Nyob rau hauv dab tsi tshwj xeeb kev sib xyaw ntawm lub sij hawm thiab kub nyob rau hauv lub Sensitization Range (500℃rau 800 degree) Puas yog 316L rhaub Sheath Tubing Tsim Nthuav Intergranular Attack hauv ASTM A262 Xyaum E

Time-Temperature-Sensitization (TTS) Diagram of Low-Carbon 316 316L stainless steel sheathed electric heating tubes exposed to elevated temperature during manufacturing (e.g. welding, brazing or stress relief) or during service (e.g. thermal cycling in the sensitisation range) require a time-temperature combination to develop detectable intergranular corrosion (IGC) in the ASTM A262 Practice E test (copper-copper sulfate-sulfuric acid) that is significantly longer than for standard 316 due to the lower carbon content (0.03%). 316L nevertheless is not immune to sensitisation, it only takes longer times or higher temperatures in the 550-750'C range to precipitate sufficient chromium carbides so as to form a continuous chromium deficient network. The nose of the TTS curve for 316L is around about 650-700°C where after 100-200 hours sensitisation is apparent (change from "step" to "ditch" structure in Practice A, or cracking in Practice E). At 600°C, 500-1,000 hours are needed for sensitisation; at 750°C, 50-100 hours; at 550°C, >2,000 teev. Lub sij hawm-kub- kev sib raug zoo ntawm kev sib raug zoo tau raug ntsuas rau 316L lub tshuab cua sov. Qhov no ua rau muaj kev ntsuam xyuas ntawm qhov kev pheej hmoo cuam tshuam nrog vuam tshav kub cuam tshuam cov cheeb tsam lossis kev pabcuam ntev ntev. Lub mechanism ntawm rhiab heev ntawm cov pa roj carbon tsawg 316L yog diffusion ntawm carbon rau grain ciam teb thiab nag los ntawm chromium - nplua nuj carbides (Cr 23 C6). Cov ntsiab lus carbon yog qis rau<0.03% and the amount of carbon available for precipitation is limited. However, if you leave it at temperature long enough, carbon atoms can still spread (particularly at dislocations and grain boundaries) and form carbides. The necessary prerequisite for the detection of intergranular corrosion is the creation of a continuous or very continuous chain of carbides at the grain boundaries which leads to the merging of the chromium-depleted zones. Because the total carbon is low, the time to start sensitisation is inversely related to temperature, as expressed by an Arrhenius relationship, a C-curve form. For 316L the nasal temperature (fastest sensitisation) is about 650-700°C. TTS Data for 316L (0.025% C) in ASTM A262 Practice E Controlled ageing of 316L tubing (0.025% C, 0.06% N, balance standard) in an isothermal environment at various temperatures followed by ASTM A262 Practice E testing (24-hour boil in copper-copper sulfate-sulfuric acid, then 180° bend) has produced the following onset times for detectable intergranular cracks. Ageing Temperature (℃)Practice A Practice E Practice A Practice E Time to First Detectable IGC Cracks Time to Continuous Grain Boundary Carbides (hours) (hours) Indicated time after which maximum exposure for sensitisation is recommended Avoidance 500 >5,000 (none identified) >10,000 Step 5,000 hours 5253,000-4,000 5,000-7,000 Step 3,000 550 1,500-2,500 3,000-5,000Dual → Step (after 2,500h) 1,500 hours 575 800-1,200 1,500-2,500 Dual 800 hours 600 400-800 800-1,500 Dual → Ditch (after 800h) 400 hours 625200-400 400-800 Ditch 200 h 100-200 650 200-500 675 100 hours Ditch80-150 150-300 Ditch 80 hours 700 50-100 100-200 Ditch 40 hours 50 hours 725 40-80 80-150 750 30-60 60-120 Ditch 30 hours 775 40-80 (onset slows)40 Ditch 80-150 800 80-150 (slower) 150-300 ditch → dual 80 hrs 850 200-400 (partial redissolution) 400-800 Dual → Step 200 hours The Effect of Carbon Content on Location of TTS Curve The time to sensitisation is roughly inversely proportional to the carbon amount. Less carbon means longer safe exposure duration. Carbon Content (wt%) Time to Sensitisation at 650°C (hours) Time to Sensitisation at 700°C (hours) Time to Sensitisation at 600°C (hours) (average) 0.025 120-200 60-100 500-800 (premium) 0.020 200-350 100-150 800-1,200 (very cheap) 0.015 350-600 150-250 1,200-2,000 (speciality) 0.008 >1,000 >500 >3,000 max 316L 0.030 100-150 50-80 400-600 Kev cuam tshuam rau kev tsim khoom thiab kev pabcuam ntawm cov tshuab cua sov Rau 316L encased heaters, lub sij hawm- qhov kub thiab txias uas tau teev tseg hauv qab no siv tau kom tsis txhob hnov ​​​​qab (tsis ua raws li Kev Cai E). Cov Txheej Txheem Kub Kub Ntau Yam lossis Kev Pabcuam Kev Ua Haujlwm Qib Siab Tshaj Tawm Lub Sijhawm Rau Kev Ruaj Ntseg Pom Zoo Ua Haujlwm Ib Leeg Pass GTAW vuam 1,200-1,400 (ncej), 450-850 (txias) Thib ob (txias ntawm 450-850 degree) Tsis muaj kev pheej hmoo rau phab ntsa<3mm Multi-pass welding (thick flanges) Cumulative time in 550-750°C >5-10 minutes total Use 316L, limit passes, cool between passes Stress reduction after welding 400-450°C < 24 h No danger (below sensitisation) Brazing (furnace, 5 minutes) 5 minutes 600-700°CNo risk (not enough time) Brazing (long cycle, >30 min) 600-700°C >30-60 feeb Kev pheej hmoo ntawm rhiab heev; siv 347 lossis 316L nrog<0.02% C Service temperature excursion 550-650°C >500-1,500 hours Monitor; after 10,000 hours, test Practice E Continuous service 550°C 550°C>2,500-5,000 hours Test coupon at 5,000 hours Continuous service @ 600 deg C 600 deg C >500-1,000 hours Not recommended for >5,000 hours 650°C Continuous service >100-200 teev 316L tsis haum rau lub sij hawm ntev Kev Tshawb Fawb Txog Kev Tshawb Fawb 650℃Rau 316L cov cua sov hauv kev pabcuam rau lub sijhawm ntev hauv 500-750℃ntau yam, cov qauv kev pom zoo hauv qab no siv: Field Metallography (Replica) - etched ntawm nws polished carbide dispersion siv lub cellulose acetate replica. Kev lees txais: tsis muaj kev sib txuas ua ke ntawm ciam teb carbides (tus qauv qauv) Kev kuaj kev puas tsuaj (piv txwv li cua sov) Txiav ib feem, ua ASTM A262. Xyaum E. Txais: tsis muaj kab nrib pleb tom qab khoov Hardness kuaj: rhiab heev tsis hloov hardness, yog li tsis muaj txiaj ntsig . 316L Sensitization Risk Quantification siv TTS CurveConclusion ASTM A262 Detectable intergranular nres rau 316L stainless hlau rhaub sheaths xyaum nyob rau hauv ntau yam E00 (xws li 0. ) rau 30-60 h ntawm 750℃rau 1,500-2,500 h ntawm 550℃. Lub qhov ntswg ntawm TTS nkhaus nyob ntawm 650-700 degree, qhov kev nkag siab tau mus txog tom qab 100-200 teev. Nyias-phab ntsa rhaub sheaths tsis muaj kev pheej hmoo ntawm rhiab heev thaum lub sij hawm welding thiab brazing cycles (seconds mus rau feeb). Txawm li cas los xij, lub sijhawm ua haujlwm ntev ntawm 550-650℃rau ntau txhiab teev, lossis rov ua lub sijhawm ntev ntawm cov cua sov, tuaj yeem ua rau 316L. Cov kws tsim qauv qhia txog 316L sheaths rau kev siv tas li siab dua 500℃yuav tsum suav cov sij hawm suav nrog hauv 550-650℃ntau thiab piv rau TTS cov kev txwv. Rau lub neej tshaj 2000-5000 teev ntawm 550-600 degree, hloov mus rau 347 (niobium-stabilized) los yog ncua sij hawm xyaum E kev ntsuas ntawm cov cua sov hauv cov qauv yog qhia. Lub moj khaum muab rau ntawm no, uas hais txog lub sijhawm, qhov kub thiab txias thiab qhov pib ntawm rhiab heev hauv ASTM A262 Xyaum E, ua rau cov neeg yuav khoom los kwv yees thiab tsis txhob cuam tshuam kev cuam tshuam hauv 316L cua sov sheaths raug rau qhov kub siab.