Ki jan fraksyon volim nan faz Laves (Fe₂Mo) presipitasyon nan 650 degre pandan sèvis alontèm -nan 316L aparèy chofaj djenn tib kontwòl ranpe rupture rediksyon duktilite

Efè Molybdène-Faz entèmetalik rich sou dekonpozisyon

For 316L stainless steel sheathed electric heating tubes operating at 650°C for long periods (5,000-20,000 hours)-such as in superheater supports, high temperature furnace heaters or petrochemical process heaters-the precipitation of Laves phase (Fe2Mo), an intermetallic compound rich in molybdenum and iron, causes a marked reduction in creep rupture ductility (measured as percent elongation at fracture) even prior to a marked reduction in creep rupture strength. The Laves phase precipitates at lower temperatures (550-700°C) with a maximum production rate at about 650°C, than sigma (σ) and chi (χ) phases which form at higher temperatures and impart significant embrittlement. Laves phase precipitates as tiny (50-200 nm) particles within grains and on grain borders. Low volume fractions (0.5-1.0%) can reduce creep rupture elongation from >20% (duktil) pou<5-10% (brittle) and hence increase the likelihood of rapid catastrophic failure with little preceding deformation (bulging or swelling). This article quantifies the association of Laves phase volume fraction, ageing period at 650°C and creep rupture ductility for 316L heater sheaths.

Mekanis nan Laves Faz Anbrittlement

Faz Laves (Fe2Mo) se yon estrikti egzagonal fèmen -chaje (HCP), rich nan molybdène (Mo 35-45%) ak Silisyòm (Si 3-5%) Li presipite nan matris la austenite supersaturated sou ekspoze tan long nan 550-700 degre. Sinetik presipitasyon yo ralanti konpare ak faz chi ak> 1,000-2,000 èdtan nan 650C pou reyalize fraksyon volim enpòtan. Faz la se difisil ak frajil ak prezans nan faz sa yo mennen nan wout ka zo kase fasil, espesyalman lè yo anbeli fwontyè grenn jaden yo. Kontrèman ak faz chi ak sigma, ki tou diminye fòs, faz Laves sitou embrittles materyèl la ak diminye kapasite nan defòme plastik anvan rupture. Sa a se sitou danjere nan djenn aparèy chofaj paske sentòm konvansyonèl yo nan echèk se anfle (ogmantasyon an dyamèt) akòz ranpe. Nan frajilman faz Laves, rupture ka rive rapidman san okenn deformation evidan anvan.

Kantite korelasyon ant fraksyon faz Laves volim ak duktilite rupture nan 650 degre

Aje kontwole nan 316L tib (2.2 wt% Mo, 0.02 wt% C, 0.08 wt% N) nan 650 degre pou jiska 20,000 h, ki te swiv pa tès ranpe rupture nan 650 degre ak 80 MPa, te demontre sa ki annapre yo Laves faz presipitasyon kinetik presipitasyon.

Tan aje (èdtan) 650 degre Laves Faz Volim Fraksyon (%) Faz Laves Gwosè Patikil (nm) Sit Presipitasyon Ranpe Rupture elongasyon nan 650 degre (%) Mode Rupture Rekòmande pou Sèvis Ki mande Duktilite (Avètisman Segondè Anvan Echèk)
0 (rekwit) 0 N/A Okenn 20-30 Ductile (transgranulaire) Wi 1,000<0.1 <50 Intragranular (sparse) 18-25 Ductile Yes 2,000 0.1-0.3 50-80 Intragranular + some grain boundaries 15-22 Mixed (ductile + some intergranular) Yes 3,000 0.3-0.6 80-120 Grain boundaries (partial coverage) 12-18 Mixed Acceptable 5,000 0.6-1.0 120-150 Grain boundaries (near-continuous) 8-12 Mixed-brittle Marginal 7,000 1.0-1.5 150-180 Continuous grain boundary network 5-8 Brittle (intergranular) Not Recommended 10,000 1.5-2.0 180-220 Continuous + Intragranular 3-6 BrittleNo 12,000 2.0-2.5 200-250 Extensive, coalesced 2-4 Very brittle No 15,000 2.5-3.0 220-300 Widespread 1-3 Extremely brittle No 20,000 >3.0 >250 Vaste<2 Extremely brittle No 
Enfliyans kontni Molybdène sou sinetik presipitasyon Laves

Pousantaj presipitasyon faz Laves pwopòsyonèl ak konsantrasyon molybdène. Lower Mo (2.0–2.2%) bon pou rezistans nan frajilman Laves.

Kontni Molybdène (wt%) Tan a 1.0% Laves nan 650 degre (èdtan) Ranpe rupture elongasyon apre 10,000h nan 650 degre (%) Tanperati maksimòm sèvis rekòmande pou 10,000h Life Steel Tanperati Range Tanperati Ranje Tanperati Ranje 2.0 (min 23}pty) 316L) 4000-6000 4-6 630 degre 2.4 2500-4000 3-5 620 degre 2.5 (max 316L) 1500-3000 2-4 610 degre 2.8 (tou pre 317L) 800-1500 1-3 600 degre Rekòmandasyon pratik pou 316L djenn aparèy chofaj nan sèvis 650 degre

For 316L heater sheaths working at 650°C, the following service duration restrictions and material criteria are applicable to preserve ductile failure mode (elongation >10-15%).

Required service life at 650 °C [hours]Maximum Permissible Laves Phase ( % )Required Minimum Creep Elongation %Recommended Max Mo Content (wt%) Inspection/Replacement Strategy2,0000.3>15 2.3None5,0000.8>10 2.2Annual diameter measurement7,5001.2>7 2.1Semi-annual diameter + hardness check10,0001.5>5 (brittle risk) 2.0Replace at 8,000-9,000h or upgrade alloy>10,000>1.5<5 <2.0Upgrade to 347 or 310
Verifikasyon faz Laves ak Duktilite diminye

Three techniques are used to evaluate the embrittlement of Laves phase of 600-700°C operated heaters. 1. Hardness testing: The creation of Laves phase increases the hardness from 160-180 HV to 200-250 HV. The second is field metallography (replica) with selective etching (e.g. electrolytic in 10% oxalic acid displays Laves phase as small white particles). The third is a simple creep or tensile test on a detached sample; a decrease in elongation from >20% pou<10% suggests embrittlement. The certain technique is to directly identify the Laves phase (Fe₂Mo) using transmission electron microscopy (TEM) or scanning electron microscopy (SEM) with EDS.

Konklizyon: Pou anpeche Laves frajilman, bay limit pou kontni Mo ak kondisyon sèvis yo.

Pou 316L Nerjaveèi asye aparèy chofaj k ap travay nan 650 degre Laves faz (Fe2Mo) presipite apre 2,000-6,000 èdtan ak rive nan fraksyon volim nan 0.6-1.5% apre 5,000-10,000 èdtan. Nan nivo sa yo, duktilite rupture ranpe a diminye soti nan > 20% a 5-8%. Sa a lakòz echèk frajil (rupture toudenkou san yo pa antecedent) olye pou yo echèk duktil (avètisman anfle vizib). Lavi sèvis nan djenn 316L limite a sa sèlman<10,000 hours for 650°C duty when molybdenum is limited to 2.0-2.2% (the low end of the specification). For service >10,000 h nan 650 degre, konsidere amelyore nan 347 (nyobyòm estabilize epi yo pa pwodwi faz Laves) oswa 310 asye pur. Fondasyon an bay isit la gen rapò fraksyon faz Laves volim nan rediksyon duktilite rupture ranpe nan 650 degre ak pèmèt kliyan yo defini chimi materyèl ak dire maksimòm sèvis ki pral anpeche echèk frajil nan alontèm sèvis chofaj tanperati ki wo.