Study of intergranular fracture mechanism by ductility reduction in Ni-Cr-Fe alloys

Abstract

Ductility-dip cracking (DDC) is a Brittle intergranular failure mechanism that occurs under high stresses within the homologous temperature range from 0.5 to 0.8 in large grain size materials with CFC crystal structure. This problem has been around for years and has been reported to be associated with continuous casting, hot rolling, forging and welding operations in different alloy systems as steels, stainless steels, Ni-base alloys, and Cu-alloys. Some practical solutions have been found to this problem for specific cases. However, the understanding of the mechanism is still incipient, making the control of this problem under production conditions a very difficult task. Therefore, this research proposes the use of in-situ thermo-mechanical testing combined with different characterization techniques to study the effect of intergranular precipitates on the DDC mechanism on Ni-Cr-Fe alloys. This system has been selected due to its relevance for the petrochemical, aerospace, nuclear and chemical industry and the high susceptibility that some of these alloys have to DDC. Specifically, this research will concentrate on the highly susceptible comercial alloy AWS A5.14 ERNiCrFe-7, also called Filler Metal 52 (FM-52), which is the matching welding filler metal used for alloy 690. Welding of this alloy can become an engineering nightmare due to its high susceptibility to DDC. Recent collaborative work between LNLS and OSU (Ohio State University) has revealed some important aspects of DDC mechanism, elucidating the effect of intergranular precipitates on DDC, which is now accepted to be a grain boundary sliding mechanism. Thus, the commercial ERNiCrFe-7 alloy will be slightly modified to induce the formation of intergranular precipitates and their effect on the DDC susceptibility will be evaluated, with special attention on the failure mechanism. The elements to be added to this alloy will be Nb, Ti, Mo and Hf. The effects of Nb, Ti and V on carbide precipitation have been already studied at LNLS. Hf will be added in very small amounts to induce slight chances on the precipitates morphology. Finally, Mo has been recently proved to improve DDC resistance of this alloy but the improvement mechanism is not clear yet. The alloys will be produced in arc furnace and the chemical compositions to study will be determined based on already established kinetic and thermodynamic calculations based on the Calphad® method.The characterization techniques to be used are: scanning and transmission electron microscopy coupled with EDS, EELS and EBSD, and X-ray diffraction. The use of these techniques together with the specially developed in-situ thermo-mechanical testing inside the SEM chamber are expected to provide a new insight into the DDC mechanism through the concurrent analysis of the chemical composition, mechanical conditions, microestrutural features, and the actual failure evolution.