Scholarly article on topic 'Study of Embrittlement of the 2.25Cr-1Mo-V Steel Weld Metal by Hydrogen Charge and High Pressure Hydrogen Gas Environment'

Study of Embrittlement of the 2.25Cr-1Mo-V Steel Weld Metal by Hydrogen Charge and High Pressure Hydrogen Gas Environment Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Y. Honma, R. Kayano

Abstract It is well-known that hydrogen would accumulate at internal defects of pressure vessels during shutdown and hydrogen embrittlement occurred. However it has not been clear that the effect of hydrogen gas environment for 2.25Cr-1Mo-V steel (22V) weld metal. For this reason, the purpose of this work is to identify and understand the potential for embrittlement of the 22V steel by hydrogen charge and high pressure hydrogen gas environment. Therefore, rising load test was carried out in this study to examine the effect of dissolved hydrogen by high temperature, high pressure hydrogen exposure and high pressure hydrogen gas environment on hydrogen embrittlement at room temperature (R.T.) and 150 ̊C. 22V forged steel base metal was used and welding was conducted by submerged arc welding (SAW) process. High and low toughness weld metals were prepared by changing PWHT condition and notch location in order to consider variation of product's weld metal. From rising load test results, hydrogen gas environment had effect on the embrittlement of the 2.25Cr-1Mo-V steel weld metal. In contrast, dissolved hydrogen had little effect on the embrittlement. Moreover, KIH value of high toughness weld metal (KIH value is about 90 MPa√m) was higher than that of low toughness (KIH value is about 70 MPa√m) at RT. However, KIH value of low toughness weld metal was the similar level (KIH value is about 110 MPa√m) of high toughness weld metal at 150 ̊C.

Academic research paper on topic "Study of Embrittlement of the 2.25Cr-1Mo-V Steel Weld Metal by Hydrogen Charge and High Pressure Hydrogen Gas Environment"

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Procedía Engineering 130 (2015) 571 - 582

Procedía Engineering

www.elsevier.com/locate/procedia

14th International Conference on Pressure Vessel Technology

Study of Embrittlement ofthe 2.25Cr-lMo-V Steel Weld Metal by Hydrogen Charge and High Pressure Hydrogen Gas Environment

Y. Honmaa,*9 R. Kayanoa

aThe Japan Steel Works, Ltd., Chatsu-machi, Muroran Hokkaido, Japan

Abstract

It is well-known that hydrogen would accumulate at internal defects of pressure vessels during shutdown and hydrogen embrittlement occurred. However it has not been clear that the effect ofhydrogen gas environment for 2.25Cr-lMo-V steel (22V) weld metal. For this reason, the purpose of this work is to identify and understand the potential for embrittlement of the 22V steel by hydrogen charge and high pressure hydrogen gas environment.

Therefore, rising load test was carried out in this study to examine the effect of dissolved hydrogen by high temperature, high pressure hydrogen exposure and high pressure hydrogen gas environment on hydrogen embrittlement at room temperature (R.T.) and 150°C. 22V forged steel base metal was used and welding was conducted by submerged arc welding (SAW) process. High and low toughness weld metals were prepared by changing PWHT condition and notch location in order to consider variation of product's weld metal.

From rising load test results, hydrogen gas environment had effect on the embrittlement of the 2.25Cr-lMo-V steel weld metal.

In contrast, dissolved hydrogen had little effect on the embrittlement. Moreover, Km value of high toughness weld metal (Km

value is about 90 MPaVm) was higher than that of low toughness (Km value is about 70 MPaVm) at RT. However, Km value of

low toughness weld metal was the similar level (Km value is about 110 MPaVm) ofhigh toughness weld metal at 150°C.

© 2015 The Authors.PublishedbyElsevierLtd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICPVT-14

Keywords: 2.25Cr-lMo-V steel; Hydrogen embrittlement; Weld metal

Corresponding author.

E-mail address: yuta_honma@jsw.co.jp

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICPVT-14

doi:10.1016/j.proeng.2015.12.269

1. Introduction

Since the heavy wall petroleum pressure vessel are operated at high temperature and high pressure hydrogen service, internal hydrogen embrittlement (I.H.E.) and hydrogen gas environment (H.E.E.) is necessary for the pressure vessel's operating procedure and startup/shutdown management. In general, I.H.E and H.E.E. mechanism of steels and metals has been reported as follows [1]. I.H.E. is the phenomenon which the embrittlement will recover by dehydrogenation. It is considered that the embrittlement happened by hydrogen atoms absorbed internal metal. The phenomenon such as a delayed fracture of a high tensile strength bolt, a cold cracking of weld metal, and a white spot are known for many years. It is well known as embrittlement of the petroleum pressure vessels operated in a high temperature, high-pressure hydrogen environment and it occurs in around room temperature after shut down. On the other hand, H.E.E. is the embrittlement by the hydrogen absorbed from the gas by metal deformation and it is the characteristic phenomenon in high pressure hydrogen gas environment. The accident which will produce a crack in NASA the first half of the 1960s on the tank for high-pressure storage made from 18%Ni Mar-aging steel. If stress is applied to steel and plastic deformation occurs, the newly-formed unoxidized steel surface which was not in contact with environment will be exposed, and hydrogen gas adsorbs and invades into the steel. It is considered that the dislocation makes contact and dissociation with hydrogen easy. H.E.E. is related with the dislocation mobility [2].

I.H.E. of 22V steel is generally recognized by previous studies [3,4] that Kih values are strikingly decreased by hydrogen charge. However, it has not been examined that the effect of H.E.E. on Kih of 22V steel, especially weld metal. It was recognized that hydrogen would accumulate at an internal defect like welded structural discontinuities of pressure vessel during shutdown. Thus, it is important to confirm hydrogen embrittlement phenomenon of 22V weld metal in order to determine minimum pressurization temperature (MPT) of hydroprocessing reactor. Accordingly, the purpose of this research is to identify and understand the effect of hydrogen gas environment on the embrittlement of the 22V steel weld metal.

2. Experimental Procedures

2.1. Material

The chemical composition of weld metal is shown in Table 1. Base metal was used 22V forged steel and welding process was submerged arc welding (SAW). Groove geometry of test block is described in Fig. 1 and welding condition was shown below. Current: 500-600A Voltage: 27-34V and Speed: 600-700mm/min. After welding, sample was treated de-hydrogen heat treatment (DHT) of 350oC for 2hrs. Moreover, it was examined that there is no harmful defect in the weld by ultrasonic testing (UT) and surface dye penetrant testing (PT). After the nondestructive examinations, sample was cut into coupon 1 and 2. Test coupon 1 was called high toughness weld material was treated PWHT of 705oC for 8hrs. Test coupon 2 was called low toughness weld which PWHT of 68O0C for 8hrs carried out in order to prepare different material of toughness.

Unit: mm

Fig. 1. Groove geometry oftestblock.

Table 1. Chemical composition of weld metal (mass%).

c Si Mn P S Ni Cr Cu Mo V N (ppm) J-Factor

0.09 0.13 1.07 0.004 0.002 0.07 2.39 0.09 1.05 0.35 85 84

J-Factor = (Si + Mn)(P +S) xlO4 / mass%

2.2. Experimental method

In order to introduce aiming lOppm hydrogen inside of the weld metal, the hydrogen charging condition was 450oC, 25MPa hydrogen gas pressure and the exposed time for 48h by autoclave was shown in Fig. 2. The specimens were rapidly taken out from the autoclave after hydrogen exposure and then cooled to water bath. After the specimens cooled to room temperature, rising load test and holding load test were conducted.

Fig. 2. Autoclave for hydrogen charging.

Table 2. Rising load test condition.

Material Hydrogen Charge Test Environment T.P.No.

High Toughness None Air H-N-A

20MPa H2Gas H-N-G

lOppm (Aiming) Air H-H-A

20MPa H2Gas H-H-G

Low Toughness None Air L-N-A

20MPa H2Gas L-N-G

1 Oppm (Aiming) Air L-H-A

20MPa HiGas L-H-G

Kih value of each specimen was evaluated by rising load test. Specimen type of rising load test was 1T-C(T) (ASTM E 1820) described in Fig. 3. Schematic illustration ofspecimen location was shown in Fig. 4. Notch location was weld center for high toughness weld material and quarter width of weld metal for low toughness weld metal. The test condition was shown in Table 2 which test temperature was room temperature (R.T.) and 150oC, hydrogen charge was carried out aiming hydrogen content lOppm and test environment was air and 20MPa hydrogen Gas. The test was conducted at a rate of 0.01mm/min (K=1.3 X 10"2 MPa V m/s) which was minimum limit of machine. After rising load test, specimen was opened in liq. N2 and fracture surface was observed by scanning electron microscope (SEM) in order to confirm stretch zone. Hydrogen content was measured using another measurement piece after the

test. Kih value was calculated using the following equation (1) where P is load of deviation point, a(m) is pre-crack length, W(m) is specimen width, B(m) is specimen thickness and BN (m) is specimen thickness at side groove.

F^a/W) = 29.6-18.55(a/W)+655.7(a/W)2-1017.0(a/W)3+638.9(a/W)4

Hydrogen charging condition: Temp.450°C, Time:45hrs, Pressure:25MPa

Fig. 3. Geometry of 1T-C(T) specimen.

Fig. 4. Schematic illustration of specimen location.

Deviation point was determined by difference in load line displacement method as shown in Fig 5. Deviation point is that the difference in load line displacement begins to rise up. In order to check whether crack growth occurs at Kih load, holding load test was carried out at RT in 20MPa gaseous hydrogen. The specimen was charged aiming lOppm hydrogen. Holding load equivalent to Kih is obtained by rising load test. In the case of high toughness weld material, holding load was 33.1kN. In the case of low toughness weld material, it was 30.1kN. Holding time was 10 days. Load was applied at rate of0.01mm/min to aiming load, and then it was kept constant. After the test, specimen was opened in liq. N2 and fracture surface was observed by SEM in order to confirm the area of propagated crack.

-Original (Hydrogen free) -Hydrogen affected i i i

- i t ^^ / / /

<-/ i >----->

/ / / / -

/ ^^Deviation point i.i.i.i.i

0 n ^E

0.10 E

0.08 a. 0:06 I 0.04 I

0.02 '3

0.00 £ Q

0.0 0.5 1.0 1.5 2.0 2.5

Crosshead displacement (mm)

Fig. 5. Determination example ofdeviation point.

3. Results

3.1. Mechanical properties of high and low toughness weld metal

Table 3. Tensile test results ofweld metal.

0.2% Yield Stress (MPa) Tensile Strength (MPa) Elongation (%) Red. of Area (%)

High Toughness Weld Metal 600 701 22.8 71.9

Low Toughness Weld Metal 688 775 18.1 68.1

Fig. 6. Charpy transition curve ofweld metal.

Tensile test results of high and low toughness weld metals are shown in Table 3. Tensile strength and yield stress of low toughness weld metal was higher than that of high toughness weld metal. Additionally, tensile property of high toughness weld metal that was treated usually PWHT (705oC for 8hrs) satisfied the spec, of ASME SA-336 F22V which is for base metal. Charpy transition curve of high and low toughness weld metal is shown in Fig. 6. Fracture appearance transition temperature (FATT) was -42°C for high toughness weld metal and it was -16°C for low toughness weld metal. Moreover, charpy impact specimens were obtained at center and quarter width of weld metal in order to investigate the effect of notch location. It doesn't have an effect on the toughness at 20oC for high toughness weld metal but the toughness of quarter width of weld metal decreased compared with the center of weld metal for low toughness weld metal. From these results, 705oC for 8hrs and center of weld metal were determined as PWHT condition and notch location for high toughness weld metal. On the other hand, 680oC for 8hrs and one-quarter width of weld metal were determined as PWHT condition and notch location for low toughness weld metal to clarify the influence of material toughness on the Km.

3.2. Rising load test results of high toughness weld metal

0.5 1.0 1.5 2.0

Crosshead displacement (mm)

0.5 1.0 1.5 2.0

Crosshead displacement (mm)

Fig. 7. Rising load test results ofT.P.No. H-N-A and T.P.No. H-H-A. (a) at R.T.; (b) at 150°C.

0.5 1.0 1.5 2.0

Crosshead displacement (mm)

0.5 1.0 1.5 2.0

Crosshead displacement (mm)

Fig. 8. Rising load test results ofT.P.No. H-N-A and T.P.No. H-N-G. (a) at R.T.; (b) at 150°C.

0.5 1.0 1.5 2.0

Crosshead displacement (mm)

0.5 1.0 1.5 2.0

Crosshead displacement (mm)

Fig. 9. Rising load test results ofT.P.No. H-N-A and T.P.No. H-H-G. (a) at R.T.; (b) at 150oC.

In this test, standard condition that was none hydrogen charge and air environment was used one specimen but other conditions were used two specimens (N1, N2) in order to confirm repeatability of the test. Fig. 7 shows Load, Difference in load line displacement and Crosshead displacement curves (L, D-C curve) of H-N-A and H-H-A condition at R.T. and 150oC. Hydrogen content after rising load test of H-H-A was also shown in the figure. As shown in the figures, there is no deviation point in H-H-A because curve of difference in load line displacement was not rise up in these conditions. Fig. 8 shows L, D-C curve of H-N-A and H-N-G condition. It is clear that deviation point was observed in H-N-G specimens. Km value at R.T. that was calculated by load of deviation point was 93 MPa V m and besides Km value at 150oC was 105 MPa V m. Thus, it was clear that hydrogen environment had effect on hydrogen embrittlement at both of test temperature. Fig. 9 shows L, D-C curve of H-N-A and H-H-G condition. These are indicative of effect of hydrogen charge and test environment. This figure shows there is deviation point for both specimens. Km value at R.T. that was calculated by load of deviation point was 90 MPa V m and Km value at 150oC was 81 MPa V m. From these results, Km value might be insensitive to dissolved hydrogen under hydrogen gas environment.

Figures 10, 11 shows fracture appearance of each condition after rising load test. Stretch zone was precisely observed and propagated crack was ductile fracture in H-N-A and H-H-A condition. However a lot of sub-crack occurred in propagated crack of H-H-A condition. Stretch zones of H-N-G and H-H-G were slightly observed, but the width was less than that of H-N-A condition, and also quasi-cleavage appearance was observed in propagated crack by hydrogen gas environment. However stretch zone width was increased at 150oC in comparison with that at R.T. Moreover, propagated crack morphology was changed to ductile fracture at 150oC even if test condition was under hydrogen gas environment. On the other hand, different between H-N-G and H-H-G were not confirm in fracture surface.

Pre C. , S.Z. , P.C. (D.F.) Pre C. , S.Z. , P.C. (D.F.)

Pre C. S.Z P.C. (Q.F.)

Pre C. S.Z. p.c. (Q.F.)

30 um 30 Pm 30 Pm

Pre C.: Pre Crack, S.Z.: Stretch Zone, P. C.: Propagated Crack D.F.: Ductile Fracture, Q.F.: Quasi-cleavege Fracture

Fig. 10. SEM image offracture surface ofeach condition at R.T. (a) H-N-A; (b) H-H-A; (c) H-N-G; (d) H-H-G.

g- — M.JJI^flMJ1 HannnVRtHtVW' ■ ^■<■■■■•■■; in—I». <■»

30 um 30 um 30 um 30 u m

Pre C.: Pre Crack, S.Z.: Stretch Zone, P. C.: Propagated Crack D.F.: Ductile Fracture, Q.F.: Quasi-cleavege Fracture Fig. 11. SEM image offracture surface ofeach condition at 150°C. (a)H-N-A; (b) H-H-A; (c) H-N-G; (d) H-H-G.

Kih values of high toughness weld metal were obtained rising load test were summarized in Table 4 and 5. Based on these results, it is evident that the effect of dissolved hydrogen on hydrogen embrittlement was a little, whereas hydrogen gas environment acts as the embrittlement factor in this condition. Therefore, Kih values at both temperatures were almost same but propagated crack morphology was changed from quasi-cleavage to ductile fracture because oftest temperature.

Table 4. Rising load test results ofhigh toughness weld metal at R.T.

T.P.No. Hydrogen content after test (PPm) Test Environment K-IH (MPa^m) Fracture Appearance

H-N-A — Air - S.Z. D.F.

H-H-A 12.1 10.1 Air Air No Deviation Point S.Z. D.F.

H-N-G : 20MPa H2 Gas 93 88 Q.F.

H-H-G 11.8 9.90 20MPa H2 Gas 90 91 Q.F.

S.Z.: Stretch Zone, D.F.: Ductile Fracture, Q.F.: Quasi-cleavege Fracture

Table 5. Rising load test results ofhigh toughness weld metal at 150°C.

T.P.No. Hydrogen content after test (PPm) Test Environment Km (MPaVm) Fracture Appearance

H-N-A — Air — S.Z. D.F.

H-H-A 8.69 6.66 Air Air No Deviation 110 S.Z. D.F.

H-N-G - 20MPa H2 Gas 105 S.Z. D.F.

H-H-G 10.5 6.82 20MPa H, Gas (81) 110 S.Z. D.F.

S.Z.: Stretch Zone, D.F.: Ductile Fracture

3.3. Rising load test results of low toughness weld metal

Table 6. Rising load test results of low toughness weld metal at R.T.

T.P.No. Hydrogen content after test (PPm) Test Environment Km (MPaVm) Fracture Appearance

L-N-A — Air — S.Z. D.F.

L-H-A 7.05 8.26 Air Air 129 116 S.Z. D.F.

L-N-G — 20MPa H2 Gas 113 109 S.Z. D.F.

L-H-G 10.5 6.82 20MPa H2 Gas 110 100 S.Z. D.F.

S.Z.: Stretch Zone, D.F.: Ductile Fracture, Q.F.: Quasi-cleavege Fracture

Table 7. Rising load test results of low toughness weld metal at 150°C.

T.P.No. Hydrogen content after test (PPm) Test Environment Km (MPaVm) Fracture Appearance

L-N-A — Air — S.Z. D.F.

L-H-A 9.04 9.77 Air Air No Deviation Point S.Z. D.F.

L-N-G : 20MPa H2 Gas 70 67 Q.F.

L-H-G 9.60 9.67 20MPa H2 Gas OO 00 — Q.F.

S.Z.: Stretch Zone, D.F.: Ductile Fracture

Pre C.: Pre Crack, S.Z.: Stretch Zone, P. C.: Propagated Crack, D.F.: Ductile Fracture, Q.F.: Quasi-cleavege Fracture Fig. 12. SEM image of fracture surface of L-H-G. (a) at R.T.; (b) at 150°C.

Low toughness weld metal was also that standard condition was used one specimen but other conditions were used two specimens (N1, N2) in order to confirm repeatability of the test. Km values of low toughness weld metal were summarized in Table 6 and 7. The results had almost same tendency of that of high toughness weld metal. In

the case of L-H-G condition, Kih values were remarkably decreased and quasi-cleavage was observed in fracture surface as shown in Fig. 12. However Kih value increased and also fracture morphology was changed from quasi-cleavage to ductile fracture due to increase test temperature because of test temperature. It is evident that the effect of dissolved hydrogen on hydrogen embrittlement was a little, whereas hydrogen gas environment acts as the embrittlement factor in this condition. However the effect might be decreased due to increasing metal temperature. Furthermore, the detail of difference between high and low toughness weld metal is described below.

3.4. Holding load test results

Time (day)

Fig. 13. Holding load test results ofhigh toughness weld metal at R.T. under 20MPa H2 Gas.

Time (day)

Fig. 14. Holding load test results oflow toughness weld metal at R.T. under 20MPa H2 Gas.

Figure 13 shows holding load test results of high toughness weld metal and crack growth length after 10 days. From this result, it is clear that crack growth occurred at Kih load. In addition, load line displacement changed discontinuously. It is considered that this phenomenon is related to hydrogen accumulation at crack tip but it needs more detail examination of fracture surface. Fig. 14 shows holding load test results of low toughness weld metal and crack growth length after 10 days. Result of low toughness weld metal was almost same as that of high toughness weld metal. Crack growth was confirmed at this load and load line displacement changed discontinuously. From SEM observation, propagated crack was quasi-cleavage and crack growth was also confirmed at Kih load. The difference between high and low toughness weld metal is crack growth length. Crack growth length of low toughness weld metal was longer than that of high toughness weld metal. Thus, it is estimated that low toughness weld metal had a tendency that crack growth occurred easily at Kih load.

4. Discussion

O H-N-G

• H-H-G A L-N-G ▲ L-H-G

-i-1-1-'-1-'-1-'-1-1-r-

Tendency of

20 40 60 80 100 120 140 160 Test Temperature(°C)

Fig. 15. Relationship between Km and test temperature under 20MPa H2 gas.

i 1 1 1 i 1 1 1 i ■ ■ 1 i

Kjlc:254MPaVm (High Toughness)

Kjc:117MPaVm (Low Toughness)

Black: High Toughness Red: Low Toughness

.............

20 40 60 80 100 120 140 160 Test Temperature(°C)

Fig. 16. Fracture toughness values ofhigh and low toughness weld metal.

Figure 16 shows relationship between Kih and test temperature under 20 MPa hydrogen gas condition. Additionally, tendency of Kih values for test temperature is shown in Fig. 15. Although Kih value of high toughness weld metal was higher than that of low toughness weld metal at R.T., Kih value of low toughness weld metal rise to the similar level of high toughness weld metal at 150oC. It is also to be noted that minimum Kih value can be estimated 60 MPa Vm at R.T. The reason why Kih become similar level between high and low toughness weld metal at 150oC is related to toughness at test temperature. Fig. 6 shows transition curve of high and low toughness weld metal.

On the other hand, fast fracture was occurred in low toughness weld metal at R.T. under 20MPa hydrogen gas condition. Fig. 17 shows fracture toughness value (Kjic, Kjc) of high and low toughness weld metal. Kjic of high toughness weld metal was 254 MPa V m, whereas Kjc of low toughness weld metal was 117 MPa V m. From these results, fracture toughness value (Kjic, Kjc) varied greatly between high and low toughness weld metal.

5. Conclusion

In this study, it was clear that the effect of hydrogen charge and hydrogen gas environment on the embrittlement of2.25Cr-lMo-V steel weld metal. The conclusions are described below:

(1) Hydrogen gas affected on the embrittlement of the 2.25Cr-lMo-V steel weld metal. In contrast, dissolved hydrogen affected little on the embrittlement of the weld metal.

(2) Deviation point of high toughness weld metal was almost same at R.T. and 150oC under hydrogen gas environment. However propagated crack morphology was changed from quasi-cleavage to ductile fracture at 150oC.

(3) Deviation point of low toughness weld metal was increased with increasing test temperature under hydrogen gas environment. In addition, propagated crack morphology was changed from quasi-cleavage to ductile fracture at 150oC. This tendency was almost same as high toughness weld metal.

(4) Kih value of high toughness weld metal was higher than that of low toughness at R.T. However Kih value of low toughness weld metal rise to the similar level ofhigh toughness weld metal at 150oC

(5) From holding load test results, it was evident that crack growth occurred at the load equivalent to Kih, which obtain by rising load test, for high and low toughness weld metal.

Acknowledgements

This work was supported by Material Properties Council (MPC) and their support was gratefully acknowledged.

References

[1] H.R.Gray, ASTM STP, 543 (1974) 133.

[2] H.G.Nelson, ASTM STP 543, (1974) 152

[3] J.Watanabe, T. Ishigro, T.Iwadate and K.Ohnishi, Hydrogen Embrittlement of 2/14Cr-lMo and 3Cr-lMo-0.25V-B Pressure Vessel Steels presented at API/MPC Task Group Meeting on Materials for Pressure Vessels, May, 1987

[4] Y.Wada, T. Hasegawa and H.Inoue, ASME PVP 2002