Pod kojom specifičnom kombinacijom vremena i temperature u opsegu senzibilizacije (500 stepeni do 800 stepeni) 316L cijev grijača s omotačem razvija uočljivi intergranularni napad u ASTM A262 praksi 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 sati. Odnos osjetljivosti na vrijeme{3}}temperatura{4}} je kvantificiran za omote grijača od 316L. Ovo omogućava procjenu rizika povezanog sa zonama pod utjecajem topline zavarivanja ili produženom ekspozicijom u radu. Mehanizam senzibilizacije niskougljičnog 316L je difuzija ugljika na granice zrna i taloženje hrom{9}}bogatih karbida (Cr 23 C6). Sadržaj ugljika je snižen na<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 Praktične implikacije za proizvodnju i servis grijača Za grijače sa kućištem od 316L, vremenska{5}}temperaturna ograničenja navedena u nastavku vrijede kako bi se izbjegla senzibilizacija (neuspjeh prakse E). Tipičan temperaturni opseg Proces ili stepen servisnog stanja Maksimalno dozvoljeno vreme do preosetljivosti Rizik Predložena radnja Jednoprolazno GTAW zavarivanje 1.200-1.400 (vrh), 450-850 (hlađenje) sekundi (hlađenje preko 450-850 stepeni) Nema rizika za zid<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 minuta Rizik od preosjetljivosti; koristite 347 ili 316L sa<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 sati 316L nije pogodno za dugotrajnu provjeru osjetljivosti 650 stepeni Za 316L grijače koji su u upotrebi tokom dužeg vremenskog perioda u opsegu od 500-750 stepeni, primjenjuju se sljedeći kriteriji odobrenja: Terenska metalografija (replika) {{36}. karbidna disperzija upotrebom replike celuloznog acetata. Prihvatanje: nema kontinualnih karbida na granici zrna (stepenasta struktura) Ispitivanje razaranja (grejač uzorka) Izrežite deo, uradite ASTM A262. Vježba E. Prihvatanje: nema pukotina nakon savijanja Ispitivanje tvrdoće: Preosjetljivost ne mijenja značajno tvrdoću, stoga nije korisna. 316L Kvantifikacija rizika od senzibilizacije pomoću TTS CurveConclusion ASTM A262 Međugranularni napad koji se može detektirati za omote grijača od nehrđajućeg čelika 316L Vježbajte u opsegu od 05 do 08 stupnjeva ekspozicije. 30-60 h na 750 stepeni do 1.500-2.500 h na 550 stepeni. Nos TTS krive leži na 650-700 stepeni, gdje se senzibilizacija postiže nakon 100-200 sati. Tankozidne obloge grijača nisu izložene riziku od senzibilizacije tokom standardnih ciklusa zavarivanja i lemljenja (od sekundi do minuta). Međutim, dugotrajan rad na 550-650 stepeni hiljadama sati, ili ponovljeni dugotrajni toplotni ciklusi, mogu izazvati senzibilizaciju 316L. Inženjeri koji specificiraju omote od 316L za kontinuiranu upotrebu iznad 500 stepeni trebali bi izračunati kumulativno vrijeme provedeno u rasponu od 550-650 stepeni i uporediti sa ograničenjima TTS-a. Za životni vek preko 2000-5000 sati na 550-600 stepeni, savetuje se nadogradnja na 347 (stabilizovan niobijumom) ili periodično testiranje E za grejače u uzorcima. Okvir koji je ovdje dat, koji povezuje vrijeme, temperaturu i početak senzibilizacije u ASTM A262 Practice E, omogućava kupcima da predvide i izbjegnu međugranularnu koroziju u omotaču grijača od 316L izloženih povišenim temperaturama.