Yuav ua li cas txhais tau tias Grain ciam teb Carbide Spacing Tom qab Sensitization Thaum tshav kub kub kho ntawm 316 Heater Sheath Tubing Control Intergranular Corrosion Rate nyob rau hauv ASTM A262 Xyaum E Test

Microstructural parameter rau qhov hnyav ntawm rhiab heev

Rau 316 stainless hlau sheathed hluav taws xob cua sov raj uas tau raug rau sensitization ntau yam kub (450-850 degree) nyob rau hauv ib co theem ntawm kev tsim khoom los yog kev pab cuam, qhov hnyav ntawm intergranular corrosion (IGC) susceptibility tsis yog ib qho yooj yim ua hauj lwm ntawm lub xub ntiag los yog tsis muaj grain ciam carbides. Qhov tseem ceeb microstructural parameter yog qhov nruab nrab ntawm qhov sib nrug ntawm chromium carbide hais ntawm cov nplej - ciam teb. Thaum cov carbide hais nyob ze txaus ua ke (qhov sib nrug tsawg dua li ntawm 0.5-1.0 µm) cov chromium depleted aav nyob ib ncig ntawm cov khoom uas nyob ib sab sib tshooj los ua ib txoj hauv kev tsis tshua muaj chromium uas tau tawm tsam sai heev hauv oxidising acids (ASTM A262 Practice E, the copper-copper sulfate test). Yog hais tias cov carbide hais tau dav dav (qhov sib nrug> 1-2 m), cov cheeb tsam depletion tsis sib tshooj thiab cov ciam teb yog resistant rau intergranular quab yuam. Tsab ntawv xov xwm no suav txog qhov sib txuas ntawm qhov nruab nrab carbide qhov sib txawv, qib ntawm kev nkag siab thiab IGC tus nqi hauv cov pa 24 h Xyaum E xeem.

Mechanism ntawm Overlapping Depleted Zones

Cov nag lossis daus ntawm chromium carbide ntawm thaj tsam ntawm 316 stainless hlau ua rau thaj tsam chromium depleted ntawm ~ 50-200 nm ntawm txhua sab ntawm cov khoom. Raws li kev ua haujlwm ntawm nag lossis daus kub thiab lub sijhawm, qhov ntau ntawm chromium depletion (feem ntau 8-12% chromium tsawg kawg nkaus concentration piv rau bulk 17-19%) yog kawm. Intergranular corrosion yog qhov tseem ceeb yog tias qhov kev ncua deb ntawm ob tus neeg nyob sib ze carbides me dua qhov sib npaug ntawm qhov dav ntawm ob thaj chaw nyob sib ze. Raws li qhov xwm txheej no, thaj chaw depletion sib tshooj thiab txoj hauv kev tsis tu ncua ntawm -chromium cov ntaub ntawv raws li cov ciam teb. Nyob rau hauv kev sib zog oxidising thiab chromium-deficient cheeb tsam- xaiv boiling tooj liab-tooj ​​liab sulfate-sulfuric acid tov (Xaiv E) cov depleted aav sib tshooj thiab yog corroded sai heev, ua rau cov nplej poob thiab qhov kawg qauv tsis ua hauj lwm nyob rau hauv dabtsi yog khoov. Rau qhov sib nrug ntawm carbide ntau dua li ntawm 1-2 μm, thaj chaw depleted raug cais tawm thiab tsis muaj qhov txuas ntxiv qis-chromium.

Quantified Link Ntawm Carbide Spacing thiab Xyaum E Cov txiaj ntsig

Cov kauj ruam hauv qab no tau tsim los ntawm kev tswj hwm ntawm 316 tubing (1.5 hli phab ntsa, 0.06% C) ntawm 650℃(qhov ceev tshaj plaws carbide nag lossis daus kub) rau ntau lub sijhawm, thiab tom qab ntsuas qhov nruab nrab carbide sib nrug los ntawm kev sib kis electron microscopy thiab ASTM E26 test.

Lub Sijhawm rhiab heev ntawm 650℃(hr) Grain Boundary Mean Carbide Spacing (µm) Estimated Depleted Zone Overlap Fraction (%) Xyaum E Result (24-hr boil Cu-CuSO4-H2SO4) Intergranular Attack Depth, µm Pom Zoo rau Kev Pabcuam hauv Oxidising
0 (annealed, Tsis muaj carbides N / A Pass (tsis muaj pob txha ntawm khoov)<5 Yes 0.1 5-10 0% Pass <10 Yes 0.5 2-4 0-20% Pass (slight etching) 10-20 Acceptable
1 1-2 20-50% Marginal (kawg ntawm khoov) 20-40Tsis qhia
2 0.8-1.2 50-80% Fail (kev tawg hnyav) 80-150 Tsis Muaj Fail (ntshiab tawg) 40-80 Tsis muaj 5 0.4-0.8 80-100%
10 0.2-0.5 100% (continuous) Fail (grain dropping) 150-250 No 24 0.1-0.3 100% (continuous) Fail (severe grain dropping) >250 Nr
Qhov cuam tshuam ntawm cov ntsiab lus carbon ntawm qhov tseem ceeb Carbide Spacing

Cov pa roj carbon ntau hauv 316 txiav txim siab tag nrho cov carbide uas tuaj yeem ua rau nag thiab yog li qhov qis tshaj carbide sib nrug uas tuaj yeem ua tiav rau cov txheej txheem rhiab heev.

Carbon Content (wt%) Minimum Carbide Spacing after Complete Sensitisation (µm) Practice E Result at Minimum SpacingMaximum Allowable Service Temperature To Avoid Sensitisation >0.020 (qis) 2-3 Pass 450℃0.020-0.030 (316L) 1.5-2.5 Pass (marginal) 425℃0.030-0.040 1.0-1.8 Marginal 400℃0.040-0.050 0.8-1.2 Fail 375℃0.050-0.060 0.5-0.8 Fail 350℃(0.06) 0.3-0.6 Fail 325 degree
Tswv yim zam ntawm rhiab heev nyob rau hauv lub rhaub tsim

Txhawm rau kom tsis txhob carbide sib nrug hauv qab qhov tseem ceeb 1-2 µm txwv nyob rau hauv 316 lub tshuab cua sov uas yuav tsum tau welded los yog yuav raug rau qhov kub thiab txias hauv kev pabcuam, cov qauv hauv qab no tau pom zoo:

Kev Tsim Kho lossis Kev Pabcuam Qhov Siab Tshaj Plaws Siab Tshaj Plaws (degree) Lub Sijhawm Siab Tshaj Plaws ntawm Qhov Kub Pom Zoo Carbon ContentPost Exposure Test Requirement
Vuam (ib zaug dhau, tsis muaj muab tub lim) ceev txias, max 1,200-1,400Seconds (HAZ txias ceev) Txhua yam (HAZ yog tias nyias tsis sensitised) Tsis muaj rau cov phab ntsa nyias (<2mm)
Vuam (thick section, multi-pass) 450-850 (tag nrho) 316L Min. (C<0.03%)Practice E Weld coupon
Ncej -weld kev kho cua sov (kev ntxhov siab) 400-450℃1-2 teev 316L (tag nrho) Tsis muaj (hauv qab qhov txwv tsis pub)
Service temperature excursion 450-550°C >10 teev cumulative 316L xav tau Xyaum E tom qab 5 xyoos
Brazing lossis soldering 500-700℃feeb rau teev 316L lossis ruaj khov (321/347) Xyaum E xav tau
ASTM A262 Xyaum E Kev Ntsuas Zoo rau Kev Txais

ASTM A262 Practice E xeem muab kev lees paub zaum kawg ntawm IGC tsis kam rau cov neeg yuav khoom hais txog 316 sheaths uas tuaj yeem hnov ​​​​qab thaum tsim khoom. Cov txheej txheem xeem yuav tsum muaj:

boiling rau 24 teev nyob rau hauv ib tug tov ntawm tooj liab - tooj liab sulphate -sulphuric acid (ASTM A262, txheej txheem E)

Cov qauv yog khoov 180℃tshaj ib lub mandrel ntawm txoj kab uas hla sib npaug rau cov qauv tuab

Saib cov fissures hauv qhov chaw khoov ntawm 10x magnification

Kev lees txais cov txheej txheem: Yog tias tsis muaj cov kab nrib pleb raug pom tom qab khoov, nws yog "dhau" (unsensitized). Ib qho "ua tsis tiav" (sensitised) raug kaw yog tias pom muaj kab nrib pleb (ntawm qhov ntev) pom. Qhov kev ntsuam xyuas yog rhiab rau carbide qhov sib nrug < ~ 1-2 μm.

Field Identification ntawm IGCs Induced los ntawm Sensitization

Hauv 316 lub tshuab cua sov uas tsis ua haujlwm oxidising acid (xws li, nitric acid, pickling solutions) los yog nyob rau hauv ib puag ncig chloride nrog oxidising tivthaiv, intergranular corrosion los ntawm sensitization yog manifested raws li ib tug network ntawm cov kab nrib pleb raws li ib thaj tsam uas yuav pom nyob rau hauv qis magnification (50-100 × . Cov fissures tej zaum yuav muaj "skeleton" qauv, qhia txog yav dhau los austenite nplej. Kev nkag siab yuav raug lees paub los ntawm ASTM A262 Practice E ntawm tus qauv coj los ntawm lub rhaub tsis ua haujlwm. Qhov sib nrug txiav txim siab metallographically siv scanning electron microscopy yuav tsawg dua 0.5-1.0 µm. Cov lus teb rau qhov ua tsis tiav no yog siv 316L lossis xav kom muaj kev tshaj tawm kev daws teeb meem anneal.

Xaus: Tswj Carbide Spacing Specification los ntawm Sensitization

Lub intergranular corrosion tus nqi raws li ASTM A262 Xyaum E rau 316 stainless hlau sheathed heaters yog raws li qhov nruab nrab grain ciam carbide sib nrug tom qab rhiab kub. Thaum qhov sib nrug ntawm carbide tsawg dua 1-2 µm, cov chromium- cov cheeb tsam depleted overlap ib leeg, tsim ib txoj kev tsis tshua muaj chromium uas corrodes sai thiab ua tsis tau qhov kev xeem E. Rau cov ntawv thov qhov chaw vuam lossis kev pabcuam kub nyob rau hauv 450-850℃ntau yam, engineers qhia 316 sheaths yuav tsum muab 316L (carbon<0.03%) to ensure that the carbide spacing will be above the critical threshold. After minimum sensitisation (1-2 hours at 650°C) carbide spacing for standard 316 (0.06-0.08% C) is less than 1 µm which results in sensitisation and IGC susceptibility. The framework offered herein, which correlates the average carbide spacing with quantitative Practice E results, allows purchasers to define combinations of material and heat treatment that will not allow overlapping depletion zones, and will retain intergranular corrosion resistance.