Ntawm Qhov Chaw Kub Kub Qhov Ncauj Ua Li Cas Cov Txheej Txheem Oxide ntawm 316 Stainless Steel Heating Tubes Hloov los ntawm Kev Tiv Thaiv mus rau Spalling-Induced Phab Ntsa Thinning

Kev txwv ntawm Duplex Oxide Structure Mechanical Stability

Rau cov hluav taws xob cua sov raj qhwv hauv 316 stainless hlau thiab siv rau hauv huab cua, combustion gases los yog superheated chav, oxide txheej tsim rau sab nrauv yog thawj zaug tiv thaiv- ua raws li qhov cuam tshuam rau kev ua kom qeeb oxidation ntxiv. Tshaj qhov kub ntawm qhov chaw pib, txawm li cas los xij, cov qauv oxide hloov los ntawm nyias, adhering, chromium- nplua nuj nplai mus rau tuab, ntau txheej, hlau- nplua nuj nplai uas nquag nquag. Txhua qhov kev tshwm sim spalling tshem tawm tsis yog tsuas yog oxide xwb tab sis me ntsis ntawm lub hauv paus hlau zoo li, ua rau cov phab ntsa nce zuj zus thiab qhov tshwm sim perforation. Qhov ntsuas kub no tsis yog tus nqi tas li tab sis yog nyob ntawm cov khoom siv hluav taws xob, thermal voj voog zaus thiab cov pa oxygen ib nrab siab. Daim ntawv no suav cov oxide hloov pauv kub rau 316 stainless hlau rhaub sheaths, muab lub moj khaum rau kev kwv yees ntawm qhov tshwm sim ntawm cov phab ntsa poob, thiab teeb tsa qhov siab tshaj plaws qhov kub thiab txias rau kev pabcuam huab cua ntev.

Ob Txoj Cai Oxidation ntawm 316 Stainless Hlau

Ntawm qhov kub thiab txias hauv huab cua qis dua li 700-750 degree, 316 stainless hlau tsim nyias (1-5 µm), cov txheej txheem txuas ntxiv ntawm chromium oxide (Cr₂O₃) nrog cov hlau me me -chromium spinel. Qhov ntsuas no nce parabolically nrog lub sij hawm piv txwv li qhov oxidation tus nqi txo raws li qhov teev ua thicker vim chromium diffusion los ntawm cov oxide uas twb muaj lawm ua tus nqi limiting. Qhov ntsuas yog ua raws, nrog lub thermal expansion coefficient tsis deb ntawm lub hauv paus hlau (∼10−12 × 10⁻⁶ /℃tiv thaiv 17 × 10−⁶ /℃rau 316), thiab feem ntau spalls tsuas yog nyob rau hauv huab cua kub poob. Ntawm qhov kub siab tshaj li ntawm 750-800℃qhov kev tiv thaiv chromium oxide ua thermodynamically tsis ruaj khov rau cov hlau oxides. Cov nplai tsim cov qauv duplex nrog rau sab nrauv txheej ntawm haematite (Fe 2 O 3) thiab magnetite (Fe 3 O 4 ) thiab ib txheej sab hauv ntawm cov hlau -chromium spinel (FeCr 2 O 4) nrog chromium oxide. Oxygen diffusion los ntawm cov ntxeem tau hlau oxide qauv yog ceev, yog li cov hlau oxides nthuav dav linearly nrog lub sijhawm. Qhov tseem ceeb tshaj, qhov thermal expansion coefficient ntawm Fe₂O₃ yog kwv yees li 15 × 10⁻⁶ / degree, thiab Fe₃O₄ nthuav dav ntawm 12-14 × 10⁻⁶ / degree. Qhov tsis sib haum xeeb nrog 316 substrate (17 × 10-6 / degree) tseem yog nruab nrab, tab sis qhov kev ntxhov siab tsim los ntawm qhov ntim ntawm cov hlau-rau-oxide hloov pauv (Pilling-Bedworth piv rau hlau yog 2.1, piv txwv li cov oxide yuav siv ntau dua li cov hlau uas tau noj) ua rau lub zog compressive nyob rau hauv lub voj voog ntau dhau. Thaum cov kev tiv thaiv chromium oxide tawg lawm, oxidation accelerates dramatically - los ntawm ib tug parabolic tus nqi tas li ntawm ib ncig ntawm 10⁻¹² g² / cm⁴·s ntawm 700℃rau 10⁻¹⁰ g² / cm⁴·s ntawm 850 degree, nce ntawm ob qhov kev txiav txim ntawm magnitude.

Spalling qhov pib kub thiab nws dependence rau thermal cycling

The shift from adhering to spalling oxide is a function of peak temperature as well as the number and magnitude of thermal cycles which determine when the accumulated strain energy surpasses the oxide-metal adhesion work. For continuous isothermal operation, 316 sheaths can survive at 750°C surface temperature for 20,000-30,000 hours with just 100-200 µm of total metal loss (acceptable for a 1.5 mm wall thickness). However, the same 750°C sheath with daily thermal cycles (cooling to 200°C between thermal cycles) will start spalling after 500-1000 cycles and the total metal loss after 5000 cycles may be >500 µm, ib feem peb ntawm phab ntsa thickness. Txawm tias nyob rau hauv isothermal tej yam kev mob thiab nyob rau ntawm ib tug kub ntawm 850 degree, ceev oxide kev loj hlob (10-20 µm ib 1,000 teev) thiab nquag spontaneous spalling tshwm sim vim hais tias cov tensions vim loj hlob ib leeg tshaj qhov txwv ntawm adhesion. Rau kev pabcuam huab cua qhov kev txwv tsis pub ntev tshaj - lub sijhawm cua sov qhov kev cia siab yog 650℃ntawm kev ua haujlwm tas mus li yog tias cov phab ntsa ua kom tawg yog yuav tsum tau khaws cia qis dua 10 µm ib xyoo. Nyob qhov twg qhov kub ntawm qhov kub siab tshaj 650℃lub rhaub yuav tsum suav hais tias yog ib qho khoom siv nrog lub sijhawm hloov pauv tau ua ntej. oxidation thiab spalling tus cwj pwm nyob rau hauv ib txwm ua hauj lwm ntau yam ntawm 316 sheathed cua sov cua sov yog sau tseg hauv qab no.

Sheath Surface Temperature (degree) Dominant Oxide Type Oxidation Kinetics Spalling Onset (thov mus rau thawj spal) Hlau poob tus nqi (µm ib 10,000 teev)Max. Pom zoo Service Life (1.5 hli phab ntsa)
<550 Cr2O3 (thin, adherent) Parabolic >10,000 cycles<5 >30 xyoo 550-650 Cr2O3 + me FeCr2O4 Parabolic 5,000-10,000 cycles5-20 15-30 xyoo
650-700 Mixed Cr2O3/Fe3O4 Transitional 2000-5000 cycles 20-50 8-15 xyoo 700-750 Duplex Fe3O4/Fe2O3 nrog Cr-nplua nuj puab txheejLinear-parabolic 500-2010} xyoo 2 xyoo Fe3O4 / Fe2O3, Cr depletion Linear 100-500 cycles 150-400 1-3 xyoo 800-850 Rapid Fe oxide, sab hauv oxidationLinear<100 cycles 400-800 6-18 months >850 Runaway oxidation, severe spalling Super-linearImmediate spalling >800 <6 months
Lub luag haujlwm ntawm Molybdenum thiab Silicon hauv kev hloov pauv ntawm qhov kub thiab txias

The transition temperatures in the above table are specific to 316 stainless steel with 2.0-2.5 % molybdenum and 0.5-1.0 % silicon. Molybdenum, contrary to certain opinions, does not strengthen the resistance to dry oxidation considerably, but instead creates, beyond 700 °C, volatile oxides (MoO3) which can disturb the protective scale. But silicon is a good thing. At levels above 1.0 % silicon stimulates the creation of a thin, continuous silica layer (SiO2) underneath the chromium oxide which functions as an additional diffusion barrier. For heater sheaths used in continuous operation at temperatures >650 °C, the spalling threshold can be improved by 25-50 °C by using 316 with the silicon concentration at the upper end of the usual range (0.8-1.0 % instead of 0.4-0.6 %). As an alternative , grade 310 stainless steel ( 25 % Cr , 20 % Ni ) or Inconel 600 should be provided for continuous operation over 750 °C . These alloys create exceptionally stable chromium oxide scales up to 1000-1100 °C . The increased expense of the nickel-based alloys is justified for intermittent duty (frequent cycling) at temperatures >650℃los ntawm lawv cov thermal qaug zog zoo dua txij li 316 yuav ua rau tsis txaus ntseeg nyob rau hauv ib lub hlis.

Kev ntsuam xyuas ntawm Oxide Spalling thiab Phab Ntsa Thinning hauv Teb

Peb qhov chaw- pom qhov ntsuas kub ntawm qhov chaw siab tshaj qhov ntsuas qhov sib txawv muaj nyob rau 316 lub tshuab cua sov tam sim no hauv kev pabcuam. 1. cov flakes dub lossis xim av tsaus muaj nyob rau hauv pem teb ntawm lub qhov rooj cua sov lossis cov chaw nyob sib ze. Cov flakes no yog magnetite (Fe3O4) thiab tuaj yeem nyiam nrog cov hlau nplaum-ib qho yooj yim kuaj. Qhov thib ob yog ib qho ntxhib, pitted saib ntawm lub plhaub saum npoo, tshwj xeeb tshaj yog yog tias tus thawj du tiav tau hloov nrog ib qho textured lossis scaly nto. Qhov thib peb, thiab feem ntau cov cim kos npe, yog qhov poob ntawm hluav taws xob tsis kam ntawm cov cua kub ntsuas ntawm chav tsev kub. Raws li cov phab ntsa sheath thins los ntawm spalling, internal resistance wire diameter kuj oxidise. Tab sis 5-10% poob rau hauv kev tiv thaiv khaub thuas los ntawm thawj spec qhia tias muaj hlau tsis zoo. Cov txheej txheej oxide raug tshem tawm los ntawm maj mam grit blasting, thiab cov phab ntsa tuab tuaj yeem ntsuas qhov tseeb nrog lub ntsuas ntsuas ultrasonic. Phab ntsa tuab hauv qab 0.8 hli ntawm nominal 1.5 hli sheath txhais tau tias hloov yuav tsum tau npaj nyob rau hauv 6-12 lub hlis tom ntej. Rau cov cua sov uas tsis paub qhov kub ntawm qhov chaw, cov thermocouple siv rau hauv lub hnab thaum ua haujlwm muaj cov ntaub ntawv tiav. Yog tias qhov ntsuas kub tau ntsuas siab tshaj 700℃thaum lub sijhawm ntau tshaj 10% ntawm lub sijhawm ua haujlwm, oxide spalling yuav nce ntxiv.

Sheath Temperature Reduction Design Strategies for Air Service

Rau cov ntawv thov uas xav tau cua sov siab tshaj 650℃tab sis qhov twg qhov kev txwv tsis pub siv 316 sheaths, peb txoj kev tsim muaj los txo qhov kub ntawm qhov chaw rau lub zog tso zis. Thawj thiab qhov yooj yim tshaj plaws yog kom nthuav dav thaj chaw los ntawm kev siv cov kab ntev los yog loj dua, los yog ntxiv cov fins. Rau ib tug muab wattage, qhov kub ntawm qhov saum npoo ntawm ambient yog inversely cuam tshuam rau qhov chaw nto. Yog hais tias lub rhuab ntev yog ob npaug, qhov nce hauv qhov kub thiab txias yog halved. Cov txheej txheem thib ob yog txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm rau txhawm. Ntawm huab cua qis (0.5-1.0 m/sec) cov cua kub hloov pauv yog 10-20 W/m2/K, uas ua rau muaj qhov kub thiab txias. Ntawm kev quab yuam convection (5-10 m / s tshaj tawm), tus coefficient nce mus rau 50-100 W / m2·K, txo qis hauv lub tsho kub los ntawm 100-200℃ntawm tib wattage. Cov txheej txheem thib peb yog siv ntau lub tshuab hluav taws xob qis dua es tsis txhob siv lub tshuab hluav taws xob uas muaj hluav taws xob ntau, uas kis tau tag nrho lub hwj chim hla thaj tsam loj dua thiab txo qhov kub ntawm qhov siab. Rau txhua daim ntawv thov cua sov, qhov siab tshaj plaws sheath kub ntawm 650℃( ntsuas los yog xam) yog ib qho kev nyab xeeb sab sauv txwv rau kev pab mus sij hawm ntev ntawm 316. Rau kev siv kub dua, tus qauv yuav tsum tau upgraded rau 310 stainless hlau los yog Inconel.

Xaus: Qhov siab tshaj qhov kub ntawm qhov txwv rau cov cua sov cua sov ntawm hom 316

Rau cov hluav taws xob cua sov cov raj qhwv hauv 316 stainless hlau uas ua haujlwm hauv huab cua lossis combustion gases cov txheej txheem oxide uas ua rau ntawm qhov chaw hloov pauv los ntawm kev tiv thaiv mus rau qhov kub ntawm li 700-750℃tsis tu ncua, tab sis ntawm qhov kub thiab txias nyob rau hauv thermal cycling tej yam kev mob. Rau kev cia siab rau lub sij hawm ntev (5+ xyoo) qhov siab tshaj plaws qhov kub thiab txias yuav tsum raug txwv rau 650℃. Rau cov kev pabcuam cuam tshuam nrog txhua hnub, qhov kev txwv raug txo mus rau 600℃kom txo qis kev puas tsuaj loj heev. Yog hais tias 316 encased heaters yuav tsum tau siv nyob rau hauv tej gaseous cua qhov chaw ntawm lub qhov chaw kub yuav tshaj 650 degree, lub wattage yuav tsum tau derated kom txo tau qhov kub thiab txias, cov huab cua txaus nce rau yuam convection txias, los yog ib tug siab dua kub alloy siv. Hauv kev ua haujlwm no, lub hauv paus ntsiab lus tau nthuav tawm uas txuas cov qhov kub thiab txias rau cov nqi ntawm oxide spalling thiab kev kwv yees ntawm phab ntsa thinning, ua rau cov neeg yuav khoom xaiv qhov kub siab tshaj plaws ua haujlwm uas tiv thaiv kev hloov pauv los ntawm kev tiv thaiv nplai mus rau kev puas tsuaj, thiab yog li zam kom tsis txhob muaj cov phab ntsa loj zuj zus uas ua rau ntxov ntxov sheath perforation.