The Effect of Sour Environments on Steel Fracture Toughness

TWI LTd, Granta Park Great Abington, Cambridge, CB21 6AL, UK

Paper Presented at Steely Hydrogen. 2nd International Conference on Metals and Hydrogen. 5-7 May 2014., Gent, Belgium.

Abstract

Hydrogen absorbed from sour environments adversely affects the fracture toughness of steel. The amount of absorbed hydrogen at the plastic zone of a crack tip in a standard fracture mechanics specimen is controlled by the applied loading rate (or K-rate) and this therefore, generally needs to be slower than the K-rate applied in standard fracture mechanics testing in air. Furthermore, for assessment of components exposed to sour environment, determination of an appropriate loading rate is the first step towards determination of lower bound fracture toughness using standard fracture mechanics specimens. This study has focussed on determining optimum K-rates for fracture mechanics tests performed on environmentally conditioned pipeline steel material (API 5L X65) both when tested in air and when tested in the sour environment. The fracture toughness data were supported by hydrogen measurements on test specimens. For a given combination of material and environment, it was found that certain K-rates (optimum K-rates) exist at which maximum deteriorating effect of absorbed hydrogen on fracture toughness was observed for tests conducted in air. When tested in the environment, a minimum K-rate was observed, below which measured fracture toughness did not decrease further. The optimum K-rate for tests conducted in the environment was somewhat lower than the optimum K-rate for tests conducted in air; however, the lower bound fracture toughness measured in both situations were close to each other.

Introduction

Fracture toughness is an important parameter used in fracture mechanics based Engineering Critical Assessment (ECA) to define alternative flaw acceptance criteria or evaluate Fitness-For-Service (FFS) of pipeline girth welds. In the oil and gas industry, pipelines are commonly made of carbon manganese steel and, if operating under a sour environment, their mechanical properties may be degraded and compromise their integrity [ , ]. The extent of deterioration of toughness depends upon a number of factors e.g. severity of the environment, type of material, temperature and pressure. All these factors control the amount of hydrogen absorbed in the steel. Furthermore, if a crack exists in the material subjected to service loading, hydrogen will accumulate in the plastic zone at the crack tip due to locally enhanced solubility, thus enhancing the hydrogen embrittlement.

For ECA of pipelines containing a sour environment, fracture toughness of the material should be evaluated with regard to the position of the flaw under consideration. If a flaw is directly exposed to the environment (internal surface cracks), the fracture toughness should be determined from specimens tested in the simulated environment. However, if a flaw is not directly exposed to the environment (embedded crack), it is likely that the hydrogen absorbed from the environment can still deteriorate the toughness. To simulate this situation, test specimens are first charged by immersing the specimens in a specific sour environment and then tested in air (hereafter referred as hydrogen charged specimens). During service, hydrogen only enters through the exposed surfaces and this can be simulated by applying suitable surface coating during exposure to the environment. This will result in lower hydrogen levels than associated with a fully saturated material.

Thus, fracture toughness of a material subjected to a sour environment can be determined by testing a pre-charged material in air or a sour environment. However, the fracture toughness determined is dependent upon a number of other factors, the most critical of which is the loading rate applied during the test, as this affects the opportunity for hydrogen to accumulate in the developing plastic zone. Fracture toughness tests on hydrogen free material are carried out at loading rates (in terms of applied K-rate) from 0.5 to 3MPam0.5s-1 according to the established standards such as BS7448: Part 1:1997[6], ASTM1820[ ] and ASTM E1921[ ]. In order to determine the fracture toughness of a material subjected to a hydrogen charging environment, the loading rate applied needs to be slower than this. However, although if the loading rate is too fast during testing, there will not be sufficient time for absorbed hydrogen atoms to migrate to plastic zone surrounding the crack tip and thus show the full effect on measured toughness, too slow a loading rate may result in such a long test duration that the specimen will lose bulk hydrogen, thus also reducing the observed effect, if tested in air [ ] (this would be less of a concern if tested in environment when hydrogen loss would not be anticipated). It is therefore very important to evaluate an optimum loading rate that should be used when conducting fracture toughness testing on pre-charged material in air and in sour environment. There is no guidance on the choice of loading rates in any of the existing standards.

A test programme was designed to study the effect of applied loading rate and surface coating on the measured hydrogen content and the fracture toughness of API 5L X65 Grade pipeline steel. The main objective of this exercise was to evaluate optimum loading rates which will give lower bound fracture toughness using standard fracture mechanics test specimens and procedures.

Material, test specimens and pre-charging

Fracture mechanics tests were performed on the parent material of a pipe made from API 5L X65 Grade steel. Single edge notched bend (SENB) specimens were machined with the specimen length parallel to the pipe axis and notched through thickness. The specimen design and size is shown in Figure 1(a). These specimens were fatigue pre-cracked according to BS7448-Part 1:1997[6] to introduce a sharp crack at the notch tip. The ratio of crack length (ao) to specimen width (W) was 0.5. The specimens subjected to fracture mechanics testing can be divided into three groups.

1. Base line tests in air: these specimens were not charged and were tested in air at a normal loading rate according to the requirements of BS7448-Part 1[6]. They are referred to ‘as received’ material
2. Specimens were exposed to a sour environment and then tested in air. They are referred to as hydrogen charged material.
3. Specimens exposed to a sour environment and then tested in the same environment (Figure 1b). They are referred to as hydrogen charged material in environment.

Prior to exposure to the environment, specimens from groups (b) and (c) were further divided into two sets. All the specimens in the first set were coated using a thin layer of ‘stopping off lacquer’. This coating was applied to all the surfaces except the surface having the notch mouth. These specimens will hereafter be referred to as ‘coated specimens’. The specimens in the second set were not coated and will be referred to as ‘un-coated specimens’. For all the specimens from groups (b) and (c), the crack was however protected from the environment during the pre-exposure period to avoid possible crack tip blunting due to corrosion. All these specimens were inserted into an environmental chamber filled with a solution of 5% sodium chloride (NaCl) in deionized water. The pH of the solution was maintained at 4.5 by sodium bi-carbonate. A mixture of 10%H2S, balance CO2, was purged into the solution. Based upon TWI’s experience, these specimens were soaked for a period of two weeks. These specimens were then removed from the chamber and stored in liquid nitrogen to prevent any loss of absorbed hydrogen due to diffusion. The test matrix is shown in the Table below

Specimen IDs	No of tests	Pre-treatment	Surface coating	Testing environment
1,2,3	3	No	NA	air
4,5	2	H charged	coated	air
6,7,8,9,10	5	H charged	un-coated	air
11	1	H charged	un-coated	environment
12,13,14,15,16	5	H charged	coated	environment

Fracture mechanics testing and hydrogen analyses

All fracture mechanics tests were performed using a servo controlled hydraulic universal materials testing machine under rising load using procedures given in BS7448-Part 1[ ]. Baseline tests in air were performed at an initial loading rate (expressed as K-rate) of ~1MPam^0.5s^-1. Tests on hydrogen charged specimens in air and environment (refer to group b and c in section 2 above) were carried out at different loading rates. Prior to fracture mechanics testing of hydrogen charged material, each specimen was first taken out of liquid nitrogen and warmed under tap water to room temperature. Applied force vs. clip gauge extension data were acquired for each test. For testing in the environment, specimens along with the jig and clip gauges were fitted inside the sealed environmental chamber (Figure 1(b)). Special clip gauges (resistant to the environment) were used to get clip gauge extension data in the fracture mechanics tests. These data were used to calculate CTOD and J values (from crack mouth opening displacement, CMOD, derived from clip gauge records).

For all the tests performed on pre-conditioned specimens (in air and in the environment), hydrogen analysis (using a Bruker G4Phoenix DHIR/TF analyser) was carried out to measure the amount of diffusible hydrogen before and after each fracture mechanics test. This task was carried out to allow the results to be related to the amount of hydrogen in the specimens. A 10x10mm slice was cut from one end of each specimen before fracture mechanics testing and subjected to hydrogen analysis. A similar analysis was performed on another slice cut from the opposite end of each specimen after completing the fracture mechanics test. The hydrogen analysis was carried out by heating the specimen in nitrogen carrier gas at 400°C for about 20 minutes.

Results

General

The fracture mechanics test results were in the form of force versus clip gauge extension records. Figure 2 shows force (F) versus crack mouth opening displacement, CMOD (derived from clip gauge extension records and used to calculate elastic plastic J integral). All the baseline tests were performed at a constant K-rate of approximately 1MPam^0.5s^-1 according to the recommendations given in the standard. The lowest curve from the baseline tests is shown in Figures 2a to 2c for reference and it is evident that the F vs CMOD curves for the tests performed on hydrogen charged specimens in air and in the environment at slow loading rates fall below this curve. These curves show that all the specimens achieved maximum force during the testing without any brittle fracture event during the test. The values of fracture toughness for each specimen were calculated in terms of J integral corresponding to the maximum force point in the F. vs CMOD data.

Figure 3 presents a chart showing the amount of diffusible hydrogen measured before and after each of the fracture toughness tests. It is evident that the measured hydrogen content (after the fracture mechanics test) in the un-coated specimens tested in air is approximately two to three times higher than the hydrogen content in the coated specimens tested in the environment. The one un-coated specimen tested in the environment showed a hydrogen level close to the hydrogen levels in the un-coated specimens tested in air. Measured hydrogen content before and after the tests showed that the specimens tested in air lost a considerable amount of hydrogen whereas those tested in the environment lost less or in some cases showed hydrogen pick up (indicated by higher hydrogen content measured after the test). The average loss of hydrogen for coated and un-coated specimens tested in air was approximately 38% and 53% respectively. The average loss of hydrogen for coated specimens tested in environment was approximately 37% and the single un-coated specimen tested in the environment showed a gain of hydrogen of approximately 37% during the test. At the lower levels in particular, uncertainties associated with the sampling technique, and inhomogeneous hydrogen distribution within the samples should be recognised.

Hydrogen charged material tested in air

Figure 4 presents the calculated values of J for different tests versus hydrogen content measured after the test. The minimum fracture toughness of the material tested in air (as received with no hydrogen) was found to be 1160 Nmm^-1

For the two tests (specimens 4 and 5) carried out on coated specimens in air, it was found that the specimen tested at the lower K-rate (K-rate of 0.06MPam^0.5s^-1) had very little remaining hydrogen (0.1ppm) after the completion of the test and the measured fracture toughness (in terms of J) was 1096Nmm-1. The second specimen (No 4) tested at a slightly higher K-rate (0.117MPam^0.5s^-1) had remaining hydrogen content of 0.14ppm and the measured toughness was 906Nmm^-1. It appears that the specimen tested at the lower K-rate did not have sufficient hydrogen content by the time the critical load had been achieved to show any noticeable effect on the measured toughness, whereas the specimen tested at a slightly higher K-rate (having slightly higher hydrogen content) retained sufficient hydrogen to reduce the fracture toughness.

Further tests were carried out in air on un-coated hydrogen charged specimens (specimens 6 to 10) at different K-rates. There are five data points (‘empty diamond’ markers in Figure 4) representing the results from these tests. It is evident that lowering the initial K-rate from 0.11 to 0.019MPam^0.5s^-1 (the test duration increased from two to four hours) has resulted in dropping the measured hydrogen content from 0.41ppm to 0.32ppm. This drop in the measured hydrogen is in accordance with the established understanding, ie lower K-rate results in longer test durations and hence the specimens are likely to lose more of the bulk hydrogen. It nevertheless is evident that the measured fracture toughness of these specimens decreased with decreasing the K-rates. The fracture toughness of these specimens is significantly lower than the fracture toughness measured from baseline tests and tests conducted on coated and pre-charged specimens in air. These observations lead to the conclusion that the un-coated specimens absorbed more hydrogen and sufficient has been retained to have a significant effect on the measured fracture toughness in slower loading rate (K-rate) rate tests.

From the data, it is evident that the fracture toughness measured from the test conducted at the lowest initial K-rate (0.019MPam^0.5s^-1) is very similar to, but slightly higher (J~83Nmm^-1) than the fracture toughness values (J~72Nmm-1 and 81Nmm-1) obtained from the tests conducted at a comparatively higher initial K-rate (0.034MPam0.5s-1, almost double the slowest K-rate). Referring to Figure 4, the hydrogen content in the specimen tested at the lowest K-rate (0.019MPam^0.5s^-1) is very close to the hydrogen content of one of the specimens tested at the comparatively higher K-rate (0.034MPam^0.5s^-1). The other specimen tested at K-rate of 0.034MPam^0.5s^-1 showed higher hydrogen content, nevertheless, the measured toughness was fairly close (81Nmm^-1) to the toughness measured for the other specimens tested at the same K-rate. This suggests that the maximum possible amount of hydrogen is diffusing into the plastic zone at the higher K-rate, and thus reducing the K-rate further has no further effect on the measured fracture toughness. It can therefore be concluded that the test conducted at the initial K-rate of 0.034MPam^0.5s^-1 has shown the maximum effect of absorbed hydrogen on the measured fracture toughness.

Hydrogen charged material tested in environment

The results of the tests conducted in the environment are shown as ‘triangle’ markers in Figure 4. Most of these tests were performed on coated and hydrogen charged specimens. A datum point shown as ‘empty triangle’ marker represents the result obtained from a test conducted on a specimen (No 11) charged and tested in the environment without surface coating.

The coated specimens would only have picked up a minimal amount of extra hydrogen during the test (they picked up most of it during pre-charging), and hydrogen pick-up during test is only indicated by the results from hydrogen analysis for the specimens tested at the slowest K-rate; specimen No 11 (un coated) and specimen No 14 (coated) (Fig 3). For both of these specimens, the measured hydrogen contents after the tests were higher than the hydrogen contents measured before the test. For other (shorter term) tests there is an apparent loss of hydrogen, which at this low level, is probably due to sampling effects rather than a true effect.

Referring to Figure 4, it is clear that reducing the K-rate has significantly reduced the fracture toughness. This is consistent with a hydrogen effect, even though the measured hydrogen content was generally less than 0.1ppm. The lowest K-rate used for these tests was approximately 0.008MPam^0.5s^-1 and the measured fracture toughness associated with this K-rate was 81Nmm^-1. A test conducted on an un-coated specimen in the environment at a slightly higher K rate (0.013MPam^0.5s^-1) had only slightly lowered the toughness (69Nmm-1) despite significantly higher bulk hydrogen content (0.74ppm). This suggests that the hydrogen in the plastic zone in the coated specimen has had time to achieve the same local concentrations as that in the un-coated specimen at this K-rate without any assistance from diffusion from the bulk material, and lower toughness is unlikely to be achieved at even lower K-rate.

Discussion

As anticipated, all fracture toughness values measured in the sour environment or after exposure to the sour environment, were lower than those measured in as-received material in air, and at the test temperature (room temperature) this is likely to be due to hydrogen introduced into the material from corrosion. For this reason, hydrogen analysis was performed, and because of the potential for hydrogen loss (after removal from solution) or continued pick-up (when tested in solution), samples for analysis were taken both before and after each test. For un-coated specimens there was a consistent loss of hydrogen when tested in air, and a small pick-up in the one specimen tested in environment (Figure 3). With regard to testing in air, the hydrogen loss was clearly associated with strain rate (and hence duration of test) as shown in Figure 5a. In the coated specimens, lower hydrogen was measured by comparison with the uncoated specimens, but there was considerable variation in behaviour, particularly when tested in environment. Care should be taken with regard to the coated specimen results, because of the difficulty in consistently sampling specimens with inhomogeneous distributions of hydrogen.

Within the environmental tests, two trends were evident. First, there was a general trend of decreasing toughness with increasing hydrogen content (Figure 4). Second, with one exception, fracture toughness fell as strain rate (in terms of K rate) decreased (Figure 6). The exception was the coated and hydrogen charged specimens, tested in air. These had the highest toughness values of all the environmentally exposed specimens, and it would appear that hydrogen loss in the slow test (0.06MPam^0.5s^-1) has dominated over the effect of strain rate, when compared with the test at 0.12MPam^0.5s^-1. When similarly coated specimens were tested in the environment (when no net hydrogen loss would be expected throughout the test) toughness was found to decrease as strain rate decreased from 0.04 to 0.008MPam^0.5s^-1, although, as discussed above, hydrogen loss/gain was not reliably demonstrated by measurement for these coated specimens.

From Figure 4, it can be seen that for similar hydrogen contents, there is an increasing embrittling effect as K rate decreases, down to a minimum of between about 70 and 100 Nmm^-1. It may be postulated that maximum embrittlement will be observed when time is allowed for accumulation of hydrogen in the plastic zone at the crack tip. The consistent lower bound toughness, obtained even at relatively high hydrogen (0.75ppm after test) in an uncoated specimen tested in the environment at slow strain rate (0.012 MPam^0.5s^-1), indicates that the effect saturates, whereas higher toughness recorded at lower hydrogen and/or at higher strain rates, particularly when the crack tip is not exposed to the environment, and therefore not able to be replenished with hydrogen directly, indicates that there is a transition to true in-air (hydrogen free) behaviour, but this is quite steep. To take advantage of this would require a good knowledge and understanding of service conditions.

Otherwise, it would be prudent to determine the lower bound, which can be most efficiently achieved by using fully exposed specimens, tested at a relatively low strain rate in the environment.

Although minimum toughness values were able to be measured in the present work with in-air tests at a sufficiently low strain rate using fully exposed specimens, the appropriate conditions would be expected to be dependent on geometry, environment and material and thus would need to be validated for each test series. With testing in the environment, although some confirmation that a suitable strain rate had been selected by carrying out tests over a range of strain rates would be advisable, the overall test matrix would be smaller, and quicker to perform.

Conclusions

The lower bound fracture toughness of environmentally treated material was found to be approximately 15 times lower than the lowest toughness value obtained from tests performed on as received material in air.

Fracture mechanics tests performed on the pre-charged specimens (un coated) in air and the pre-charged specimens (coated and un-coated) in the environment showed that an initial K-rate of 0.034MPam^0.5s^-1 and 0.008MPam^0.5s^-1, respectively, resulted in lower bound fracture toughness which is envisaged to have incorporated the maximum effect of absorbed hydrogen.

The lower bound fracture toughness of the pre-charged material tested in air and in the environment are very close to each other and for the material and environment considered in the this test programme, the deteriorating effect of absorbed hydrogen on the measured fracture toughness was able to be reliably assessed by testing pre-conditioned specimens in air.

Test approach and conditions required to determine lower bound toughness need to be validated for each material / environment combination under consideration.

Acknowledgment

TWI acknowledges Technip Ltd for supplying material and sponsoring the test programme.

References

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3. ASTM E1820-11e2: Standard Test Method for Measurement of Fracture Toughness, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2011
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Figure 2 Comparison of force versus CMOD (derived from clip gauge) records for tests performed on as received material in air with (a) pre-charged material (un-coated) tested in air (b) pre-charged material (coated) tested in air and (c) pre-charged material (coated and un-coated) tested in the environment. Note: K rates are in MPam^0.5s^-1