Qhov Volume Fraction ntawm Primary Niobium Carbide (NbC) li cas hauv Stabilized 347 Heater Sheath Tubing Control Sensitization Resistance Thaum Lub Sij Hawm Ntev -Tsev Kev Pabcuam ntawm 550℃Piv rau 316L

Cov nyhuv Stabilization ntawm Extended Exposure rau High Temperature

Rau lub sij hawm ntev, qhov kub siab (550 degree) daim ntawv thov ntawm sheathed hluav taws xob cua sov raj, xws li rau cov cua sov cua sov, petrochemical rauv preheaters los yog cua sov- cov khoom siv kho mob, kev xaiv ntawm 316L (tsawg carbon) piv rau 347 (niobium stabilized) stainless hlau yog tsav los ntawm lawv cov kev muaj peev xwm tiv taus { 4 } ntawm chromium carbide rau nws cov ntsiab lus qis carbon (<0.03%). However even 316L can sensitise at 550 C after long service (5000-10000 hours). Carbon diffusion is sluggish, but will allow carbides to develop at grain boundaries. Type 347 has niobium (Nb, usually 10× C content, 0.3-0.8% Nb) which forms stable niobium carbide (NbC) precipitates in the grains, binding up carbon and preventing chromium depletion. The long term resistance to sensitisation is totally controlled by the volume fraction of the primary NbC (0.05-0.2%) and the fine secondary NbC which precipitates in service. Properly stabilised 347 is not sensitised at 550°C for >50,000 teev. 316L pib sensitize ntawm 5,000-10,000 teev. Tsab xov xwm suav txog qhov kev sib koom ntawm NbC ntim feem pua, lub sijhawm ua haujlwm thiab kev nkag siab tsis zoo hauv 347 piv rau 316L.

Carbon Fixation Mechanism thiab Stabilization ntawm Niobium

Hauv 347 stainless hlau (18Cr-11Ni-Nb), niobium muaj kev sib raug zoo rau cov pa roj carbon ntau dua li chromium. Thaum cov tshuaj annealing (1,050-1,100 degree) feem ntau ntawm NbC dissolves, tab sis thawj NbC hais (1-5 µm) yog sab laug undissolved. Fine Secondary NbC precipitates (10-50 nm) nyob rau hauv lub austenite matrix, feem ntau ntawm dislocations thiab nyob rau hauv nplej es tsis yog nyob rau hauv grain ciam teb, nyob rau hauv txias thiab nyob rau hauv kev pab cuam ntawm 450-750 degree. Cov pa roj carbon yog khi li NbC yog li cov pa roj carbon monoxide hauv matrix tsawg heev. Qhov no tiv thaiv Cr carbides (Cr2 3 C6) los ntawm kev tsim ntawm cov ciam teb . Qhov ua tau zoo ntawm kev ruaj khov yog nyob ntawm qhov piv ntawm Nb: C (kwv yees li 8: 1 rau NbC) thiab qhov ua tiav ntawm stabilization anneal (feem ntau 850-900℃rau 2-4 teev tom qab cov tshuaj annealing).

Kev sib raug zoo ntawm NbC Volume Fraction thiab Resistance to Sensitization ntawm 550 degree

347 tubing (nrog rau ntau yam niobium thiab carbon concentrations) thiab 316L tubing ntawm 550℃mus txog 50,000 teev nrog lub sij hawm ASTM A262 Xyaum A (oxalic acid etch) thiab Xyaum E (tooj liab- tooj liab sulphate) kuaj, tau txiav txim siab cov sij hawm hauv qab no rau kev kuaj mob.

Alloy Niobium (wt %) Carbon (wt %)Nb:C weight ratio NbC fraction Volume (%) Sensitisation time at 550°C (h) ASTM A262 Practice A after 10,000hRecommended for >10,000 hours at 550°C 316L 0 0.020 0 0 5,000-10,000 Ditch (sensitized) No 316L (extra low C) 0 0.010 0 0 10,000-20,000 Dual (transition) Marginal 347 (under-stabilized) 0.15 0.040 3.8 0.02-0.05 10,000-20,000 Dual Acceptable 347 (minimally stabilized) 0.20 0.040 5.0 0.04-0.08 20,000-30,000 Step (mostly) Recommended 347 (properly stabilized) 0.40 0.040 10 0.08-0.15 >50,000 Step Best 347 (fully stabilized) 0.60 0.040 15 0.12-0.20 >50,000 Step Best 347 (over-stabilized) 0.80 0.040 20 0.15-0.25 >50,000 (tab sis NbC stringers) Kauj Ruam (tab sis muaj kev pheej hmoo ferrite) Ceeb Toom Txais 347 (siab C) 0.40 0.060 6.7 0.10-0.18 30,000-40,000 Kauj Ruam Pom Zoo
Cov nyhuv ntawm stabilization annealing ntawm nag lossis daus ntawm NbC

Tom qab cov tshuaj annealing, lub stabilization anneal ntawm 850-900℃induces cov nag lossis daus ntawm nplua, intragranular NbC, maximizing carbon fixation thiab tiv thaiv tom qab rhiab heev.

Thaum tshav kub kub Treatment Sequence NbC Morphology Lub Sijhawm Rau Kev Ruaj Ntseg ntawm 550oC (hrs) Pom Zoo Rau Lub Sijhawm Ntev (LTS)
Cov tshuaj anneal nkaus xwb (1050 degree, dej quenched) Thawj NbC nkaus xwb (coarse, qee C hauv cov tshuaj) 15,000-30,000 Txais
Solution anneal + stabilisation (850°C, 2 h)Fine intragranular NbC (most C fixed) >50,000 Qhov zoo tshaj plaws
Solution anneal + stabilisation (900°C, 4h)>50,000 Beste Zoo heev NbC, tag nrho C fixation
Rough stabilization tsis muaj tshuaj anneal NbC nkaus xwb (tsis tiav) 10,000-20,000 Borderline
Tsis muaj stabilization (raws li welded) Mixed 5,000-15,000 Tsis qhia
Cov lus pom zoo rau kev xaiv lub rhaub rau 550℃Kev Pabcuam

Rau kev ua haujlwm tas mus li ntawm cov cua kub cua sov ntawm 550 degree, cov hauv qab no tsis siv rau kev xaiv alloy siv nyob ntawm qhov kev pab cuam lub neej:

Kev Pabcuam Lub Neej ntawm 550℃(teev) Pom zoo Alloy Yam tsawg kawg nkaus Stabilization RequirementExpected Sensitization Resistance Cost Factor<5,000 316L (0.03%C) None Acceptable 1.0× 5,000-10,000 316L (0.02%C) or 347 (minimum)1.1×10,000-20,000 347 (well stabilised) 850°C, 2h Marginal None for 316L; stabilisation optional for 347Good 1.3× 20,000-50,000 347 (completely stabilized) 900°C, 4h Excellent 1.4× >50,000 347 + high Nb (0.6-0.8%) Full stabilisation + fine grain Best 1.5× >50,000 (ib puag ncig corrosive) 310 stainless hlau (tsis muaj rhiab heev) N / A Tsis siv tau 2.0 × Cov txiaj ntsig ntawm Stabilization Verification

Cov kev pom zoo hauv qab no yuav tsum tau siv rau cov neeg yuav khoom tshwj xeeb 347 rau kev pabcuam kub kub mus ntev:

ASTM A262 Xyaum A: Cov qauv txheej txheem (txhim kho cov ciam teb) tom qab kev pabcuam lub neej tag nrho lossis tom qab kev laus zuj zus (1,000 h ntawm 650 degree)

ASTM A262 Xyaum E: Tom qab kev laus dhau (tsis muaj tawg tom qab khoov)

Niobium to carbon ratio: Nb/C weight ratio >= 10 (zoo tshaj 12-15) ntawm daim ntawv pov thawj zeb

Stabilization anneal ntawv pov thawj: lub sij hawm thiab kub (ie . 900 deg C rau 4 teev) kaw

Xaus: 347 Tshaj 316L rau Extended 550℃Kev Pabcuam

Carbon diffusion in 316L stainless steel heater sheaths at 550°C leads to chromium carbide precipitation and sensitisation after 5,000-10,000 hours of operation, making it susceptible to intergranular corrosion. Type 347 (niobium stabilised) with correct ratio of Nb:C (≥10:1) and stabilisation annealing (850-900°C, 2-4 hours) is not sensitised for >50,000 teev vim yog cov pa roj carbon monoxide tso mus tas li raws li intragranular NbC. Rau kev pabcuam txuas ntxiv siab tshaj 500℃nrog lub neej xav tau ntau tshaj 5,000-10,000 teev, engineers qhia cov khoom siv cua sov yuav tsum xaiv 347 es tsis yog 316L. Rau qhov xav tau tshaj plaws ntawm daim ntawv thov (cov kab mob corrosive + kub siab) 310 stainless hlau (25Cr-20Ni) yuav muab kev tiv thaiv ntau dua tab sis ntawm tus nqi siab dua. Lub moj khaum no tso cai rau cov neeg yuav khoom xaiv cov hlau ruaj khov tshaj plaws rau kev ua haujlwm mus sij hawm ntev hauv lub tshuab cua kub kub los ntawm kev sib txheeb ntawm NbC ntim feem ntau nrog qhov kev xav pib lub sijhawm ntawm 550 degree.