Qhov Volume Fraction ntawm Chi (χ) Theem Ntog li cas ntawm 700℃Thaum Lub Sijhawm Ntev -Term Service ntawm 316L Heater Sheath Tubing Control Room Temperature Impact Toughness Reduction

Lub embrittling intermetallic theem tsim ntawm 600-800 degree

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 (ductile) rau<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.

Mechanism ntawm embrittlement ntawm Chi theem

Lub χ theem muaj ib lub cev nruab nrab cubic (BCC) qauv nrog ib tug loj chav tsev cell (a≈ 8.9 Å) thiab yog nplua nuj nyob rau hauv molybdenum (feem ntau 20-30% Mo) thiab chromium (25-35% Cr). Nws precipitates nyiam nyob rau hauv ib thaj tsam ntawm grain, ferrite-austenite hu thiab ntawm inclusions. Lub sij hawm yog ib qho nyuaj thiab nkig thiab nws lub xub ntiag muab txoj hauv kev yooj yim rau kev puas tsuaj raws cov kab lis kev cai. Tsis zoo li sigma theem, chi theem yog ib tug muaj zog embrittling tus neeg saib xyuas raws li nws cov ntaub ntawv raws li me me, nyob ze qhov chaw uas ua rau ib tug nruam nkig network ntawm qis ntim feem. Qhov siab tshaj plaws yog pom ntawm 700℃thiab 1,000-5,000 teev ntawm kev laus nyob ntawm cov ntsiab lus molybdenum. Standard 316L (2.0-2.5% Mo) yog raug; nce molybdenum (2.5-3.0% Mo, ze rau 317L) txhim kho tus nqi ntawm chi ntau lawm.

Quantitative Relation ntawm Thaum tshav kub kub-Kho Volume Fraction thiab Impact Toughness ntawm 700 degree

Kev loj hlob ntawm embrittlement txiav txim siab los ntawm kev tswj kev laus ntawm 316L tubing ntawm 700℃(lub qhov ntswg ntawm chi theem TTT nkhaus) nrog lub sijhawm laus sib txawv ua raws li chi theem quantification (X-ray diffraction, SEM) thiab Charpy V-notch impact testing at room temperature (20 degree).

Lub Sijhawm Kev laus ntawm 700℃(h) Chi Phase Volume Fraction (%Chi Particle Loj (nm) Lub Sij Hawm Ntawm Qhov Chaw Kub Kub Kub Hluav Taws Xob Hluav Taws Xob (J, Charpy V-notch) Fracture Surface Appearance Pom Zoo rau Kev Pabcuam Qhov Toughness Yog Xav Tau
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 5-10Hmoob tawg yooj yim No
TTT Daim duab rau Chi Phase hauv 316L

Daim duab TTT ntawm chi theem hauv 316L (2.3% Mo) qhia tias nag lossis daus sai tshaj plaws tshwm sim ntawm 700 o C thiab qhov ntswg yog li 1,000-2,000 teev.

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
Kev cuam tshuam ntawm Molybdenum Cov ntsiab lus ntawm tus nqi Chi Precipitation

Molybdenum siab dua txhawb cov nag lossis daus ntawm chi theem. Tsawg molybdenum yog qhov zoo rau kev pabcuam ntawm 700 degree.

Molybdenum Cov ntsiab lus (wt%) Lub Sijhawm mus rau 1.0% Chi ntawm 700℃(teev) Kev cuam tshuam Lub zog tom qab 2,000 teev ntawm 700℃(J) Pom Zoo Siab Tshaj Lij rau 10,000h Lub Neej 2.0 (yam tsawg 316) 1,500-2,{1211}degree (tus qauv) 1,000-1,800 25-50 640℃2.5 (siab tshaj 316L) 600-1,000 15-35 620℃2.8 (txog 317L) 300-600 10-25 600℃3.2 (317L) 150-300 5-15 580℃Cov Lus Qhia Txog Kev Pabcuam rau 7006L

To maintain adequate toughness (impact energy >50 J) rau 316L cua sov sheaths ntawm lossis ib ncig ntawm 700 degree, cov kev txwv hauv qab no yog siv tau.

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 (tsis tau)<600 310 or Inconel 600
Txhua yam kev kawm rau kev caij tsheb kauj vab 50% lub sij hawm txuag Saum toj no txhawb rau qib tom ntej
CHARACTERIZATION OF CHI-PHASE EMBRITTLEMENT

Peb txoj kev ntsuas chi theem embrittlement rau cov cua sov uas tau ua haujlwm ntawm 600-750℃. Ua ntej, kev sim hardness. Lub hardness nce los ntawm 160-180 HV rau 220-300 HV vim lub chi theem kev loj hlob. Qhov thib ob yog teb metallography (replica). Ib qho polished thiab etched nto pom ntawm 500-1,000 tuaj yeem pom chi precipitates ntawm cov ciam teb. Qhov thib peb yog kev simplified cuam tshuam. Yog tias ib feem ntawm lub rhaub tuaj yeem raug tshem tawm, ib qho me me ntawm Charpy test tuaj yeem ntsuas qhov poob qis. X-ray diffraction (XRD) lossis SEM / EDS yog txoj hauv kev los txheeb xyuas cov theem chi.

Xaus: Txhais kev txwv kev pab cuam los tiv thaiv Chi Embrittlement

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 mus rau 30-60 J (nkig tawg). Cov kws tsim qauv qhia txog 316L sheaths rau kev pabcuam tshaj 650℃yuav tsum txiav txim siab tag nrho lub sijhawm siv nyob rau hauv 600-750℃cheeb tsam thiab muab piv nrog cov kev txwv uas qhia hauv daim duab TTT. Rau 1000 txog 2000 teev ntawm 700℃lossis 5000 txog 10000 teev ntawm 650℃nws raug pom zoo kom hloov mus rau 347 (niobium-stabilized, tsis tsim chi) lossis 310 stainless hlau (ua tiav austenitic, siab dua chromium). Txoj hauv kev uas tau hais los ntawm no hais txog chi theem ntim feem pua ​​​​rau qhov ntsuas qhov kub ntawm chav tsev qhov cuam tshuam qhov hnyav poob thiab tso cai rau cov neeg yuav khoom los tsim cov kev txwv kub-lub sij hawm uas txwv tsis pub embrittlement ntawm 316L rhaub sheaths nyob rau hauv lub sij hawm ntev siv kub.