Jorge Hau and Antonio Seijas Lloyd’s Register Capstone, Inc

1505 Highway 6 South, Suite 250 Houston, TX 77077 jhau@cap-eng.com

ABSTRACT

This paper describes assessment of sigma phase embrittlement in austenitic stainless steels such as Type 304H, commonly used in fluid catalytic cracking (FCC) units. Other austenitic stainless steels used in other refining process units are also discussed. The detection and measuring of the amount of sigma phase were made using metallography. It was found that the relationship of the amount of sigma phase with time in refining service has not yet been established and that, rather than the amount, the most important parameter is to assess the degree of embrittlement attained. This depends not only on the amount of sigma phase but also on the size and distribution, as well as the presence, amount, size and distribution of other intermetallic particles that also precipitate during service. Charpy V-notch (CVN) tests indicate the overall effect and contribution of all these factors. It is proposed to use the requirement of meeting 20 J (15 ft-lbf) at 0°C (32°F) with no single value less than 13 J (10 ft-lbf). Although no criterion was given for hot impact testing, it is considered that CVN tests conducted at service temperature provides useful information about the degree of embrittlement that applies when the metal is hot.

Keywords: Sigma phase, embrittlement, austenitic stainless steel, FCC, Charpy Test, degradation mechanisms.

INTRODUCTION

Sigma phase is a non-magnetic intermetallic phase composed mainly of iron and chromium which forms in ferritic and austenitic stainless steels during exposure at 560º-980ºC (1,050º-1,800ºF)1. It causes loss of ductility, toughness and is generally strain intolerant at temperatures under 120º-150ºC (250º-300ºF) but it is believed it has little effect on properties in the temperature range where it forms. If this is so it would appear that there should be little consequence as long as the affected components continuously operate at the elevated temperature. However, cracking could occur if the components were impact loaded or excessively stressed during maintenance work.

Austenitic stainless steel Type 304H (TP 304H SS) has traditionally been used in FCC regenerator internals, associated equipment, and piping involving temperatures about 650º-760ºC (1,200º-1,400ºF). This stainless steel is chosen as it meets a cost effective solution for a material with the necessary oxidation resistance, strength, and creep properties for this service. Over time, however, as the ductility and toughness decrease because of the presence of sigma phase, the question is often asked as to replacement timing or criteria for replacement. The effect of sigma phase on the degradation of creep properties has also been a concern but this issue was not

Sigma Phase Embrittlement of Stainless Steel in FCC Service

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addressed in this paper. To assess the effect of sigma phase on reducing creep properties and ductility, creep testing of samples removed from service may be necessary.

After having been involved in several studies in different FCC and other oil refining process units, the opportunity was taken to select some relevant findings and testing results obtained when examining sigmatized austenitic stainless steels. This study is intended to complement the information available in previous publications on the same subject2,3,4 of sigma phase embrittlement and to propose a criterion to assist in the decision making process.

EXPERIMENTAL PROCEDURE

The presence of sigma phase in TP 304H SS is determined by metallography, either using samples removed from service or field metallography replication (FMR or in-situ metallography). This steel is delivered in the solution-annealed condition when it contains all its alloying elements in solid solution and hence its microstructure appears as a fairly homogeneous single phase alloy. This microstructure changes upon aging. Figure 1a shows stainless steel 304H with 14 years in service at a nominal temperature of ≈716ºC (≈1,320ºF). The sample was taken from a FCC regenerator cyclone. The microstructure could be revealed by etching in Vilella’s reagent (5 ml HCl, 1 g picric acid, in 100 ml ethanol or methanol) but usually this steel would not respond to this etching unless it is sensitized. In this case the matrix darkened slightly and the sigma phase was revealed as white-etching particles at grain boundaries. The carbides were revealed exactly the same as sigma phase and, therefore, they could not be distinguished from each other. Electrolytic etching in KOH aqueous solution revealed the sigma phase as dark-etching particles at grain boundaries, Figure 1b. Under the optical microscope, they may actually appear with a red-brownish-orange color, which constitutes the basis to identify sigma phase with this etching method. However, over etching tends to blacken the sigma phase particles.

Figure 2 shows the microstructure of TP 304H SS as seen in a FMR taken after electrolytic etching in 10% oxalic acid solution. The location where this FMR was taken was the outside surface of a FCC regenerator flue gas line. Notice the contrast achieved in the optical microscope is the same as it would be if the steel had been examined directly, except for color. Under the optical microscope the microstructure is seen in color while the microstructure in the FMR is seen in black and white. The FMR method does not copy any color, only the surface topography. Electrolytic etching using an aqueous solution of 10% oxalic acid tends to reveal the general steel microstructure, making grain boundaries, grains, sigma phase, carbides and other intermetallic phases visible. Apart from the sigma phase, recognized because of its characteristic morphology and location at grain boundaries, there are numerous tiny particles within the grains appearing as black dots and there are also needles. Some of the thicker needles and larger particles within the grains also appeared to be brownish-orange in color when electrolytically etched in aqueous KOH solution. This suggests that they are sigma phase but the finer particles and needles still appeared black.

Lack of color of the phases revealed by electrolytic etching in aqueous KOH solution does not necessarily mean they are not sigma phase. Etching does not reveal color in the tiny particles and fine needles. In these studies, no attempts were made to positively identify these particles using, for example, particle extraction and x-ray diffraction analysis or diffraction pattern analysis in thin-foil transmission electron microscopy (TEM) to determine if they were sigma phase, carbides, or other intermetallic phases. Also, the impact property losses were not evaluated in terms of the morphology of sigma phase to determine if needle-shaped sigma was much more damaging than blocky-intergranular sigma. The sigma phase was measured as a total amount without making any discrimination between these two sigma phase morphologies.

The amount of sigma phase present can be measured by using the point counting method5 or simply by using a digital image analyzer. For the former method, a point-count grid is used and randomly placed on several different fields of view. The amount of intersections falling on a sigma phase particle is counted. Ideally, there should not be more than one intersection falling on a single sigma phase particle as this would count as one intersection. The particle is counted as half if the grid intersection does not fall on to the particle but only touches it. The percentage of sigma phase is calculated out of the total amount of points counted; the larger the amount of

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views and points counted, the more accurate the measurement is. Digital image analyzers work by measuring the area occupied in the view by the second phase particle of interest. The phase to be analyzed is selected by adjusting the control on the image of a properly etched metallographic specimen. The selected phase turns blue in color and the tuning is done by carefully examining the screen until including all the particles that were previously identified to be sigma phase. In both methods, it is necessary to choose the right magnification and take as many fields of view as necessary to achieve the required accuracy. The numerous tiny particles and small and thin black needles when present within the grains were not included in the count but the thicker needles and larger particles within the grains with a brownish-orange color were all included in the count.

Fracture surface morphology was examined in the scanning electron microscope (SEM). Elemental microanalysis was performed by energy dispersive x-ray spectroscopy (EDS) in conjunction with the SEM.

CVN testing was performed at both ambient and elevated temperature. CVN specimens were placed inside a furnace and soaked until achieving slightly higher temperature than the selected level. The intent was not to further age the CVN specimen but rather to raise its temperature to the desired level for conducting the Charpy test when the steel reaches a temperature similar to the nominal operating temperature. The CVN specimens were then quickly placed in the impact testing machine and broken within seconds to avoid significant cooling. The actual metal temperature at the time of the breakage may have fallen slightly during the transfer and upon contacting the colder testing machine specimen holder but this cooling rate was not measured. The test provided hot impact strength properties as compared with the room temperature properties.

Tensile tests were also performed at both room temperature and at high temperatures. Tensile test results were not as revealing as CVN test results. The level of tensile strength, yield strength, and hardness increased slightly but the differences between specimens removed from service and unaged steel was not always significant. The elongation and reduction in area for the specimens removed from service were lower that for unaged steel. Because the differences obtained in the results from CVN test were more marked, they were used instead of the results from tensile test.

RESULTS

Sigma Phase Formation

Typical measured amounts of sigma phase in 9 to 12 fields from the same base metal samples were 1.9 ± 0.6%, 2.6 ± 0.8%, 3.5 ± 0.6%, and 12.0± 2.0%. Typical amounts of weld metal sigma phase were 4.9 ± 1.9%, 6.0 ± 1.4%, 8.9 ± 0.9%, 11.4 ± 2.4%, and 13.8 ± 3.4%. These measurements were obtained with image analyzer that is based on area %. Precision was calculated by assuming a normal (Gaussian) distribution and finding the 95% confidence limits of individual values around the mean. These values give some idea of the scatter and are calculated by using the following expression: mean value ± 1.96 x standard deviation. The normal distribution table gives the value of 1.96 for 95% confidence limits6. The variation is higher when including different metallographic specimens or locations within the same component or from different components of the same system. The amount of sigma phase and associated variation measured when examining several metallographic specimens or several FMR locations from the same equipment or component was used to represent its sigma phase content. These measurements of sigma phase content could be for a particular FCC regenerator cyclone, cyclone system or the FCC regenerator overhead line.

Figure 3 shows a histogram of 33 different fields of view where the amount of sigma phase was estimated using the point-count grid method on metallographic specimens and also on FMR’s taken from Type 304H cyclones and the regenerator overhead line in a FCC unit. The time in service was 8 years and the nominal metal temperature was assumed to be ≈716°C (≈1,320°F). In this case, the measurement of sigma phase content was performed only on base metal; weld metal was not included. The horizontal axis (abscissa) represents sigma phase content intervals (e.g. 1 to 2%, 2 to 3%, up to 9 to 10%). There were four cases where the amount of sigma phase measured fell within the interval 1 to 2%; another four cases where it fell within the interval 2 to 3%; ten cases where it fell within the interval 3 to 4%; and so on. These numbers of cases are referred to as frequency or counts

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and are represented on the vertical axis (ordinate) so that the height of the bar on the interval of 1-2% sigma phase is four, on the interval of 2-3% sigma phase is four, on the interval of 3-4% sigma phase is ten, and so on. The mean or average value of sigma phase content was 4.3% but the data scattering was relatively large, the minimum value found being 1.1% and the maximum, 9.4%. Applying basic statistics, it is possible to state with 95% confidence that the true mean value for the sigma phase content was between 3.6% and 5.0%. This was calculated by using the t-student distribution6 and the standard error ( ± t95 (standard deviation/√ n, where n is the number of individual measurements).

Figures 4 and 5 are corresponding histograms for TP 304 SS base and weld metal from regenerator cyclones after 22 years of service at a nominal metal temperature assumed to be ≈716°C (≈1,320°F). If the data fits a normal distribution the histogram should eventually result in a bell-shaped curve by indefinitely increasing the amount of fields of view. The bell-shaped curve depicted on the histogram in Figure 4 was drawn using the corresponding formula for a normal distribution based on the estimated mean value and standard deviation. The estimated mean value of sigma phase content was 5.5%. Notice that there are four sigma phase measurements that fell within the interval 7 to 9%, one within the interval 9 to 11%, and one within the interval 11 to 13%. The 95% confidence limits for the mean value were 4.1% and 6.9%. In the case of weld metal in the same cyclones, Figure 5, the 95% confidence limits for the mean value were 4.4% and 9.8%. The sigma phase content typically appeared higher in the weld metal than in the base metal in aged austenitic stainless steel.

It is important to realize that the amount of sigma phase measured will vary from place to place, not only as a result of the experimental error but also because it varies from place to place within the same sample. This is why, it was considered to be more appropriate to provide an interval estimate for the average amount of measured sigma phase, rather than give a point estimate of the amount of sigma phase found. Sigma phase content was measured in several places in the same cyclone, cyclone system, FCC regenerator overhead line, and the 95% confidence intervals for the mean value were calculated.

In another study, the base and weld metal of a regenerator overhead line fabricated with a modified version TP 304H SS (304 mod) was examined to determine the amount of sigma phase and level of embrittlement. The results of measuring the sigma phase content are shown in Figures 6 and 7. This regenerator overhead line had 17 years in service at the time these measurements were taken. The nominal metal temperature was assumed to be ≈716°C (≈1,320°F). The 95% confidence limits for the mean value of sigma phase measured were 2.5% and 5.5% for the base metal and 5.9% to 11.5% for the weld metal.

The overhead line (large size pipe) where the results in Figures 6 and 7 were obtained is a hot wall design having no internal refractory lining, containing three mitered 90° elbows, two of which form a U-bend at the top of the regenerator. The welds at the short radius side (intrados) of these two elbows are often found with cracks at every turnaround, Figure 8. Typically the crack indications have been removed by grinding. Crack removal has been followed by liquid penetrant (LP) testing to verify successful crack removal. In the case of Figure 8, a location in a weld at the short radius side of one of these two elbows, grinding was performed to about half the wall thickness but liquid penetrant testing still revealed cracks in the weld.

This study (Figures 6 and 7) found that sigma phase can precipitate in weld metal after as little as one year of service at a nominal temperature of ≈716°C (≈1,320°F). This is not surprising as other literature shows sigma forming from weld delta ferrite within several hundred hours after elevated temperature exposure7. Welds were made with electrode TP 308H SS and were specified to have a ferrite content ranging from 3 to 8%. The ferrite phase tends to nucleate sigma phase faster than from the austenite phase. The stainless steel electrode specified for shielded metal arc welding is AWS classification E-308H which has 18.0-21.0 % Cr and 9.0-11.0 % Ni, as compared with 18.0-20.0 % Cr and 8.0-10.5 % Ni specified for Type 304H stainless steel base metal. The cracking shown in Figure 8 was attributed to sigma phase based on metallography and the fact that sigma phase content reached values up to 14%. It is realized that such cracking may also be due to reduced creep ductility because of the presence of sigma phase but this could not be confirmed based solely on metallography.

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In general, in several examinations that have been performed in different refineries, there has been no clear tendency of increasing sigma phase formation measured in subsequent turnarounds. Measurements have been repeated with a frequency of 2 to 6 years and the amount of sigma phase appeared unchanged. Either the scatter in the amount of sigma phase measured hid any increase in sigma phase content or the steel attained a certain equilibrium amount of sigma phase. Even though all of these steels belong to the same specification, the amount of sigma phase formed during service appeared to be more dependent on steel chemistry than on time in service, even though all these steels have the same nominal chemistry within the ASTM specification. This statement is based on the fact that steels with the same service time may exhibit significantly different amount of sigma phase.

Cold Work Effect

Several factors are known to influence the sigma phase transformation. Cold work prior to aging and the presence of ferrite and some ferrite forming alloy elements, are known to significantly accelerate the kinetics of sigma phase formation. Figure 9 shows the microstructure observed near the outside surface of a FCC regenerator internal plenum chamber, after electrolytic etching (aqueous 33% KOH solution). The condition found through-thickness is illustrated in Figure 10 showing the microstructure of the same sample but at midwall. It is understood that this steel had been in service for 8 years at a nominal temperature of ≈716ºC (≈1,320ºF). There are a certain amount of thin platelets within the grains. While some of these platelets may be sigma phase, the bulk of the sigma phase formed at grain boundaries. The estimated amount of sigma phase was 6.0%. Near the outside surface (Figure 9), there was a region heavily populated with sigma phase particles that formed indiscriminately in the microstructure, at grain boundaries as well as within the grains. A rather thin layer of metal, less than 0.15 mm (0.006 in) deep from the outside surface, did not have any sigma phase particles.

The reason for having an increased amount of sigma phase near the surface became evident when electrolytic etching with 10% oxalic acid, Figure 11. There was some superficial cold work in this region, possibly introduced when manufacturing the plenum chamber. Cold work was recognized due to the presence of deformation twins and slip bands within the grains. The presence of cold work prior to aging induced the formation of a large quantity of sigma phase particles.

Mechanical Testing

The presence of sigma phase in the steel produces a strengthening effect noticed most evidently in the ambient yield strength and hardness of the steel, as compared with unaged condition. Since unaged material from the same heat was not available for establishing a comparison, the approach used in the literature2-3 was to solution anneal pieces from the same steel sample to produce a base-line reference condition. Sigma phase and other second phase particles redissolved by holding the material at 1,010º-1,066ºC (1,850º-1,950ºF) for 1 to 4 hours depending on thickness and amount of sigma that has formed. Annealing did not change chemical composition and was used to establish comparison between sigmatized and non-sigmatized steel. Their results clearly indicated the effect of sigma phase on the mechanical properties. The most evident effect of sigma phase on stainless steel is the loss of room temperature ductility and toughness. In this paper, new unaged TP 340H SS material was used as a reference in some studies and the annealing treatment of samples removed from service was used in other studies.

The obtained results from mechanical testing have confirmed the findings already reported in the literature. Figure 12 illustrates the morphology of brittle fracture produced during tensile testing at room temperature samples removed from the FCC flue gas line made of TP 304H SS with 12% sigma phase. Fracture surfaces are perpendicular to the longitudinal direction of the specimen and do not show necking, a sign of brittleness (Figure 12a). Tensile testing at service temperature, the same steel exhibited a 45° ductile fracture surface and some amount of necking (Figure 12b). At service temperature the steel recovered some ductility, as expected.

CVN tests (full size specimens) done at both room temperature and service temperature tests are shown in Table 1 and Figure 13, which summarize some of the results obtained in studies done in different units and different refineries. When comparing results from the base and weld metal, it is obvious that the weld metal

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suffered the most loss of ductility, not surprising given the original weld delta ferrite. This was best demonstrated by samples from refinery “A” (17 years of service) and is illustrated by the two upper lines in Figure 13. Cracking in welds is a common problem in FCC regenerator cyclone systems or in regenerator overhead lines. The lower CVN energy values for weld metal was corroborated by metallographic examination , where the measured sigma phase for the base metal and weld metal was 4.0% and 8.7%, respectively.

The correlation of the degree of embrittlement with the amount of sigma phase has not been established for the steels that have been evaluated by the authors in the time scale applicable to process units in the oil refining industry. The CVN energy values for base metal samples from refinery “A” appeared acceptable even though the amount of sigma phase was 4.0%. Conversely, the CVN energy values for base metal samples from refinery “C”, with only 1.5% sigma phase, appeared as low as those for the weld metal with 8.7% sigma phase at room temperature in Figure 13. Therefore, CVN test results are more revealing than just the knowledge of the amount of sigma phase. Formation of carbides during exposure in the above elevated temperature range also causes low temperature embrittlement. These precipitates are smaller than sigma phase particles and are more evenly distributed. They were not included in the metallographic sigma phase measurements.

Hot CVN test results always produced values above 27 J (20 ft-lbf), except for one sample that produced 15 J (11 ft-lbf) when impact tested at ≈716°C (≈1,320°F). Overall, there was a significant improvement in ductility when compared to room temperature test results. Room temperature values were low for the case of refinery “B” with TP 304 SS having average values of 12% sigma phase, lower line in Figure 13. As an average, the CVN value was only 12 J (9 ft-lbf).

There is no available criterion establishing CVN impact requirement for sigmatized TP 304H SS. In the case of carbon and Cr-Mo pressure vessel steels, the minimum requirement is often specified to be either 20 J or 27 J (15 or 20 ft-lbf), depending on the minimum specified yield strength and applicable wall thickness8. If these limits were to be used for all the cases in Table 1 and Figure 13, the lower line for refinery “B” would not meet this requirement. Room temperature CVN results for TP 304H SS having 19 years of service; were below 20 J (15 ft-lbf). At service temperature, the average for this same steel was 43 J (32 ft-lbf) but with a single value giving 15 J (11 ft-lbf). This low value should be considered real taking into consideration the scatter that is usually observed in the amount of sigma phase present (Figures 3 through 7). The average amount of sigma phase present is a measure of tendency to embrittle, and as such, there were obviously places including welds in this steel that had sigma phase content higher than the average value of 12%.

The straight lines joining the data at room and service temperature in Figure 13 were best-fitted lines. For the worst case at the bottom of the graph in Figure 13, the line crosses the limit of 20 J (15 ft-lbf) at about 204°C (400°F) and the limit of 27 J (20 ft-lbf) at about 371°C (700°F). Figure 14 shows bend test results performed at the indicated temperature in TP 304H SS with 12% sigma phase content. The material passes the bend test at temperatures of 316°C (600°F) and 704°C (1,300°F), as expected, but it failed at both room temperature and 204°C (400°F), demonstrating that the steel does not have to reach room temperature to become brittle. At 204°C (400°F), it was as brittle as at room temperature. It is not possible to be more accurate in plotting a best fitted line with data so scattered but it is no coincidence that up to about 204°C (400°F) this steel did not meet the 20 J (15 ft-lbf) criterion and did not pass the bending test. This seems to confirm that the degree of embrittlement achieved when the Charpy V-notch test result is below this limit may be significant.

Fractography

Austenitic TP 304H SS and, in general, any of 300 series stainless steels are fairly ductile and tough. Figure 15 shows the fracture surface obtained after tensile testing unaged 304H material at room temperature. There was considerable necking before the round specimen finally fractured, producing the typical cup-and-cone fracture and 100% ductile fracture containing dimples of different sizes. The fracture occurred by pure microvoid coalescence.

Figure 16 shows the fracture surface of a steel sample removed from a cyclone in a FCC unit. The steel had been in service 8 years at a nominal temperature of ≈716°C (≈1,320°F) and exhibited an average 4.4% sigma

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phase. The small boat sample was cut with a small grinding wheel. There were no cracks in the area this sample was taken from. The small piece was made to fracture by pulling it away from the wall. The fracture surface examined in Figure 16 corresponded to this last fracture. The surface contained dimples of a great variety of sizes but with broken sigma phase particles exhibiting cleavage fracture.

The relative elemental concentrations of iron and chromium were obtained across the surface using SEM/EDS. The corresponding dot maps were recorded using image brightness intensity as a direct function of the local concentration of the element present. Additionally, three regions of interest were selected and identified with the numbers 1, 2, and 3 on the SEM photomicrograph in Figure 16a. The region identified as number 1 corresponded to a sigma phase particle. There is another similar particle on the right-hand side of this. The region identified as number 2 corresponded to an area with dimples and with a secondary crack. The region identified as number 3 corresponded to an area with dimples and represented the austenitic matrix or base metal.

The distribution of iron in Figure 16b appeared to be uniform, except for multiple discrete spots on the left-hand side lower corner of the photomicrograph, where less iron was observed. There are two additional iron-depleted areas appearing just below the second sigma phase particle on the right-hand side and an area on the right-hand side lower corner of the photomicrograph. These depleted zones corresponded to deep depressions, and could be due to surface topography. The edges or borders of the two indicated sigma phase particles could not be clearly distinguished from the background in the iron map (Figure 16b) but were easily recognized in the chromium map in Figure 16c. The image brightness intensity was similar for the sigma phase particle and the background as they both have similar concentration of iron. Regarding chromium content, the sigma phase particles show higher image brightness intensity than the background, indicating that the chromium concentration in the sigma phase particle was higher than of the matrix. Multiple discrete spots on the left-hand side lower corner of the photomicrograph that showed no iron appeared brightened by high chromium concentration. This means that in these spots there is more chromium and less iron. These are carbide particles that had not been seen in Figure 16a because they are located deep inside these dimples and were not obvious.

The EDS spectrum in Figure 16d confirmed that the particle indicated with the number 1 in Figure 16a was sigma phase. As is typical of sigma phase, Kα energy peaks for iron and chromium appeared at the same height in the EDS spectrum in Figure 16d. The quantities of Cr and Fe were estimated to be 35 and 55%, respectively. The EDS spectrum taken at region number 2 (Figure 16e), indicated the predominant presence of chromium, as expected from carbide particles. The quantities of Cr and Fe were estimated to be 67 and 27%, respectively. Typical carbides found in 300 series stainless steels (Cr23C6) are expected to contain nil iron so this iron must be from the matrix behind or around the carbides or the presence of some Fe carbides. The EDS spectrum obtained from the region number 3 (Figure 16f) gives a composition matching the base metal. The quantities of Cr and Ni were estimated to be 20 and 8%, respectively, which are basically the same values given by the nominal chemical composition of this steel.

The fracture surface of the tensile test specimens and also of the broken Charpy V-notch specimens were all examined. There were observed differences between specimens broken at room temperature and specimens broken near service temperature. The broken sigma phase particles or areas having the sigma phase were more evident in the specimens broken at room temperature than at service temperature. There were more dimples in the fracture of specimen broken at service temperature but these also had spots with smoother fracture surface that were identified as sigma phase, Figure 17. The smoother and flat surface seen in these spots suggested cleavage fracture. The carbide particles were obvious at the center on many dimples in the fracture surface produced at high temperature. Therefore, in essence, the room temperature and hot fracture in aged steel with sigma phase and carbide particles was similar in that they both have cleavage and ductile features. At high temperature, however, the amount of spots with cleavage fracture appeared lower than at room temperature. The fracture morphology in places away from sigma phase particles was consistent with microvoid coalescence.

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Sigma Phase Formation in Steels Other Than TP 304 SS

Over time, sigma phase formation is unavoidable in many of the commercial austenitic stainless steel alloys used within the temperature range of 560°-980°C (1,050°-1,800°F). Series 300stainless steels are all susceptible to sigma phase embrittlement. Type 310 stainless steel (24-26% Cr compared with 18-20% Cr for Type 304) is very prone to significant sigma phase precipitation if exposed to these high temperatures. Types 316, 321, and 347stainless steels are commonly used in the oil refining industry, particularly in heater tubes or as weld overlay or cladding. Hence, it should not be a surprise to see sigma phase when examining furnace tubes made of these steels.

Figure 18 shows sigma phase particles found in a TP 347H SS heater tube after 3.6 years in delayed coker service at skin metal temperature varying from 538ºC to 766ºC (1,000º to 1,410ºF). The tube wall at this location in the furnace was approximately 4.3 mm (0.169 in) thick; the inside surface of the tube is on the left-hand side. Because of coke deposition within the tube, the inside surface was severely carburized. The carburized layer contained a crack. Note that there is a uniform distribution of sigma within the tube, except in the carburized region. Microhardness measurements as high as 450 HV were obtained (equivalent to 45.2 HRC) on the carburized layer. Omega creep tests were performed in this case and this amount of sigma phase did not result in any significant degree of creep strength deterioration. The remaining life was more severely affected by wall thinning due to high-temperature corrosion of the heavily carburized layer in the steel and this superseded the possible effect of sigma phase presence. The total creep strains achieved after completion of the tests were 13 to 37%, the only specimen that ruptured achieved 13% creep strain.

Lower amounts of sigma phase near and in the carburized layer have also been observed when examining TP 304H SS. At the surface and near surface, a carburized structure is the predominant feature, and sigma phase is no longer evident. Ideally, a metallographic sample should be removed to be able to examine the material in the through wall direction but if this is not possible and replication is to be done on a carburized surface, it is recommended to remove at least 1.6 mm (1/16 in) from the surface before attempting to obtain a FMR. Magnetic testing may be performed to detect the presence of carburization.

DISCUSSIONS

Sigma phase definitely affects the mechanical properties of TP 304H SS. Fortunately, few on-stream failures have been directly attributed to it. Typical failures are expected to occur if the steel is exposed to temperatures under the critical temperature range (where toughness values drop) and subjected to adverse loading conditions or to shock loadings. This may occur under a number of scenarios, such as during cool down when differential thermal contraction forces arise, or with maintenance activities like welding, straightening, or refractory chipping (e.g. impact loading to remove refractory) the affected components or parts. Some precautions used for welding onto sigmatized stainless steel include a solution anneal before welding. Where the metallurgical condition is unknown, care must be taken to minimize or try avoiding altogether impact or suddenly applied high stress when the unit is out of service.

Sigma phase formation is due to thermodynamic instability, and precipitation of second phase intermetallic precipitates. Its presence can be easily identified by metallography. Bend or impact testing is required to determine the degree of embrittlement. Tensile strength, yield strength, and hardness may change from the original values but usually without adversely affecting the integrity of the metal and the components. These tests do not give adequate indication of loss of ductility due to sigma phase.

The presence, amount, distribution, and morphology of sigma phase may also affect creep properties and reduce creep ductility. Creep testing will probably be required to be able to assess the degree of deterioration; in this case, the acceptable limits would probably be defined in terms of remaining life but this was outside the scope of these studies.

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It has not yet been possible to derive a valid trend indicating the amount of sigma phase formed as a function of time in service for refinery process units. This would require establishing a program starting from the installation of new TP 304H SS and measuring the amount of sigma phase every opportunity available during a prolonged time in service. However, refinery process units are run continuously for at least 2 to 4 years. This means that the first measurement for new steel could be taken after 17,000 to 35,000 hours exposure time; the second and third opportunities could occur after 34,000 to 70,000 hours and 51,000-105,000 hours, respectively.

An evaluation of welds was conducted and it was found that a significant amount of sigma phase formed after only 1 year of service. The literature9 refers to C-Type reaction curve for sigma phase formation. These are curves indicating the start and zone of sigma phase formation at any given combination of temperature and exposure time, plotted as a logarithm of time versus temperature. There is a critical temperature range represented by the nose of the C-curve where sigma phase forms faster. Above and below this critical temperature range, sigma phase formation is more sluggish and takes much longer time to precipitate. If the exposure temperature is too far apart from this critical temperature range, sigma phase formation may not form at all. The time required to start detecting sigma phase at conditions corresponding to the nose of the C-curve has been measured in minutes, hours, or days depending on temperature and steel condition. When the cold working factor has been introduced in these studies reported in the literature9, it has been demonstrated that cold working is a most important factor in reducing the time required to form sigma phase.

Sigma phase formation in stainless steel base metal is more sluggish than in weld metal, since the original weld delta ferrite easily transforms. The amount of sigma phase may vary within the same piece of equipment from one area to another. Depending of the amount of sigma phase, it has been found that the amount of measured sigma may vary within intervals of ± 0.4% to ± 3.4%. Due to so much data scatter, measuring the amount of sigma phase observed from one turnaround to another requires adoption of a statistical approach. Only a statistical approach will be able to distinguish the effect of time on the increase in the amount of sigma phase.

Although undoubtedly more data would be required to draw valid conclusions, the indication is that precipitation of sigma phase becomes so slow after some time that it appears as if an equilibrium amount of sigma phase is reached in a few years of service10. Under these circumstances, the remaining life (based solely on the amount of sigma phase) may not be adequately predicted because of the lack of a degradation mechanism-time relationship. Also, there is not yet any criterion for an allowable limit in the amount of sigma phase. It appears that relatively low percentage of sigma phase may cause significant reduction in fracture toughness. This reduction probably depends not only on the amount of sigma phase but also on size and distribution, as well as the presence of other second phase particles (carbides), some of which may be submicroscopic.

In general, the decision to replace refinery components should not depend on the amount of sigma phase alone but rather on Charpy V-notch test results such as those illustrated in Figure 13. The criterion adopted in this paper assumes, as in the case of carbon and Cr-Mo steels, a minimum Charpy V-notch test result requirement of either 20 J or 27 J (15 or 20 ft-lb). This implies a first step to determine the amount of sigma phase present and, if there are concerns about excessive embrittlement, a second step extracting samples and performing CVN tests using at least one set of three specimens. The average value should meet the minimum requirement, no single value could be less than 2/3 the average value or 13 J (10 ft-lbf), if the criterion of 20 J (15 ft-lb) is adopted.

If there is great concern for creep property degradation due to sigma phase presence, creep testing may be necessary. Omega analyses have been used to determine fitness for service of partially repaired welds in 304H stainless steel FCC regenerator overhead piping to the power recovery turbine11. Omega tests were performed only for the case described in Figure 18 but the effect of wall thinning by corrosion greatly superseded the effect of sigma phase on creep properties. In spite of the presence of sigma phase and the carburized layer at the inside surface of the tubes, the specimens were able to undergo considerable amount of creep strain (13 to 37%).

Proper conversions will have to be made in case of using subsize Charpy V-notch specimens. In case of doubts, full size specimen results should prevail. If a given test temperature is chosen in a standard procedure (0°C), adjustment will also have to be made for the required temperature shift if subsize Charpy V-notch

9

specimens are used. For instance, a 0°C (32°F) requirement would convert to -11°C (12°F) for half size specimens. High temperature CVN tests could also be specified but we would not know what the requirement and the testing temperature should be.

Further debate may be required to reach a consensus on the criterion to be used to decide when TP 304H SS needs to be replaced, based on the extent of sigma phase embrittlement. As stated previously, the amount of sigma phase alone may not be enough to accurately provide the required information. There is significant amount of scatter in the results obtained when measuring sigma phase content and the correlation between the amount of sigma phase and the degree of embrittlement has not yet been established.

The greatest concern is excessive sigma phase embrittlement developing with time without giving any early warning and then suddenly causing a costly one-time failure. This is why the degree of sigma phase embrittlement needs to be assessed and for this it is proposed to implement a program to monitor the amount of sigma phase formation and embrittlement. It is proposed to consider replacement only when not meeting the 20 J (15 ft-lb) criterion at 0°C (32°F).

The amount of sigma phase formed during a first typical run length of a refinery unit (2 to 4 years) may be significant and in the case of weld metal it is known that as much sigma phase as the original amount of delta ferrite ( 3 to 8%) or higher may form within one year of service (8,760 hours). Since in most of the cases the welds are the areas that experience cracking due to sigma phase, the question could be asked if it is necessary to assess the embrittlement only at weld metal. Recommending replacing equipment or piping when finding that the welds do not meet this requirement may be difficult to accept when the base metal is still in good condition. In principle, welds can be repaired by removing the welds and rewelding. The use of electrodes with reduced ferrite number may be encouraged.

CONCLUSIONS

Sigma phase formation is a natural aging process that will occur in TP 304H SS and in other similar austenitic stainless steels when exposed to high temperature service, in the case of FCC units, about ≈650°-716°C (≈1,200-1,400°F). It will also occur in furnace tubes or any other refining service using this and other austenitic stainless steels at similar high metal temperatures.

The amount of sigma phase formed during a first typical run length of a refinery unit (2 to 4 years) may be significant.

In the case of weld metal it is known that as much sigma phase as the original amount of delta ferrite (3 to 8%) or higher may form within one year of service. The use of electrodes with reduced ferrite number may be encouraged.

For new FCC cyclone installations, operators should consider adding sample welded test plates to their cyclone assemblies, similar to what is done for Cr-Mo reactors. These could be easily sampled in future T/A's to destructively determine the loss of properties with time.

For the base metal, a relationship of the amount of sigma phase formed with time in refining units has not been established. It is known that the amount of sigma phase varies significantly from one installation to another. For the same temperature and exposure time, the amount of sigma phase seems to depend more on particular conditions and chemistry of the steel (within the specified chemical composition ranges). Also, tiny black precipitates and needles are often seen in the microstructure suggesting that the aging process involves precipitation of intermetallic particles other than just sigma phase and they could also contribute to the embrittlement effect.

The larger needle-like particles within the grains were metallographically identified as sigma phase. The smaller and thinner needle-like particles may also be sigma phase but the scope of these studies did not include

10

any particle extraction and X-ray diffraction or diffraction pattern analysis in thin-foil transmission electron microscopy to positively identify them.

The amount of sigma phase found within the same unit, equipment, component, or heat of steel may also vary. The scatter in the data has been large, and this has made it difficult to clearly establish an increase in the amount of sigma phase as measured from one turnaround to the next. It would appear as the amount of sigma phase reaches an equilibrium level, and thereafter remains constant.

In the absence of a relationship governing the amount of sigma phase as a function of time in service of refining units and of specific criterion stating how much sigma phase is acceptable, the advice has been to not depend on the amount of sigma phase alone but rather on Charpy V-notch test results to assist on making decisions about replacement. If there is concern for creep property degradation due to sigma phase presence, creep testing may also be necessary.

It is proposed to consider replacement only when CVN test values do not meet a minimum Charpy V-notch test requirement of 20 J (15 ft-lb) at 0°C (32°F) for at least a set of three specimens, with no single value being less than 13 J (10 ft-lbf). This would imply to determine the amount of sigma phase present and extract samples to carry out Charpy V-notch tests.

The scope of the study did not include creep testing but it is recognized that sigma phase presence, distribution, size, and morphology can affect creep strength and reduce creep ductility. Therefore, this may also need to be assessed.

ACKNOWLEDGMENT

The authors acknowledge the contribution of Stan Daigle, David Johnson, Tim Munsterman, Patricia Chacon, and Luis Silva. Acknowledgement is extended to the owners and custodians of the equipment and units where these projects have been performed, to the reviewers of the papers; to the symposium chairman, and to Lloyd’s Register Capstone Inc., for consenting to publishing this paper.

REFERENCES

1 Metals Handbook, Vol. 13: Corrosion, ASM International, Metals Park, Ohio, 1987, p. 11.

2 Dean J. Gaertner: “Characterization of Sigmatized Austenitic Stainless Steels”, Materials Performance, January 1985 (NACE International, Houston Texas, 1985).

3 Victor E. May: “Sigma Phase Embrittlement of Austenitic Stainless Steel FCCU Regenerator Internals”, paper 378, Corrosion 1986 (NACE International, Houston Texas, 1986).

4 Michael R. Finlay and Frank Orszag: “Sigma Phase Embrittlement of 304H Austenitic Stainless Steel”, PVP-Vol 359, Fitness for Service for Adverse Environments in Petroleum and Power Equipment, ASME, United Engineering Center, 344 East 47th Street, New York, N. Y.10017, 1997

5 Metals Handbook, 8th Ed., Vol. 8: “Metallography, Structure and Phase Diagrams”, American Society for Metals (ASM International), Metals Park, Ohio, 1973, pp. 37-47.

6 Murray R. Spiegel, and Larry J. Stephens: “Theory and Problems of Statistics”, 3rd Ed., Schaum’s Outline Series, McGraw-Hill, New York, 1999, p. 203.

7 J. M. Vitek and S. A. David: “The Sigma Phase Transformation in Austenitic Stainless Steels”, Welding Research Supplement, April 1986, pp.106s-111s.

11

8 FIG. UG-84.1, ASME Boiler and Pressure Vessel Code, Vo. VIII, Division 1, The American Society of Mechanical Engineers, New York, NY, 2004.

9 G. F. Tisinai, J. K. Stanley, and C. H. Samans: “Sigma Nucleation Times in Stainless Steels”, Transaction AIME, Vol. 206, May 1956, Journal of Metals, pp.600-604.

10 A. M. Talbot and D. E. Furman: “Sigma Formation and its Effect on the Impact Properties of Iron-Nickel-Chromium Alloys”, Trans. ASM, Vol 43, 1953, pp. 429-440.

11 Gerald W. Wilks: “Weld Cracking in a 72-inch (1,829 mm) Stainless Steel FCC Duct”, paper 01522, Corrosion 2001 (NACE International, Houston Texas, 2001).

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TABLE 1. Average Charpy V-Notch Impact Test Results, absorbed energy in Joules (ft-lbf) Refinery Location Years of Service Sigma Phase

Content (%) Temperature

RT Service A Base Metal 17 4.0 85 (63) 145 (107) A Weld Metal 17 8.7 37 (27) 100 (74) B Base Metal 19 12.0 12 (9) 43 (32) C Base Metal 13 1.5 35 (26) 75 (55)

(a)

(b)

FIGURE 1. Stainless steel 304H with 14 years operating at nominal temperature of 716°C (1,320°F) after (a) etching in Vilella’s reagent, 100X; and (b) electrolytic etching in KOH, 200X. The estimated amount of sigma phase was

5.0%.

13

FIGURE 2. Microstructure as seen in a FMR taken on stainless steel 304H FCC flue gas line, electrolytic etching in 10% oxalic acid, 200X. The estimated amount of sigma phase was 3.0%.

14

FIGURE 3. Sigma phase content in 304H stainless steel base metal, 8 years at nominal temperature of 716°C

(1,320°F), from FCC regenerator cyclones and overhead line.

FIGURE 4. Sigma phase content in 304H stainless steel base metal, 22 years at nominal temperature of 716°C

(1,320°F), from FCC regenerator cyclones.

0

1

2

3

4

5

6

7

Count

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14Sigma Phase Content Base Metal %

Histogram Count = 18 Std. Dev. = 2.89% Minimum = 1.3% Maximum = 12.3% Mean = 5.5% Precision = ± 1.4% 95% Confidence 4.1% - 6.9%

Mean = 5.5%

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10

Count

Sigma Phase Content Base Metal %

Count = 33 Std. Dev. = 1.93% Minimum = 1.1% Maximum = 9.4% Mean = 4.3% Precision = ± 0.7% 95% Confidence 3.6% - 5.0%

Histogram Base Metal Mean = 4.3%

15

FIGURE 5. Sigma phase content in 304H stainless steel weld metal, 22 years at nominal temperature of 716°C

(1,320°F), from FCC regenerator cyclones.

FIGURE 6. Sigma phase content in 304H stainless steel base metal, 17 years at nominal temperature of 716°C

(1,320°F), from FCC regenerator overhead.

0

1

2

3

4

5

6

7

Count

0 2 4 6 8 10 12 14 16 18 20 Weld Metal

Histogram Count = 11 Std. Dev. = 4.03% Minimum = 3.2% Maximum = 16.7% Mean = 7.1% Precision = ± 2.7% 95% Confidence 4.4% - 9.8%

Mean = 7.1%

0

1

2

3

4

Count

0 2 4 6 8 10 12

Histogram

Sigma Phase Content Base Metal %

Count = 15 Std. Dev. = 2.7% Minimum = 1% Maximum = 11% Mean = 4.0% Precision = ± 1.5% 95% Confidence 2.5% - 5.5%

Mean = 4.0%

16

FIGURE 7. Sigma phase content in 304H stainless steel weld metal, 17 years at nominal temperature of

716°C (1,320°F), from FCC regenerator overhead.

FIGURE 8. Liquid penetrant testing still revealed numerous transverse cracks in a weld that was partially removed

by grinding. The weld suffered sigma phase embrittlement.

Count = 10 Std. Dev. = 3.0% Minimum = 5% Maximum = 14% Mean = 8.7% Precision = ± 2.8% 95% Confidence 5.9% - 11.5%

Sigma Phase Content Weld Metal %

0

1

2

3

Count

0 5 10 15 20

Histogram

Mean = 8.7%

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FIGURE 9. Dark-etching particles are sigma phase that are concentrated near the outside surface of FCC

regenerator plenum chamber, electrolytic etch in 33% KOH, 100X.

FIGURE 10. Microstructure of FCC regenerator plenum chamber wall, electrolytic etch in 33% KOH, 100X. Dark-

etching particles are sigma phase, estimated amount 6.0%.

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FIGURE 11. Microstructure near outside surface of FCC regenerator plenum chamber, electrolytic etch in 10%

oxalic acid, 100X

(a)

(b)

FIGURE 12. Tensile tests from FCC flue gas stainless steel type 304H line having 12% sigma phase: (a) room temperature and (b) 716°C (1,320°F).

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FIGURE 13. Charpy V-Notch Impact Test Results as absorbed energy in ft-lbf, at room temperature and

service temperature.

(a) 21°C (70°F) (b) 204°C (400°F) (c) 316°C (600°F) (d) 704°C (1,300°F)

FIGURE 14. Bending test results performed at the indicated temperature. Stainless steel type 304H base metal, 19 years at nominal temperature of 716°C (1,320°F), 12 ± 2 % sigma phase content.

(a) (b)

FIGURE 15. Room temperature tensile fracture of new stainless steel type 304H: (a) typical cup and cone fracture and (b) dimples at the center, typical of ductile fracture.

14001200 1000800600400200 0 0

20

40

60

80

100

120

Base Metal, 4.0%17 years

Weld Metal, 8.7%17 years

Base Metal, 1.5%13 years

Base Metal, 12%19 years

Charpy V-Notch Test Temperature, ºF

Absorbed Energy ft-lb

% Sigma Phase

Refinery A

Refinery A

Refinery B

Refinery C

New (specimens did not break)

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(a) (b) Fe

(c) Cr (d) Point 1

(e) Point 2 (f) Point 3

FIGURE 16. Fracture surface on 304H steel with 4.4% sigma phase. (a) Fractograph, (b) EDS elemental map for Fe, (c) EDS elemental map for Cr, (d) EDS spectrum at point 1, (e) EDS spectrum at point 2, and (f) EDS spectrum at

point 3.

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FIGURE 17. Fracture surface in Charpy V-notch specimen with about 12 ± 2 % sigma phase, broken at

about 716ºC (1,320°F). The area within the circle was identified as sigma phase.

(a)

(b)

FIGURE 18. Sigma phase found in stainless steel heater tube type 347H, 32,000 hours in service, at 538°C to 766°C (1,000° to 1,410°F). (a) General view showing a crack through the severely carburized layer and sigma phase distribution across the wall, 40X, and (b) detail of the sigma phase within the rectangle, 250X. The tube inside

surface is on the left-hand side. Electrolytic etch in KOH.

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