The study of the elements migration from the matrix phases to the grain boundary effects on the creep rupture of G-NiCrTi۲۸W alloy at elevated temperature

The microstructure and mechanical properties of a centrifugally Cast G-NiCrTi۲۸W heat resistant alloy and effects of the elements migration from matrix phase to grain boundary have been stated and the effects of microstructure and composition changes on the life of reformer tubes from a direct reduction iron plant have been assessed. For this purpose, one sample from the as-cast tubes, and one from the aged tubes at ۱۱۰۰ °C for about ۸ years, were used. Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) analysis of two tubes were done. In as-cast tubes, the Cr۷C۳ primary chromium carbides and niobium-tungsten carbides were identified. Aging promoted the occurrence of different phenomena such as the transformation of Cr۷C۳ primary carbides to Cr۲۳C۶ secondary carbides, and the transformation of niobium-tungsten carbide to the M۶C tungsten carbides. It was found that aging promoted the reduction in tensile strength from ۵۸۰ MPa to ۱۰۰ MPa and the increase in elongation from around ۶% to around ۳۰%. According to the results of the creep test at ۹۶۰ °C and ۵۲ MPa, no rupture occurs on the as-cast reformer tube during the test time.

Keywords:

Heat Resistant Alloy , Creep rupture , Reformer tubes , Modified HV , G۴۸۷۹ Micro