Ki jan fraksyon volim nan Chi (χ) presipitasyon faz nan 700 degre pandan sèvis alontèm -nan 316L aparèy chofaj djenn tib kontwòl tanperati chanm enpak rediksyon solidite

Faz entèmetalik embrittling fòme nan 600-800 degre

Precipitation of the intermetallic Fe₃₆Cr₁₂Mo₁₀ phase, called chi (χ) phase, severely embrittles 316L stainless steel sheathed electric heating tubes in service for extended periods (thousands of hours) in the 600-800°C range, such as superheater supports, high-temperature furnace heaters, and petrochemical process heaters, as measured by room temperature impact toughness (Charpy V-notch testing). Chi phase has an even more negative influence on toughness. Unlike the well known sigma (σ) phase formation at higher temperature (650-900°C), chi phase precipitates more quickly at lower temperature (600-750°C). Molybdenum and chromium rich chi phase is found on the ferrite-austenite boundaries and on the grain boundaries. Even small volume fractions of chi phase (i.e. 0.5-1.0%) can lower the room temperature impact energy of 316L from >200 J (duktil) pou<50 J (brittle), and at 2-3% chi phase the impact energy is below 20 J (severe embrittlement). This paper evaluates the link between chi phase volume fraction, precipitation kinetics at 700oC, and consequent room temperature toughness for 316L heater sheaths.

Mekanis frajilman nan faz Chi

Faz χ a gen yon estrikti kib ki santre kò (BCC) ak yon selil inite gwo (a≈ 8.9 Å) epi li rich nan molybdène (anjeneral 20-30% Mo) ak chromium (25-35% Cr). Li presipite preferansyèlman nan fwontyè grenn jaden, kontak ferit-austenite ak nan enklizyon. Faz la difisil ak frajil ak prezans li bay wout pratik pou ka zo kase sou fwontyè grenn jaden yo. Kontrèman ak faz sigma, faz chi se yon ajan ki pi solid pandan li fòme tankou ti patikil ki byen espace, ki fè yon rezo frajil kontinyèl nan fraksyon volim ki pi ba yo. Se maksimòm nan obsève nan 700 degre ak 1,000-5,000 èdtan nan aje depann sou kontni molybdène. Creole 316L (2.0-2.5% Mo) se sansib; ogmante molybdène (2.5-3.0% Mo, fèmen nan 317L) amelyore pousantaj pwodiksyon chi.

Relasyon Kantitatif ant Chalè-Fraksyon Volim Trete ak Enpak Severite a 700 degre

Pwogresyon estabilize detèmine pa aje kontwole nan 316L tib nan 700 degre (nen nan koub chi faz TTT) ak diferan tan aje ki te swiv pa chi faz quantification (X-difraksyon reyon, SEM) ak Charpy V-dan tès enpak nan tanperati chanm (20 degre).

Tan aje nan 700 degre (h) Chi Faz Volim Fraksyon (% Chi Gwosè Patikil (nm) Sit Presipisyon Tanperati Chanm Enpak Enèji (J, Charpy V-dan) Frakti Sifas Aparans Rekòmande pou Sèvis Kote Severite Bezwen
0 (annealed) 0 N/A None >200 Ductile (100% shear)Yes 100 < 0.1 < 50Grain boundaries (sparse) 180-200 Mostly ductile Yes 250 0.1-0.3 50-100 Grain boundaries (scattered) 150-180Ductile, with some cleavage Yes 500 0.3-0.6 100-150 Grain boundaries (partial coverage) 100-150 Mixed (50% cleavage) Acceptable 750 0.6-1.0 150-200 Grain boundaries (near-continuous) 60-100Marginal Mostly cleavage 1,000 1.0-1.5 150-250 Continuous grain boundary network 30-60 Cleavage (brittle)Not recommended 1500 1.5-2.0 200-300Continuous + intragranular 15-30 Brittle (100% cleavage) No 2,000 2.0-2.5 250-350 Extensive, coalesced 10-20Very Brittle No 3,000 2.5-3.0 300-400 Widespread, coarse 5-15Very brittle No. 5,000 3.0-4.0 >400 Wide, overaged 5-10Extremely frajil No
Dyagram TTT pou faz Chi nan 316L

Dyagram TTT nan faz chi a nan 316L (2.3% Mo) endike ke presipitasyon ki pi rapid la rive nan 700 o C ak nen an se apeprè 1,000-2,000 èdtan.

Temperature ( o C) Time to 0.5 % Chi Phase (hours) Time to 1.5 % Chi Phase (hours) Maximum Recommended Service Time Before Embrittlement (Hours) 550 >10,000 >20,000 20,000 600 2,000-4,000 5,000-8,000 5,000 650 800-1,500 2,000-3,500 2,000 675 500-800 1,200-2,000 1,500 700 400-700 1,000-1,500 1,000 725 500-900 1,200-2,000 1,500 750 800-1,500 2,000-4,000 2,500 800 2,000-4,000 5,000-10,000 5,000
Enfliyans nan kontni Molybdène sou pousantaj la presipitasyon Chi

Pi wo molybdène ankouraje presipitasyon nan faz chi. Lower molybdène bon pou sèvis nan 700 degre.

Kontni Molybdène (wt%) Tan a 1.0% Chi nan 700 degre (èdtan) Enèji enpak apre 2,000 èdtan nan 700 degre (J) Tanperati Maksimòm Rekòmande pou 10,000h Lavi 2.0 (minimòm 316) 1,500-2,{11}26}L ({11}, 26) tipik 1,000-1,800 25-50 640 degre 2.5 (maksimòm 316L) 600-1,000 15-35 620 degre 2.8 (apwoche 317L) 300-600 10-25 600 degre 3.2 (317L) 150-300 5-15 580 degre Rekòmandasyon pratik pou sèvis 370ath degre

To maintain adequate toughness (impact energy >50 J) pou djenn aparèy chofaj 316L nan oswa alantou 700 degre, limit sèvis sa yo aplikab.

Desired service life at 700 °C (hours)Maximum Allowable Chi Phase (%) Recommended Maximum Service Temperature (°C) Alternative Alloy 500 0.5 700 316L acceptable 1,000 1.0 690 316L with lower Mo (2.0-2.2%) 2,000 1.5 670 347 or 316L with annual toughness check 5,000 2.0 650 (or less) 347 (no chi) or 310 10,000 >2.0 (pa akseptab)<600 310 or Inconel 600
Nenpòt sijè a tèmik bisiklèt 50% ekonomi tan Anwo a Ankouraje nan pwochen klas
KARAKTERIZASYON CHI-FAZ ANBRITLEMENT

Twa metòd mezire frajilman faz chi pou aparèy chofaj ki te opere nan 600-750 degre. Premyèman, tès la dite. Dite a ogmante soti nan 160-180 HV a 220-300 HV akòz devlopman faz chi. Dezyèm lan se metalografi jaden (repli). Yon sifas poli ak grave wè nan 500-1,000 ka montre chi presipite nan limit grenn yo. Twazyèm lan se yon tès enpak senplifye. Si yo ka retire yon pati nan aparèy chofaj la, yon ti tès Charpy subsize ka evalye pèt severite. X-ray diffraction (XRD) oswa SEM/EDS se fason definitif pou idantifye dirèkteman faz chi.

Konklizyon: Defini limit sèvis pou anpeche Chi Angajman

For 316L stainless steel heater sheaths running at 700°C, chi (χ) phase precipitation was observed after 400-1500 h, with a volume fraction of 1.0-1.5% after 1000-2000 h. The room temperature impact toughness at this level diminishes from >200 J a 30-60 J (frajil ka zo kase). Enjenyè ki espesifye djenn 316L pou sèvis plis pase 650 degre yo ta dwe detèmine tan total ki pase nan rejyon 600-750 degre epi konpare li ak restriksyon yo montre nan dyagram TTT la. Pou 1000 a 2000 èdtan nan 700 degre oswa 5000 a 10000 èdtan nan 650 degre li rekòmande ajou 347 (niobyòm-estabilize, pa fòme chi) oswa 310 Nerjaveèi asye (konplètman austenitic, pi wo Kwòm). Apwòch ki pwopoze isit la gen rapò chi faz volim pousantaj nan tanperati chanm mezirab enpak pèt solidite ak pèmèt achtè yo etabli restriksyon tanperati-tan ki anpeche embrittlement nan djenn aparèy chofaj 316L nan alontèm itilizasyon tanperati ki wo.