Yuav Ua Li Cas Lub Hwj Chim Ntawm Thawj MC Carbides (NbC/TiC) los ntawm Tshaj Niobium (0.60-0.80 wt%) nyob rau hauv 347 Heater Sheath Tubing Tswj lub qhov chaw pib qhov ntom ntom thiab Perforation sij hawm hauv 1,000 ppm Chloride Dej ntawm 80 degree

Kev Pabcuam Chloride Tshaj -Stabilization Rau Pitting Resistance

For sheathed electric heating tubes of 347 stainless steel (Nb-stabilized) in moderate chloride water (1,000 ppm Cl⁻) at 80°C, the excess niobium beyond the stoichiometric requirement for carbon fixation (Nb:C ratio >10:1) leads to the formation of primary MC carbides (NbC) not dissolved during solution annealing. The initial carbides are usually 2-10 m in size and are cathodic sites for pit start. At 0.30-0.40 wt % niobium (Nb:C = 8:1 to 10:1) principal MC volume percentage is 0.04-0.06% and service life >10 xyoo. Thawj MC ntim feem ntau nce mus rau 0.12-0.20% (over-stabilized (0.60-0.80 wt% niobium)) Qhov ntom ntom nce 5-10 zaug Perforation lub sij hawm txo mus rau 2-4 xyoo Daim ntawv no ntsuas qhov kev sib txuas ntawm ntau tshaj Nb, thawj qhov sib npaug ntawm qhov ntim ntawm carbide, thawj qhov ntim ntawm lub sijhawm pib 1,000 ppm Cl⁻ dej ntawm 80 degree.

Lub Mechanism ntawm Pitting Tsav los ntawm Thawj MC Carbide

Thaum lub sij hawm solidification ntawm 347 stainless hlau, thawj MC carbides (NbC, TiC) yog tsim. Vim lawv cov hluav taws xob hluav taws xob siab thiab qhov zoo ntawm qhov zoo, lawv yog cathodic rau austenitic matrix. Yog tias cov ntsiab lus niobium siab dua li ntawm 0.45-0.50 wt% (cov ntsiab lus carbon ib txwm yog 0.04-0.06%) tom qab ntawd ib qho ntxiv niobium tshaj li qhov xav tau rau tag nrho stabilization tshwm sim hauv kev tsim cov carbide ntau dua. Cov khoom no tsis soluble thaum cov tshuaj annealing (1,050-1,100 degree) thiab khaws cia rau hauv qhov kawg tubing. Hauv kev daws teeb meem chloride cov carbides nyiam koom nrog hauv cathodic oxygen txo cov tshuaj tiv thaiv thiab tsim ib khub galvanic uas nyiam cov anodic dissolution ntawm cov matrix ib puag ncig. Qhov ceev ntawm qhov pib pib yog proportional rau tus naj npawb ntawm thawj MC carbide hais tawm ntawm qhov chaw.

Kev sib raug zoo ntawm Excess Niobium, MC Volume Fraction thiab Pitting Performance

Tswj kev ntsuas immersion nrog 347 tubing nrog ntau qib niobium (qhov tsis tu ncua carbon 0.05-0.06%, titanium<0.01%) in 1,000 ppm Cl- water (pH 7.0, aerated) at 80°C for 5,000 hours have established the following pit initiation densities and perforation times.

Niobium (wt%) Nb: C Ratio Carbon (wt%) MC Thawj Volumetric Fraction (%)MC Particle Loj (µm) Qhov ntom ntom tom qab 5,000h (pits / cm2) Qhov siab siab tshaj plaws tom qab 5,000h (µm) Lub Sijhawm rau Perforation (1.5 mm phab ntsa, xyoo) 100℃pom zoo, pp. Cl- Kev Pabcuam 0.20 4:1 0.05 0.02-0.03 1-2 0.2-0.5 15-25 10-15 Txais tau (hauv- ruaj khov, txaus ntshai ntawm rhiab heev)
0.30 6:1 0.05 0.03-0.04 1-2 0.3-0.6 18-28 9-12 Txais tau 0.35 7:1 0.05 0.04-0.05 1-2 0.5-1.0 20-35 8-11 Txais tau 0.40 8:1 0.05 0.05-0.06 1-3 0.8-1.5 25-40 6-9 Txais tau (qhov zoo tshaj qhov sib npaug) 0.45 9:1 0.05 0.06-0.08 2-4 1-2 30-50 5-7 Marginal 0.50 10:1 0.05 0.08-0.10 2-5 2-3 40-60 4-5 Tsis pom zoo 0.60 12:1 0.05 0.12-0.15 3-6 4-8 50-80 2.5-4 Tsis yog {{12}5}:{13} Tsis yog {{12}5}:{13} 1.00 20:1 0.05 0.25-0.30 6-12 25-40 100-150 0.8-1.2 Tsis yog
Kev cuam tshuam ntawm kev daws qhov ntsuas kub ntawm qhov pib MC Dissolution

Qhov kev daws teeb meem siab dua tuaj yeem ua rau cov MC carbides ib nrab, tab sis muaj kev pheej hmoo ntawm cov nplej coarsening thiab ferrite tsim tshaj 1,150 degree.

Kev daws qhov kub thiab txias (degree) Thawj MC Volume Fraction ntawm 0.60% Nb (%) MC Particle Loj (µm) Qhov ntom ntom (pits/cm2) Pom zoo 1,050 0.12-0.15 3-6 4-8 Tsis yog 1,080 0.10-0.12 3-5 3-6 Tsis yog 1,100 0.08-0.10 3-4 2-4 Marginal 1,120 0.06-0.08 2-4 1-3 Kev loj hlob txaus ntshai
1, 150 0.05-0.07 2-3 1-2 Txais tau (grain coarsening → qis creep zog)
347 Cov Lus Pom Zoo rau Kev Pabcuam Chloride

Kev Pabcuam Lub Neej (xyoo) Qhov siab tshaj plaws Niobium (wt%) Yam tsawg kawg nkaus kev daws qhov kub thiab txias (oC) Qhov siab tshaj plaws Primary MC Vol. Fraction (%) Stabilisation suggested10 0.40 1,050 0.06 Tsis muaj (kev daws teeb meem anneal)
8-10 0.42 1,060 0.07 Tsis muaj 5-8 0.45 1,080 0.08 Tsis muaj (Tsis txhob muaj kev ruaj ntseg)
3-5 0.50 1,100 (nrog kev hloov pauv loj) 0.10 Marginal<3 >0.50Not recommended >0.10 Hloov mus rau 316L lossis duplex 2205 Thawj MC Carbide Cov Ntsiab Lus Tshawb Fawb

Rau cov neeg siv khoom uas xav tau qee qhov pitting kuj muaj ob txoj hauv kev los xyuas. Thawj yog SEM metallographic tshuaj xyuas ntawm polished thiab me ntsis etched hla ntu. Suav thawj MC carbides (angular, 2–10 µm) ntawm 1,000 × Txais:<5 particles/mm2 for >8 xyoo lub neej