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The influence of a coal gasifier atmosphere on fatigue crack growth rates in BS 4360 steel
Int J Fatigue 15 No 1 (1993) pp 27-30
The influence of a coal gasifier atmosphere on fatigue crack growth rates in BS 4360 steel P.J. Cotterill and J.E. King
Fatigue crack growth tests have been carried out in a number of gaseous environments in order to assess their effects on the crack propagation resistance of BS 4360 grade 50EE, a weldable structural steel. Crack growth rates at 25 °C are up to 20 times higher in hydrogen than in air, but there is no effect when hydrogen is present as a 30% constituent of a simplified product gas (SPG). Indeed, crack growth rates in such a mixture are slightly lower than those measured in air, being comparable with those observed in an inert environment. The other gases present in the SPG are CO, CO2 and CH4, and it is probable that the carbon monoxide is responsible for nullifying the embrittling effects of hydrogen, by preferentially adsorbing on to the surface of the steel and thus blocking hydrogen entry. Experimental observations suggest that oxygen has the same effect when small quantities are allowed to diffuse into a non-flowing hydrogen environment around a propagating crack. The results are encouraging in terms of the suitability of conventional structural steels such as BS 4360 for gas plant applications. The gas mixtures present in such an environment would not have the severe detrimental effects on fatigue crack growth resistance which result from the presence of 'pure' hydrogen.
Key words: structural steel; embrittlement; gas production
The work presented here was carried out as part of a project designed to assess the effect of the presence of hydrogencontaining environments on fatigue crack growth resistance in BS 4360 steel. BS 4360 grade 50EE is a specification for weldable structural steels which are candidate materials for use in gas-production plants. Of particular interest was the effect on crack propagation of the presence of a simplified product gas (SPG). This is a gas mixture containing H2, CO, CO2 and CH4, which are the major products of coal gasification. The first part of the project was concerned with the effects of a 'pure' hydrogen environment on fatigue crack growth rates, and these results are already in the literature) They showed that the presence of hydrogen could increase da/dN by up to 20 times at 25 °C, and to a lesser extent at 50 °C. The higher growth rates in hydrogen were associated with the incidence of transgranular quasi-cleavage cracking, in contrast to the ductile striation mechanism of fatigue in air. The significant effect of hydrogen on the fatigue resistance of BS 4360 steel is a cause for concern. It has been observed, 2-4 however, that the presence of other species in a hydrogencontaining gas mixture can diminish such growth rate enhancements in steels, and a determination of the extent to which this occurs in the simplified product gas was the major aim of the work presented here.
Experimental procedure The simplified product gas used for this work consisted of 60% CO, 30% H2, 5% CO2 and 5% CH4. Fatigue tests were
performed on adapted compact tension (CT) specimens (Fig. 1), which were machined from a cross-rolled BS 4360 steel plate, so that cracks could be grown in the shorttransverse direction. These were tested in a 50 kN capacity screw-driven Dartec machine (operating in load control), fitted with an environmental chamber which formed part of a gas-supply system. The system was capable of providing a near-constant flow of either hydrogen, SPG or argon around
6 BA threaded holes for connection of current leads
1.6
mm ~-~
I III I I
1S m m
30 mm
•
10 mm I t
4 mm
12
mm[ •
----
L
lI
¢~
~,~
6.9ram
• angle
I
27 mm
20 mm
Loading pin diameter D= 6.25 mm
Fig. 1 Design and dimensions of the adapted compact tension
specimen
0142-1123/93/010027-04 © 1993 Butterworth-Heinemann Ltd Int J Fatigue January 1993
27
the specimen at very low flow rates (2.5 x 10 -6 m 3 s -1 for this work). Heating was provided by a tape, wrapped around the outside of the chamber, and the temperature was controlled to within 1 °C. Crack growth was monitored using a d.c. potential drop technique and all tests followed a constant load/increasing AK procedure, using a load ratio of R = 0.1. Each specimen was pre-cracked to a length of 1 mm prior to loading in the Dartec, and all testing was carried out at a frequency of 0.1 Hz. After each test, one half of the broken specimen was examined fractographically using a Camscan $4 scanning electron microscope. Further details of the testing procedure, the validation of the adapted CT specimen design and a full characterization of the microstructure (ferrite/pearlite) of the BS 4360 plate may be found elsewhere.1
of d a / d N are insensitive to temperature in the range 25-80 °C and similar to those measured in argon (Fig. 2(a)). This is particularly evident at 80 °C (Fig.2(c)), where an oxidauon contribution ~'5 to crack growth in air leads to noticeably higher growth rates than those measured in the SPG. The absence of embrittlement by the SPG is confirmed by the fractographic evidence. The dark quasi-cleavage facets (Fig. 3(b)), which are characteristic of the mechanism of embrittled crack growth in hydrogen, ~ are not observed. Crack propagation is by a ductile striation mechanism, and this is reflected in the values of Paris gradient measured in the SPG (Table 1), which are relatively constant, and significantly lower than those measured in hydrogen. Similar observations have previously been reported 2-+ concerning other hydrogen-containing gas mixtures. Fransden and Marcus, 2 who investigated fatigue of steels in a number of primary and binary gas systems, found that fatigue crack growth rates in hydrogen were an order of magnitude higher than those in vacuum, but in gas mixtures of H2 with CO and H2 with 02 the effect was dramatically reduced to around a factor of two. There was, however, little influence of the presence of either CO2 or CH4 on the embrittling effect of hydrogen. Spitzig et aP reported that the presence of water vapour also has an affect on the severity of hydrogen embrittlement, similar to that of 02 and CO. Fatigue crack growth rates in moist hydrogen were observed to be three times lower than those measured in dry hydrogen, and close to the values recorded in an inert environment. The fractographic observations in this case were similar to the present findings, in that cleavage-type features were noted on the fatigued surfaces of specimens tested in dry hydrogen, but not on those tested in moist hydrogen, where no embrittlement occurred. The role of gases such as 02 and CO in diminishing the effects of hydrogen embrittlement is generally explained in terms of the relative affinities for adsorption of each gas species. 2-4 If oxygen and carbon monoxide preferentially adsorb on to the freshly created steel surface produced by fatigue, they will quickly occupy the available sites. These would otherwise have been used by hydrogen, and hence embrittlement is reduced by the limitation of hydrogen entry into the steel. Such a theory is supported by the adsorption studies on steel of Srikrishnan and Ficalora, 6 which show that
Results Figure 2 shows the fatigue crack propagation data collected in each gaseous environment at (a) 25 °C, (b) 50 °C and (c) 80 °C. At each temperature, crack growth rates in the simplified product gas are similar to or less than those measured in air. The enhancement in d a / d N due to embrittlement, which was seen during testing in hydrogen, is not observed in the SPG and crack growth rates are independent of temperature and similar to those seen in argon (Fig. 2(a)). These trends in the fatigue crack growth data are supported by the fractographic evidence presented in Fig. 3. The surface features produced at 25 °C in the SPG (Fig. 3(c)) are very similar to those resulting from crack growth in air (Fig. 3(a)) and argon. There is no evidence of the formation of quasi-cleavage facets of the type noted on fatigued surfaces produced in hydrogen (Fig. 3(b)).
Discussion Fatigue in the simplified product gas The results presented in Fig. 2 show that the embrittling effect of hydrogen on BS 4360 steel is severe when it is present as a 'pure' gas, but non-existent when it is a 30% constituent of the simplified product gas. Indeed, the SPG appears to be an inert environment in terms of fatigue crack growth. Values
. . . .
,
....
Simplified Laboratory Hydrogen Argon
• O •
, ....
, ....
, ....
,
..,...,
,
gas
product
air
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•
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,
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°°
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. . . .
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....
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I . . . . . . . . .
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~K(MPa ~/m)
,..+.,
45 50 55
.
b
.
.
.
i
25
. . . .
i
. . . .
,
....
,
.,
....
,
•
. . . .
,
30 35 40 0*5 50 55 ~K(MPa/rn)
c
,
....
25
, . . . . . . .
30 35
, ....
40
AK(MPa/m)
,
...,....,
45 50 55
Fig. 2 Fatigue crack growth rates measured in a r a n g e o f gaseous environments at (a) 25 °C, (b) 50 °C a n d (c) 80 °C
28
Int J Fatigue January 1993
! -
Direction of crack growth
Fig, 3 Fraetographs of fatigue surfaces produced at 25°C and z~K= 35 MPa ~/m in (a) air, (b) hydrogen and (c) simplified product gas
Table1. Paris gradients (m) measured from fatigue data produced at 25, 50 and 80 °C in each environment
Intermittent hydrogen Flowing hydrogen Simplified product gas Resumptions of hydrogen flow
o x
Testing temperature Value of m in air Value of m in hydrogen Value of m in the SPG Value of m in argon
25 °C 4.2 8.7 4.9 4.6
50 °C 4.1 6.6 3.9
80 °C 4.0 6.1 5.0
• •
10-3
•
•
l
.
e
I u >. u
E E
10-" the presence of both oxygen and carbon monoxide inhibits hydrogen entry, whereas the presence of carbon dioxide has little effect. These results confirm the findings of Fransden and Marcus, 2 and indicate that it is the carbon monoxide in the simplified product gas which suppresses the embrittling effects of hydrogen.
Fatigue in intermittently flowing hydrogen The tests conducted in the SPG have shown the effectiveness of carbon monoxide in preventing hydrogen embrittlement, but the influence of oxygen has also been noted during the current work, during a test where the specimen was subjected to intermittent hydrogen flow. In the environmental chamber, the maintenance of the test environment depended on there being a steady flow of the test gas around the specimen. In this particular test, however, there were several periods where the hydrogen flow stopped and was then restarted. The crack growth rate variations throughout the test are shown in Fig. 4. The data for crack growth in intermittent hydrogen show that when flow is maintained, crack growth rates are characteristically high, but that when it ceases da/dN gradually drops to values similar to those measured in the simplified product gas. For values of ~ ~ 26 MPa V~m, when hydrogen flow recommences (at the values of fia~ indicated by the × symbols in Fig. 4), growth rates rapidly recover to levels characteristic of fatigue in flowing hydrogen, and follow the hydrogen curve until the next incidence of flow cessation. At values of M~ above this, however, the rise in da/dN is insufficient for such a complete recovery, and growth rates are no longer significantly higher than those measured in the simplified product gas. To explain this behaviour it is important to note that the environmental chamber was not entirely gas-tight and that it contained a number of 'stagnant zones' remote from the specimen, where no flow occurred under normal testing conditions. During the periods (of up to 15 h) in the
Int J Fatigue January 1993
"O
10-5
X l
21
,
X I
22
X .
t
X .
I
.
i
X ,
t
.
l
.
X l
,
I
,
I.
I
,
I
.
I
.
I
.
i
23 24 25 26 27 28 29 30 31 32 33 34 35 AK(MPa ~'m)
Fig. 4 Fatigue crack growth rates measured in an intermittent supply of flowing hydrogen at 25 °C. The x symbols mark values o f / t K at which hydrogen flow was resumed
intermittent test when there was no hydrogen flow, air could diffuse from these zones into the region around the test piece, and it is likely that the gradual reduction in growth rates (Fig. 4) results from preferential adsorption of the oxygen in the air on to the fatigued steel surfaces, particularly those formed at the crack tip. The embrittling effects of hydrogen are progressively reduced as more 02 diffuses into the region around the specimen and, when the concentration of oxygen is sufficiently high, crack growth rates fall as low as those measured in the SPG. When hydrogen flow is recommenced, any oxygen adjacent to the crack tip is rapidly flushed away, allowing the return of the faster crack growth associated with hydrogen embrittlement. These observations suggest that the role of oxygen in this case, which is to reduce the embrittling effect of hydrogen by preferential adsorption, is similar to that played by the carbon monoxide in the SPG. This conclusion is supported by fractographic evidence (Fig. 5), which shows that the fatigued surface of the specimen tested in intermittent flowing hydrogen consists of a series of light and dark bands (Fig. 5(a)). The dark bands result from crack growth while a flow was being maintained, and closer examination (Fig. 5(b))
29
30 ~Jm -
b
30 IJm . . . .
Direction of crack growth
Fig. 5 Fatigue surface produced by crack growth at 25 °C under intermittent hydrogen flow, showing: (a) light and dark bands of growth; (b) quasi-cleavage facets in the dark bands; and (c) ductile striation growth in the light bands
shows that they contain quasi-cleavage markings, which are characteristic of crack growth in hydrogen (Fig. 3(b)). In contrast, the light bands were formed during periods when there was no hydrogen flow, and the fatigued surface displays ductile striation markings (Fig. 5(c)), which are similar to those produced in the simplified product gas (Fig. 3(c)).
the Fellowship of Engineering for financial support. They are particularly grateful to Robert Owen of British Gas Midland Research Station for his continual support throughout the project, and to those members of the department in Cambridge who helped in the preparation of the testing apparatus.
References Conclusions 1) 2)
3)
4)
The embrittling effect of a hydrogen atmosphere on BS 4360 steel is not observed when hydrogen is present as part of a simplified product gas mixture. Embrittlement is prevented in the SPG environment by the preferential adsorption of carbon monoxide on to the freshly created fatigued surface, which limits hydrogen entry into the steel. As there is no embrittlement, and in the absence of an oxidizing environment, fatigue crack growth rates in the SPG are insensitive to temperature (25-80 °C) and similar to those in an inert gas. Embrittlement is also prevented by the preferential adsorption of oxygen, if air is allowed to diffuse into the region around a specimen while it is being fatigued in a non-flowing hydrogen environment.
1.
2.
3.
4.
5. 6.
Cotterill, P.J. and King, J.E. 'Hydrogen embrittlement contributions to fatigue crack growth in a structural steel' /nt J Fatigue 13 (1991) pp 447-452 Fransden, J.D. and Marcus, H.L. 'Environmentally assisted fatigue crack propagation in steels' Met Trans 8A (1977) pp 265-272 Spitzig, W.A., Talda, P.M. and Wei, R.P. 'Fatigue crack propagation and fractographical analysis of 18Ni (250) maraging steel tested in argon and hydrogen environments' Eng Fract Mech 1 (1968) pp 155-166 Sudarashen, T.S. and Louthan, M.R. 'Gaseous environment effects on fatigue behaviour of metals' Int Mater Rev 32 (1987) pp 121-152 King, J.E. and Cotterill, P.J. 'The role of oxides in fatigue crack propagation' J Mater Sci Tech 6 (1989) pp 19-31 Srikrishnan, V. and Ficalora, P.J. 'Selective adsorption and hydrogen embrittlement' Met Trans 7A (1976) pp 1669-1675
Authors Acknowledgements The authors are grateful to Professor D. Hull, FRS for provision of laboratory facilities in the Department of Materials Science and Metallurgy (Cambridge) and to British Gas and
30
P.j. Cotterill is with the IRC in Materials for High Performance Applications, Edgbaston, Birmingham B 15 2TT, UK. J.E. King is with the Department of Materials Science and Metallurgy, University of Cambridge, UK. Received 1 July 1992; accepted 17 July 1992.
Int J Fatigue J a n u a r y 1993
The influence of a coal gasifier atmosphere on fatigue crack growth rates in BS 4360 steel P.J. Cotterill and J.E. King
Fatigue crack growth tests have been carried out in a number of gaseous environments in order to assess their effects on the crack propagation resistance of BS 4360 grade 50EE, a weldable structural steel. Crack growth rates at 25 °C are up to 20 times higher in hydrogen than in air, but there is no effect when hydrogen is present as a 30% constituent of a simplified product gas (SPG). Indeed, crack growth rates in such a mixture are slightly lower than those measured in air, being comparable with those observed in an inert environment. The other gases present in the SPG are CO, CO2 and CH4, and it is probable that the carbon monoxide is responsible for nullifying the embrittling effects of hydrogen, by preferentially adsorbing on to the surface of the steel and thus blocking hydrogen entry. Experimental observations suggest that oxygen has the same effect when small quantities are allowed to diffuse into a non-flowing hydrogen environment around a propagating crack. The results are encouraging in terms of the suitability of conventional structural steels such as BS 4360 for gas plant applications. The gas mixtures present in such an environment would not have the severe detrimental effects on fatigue crack growth resistance which result from the presence of 'pure' hydrogen.
Key words: structural steel; embrittlement; gas production
The work presented here was carried out as part of a project designed to assess the effect of the presence of hydrogencontaining environments on fatigue crack growth resistance in BS 4360 steel. BS 4360 grade 50EE is a specification for weldable structural steels which are candidate materials for use in gas-production plants. Of particular interest was the effect on crack propagation of the presence of a simplified product gas (SPG). This is a gas mixture containing H2, CO, CO2 and CH4, which are the major products of coal gasification. The first part of the project was concerned with the effects of a 'pure' hydrogen environment on fatigue crack growth rates, and these results are already in the literature) They showed that the presence of hydrogen could increase da/dN by up to 20 times at 25 °C, and to a lesser extent at 50 °C. The higher growth rates in hydrogen were associated with the incidence of transgranular quasi-cleavage cracking, in contrast to the ductile striation mechanism of fatigue in air. The significant effect of hydrogen on the fatigue resistance of BS 4360 steel is a cause for concern. It has been observed, 2-4 however, that the presence of other species in a hydrogencontaining gas mixture can diminish such growth rate enhancements in steels, and a determination of the extent to which this occurs in the simplified product gas was the major aim of the work presented here.
Experimental procedure The simplified product gas used for this work consisted of 60% CO, 30% H2, 5% CO2 and 5% CH4. Fatigue tests were
performed on adapted compact tension (CT) specimens (Fig. 1), which were machined from a cross-rolled BS 4360 steel plate, so that cracks could be grown in the shorttransverse direction. These were tested in a 50 kN capacity screw-driven Dartec machine (operating in load control), fitted with an environmental chamber which formed part of a gas-supply system. The system was capable of providing a near-constant flow of either hydrogen, SPG or argon around
6 BA threaded holes for connection of current leads
1.6
mm ~-~
I III I I
1S m m
30 mm
•
10 mm I t
4 mm
12
mm[ •
----
L
lI
¢~
~,~
6.9ram
• angle
I
27 mm
20 mm
Loading pin diameter D= 6.25 mm
Fig. 1 Design and dimensions of the adapted compact tension
specimen
0142-1123/93/010027-04 © 1993 Butterworth-Heinemann Ltd Int J Fatigue January 1993
27
the specimen at very low flow rates (2.5 x 10 -6 m 3 s -1 for this work). Heating was provided by a tape, wrapped around the outside of the chamber, and the temperature was controlled to within 1 °C. Crack growth was monitored using a d.c. potential drop technique and all tests followed a constant load/increasing AK procedure, using a load ratio of R = 0.1. Each specimen was pre-cracked to a length of 1 mm prior to loading in the Dartec, and all testing was carried out at a frequency of 0.1 Hz. After each test, one half of the broken specimen was examined fractographically using a Camscan $4 scanning electron microscope. Further details of the testing procedure, the validation of the adapted CT specimen design and a full characterization of the microstructure (ferrite/pearlite) of the BS 4360 plate may be found elsewhere.1
of d a / d N are insensitive to temperature in the range 25-80 °C and similar to those measured in argon (Fig. 2(a)). This is particularly evident at 80 °C (Fig.2(c)), where an oxidauon contribution ~'5 to crack growth in air leads to noticeably higher growth rates than those measured in the SPG. The absence of embrittlement by the SPG is confirmed by the fractographic evidence. The dark quasi-cleavage facets (Fig. 3(b)), which are characteristic of the mechanism of embrittled crack growth in hydrogen, ~ are not observed. Crack propagation is by a ductile striation mechanism, and this is reflected in the values of Paris gradient measured in the SPG (Table 1), which are relatively constant, and significantly lower than those measured in hydrogen. Similar observations have previously been reported 2-+ concerning other hydrogen-containing gas mixtures. Fransden and Marcus, 2 who investigated fatigue of steels in a number of primary and binary gas systems, found that fatigue crack growth rates in hydrogen were an order of magnitude higher than those in vacuum, but in gas mixtures of H2 with CO and H2 with 02 the effect was dramatically reduced to around a factor of two. There was, however, little influence of the presence of either CO2 or CH4 on the embrittling effect of hydrogen. Spitzig et aP reported that the presence of water vapour also has an affect on the severity of hydrogen embrittlement, similar to that of 02 and CO. Fatigue crack growth rates in moist hydrogen were observed to be three times lower than those measured in dry hydrogen, and close to the values recorded in an inert environment. The fractographic observations in this case were similar to the present findings, in that cleavage-type features were noted on the fatigued surfaces of specimens tested in dry hydrogen, but not on those tested in moist hydrogen, where no embrittlement occurred. The role of gases such as 02 and CO in diminishing the effects of hydrogen embrittlement is generally explained in terms of the relative affinities for adsorption of each gas species. 2-4 If oxygen and carbon monoxide preferentially adsorb on to the freshly created steel surface produced by fatigue, they will quickly occupy the available sites. These would otherwise have been used by hydrogen, and hence embrittlement is reduced by the limitation of hydrogen entry into the steel. Such a theory is supported by the adsorption studies on steel of Srikrishnan and Ficalora, 6 which show that
Results Figure 2 shows the fatigue crack propagation data collected in each gaseous environment at (a) 25 °C, (b) 50 °C and (c) 80 °C. At each temperature, crack growth rates in the simplified product gas are similar to or less than those measured in air. The enhancement in d a / d N due to embrittlement, which was seen during testing in hydrogen, is not observed in the SPG and crack growth rates are independent of temperature and similar to those seen in argon (Fig. 2(a)). These trends in the fatigue crack growth data are supported by the fractographic evidence presented in Fig. 3. The surface features produced at 25 °C in the SPG (Fig. 3(c)) are very similar to those resulting from crack growth in air (Fig. 3(a)) and argon. There is no evidence of the formation of quasi-cleavage facets of the type noted on fatigued surfaces produced in hydrogen (Fig. 3(b)).
Discussion Fatigue in the simplified product gas The results presented in Fig. 2 show that the embrittling effect of hydrogen on BS 4360 steel is severe when it is present as a 'pure' gas, but non-existent when it is a 30% constituent of the simplified product gas. Indeed, the SPG appears to be an inert environment in terms of fatigue crack growth. Values
. . . .
,
....
Simplified Laboratory Hydrogen Argon
• O •
, ....
, ....
, ....
,
..,...,
,
gas
product
air
•° •
:++acT+ .................. f
: .%r~tOn~Y+,r
10 -2 ~
o
ooO ° •
...
•
:-"
a
,
,
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. . . .
°°
f
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...:"
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,,','o~
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•
•
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,
product
Hydrogen
10 -2
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A
oo~
:
u
"0
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•
10 -2
uE 1 O_3 E
• o •
f
7f
i0-4
i0_4
coo
,
. . . .
30
,
....
35
I . . . . . . . . .
40
~K(MPa ~/m)
,..+.,
45 50 55
.
b
.
.
.
i
25
. . . .
i
. . . .
,
....
,
.,
....
,
•
. . . .
,
30 35 40 0*5 50 55 ~K(MPa/rn)
c
,
....
25
, . . . . . . .
30 35
, ....
40
AK(MPa/m)
,
...,....,
45 50 55
Fig. 2 Fatigue crack growth rates measured in a r a n g e o f gaseous environments at (a) 25 °C, (b) 50 °C a n d (c) 80 °C
28
Int J Fatigue January 1993
! -
Direction of crack growth
Fig, 3 Fraetographs of fatigue surfaces produced at 25°C and z~K= 35 MPa ~/m in (a) air, (b) hydrogen and (c) simplified product gas
Table1. Paris gradients (m) measured from fatigue data produced at 25, 50 and 80 °C in each environment
Intermittent hydrogen Flowing hydrogen Simplified product gas Resumptions of hydrogen flow
o x
Testing temperature Value of m in air Value of m in hydrogen Value of m in the SPG Value of m in argon
25 °C 4.2 8.7 4.9 4.6
50 °C 4.1 6.6 3.9
80 °C 4.0 6.1 5.0
• •
10-3
•
•
l
.
e
I u >. u
E E
10-" the presence of both oxygen and carbon monoxide inhibits hydrogen entry, whereas the presence of carbon dioxide has little effect. These results confirm the findings of Fransden and Marcus, 2 and indicate that it is the carbon monoxide in the simplified product gas which suppresses the embrittling effects of hydrogen.
Fatigue in intermittently flowing hydrogen The tests conducted in the SPG have shown the effectiveness of carbon monoxide in preventing hydrogen embrittlement, but the influence of oxygen has also been noted during the current work, during a test where the specimen was subjected to intermittent hydrogen flow. In the environmental chamber, the maintenance of the test environment depended on there being a steady flow of the test gas around the specimen. In this particular test, however, there were several periods where the hydrogen flow stopped and was then restarted. The crack growth rate variations throughout the test are shown in Fig. 4. The data for crack growth in intermittent hydrogen show that when flow is maintained, crack growth rates are characteristically high, but that when it ceases da/dN gradually drops to values similar to those measured in the simplified product gas. For values of ~ ~ 26 MPa V~m, when hydrogen flow recommences (at the values of fia~ indicated by the × symbols in Fig. 4), growth rates rapidly recover to levels characteristic of fatigue in flowing hydrogen, and follow the hydrogen curve until the next incidence of flow cessation. At values of M~ above this, however, the rise in da/dN is insufficient for such a complete recovery, and growth rates are no longer significantly higher than those measured in the simplified product gas. To explain this behaviour it is important to note that the environmental chamber was not entirely gas-tight and that it contained a number of 'stagnant zones' remote from the specimen, where no flow occurred under normal testing conditions. During the periods (of up to 15 h) in the
Int J Fatigue January 1993
"O
10-5
X l
21
,
X I
22
X .
t
X .
I
.
i
X ,
t
.
l
.
X l
,
I
,
I.
I
,
I
.
I
.
I
.
i
23 24 25 26 27 28 29 30 31 32 33 34 35 AK(MPa ~'m)
Fig. 4 Fatigue crack growth rates measured in an intermittent supply of flowing hydrogen at 25 °C. The x symbols mark values o f / t K at which hydrogen flow was resumed
intermittent test when there was no hydrogen flow, air could diffuse from these zones into the region around the test piece, and it is likely that the gradual reduction in growth rates (Fig. 4) results from preferential adsorption of the oxygen in the air on to the fatigued steel surfaces, particularly those formed at the crack tip. The embrittling effects of hydrogen are progressively reduced as more 02 diffuses into the region around the specimen and, when the concentration of oxygen is sufficiently high, crack growth rates fall as low as those measured in the SPG. When hydrogen flow is recommenced, any oxygen adjacent to the crack tip is rapidly flushed away, allowing the return of the faster crack growth associated with hydrogen embrittlement. These observations suggest that the role of oxygen in this case, which is to reduce the embrittling effect of hydrogen by preferential adsorption, is similar to that played by the carbon monoxide in the SPG. This conclusion is supported by fractographic evidence (Fig. 5), which shows that the fatigued surface of the specimen tested in intermittent flowing hydrogen consists of a series of light and dark bands (Fig. 5(a)). The dark bands result from crack growth while a flow was being maintained, and closer examination (Fig. 5(b))
29
30 ~Jm -
b
30 IJm . . . .
Direction of crack growth
Fig. 5 Fatigue surface produced by crack growth at 25 °C under intermittent hydrogen flow, showing: (a) light and dark bands of growth; (b) quasi-cleavage facets in the dark bands; and (c) ductile striation growth in the light bands
shows that they contain quasi-cleavage markings, which are characteristic of crack growth in hydrogen (Fig. 3(b)). In contrast, the light bands were formed during periods when there was no hydrogen flow, and the fatigued surface displays ductile striation markings (Fig. 5(c)), which are similar to those produced in the simplified product gas (Fig. 3(c)).
the Fellowship of Engineering for financial support. They are particularly grateful to Robert Owen of British Gas Midland Research Station for his continual support throughout the project, and to those members of the department in Cambridge who helped in the preparation of the testing apparatus.
References Conclusions 1) 2)
3)
4)
The embrittling effect of a hydrogen atmosphere on BS 4360 steel is not observed when hydrogen is present as part of a simplified product gas mixture. Embrittlement is prevented in the SPG environment by the preferential adsorption of carbon monoxide on to the freshly created fatigued surface, which limits hydrogen entry into the steel. As there is no embrittlement, and in the absence of an oxidizing environment, fatigue crack growth rates in the SPG are insensitive to temperature (25-80 °C) and similar to those in an inert gas. Embrittlement is also prevented by the preferential adsorption of oxygen, if air is allowed to diffuse into the region around a specimen while it is being fatigued in a non-flowing hydrogen environment.
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Cotterill, P.J. and King, J.E. 'Hydrogen embrittlement contributions to fatigue crack growth in a structural steel' /nt J Fatigue 13 (1991) pp 447-452 Fransden, J.D. and Marcus, H.L. 'Environmentally assisted fatigue crack propagation in steels' Met Trans 8A (1977) pp 265-272 Spitzig, W.A., Talda, P.M. and Wei, R.P. 'Fatigue crack propagation and fractographical analysis of 18Ni (250) maraging steel tested in argon and hydrogen environments' Eng Fract Mech 1 (1968) pp 155-166 Sudarashen, T.S. and Louthan, M.R. 'Gaseous environment effects on fatigue behaviour of metals' Int Mater Rev 32 (1987) pp 121-152 King, J.E. and Cotterill, P.J. 'The role of oxides in fatigue crack propagation' J Mater Sci Tech 6 (1989) pp 19-31 Srikrishnan, V. and Ficalora, P.J. 'Selective adsorption and hydrogen embrittlement' Met Trans 7A (1976) pp 1669-1675
Authors Acknowledgements The authors are grateful to Professor D. Hull, FRS for provision of laboratory facilities in the Department of Materials Science and Metallurgy (Cambridge) and to British Gas and
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P.j. Cotterill is with the IRC in Materials for High Performance Applications, Edgbaston, Birmingham B 15 2TT, UK. J.E. King is with the Department of Materials Science and Metallurgy, University of Cambridge, UK. Received 1 July 1992; accepted 17 July 1992.
Int J Fatigue J a n u a r y 1993