Ki jan fraksyon volim G-faz (Ni₁₆Nb₆Si₇) presipitasyon nan djenn aparèy chofaj 347 apre 20,000 èdtan nan 600 degre kontwole tanperati chanm nan Charpy enpak enèji pou pòs-demantèlman ak transpò sèvis

Konplèks faz silisid la ak efè ajitasyon nan Niobyòm-Stabilize Nerjaveèi

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%, ki reprezante yon risk pou yo fann pandan apre sèvis demantèlman, manyen ak anbake. Atik sa a mezire lyen ki genyen ant solidite enpak Charpy ak fraksyon volim faz G apre 20,000 èdtan nan 600 C.

Mekanis G-faz pwovoke frajilman

Faz G-(Ni₁₆Nb₆Si₇) gen ladann ~50-60% Ni, 20-25% Nb ak 10-15% Si pa pwa. Li presipite nan fwontyè grenn jaden apre lontan aje nan 550-650 degre, sitou nan niobyòm estabilize asye pur (347, 348). Faz la difisil (400-600 HV) ak frajil. Tès enpak tanperati chanm nan montre ke fant nwaye nan patikil G-faz epi vwayaje sou fwontyè grenn jaden ak ti deformation plastik. Fragilizasyon an se sibtil: dite ka amelyore jis yon ti kras (soti nan 160-180 HV a 190-220 HV) pandan y ap dite enpak la diminye konsiderableman. Faz G se souvan neglije paske nan dire aje pwolonje (10,000+ èdtan) ak patikilye pè niobyòm-silikon.

Kantite relasyon ant G-Fraksyon Volim Faz ak Enèji enpak Charpy

Pwosesis frajilman 347 tib (0.45% Nb, 0.50% Si, 0.08% C) ki gen laj nan 600Â degre pou jiska 25,000 èdtan detèmine pa G-faz quantification ak SEM/EDS ak Charpy V-degre enpak enpak nan 25ÂÂ.

Tan aje a 600 degre (èdtan) G-Fraksyon Volim Faz (%)G-Grasè Patikil Faz (nm) Kote Presipitasyon Enèji Enpak Charpy a 25 degre (J) Aparans Frakti Rekòmande pou Post-Manitasyon Sèvis (Demantèlman, Transpò)
0 (rekwit) 0 N/A Okenn 200-240 Ductile (100% Chea) Wi 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 Kontinyèl + intragranulaire 30-50 Trè frajil Non 25,000 1.30-1.70 100-150 Vaste, konsolide 20-35 Trè frajil Non
Enfliyans Silisyòm ak Niobyòm sou G-Presipasyon Faz

Faz G-la bezwen niobyòm ak Silisyòm. Mwens Silisyòm ak mwens niobyòm ralanti presipitasyon faz 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 Wi 0.35 0.45 15,000-20,000 70-100 Akseptab (moniteur) 0.50 0.45 10,000-15,000 45-65 Pa rekòmande 0.65 0.50 8,000-12,000 35-55 Non 0.80 0.55 6,000-10,{13}}Aprè rekòmandasyon pou rekòmandasyon pratik. 600 degre ak Post Service Handling

Limit G-faz sa yo ak paramèt materyèl yo bay severite apwopriye pou anpeche fann nan manyen pou 347 aparèy chofaj ki anvlòp yo ke yo pral retire, demonte, transpòte oswa jete apre 20,000 èdtan nan 600 degre.

Aktivite apre sèvis yo dezire. Minimòm enpak Charpy pou manyen an sekirite (J) Maksimòm G-faz (%) Max. Silisyòm (wt%) Niobyòm maksimòm (wt%}Limit lavi sèvis rekòmande (èdtan)
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-Faz Konfimasyon Konfimasyon

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 pou manyen san danje Dezyèm lan se tès dite: G-faz ogmante dite soti 160-180 HV rive 190-220 HV (modere). Twazyèmman se envestigasyon metalografik (SEM/EDS) pou deteksyon G-faz (briyan, patikil blòk nan fwontyè grenn ak konpozisyon Ni-Nb-Si). Yon tès jaden senp se pliye yon koupon ti. Si li fann lè bese 90 degre atravè yon reyon 10 mm, frajilman grav.

Konklizyon: Sèvi ak Low Si + Low Nb pou Evite G-Faz Faz Pandan Post-Manisyon Sèvis

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 a 45-65 J (frajil) pwodwi yon gwo danje nan fann pandan sèvis pòs demonte, leve teknik oswa transpò piblik. 347 djenn ki espesifye yo dwe genyen ladan l<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.