The high temperature wear mechanisms of iron-nickel steel (NCF 3015) and nickel based superalloy (inconel 751) engine valves

Abstract

High temperature sliding wear tests were performed on samples taken from NCF 3015 austenitic steel (Fe-32% Ni-14% Cr based 0.8% C alloy with Al, Ti, Nb, Mo) and Inconel 751 superalloy (Ni-16%Cr alloy with Al, Ti, Nb) engine valves using the counterfaces made of PL12 chromium alloyed cast iron seat insert. The contact surfaces were characterized by SEM-EDS, Optical Profilometer, Raman, XRD. The wear mechanisms of the valve systems were explored using the results of these analyses that determined compositions of the tribolayers that formed on the contact surfaces. The Calphad based thermodynamic analyses that revealed stability of the precipitates in the alloys at the test temperatures. An increase of wear occurred between 25 degrees C and 350 degrees C, but for T > 350 degrees C, a continuous reduction in volumetric wear losses were observed with increasing temperature for both alloys. At all temperatures Inconel 751 showed higher wear performance than NCF 3015. It was found that the formation of (Fe1-x-yCrxNiy)(3)O-4 mixed spinel oxides controlled the high temperature wear behavior of these alloys and these structures support the formation of tribolayer under local pressure and high temperature during sliding. Besides the direct effect of oxide compositions within tribolayer, the formation of high amount of (Ti,Nb)C carbides, Ni-3(Ti,Nb,Al) and Ni3Nb precipitates play important role in transition from severe to mild wear regime above 350 degrees C.