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Identification and quantification of hydrogen attack
Int. J. Pres. Ves. & Piping 46 (1991) 113-124
l Identification and Quantification of Hydrogen Attack
Y. Houbaert & J. Dilewijns Lab. Algemene Metallurgie, Siderurgie en Fysische Metaalkunde, Rijksuniversiteit Gent, Sint Pietersnieuwstraat 41 B-9000 Gent, Belgium (Received 23 August 1990; accepted 15 November 1990)
ABSTRACT The occurrence of hydrogen attack related to the presence of methane in the steel cavities can be identified. This can be done by using the described annealing and metallographic testing procedure. To quantify the remaining safety of hydrogen-attacked industrial equipment, the degree of hydrogen damage can be measured by means of metaUography, ultrasonic testing and chemical H-determination. A relation has been established between these different testing procedures. The fracture-mechanical properties of the damaged material have been determined. The results are expressed in terms of strength ratio. A first attempt is made to give an approximation of the remaining safety of the damaged material.
1 INTRODUCTION Several European fertilizer and petrochemical industries were built some 15-20 years ago. Some parts of these plants are working under conditions that might cause hydrogen attack. The industry is worried about the possible occurrence of hydrogen attack and about the problem of how much remaining safety does still exist in partially attacked pipes and how this can be checked by a non-destructive method. In the present paper, a metallographic examination procedure is described and some mechanical test results on a ferritic steel (16Mo5) 113 Int. J. Pres. Ves. & Piping 0308-0161/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
114
Y. Houbaert, J. Dilewijns
with 0.2% C and 0.5% Mo are reported. Most of the examined material was exposed to an atmosphere containing hydrogen at 150 bar partial hydrogen pressure, 220 bar total gas pressure and a temperature of 285 °C during more than 15 years. The investigated samples were taken from a pipe with 75 mm wall thickness.
2 EFFECTS OF H Y D R O G E N ON STEEL PIPE A ferritic steel exposed to a hydrogen atmosphere may experience the following problems. - - H y d r o g e n embrittlement: this occurs mainly in high-strength steels, exposed to hydrogen at temperatures between - 7 0 and +140 °C. No problems of this type to be expected for the fertilizer industries, working at temperatures about 300 °C. - - H y d r o g e n blistering: when dissolved hydrogen atoms recombine to molecular hydrogen, blisters may grow on steel inclusions. This is not a major industrial problem, because the blisters are easily detectable and they have an orientation that is parallel to the maximum stresses in the pipe. --Decarburization: here we have to distinguish between surface and volume decarburization. The first one is generally not an important problem, but the internal decarburization, also called hydrogen attack, has caused a lot of damage in industrial installations over recent years. The basic mechanism of hydrogen attack is as follows: atomic hydrogen, diffusing through the steel, reacts locally with the carbon of cementite in pearlite colonies, leading to the production of cavities filled with methane gas at a high pressure. The reaction equation can be simplified to: C + 4H = CH4
(1)
The 'hydrogen attack' or 'hot hydrogen damage' is a complex problem, because it is not easily recognized and it often extends over some localized, but large areas of the equipment, reducing drastically the toughness of the material. When a critical degree of attack is reached, it may lead to a sudden catastrophic crack propagation and explosion. Such accidents have already occurred in hydrogen treating installations, with considerable human and material injury.
Identification and quantification of hydrogen attack
115
3 I D E N T I F I C A T I O N OF H Y D R O G E N A T T A C K BY METALLOGRAPHY Figure 1 shows the usual appearance of a plain carbon steel (e.g. St.35.8, DIN 17175) after heavy hydrogen attack: the attack was localized near a weld. Molybdenum is known to reduce the likeliness of steel to present hydrogen attack, but Fig. 2 shows a 16Mo5-steel (0-2% C, 0.5% Mo) with evident hydrogen damage: part of a pearlite colony was transformed to ferrite, the carbon reacted with hydrogen (see reaction (1)), producing a methane cavity in the grain boundary. These are two clear examples of hydrogen attack, but in some occasions the metallographic aspect and the position of the cavities does not give a definite indication as to whether there has been hydrogen attack or not. In these cases, an additional study must be carried out to check the origin of the observed cavities, which may be creep cavities as well as H A cavities. The samples are normally polished in a conventional way: grinding with silicon carbide and polishing with diamond paste on a relatively hard cloth (in order to avoid the smearing of material into the cavities). It is advisable to observe the samples without etching, to avoid confusion with pitting effects due to etching.
Fig. 1. Plain carbon steel St.35.8 exposed to H 2 Of 1-32 MPa = 13.2 bar at 290°C during 15 years---not etched---originalmagnification5-3x. (Wall thickness, 7-5 mm.)
Y. Houbaert, J. Dilewijns
116
Fig. 2.
Steel 16Mo5 exposed to H2 at 175 bar and 295 °C for 10 years---SEM, etched--original magnification 3100x.
The prepared samples can be observed with a light microscope or even better with a scanning electron microscope. We have found 1 that, if the gas in the cavities is methane, it can be dissociated by heating at a temperature above a definite equilibrium value. Figures 3 and 4 give an example of these temperatures: the dissociation occurs at 450 °C (6 h) for a plain carbon steel (St.35.8) and for a 16Mo5 steel, at 560 °C (6 h) ppm 60
16 Mo 5
50 1.0 30 20 10
300
"~
450
l,h
500 520
560
"C
6h
Fig. 3. Variation of measured H2 content with changing annealing treatment. Material: 16Mo5.
Identification and quantification of hydrogen attack
117
ppmH2~ 50
13
Cr Mo
4 4
40 30 20 10 o •~ (
3OO"
~h
f
~so
I
I
el
|~v
soo s2o s.6o STO 6oo~ 'c 6~
Fig. 4. Variation of measured H2 content with changing annealing treatment. Material: 13CrMo44.
for a 13CrMo44 steel. The mechanism is supposed to be the following: at the appropriate temperature, the methane dissociates, the atomic hydrogen diffuses away through the lattice and the carbon precipitates again as cementite in the adjacent grain boundaries. The phenomenon of cementite precipitation can be observed in Fig. 5. The carbides present a clear and definite crystallographic orientation which may easily be measured and identified on SEM images.
Fig. 5.
Steel 16Mo5 (same working conditions as Fig. 2) after vacuum annealing of 6 h at 450 °C---etched, SEM--original magnification 1550x.
118
Y. Houbaert, J. Dilewijns
4 QUANTIFICATION OF THE HYDROGEN ATTACK
4.1 Metailography The above described metallographic technique is not quantitative and it is a destructive examination, except if a surface replica is taken. In many industrial applications, this method would not be applicable, because what has to be investigated is precisely volume decarburization. A usual, partially destructive, method is to take a root sample through the thickness of the pipe wall and afterwards to close this hole by welding.
4.2 Ultrasonic testing It should be emphasized that ultrasonic testing is a non-destructive testing procedure, but that the relation between a certain degree of hydrogen attack and the corresponding attenuation phenomenon of the ultrasonic signal is generally not well known. It is not easy to carry out accurate ultrasonic testing in connection with the problem of hydrogen attack. However, the Royal Dutch Shell Laboratories in Amsterdam performed very accurate scanning ultrasonic testing on some of our samples and obtained a clear map of the degree of attack. After selecting some adequate reference level of the ultrasonic signal, the depth and the degree of hydrogen attack can be represented in terms of transit time or amplitude. The transit time corresponds to a definite depth of attack defined by the degree of reflection of the ultrasonic signal on methane cavities. The amplitude of the ultrasonic signal at a definite depth of the material gives also an indication of the local degree of hydrogen attack. These procedures lead to quantitative diagrams where the fractional transit time or the amplitude are represented using a colour or a grey scale.
4.3 Hydrogen determination The most accurate method to determine the degree of hydrogen attack is the measurement of the hydrogen content in the sample. A 'Hydrogen Determinator' was used, based on the thermal conductivity of the gases escaping from a molten specimen. During the melting process, the methane is dissociated into carbon and molecular hydrogen and detected as such.
119
Identification and quanttfication of hydrogen attack Steel : 0J5 °l,C -0.50 %Mo l&O
130 120 '~' LLI
II0 1oo 9o-
ou z o "" O
8070605040-
302010i
0 INSIDE PIPE F ~ o 6o
20
i
4'0 DISTANCE [mm~
'
6'0
80 OUTSIDE PIPE
Typical variation of the hydrogen content (present as methane) versus wall thickness for a hydrogen damaged pipe.
Figure 6 shows the typical variation of the hydrogen content in a damaged specimen as a function of the thickness of the pipe wall. Three different zones can be distinguished: 1. A kind of saturation phenomenon occurs up to a depth of about 15 mm: the hydrogen content remains constant at about 120130 ppm. 2. A clear gradient is visible between 15 and 55 mm depth. 3. A zone without excessive hydrogen content is observed for depths over some 55 mm. Considering the methane reaction (1), it follows that the chemical activity (and in this manner the tendency to form cavities) increases according to a fourth power law with the dissolved hydrogen concentration. Supposing in a first approximation that the hydrogen gradient caused by diffusion is linear (steady state situation), the degree of hydrogen attack should present a fourth power law with the depth. Indeed, using a regression technique on the gradient zone, power laws with exponents varying between 3.8 and 4.2 are found.
4.4 Correlation of ultrasonic testing with hydrogen analysis As mentioned above, a correlation between non-destructive ultrasonic testing and destructive hydrogen determination is necessary to solve the problem described in the present paper.
Y. Houbaert, J. Dilewijns
120
Some of the hydrogen profiles which have been discussed in the previous paragraph were determined on a material after ultrasonic testing, in order to establish an exact relation between the attenuation factor and the hydrogen content. At the moment, it seems that the ultrasonic detection limit is some 80 ppm hydrogen, which is a very high value. A better resolution can surely be obtained by modifying the settings of the ultrasonic measurement equipment.
5 BASIC F R A C T U R E - M E C H A N I C A L A P P R O A C H
5.1 Principle A non-destructive determination of the remaining security of hydrogen damaged equipment can be performed using a basic fracturemechanical approach, consisting of the following steps: .
.
Elaboration of a model for the description of the stress intensity of the most critical crack, propagating through the wall of an industrial element (determination of the K~ value at the tip of a crack). Determination of the fracture-mechanical properties of the material as a function of the degree of hydrogen attack. This has to be done in two steps: first a quantification of the degree of hydrogen attack, which can be done in several ways, such as: metallography, hydrogen determination and ultra-sonic testing. The second step is the determination of the fracture-mechanical properties of the material, using different types of procedure, such as KIc test, COD test or J-integral.
5.2 Calculations The remaining resistance of the pipe wall to unstable crack propagation can be calculated using the procedure shown in Fig. 7. The calculation of the maximum stress in the pipe wall gives the curve No. 1 indicated by a. The calculation of the stress intensity K, at the tip of a crack growing radially through the wall thickness gives the curve No. 2. No details will be given here about the calculations of the curves 1 and 2, as these are problems of mechanical engineering which can easily be solved for different types of configurations (see, e.g. (Ref. 2)). Curve No. 3 represents the remaining fracture toughness as a function of the depth in the pipe wall. This curve should be obtained
Identification and quantification of hydrogen attack
121
® = f (H.A.)
i i i
I I I I
INNER SIDE 0
OUIER SIDE ?5ram
®
INNER SIDE
OUIER SIDE
II CRACK
NO CRACK GROWIII
75mm
X
Fig. 7. Schematic representation of the fracture-mechanicalapproach. from a relation between the fracture-mechanical data (e.g. Kic) and the quantified hydrogen damage (e.g. through H-determination, as in Fig.
6). It appears in this example that crack propagation may be expected up to a depth of x mm, but that further crack growth would normally not occur, unless the hydrogen attack would proceed to a greater depth. 5.3 Sampling The actual samples for this paper were taken from a pipe with 75 mm wall thickness. Although possible crack propagation would occur radially, the fracture-mechanical properties were not measured with cracks in this orientation, but instead as indicated in Fig. 8. This was
Y. Houbaert, J. Dilewijns
122
OUTSIDE PIPE
DEGREE OF HYDROGEN ATTACK m
@ -- ----J
_~POWER
LAW
z
1 INSIDE PIPE (~ PROBABLE GROWING DIRECTION OF CRACK UNDER WORKING CONDITIONS (~) CRACK GROWING THROUGH HOMOGENEOUS ATTACK ZONE
S
_
~
L WELDING "/ /
SAMPLES
FOR
KIc
-
SAMPLES FOR coo
J-INTEGRAL
Fig. 8. Schematic representation of the crack orientation and the sampling. done in order to let the crack grow through a zone with almost constant degree of hydrogen attack. Most of the samples had a thickness of 25 mm, allowing the selection of specimens with correct crack orientation and some possible variation of the crack position relative to the attack gradient. By welding some additional pieces, the crack location can be moved closer to the inner and outer sides of the pipe wall. As indicated in Fig. 8, samples have been taken according to the procedures for K~c, COD and J-integral testing. The welding of supporting pieces had no influence on the H-determination nor on the fracture-mechanical properties, if the crack was not too close to the heat affected zone. 5.4 Testing and results Different series of samples have been tested, with the crack advancing through zones with variable hydrogen content and variable response to ultrasonic testing.
Identification and quantification of hydrogen attack RSC 1.B5
1.75
1.65
1.66
123
\\
\\ \
RSC =
2 Pmctx (2W.a) = B (W-a) 2 YS
~notched ~'unnotched
\
*Xx
lJ.5
1.3S
1.25 0 Fig. 9.
~÷ 1
I
26
6o
r
r
,o0
HYDROGEN
,Z pm]
Strength ratio R,~ versus H content for hydrogen damaged steel 16Mo5.
Crack propagation tests were first performed on the K~¢ specimens. No relation could be found between the kinetics of the crack propagation (Paris-Erdogan) and the hydrogen content of the samples. Fracture-mechanical tests for the first series were all planned as Kk tests. Unfortunately, due to the still high ductility of the material, the results are not acceptable according to the ASTM standards. Nevertheless, one still can get some indication about the fracture-mechanical behaviour of the material by using the so-called 'strength ratio': R~. 3 This result is very hopeful and Fig. 9 shows that a clear relation exists between valid fracture-mechanical data and the degree of hydrogen attack measured by hydrogen analysis. 6 CONCLUSION The proposed metallographic technique (including an annealing treatment) may be used to prove the presence of methane in steel cavities. A correlation was found between ultrasonic testing and the H content and also between the H content and the fracture-mechanical data. It is possible to obtain fracture-mechanical information about a hydrogen attacked material using the results of a non-destructive ultrasonic test.
124
Y. Houbaert, J. Dilewijns
ACKNOWLEDGEMENT We want to thank Kemira b.v. for the industrially damaged material and the Royal Dutch Shell Laboratories for the US-testing.
REFERENCES 1. Mertens, D., Houbaert, Y. & Dilewijns, J., Metallographic indication of methane in steel cavities. In High Temperature Alloys: their Exploitable Potential, ed. J. B. Marriott, M. Merz, J. Nihoul & J. Wars. Elsevier Applied Science, London and New York, 1987, pp. 39-48. 2. Buchalet, C. B. & Bamford, W. H., Stress Intensity Factor Solutions for Continuous Surface Flaws in Reactor Pressure Vessels. ASTM, STP 590, 1976, pp. 285-402. 3. ASTM E399-83, Plane-strain Fracture Toughness of Metallic Materials, 1983.
BIBLIOGRAPHY 1. Houkaert, Y. & Dilewijns, J., Study of the toughness of hydrogen attacked steel. Proceedings of the International Conference on Interaction of Steels with Hydrogen in Petroleum Industry Pressure Vessel Service. Materials Properties Council, Inc., Paris, March 1989, pp. 167-71. 2. Derycke, L., Breukmechanische Beraderig van de Genolgen van Waterstofaanfasting. Thesis, University of Ghent, Faculty of Applied Sciences, Belgium, 1988.
l Identification and Quantification of Hydrogen Attack
Y. Houbaert & J. Dilewijns Lab. Algemene Metallurgie, Siderurgie en Fysische Metaalkunde, Rijksuniversiteit Gent, Sint Pietersnieuwstraat 41 B-9000 Gent, Belgium (Received 23 August 1990; accepted 15 November 1990)
ABSTRACT The occurrence of hydrogen attack related to the presence of methane in the steel cavities can be identified. This can be done by using the described annealing and metallographic testing procedure. To quantify the remaining safety of hydrogen-attacked industrial equipment, the degree of hydrogen damage can be measured by means of metaUography, ultrasonic testing and chemical H-determination. A relation has been established between these different testing procedures. The fracture-mechanical properties of the damaged material have been determined. The results are expressed in terms of strength ratio. A first attempt is made to give an approximation of the remaining safety of the damaged material.
1 INTRODUCTION Several European fertilizer and petrochemical industries were built some 15-20 years ago. Some parts of these plants are working under conditions that might cause hydrogen attack. The industry is worried about the possible occurrence of hydrogen attack and about the problem of how much remaining safety does still exist in partially attacked pipes and how this can be checked by a non-destructive method. In the present paper, a metallographic examination procedure is described and some mechanical test results on a ferritic steel (16Mo5) 113 Int. J. Pres. Ves. & Piping 0308-0161/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
114
Y. Houbaert, J. Dilewijns
with 0.2% C and 0.5% Mo are reported. Most of the examined material was exposed to an atmosphere containing hydrogen at 150 bar partial hydrogen pressure, 220 bar total gas pressure and a temperature of 285 °C during more than 15 years. The investigated samples were taken from a pipe with 75 mm wall thickness.
2 EFFECTS OF H Y D R O G E N ON STEEL PIPE A ferritic steel exposed to a hydrogen atmosphere may experience the following problems. - - H y d r o g e n embrittlement: this occurs mainly in high-strength steels, exposed to hydrogen at temperatures between - 7 0 and +140 °C. No problems of this type to be expected for the fertilizer industries, working at temperatures about 300 °C. - - H y d r o g e n blistering: when dissolved hydrogen atoms recombine to molecular hydrogen, blisters may grow on steel inclusions. This is not a major industrial problem, because the blisters are easily detectable and they have an orientation that is parallel to the maximum stresses in the pipe. --Decarburization: here we have to distinguish between surface and volume decarburization. The first one is generally not an important problem, but the internal decarburization, also called hydrogen attack, has caused a lot of damage in industrial installations over recent years. The basic mechanism of hydrogen attack is as follows: atomic hydrogen, diffusing through the steel, reacts locally with the carbon of cementite in pearlite colonies, leading to the production of cavities filled with methane gas at a high pressure. The reaction equation can be simplified to: C + 4H = CH4
(1)
The 'hydrogen attack' or 'hot hydrogen damage' is a complex problem, because it is not easily recognized and it often extends over some localized, but large areas of the equipment, reducing drastically the toughness of the material. When a critical degree of attack is reached, it may lead to a sudden catastrophic crack propagation and explosion. Such accidents have already occurred in hydrogen treating installations, with considerable human and material injury.
Identification and quantification of hydrogen attack
115
3 I D E N T I F I C A T I O N OF H Y D R O G E N A T T A C K BY METALLOGRAPHY Figure 1 shows the usual appearance of a plain carbon steel (e.g. St.35.8, DIN 17175) after heavy hydrogen attack: the attack was localized near a weld. Molybdenum is known to reduce the likeliness of steel to present hydrogen attack, but Fig. 2 shows a 16Mo5-steel (0-2% C, 0.5% Mo) with evident hydrogen damage: part of a pearlite colony was transformed to ferrite, the carbon reacted with hydrogen (see reaction (1)), producing a methane cavity in the grain boundary. These are two clear examples of hydrogen attack, but in some occasions the metallographic aspect and the position of the cavities does not give a definite indication as to whether there has been hydrogen attack or not. In these cases, an additional study must be carried out to check the origin of the observed cavities, which may be creep cavities as well as H A cavities. The samples are normally polished in a conventional way: grinding with silicon carbide and polishing with diamond paste on a relatively hard cloth (in order to avoid the smearing of material into the cavities). It is advisable to observe the samples without etching, to avoid confusion with pitting effects due to etching.
Fig. 1. Plain carbon steel St.35.8 exposed to H 2 Of 1-32 MPa = 13.2 bar at 290°C during 15 years---not etched---originalmagnification5-3x. (Wall thickness, 7-5 mm.)
Y. Houbaert, J. Dilewijns
116
Fig. 2.
Steel 16Mo5 exposed to H2 at 175 bar and 295 °C for 10 years---SEM, etched--original magnification 3100x.
The prepared samples can be observed with a light microscope or even better with a scanning electron microscope. We have found 1 that, if the gas in the cavities is methane, it can be dissociated by heating at a temperature above a definite equilibrium value. Figures 3 and 4 give an example of these temperatures: the dissociation occurs at 450 °C (6 h) for a plain carbon steel (St.35.8) and for a 16Mo5 steel, at 560 °C (6 h) ppm 60
16 Mo 5
50 1.0 30 20 10
300
"~
450
l,h
500 520
560
"C
6h
Fig. 3. Variation of measured H2 content with changing annealing treatment. Material: 16Mo5.
Identification and quantification of hydrogen attack
117
ppmH2~ 50
13
Cr Mo
4 4
40 30 20 10 o •~ (
3OO"
~h
f
~so
I
I
el
|~v
soo s2o s.6o STO 6oo~ 'c 6~
Fig. 4. Variation of measured H2 content with changing annealing treatment. Material: 13CrMo44.
for a 13CrMo44 steel. The mechanism is supposed to be the following: at the appropriate temperature, the methane dissociates, the atomic hydrogen diffuses away through the lattice and the carbon precipitates again as cementite in the adjacent grain boundaries. The phenomenon of cementite precipitation can be observed in Fig. 5. The carbides present a clear and definite crystallographic orientation which may easily be measured and identified on SEM images.
Fig. 5.
Steel 16Mo5 (same working conditions as Fig. 2) after vacuum annealing of 6 h at 450 °C---etched, SEM--original magnification 1550x.
118
Y. Houbaert, J. Dilewijns
4 QUANTIFICATION OF THE HYDROGEN ATTACK
4.1 Metailography The above described metallographic technique is not quantitative and it is a destructive examination, except if a surface replica is taken. In many industrial applications, this method would not be applicable, because what has to be investigated is precisely volume decarburization. A usual, partially destructive, method is to take a root sample through the thickness of the pipe wall and afterwards to close this hole by welding.
4.2 Ultrasonic testing It should be emphasized that ultrasonic testing is a non-destructive testing procedure, but that the relation between a certain degree of hydrogen attack and the corresponding attenuation phenomenon of the ultrasonic signal is generally not well known. It is not easy to carry out accurate ultrasonic testing in connection with the problem of hydrogen attack. However, the Royal Dutch Shell Laboratories in Amsterdam performed very accurate scanning ultrasonic testing on some of our samples and obtained a clear map of the degree of attack. After selecting some adequate reference level of the ultrasonic signal, the depth and the degree of hydrogen attack can be represented in terms of transit time or amplitude. The transit time corresponds to a definite depth of attack defined by the degree of reflection of the ultrasonic signal on methane cavities. The amplitude of the ultrasonic signal at a definite depth of the material gives also an indication of the local degree of hydrogen attack. These procedures lead to quantitative diagrams where the fractional transit time or the amplitude are represented using a colour or a grey scale.
4.3 Hydrogen determination The most accurate method to determine the degree of hydrogen attack is the measurement of the hydrogen content in the sample. A 'Hydrogen Determinator' was used, based on the thermal conductivity of the gases escaping from a molten specimen. During the melting process, the methane is dissociated into carbon and molecular hydrogen and detected as such.
119
Identification and quanttfication of hydrogen attack Steel : 0J5 °l,C -0.50 %Mo l&O
130 120 '~' LLI
II0 1oo 9o-
ou z o "" O
8070605040-
302010i
0 INSIDE PIPE F ~ o 6o
20
i
4'0 DISTANCE [mm~
'
6'0
80 OUTSIDE PIPE
Typical variation of the hydrogen content (present as methane) versus wall thickness for a hydrogen damaged pipe.
Figure 6 shows the typical variation of the hydrogen content in a damaged specimen as a function of the thickness of the pipe wall. Three different zones can be distinguished: 1. A kind of saturation phenomenon occurs up to a depth of about 15 mm: the hydrogen content remains constant at about 120130 ppm. 2. A clear gradient is visible between 15 and 55 mm depth. 3. A zone without excessive hydrogen content is observed for depths over some 55 mm. Considering the methane reaction (1), it follows that the chemical activity (and in this manner the tendency to form cavities) increases according to a fourth power law with the dissolved hydrogen concentration. Supposing in a first approximation that the hydrogen gradient caused by diffusion is linear (steady state situation), the degree of hydrogen attack should present a fourth power law with the depth. Indeed, using a regression technique on the gradient zone, power laws with exponents varying between 3.8 and 4.2 are found.
4.4 Correlation of ultrasonic testing with hydrogen analysis As mentioned above, a correlation between non-destructive ultrasonic testing and destructive hydrogen determination is necessary to solve the problem described in the present paper.
Y. Houbaert, J. Dilewijns
120
Some of the hydrogen profiles which have been discussed in the previous paragraph were determined on a material after ultrasonic testing, in order to establish an exact relation between the attenuation factor and the hydrogen content. At the moment, it seems that the ultrasonic detection limit is some 80 ppm hydrogen, which is a very high value. A better resolution can surely be obtained by modifying the settings of the ultrasonic measurement equipment.
5 BASIC F R A C T U R E - M E C H A N I C A L A P P R O A C H
5.1 Principle A non-destructive determination of the remaining security of hydrogen damaged equipment can be performed using a basic fracturemechanical approach, consisting of the following steps: .
.
Elaboration of a model for the description of the stress intensity of the most critical crack, propagating through the wall of an industrial element (determination of the K~ value at the tip of a crack). Determination of the fracture-mechanical properties of the material as a function of the degree of hydrogen attack. This has to be done in two steps: first a quantification of the degree of hydrogen attack, which can be done in several ways, such as: metallography, hydrogen determination and ultra-sonic testing. The second step is the determination of the fracture-mechanical properties of the material, using different types of procedure, such as KIc test, COD test or J-integral.
5.2 Calculations The remaining resistance of the pipe wall to unstable crack propagation can be calculated using the procedure shown in Fig. 7. The calculation of the maximum stress in the pipe wall gives the curve No. 1 indicated by a. The calculation of the stress intensity K, at the tip of a crack growing radially through the wall thickness gives the curve No. 2. No details will be given here about the calculations of the curves 1 and 2, as these are problems of mechanical engineering which can easily be solved for different types of configurations (see, e.g. (Ref. 2)). Curve No. 3 represents the remaining fracture toughness as a function of the depth in the pipe wall. This curve should be obtained
Identification and quantification of hydrogen attack
121
® = f (H.A.)
i i i
I I I I
INNER SIDE 0
OUIER SIDE ?5ram
®
INNER SIDE
OUIER SIDE
II CRACK
NO CRACK GROWIII
75mm
X
Fig. 7. Schematic representation of the fracture-mechanicalapproach. from a relation between the fracture-mechanical data (e.g. Kic) and the quantified hydrogen damage (e.g. through H-determination, as in Fig.
6). It appears in this example that crack propagation may be expected up to a depth of x mm, but that further crack growth would normally not occur, unless the hydrogen attack would proceed to a greater depth. 5.3 Sampling The actual samples for this paper were taken from a pipe with 75 mm wall thickness. Although possible crack propagation would occur radially, the fracture-mechanical properties were not measured with cracks in this orientation, but instead as indicated in Fig. 8. This was
Y. Houbaert, J. Dilewijns
122
OUTSIDE PIPE
DEGREE OF HYDROGEN ATTACK m
@ -- ----J
_~POWER
LAW
z
1 INSIDE PIPE (~ PROBABLE GROWING DIRECTION OF CRACK UNDER WORKING CONDITIONS (~) CRACK GROWING THROUGH HOMOGENEOUS ATTACK ZONE
S
_
~
L WELDING "/ /
SAMPLES
FOR
KIc
-
SAMPLES FOR coo
J-INTEGRAL
Fig. 8. Schematic representation of the crack orientation and the sampling. done in order to let the crack grow through a zone with almost constant degree of hydrogen attack. Most of the samples had a thickness of 25 mm, allowing the selection of specimens with correct crack orientation and some possible variation of the crack position relative to the attack gradient. By welding some additional pieces, the crack location can be moved closer to the inner and outer sides of the pipe wall. As indicated in Fig. 8, samples have been taken according to the procedures for K~c, COD and J-integral testing. The welding of supporting pieces had no influence on the H-determination nor on the fracture-mechanical properties, if the crack was not too close to the heat affected zone. 5.4 Testing and results Different series of samples have been tested, with the crack advancing through zones with variable hydrogen content and variable response to ultrasonic testing.
Identification and quantification of hydrogen attack RSC 1.B5
1.75
1.65
1.66
123
\\
\\ \
RSC =
2 Pmctx (2W.a) = B (W-a) 2 YS
~notched ~'unnotched
\
*Xx
lJ.5
1.3S
1.25 0 Fig. 9.
~÷ 1
I
26
6o
r
r
,o0
HYDROGEN
,Z pm]
Strength ratio R,~ versus H content for hydrogen damaged steel 16Mo5.
Crack propagation tests were first performed on the K~¢ specimens. No relation could be found between the kinetics of the crack propagation (Paris-Erdogan) and the hydrogen content of the samples. Fracture-mechanical tests for the first series were all planned as Kk tests. Unfortunately, due to the still high ductility of the material, the results are not acceptable according to the ASTM standards. Nevertheless, one still can get some indication about the fracture-mechanical behaviour of the material by using the so-called 'strength ratio': R~. 3 This result is very hopeful and Fig. 9 shows that a clear relation exists between valid fracture-mechanical data and the degree of hydrogen attack measured by hydrogen analysis. 6 CONCLUSION The proposed metallographic technique (including an annealing treatment) may be used to prove the presence of methane in steel cavities. A correlation was found between ultrasonic testing and the H content and also between the H content and the fracture-mechanical data. It is possible to obtain fracture-mechanical information about a hydrogen attacked material using the results of a non-destructive ultrasonic test.
124
Y. Houbaert, J. Dilewijns
ACKNOWLEDGEMENT We want to thank Kemira b.v. for the industrially damaged material and the Royal Dutch Shell Laboratories for the US-testing.
REFERENCES 1. Mertens, D., Houbaert, Y. & Dilewijns, J., Metallographic indication of methane in steel cavities. In High Temperature Alloys: their Exploitable Potential, ed. J. B. Marriott, M. Merz, J. Nihoul & J. Wars. Elsevier Applied Science, London and New York, 1987, pp. 39-48. 2. Buchalet, C. B. & Bamford, W. H., Stress Intensity Factor Solutions for Continuous Surface Flaws in Reactor Pressure Vessels. ASTM, STP 590, 1976, pp. 285-402. 3. ASTM E399-83, Plane-strain Fracture Toughness of Metallic Materials, 1983.
BIBLIOGRAPHY 1. Houkaert, Y. & Dilewijns, J., Study of the toughness of hydrogen attacked steel. Proceedings of the International Conference on Interaction of Steels with Hydrogen in Petroleum Industry Pressure Vessel Service. Materials Properties Council, Inc., Paris, March 1989, pp. 167-71. 2. Derycke, L., Breukmechanische Beraderig van de Genolgen van Waterstofaanfasting. Thesis, University of Ghent, Faculty of Applied Sciences, Belgium, 1988.