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Embrielement in Super Austenic Stainless Steel Şermin Özlem TURHAN Contact: Assistant Prof. Y. Eren Kalay Department of Metallurgical and Materials Engineering Middle East Technical University Producon and Heat Treatment Embrilement Problem and Characterizaon Defects Analysis under TEM Future Direcons… • If the alloy is incorrectly heat-treated as lile as 15 minutes, it embriles and the toughness is decreased by 50% (Fig.1). • Catastrophic failure • Disastrous consequences - environmental polluon In this study the possible reasons of embrilement in CN3MN superaustenic stainless steel are invesgated. What are the possible reasons for embrilement? • Brile secondary phases - nanoprecipitates • Defects; dislocaon tangles - stacking faults Failure Analysis The failure analysis results with SEM indicated a ducle to brile fracture transion with respect to annealing me. (Fig. 4.) • Superaustenic stainless steels are Fe – C systems that are alloyed with Cr, Ni, Mo and N. For a steel to be stainless, it must have the chromium concentraon at least 11 wt %. Chromium also cause the austenite phase field to extent to room temperature with the help of Ni concentraon at least 8 wt %. •Higher Mo content ( 6 – 7 %) •Higher Ni content •N addions • Due to high alloying, these steels have high resistance to corrosion with superior toughness. • Aributed to these properes, they are used in extreme environments such as; » Off - shore oil wells » Seawater systems » Heat exchangers and piping in purificaon systems. • For sample producon and heat treatment processes, a parcular alloy, CN3MN is chosen. Table 1 shows the nominal composion of CN3MN as cast condion. Table 2. Annealing mes for samples heat treated at 927 °C Samples 1 2 3 4 5 6 7 Annealing Times (min) 0 min 30 sec 1 min 2 min 4 min 8 min 16 min SAMPLE PRODUCTION HEAT TREATMENT • The alloys CN3MN were produced in Steel Founders’ Society of America (SFSA) foundaries, then recivecd as cast condion for heat treatment. • Held for 4 hours at 1205 ºC and water quenched • 7 Charpy impact specimens prepared • Specimens were encapsulated in a quartz tube under inert argon atmosphere • Held for 0 - 16 min at 927 ºC and water quenched (Table 2) CN3MN Components Wt % Carbon, C < = 0.030 % Chromium, Cr 20.0 – 22.0 % Copper, Cu < = 0.75 % Iron, Fe 41.4 – 50.3 % Manganese, Mn < = 2.0 % Molybdenum, Mo 6.0 – 7. 0 % Nitrogen, N 0.18 – 0.26 % Phosphorus, P < = 0.04 % Silicon, Si < = 1.0 % Sulfur, S < = 0.01 % Nickel, Ni 23.5 – 25.5 % Table 1. The nominal composion of CN3MN Fig.1. Impact strength as a funcon of me-at – temperature for 872 C heat treatment Fig. 3. Hardness with impact energy as a funcon of annealing me for CN3MN heat treated at 927 °C Fig. 4. SE images of CN3MN heat treated at 927 °C for different mes: (a) 0 second, (b) 4 minutes and (c) 16 minutes (200X) Fig. 2. Examples to catastrophic failure due to brile fracture. (a) broken pipe, (b) offshore well, (c) an oil tanker fractured Annealing Time (min) Room Temperature - 40 °C HARDNESS (HRB) IMPACT ENERGY (J) ANNEALING TIME (MIN) 0 sec 4 min 16 min Characterizaon When superaustenic stainlesss steel is heat treated at high temperatures for long periods of me, intermetallic sigma and laves secondary phases (Fig. 5). Since the transformaon kinecs of secondary phases are sluggish for mes as short as 16 min, only austenite phase is observed with XRD and EBSD (Fig. 6). The nanoprecitates with similar composions are observed along the grain boundaries of 0, 4, 8, and 16 minutes heat treated samples with EDS mode of TEM also. The corresponding EDS results show that the precipitates are rich in Mo and Si (Table 4). The backscaered electron (BSE) microscopy, corresponding energy dispersive analyses results and EBSD results have shown intergranular precipitaon of extremely small Fig. 7. Back scaered imaging of precipitates rich in Mo and Si along the grain boundaries. MICROSTRUCTURAL ANALYSIS Nanoprecipitates Defects Nanoprecipitates Secondary Phases Fig. 5. Experimental Ϭ and Laves volume percents for CN3MN at 900°C heat treatment and BSE image at 125 hour heat treatment. Fig. 9. BF image of the nanoprecipitates seen at grain boundaries of (a) 0 second (b) 4 minutes (c) 8 minutes (d) 16 minutes heat treated samples Fig.10. BF Images of brile fractured CN3MN heat treated for 16 minutes. (a) Cracks seen in brile fractured regions (b) Boundaries near cracks Fig 11. (a) BF image of birle sample showing the boundaries and the inset showing the corresponding SAED paern. (b) BF and DF image showing the same boundaries at higher magnificaons. (c) High resoluon TEM image of the boundaries showing the atomic arrangement along them. Fig. 12 shows the disncve structure of cracks and the step like fracture in micro scale. The similar step like structure was also observed in macro scale with SEM with examining the fracture surfaces of the brile regions (Fig. 13). Fig.14. HRTEM and BF images showing similar step like structure Table 4. EDS results from grains and the nanoprecipitates along grain boundaries of 0, 4, 8 and 16 minutes heat treated samples Fig. 6. XRD results from ducle and brile samples(a) and EBSD image showing the austenite (b) grains Fig. 7. Back scaered imaging of CN3MN heat treated at 927 °C for 16 min. Arrows indicate nanoprecipitates along the grain boundary. Fig. 8. (a) Backscaered electron microscopy image for the brile specimen (b) The nano precipitates that are seen in Fig. 8 (a) are observed to precipitate along austenite grain boundaries with EBSD techique with SEM Table 3. EDS results of CN3MN [heat treated at 927 °C for 16 min] from boundary and matrix Volume Percent Time (Min) Austenite matrix Ϭ phase Laves phase 20 30 40 50 60 70 80 90 100 110 INTENSITY (A.U) BRAGG ANGLE (2θ) 0 min 16 min At % Fe Ni Cr Mo Si Mn Total Matrix 45.05 ± 0.46 24.42 ± 0.34 21.12 ± 0.20 4.78 ± 0.27 2.89 ± 0.29 1.75 ± 0.13 100.00 Bounday 39.27 ± 0.88 23.73 ± 0.47 21.43 ± 0.15 8.44 ± 0.47 5.46 ± 0.51 1.68 ± 0.09 100.00 a b Grain Boundary Grain 2 Grain 1 Fig.12. BF image from brile regions showing the step like fracture in micro scale c d a b DF 125� <200> <111> ḡ {200} {111} BF TEM invesgaon from the brile fractured regions of 16 minutes heat treated sample shows a microstructure with some kind of boundaries and dislocaon clusters. These boundaries and the dislocaon clusters are observed to be near cracks seen only in brile regions. Long term future studies includes; • The determinaon of possible reasons for the formaon of boundaries • Causes of embrilement • The relaonship between the nano boundaries and step like cracking 2A a b c Large drop in toughness can cause catastrophic failure Mechanical Analysis While impact strength decreases with annealing me, hardness doesn’t change (Fig. 3) Step like fracture in micro and macro scale Nanoprecitates in all samples heat treated at different mes Fig.13. SE image from brile regions showing the step like fracture in macro scale Locaon Cr Ni Mo Fe Si Mn 0 minute heat-treated specimen PRECIPITATES 25.25 ± 0.53 22.88 ± 0.8 20.57 ±1.17 13.23 ± 0.40 17.61 ± 0.9 0.46 ± 0.12 MATRIX 22.58 ± 0.31 23.65 ± 0.26 3.67 ±0.33 46.98 ± 0.32 1.93 ± 0.16 1.20 ± 0.12 16 minute heat-treated specimen PRECIPITATES 24.77 ± 0.55 22.88 ± 0.31 20.02 ± 0.36 14.75 ± 0.19 13.13 ± 0.52 0.56 ± 0.07 MATRIX 22.43 ± 0.22 23.30 ± 0.1 4.01 ± 0.14 47.16 ± 0.22 1.97 ± 0.09 1.14 ± 0.07
