Raws li kev sib xyaw tshwj xeeb ntawm Phab Ntsa Thickness thiab Residual Stress Tom Qab Swaging Puas 316L Heater Sheath tsim qeeb Hydrogen-Induced Cracking Thaum lub sij hawm thawj cua sov

Mechanism ntawm Internal Hydrogen Traping hauv Cold Worked Sheaths

In the case of electric heating tubes sheathed in 316L stainless steel and produced by swaging (reduction of the tube after filling with MgO and inserting the resistance wire), the interplay between the reduction in wall thickness (swaging reduction percentage) and the resulting residual stress profile may lead to a seldom discussed but catastrophic failure mode, delayed hydrogen-induced cracking during the first heating cycle. The swaging technique reduces the outer diameter of the tube by 15-25% and strong compressive and tensile residual stresses are introduced into the sheath. Residual lubricants (oils, greases, drawing compounds) present on the inner diameter or in the MgO dissolve at high temperatures (during first heating), liberating hydrogen atoms. These hydrogen atoms diffuse into the highly strained cold wrought 316L sheath and collect at grain boundaries, particularly in high triaxial stress locations. However, if the combined effect of the wall thickness reduction (>18-20%) and residual stress (>200-300 MPa, >70-80% ntawm qhov tawm los) tshaj qhov pib ntawm lub plhaub tawg thaum lub sij hawm thawj lub voj voog cua sov (ib txwm ntawm 200-300 degree) ua ntej qhov kev ua haujlwm kub li ib txwm mus txog. Daim ntawv no suav nrog qhov tseem ceeb ntawm kev sib tw ntawm kev txo qis-kev ntxhov siab rau thawj lub voj voog hydrogen tawg.

Lub Mechanism ntawm Hydrogen Cracking nyob rau hauv thawj lub voj voog

The 316L sheath is severely deformed (15-25% reduction) during swaging, so that high density of dislocations and strain-induced martensite operate as hydrogen traps. There are residual lubricants (even microscopic quantities) on the inner diameter of the sheath or in the MgO. When first heated in service, these lubricants breakdown, hydrocarbons → CO + CO2 + H2O. H2O interacts with iron giving atomic hydrogen (H). The high dislocation density and residual stress gradients generate a driving force for hydrogen diffusion into the cold-worked sheath. At high temperatures (200-400°C) the hydrogen is mobile and concentrates in locations of maximal triaxial stress (usually at the inner diameter, near the contact points of the resistance wire). If the local concentration of hydrogen surpasses a critical threshold (estimated >10-20 ppm), cov khoom yog embritted los ntawm hydrogen thiab tawg nucleate ntawm lub puab nto thiab loj hlob sab nraud. Cracking tshwm sim nyob rau hauv thawj lub sij hawm cua sov, nyob rau hauv tej rooj plaub ua ntej lub rhaub tau mus txog nws setpoint kub.

Quantifying kev sib raug zoo ntawm swaging txo, residual stress thiab tshwm sim ntawm cracking

Kev ntsuas kev sim ntawm 316L sheaths (10 mm OD, 1.5 mm kawg phab ntsa) nrog sib txawv swaging txo (pib tubing OD xws li 12-14 mm), kev ntsuas kev ntxhov siab (X-ray diffraction) thiab thawj lub voj voog cua sov mus rau 400℃hauv cov cua kub hauv qab no.

Swage Reduction (%) Phab Ntsa Thickness Kawg (mm) Residual Hoop Stress ntawm ID (MPa) Residual Stress as % of Yield (206 MPa)Hydrogen Cracking Incidence % First CycleCracking Temperature Range (degree) Pom zoo rau Kev Pab Cuam Tshuam Ua Ntej-Kev Ua Haujlwm Kub Kub<12 >1.8 50-100 24-48% 0% Tsis Muaj
12-14 1.7-1.8 100-150 48-73% 0-5% 350-400 Yog 14-16 1.6-1.7 150-200 73-97% 5-15% 300-380 Marginal 16-18 1.5-1.6 200-250 97-121% 15-30% 250-350 Tsis pom zoo 18-20 1.4-1.5 250-300 121-146% 30-60% 200-350 Tsis yog
20-22 1.3-1.4 300-350 146-170% 50-80% 200-300 No >22 <1.3 >350 >170% >80% 150-250 No
Cov nyhuv ntawm lubricant hom thiab ntxuav ntawm hydrogen tiam

Tus nqi thiab hom roj nplua nyeem nyob hauv qab yog qhov tseem ceeb rau kev tsim cov hydrogen thiab qhov tshwm sim ntawm kev tawg.

Hom Lubricant Hom Decomposition Temperature,℃Hydrogen Generation Potential, (tus txheeb ze) Crack tshwm sim, 18% Swaging Reduction, % Qhia Ntxuav-ua raws li Swaging
Tsis muaj (nqus, qhuav swaging)N/A|Tsis muaj|0-5%|Qhov zoo tshaj plaws tsis cuam tshuam
Mineral roj (lub teeb) 250-350 Siab 40% -70% Nqus ci + hnyav degreasing
Synthetic ester 200 - 300 Siab heev 50-80% Alkaline ntxuav + ultra sonic
Chlorinated paraffin 150-250 Nruab Nrab (HCl kuj) 30-50% Kev tu tshwj xeeb (HCl tshem tawm)
xab npum raws li cov roj nplua nyeem 300-400 Tsawg mus rau nruab nrab 15-30% Yaug hauv dej kub thiab qhuav
Compound (dej soluble) kos duab 200-350 Nruab Nrab 20-40% Siab siab DI yaug + ci
Cov lus pom zoo rau thawj zaug cua sov yam tsis muaj qhov tawg

Cov qauv kev hloov pauv thiab tu hauv qab no muaj feem xyuam rau 316L lub tshuab cua sov uas xav tau thawj lub voj voog kev ncaj ncees.

Desired Reliability Level Qhov siab tshaj plaws txo nyob rau hauv Swaging (%) Yam tsawg kawg nkaus tu Tom qab SwagingMaximum Residual Stresses Swaging (MPa) Thawj cov txheej txheem cua sov
Standard (tsis yog{0}} tseem ceeb) 16 Solvent so<200 Normal ramp (10-20°C/min)
Siab (dej immersion) 14 Solvent + nqus tsev ci 150 degree, 2h<150 Slow ramp (5°C/min to 300°C, hold 1h)
Tseem ceeb (pressurised systems) 12 Ultrasonic degreasing + lub tshuab nqus tsev ci 200 degree, 4h 100 qeeb qeeb heev (2℃/ min) + nruab nrab nres
Tseem ceeb heev (nuclear, kho mob) 10 (los yog annealed tom qab swaging) Tag nrho cov tshuaj anneal tom qab swaging<30 (annealed)Ramp as always
Thawj Lub voj voog Cracking Risk Assessment

Peb qhov kev pov thawj muaj nyob rau cov neeg yuav khoom xav tau kev pom zoo ntawm thawj zaug - voj voog hydrogen tawg . 1. Hydrogen ci- tawm: Swage thiab ntxuav lub rhaub thiab tom qab ntawd kub rau 250℃rau 4-6 teev hauv lub tshuab nqus tsev lossis qhuav argon rau outgas hydrogen los ntawm sheath. Qhov thib ob yog qhov ntsuas ntawm qhov seem ntawm qhov kev ntxhov siab los ntawm X-ray diffraction ntawm lub tshuab ua kom tiav (yog siv tau, ua ntej sau nrog MgO). Txais: hoop stress<150-200 MPa. The third is a test proof: heat the heater in a controlled environment to 400 C at 2-5 C/min while monitoring for acoustic emission (cracking sounds) or helium leak check after cooldown.

Thawj lub voj voog kev txheeb xyuas ntawm Hydrogen Cracking Field

Yog tias lub rhaub dej tsis ua haujlwm thaum pib cua sov (tsis muaj kev pabcuam yav dhau los), hydrogen-vim tawg yuav ua rau cov kab nrib pleb ntev ntawm lub plhaub, feem ntau ncaj, tsis muaj pitting, scaling lossis lwm yam corrosion. Cov pob txha tawg tuaj yeem pom qhov sib txawv lossis quasi - cov yam ntxwv sib cais nrog cov dimples zoo. Koj yuav pom cov cim ntawm cov roj nplua nyeem (xim av, carbonaceous deposits) nyob rau sab hauv txoj kab uas hla ze ntawm qhov tawg. Txoj kev kho yog yuav tsum tau nruj dua ncej-swaging tu thiab ib tug hydrogen ci- tawm lub voj voog ua ntej zaum kawg los ua ke los yog kom txo tau swage txo mus rau<12-14%.

Xaus: Kev tswj ntawm swage txo thiab lubricant residue yog tsim nyog los tiv thaiv thawj zaug - voj voog tawg

Delayed hydrogen-induced cracking during the first heating cycle (150-350°C) of 316L stainless steel heater sheaths can be initiated by swaging reductions >16-18% in conjunction with residual stress >200-250 MPa thiab residual lubricants los ntawm cov txheej txheem swaging. Qhov tshwm sim ntawm kev tawg yog 15-60% ntawm 18-20% txo hauv cheeb tsam. Cov kws tshaj lij hais txog 316L sheaths yuav tsum txwv kev txo qis mus rau 14-16% rau cov ntawv thov tseem ceeb thiab yuav tsum tau ua tom qab-swaging tu (cov kuab tshuaj degreasing + lub tshuab nqus tsev ci ntawm 200℃rau 4 teev) kom tshem tawm cov roj nplua nyeem. Rau qhov kev ntseeg siab tshaj plaws, qhia lub voj voog hydrogen ci tawm ua ntej kev sib dhos zaum kawg. Txoj hauv kev uas tau muab los ntawm no cuam tshuam txog kev txo qis, kev ntxhov siab, thiab cov roj nplua nyeem nyob rau hauv qhov tshwm sim ntawm thawj lub voj voog hydrogen tawg, ua rau cov neeg siv khoom hais txog cov txheej txheem tsim khoom uas yuav tiv thaiv kev puas tsuaj rau thawj zaug cua sov tsis ua haujlwm ntawm 316L cua sov sheaths.