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.

Automated summary

Investigating the microstructure of components exposed to extended service in refineries and petrochemical plants at high temperatures reveals the influence of intergranular relationships on carbide growth, resulting in intricate 3D structures.