Kif il-Frazzjoni tal-Volum tal-Preċipitazzjoni G-Fażi (Ni₁₆Nb₆Si₇) f'347 Heater Sheath Wara 20,000 Siegħa f'600 grad Tikkontrolla l-Enerġija tal-Impatt Charpy tat-Temperatura tal-Kamra għal Żarmar u Trasport ta' Post-Servizz

Il-Fażi Kumpless tas-Siliċidu u l-Effett ta' Ħeffa tan-Nijobju-Azzar Stabbilizzat Stainless

Precipitation of G-phase (Ni 16 Nb 6 Si 7 ), a complex nickel-niobium-silicide intermetallic phase, can cause significant embrittlement, as measured by room temperature Charpy V-notch impact energy, for 347 stainless steel (Nb-stabilized) sheathed electric heating tubes operating at 600 C for extended periods (20,000+ hours)-such as in long-term industrial furnace heaters, petrochemical reformer tubes or superheater supports. The G-phase is formed at lower temperatures (550-650°C) during very lengthy ageing times (10,000-30,000 hours) while chi (χ) and Laves phases form at higher temperatures (650-750°C). G-phase is brittle with face centred cubic (FCC) structure and is mostly formed at the grain boundaries. G-phase at volume fractions as low as 0.2-0.5% reduces impact energy from >200 J (ductile) to 100-150 J (acceptable). At 1.0-1.5 vol% the impact energy is reduced only to 50-80 J. Impact energy drops to below 40 J (brittle) at >2.0-2.5%, li joħolqu riskju ta' qsim waqt iż-żarmar, l-immaniġġjar u t-tbaħħir ta' wara s-servizz. Dan l-artikolu jikkwantifika r-rabta bejn it-toughness tal-impatt Charpy u l-frazzjoni tal-volum tal-fażi G wara 20,000 siegħa f'600 C.

Il-mekkaniżmu tal-fraġilità indotta mill-fażi G-

Il-fażi G-(Ni₁₆Nb₆Si₇) tinkludi ~50-60% Ni, 20-25% Nb u 10-15% Si bil-piż. Jippreċipita fil-konfini tal-qamħ wara tixjiħ twil f'550-650 grad, prinċipalment fi azzar li ma jissaddadx stabilizzat b'nijobju (347, 348). Il-fażi hija iebsa (400-600 HV) u fraġli. L-ittestjar tal-impatt tat-temperatura tal-kamra juri li xquq jinnukleaw fil-partiċelli tal-fażi G u jivvjaġġaw tul il-konfini tal-qamħ bi ftit deformazzjoni tal-plastik. Il-fraġilità hija sottili: l-ebusija tista 'titjieb biss b'mod marġinali (minn 160-180 HV għal 190-220 HV) filwaqt li l-ebusija tal-impatt tonqos b'mod konsiderevoli. Il-fażi G hija komunement injorata minħabba t-tul ta 'tixjiħ estiż (10,000+ sigħat) u tqabbil partikolari tan-nijobju-silikon.

Relazzjoni Kwantifikata Bejn G-Frazzjoni tal-Volum tal-Fażi u l-Enerġija tal-Impatt Charpy

Il-proċess ta 'fraġilità ta' 347 tubi (0.45% Nb, 0.50% Si, 0.08% C) imqabbad f'600Â grad sa 25,000 siegħa huwa determinat minn kwantifikazzjoni tal-fażi G-b'SEM/EDS u Charpy V-grad tal-impatt tal-grad ta '25Â.

Ħin ta' Tixjiħ f'600 grad (siegħa) G-Frazzjoni tal-Volum tal-Fażi (%)G-Daqs tal-Partiċelli tal-Fażi (nm) Post tal-Preċipitazzjoni Enerġija tal-Impatt Charpy f'25 grad (J) Dehra tal-Ksur Rakkomandat għall-Immaniġġjar ta' Post-Servizz (Żarmar, Trasport)
0 (ittemprat) 0 N/A Xejn 200-240 Duttili (100% shear) Iva 5,000 0.05<20 Intragranular (sparse) 190-220 DuctileYes 10,000 0.05-0.10 20-40 Grain boundaries (few) 160-190 DuctileYes 12,000 0.10-0.20 30-50 Grain boundaries (scattered) 130-160Mixed (some cleavage) Yes 14,000 0.20-0.35 40-60 Partial grain borders 100-130 Mixed Acceptable 16,000 0.35-0.50 50-70 Near-continuous grain boundaries 80-100 Mixed-brittle Marginal 18,000 0.50-0.70 60-80 Continuous grain boundaries 60-80 Brittle (cleavage)Not recommended 20,000 0.70-1.00 70-10045-65 Brittle continuous netNo (high risk when handled)
22,000 1.00-1.30 80-120 Kontinwu + intragranulari 30-50 Fraġli ħafna Nru 25,000 1.30-1.70 100-150 Estensiv, konsolidat 20-35 Fraġli ħafna Le
Influwenza tas-Silikon u Nijobju fuq il-Preċipitazzjoni tal-Fażi G-

Il-fażi G- teħtieġ nijobju u silikon. Inqas silikon u inqas nijobju jnaqqsu l-preċipitazzjoni tal-fażi G-.

Silicon (wt%) Niobium (wt%) Time to 0.7% G-Phase at 600°C (h) Charpy Impact after 20,000h at 600°C (J)Recommended for 20,000h Service with Post Service Handling 0.20 0.40 >25,000 120-160 Iva 0.35 0.45 15,000-20,000 70-100 Aċċettabbli (immonitorja) 0.50 0.45 10,000-15,000 45-65 Mhux rakkomandat 0.65 0.50 8,000-12,{000 35-55 Le 0.80 0.55 6,000-10,{13}} Rakkomandazzjonijiet Prattiċi ta' 7 ta' Fuq 600 grad bi Post Service Handling

Il-limitazzjonijiet tal-fażi G-li ġejjin u l-parametri materjali jipprovdu toughness xierqa biex jipprevjenu l-qsim mill-immaniġġjar għal 347 ħiters magħluqa li se jitneħħew, jiġu żarmati, ittrasportati jew mormija wara 20,000 siegħa f'600 grad .

Attività ta' Post Servizz Mixtieqa Impatt Charpy Minimu għal Immaniġġjar Sikur (J) Fażi Massimu Permissibbli G- (%) Max. Silikon (wt%) Nijobju Massimu (wt%}Limitu Rakkomandat tal-Ħajja tas-Servizz (sigħat)
Removal and disposal (no impact loading) >30 1.3 0.50 0.45 25000
Removal with crane lifting >50 0.9 0.45 0.45 18000
Dismantling (cutting, welding) >70 0,6 0,40 0,40 15 000
Re-use or Refurbishment >100 0.3 0.35 0.40 12,000
Critical handling (people nearby) >120 0.2 0.30 0.40 10 000
G-Konferma tal-Infraġilità tal-Fażi

Three approaches are used to evaluate G-phase embrittlement for heaters retired after long-term service at 600°C. The first is Charpy impact testing on a deleted sample (if available). Acceptance: >50 J għal immaniġġjar sigur It-tieni huwa l-ittestjar tal-ebusija: G-fażi tgħolli l-ebusija minn 160-180 HV għal 190-220 HV (moderata). It-tielet hija l-investigazzjoni metallografika (SEM/EDS) għall-iskoperta tal-fażi G-(partiċelli brillanti, imblokkati fil-konfini tal-qamħ b'kompożizzjoni Ni-Nb-Si). Test sempliċi fuq il-post huwa li tgħawweġ kupun żgħir. Jekk tinqasam meta titgħawweġ 90 grad fuq raġġ ta '10 mm, l-fraġilità hija serja.

Konklużjoni: Uża Si Baxx + Nb Baxx biex Tevita l-Infraġilità G-Fażi Waqt l-Immaniġġjar ta' Post-Servizz

For 347 stainless steel heater sheaths after 20,000 h at 600°C, G-phase (Ni16Nb6Si7) precipitation at volume fractions as low as 0.7-1.0% decreases room temperature Charpy impact energy from >200 J sa 45-65 J (fraġli) li jipproduċu periklu għoli ta 'qsim waqt iż-żarmar tas-servizz ta' wara, l-irfigħ tal-krejn jew it-transitu. It-347 għantijiet speċifikati għandhom jinkludu<0.35% silicon and <0.40% niobium to reduce G-phase to <0.5% after 20,000 hours for the recommended 20,000 hour service at 600°C with expected post-service handling. For service beyond 15,000-18,000 hours, it is recommended to upgrade to 310 stainless steel (which does not create G-phase) or Inconnel 600. The current system allows the buyers to define 347 chemical limits to avoid brittle failure during retirement and handling of long-service heater sheaths, by correlating G-phase volume fraction after 20,000 hours at 600°C to Charpy impact energy reduction.