1. 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 -120o
C under pressure 43 kg/cm2
. The

2. 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

3. 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

4. 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)

5. 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

6. 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

7. 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

8. 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 -196o
C 10
which is quite far from this case (-130o
C). 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

9. 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 -100o
C requires time more than 100 years to obtain the target
concentration. Then, both conditions during startup (30o
C to sub-zero) and during shutdown (sub-zero
to 30o
C) 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

10. 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

11. 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.
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