Hydrogen Induced Cracking (HIC) Investigation on High Cold Work Austenitic Stainless Steel Pressure Vessel

Pongpat Lortrakul

REPCO / Industrial Solution / SCG-Chemicals 271 Sukhumvit

Map Ta Phut, Muang, Rayong, 21150 Thailand

Panupong Srisasrat

REPCO / Industrial Solution / SCG-Chemicals 271 Sukhumvit

Map Ta Phut, Muang, Rayong, 21150 Thailand

ABSTRACT A cryogenic pressure vessel of one client leaks during plant startup. The problem causes significant plant opportunity loss and RCA is required to prevent this serious issue. Straight cracks without branch are initiated and grown from inside to outside at highest stress location where knuckle area is; and at one closing to welding toe. Strain induced martensite from cold forming, approximately 30%, causes increase of hydrogen absorption in material. Intergranular cracking indicates influence of hydrogen and trangranular cracking at dislocation path is observed. With reduction of temperature during startup, hydrogen concentration in material could possibly reach saturated point; these increase degradation of toughness and increase chance of crack formation. In this case, cracks occur with the combination of stress, martensitic phase, temperature, and hydrogen diffusion. Hot forming head and PWHT at welding seam are recommended for cryogenic pressure vessel to prevent HIC. Hardness test, ferrite number measurement, replication consider to be sufficient technique to evaluate HIC susceptibility. Key words: HIC, Hydrogen induced cracking, austenitic stainless steel, crack, cryogenic, stress induced martensite, intergranular cracking, trangranular cracking

INTRODUCTION

One of stainless steel cryogenic pressure vessel experienced cracks at bottom head result in leakage during first turnaround. The problem delays plant startup and requires immediate repair. There are significant opportunity loss and high maintenance cost for unplanned repair. Systematic root cause analysis (RCA) is performed in order to establish suitable preventive action. The pressure vessel contains liquid and gas consisting of hydrogen about 6% and others about 94%. Liquid level is about 50% in basic. Normal operating condition is -120oC under pressure 43 kg/cm2. The

dimension is 6,100 mm in height and 2,300 mm in diameter. Top and bottom head is ellipse 2:1 type made by cold forming process. Shell thickness is 37 mm and bottom head thickness is 42 mm. Material for head and shell are SA240-TP304, austenitic stainless steel. The field picture is shown in Figure 1. As cryogenic media is operated at sub-zero temperature. That gas leaks into atmosphere results to ice formation on the surface. Figure 2 shows the condition after cold insulation is removed. Further detail investigation and testing is performed.

Figure 1: Client’s cryogenic pressure vessel

Figure 2: Ice forming at leak point

FIELD INSPECTION

Various inspections in the field acquire failure data and material conditions. Visual inspection, penetrant test, UTPA, hardness test, replication, and ferrite number measurement are selected to find defects and gather more information. A straight crack without branch toward longitudinal direction is observed by visual inspection at leak point (outside). The crack locates at bottom head below circumferential welding seam and the crack is perpendicular to welding seam as shown in Figure 3. Welding joint configuration is shown in Figure 4. because bottom head is thicker than shell. Knuckle thickness for butt-weld is reduced by tapering.

Figure 3: “a” shows crack observed at external surface at leaked point. “b” shows the result of

penetrant test at the crack

Figure 4: Illustrative joint configuration between shell and bottom head

After temperature reaches to ambient, penetrate test (PT) is selected to find small cracks both inside and outside surface of the pressure vessel. 16 longitudinal cracks are randomly found both inside and outside (through crack). The length of inside crack is about 12-140 mm and the one of outside crack is about 11-25 mm. Generally, inside cracks are longer than outside ones; and cracks often close to welding toe as shown in Figure 5a and 5b. No crack is observed in other area. For double confirmation, ultrasonic test phase array (UTPA) is performed at all welding seam as well.

Figure 5: “a” shows short length of crack. One of the tips closes to welding toe. “b” shows

most severe crack. Most length of crack occurs on bottom head. Minor length crack is on welding seam and shell

High hardness is one of the causes to make stainless steel crack. Hardness test is performed at top head, shell, bottom head both inside and outside at various points. Average hardness result is summarized in Table 1a. The result reveals high hardness at top and bottom head which is greater than one of shell about 55%. In case of reference plant (no crack), both top head and shell have similar hardness as shown in Table 1b. Thus, more detail is required to observe this abnormality. Table 1: “a” shows average hardness at metal (welding seam excluded) “b” shows data from reference plant (no crack)

Replication at crack tip reveals mixture of both intergranular and trangranular cracks as shown in Figure 6a and 6b. The trangranular crack occurs at the area of high slip band density and likely to grow in the direction of dislocation (parallel to slip band). High slip band density represents experience of high cold work (%CW) or high deformation. Based on microstructure analysis comparison, %CW is approximately 30% and martensitic phase increases according to %CW due to strain induced martensite mechanism.1 Replication also performs at the base metal close to cracks. High %CW and martensitic phase in grains can be observed as shown in Figure 7a, 7b which are completely different to normal material at shell as shown in Figure 8.

Figure 6a : microstructure at one of the cracks. Intergranular and trangranular cracking types are observed

Ferrite number measurement is selected to estimate martensitic phase quantity as its magnetic property. The result summarized in Table 2a shows the inside of bottom head reveals ferrite number about 30 times as much as the one of shell part. Additionally, Ferrite number at outside of both top and bottom head are about 10 times as much as one of shell part. In case of reference plant, both top head and shell have similar ferrite number as shown in Table 2b. This abnormality requires further detail investigation. In conclusion, straight cracks without branch are randomly found at only bottom head. No crack is observed at shell and top head. Tip of cracks generally close to welding toe and crack direction is

usually longitudinal and perpendicular to circumferential welding seam. Intergranular and trangranular cracking are observed. High %CW and high martensitic phase are found. Hardness and ferrite number data support high chance of crack formation at bottom head rather than shell.

Figure 6b : microstructure at another crack. Intergranular and trangranular cracking types are observed.

Figure 7a : microstructure close to a crack at bottom head. High density of slip band is found and martensitic phase is expected for whole black grain

Figure 7b : microstructure close to a crack at bottom head for another location. Martensitic phase is expected for whole black grain

Figure 8 : microstructure close at shell side. Normal austenite phase is found and no high

slip band and expected martensite in the microstucture

Table 2: “a” shows average ferrite number at metal (welding seam excluded). “b” shows data from another reference (no crack)

DISCUSSION

Effect of hydrogen absorption on carbon steel degradation is well known in industry. Hydrogen causes reduction of strength, toughness, and it possibly changes the mode of fracture from ductile to brittle with increase of hydrogen absorption 2,3,4,5. For high temperature hydrogen attack (HTHA), carbon steel generally follows Nelson curve which guides material limitation in term of both temperature and hydrogen partial pressure. Nevertheless, austenitic stainless is out of the curve because it is not susceptible to HTHA. 5 However, phase transformation of stainless steel can change properties from ductile to brittle fracture similar to carbon steel due to strain induce martensite machanism. Cold working process can reduce material strength and toughness due to increase of martensitic phase which enhance hydrogen absorption. Affected material trends to behave as brittle material. 6,7,8,9 As effect of temperature, brittleness increases as reduction of temperature. Ductile to brittle transition temperature (DBTT) is generally considered for cryogenic application. For austenitic stainless steel, the lowest temperature is designated at -196oC 10 which is quite far from this case (-130oC). Direction of crack is depended on stress consisting of applied stress from operation and residual stress from fabrication. Then, failure of austenitic stainless steel in cryogenic hydrogen environment is likely to be contributed by stress, martensitic phase from fabrication, temperature, and hydrogen absorption. Crack morphology is straight line without branch in longitudinal direction. This is one of the hydrogen cracking types. Cracks usually occur at the highest stress. Longitudinal crack perpendicular to welding seam indicates hoop stress from pressure taking into account. 11 Tip of cracks usually close to welding toe which act as stress concentration. At this point, stress can raise up more than twice as much as stress in the base metal.12 Additionally, knuckle end is the most deformation part. Thus, the highest residual stress is expected.13 Moreover, the effect of welding process can rise up residual stress to more than 50% of yield stress.14,15,16,17 Then, in term of stress, crack likely to initiates at welding toe and grow to bottom head where high residual stress and low toughness are. Abnormal material of bottom head can be obviously found as evidences of high hardness, high ferrite number, and high % CW that are compared to ones of shell. These abnormalities possibly come from high deformation from cold forming of bottom head. When these abnormal data are compared to reference plant, they show significant difference. Reference plant shows similar hardness and ferrite for both shell and head. It is due to the head of reference plant made by hot forming process which may result to much lower martensite, hardness, and ferrite number.6,7 No crack has been experienced for 17 years in this equipment. Microstructure of crack indicates the effect of corrosion agent, probably hydrogen, as intergranular cracking is observed. Grain boundary is the weak point for crack formation due to ease for the interaction of corrosion agent to material and where material defects generally locate. Transgranular cracking in microstructure indicates high stress effect which may come from residual stress from cold

forming. The other effect is an increase of martensitic phase as strain induced martensite mechanism. Therefore, hydrogen can solute into martensitic phase weaken material by increasing brittleness.6,7,8,9 Location of crack is close to heat affected zone (HAZ) where chromium content is lower than unaffected metal because of chromium carbide precipitation. Low chromium content has low stacking fault energy resulting to more susceptible to martensitic transformation.18

Figure 9 : calculation of diffusion at operating condition

Diffusion of hydrogen depends on material, partial pressure, and temperature. With high hydrogen partial pressure and high temperature, diffusion increases according to Fick’s second law.19 Diffusion coefficient information can be found in the reference.18 To study operating condition generating high risk of crack from hydrogen absorption, trial calculation is set up to find required time for hydrogen to diffuse into material for 1 micron at concentration about 1/53 of hydrogen partial pressure at inside surface. Figure 9 illustrates the model and assumption. The result of calculation is shown in Figure 10. It is obvious that temperature below -100oC requires time more than 100 years to obtain the target concentration. Then, both conditions during startup (30oC to sub-zero) and during shutdown (sub-zero to 30oC) have high possibility for hydrogen absorption because temperature is high enough. In this case, hydrogen possibly charges into pressure vessel wall and reaches to saturated point during startup because solubility limit is decreased while temperature decrease as illustrated in Figure 11. These increase chance for crack formation.

Figure 10 : Result of calculation

Figure 11 : calculation of diffusion at operating condition

The possible cause of crack is hydrogen induced cracking. Bottom head constructed by cold forming causes high residual stress and high martensitic phase. During plant startup, hydrogen charges into material while temperature is also decreased. Solubility limitation decreases and hydrogen concentration reaches to saturated point. Crack possibly initiates internally at welding toe and grows toward bottom head in longitudinal direction and cracks propagate to outside afterward to cause leakage. The conclusion is shown in Figure 12.

Figure 12 : Conclusion of root cause

For remediation, new pressure vessel is fabricated as shown in Figure 13. For alleviating residual stress, hot forming for top and bottom head is specified and PWHT at welding is required. Hardness, ferrite number measurement, and replication are included into pressure vessel for quality control.

Figure 13 : New pressure vessel

CONCLUSIONS

At saturated hydrogen solubility, sufficient low temperature and existing residue stress & operation stress, hydrogen induced cracking possibly occurs at high cold work austenitic stainless steel.

High residual stress comes from cold forming and effect of welding process.

Strain induced martensite from cold work play a role in increasing hydrogen absorption of material that increases brittleness and increases chance of crack formation.

Hot forming head and PWHT at welding seam are suggested for cryogenic pressure vessel in hydrogen environment.

Ferrite number, hardness test, and replica test are effective to evaluate condition of material.

REFERENCES

1. A. Rezaee, “Investigation of Cold Rolling Variables on the Formation of Strain-Induced Martensite in 201L Stainless Steel,” Materials & Design, Volume 46, April 2013, p. 49-53. 2. T.G. Martin, “Hydrogen Induced Cracking Failure of SA516 Grade 70 Steel in Near Neutral to High pH Solution”, CORROSION/2003, paper no 03532. 3. API Recommendation Practice 581 Second edition (SEPTEMBER 2008),“Risk-Based Inspection Technology” 4. API Recommendation Practice 571 first edition (DECEMBER 2003),“Damage Mechanisms Affecting Fixed Equipment in The Refining Industry” 5. API RP 941 Seventh Edition, ”Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants”

6. MR Louthan, “Hydrogen Embrittlement of metals”, Mater Sci Eng, Volume 10, 1972, p. 357-368. 7. M. Martin, “Effect of Alloying Element on Hydrogen Environment Embrittlement of AISI Type 304 Austenitic Stainless Steel,” International Journal of Hydrogen Energy, Volume 36, Issue 24, December 2011, p. 15888–15898. 8. Ali Hedayati, “The Effect of Cold Rolling Regime on Microstructure and Mechanical Properties of AISI 304L Stainless Steel”, Journal of Materials Processing Technology, Volume 210, 2010, p. 1017-1022. 9. Huang Jun-xia, “Effect of Cold Rolling on Microstructure and Mechanical Properties of AISI 301LN Metastable Austenitic Stainless steel”, Journal of Iron and Steel Research International, Volume 19(10), 2012, p. 59-63. 10. David A. Hansen, Materials Selection for Hydrocarbon and Chemical Plants, CRC Press; 1 edition (August 8, 1996), p. 316. 11. Donald J. Wulpi, Understand How Component Fail, Second Edition, ASM 12. Lloyd’s Register of Shipping, “Stress Concentration Factor for Simple Tubular Joints-Assessment of Existing and Development of New Parametric Formulae”, OTH354, HSE 13. B. N. Shevelkin, “The Properties of the Metal of Heads Made by Cold and Hot Forming”, Equipment-Manufacturing Technology, Chemical and Petroleum Engineering, Volume 13, Issue 2, February 1977, p. 149-152. 14. D. P. G. Lidbury,”The Significance of Residual Stresses in Relation to The Integrity of LWR Pressure Vessel”, Int. J. Pres. Ves. & Piping, Volume 17, 1984, p. 197-328. 15. Chin-Hyung Lee, “Three-Dimensional Finite Element Simulation of Residual Stresses in Circumferential Welds of Steel Pipe Including Pipe Diameter Effects”, Materials Science and Engineering, Volume A 487, 2008, p. 210-218. 16. P. Dong, “Analysis of Residual Stresses at Weld Repairs”, International Journal of Pressure Vessels and Piping, Volume 82, 2005, p. 258-269. 17. R. B. Tait, “An Experimental Study of the Residual Stress, and Their Alleviation, in Tube to Tube-sheet Welds of Industrial Boilers, Engineering Failure Analysis, Volume 8, 2001, p. 15-27. 18. C. San Marchi, “Technical Reference on Hydrogen Compatibility of Material – Austenitic stainless steels : Type 304 & 304L (code 2101)”, Sandia National Laboratories 19. William D. Callister, Fundamentals of Material Science and Engineering, Third Edition, p. 168.