Origin of fine needle-like M23C6 carbides in a heat resistant steel after extended service

Components in refineries and petrochemical plants exposed to extended service offer a unique opportunity to investigate the effect of a long heat exposure on the microstructure. Twenty years of service at 900 degrees C of a super austenitic Fe-0.5C-25Cr-35Ni-1Nb-0.1Ti alloy led to the formation of both coarse intergranular equiaxed as well as fine intragranular M23C6 needle-like carbides. Cube-cube and twin-like Sigma 3 orientation relationships are identified between the matrix and fine needle-like carbides. The former relationship maintains {111}(CC)//{111}(M) parallelism while the latter satisfies only one (111)(TC)//(111)(M) with 60 twist misorientation similar to that of twins or Sigma 3 in FCC crystals. Both planar relationships generate low energy {111} carbide-matrix phase boundaries. Cube relationship generates Sigma 3 phase boundary and a superlattice with 3 times lattice parameter of the matrix while twin-like relationship results in Sigma 9 phase boundary and a superlattice with 3 root 3 times lattice parameter of the matrix. The carbide growth direction is determined as 011(C). All six < 011 >(M) directions in the matrix accommodate the growth of needle-like carbides resulting in an intricate 3D structure. Twinning is observed in some needle-like carbides resulting in coexisting cube-cube and twin-like orientation relationships with the matrix. Carbide twinning leads to three in-common < 011 >(C) directions for preferential growth. Twinning also allows needle-like M23C6 to form serrated cube-cube and straight twin-like {111} phase boundaries with the matrix.