جوشكاري غير مشابه فولاد مقاوم به حرارت25 ‍Cr-35Ni به فولاد مقاوم به حرارت 30Cr-48Ni و ارزيابي خواص اتصال

چكيده انگليسي :

Dissimilar Welding of 25Cr 35Ni 30Cr 48Ni Cast Heat Resistant Steels and Evaluation of Joint Properties Mohammad Mostafaei m mostafaei@ma iut ac ir April 18 2011 Department of Materials Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiDr M Shamanian Supervisor Dr A Saatchi Supervisor AbstractIn this study 25Cr 35Ni 30Cr 48Ni HP HV cast heat resistant alloys dissimilar joint wasinvestigated This joint is commonly used in reformer tubes in direct reduction unit of steelmaking industries The investigation by SEM equipped with EDS elemental analysis systemindicated that HP alloy in as cast condition contains M7C3 M23C6 and NbC primary eutecticlike carbides embedded in an austenitic matrix Also in HV alloy which has 5 tungsten includes M23C6 and M6 C carbides embedded in an austenitic matrix HP and HV alloys haveappropriate ductility and relatively good weldability in as cast condition But during 8 yearsex service aging at 1050 C chromium rich carbides grow gradually and their morphologychanges to blocky and angular as a result ductility of these reformer tubes decreasesdramatically and thus repair welding of aged HP HV dissimilar joint may be at a risk Foreconomical saving by rewelding of aged reformer tubes and increasing the weldabilitypropertices partially solution annealing heat treatment at 1200 C for 2 4 and 6 hours wascarried out Tensile test results showed that by partially solution annealing heat treatment at1200 C for 6 hours can increase the elongation of aged HP alloy increases from 3 to 15 and HV alloy from 2 6 to 3 4 Also study of Cr rich carbides by EDS line scan analysisindicated that during partially solution annealing heat treatment Cr atoms diffuse from outerparts of carbide to the austenite matrix and lead to raising of tensile strength at solutionannealed condition Also constitutional liquation cracking mechanism of HAZ carbides atservice aged condition was fully evaluated and analyzed In the HAZ of aged HP HV weldedjoint the cracks around the locally melted carbides were initiated and propagated duringsolidification step of remelted carbides at cooling cycle of welding This causes a decrease inductility of aged HP and HV alloys The mechanism of cracking around the locally meltedcarbides is similar to solidification cracking of weld metal Varestraint weldability testshowed that susceptibility to hot cracking and constitutional liquation cracking decreaseswith partially solution annealing heat treatment at 1200 C for 6 hours Also this test resultsshowed that solution annealed 25Cr 35Ni base metal has least susceptibility to hot crackingwhile aged 30Cr 48Ni 5W base metal has most susceptibility Thus it is recommended thatbefore repair welding of aged reformer tubes they should be solution annealed at 1200 C for6 hours Keywords Dissimilar welding Heat resistant steel Constitutional liquation cracking

مصطفايي، محمد

جوشكاري غير مشابه فولاد مقاوم به حرارت25 ‍Cr-35Ni به فولاد مقاوم به حرارت 30Cr-48Ni و ارزيابي خواص اتصال