Paper No.06578 Sigma Phase Embrittlement of Stainless Steel in FCC Service Jorge Hau and Antonio Seijas Lloyd’s Register Capstone, Inc 1505 Highway 6 South, Suite 250 Houston, TX 77077
[email protected] 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 Copyright ©2006 NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International, Conferences Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A. 1 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 whiteetching 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 xray 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 2 The test provided hot impact strength properties as compared with the room temperature properties. The CVN specimens were then quickly placed in the impact testing machine and broken within seconds to avoid significant cooling. 1 to 2%. Precision was calculated by assuming a normal (Gaussian) distribution and finding the 95% confidence limits of individual values around the mean. and hardness increased slightly but the differences between specimens removed from service and unaged steel was not always significant.0%. weld metal was not included.5 ± 0. 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. cyclone system or the FCC regenerator overhead line.9 ± 0. These numbers of cases are referred to as frequency or counts 3 .0± 2. 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 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. The horizontal axis (abscissa) represents sigma phase content intervals (e. and 13. Elemental microanalysis was performed by energy dispersive x-ray spectroscopy (EDS) in conjunction with the SEM. Fracture surface morphology was examined in the scanning electron microscope (SEM). they were used instead of the results from tensile test. There were four cases where the amount of sigma phase measured fell within the interval 1 to 2%. Digital image analyzers work by measuring the area occupied in the view by the second phase particle of interest.6%. CVN testing was performed at both ambient and elevated temperature. ten cases where it fell within the interval 3 to 4%. yield strength. 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.g. These values give some idea of the scatter and are calculated by using the following expression: mean value ± 1.6%.4%. CVN specimens were placed inside a furnace and soaked until achieving slightly higher temperature than the selected level.9 ± 1. the measurement of sigma phase content was performed only on base metal. 3. In this case. Tensile tests were also performed at both room temperature and at high temperatures. In both methods. The normal distribution table gives the value of 1. The phase to be analyzed is selected by adjusting the control on the image of a properly etched metallographic specimen. The level of tensile strength. 6. and 12. another four cases where it fell within the interval 2 to 3%. it is necessary to choose the right magnification and take as many fields of view as necessary to achieve the required accuracy. 2 to 3%.9%. 11. 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.96 x standard deviation.8 ± 3. Typical amounts of weld metal sigma phase were 4.4 ± 2.4%. The time in service was 8 years and the nominal metal temperature was assumed to be ≈716°C (≈1. These measurements of sigma phase content could be for a particular FCC regenerator cyclone.6 ± 0.9%.8%. 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 elongation and reduction in area for the specimens removed from service were lower that for unaged steel. and so on. up to 9 to 10%).0 ± 1. Because the differences obtained in the results from CVN test were more marked.9 ± 0. These measurements were obtained with image analyzer that is based on area %. 8. Tensile test results were not as revealing as CVN test results. 2. the more accurate the measurement is.320°F).views and points counted.96 for 95% confidence limits6. RESULTS Sigma Phase Formation Typical measured amounts of sigma phase in 9 to 12 fields from the same base metal samples were 1.4%. The variation is higher when including different metallographic specimens or locations within the same component or from different components of the same system. The sigma phase content typically appeared higher in the weld metal than in the base metal in aged austenitic stainless steel. Figure 8. Sigma phase content was measured in several places in the same cyclone. Figure 5.9%.0%. Welds were made with electrode TP 308H SS and were specified to have a ferrite content ranging from 3 to 8%. The stainless steel electrode specified for shielded metal arc welding is AWS classification E-308H which has 18. The 95% confidence limits for the mean value were 4. cyclone system. 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%. 9.4%.0-11.320°F).9% to 11.3% but the data scattering was relatively large. 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. In the case of weld metal in the same cyclones.0 % Cr and 9. This regenerator overhead line had 17 years in service at the time these measurements were taken. one within the interval 9 to 11%. FCC regenerator overhead line. This is why.6% and 5. it is possible to state with 95% confidence that the true mean value for the sigma phase content was between 3.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. The nominal metal temperature was assumed to be ≈716°C (≈1. where n is the number of individual measurements). 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.5%. a location in a weld at the short radius side of one of these two elbows. The mean or average value of sigma phase content was 4. 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.0-20. The welds at the short radius side (intrados) of these two elbows are often found with cracks at every turnaround.1% and 6.8%. In the case of Figure 8. Typically the crack indications have been removed by grinding.0 % Ni. the minimum value found being 1. Crack removal has been followed by liquid penetrant (LP) testing to verify successful crack removal.1% and the maximum. and one within the interval 11 to 13%.4% and 9. 4 . It is important to realize that the amount of sigma phase measured will vary from place to place.320°F). not only as a result of the experimental error but also because it varies from place to place within the same sample. two of which form a U-bend at the top of the regenerator. This was calculated by using the t-student distribution6 and the standard error ( ± t95 (standard deviation/√ n. 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. it was considered to be more appropriate to provide an interval estimate for the average amount of measured sigma phase. 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. containing three mitered 90° elbows.5% and 5. on the interval of 3-4% sigma phase is ten.320°F). The estimated mean value of sigma phase content was 5. as compared with 18. In another study. 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.0 % Cr and 8.5% for the base metal and 5. grinding was performed to about half the wall thickness but liquid penetrant testing still revealed cracks in the weld. and the 95% confidence intervals for the mean value were calculated. the 95% confidence limits for the mean value were 4.0-10. on the interval of 2-3% sigma phase is four. and so on. 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. Notice that there are four sigma phase measurements that fell within the interval 7 to 9%. The 95% confidence limits for the mean value of sigma phase measured were 2.5 % Ni specified for Type 304H stainless steel base metal.0-21. rather than give a point estimate of the amount of sigma phase found.5% for the weld metal. Applying basic statistics. The ferrite phase tends to nucleate sigma phase faster than from the austenite phase. The results of measuring the sigma phase content are shown in Figures 6 and 7. This is not surprising as other literature shows sigma forming from weld delta ferrite within several hundred hours after elevated temperature exposure7. it is obvious that the weld metal 5 . Cold work was recognized due to the presence of deformation twins and slip bands within the grains. The obtained results from mechanical testing have confirmed the findings already reported in the literature. after electrolytic etching (aqueous 33% KOH solution). are known to significantly accelerate the kinetics of sigma phase formation. When comparing results from the base and weld metal. as compared with unaged condition. there was a region heavily populated with sigma phase particles that formed indiscriminately in the microstructure. the bulk of the sigma phase formed at grain boundaries. less than 0. the amount of sigma phase formed during service appeared to be more dependent on steel chemistry than on time in service. A rather thin layer of metal. Sigma phase and other second phase particles redissolved by holding the material at 1. Tensile testing at service temperature. There was some superficial cold work in this region. in several examinations that have been performed in different refineries. which summarize some of the results obtained in studies done in different units and different refineries. Since unaged material from the same heat was not available for establishing a comparison.In general. Figure 9 shows the microstructure observed near the outside surface of a FCC regenerator internal plenum chamber. Annealing did not change chemical composition and was used to establish comparison between sigmatized and non-sigmatized steel. CVN tests (full size specimens) done at both room temperature and service temperature tests are shown in Table 1 and Figure 13. While some of these platelets may be sigma phase. Measurements have been repeated with a frequency of 2 to 6 years and the amount of sigma phase appeared unchanged.850º-1.320ºF).066ºC (1. Fracture surfaces are perpendicular to the longitudinal direction of the specimen and do not show necking. did not have any sigma phase particles. 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. the same steel exhibited a 45° ductile fracture surface and some amount of necking (Figure 12b). Cold work prior to aging and the presence of ferrite and some ferrite forming alloy elements. the approach used in the literature2-3 was to solution anneal pieces from the same steel sample to produce a base-line reference condition. Near the outside surface (Figure 9). This statement is based on the fact that steels with the same service time may exhibit significantly different amount of sigma phase. The presence of cold work prior to aging induced the formation of a large quantity of sigma phase particles.006 in) deep from the outside surface.0%. It is understood that this steel had been in service for 8 years at a nominal temperature of ≈716ºC (≈1. The most evident effect of sigma phase on stainless steel is the loss of room temperature ductility and toughness.010º-1. as expected. Even though all of these steels belong to the same specification. Cold Work Effect Several factors are known to influence the sigma phase transformation. Figure 11. 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. there has been no clear tendency of increasing sigma phase formation measured in subsequent turnarounds. The reason for having an increased amount of sigma phase near the surface became evident when electrolytic etching with 10% oxalic acid. The estimated amount of sigma phase was 6. In this paper. at grain boundaries as well as within the grains. Their results clearly indicated the effect of sigma phase on the mechanical properties. 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. There are a certain amount of thin platelets within the grains. At service temperature the steel recovered some ductility. 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. a sign of brittleness (Figure 12a).950ºF) for 1 to 4 hours depending on thickness and amount of sigma that has formed.15 mm (0. The condition found throughthickness is illustrated in Figure 10 showing the microstructure of the same sample but at midwall. even though all these steels have the same nominal chemistry within the ASTM specification. possibly introduced when manufacturing the plenum chamber. Cracking in welds is a common problem in FCC regenerator cyclone systems or in regenerator overhead lines. Figure 15 shows the fracture surface obtained after tensile testing unaged 304H material at room temperature. For the worst case at the bottom of the graph in Figure 13. Figure 16 shows the fracture surface of a steel sample removed from a cyclone in a FCC unit. the CVN energy values for base metal samples from refinery “C”. not surprising given the original weld delta ferrite. The CVN energy values for base metal samples from refinery “A” appeared acceptable even though the amount of sigma phase was 4. lower line in Figure 13. were below 20 J (15 ftlbf). but it failed at both room temperature and 204°C (400°F). This seems to confirm that the degree of embrittlement achieved when the Charpy V-notch test result is below this limit may be significant.4% sigma 6 . and as such. it was as brittle as at room temperature. Overall. If these limits were to be used for all the cases in Table 1 and Figure 13. These precipitates are smaller than sigma phase particles and are more evenly distributed. producing the typical cup-and-cone fracture and 100% ductile fracture containing dimples of different sizes. Hot CVN test results always produced values above 27 J (20 ft-lbf). the average for this same steel was 43 J (32 ft-lbf) but with a single value giving 15 J (11 ft-lbf). with only 1. except for one sample that produced 15 J (11 ft-lbf) when impact tested at ≈716°C (≈1. Room temperature values were low for the case of refinery “B” with TP 304 SS having average values of 12% sigma phase. The average amount of sigma phase present is a measure of tendency to embrittle. as expected. The material passes the bend test at temperatures of 316°C (600°F) and 704°C (1. The straight lines joining the data at room and service temperature in Figure 13 were best-fitted lines. there was a significant improvement in ductility when compared to room temperature test results. where the measured sigma phase for the base metal and weld metal was 4. depending on the minimum specified yield strength and applicable wall thickness8.300°F). There was considerable necking before the round specimen finally fractured. demonstrating that the steel does not have to reach room temperature to become brittle. appeared as low as those for the weld metal with 8. The fracture occurred by pure microvoid coalescence. the CVN value was only 12 J (9 ft-lbf). in general.5% sigma phase. At service temperature. As an average. Fractography Austenitic TP 304H SS and. Formation of carbides during exposure in the above elevated temperature range also causes low temperature embrittlement. 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. 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. the lower line for refinery “B” would not meet this requirement. They were not included in the metallographic sigma phase measurements. Figure 14 shows bend test results performed at the indicated temperature in TP 304H SS with 12% sigma phase content.7%. The lower CVN energy values for weld metal was corroborated by metallographic examination . 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).320°F) and exhibited an average 4.0%.0% and 8. This was best demonstrated by samples from refinery “A” (17 years of service) and is illustrated by the two upper lines in Figure 13.suffered the most loss of ductility. In the case of carbon and Cr-Mo pressure vessel steels. At 204°C (400°F).320°F). the minimum requirement is often specified to be either 20 J or 27 J (15 or 20 ft-lbf). CVN test results are more revealing than just the knowledge of the amount of sigma phase. There is no available criterion establishing CVN impact requirement for sigmatized TP 304H SS. any of 300 series stainless steels are fairly ductile and tough. Therefore. 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 steel had been in service 8 years at a nominal temperature of ≈716°C (≈1. respectively. Room temperature CVN results for TP 304H SS having 19 years of service. there were obviously places including welds in this steel that had sigma phase content higher than the average value of 12%.7% sigma phase at room temperature in Figure 13. Conversely. The distribution of iron in Figure 16b appeared to be uniform. 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. 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. indicating that the chromium concentration in the sigma phase particle was higher than of the matrix. The image brightness intensity was similar for the sigma phase particle and the background as they both have similar concentration of iron. except for multiple discrete spots on the lefthand side lower corner of the photomicrograph. and could be due to surface topography. The small boat sample was cut with a small grinding wheel. These are carbide particles that had not been seen in Figure 16a because they are located deep inside these dimples and were not obvious. in essence. The fracture surface examined in Figure 16 corresponded to this last fracture. which are basically the same values given by the nominal chemical composition of this steel. The region identified as number 2 corresponded to an area with dimples and with a secondary crack. 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. The quantities of Cr and Fe were estimated to be 35 and 55%. The smoother and flat surface seen in these spots suggested cleavage fracture. The EDS spectrum obtained from the region number 3 (Figure 16f) gives a composition matching the base metal. The surface contained dimples of a great variety of sizes but with broken sigma phase particles exhibiting cleavage fracture. There is another similar particle on the right-hand side of this. The relative elemental concentrations of iron and chromium were obtained across the surface using SEM/EDS. indicated the predominant presence of chromium. Figure 17. The fracture morphology in places away from sigma phase particles was consistent with microvoid coalescence. As is typical of sigma phase. respectively. the amount of spots with cleavage fracture appeared lower than at room temperature. respectively. as expected from carbide particles. The EDS spectrum in Figure 16d confirmed that the particle indicated with the number 1 in Figure 16a was sigma phase. and 3 on the SEM photomicrograph in Figure 16a. The region identified as number 3 corresponded to an area with dimples and represented the austenitic matrix or base metal. 2. Regarding chromium content. These depleted zones corresponded to deep depressions. respectively. There were observed differences between specimens broken at room temperature and specimens broken near 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. however. The quantities of Cr and Ni were estimated to be 20 and 8%. Additionally. the sigma phase particles show higher image brightness intensity than the background. The corresponding dot maps were recorded using image brightness intensity as a direct function of the local concentration of the element present.phase. three regions of interest were selected and identified with the numbers 1. There are two additional irondepleted 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. 7 . 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. The fracture surface of the tensile test specimens and also of the broken Charpy V-notch specimens were all examined. 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 carbide particles were obvious at the center on many dimples in the fracture surface produced at high temperature. Therefore. The region identified as number 1 corresponded to a sigma phase particle. 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. At high temperature. The quantities of Cr and Fe were estimated to be 67 and 27%. where less iron was observed. The EDS spectrum taken at region number 2 (Figure 16e). Kα energy peaks for iron and chromium appeared at the same height in the EDS spectrum in Figure 16d. 169 in) thick.Sigma Phase Formation in Steels Other Than TP 304 SS Over time. distribution. Where the metallurgical condition is unknown. in this case.g. Note that there is a uniform distribution of sigma within the tube. These tests do not give adequate indication of loss of ductility due to sigma phase. 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. 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.000º to 1. such as during cool down when differential thermal contraction forces arise.050°-1.6 years in delayed coker service at skin metal temperature varying from 538ºC to 766ºC (1. DISCUSSIONS Sigma phase definitely affects the mechanical properties of TP 304H SS.3 mm (0. 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. Sigma phase formation is due to thermodynamic instability. Because of coke deposition within the tube. it is recommended to remove at least 1. impact loading to remove refractory) the affected components or parts. Types 316. Lower amounts of sigma phase near and in the carburized layer have also been observed when examining TP 304H SS. and 347stainless steels are commonly used in the oil refining industry. or refractory chipping (e. Hence. 321. the acceptable limits would probably be defined in terms of remaining life but this was outside the scope of these studies. Bend or impact testing is required to determine the degree of embrittlement.410ºF). Magnetic testing may be performed to detect the presence of carburization. or with maintenance activities like welding. The tube wall at this location in the furnace was approximately 4.800°F). and sigma phase is no longer evident. the inside surface of the tube is on the left-hand side. The total creep strains achieved after completion of the tests were 13 to 37%. Figure 18 shows sigma phase particles found in a TP 347H SS heater tube after 3. Tensile strength. the only specimen that ruptured achieved 13% creep strain. Creep testing will probably be required to be able to assess the degree of deterioration. yield strength. The carburized layer contained a crack. Fortunately. This may occur under a number of scenarios.2 HRC) on the carburized layer. Its presence can be easily identified by metallography. care must be taken to minimize or try avoiding altogether impact or suddenly applied high stress when the unit is out of service. particularly in heater tubes or as weld overlay or cladding. straightening. a carburized structure is the predominant feature. Series 300stainless steels are all susceptible to sigma phase embrittlement. Microhardness measurements as high as 450 HV were obtained (equivalent to 45. and hardness may change from the original values but usually without adversely affecting the integrity of the metal and the components. it should not be a surprise to see sigma phase when examining furnace tubes made of these steels. the inside surface was severely carburized. The presence. 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. except in the carburized region. and precipitation of second phase intermetallic precipitates. 8 . 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. few on-stream failures have been directly attributed to it.6 mm (1/16 in) from the surface before attempting to obtain a FMR. At the surface and near surface. Some precautions used for welding onto sigmatized stainless steel include a solution anneal before welding. and morphology of sigma phase may also affect creep properties and reduce creep ductility. sigma phase formation is unavoidable in many of the commercial austenitic stainless steel alloys used within the temperature range of 560°-980°C (1. Ideally. amount. If there is great concern for creep property degradation due to sigma phase presence. If the exposure temperature is too far apart from this critical temperature range. The average value should meet the minimum requirement. refinery process units are run continuously for at least 2 to 4 years. since the original weld delta ferrite easily transforms. no single value could be less than 2/3 the average value or 13 J (10 ft-lbf). The literature9 refers to C-Type reaction curve for sigma phase formation. If a given test temperature is chosen in a standard procedure (0°C). The time required to start detecting sigma phase at conditions corresponding to the nose of the C-curve has been measured in minutes. if there are concerns about excessive embrittlement. a second step extracting samples and performing CVN tests using at least one set of three specimens. as in the case of carbon and Cr-Mo steels. it has been found that the amount of measured sigma may vary within intervals of ± 0. plotted as a logarithm of time versus temperature. These are curves indicating the start and zone of sigma phase formation at any given combination of temperature and exposure time. respectively. Although undoubtedly more data would be required to draw valid conclusions.000 to 70. as well as the presence of other second phase particles (carbides). Only a statistical approach will be able to distinguish the effect of time on the increase in the amount of sigma phase. Above and below this critical temperature range. However. This implies a first step to determine the amount of sigma phase present and. It appears that relatively low percentage of sigma phase may cause significant reduction in fracture toughness. 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 service10.000 hours. Due to so much data scatter.4%. measuring the amount of sigma phase observed from one turnaround to another requires adoption of a statistical approach.000-105. 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.000 hours and 51. 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. hours. or days depending on temperature and steel condition. When the cold working factor has been introduced in these studies reported in the literature9. Under these circumstances. In general. There is a critical temperature range represented by the nose of the C-curve where sigma phase forms faster. sigma phase formation may not form at all. there is not yet any criterion for an allowable limit in the amount of sigma phase.4% to ± 3. if the criterion of 20 J (15 ft-lb) is adopted. Sigma phase formation in stainless steel base metal is more sluggish than in weld metal. This means that the first measurement for new steel could be taken after 17.000 to 35. creep testing may be necessary. Depending of the amount of sigma phase. 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.000 hours exposure time. sigma phase formation is more sluggish and takes much longer time to precipitate. Also. The criterion adopted in this paper assumes. adjustment will also have to be made for the required temperature shift if subsize Charpy V-notch 9 . This reduction probably depends not only on the amount of sigma phase but also on size and distribution. The amount of sigma phase may vary within the same piece of equipment from one area to another. the second and third opportunities could occur after 34. An evaluation of welds was conducted and it was found that a significant amount of sigma phase formed after only 1 year of service.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. 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. full size specimen results should prevail. In spite of the presence of sigma phase and the carburized layer at the inside surface of the tubes. a minimum Charpy V-notch test result requirement of either 20 J or 27 J (15 or 20 ft-lb). 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 case of doubts. some of which may be submicroscopic. it has been demonstrated that cold working is a most important factor in reducing the time required to form sigma phase. 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. 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. It is proposed to consider replacement only when not meeting the 20 J (15 ft-lb) criterion at 0°C (32°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 larger needle-like particles within the grains were metallographically identified as sigma phase. a relationship of the amount of sigma phase formed with time in refining units has not been established. For new FCC cyclone installations. 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. Since in most of the cases the welds are the areas that experience cracking due to sigma phase. 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. 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 amount of sigma phase alone may not be enough to accurately provide the required information. The use of electrodes with reduced ferrite number may be encouraged. based on the extent of sigma phase embrittlement. It is known that the amount of sigma phase varies significantly from one installation to another. the amount of sigma phase seems to depend more on particular conditions and chemistry of the steel (within the specified chemical composition ranges). The smaller and thinner needle-like particles may also be sigma phase but the scope of these studies did not include 10 . For the base metal. In principle. 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. As stated previously.200-1. welds can be repaired by removing the welds and rewelding. a 0°C (32°F) requirement would convert to -11°C (12°F) for half size specimens. For instance. Also. operators should consider adding sample welded test plates to their cyclone assemblies. High temperature CVN tests could also be specified but we would not know what the requirement and the testing temperature should be. These could be easily sampled in future T/A's to destructively determine the loss of properties with time. The amount of sigma phase formed during a first typical run length of a refinery unit (2 to 4 years) may be significant.400°F). 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. in the case of FCC units. 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.specimens are used. 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. about ≈650°-716°C (≈1.760 hours). The use of electrodes with reduced ferrite number may be encouraged. For the same temperature and exposure time. similar to what is done for Cr-Mo reactors. Acknowledgement is extended to the owners and custodians of the equipment and units where these projects have been performed. and to Lloyd’s Register Capstone Inc. this may also need to be assessed. The scatter in the data has been large. Michael R. May: “Sigma Phase Embrittlement of Austenitic Stainless Steel FCCU Regenerator Internals”. or heat of steel may also vary. and Luis Silva. distribution. United Engineering Center.. Ohio. 8: “Metallography. 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. Therefore. 1985). and thereafter remains constant. The amount of sigma phase found within the same unit. to the reviewers of the papers. Stephens: “Theory and Problems of Statistics”. Patricia Chacon. equipment.106s-111s. J. 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. 344 East 47th Street. Houston Texas. David Johnson. Victor E. It would appear as the amount of sigma phase reaches an equilibrium level. paper 378. If there is concern for creep property degradation due to sigma phase presence. Metals Park. REFERENCES 1 2 Metals Handbook. Structure and Phase Diagrams”. Metals Park. Corrosion 1986 (NACE International. 1997 Metals Handbook. January 1985 (NACE International. 1986).. American Society for Metals (ASM International). Spiegel. Ohio. 1973. McGraw-Hill. 8th Ed. ASME. ASM International. ACKNOWLEDGMENT The authors acknowledge the contribution of Stan Daigle. Vol. A. Materials Performance. 3 4 5 6 7 11 . 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. for consenting to publishing this paper. p. 3rd Ed. Y. April 1986. N.. Fitness for Service for Adverse Environments in Petroleum and Power Equipment. 203. Houston Texas. pp. to the symposium chairman. Schaum’s Outline Series. This would imply to determine the amount of sigma phase present and extract samples to carry out Charpy V-notch tests. 13: Corrosion. 1987. with no single value being less than 13 J (10 ft-lbf). 37-47. PVPVol 359. New York. Welding Research Supplement. Murray R. Vitek and S. Vol. p. New York. and morphology can affect creep strength and reduce creep ductility. Tim Munsterman. The scope of the study did not include creep testing but it is recognized that sigma phase presence.10017. Dean J. 11. creep testing may also be necessary. and Larry J. 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.any particle extraction and X-ray diffraction or diffraction pattern analysis in thin-foil transmission electron microscopy to positively identify them. 1999. component. M. Finlay and Frank Orszag: “Sigma Phase Embrittlement of 304H Austenitic Stainless Steel”. pp. size. Gaertner: “Characterization of Sigmatized Austenitic Stainless Steels”. David: “The Sigma Phase Transformation in Austenitic Stainless Steels”. Vo. ASM. K.600-604. G. A. New York. Furman: “Sigma Formation and its Effect on the Impact Properties of Iron-NickelChromium Alloys”. 2004. Houston Texas. M. 2001). F. NY. Stanley. H. and C.829 mm) Stainless Steel FCC Duct”. Tisinai. Gerald W. The American Society of Mechanical Engineers.1. Vol 43. paper 01522. Vol. VIII. ASME Boiler and Pressure Vessel Code. Wilks: “Weld Cracking in a 72-inch (1.8 FIG. 9 10 11 12 . J. E. 1953. pp. pp. UG-84. Journal of Metals. Corrosion 2001 (NACE International. 206. May 1956. Trans. Division 1. Talbot and D. Samans: “Sigma Nucleation Times in Stainless Steels”. Transaction AIME. 429-440. Average Charpy V-Notch Impact Test Results. The estimated amount of sigma phase was 5. 100X. 200X. absorbed energy in Joules (ft-lbf) Refinery A A B C Location Base Metal Weld Metal Base Metal Base Metal Years of Service Sigma Phase Content (%) 17 4.320°F) after (a) etching in Vilella’s reagent. Stainless steel 304H with 14 years operating at nominal temperature of 716°C (1.TABLE 1.7 19 12.5 Temperature RT Service 85 (63) 145 (107) 37 (27) 100 (74) 12 (9) 43 (32) 35 (26) 75 (55) (a) (b) FIGURE 1. and (b) electrolytic etching in KOH.0%.0 13 1.0 17 8. 13 . electrolytic etching in 10% oxalic acid.FIGURE 2. Microstructure as seen in a FMR taken on stainless steel 304H FCC flue gas line. 200X. The estimated amount of sigma phase was 3.0%. 14 . Sigma phase content in 304H stainless steel base metal.6% .3% Base Metal Count = 33 Std. = 2. 15 . Dev. Dev.7% 95% Confidence 3.1% Maximum = 9.320°F).4% 95% Confidence 4.89% Minimum = 1.5.12 10 C o 8 u n 6 t 4 2 0 1 2 3 4 5 6 7 8 9 10 Sigma Phase Content Base Metal % FIGURE 3.3% Precision = ± 0.93% Minimum = 1. = 1. from FCC regenerator cyclones.4% Mean = 4.0% 7 6 C 5 o u 4 n 3 t 2 1 0 Histogram Mean = 5.5% Precision = ± 1. 8 years at nominal temperature of 716°C (1.320°F). Sigma phase content in 304H stainless steel base metal. from FCC regenerator cyclones and overhead line. Histogram Mean = 4.1% .5% Count = 18 Std.3% Mean = 5.9% 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Sigma Phase Content Base Metal % FIGURE 4.3% Maximum = 12. 22 years at nominal temperature of 716°C (1.6. Dev. from FCC regenerator cyclones. 16 .7% 95% Confidence 4.8% 2 4 6 8 10 12 Weld Metal 14 16 18 20 FIGURE 5. 4 Histogram Mean = 4.0% Precision = ± 1. from FCC regenerator overhead.9.1% Precision = ± 2.5. Sigma phase content in 304H stainless steel base metal. = 4.5% 95% Confidence 2.4% . = 2. Dev.5% . 22 years at nominal temperature of 716°C (1.7 6 5 C o 4 u n 3 t 2 1 0 0 Histogram Mean = 7.320°F).0% C3 o u n2 t 1 Count = 15 Std.7% Mean = 7.7% Minimum = 1% Maximum = 11% Mean = 4.320°F).2% Maximum = 16. Sigma phase content in 304H stainless steel weld metal.1% Count = 11 Std.5% 0 0 2 4 6 8 10 Sigma Phase Content Base Metal % 12 FIGURE 6. 17 years at nominal temperature of 716°C (1.03% Minimum = 3. 7% Histogram Count = 10 Std.320°F). Dev.0% Minimum = 5% Maximum = 14% Mean = 8. 17 years at nominal temperature of 716°C (1.5% 0 0 5 10 15 Sigma Phase Content Weld Metal % 20 FIGURE 7.11. The weld suffered sigma phase embrittlement. Liquid penetrant testing still revealed numerous transverse cracks in a weld that was partially removed by grinding. = 3. 17 .3 C o2 u n t 1 Mean = 8.9% . FIGURE 8. from FCC regenerator overhead.8% 95% Confidence 5. Sigma phase content in 304H stainless steel weld metal.7% Precision = ± 2. electrolytic etch in 33% KOH. Microstructure of FCC regenerator plenum chamber wall.0%. estimated amount 6. FIGURE 10.FIGURE 9. 100X. Darketching particles are sigma phase. Dark-etching particles are sigma phase that are concentrated near the outside surface of FCC regenerator plenum chamber. 100X. 18 . electrolytic etch in 33% KOH. Microstructure near outside surface of FCC regenerator plenum chamber. 19 . 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.FIGURE 11. electrolytic etch in 10% oxalic acid.320°F). 4.5% 13 years Base Metal.0% 17 years Refinery A Weld Metal. (a) (b) FIGURE 15. (a) 21°C (70°F) (b) 204°C (400°F) (c) 316°C (600°F) (d) 704°C (1. 12 ± 2 % sigma phase content. Bending test results performed at the indicated temperature.120 A b s o r b e d E n er g y ft lb New (specimens did not break) % Sigma Phase 100 80 60 40 20 Refinery A Base Metal. 19 years at nominal temperature of 716°C (1. Charpy V-Notch Impact Test Results as absorbed energy in ft-lbf. at room temperature and service temperature. Stainless steel type 304H base metal. typical of ductile fracture. 20 . Room temperature tensile fracture of new stainless steel type 304H: (a) typical cup and cone fracture and (b) dimples at the center.320°F). 12% 19 years 1200 1400 Refinery C Refinery B 0 0 200 400 600 800 1000 Charpy V-Notch Test Temperature. ºF FIGURE 13. 8.300°F) FIGURE 14.7% 17 years Base Metal. 1. (a) (b) Fe (c) Cr (d) Point 1 (e) Point 2 (f) Point 3 FIGURE 16. (a) Fractograph. and (f) EDS spectrum at point 3. (b) EDS elemental map for Fe.4% sigma phase. (d) EDS spectrum at point 1. (c) EDS elemental map for Cr. 21 . Fracture surface on 304H steel with 4. (e) EDS spectrum at point 2. 250X. 32. 40X. 22 . The area within the circle was identified as sigma phase. (a) General view showing a crack through the severely carburized layer and sigma phase distribution across the wall. Fracture surface in Charpy V-notch specimen with about 12 ± 2 % sigma phase.410°F).000° to 1.320°F). The tube inside surface is on the left-hand side.FIGURE 17.000 hours in service. Electrolytic etch in KOH. Sigma phase found in stainless steel heater tube type 347H. and (b) detail of the sigma phase within the rectangle. (a) (b) FIGURE 18. broken at about 716ºC (1. at 538°C to 766°C (1.