Microstructural evolution in austenitic heat-resistant cast steel 35Cr25Ni12NNbRE during long-term service

The microstructural evolution of austenitic heat-resistant cast steel 35Cr25Ni12NNbRE during aging and long-term service was investigated using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The microstructure of the as cast steel consists of the dendritic austenite, the block-like eutectic carbide M(7)C(3) spreaded among austenitic dendrite, and a small quantity of M(23)C(6) carbide. The microstructure of the steel aged at 600 degrees C consists of eutectic carbide M(23)C(6) transformed from eutectic carbide M(7)C(3) and dendritic austenite in which fine secondary carbide particles M(23)C(6) precipitated. The precipitated carbide M(23)C(6) kept a cubic-cubic orientation relationship (OR) with austenite matrix. There existed a carbide precipitation free zone (PFZ) around the eutectic carbide. For the long-term serviced samples, the secondary carbide precipitated in the austenite strikingly increased and the FEZ disappeared. Part of the M(23)C(6) transformed into M(6)C, which always kept a twin OR, [1 1 4](M6C)//[1 1 0](A)//[1 1 0]M(23)C(6), with the austenite and the M(23)C(6) secondary carbide. In addition, a small quantity of sigma phase FeCr and epsilon-Cr(2)N were also identified. The effects of alloy composition and service condition on the microstructural evolution of the steel were discussed. (C) 2010 Elsevier B.V. All rights reserved.