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Jeklo A-286 je superzlitina na osnovi železa, niklja in kroma z dodatkom titana in molibdena ter spada med izločevalno utrjena avstenitna nerjavna jekla. Uporabno je predvsem za plinske turbine, v petrokemijski industriji ter v letalski in vesoljski industriji. Ker se jeklo prodaja v različnih presekih, v izločevalno utrjenem stanju, smo ugotavljali kako različne stopnje deformacije, hitrost ohlajanja po kovanju in različni parametri naknadne toplotne obdelave vplivajo na njegovo interkristalno korozijsko odpornost. V podjetju SIJ-Metal Ravne d.o.o. se preizkus odpornosti zoper interkristalno korozijo izvaja tako po standardu EN ISO 3651-2 kot po standardu ASTM A262- 15. Zaradi tega smo vzorce testirali po obeh standardih in najprej preverili ali je mogoče za negativne rezultate korozijskih testov predhodnih šarž lahko odgovorna slaba regulacija temperature sensitizacijskega žarjenja oz. ali bi lahko prihajalo do kakšnih neskladnosti pri izvajanju korozijskih testov. Pri študiju vpliva deformacije na korozijsko odpornost tega jekla smo uporabili tri različne stopnje deformacije, pri študiju vpliva hitrosti ohlajanja jekla po kovanju pa smo preizkušance ohlajali v štirih ohlajevalnih medijih (voda, zrak, olje, peč). Korozijsko smo testirali tudi toplotno obdelane vzorce. Toploto obdelavo smo izvajali pri temperaturi raztopnega žerjenja 980 °C/ različni časi in staranja na 720 °C/16 ur. Pri testiranju toplotno obdelanih vzorcev je prišlo do loma pri enem izmed vzorcev. Z analizo mikrostrukture (OM in SEM-EDS) smo potrdili prisotnost večjih izločkov TiC, TiN in Ti(C, N), ki se nahajajo predvsem v notranjosti kristalnih zrn ter mešanih (titan, molibden, krom, niobij, vanadij, …) karbidov in fosfidov po kristalnih mejah. Korozijskih produktov (oksidov, kloridov) v mikrostrukturi korozijsko testiranih vzorcev nismo opazili, prav tako ne škodljivih kromovih karbidov (Cr23C6) po mejah zrn, ki so vzrok za interkristalno korozijo. Opravili smo tudi analizo prelomnih površin in nekaterih mesti izmerili povišano koncentracijo fosforja. Na podlagi tega smo sklepali, da za lom materiala ni bila odgovorna zmanjšana odpornost zoper korozijo temveč zmanjšana duktilnost jekla zaradi segregacij fosforja na kristalne meje. Alloy A-286 is a superalloy based on iron, nickel and chromium with the addition of titanium and molybdenum and belongs to the family of precipitation hardened austenitic stainless steels. Since this steel is sold in the form of bars (different diameters) in the precipitation hardened condition, we want to investigate how different degrees of deformation, cooling rate after forging, and different heat treatment parameters affect the microstructure of the steel and, consequently, its resistance to intergranular corrosion. In the company SIJ Metal Ravne d. o. o. the tests against intergranular corrosion are carried out according to the standards EN ISO 3651-2 or ASTM A262 15. For this reason, we tested the samples according to both standards and first checked whether it is possible that poor temperature control of sensitization annealing could be responsible for the negative results of the corrosion tests of the previous batches, or whether there could be any inconsistencies in the performance of the corrosion tests. In the study of the effect of deformation on the corrosion resistance of this steel, three different degrees of deformation were used, and in the study of the effect of the cooling rate of the steel after forging, the test samples were cooled in four cooling media (water, air, oil, furnace). The heat-treated specimens were also tested for intergranular corrosion susceptibility. Heat treatment was performed at a solution annealing temperature of 980 °C/different times and aging at 720 °C/16 hours. Because one of the heat treaded specimens didn’t pass the corrosion test, we performed microstructure analysis with optical and electron microscopy (SEM-EDS). We confirmed the presence of large TiC, TiN and Ti(C, N) inclusions within the crystal grains and of mixed carbides (titanium, molybdenum, chromium, niobium, vanadium, ...) and phosphides along the crystal boundaries. We could not find any corrosion products (oxides, chlorides) in the microstructure and also no chromium carbides at the grain boundaries, which are the cause of intergranular corrosion. We also performed an analysis of the fracture surfaces and measured elevated phosphorus concentrations in some locations. From this we concluded that the reason for the fracture of the material was not the reduced corrosion resistance, but the reduced ductility of the steel due to the segregation of phosphorus at the crystal boundaries. |
