PROPERTIES OF SUMITOMO 347AP STEEL TUBE

Transcription

1 PROPERTIES OF SUMITOMO 347AP STEEL TUBE FOR HYDRO-TREATER IN COMPLEX REFINERY Presenter: YUYA MATSUDA Specification & quality control, research and development for seamless stainless steel tubes and pipe for 3 years in Sumitomo Metal Ind., LTD. and continuing as a technical staff. 1

2 Properties of Sumitomo 347AP Steel tube for hydro-treater in complex refinery YUYA MATSUDA, JUNICHI HIGUCHI, and HIROYUKI ANADA Sumitomo Metal Ind., LTD., Amagasaki, Japan KEYWORDS: stabilized stainless steel, polythionic acid stress corrosion cracking, intergranular corrosion stabilization, post-weld heat treatment, desulfurizing plant, weldability, hot weld cracking, hydro-treater, hydro-cracker Abstract: Sumitomo 347AP(UNS S34751) tube has been widely used for hydro-treater in Japanese complex refinery as a substitute for 9-Cr, 321 and 347 for more than 15 years since its especially excellent resistance for sensitization gives reduction of maintenance cost and construction work to omit post-weld heat treatment and neutralize cleaning. This paper describes fundamental property of 347AP as well as its long term usage review and advantages of its application on actual plant. Foremost feature on Sumitomo 347AP is remarkable reduction of SCC sensitivity in polythionic acid to prevent sensitization by lower carbon content up to 0.02%. Resistance of Polythionic acid SCC (PTA-SCC) examinations (ASTM G35) reveal that SCC did not occur even after 10000h aging at 550 deg. C. Strength and creep rupture properties of 347AP are equaling or surpassing 321 or 347 to add optimum amount of nitrogen, which compensates for the reduction in strength due to the lower carbon content. Furthermore, lower Carbon resulted lower Niobium requirement achieves improved weldability which is almost same as TP304 and significantly better than 347 by the evaluation of Varestraint test. Therefore 347AP has been applied as an optimum material for dozens of actual hydro-treater without any treatment for the maximum of 15 years or more, and actually, not only SCC but also any other troubles resulted from material property haven t been observed. 2

3 1 Introduction Chemically stabilized austenitic stainless steels such as TP321 and TP347 are widely used for hydro-treater and hydro-cracker in complex refinery, because of their resistance to high temperature corrosion by desulfurizing process. However, austenitic stainless steels are susceptible to stress corrosion cracking (SCC) after welding. Among them, the SCC due to polythionic acid (H 2 SxO 6, x = 3-6) is one of the most serious corrosion issue affecting equipment for hydro-treater and hydrocracker in complex refinery. The PTA-SCC of stainless steels is generally caused by the formation of chromium depleted zone in HAZ (Heat Affected Zone) of welded joints due to the precipitation of chromium carbides on grain boundaries during fabrication or operation. 1) To prevent PTA-SCC, post-weld heat treatment (PWHT) at about 900deg.C is necessary for TP321 and TP347 to stabilize TiC and NbC in HAZ. Table 1 shows a modification of alloying elements and PWHT for conventional TP321 and TP347 steels in order to prevent the PTA-SCC for welded joints. Table 1: Conventional steels used for polythionic acid condition Steel Recommendation Base metal PWHT TP321 min.ti/c: 7 Mandatory at about 900deg.C TP347 min. Nb/C: 10 Mandatory at about 900deg.C Furthermore, several methods have been well-known such as NACE RP standard which lists the main shutdown procedure to reduce the probability of the PTA-SCC during shutdown. Although TP321 and TP347 are generally used to apply above methods in the petroleum refinery industries, PWHT and shutdown procedure affect not only cost but also production since they take certain period of time. Hence a steel tube material which has excellent resistance for sensitization to prevent chromium depleted zone on grain boundary without above methods is ideal for hydro-treater and hydro-cracker in complex refinery. This paper describes alloy design, performance properties, and service experience of 347AP which expected to be the alternative to TP321 or TP347 to have excellent resistance for sensitization to omit PWHT and shutdown procedure. 3

4 2 Design of Alloy Composition of Sumitomo 347AP (UNS S34751) Table 2 shows chemical compositions of 347AP (ASTM TP347LN / UNS S34751). 347AP have a low carbon content with less than 0.02mass% and optimum nitrogen content with mass%. The addition of nitrogen compensates for the reduction in strength due to the lower carbon content. Low carbon content results lower niobium requirement for stabilization, and lower niobium content contributes improved weldability. Figure 1 shows the effect of carbon and nitrogen on the precipitation of carbonitride for type347 aged at deg.c for 24 hours. When the carbon content is 0.02mass% or more, the chromium carbides precipitate continuously along the grain boundaries. When the nitrogen content is 0.1mass% or more, the chromium nitrides precipitate on the grain boundaries. To prevent carbide and nitride precipitation on grain boundaries of welded joint without PWHT, the 347AP has been optimized carbon content with less than 0.02mass% and nitrogen content with 0.06 to 0.10mass%. Table 2: Typical composition of 347AP (UNS No.S34751) (mass%) C Si Mn P S Ni Cr Nb Nb/C N TP347 Max. Max. Max. Max. Max Max min 347AP Max. Max. Max. Max Min [TP347LN UNS No.S34751] µm 5µm deg.c x 24 h 0.15 CrN Cr 2 (C,N) NbC NbCrN N (%) NbC NbCrN Cr 23 C 6 Cr 7 C 3 NbC 347AP C (%) Figure 1: Effect of C and N on precipitation of Type 347 (Solution heat treatment + deg.c x 24hrs aging) 5µm 4

5 3 Corrosion resistance of GTAW welded joints Table 3 shows the GTAW matching filler for 347AP. TP321 and TP347 steel welded joints were also prepared with conventional TP347 GTAW filler and welding condition is shown in Table 4. Test pieces were cut from welded joints of TP321, TP347 and 347AP steels precisely shown in Figure 2. U-bend specimen was prepared for the corrosion test. Two corrosion tests were conducted for these welded joints without and with stabilization at 900 deg.c for one hour. Intergranular corrosion tests were conducted by the copper sulfate-sulfuric acid test in accordance with ASTM A262 Practice E for 72h(CuSO 4 -H 2 SO 4, solution ) 2). The polythionic acid stress corrosion cracking tests 3), 4) were conducted by Wackenroder liquid in accordance with ASTM G35. Table 3: Chemical composition of filler for welding Sumitomo 347AP (%) Chemical C Si Mn P S Ni Cr Nb N composition #STG347AP <0.03 < <0.03 < xC-1.0 <0.1 GTAW filler #SM347AP <0.03 < <0.03 < xC-1.0 <0.1 MIG filler Actual case Table 4: Welding conditions (Manual TIG welding) Figure 2: Position of sampling test piece from welded portion 5

6 Figure 3 shows the results of intergranular corrosion test following ASTM A262 Practice E on 347AP, TP321, and TP347 without stabilization. Serious intergranular corrosion were observed in TP321 welded joints without stabilization after sensitized at deg.C for less than 10 hours. Also, intergranular corrosion was observed in TP347 welded joints without stabilization after sensitized at 550deg.C for 30 hours or more. 347AP welded joints without stabilization were not observed any intergranular corrosion even after sensitized at deg.c for 10 4 hours. After stabilization at 900deg.C for one hour, no intergranular corrosion was observed in TP321 and TP347 welded joints. Figure 4 shows the results of polythionic acid stress corrosion cracking test following ASTM G35 on 347AP, TP321, and TP347 without stabilization. TP321 welded joints without stabilization were observed SCC after sensitized at deg.c for 100 hours or more. 347AP welded joints without stabilization were not observed any polythionic acid stress corrosion cracking after sensitized at deg.c for 10,000 hours. 6

7 Temperature( ) Holding time (hr) : No intergranular corrosion Temperature( ) (a)347ap Temperature( ) Temperature( ) Holding time (hr) : No SCC (a)347ap Holding time (hr) Holding time (hr) 750 : No IGC : Slight IGC : Heavy IGC (b)tp :N o SC C :SC C (b)tp321 Temperature( ) Temperature( ) Holding time (hr) Holding time (hr) :N o IG C :Slight IG C :H eavy IG C (c)tp347 (c)tp347 Figure 3: Effect of aging conditions Figure 4: Effect of aging conditions on IGC resistance of weldment on SCC resistance of weldment (ASTM A262 Practice E) (ASTM G35) 7

8 Photo 1 and Photo 2 shows extraction replicas of HAZ in TP321, TP347 and 347AP welded joints after sensitized at 550deg.C for 300 hours and deg.c for 1,000 hours. In case of conventional TP321 and TP347 steels, chromium carbides of M 23 C 6 type were observed continuously along grain boundaries. 347AP was free from carbide and nitride precipitation on grain boundaries. Cromium carbide precipitation Cromium carbide precipitation (a) TP321 (b) TP347 (c) 347AP Photo 1: Extraction replicas of HAZ sensitized at 550deg.C for 300 hrs 10µm (a) TP347 (b) 347AP Photo 2: Extraction replicas of HAZ sensitized at deg.c for 1000 hrs 4 Tensile properties The elevated temperature tensile properties of 347AP are shown in Figure 5. Although the carbon content of the steel is lower to prevent stress corrosion cracking, the tensile strength at each temperature exceed allowable stress/0.9 of ASME TP347 because of the addition of nitrogen. TS, YS (N/mm 2 ) TS YS (0.2% proof stress) 100 ASME TP347 0 Allowable stress/ Testing temperature (deg.c) Figure 5: Elevated temperature tensile properties of 347AP 8

9 5 Creep rupture properties The creep rupture properties of 347AP are shown in Figure 6. Creep rupture tests of 347AP were conducted for over 10,000hours at 550 to 750deg.C. It is concluded that the 100,000 hours extrapolated strength at 550 to deg.c satisfies allowable stress/0.83 of the JIS SUS347 (equivalent to conventional TP347). It is estimated that finely dispersed precipitation such as NbCr-nitride and Ni-nitride contribute to improve the creep rupture strength for 347AP. Stress (kgf/mm 2 ) deg.c x10 5 hr 550 deg.c x 10 5 hr deg.c x 10 5 hr 650deg.C x 0 5 hr deg.c x 10 5 hr Temperature (deg.c) : SUS 347TF/0.83 Allowable stress T( log t) x 10-3 Figure 6: Creep rupture strength curve of 347AP (Larson-Miller parametric curve) 6 Effect of aging on toughness The effect of aging on charpy impact values of 347AP and TP347H is shown in Figure 7. Aging at 500 to 650deg.C up to 10,000hours reduces the impact value at 0deg.C for both 347AP and TP347H, but the impact values of 347AP is still enough high compared with the conventional TP347H Impact value (kgf-m/cm 2 ) TP347H 347AP Aging temp.( ) x10x2V(mm) Aging time (hr) Figure 7: Charpy impact values at RT of 347AP and TP347H after long-term aging 9

10 7 Weldability The Varestraint weld-cracking test is one of the methods for evaluation of hot weldcracking. Figure 8 gives an outline of the test method. Figure 9 shows the result of the test describing crack length of 347AP, TP347, TP321, TP304 and TP316. Whereas TP347 and TP316 indicate relatively longer crack length than others, 347AP indicates improved susceptibility of weld-cracking since the crack length was same as TP304 and significantly smaller than TP347. It s suggested that lower niobium content up to 0.5mass % contribute improved weldability. 20 Test plate thickness(mm):12 Strain added(%):2.0 Weld heat input(j/cm):12,000 Filler metal:none Shield gas:ar Varestraint crack length in weld metal (mm) Added strain = 2% Figure 8: Outline of Varestraint test (welding crack susceptibility) 0 347AP Figure 9: Result of Varestraint test 8 Service experience in practical plants Sumitomo 347AP steel tubes have been installed in Japanese practical plants since ,500 metric tons in total of 347AP have been used for furnace tubes, pipes and heat exchanger tubes in hydro-treater and hydro-cracker units. Table 5 shows some examples of the service experience in the petroleum refinery industry. In most of cases, 347AP steel tube have been applied under the temperature higher than 400deg.C without PWHT and no trouble resulted from material property has been reported. 10

11 Table 5: Service experience of 347AP Plant Plant A Plant B Plant C Application Furnace Tube Furnace Tube Furnace Tube Size (outer diameter x wall thickness) Service conditions x 13.2mm x 11.9mm x 7.0mm x 7.5mm x 6.3mm Metal temperature 500 deg.c 460 deg.c 320 deg.c Feed product Residue of crude oil Naphtha Naphtha S (wt%) 3% max. 0.2% max. 0.2% max. PWHT N/A N/A N/A Year delivered Nitrogen purge applied applied applied Alkaline wash N/A N/A N/A Plant Plant D Plant E Plant F Application Heat Exchanger Furnace Tube Furnace Tube Size (outer diameter x wall thickness) Service conditions 19.0 x 2.1mm x 7.1mm x 11.1mm Metal temperature 388 deg.c 460 deg.c 410 deg.c Feed product Light oil Heavy oil Diesel oil S (wt%) 2.3% max. 1.0% max. 1.7% max. PWHT N/A N/A N/A Year delivered Nitrogen purge applied applied N/A Alkaline wash N/A N/A N/A Plant Application Size (outer diameter x wall thickness) Service conditions Metal temperature Feed product S (wt%) PWHT Plant G Furnace Tube x 7.0mm x 9.2mm 400 deg.c Light oil 5.0% max. applied Year delivered 1996 Nitrogen purge Alkaline wash N/A N/A 11

12 To see the resistance for sensitization of long term applied 347AP, furnace tube of Plant E which was used for 12 years under 460deg.C condition without PWHT (described in Table 5) has been evaluated. Photo 3 shows furnace tubes appearance after penetration test on weld joint. No indication has been observed. Photo 3: 347AP Furnace tube appearance after penetration test Photo 4 and Photo 5 show the microstructure and TEM examination of HAZ taken by extraction replicas, and they were free from precipitation on grain boundaries. These results suggest no sensitization has occurred on 347AP since it has applied. Photo 4: Microstructure of 347AP HAZ on furnace tubes of plant E 12

13 Grain boudary 5µm Photo 5: TEM examination of 347AP HAZ on furnace tubes of plant E Figure 10 shows the result of identification of precipitates in the grain. Finely dispersed NbCr nitride were detected. Any harmful precipitates such as chromium carbides were not found. Table 6 shows the result of EPR (Electrochemical Reactivation) test which has conducted according to ASTM G108 on HAZ of the 347AP tube. Pa value 0.0 coulombs/cm 2 suggests no sensitization since G108 interpret unsensitized microstructure when Pa value was less than 0.10 coulombs/cm 2 in 304 and 304L. 5) 200nm Nb Cr Bright field EDX analysis Electron diffraction pattern Result Figure 10: Precipitate identification of 347AP HAZ on furnace tubes of plant E 13

14 Table 6: EPR test results of 347AP on furnace tubes of plant E Portion Initial Open Circuit Q Ir Grain Pa Potential (mv vs. SCE) (coulombs) (ma/cm 2 ) Size (coulombs/cm 2 ) HAZ Base metal Many products forms of 347AP, such as pipe and plate have been successfully used in desulfurizing plant in Japan and its applications are expanding to other chemical plants under sensitization condition in recent years. 347AP steel tube has been designated in ASME cord case 2196 and in ASTM A312/A418 as TP347LN (UNS S34751). 9 Conclusion Sumitomo 347AP stainless steel has improved properties and advantages over conventional TP321 and TP347 stainless steels. The main conclusion can be summarized as follows. (1) 347AP has high resistance to PTA-SCC on the welded joint without PWHT with optimized chemical composition (low carbon content less than 0.02mass %, high Nb/C min.15). (2) 347AP has equivalent elevated temperature strength to that of TP347 with optimum contained nitrogen of mass %. (3) Weldability of 347AP is better than TP347 and equal to that of TP304 with low niobium less than 0.5mass %. (4) 347AP steel tubes have been applied since 1985 mainly as heat exchanger tubes in hydro-treater and hydro-cracker units without PWHT, and no trouble resulted from material property has been reported. (5) No sensitization has been observed on 347AP furnace tube in actual hydrotreater which has used for 12 years without PWHT. 14

15 10 References 1) M. Kowaka Metal Corrosion Damage and Protection Technology (1983) 2) ASTM A262 Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (2005) 3) T. Kudo, Y.Tarutani, M. Miura, Y.Sawaragi and M.Nishi The Sumitomo Search 3 (1988), No36, p73 4) ASTM G35 Determining the Susceptibility of Stainless Steels and Related Nickel- Chromium-Iron Alloys to Stress-Corrosion Cracking in Polythionic Acids, (2004) 5) ASTM G108 Electrochemical Reactivation (EPR) for Detecting Sensitization of AISI Type 304 and 304L Stainless Steels (2004) 15