directions of the cylindrical indenter and the 45 ° shear stress (r), which can be evaluated by means of the elasticity equations [10] (Fig. 2). With an uniform pressure q, the normal stresses decrease progressively with increasing the distance from the surface, whereas the shear stress reaches a maximum value r * -=-q / 3 for a depth z* - 0.3~b. This behaviour is consistent with the assumption that the stage of non-linear work hardening, for loads q > q r , depends on the onset of yielding when the maximum shear stress overcomes the yield stress of the material. Moreover, when q > qy plastic deformation with large protrusion of material around the imprint can be observed (Fig. 3a); SEM images have also shown that zones with rough surface and microcracks are present all around the imprint bottom (Fig. 3b). The first stage of linear work hardening, from qL to qy, is related to the deformation in the cylindrical core within the circular cut, until the deformation spreads over when the yield stress is overcome at the depth of maximum shear stress. A more detailed discussion is in Ref. [11]. The effect of temperature is shown by the LP curves of MANET-2, BATMAN and modified F82H steels plotted in Fig. 4a-c, which have been recorded performing tests at three different temperatures ( - 180, + 2 0 and + 100°C). The LP curves exhibit a higher work hardening rate ( d q / d # ) during the third stage when penetrations are made at low temperatures. The temperature of - 180°C corresponds to the range where the steels show brittle behaviour in Charpy impact tests, whereas the other ones are in the ductile range: the transition from ductile to brittle behaviour is accompanied also by a small increase of qr and by a more evident 5
200
E /=7=
:s ~4
100 '-'~
T
3 - 200
-
-
-
,
-
-100
6
0
100
T (°C)
Fig. 5. Charpy fracture energies (dashed line) and average work hardening rates of the LP curves (solid line) vs. test temperature for MANET-2 steel.
increase of qs as a consequence of the high values of the work hardening rate. Therefore, average values of d q / d 8, which can be obtained by the evaluation of the LP curve slopes in the second stage of non-linear work hardening, have been considered to characterize the transition from ductile to brittle regime. In Fig. 5 the average work hardening rates ( A q / A S ) vs. test temperature (T) for MANET steel are plotted; in the same figure the fracture energies ( J ) obtained by Charpy tests at various temperatures are also reported. Both tests show a larger scattering of data in the transition region, which takes place in the same range of temperature. This correspondence gives the possibility to have indications on fracture behaviour and ductile to brittle transition temperature from penetration tests.
Acknowledgements The authors are grateful to Mr. Piero Plini and Mr. Benedetto Iacovone for assistance in mechanical tests.
References [1] P. Jung and J. Chen, Miniaturized Specimens for Testing of Irradiated Mater. lEA Int. Symp., (Jfllich, 1994) p. 85. [2] A.B. Tsepelev, L.I. lvanov and I.L. Poymenov, Miniaturized Specimens for Testing of Irradiated Mater. lEA Int. Symp., (Jiilich, 1994) p. 59. [3] M.L. Hamilton, M.B. Toloczko and G.E. Lucas, Miniaturized Specimens for Testing of Irradiated Mater. IEA Int. Symp. (Jiilich, 1994) p. 46. [4] T. Misawa, T. Adachi, M. Saito and Y. Hamaguchi, J. Nucl. Mater. 150 (1987) I94. [5] N.N. Gabuiev, Yu.N. Korolyov, M.E. Korshunov, E.A. Krasikov and D.Yu. Tuochmenev, MSTIM Symp., (Julich, 1995) p. 123. [6] N. Igata, K. Miyahara, C. Tada, D. Basl and G. Lukas, Rad. Effects 101 (1986) 131. [7] H. Kurishita, H. Kayano and M. Narui, Miniaturized Specimens for Testing of Irradiated Mater. lEA Int. Symp., (Jhlich, 1994) p. 14O. [8] A. Kimura, T. Morimura and H. Matsui, Miniaturized Specimens for Testing of Irradiated Mater. lEA Int. Syrup., (JiJlich, 1994) p. 167. [9] K. Yasuda, K. Shinohara, C. Kinoshita, M. Yamada and M. Arai, J. Nucl. Mater. 212-215 (1994) 1698. [10] P. Gondi and A. Sill, Z. Metallkunde 82 (1991) 377. [11] P. Gondi, R. Montanari and A. Sili, J. Nucl. Mater. 212-215 (1994) 1688. [I 2] P. Gondi, R. Montanari and A. Sili, Miniaturized Specimens for Testing of Irradiated Mater. IEA Int. Syrup., (Jiilich, 1994) p. 79.
Journal of Nuclear Materials 233- 237 (1996) 1557-1560
journalof nuclear materials
A miniaturized test method for the mechanical characterization of structural materials for fusion reactors P. Gondi a, *, A. Donato b, R. Montanari a, A. Sill a a Mechanical Engineering Department, University ofRome-Tor Vergata, Rome, Italy b ENEA CRE, Fusion Sector, Frascati, Rome, Italy
Abstract This work deals with a non-destructive method for mechanical tests which is based on the indentation of materials at a constant rate by means of a cylinder with a small radius and penetrating flat surface. The load versus penetration depth curves obtained using this method have shown correspondences with those of tensile tests and have given indications about the mechanical properties on a reduced scale. In this work penetration tests have been carried out on various kinds of Cr martensitic steels (MANET-2, B A T M A N and modified F82H) which are of interest for first wall and structural applications in future fusion reactors. The load versus penetration depth curves have been examined with reference to data obtained in tensile tests and to microhardness measurements. Penetration tests have been performed at various temperature (from - 180 to 100°C). Conclusions, which can be drawn for the ductile to brittle transition, are discussed for MANET-2 steel. Preliminary results obtained on BATIVIAN and modified F82H steels are reported. The characteristics of the indenter imprints have been studied by scanning electron microscopy.
1. Introduction The possibility to have general information on mechanical properties by a non-destructive method working on a miniaturized scale appears suitable for local determination of these properties or for the characterization of materials directly in use. Small specimen techniques could be useful to test materials in particular after irradiation in the IFMIF (International Fusion Material Irradiated Facility), a source which will be built to irradiate materials with fusion-equivalent high energy (14 MeV) neutrons in a foreseen useful volume not exceeding 1 1. Miniaturized techniques for the evaluation of mechanical properties on samples after irradiation have been investigated considering microhardness measurements [1], disk bend [2] and shear punch tests [3] or Charpy tests performed with V-notched specimens smaller than the standard requirements [4,5]. Data of mechanical properties obtained with sub-sized specimens differ in general from data obtained in standard tests [6,7]. In
* Corresponding author. Tel.: +39-6-72594617; fax: +39-62021351; e-mail: [email protected].
order to assess bulk properties it is necessary to use empirical expressions or physical models. For instance, the radius of the plane stress state at the specimen surface is considered to be a material independent factor controlling size effects in Charpy tests (Kimura et al. [8]). Following a different approach, other methods are based on indentation tests performed on small areas of specimens. Yasuda et al. [9], for example developed an ultra microhardness technique for evaluating stress-strain properties of metals. The method developed by us [10,11] is based on the indentation of materials by means of a cylinder with a small radius and a flat penetrating surface. In this case miniaturization regards not the samples but only the tested zone: an advantage of this method is the possibility to test irradiated zones of particular interest such as welding joints. The load vs. penetration depth diagrams obtained with this test have shown correspondences with o--e curves obtained in tensile tests. Moreover, experiments carried out on some martensitic steels at different temperatures have given indications on the ductile to brittle transition temperature [12]. In this paper results of penetration tests regarding different Cr martensitic steels, candidates for structural appli-
0022-3115/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 I 15(96)003 15-7
1558
P. Gondi et al./Journal of Nuclear Materials 233-237 (1996) 1557-1560
Table l Steels compositions, Fe to balance (wt%)
cations in future fusion reactors, are presented and discussed.
MANET-2 C Cr Mo Ni Mn Nb V Ti Ta W B Nb Co Si AI Cu As Sb Sn N P S
2. E x p e r i m e n t a l Penetration tests were p e r f o r m e d by m e a n s o f cylindrical indenters with d i a m e t e r ~b = 1.0 m m and h e i g h t h = 1.5 ram. T h e indenters were m a d e o f W C , w h i c h g u a r a n t e e s g o o d rigidity and n o n - d e f o r m a b i l i t y also w h e n applied to s a m p l e s of hard materials. T h e e x p e r i m e n t a l apparatus allowed m e a s u r e m e n t s of the indenter a d v a n c e m e n t 6 in d e p e n d e n c e o f the applied force, P. U n d e r the a s s u m p t i o n that P is u n i f o r m l y distributed on the contact surface o f the s a m p l e s with constant pressure q = 4P/(Trq52) [10], q load vs. 6 penetration depth c u r v e s are obtained (in the following indicated as LP curves). T h i s work deals with tests carried out at c o n s t a n t penetration rate and at various temperatures (from - 180 to 100°C). T h e effect o f the penetration rate on the LP curves w a s also taken into a c c o u n t p e r f o r m i n g tests with different rates in the range from 0.1 to 2 r a m / r a i n . T h e c o m p o s i t i o n s o f the investigated Cr martensitic steels ( M A N E T - 2 , B A T M A N and modified F 8 2 H ) are g i v e n in Table 1. T h e preliminary thermal treatments c o n s i s t o f q u e n c h i n g and t e m p e r i n g for 2 h at 700°C for the M A N E T - 2 steel and o f n o r m a l i z i n g and t e m p e r i n g for 1 h at 730°C for the B A T M A N steel and for 1 h at 750°C for the m o d i f i e d F82H steel. T h e s p e c i m e n s were subjected to electrochemical polishing to avoid surface harde n i n g effects. Indenter and s p e c i m e n were dipped into chilled w a t e r ethanol solutions or into liquid nitrogen for tests p e r f o r m e d b e l o w r o o m temperature and into water baths for tests p e r f o r m e d above room temperature up to 100°C. O n the s a m e materials Vickers m i c r o h a r d n e s s m e a s u r e m e n t s , tensile tests with a strain rate o f ~ = 10 3 s 1 and C h a r p y impact tests h a v e been carried out.
6000qs
BATMAN
Mod. F82H
0.12 8.32 0.0154 0.0275 0.52 0.0021 0.20 0.09 0.00014 0.0062 0.0021
0.09 7.68 0.0030 0.020 0. I 1 0.0001 0.16 0.01 0.02 0.0002 0.0002 0.0001
0.0260 0.0085 0.0051
0.11 0.0030 0.01
0.0014 0.0072 0.0050 0.0026
0.0070 0.0020 0.0020
0.10 10.37 0.58 0.65 0.76 0.16 0.21
0.0075 0.005 0.18 0.007 0.010 0.010 0.0002 < 0.01 0.032 0.004 0.005
Characteristics o f the indentations have been studied by s c a n n i n g electron m i c r o s c o p y (SEM).
3. Results a n d discussion Previous e x p e r i m e n t s p e r f o r m e d on various metals and alloys s u c h as A1, Cu, Zn and plain carbon steel AISI 1040, have s h o w n that LP c u r v e s exhibit different stages [10]. T h e d e f o r m a t i o n is elastic for q values not e x c e e d i n g a critical limit qL w h e n q > qL a p e r m a n e n t indentation with sharp edge is observed. T h e first stage o f workh a r d e n i n g is linear (see also Fig. 1).
20000"
a)
5ooo-~,
~ " 150004000
13.
CL
~E
10000-
.2500
2000 5000-
qv
o~
- -&T
0,4
(mm)
HV 0
0
q
i
~
u
0 (~v
d(~Idt (mm/min)
Fig. 1. (a) LP curve for MANET-2 steel with indication of qy, qs limit loads by means of the arrows on the ordinate; (b) trends of the qr ~md qs limit loads vs. d ~ / d t penetration rate (indication of the yield stress o-r and ultimate tensile o-u from tensile tests are given by the arrows on the right ordinate; the HV microhardness value is given by the arrow on the left ordinate).
1.0~
P. Gondi et al. / Journal of Nuclear Materials 233-237 (1996) 1557-1560
1559
0.5
- 0"5t/z - 1.0 II 0
,
5
,
Z/O
10
15
Fig. 2. Trends o f axial stress (o-z), radial stress (o"x) and 45 ° shear stress ( r ) for a uniform q load applied by the cylindrical indenter.
With increasing penetration depth, starting from a load q r , LP curves show a second stage of non-linear work hardening which is followed by a third stage where they slowly tend towards saturation values qs, which are comparable to the hardness values for the smallest penetration rate. In the second and third stages large plastic deformations with protrusion of material around the indentation are observed. With a suitable choice of the coordinates correspondences exist between LP curves and o--E diagrams of tensile test. In particular, at small penetration rates (0.1 ram/rain), the ultimate tensile stress o"u equals about q s / 3 and the yield stress o-r about q y / 3 . The effects of penetration rates and test temperatures have been also considered. As discussed in detail in a previous work [11], by increasing the penetration rate a little increase of q r and a more evident increase of qs, that reaches values much higher than the hardness, have been found. As regards the test temperature, the transition for steels from ductile to brittle regime is accompanied by an increase of the work hardening rate during the third loading stage where the curves tend to the saturation values [ 12]. The above mentioned behaviour during penetration tests has been confirmed also for the materials examined in this
6000" 5000-
a)
Fig. 3. SEM images of the imprint for MANET-2 steel obtained with load q > qr (a) and of the zone around the imprint bottom (b).
work. Fig. la shows the LP curve obtained for MANET-2 steel at room temperature and a penetration rate of 0.3 m m / m i n : the loads q r and qs are indicated by arrows on the ordinate. The values of q r and qs vs. the penetration rate ( d 6 / d t ) are plotted in Fig. lb for MANET-2 steel. In this figure ~ry and crU obtained in tensile tests and the microhardness (HV) are also indicated by arrows on the ordinate. Comparable values of qs and HV are verified only at
b)
c)
4000"
~ 30002000 ~ 1000 0
0
0.1 6,2 013 0'.4
0
0.1 0.2 0.3 0.4 0
(mm)
0'.1 0'.2 013 0'.4 0.5
Fig. 4. LP curves for MANET-2 (a), BATMAN (b) and modified F82H (c) steels at T= - 180°C (~x), T= 20°C (/3) and T= + 100°C (y).
1560
P. Gondi et al./Journal of Nuclear Materials 233-237 (1996) 1557-1560
the smallest penetration rate, while the q r values show only small variations confirming the general correspondence with the yield strength. This behaviour can be interpreted considering the trends of stresses in radial (o R) and axial (o"z) directions of the cylindrical indenter and the 45 ° shear stress (r), which can be evaluated by means of the elasticity equations [10] (Fig. 2). With an uniform pressure q, the normal stresses decrease progressively with increasing the distance from the surface, whereas the shear stress reaches a maximum value r * -=-q / 3 for a depth z* - 0.3~b. This behaviour is consistent with the assumption that the stage of non-linear work hardening, for loads q > q r , depends on the onset of yielding when the maximum shear stress overcomes the yield stress of the material. Moreover, when q > qy plastic deformation with large protrusion of material around the imprint can be observed (Fig. 3a); SEM images have also shown that zones with rough surface and microcracks are present all around the imprint bottom (Fig. 3b). The first stage of linear work hardening, from qL to qy, is related to the deformation in the cylindrical core within the circular cut, until the deformation spreads over when the yield stress is overcome at the depth of maximum shear stress. A more detailed discussion is in Ref. [11]. The effect of temperature is shown by the LP curves of MANET-2, BATMAN and modified F82H steels plotted in Fig. 4a-c, which have been recorded performing tests at three different temperatures ( - 180, + 2 0 and + 100°C). The LP curves exhibit a higher work hardening rate ( d q / d # ) during the third stage when penetrations are made at low temperatures. The temperature of - 180°C corresponds to the range where the steels show brittle behaviour in Charpy impact tests, whereas the other ones are in the ductile range: the transition from ductile to brittle behaviour is accompanied also by a small increase of qr and by a more evident 5
200
E /=7=
:s ~4
100 '-'~
T
3 - 200
-
-
-
,
-
-100
6
0
100
T (°C)
Fig. 5. Charpy fracture energies (dashed line) and average work hardening rates of the LP curves (solid line) vs. test temperature for MANET-2 steel.
increase of qs as a consequence of the high values of the work hardening rate. Therefore, average values of d q / d 8, which can be obtained by the evaluation of the LP curve slopes in the second stage of non-linear work hardening, have been considered to characterize the transition from ductile to brittle regime. In Fig. 5 the average work hardening rates ( A q / A S ) vs. test temperature (T) for MANET steel are plotted; in the same figure the fracture energies ( J ) obtained by Charpy tests at various temperatures are also reported. Both tests show a larger scattering of data in the transition region, which takes place in the same range of temperature. This correspondence gives the possibility to have indications on fracture behaviour and ductile to brittle transition temperature from penetration tests.
Acknowledgements The authors are grateful to Mr. Piero Plini and Mr. Benedetto Iacovone for assistance in mechanical tests.
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