Tech Paper1-High Temp 310 SS

Dr.Edwin Jong 09/01/2007

Industrial Experiences with High Temperature Stainless Steel Materials

Ir. Dr. Edwin Jong Nyon Tchan, BSc (Hons), PhD (Imperial College), MIEM, MICorr, MIM, PEng, CEng

Principal Materials and Corrosion Engineer, Sarawak Shell Berhad, Miri

Abstract: A highly-alloyed austenitic stainless steel (AISI 310) plate material was used to fabricate the HP/LP flare tip and flare stack in an offshore platform. Shortly a year after commissioning, excessive through-thickness cracks along the heat-affected zone (HAZ) of the circumferential weldment were observed on the HP flare tip ring. This paper reveals the different analytical techniques being employed to find out some evidence on the pre-mature failure of the flare tip and thus to determine the root cause of its eventual failure after a short duration of fewer than two years in service as compared to the 20-year design intent. From this analytical work, the original microstructure of austenitic phase has in fact transformed completely into the detrimental brittle sigma phase in the welded regions as well as in the parent metal resulting a substantial loss of ductility. Background The M3 complexes have only been operating for about a year previously (since early January-1996). However, during the baseline inspection and maintenance survey that was conducted immediately after the most recent outbreak of fire on the flare deck in March-1997, it was found that the galvanised steel gratings on the flare deck were badly distorted. The HP stack revealed severe visible through-thickness cracks along the circumferential and the vertical welds. Besides, the "wine-cup" flare tip (as depicted in Figure-1) which was attached to the top end of the LP flare stack also found to have the similar type of cracks. Site Observations / Findings After having known the severity of these visible cracks in the structures from the site inspection, an investigation team was immediately formed to conduct an assessment on the flare-tip technical integrity. During the assessment / investigation work, the HP lip ring (about 200 mm high) was removed, the following surface conditions of the HP / LP stack and the external surface of the LP flare tip revealed the following abnormal conditions.

• Bulging area with ~2" bulge size was observed around the section that was just below the LP welded flange area. It appeared to be caused by metal weakening after prolonged exposure to excessive thermal and axial compressive effect acting directly from the 3-ton weight of the "wine-cup" flare tip;


• The upper section of the annular surfaces between the HP inner surface and the LP outer surface of the flare stacks was found to be covered with a thick blackish layer of coke. This was an evidence clearly caused by incomplete internal combustion during flaring.

• Numerous visible transverse cracks in a highly branching manner along vertical welds were also found;

• 6 out of the 12 pairs of guide ring supports in the HP chamber were found missing. All of the remaining ring supports were detached and bent;

• From the flare deck, the flare tip appeared to tilt slightly towards the east side where all the production coolers and separators are located.

Judging from the extent of the numerous damages caused to the HP / LP flare stack, the technical integrity of the flare tip to operate a flare was highly questionable; it was NOT safe to operate this flare tip as a flare. The team decided that it was prudent to analyse the material further and thus to determine the root cause of the premature failure in order to prevent its reoccurrence in the future. Scopes of Work Following the decision made to have an insight to this premature failure and to determine the possible root cause of its failure, several as-received failed sections (as shown in Figure-2) were retrieved for detailed laboratory analysis that comprises of the following scopes of work was jointly executed using the DNV’s laboratory facilities..

1. Visual Examination including Photographical Documentation:

• To examine and closely study the overall features of the as received failed sections.

2. Fractography:

• To identify the mode of failure and to determine the crack initiation point by scanning electron microscopy (SEM).

3. Hardness Measurements:

• To conduct a micro-hardness measurement on the austenitic matrix grains of the as-received samples.

• To check for any possible increment of hardness values after exposure to excessive burning during internal combustion inside the HP flare stack.

4. Chemical Analysis:

• To evaluate any possible influence by the detected concentration of H2S (less than 200ppm) which may lead to the eventual failure of the HP flare stack.

• To confirm the types and grade of austenitic stainless steel material.

5. Metallography:

• To evaluate and detect any abnormalities present within the microstructure of the base material,

• To identify and confirm the likely causes of the high-temperature oxidation of the base material in the presence of low percent H2S gas.

• To detect any presence of sigma phase in the material.


Laboratory Examination and Analysis Results Visual examination on the as-received cracked samples revealed that one of the main cracks in the circumferential direction was propagating along the machining marks and at the transition area from thick to thin section. Besides, another major crack that was observed to be in a highly branching manner was propagating in the vertical direction as shown in Figure-2. The fractography study that was conducted along the as-received fracture surfaces distinctively revealed the material’s brittleness in nature as depicted in Figure-3. The results indicated highly branching cracks; and these cracks appeared to follow the grain boundaries within the weld solidification pattern as clearly revealed in Figure-4. The crack morphology appeared to outline sigma phase precipitates indicating that sigma phase caused the crack branching and brittle nature of the fracture surface. Micro-hardness measurement using a Vickers Hardness Tester was carried out on the parent metal as well as on the weldment. The measured hardness values (higher than 315Hv as compared to the normal hardness of 170Hv) were unusually high for an AISI 310 stainless steel material indicating that the presence of sigma phase. Employing the energy dispersive spectrometric (EDS) technique for analysing the parent metal and the weldment, the analytical results (as depicted in Figure-5) strongly revealed that an intermetallic phase consisted of mainly FeCr (i.e. 50%Cr and 50%Fe) was detected on both the parent metal and the weldment. This is of course the typical composition of the sigma phase intermetallic compound as confirmed from the iron-chromium phase diagram (as indicated in Figure-6) where the sigma phase formation is shown to occur in the sigma loop. From the metallographic study on the as-received failed samples, it clearly revealed that the original microstructure of AISI 310 stainless steel material (mainly austenite as depicted in Figure-7) had completely transformed into the detrimental sigma phase. The presence of the detrimental sigma phase was detected in both the parent metal and the weldment as evident in Figure-8. This sigma phase was observed to be more elongated at the grain boundary. From the interpretation of the Fe-Cr Binary Equilibrium Phase Diagram (see Figure-6), it is clearly illustrated that the flare tip had been exposed to the temperature range of 600oC and above. Figure-9 illustrating the effect on the ratio of Ni to Cr on the formation of sigma phase after a 3000-hour exposure at three selected elevated temperatures further explains the likelihood of sigma phase formation in the 310 type stainless steel materials. Discussion In general, the studies from metallography, energy dispersive spectrometric (EDS) and micro-hardness testing have in fact confirmed to each other on the presence of sigma phase, which at room temperature is hard, brittle and non-magnetic phase consisting of intermetallic compound FeCr with a complex tetragonal crystal lattice. Commercially available austenitic stainless steels contain sufficient carbon, as a rule, to make plain 18Cr-8Ni type (e.g. type 304, type 304L) rather immune to sigma


phase formation, but with the higher Cr types are prone to sigma formation. For this case, the type 310 contains 25%Cr and 21%Ni has possessed adequate Cr to promote sigma formation during exposure for a prolonged duration in the temperature range above 600oC. Judging from the analyses, chromium carbide formation is very unlikely due to the low percentage of carbon (only 0.02wt%) presence in the said material. For the 300 series of austenitic stainless steel with less than 0.03wt% carbon, the formation of chromium carbide at the temperature range between 450oC to 870oC requires very long exposure times. This is confirmed by the EDS analysis on the intermetallic phase that significant carbide formation has not occurred. Also, it is found that the presence H2S had not caused any visible degradation to the flare tip. The mechanism of failure can indeed be explained as follows:- Type 310 stainless steel material is susceptible to sigma phase formation once it is subjected to temperature above 600oC, within an order of weeks of operation for the extreme case. The amount of sigma phase will kinetically grow with times during the operation of the flare tip as a flare. As the amount of sigma phase increases, the material of the flare tip will become more brittle, harder but less ductile. Whenever the system is shut down for inspection, or due to other possibilities such as fluctuation of temperatures, vibration, the flare tip is subjected to high thermal stresses. As the exposure times increase, the flare tip becomes more brittle and its ductility is drastically reduced. Cracking starts to develop at the weaker area within the grain boundary especially in the weldment (see Figures 2 & 3) and at the transition area between the thick and thin sections. Conclusion The failure of the flare tip was due to its prolonged exposure to inappropriate operating temperature range (above 600oC) and the material’s inherent susceptibility to sigma phase formation, which have embrittled the material during service and lead to its eventual failure. Lesson Learnt / Experiences

•••• Operating equipment outside the design operating envelops The bulging section of the LP stack was believed to be caused by excessive weight of the "wine cup" (3 tons in weight) constantly acting / compressing on the LP stack. During the most recent fire incident, this section had revealed that it had frequently been heated to excessive high temperatures (this is most likely caused by production / process upset) causing softening effect to the metal together with the 3-ton weight directly compressing on it. This resulted the top end of this conical LP flare stack next to the butt-welded joint of the flange connection to "BULGE" (see Figure-1 attached). This following evidence is essential to warrant conducting a review to the existing design of the flare tip structure, and possibly needs to revise the design to suit the operating environment.


a) The smooth surface on the base metal as if it has been stressed beyond its plastic region; this evidence confirms the LP flare stake has been excessively compressed.

b) All hair-line cracks on the weldment have revealed a similar type of crack orientation as if they have been subjected to shear forces; this evidence reveals that the high temperatures generated by flaring has indeed approached at least 50% of the material’s melting temperature (~1543oC).

c) The thick blackish layer of coke adhered onto the HP / LP annular surfaces

revealing that the structures had been subjected to incomplete internal combustion.

•••• Inappropriate Design with Wrong Material Specification AISI 310 austenitic stainless steel containing 25%Cr and 21%Ni as the main alloying elements is a material usually for high temperature application. However, it has high percentage of chromium to promote sigma formation after prolonged exposure for appropriate times in the range of 590 - 870oC (see Figure-6) which reveals the Fe-Cr binary phase diagram at varying isothermal temperature, where the sigma (FeCr) phase formation occurs in the sigma loop. Figure-9 depicts the effect of the ratio of nickel to chromium on the formation of sigma phase at three temperatures after a 3,000-hour exposure. From the metallurgical analysis, it was confirmed that the detrimental brittle sigma phase (see Figure-6) was detected and identified both in the parent material and in the weldment indicating the flare tip had been exposed to elevated temperatures above 600oC. With our recommendation to change out the single combined HP / LP flare stack design to a separate individual HP / LP flare stack design and thus to reduce the dead load (3-ton weight) of the "wine cup" flare tip but is still serving the same intended purpose, the new M3 HP / LP flare tips have been successfully commissioned since September, 1997. At the same time, a tangible saving of more than RM 2-million on materials alone is realised as compared to the original design. So far, there is no single report related to excessive burning on the flare tip. The baseline inspection being conducted in August 1998 when they are due for inspection and maintenance has proven their effectiveness and reliability. Reference 1. ASM/MEI Stainless Steels, 18: Physical Metallurgy of Stainless Steels. 2. Powder Diffraction File – Alphabetical Index, Inorganic Phases, pub. International

Centre for Diffraction Data, Swarthmore, USA (1988) 3. CRC Handbook of Chemistry and Physics, 67th Edn., eds. R.C. Weast, M.J. Astle

and W.H.Beyer, pub. CRC Press, Florida (1987)


Figure-1: The original "wine-cup" flare tip as installed in M3.

Figure-2: A section of the as-received cracked sample from the region as indicated.

Figure-3: A micrograph showing the brittle nature of the fracture surface


Figure-4: A micrograph showing that cracks appear to follow the grain boundaries within the weld solidification pattern

Figure-5: A typical spectrograph revealing the main chemical elements detected on both the parent metal and the weldment

Figure-6: The Fe-Cr Binary Phase Diagram

Figure-7:

Micrographs showing the original microstructure of AISI 310 stainless steel material consisting of mainly the austenitic phase

Cr

Fe


In the weldment In the parent metal

Figure-8: Micrographs showing the presence of brittle Intermetallic sigma phase on both the parent metal and the weldment

Figure-9: The effect on the ratio of nickel to chromium on the formation of sigma phase after a 3000-hour exposure at three different selected elevated temperatures

Cr

Ni

Tech Paper1-High Temp 310 SS

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