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Embrittlement due to hydrogen in ferritic and martensitic structural steels
ht. J. Hydrogen Enerjg, Vol. 20, No. 2, pp. 133-139, 1995 Copyright
(Q 1994 International
Association
for Hydrogen Energy Eisevier Science Ltd Punted m Great B&am. Atl rights reserved
0360.3199,‘95$9.50 c 0.00
0360-3199(94)EOOO41
EMBRTTTLEMENT DUE TO HYDROGEN IN FERRITIC AND MARTENSITIC STRUCTURAL STEELS G. BIGGIERO, A. BORRUTO and F. GAUDINO University of Rome, I.C.M.M.P.M. Department, Metallurgy Section, Via Eudossiana 18, I-00184 Roma. Italy 18 January 1994)
(Received for publication
Abstract-AISI 420 steel test-pieces were submitted to hydrogen discharge by electrolyzation at a constant current density (8 mA cmM2)under a constant load (14 daN mme2), with different charging times (from 3600 to 450,WOs). Two test-pieces were hydrogenated simultaneously, then one was tensioned by a tensile machine, while the other was tested by a hydrogen analyzer. Tensile tests results have shown that as the charging time increases,the material toughness (obtained by the value of reduction in area) decreases,passesthrough a minimum value and then later tends to recover its origirral value. Hydrogen-analyzer results have highlighted that, in parallel with the decreaseand the recoveryin material toughness, there is an increase and a decreasein the amount of hydrogen extractable at 4OO”C,respectively, while at 600°C and 800°C respectively, it remains practically constant and subsequently increases.This phenomenon has been confirmed by fractographic analysis
The present work is aimed at establishing a correlation between the variations of mechanical characteristics of steel due to hydrogen penetration.
NOMENCLATURE
RS Yield stress %I Tensile stress R” Ultimate tensile J Charging-current k I.
stress density
MATERIAL
Hydrogenation time Percent reduction in area Work per unit volume (specific work)
EMPLOYED AND EXPERIMENTAL TECHNOLOGY
The material utilized in this study was AISI 420, a medium-carbon stainless steel, characterized by a partially tempered martensitic structure particularly susceptible to embrittlement. Its main mechanical chatacteristics are:
INTRODUCTION Steel penetration by hydrogen, which leads to changes in its tensile and toughness characteristics, is a dangerous phenomenon, often resulting in catastrophic conse-
R, (kg, mm-?
R, (kgf mm-*)
quences. In a tensile test on pre-hydrogenated steel test pieces, this phenomenon is mainly visible through a decreasein the steel’s toughness, which leads to an early
68.6 -.
95.3
failure of the test piece. Such decrease is shown by measuring the percent reduction in area and calculating
Steel test-pieceswere charged with hydrogen at room pressure and temperature with a constant pm-load and at different hydrogenation times (3600 s, lO,&oos, 21,6QO s, 86,400 s, 172,800s, 202,000 s, 250,ooOs, 4%&000s). Hydrogenation was realized by cathodic discharge in a 0.1 M solution of H,SO, on a stress corrosion test piece (Fig. 1) submitted to a static pm-load. Hydrogenation was carried out keeping the charging current density constant (8 mA cm-‘), with the pm-load value equal to 14 daN mm-‘. The 8 mA cm-’ current density value was selected since previous studies [l-7] showed it to be the most dangerous. The pre-load value was selected
the specific strain work. The conditions of hydrogen-steel interaction causing embrittlement, which are still far from clear, mainly depend on the type and strength of interaction. In fact, after steel penetration, hydrogen may be localized in the metal lattice-in interatomic voids, relatively free to move and capable of strong interaction with dislocations. It can be also found on the interfaces of microscopic defects (micro-voids and micra-induaions) in a molecular form or in the form of actual compounds as metal hydrides. 133
HV (kg mm-.‘) _~-.--.
300
134
G. BIGGIERO
h
et al.
H
17
9
21.5
7
1r
-.-0.-.
JL
Fig. 1. Geometryof the test-pieces(mm).
considering the value of the material ratio RJR, in order to prevent the early fracture of the test piece during the 50 */** * hydrogenation. Hydrogen charging was carried out with each selected hydrogenation time on two test pieces simultaneously. s40 f / After hydrogen charging one of the test pieces was submitted to a tensile test, after a standard time (3 min) of exposure to air, by an INSTRON machine at a constant cross-head speed (0.5 mm min-I), while the remaining one was analysed by a LECO 200 hydrogen analyzer. The main tensile characteristics of steel were obtained lOA (R,, R,, R,, Z%) and the specific strain work (L) was 2 5 10’ 2 s 10s 2 calculated. Seconds Test-pieces submitted to the hydrogen analyzer have Fig. 3. Specificstrain work vs chargingtime. Current density: been previously kept in a furnace at 150°C for 48 h. Tests 8 mA cm-‘, pre-load: 14 daN mm-‘. were carried out, dividing the total analysis time into three subsequent intervals, at the constant temperatures of 4QO”C,600°C and 8OO”C,lasting 300 s, 300 s and 600 s respectively. For each test, the hydrogen analyzer gave RESULTS AND DISCUSSION the pattern of hydrogen extracted from the test piece as Test results are shown in Figs 2-6, in logarithmic scale. a function of the analysis time and temperature, together Figure 2 shows the pattern of percent reduction in area with the amounts of hydrogen extracted in each temperature interval and the total amount of hydrogen extracted, as a function of the charging time. It is worth noting that as the charging time increases,the value of reduction in in ml (100 g)-’ of material. After fracture, all test pieceswere analyzed by the SEM area decreases,passesthrough a minimum value which is found at 10,800 s charging time, and then later tends microscope.
800 *,f*** / :P*\*,*
moo
r
d
100I- -* 90
t
-
‘*
\
Rm
l* -
-
-
*- /f
+**
200
Fig. 2. Reductionin area vs charging time. Current density: 8 mA cm-‘, pre-load: 14daN mm-*.
Fig. 4.R,, R,, R, vschargingtime.Currentdensity:8 mA cm-*, pre-load: 14daN mm-*.
STEEL EMBRITTLEMENT
DUE TO HYDROGEN
13s
non-hydrogenated test-pieces turned out to be approx. 0.2 ml (100 g- “). 0.80 Figure 6 shows, as a function of charging time, the F total amounts of hydrogen extracted in the test and the amounts extracted at each temperature interval. Five different hydrogen behaviour fields are identified as the charging time increases. The first field (CrlO,SOO s, when there is a decrease in the material toughness) shows that the total amount of hydrogen extracted is rapidly increasing-the amount of hydrogen extracted at 400°C increasing equally rapidly, while at 600°C and 800°C it remains quite constant. 5 10’ f 5 ld 2 The second field [10,80&21,600 s, in which the phenomenon of gradually recovery of toughness is evident Seconds (Fig. 2)] shows a still rapid increase in the total amount Fig. 5. Ratio RJR, vs charging time. Current density: 8 mA of hydrogen extracted-a slight increase in the fraction cm-2, pre-load: 14 daN mm-*. extracted at 4OO”C,a strong increase in that extracted at 6OO’C,while the one extracted at 800°C remains rather constant. The third field (21,~86,400 s, when the toughness to recover its original value [l-7]. This recovery is almost is almost fully recovered) shows a slower growth of the complete in the test at 86,400 s. total amount of hydrogen extracted-a sharp drop of the Figure 3 shows the pattern of the specific strain work amount extracted at 400°C and an equally sharp increase which is fully similar to the reduction in area one. in fractions extracted at 600°C and 800°C. Figure 4 shows the patterns of yield stress, maximum The fourth field (86,400-202,000 s) shows a decrease tensile stress and ultimate tensile stress as a function of in the total amount of hydrogen extracted--a sharp drop the charging time. In particular, R, and R, show a slight in fractions extracted at 600°C and 8OO”C,while the one decrease,reaching the minimum value at 21,600 s. Both extracted at 4W’C slightly increases. of them recover their values with longer charging times; The fifth and last field (202,00@-450,000 s) shows a new these values decreaseagain at 250,000s. On the contrary, increase in the total amount of hydrogen extracted. R, increases up to 10.800 s, later to decreasein longer Between 202,000 and 250,000s an increase is noticed in charging-time tests. The pattern of the ratio R,IR, the hydrogen extracted at 600°C and 8OO”C,while the shown in Fig. 5 seemsanalogous to the one of reduction fraction extracted at 4OO’Cshows a slight decrease.Then in area. between 250,ooOand 450,000s there is a stabilization Tests by the hydrogen analyzer were also carried out trend at 600°C and XW’C, while at 400°C there is a new on non-hydrogenated test-pieces to evaluate the hydro- increase. This trend indicates a phenomenon iteration. gen amount already present in the material as received. Another important consideration concerns the hydroRepeating the sametests on non-hydrogenated test-pieces genemission peaksshapein theanalysis interval at 400°C. kept in a furnace for 48 h at 15O”C, resulted in the It has been noticed that, at times coincident with the area disappearance of the peak at 6OO”C,while the one at reduction decrease, hydrogen emission peaks are high 800°C was unchanged. The total amount of hydrogen in and narrow, and appear as soon as the analysis temperature is reached. Instead, at longer hydrogenation times. when there is a total recovery of toughness, peaks arc low and wide and tend definitely to appear later than 0.60 when the analysis temperature is reached. This phenom*/y */* 0.50 enon leads to the supposition that at short charging times, *d penetrated hydrogen concentrates not far from the ma.~ / 7WI 0.40 terial’s surface, so that during the analysis, it rapidly *-+ 8 0.30 MNPC! 7 leaves the metal. At longer hydrogenation times, pen/ '*\ *' .* iji etrated hydrogen is more diffused in the metal matrix. so 0.20 * -0' aoo-C -. 0.85
.* o 1o . ffb=*rc'* ,
d + -*\ - - \ 4oo*c-*
+*
o :g.
,*
r*
Fig. 6. Hydrogen extracted in the test: total amount and amount in each temperature interval vs charging time.
that it progressively
leaves over a longer time period.
Fractographic analysis of test pieces which showed an increasein brittleness has evidenced fractures propagated from one or more notches, with a partially inkgranular aspect and visible surface damage having de-cohesions perpendicular to its axis (Figs 7 and 8). As for test-pieces which showed the total recovery of toughness and non-hydrogenated test-pieces, the fracture propagated from several notches, while the exterior surface did not show any damage (Fig. 9).
136
G. BIGGIERO
et al.
Figs 7 and 8. AISI 420. Fracture surface. Current density: 8 mA err-‘,
pre-load:
14 daN mm-*, hydrogenation
time: 10,800 s.
STEEL EMBRITTLEMENT
DUE TO HYDROGEN
I 3.1
Fig. 8. Detail: edge of fracture surface.
CONCLUSIONS
dislocations, thus having a slip-blocking action [8-l I]. With longer charging times, there is an energetically The analysis of the above results seemsto prove that stronger hydrogen fixation: it can be supiposed, for the decreasein the steel toughness is due to the hydrogen example, that hydride formation, in the form of mioroextractable at 4OO”C,which is more weakly bonded and precipitates, takes place in the sites where su%ient more widespread, and whose amount, in fact, increases concentration levels are reached. This fact could lead to as percent reduction in area and specific strain work the subtraction of the hydrogen in its most d decrease.In contrast, when fractions extracted at 600°C form, which results in the recovering of mater&I to@+and goO”C,with a higher bond-energy, increase,and those ness tin accordance with the increase in hydFOf#@I exextracted at 400°C decrease,the toughness of the material tractable at 600°C and 800°C). Further increasing the charging time, and therefore is recovered. Therefore, it is considered that, at first, hydrogen penetrates the material, concentrating near its probable saturation of the sites with a stronger bondsurface and, being weakly bonded, condensates around energy, leads to a phenomenon iteration which is ex-
138
G. BIGGIERO et al.
Fig. 9. Fracture surface. Current density: 8 mA cm-2, pre-load: 14 daN mm-‘, hydrogenation time: 86,400s.
pressed by a new increase and subsequent decrease of the hydrogen fraction extracted at NOT, and consequently, by a new decrease and subsequent increase of the one extracted at 6OO’C and 800°C.
REFERENCES 1. G. Biggiero, A. Borruto, F. Marafini and R. Cicione, Interazione idrogeno-acciaio. L ‘Ingegnere, pp. 497-503 (1980). 2. G. Biggiero, A. Borruto and F. Marafini, Embrittlement break-down of steel upon electrolytic hydrogen saturation. Atonmega Energia 49, 22-25 (1980).
3. G. Biggiero and A. Borruto, Delayed fracture as parficular case of premature fracture. Miami Int. Symp. Metal-Hydrogen Systems,pp. 259-270. Pergamon Press, Oxford (1981). 4. G. Biggiero and A. Borruto, The alternative fragilization and the recovery of mechanical properties emphasized with the method of preaged fracture. Alternative Energy Sources VI- Wind/Ocean/Nuc~ear/Hydroger& 587-605 (1983). 5. G. Biggiero and A. Borruto, Mechanical behaviour of sorbitic and austenitic steels in strongly hydrogenating environments: relevant use prospects. ht. J. Hydrogen Energy 13,743-747 (1988). 6. G. Biggiero and A. Borruto, The behaviour of AISI 304 austenitic stainless steel under traction during hydrogen discharge. Int. J. Hydrogen Energy 15, 187-191 (1990).
STEEL EMBRITTLEMENT 7. G. Biggiero, A. Borruto, and S. Amato, Differences in the trend of strain and fracture of AISI 9840 steel pre-charged in hydrogen in cases of traction with or without hydrogen discharge. Proc. 2nd Cairo Znt. Symp. Renewable Energy Sources 2, 659-668 (1990). 8. H. Margot Marette, B. Marandet and J. C. Charbonnier, MJmoires Scientijiques Revue Metalluryie, pp. 175, March 1976 I 1990).
DUE TO HYDROGEN
Ii9
9. P. Bastien, V” Colloquie de Metallurgic. Le Ga: duns ic Metuux, p. 2. Universiti de France (1962). 10. P. Bastien, VIII” Colloquie de Metullurgie. Lu Rupiurc Differ&s, p. I. Universite de France (1965). 11. J. P. Hirth, Met&. Trans. llA, 869 (1980). 12. T. Asaoka, Proc. Miami ht. Symp Metal--H.vdrt>s~en .~~.cfcms. pp. 197. Pergamon Press, Oxford (1984).
(Q 1994 International
Association
for Hydrogen Energy Eisevier Science Ltd Punted m Great B&am. Atl rights reserved
0360.3199,‘95$9.50 c 0.00
0360-3199(94)EOOO41
EMBRTTTLEMENT DUE TO HYDROGEN IN FERRITIC AND MARTENSITIC STRUCTURAL STEELS G. BIGGIERO, A. BORRUTO and F. GAUDINO University of Rome, I.C.M.M.P.M. Department, Metallurgy Section, Via Eudossiana 18, I-00184 Roma. Italy 18 January 1994)
(Received for publication
Abstract-AISI 420 steel test-pieces were submitted to hydrogen discharge by electrolyzation at a constant current density (8 mA cmM2)under a constant load (14 daN mme2), with different charging times (from 3600 to 450,WOs). Two test-pieces were hydrogenated simultaneously, then one was tensioned by a tensile machine, while the other was tested by a hydrogen analyzer. Tensile tests results have shown that as the charging time increases,the material toughness (obtained by the value of reduction in area) decreases,passesthrough a minimum value and then later tends to recover its origirral value. Hydrogen-analyzer results have highlighted that, in parallel with the decreaseand the recoveryin material toughness, there is an increase and a decreasein the amount of hydrogen extractable at 4OO”C,respectively, while at 600°C and 800°C respectively, it remains practically constant and subsequently increases.This phenomenon has been confirmed by fractographic analysis
The present work is aimed at establishing a correlation between the variations of mechanical characteristics of steel due to hydrogen penetration.
NOMENCLATURE
RS Yield stress %I Tensile stress R” Ultimate tensile J Charging-current k I.
stress density
MATERIAL
Hydrogenation time Percent reduction in area Work per unit volume (specific work)
EMPLOYED AND EXPERIMENTAL TECHNOLOGY
The material utilized in this study was AISI 420, a medium-carbon stainless steel, characterized by a partially tempered martensitic structure particularly susceptible to embrittlement. Its main mechanical chatacteristics are:
INTRODUCTION Steel penetration by hydrogen, which leads to changes in its tensile and toughness characteristics, is a dangerous phenomenon, often resulting in catastrophic conse-
R, (kg, mm-?
R, (kgf mm-*)
quences. In a tensile test on pre-hydrogenated steel test pieces, this phenomenon is mainly visible through a decreasein the steel’s toughness, which leads to an early
68.6 -.
95.3
failure of the test piece. Such decrease is shown by measuring the percent reduction in area and calculating
Steel test-pieceswere charged with hydrogen at room pressure and temperature with a constant pm-load and at different hydrogenation times (3600 s, lO,&oos, 21,6QO s, 86,400 s, 172,800s, 202,000 s, 250,ooOs, 4%&000s). Hydrogenation was realized by cathodic discharge in a 0.1 M solution of H,SO, on a stress corrosion test piece (Fig. 1) submitted to a static pm-load. Hydrogenation was carried out keeping the charging current density constant (8 mA cm-‘), with the pm-load value equal to 14 daN mm-‘. The 8 mA cm-’ current density value was selected since previous studies [l-7] showed it to be the most dangerous. The pre-load value was selected
the specific strain work. The conditions of hydrogen-steel interaction causing embrittlement, which are still far from clear, mainly depend on the type and strength of interaction. In fact, after steel penetration, hydrogen may be localized in the metal lattice-in interatomic voids, relatively free to move and capable of strong interaction with dislocations. It can be also found on the interfaces of microscopic defects (micro-voids and micra-induaions) in a molecular form or in the form of actual compounds as metal hydrides. 133
HV (kg mm-.‘) _~-.--.
300
134
G. BIGGIERO
h
et al.
H
17
9
21.5
7
1r
-.-0.-.
JL
Fig. 1. Geometryof the test-pieces(mm).
considering the value of the material ratio RJR, in order to prevent the early fracture of the test piece during the 50 */** * hydrogenation. Hydrogen charging was carried out with each selected hydrogenation time on two test pieces simultaneously. s40 f / After hydrogen charging one of the test pieces was submitted to a tensile test, after a standard time (3 min) of exposure to air, by an INSTRON machine at a constant cross-head speed (0.5 mm min-I), while the remaining one was analysed by a LECO 200 hydrogen analyzer. The main tensile characteristics of steel were obtained lOA (R,, R,, R,, Z%) and the specific strain work (L) was 2 5 10’ 2 s 10s 2 calculated. Seconds Test-pieces submitted to the hydrogen analyzer have Fig. 3. Specificstrain work vs chargingtime. Current density: been previously kept in a furnace at 150°C for 48 h. Tests 8 mA cm-‘, pre-load: 14 daN mm-‘. were carried out, dividing the total analysis time into three subsequent intervals, at the constant temperatures of 4QO”C,600°C and 8OO”C,lasting 300 s, 300 s and 600 s respectively. For each test, the hydrogen analyzer gave RESULTS AND DISCUSSION the pattern of hydrogen extracted from the test piece as Test results are shown in Figs 2-6, in logarithmic scale. a function of the analysis time and temperature, together Figure 2 shows the pattern of percent reduction in area with the amounts of hydrogen extracted in each temperature interval and the total amount of hydrogen extracted, as a function of the charging time. It is worth noting that as the charging time increases,the value of reduction in in ml (100 g)-’ of material. After fracture, all test pieceswere analyzed by the SEM area decreases,passesthrough a minimum value which is found at 10,800 s charging time, and then later tends microscope.
800 *,f*** / :P*\*,*
moo
r
d
100I- -* 90
t
-
‘*
\
Rm
l* -
-
-
*- /f
+**
200
Fig. 2. Reductionin area vs charging time. Current density: 8 mA cm-‘, pre-load: 14daN mm-*.
Fig. 4.R,, R,, R, vschargingtime.Currentdensity:8 mA cm-*, pre-load: 14daN mm-*.
STEEL EMBRITTLEMENT
DUE TO HYDROGEN
13s
non-hydrogenated test-pieces turned out to be approx. 0.2 ml (100 g- “). 0.80 Figure 6 shows, as a function of charging time, the F total amounts of hydrogen extracted in the test and the amounts extracted at each temperature interval. Five different hydrogen behaviour fields are identified as the charging time increases. The first field (CrlO,SOO s, when there is a decrease in the material toughness) shows that the total amount of hydrogen extracted is rapidly increasing-the amount of hydrogen extracted at 400°C increasing equally rapidly, while at 600°C and 800°C it remains quite constant. 5 10’ f 5 ld 2 The second field [10,80&21,600 s, in which the phenomenon of gradually recovery of toughness is evident Seconds (Fig. 2)] shows a still rapid increase in the total amount Fig. 5. Ratio RJR, vs charging time. Current density: 8 mA of hydrogen extracted-a slight increase in the fraction cm-2, pre-load: 14 daN mm-*. extracted at 4OO”C,a strong increase in that extracted at 6OO’C,while the one extracted at 800°C remains rather constant. The third field (21,~86,400 s, when the toughness to recover its original value [l-7]. This recovery is almost is almost fully recovered) shows a slower growth of the complete in the test at 86,400 s. total amount of hydrogen extracted-a sharp drop of the Figure 3 shows the pattern of the specific strain work amount extracted at 400°C and an equally sharp increase which is fully similar to the reduction in area one. in fractions extracted at 600°C and 800°C. Figure 4 shows the patterns of yield stress, maximum The fourth field (86,400-202,000 s) shows a decrease tensile stress and ultimate tensile stress as a function of in the total amount of hydrogen extracted--a sharp drop the charging time. In particular, R, and R, show a slight in fractions extracted at 600°C and 8OO”C,while the one decrease,reaching the minimum value at 21,600 s. Both extracted at 4W’C slightly increases. of them recover their values with longer charging times; The fifth and last field (202,00@-450,000 s) shows a new these values decreaseagain at 250,000s. On the contrary, increase in the total amount of hydrogen extracted. R, increases up to 10.800 s, later to decreasein longer Between 202,000 and 250,000s an increase is noticed in charging-time tests. The pattern of the ratio R,IR, the hydrogen extracted at 600°C and 8OO”C,while the shown in Fig. 5 seemsanalogous to the one of reduction fraction extracted at 4OO’Cshows a slight decrease.Then in area. between 250,ooOand 450,000s there is a stabilization Tests by the hydrogen analyzer were also carried out trend at 600°C and XW’C, while at 400°C there is a new on non-hydrogenated test-pieces to evaluate the hydro- increase. This trend indicates a phenomenon iteration. gen amount already present in the material as received. Another important consideration concerns the hydroRepeating the sametests on non-hydrogenated test-pieces genemission peaksshapein theanalysis interval at 400°C. kept in a furnace for 48 h at 15O”C, resulted in the It has been noticed that, at times coincident with the area disappearance of the peak at 6OO”C,while the one at reduction decrease, hydrogen emission peaks are high 800°C was unchanged. The total amount of hydrogen in and narrow, and appear as soon as the analysis temperature is reached. Instead, at longer hydrogenation times. when there is a total recovery of toughness, peaks arc low and wide and tend definitely to appear later than 0.60 when the analysis temperature is reached. This phenom*/y */* 0.50 enon leads to the supposition that at short charging times, *d penetrated hydrogen concentrates not far from the ma.~ / 7WI 0.40 terial’s surface, so that during the analysis, it rapidly *-+ 8 0.30 MNPC! 7 leaves the metal. At longer hydrogenation times, pen/ '*\ *' .* iji etrated hydrogen is more diffused in the metal matrix. so 0.20 * -0' aoo-C -. 0.85
.* o 1o . ffb=*rc'* ,
d + -*\ - - \ 4oo*c-*
+*
o :g.
,*
r*
Fig. 6. Hydrogen extracted in the test: total amount and amount in each temperature interval vs charging time.
that it progressively
leaves over a longer time period.
Fractographic analysis of test pieces which showed an increasein brittleness has evidenced fractures propagated from one or more notches, with a partially inkgranular aspect and visible surface damage having de-cohesions perpendicular to its axis (Figs 7 and 8). As for test-pieces which showed the total recovery of toughness and non-hydrogenated test-pieces, the fracture propagated from several notches, while the exterior surface did not show any damage (Fig. 9).
136
G. BIGGIERO
et al.
Figs 7 and 8. AISI 420. Fracture surface. Current density: 8 mA err-‘,
pre-load:
14 daN mm-*, hydrogenation
time: 10,800 s.
STEEL EMBRITTLEMENT
DUE TO HYDROGEN
I 3.1
Fig. 8. Detail: edge of fracture surface.
CONCLUSIONS
dislocations, thus having a slip-blocking action [8-l I]. With longer charging times, there is an energetically The analysis of the above results seemsto prove that stronger hydrogen fixation: it can be supiposed, for the decreasein the steel toughness is due to the hydrogen example, that hydride formation, in the form of mioroextractable at 4OO”C,which is more weakly bonded and precipitates, takes place in the sites where su%ient more widespread, and whose amount, in fact, increases concentration levels are reached. This fact could lead to as percent reduction in area and specific strain work the subtraction of the hydrogen in its most d decrease.In contrast, when fractions extracted at 600°C form, which results in the recovering of mater&I to@+and goO”C,with a higher bond-energy, increase,and those ness tin accordance with the increase in hydFOf#@I exextracted at 400°C decrease,the toughness of the material tractable at 600°C and 800°C). Further increasing the charging time, and therefore is recovered. Therefore, it is considered that, at first, hydrogen penetrates the material, concentrating near its probable saturation of the sites with a stronger bondsurface and, being weakly bonded, condensates around energy, leads to a phenomenon iteration which is ex-
138
G. BIGGIERO et al.
Fig. 9. Fracture surface. Current density: 8 mA cm-2, pre-load: 14 daN mm-‘, hydrogenation time: 86,400s.
pressed by a new increase and subsequent decrease of the hydrogen fraction extracted at NOT, and consequently, by a new decrease and subsequent increase of the one extracted at 6OO’C and 800°C.
REFERENCES 1. G. Biggiero, A. Borruto, F. Marafini and R. Cicione, Interazione idrogeno-acciaio. L ‘Ingegnere, pp. 497-503 (1980). 2. G. Biggiero, A. Borruto and F. Marafini, Embrittlement break-down of steel upon electrolytic hydrogen saturation. Atonmega Energia 49, 22-25 (1980).
3. G. Biggiero and A. Borruto, Delayed fracture as parficular case of premature fracture. Miami Int. Symp. Metal-Hydrogen Systems,pp. 259-270. Pergamon Press, Oxford (1981). 4. G. Biggiero and A. Borruto, The alternative fragilization and the recovery of mechanical properties emphasized with the method of preaged fracture. Alternative Energy Sources VI- Wind/Ocean/Nuc~ear/Hydroger& 587-605 (1983). 5. G. Biggiero and A. Borruto, Mechanical behaviour of sorbitic and austenitic steels in strongly hydrogenating environments: relevant use prospects. ht. J. Hydrogen Energy 13,743-747 (1988). 6. G. Biggiero and A. Borruto, The behaviour of AISI 304 austenitic stainless steel under traction during hydrogen discharge. Int. J. Hydrogen Energy 15, 187-191 (1990).
STEEL EMBRITTLEMENT 7. G. Biggiero, A. Borruto, and S. Amato, Differences in the trend of strain and fracture of AISI 9840 steel pre-charged in hydrogen in cases of traction with or without hydrogen discharge. Proc. 2nd Cairo Znt. Symp. Renewable Energy Sources 2, 659-668 (1990). 8. H. Margot Marette, B. Marandet and J. C. Charbonnier, MJmoires Scientijiques Revue Metalluryie, pp. 175, March 1976 I 1990).
DUE TO HYDROGEN
Ii9
9. P. Bastien, V” Colloquie de Metallurgic. Le Ga: duns ic Metuux, p. 2. Universiti de France (1962). 10. P. Bastien, VIII” Colloquie de Metullurgie. Lu Rupiurc Differ&s, p. I. Universite de France (1965). 11. J. P. Hirth, Met&. Trans. llA, 869 (1980). 12. T. Asaoka, Proc. Miami ht. Symp Metal--H.vdrt>s~en .~~.cfcms. pp. 197. Pergamon Press, Oxford (1984).