Under hvilken specifik kombination af tid og temperatur i sensibiliseringsområdet (500 grader til 800 grader) udvikler 316L varmekapperør påviselig intergranulært angreb i ASTM A262 Praksis 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 timer. Tids-temperatur-sensibiliseringsforholdet er blevet kvantificeret for 316L varmekapper. Dette muliggør en vurdering af risikoen forbundet med svejsevarmepåvirkede zoner eller langvarig serviceeksponering. Mekanismen for sensibilisering af 316L med lavt kulstofindhold er diffusionen af ​​kulstof til korngrænser og udfældningen af ​​chrom-rige carbider (Cr 23 C6). Kulstofindholdet sænkes til<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 maks. 316L 0.030 100-150 50-80 400-600 Praktiske implikationer for fremstilling og service af varmelegemer For 316L indkapslede varmelegemer er nedenstående tids-temperaturbegrænsninger gyldige for at undgå sensibilisering (fejl i praksis E). Typisk temperaturområde Proces eller servicetilstand grad Maksimal tilladt tid til sensibilisering Risiko Foreslået handling Single Pass GTAW svejsning 1.200-1.400 (peak), 450-850 (køling) sekunder (køling via 450-850 grader ) Ingen risiko for væggen<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 minutter Risiko for sensibilisering; brug 347 eller 316L med<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 timer 316L ikke egnet til langtidsverifikation af sensibilisering 650 grader For 316L varmeapparater i drift i lang tid i 500-750 graders området, gælder følgende godkendelseskriterier: Feltmetallografi (replika) - Den polerede overflade kan være poleret til 50 og osv. dispersion under anvendelse af en celluloseacetat-replika. Accept: ingen kontinuerlige korngrænsekarbider (trinstruktur) Destruktiv test (prøvevarmer) Skær en del, udfør ASTM A262. Praksis E. Accept: ingen revner efter bøjning. Hårdhedstestning: Sensibilisering ændrer ikke i væsentlig grad hårdheden, og derfor ikke nyttig. 316L Sensibiliseringsrisikokvantificering ved hjælp af TTS CurveConclusion ASTM A262 Detekterbart intergranulært angreb for 316L varmekapper i rustfrit stål. 30-60 timer ved 750 grader til 1.500-2.500 timer ved 550 grader. TTS-kurvens næse ligger på 650-700 grader, hvor sensibilisering nås efter 100-200 timer. Tyndvæggede varmekapper er ikke i risiko for sensibilisering under standard svejse- og loddecyklusser (sekunder til minutter). Langtidsdrift ved 550-650 grader i tusindvis af timer, eller gentagne langvarige varmecyklusser, kan dog sensibilisere 316L. Ingeniører, der specificerer 316L hylstre til kontinuerlig brug over 500 grader, bør beregne kumulativ tid brugt i 550-650 graders området og sammenligne med TTS-begrænsningerne. For levetid ud over 2000-5000 timer ved 550-600 grader anbefales det at opgradere til 347 (niobium-stabiliseret) eller periodisk Practice E-test af varmelegemer i prøver. Den her angivne ramme, som relaterer tid, temperatur og begyndelsen af ​​sensibilisering i ASTM A262 Practice E, gør det muligt for købere at forudsige og undgå intergranulær korrosion i 316L varmekapper udsat for forhøjede temperaturer.