Prior Austenite Grain Boundary Detection in an Ultra-Cleaned Cr Bearing Microalloyed Steel

摘要

In this study, several single chemical and double thermal-chemical etching techniques have been used in order to identify in details the prior austenite grain boundaries (PAGBs) with a good contrasting resolution in an ultra cleaned high strength low alloy Cr bearing microalloyed steel. The general chemical etching techniques are based on the various conventional metallographic reagents involving: Picral; 60℃ hot Picral; and Vilella solutions; while the double stage of thermal-chemical procedures have been classified into 400℃ low and 650℃ high temperature thermally treated for 3 hours followed with general chemical etching. Light and scanning electron microscopic observations have been supplemented with energy dispersive X-ray spectroscopy to follow the PAGBs detection in conjunction with that of carbide precipitation. Evidences are presented which indicate that only a tracing detection of PAGBs can be identified with general single metallography using conventional chemical etching reagents, while the double stage of thermal-chemical etching treated samples are associated with sharp contrasting resolution of PAGB areas. These observations are rationalized in terms of higher stimulation of chromium rich carbide formation and consequently the higher dilution of solute chromium at grain boundary areas of thermal-chemical treated samples. The PAGBs are considerably enriched from solute chromium in the direct water quenched fresh martensitic samples, while the thermal-chemical treated samples have been associated with chromium rich carbide precipitation at grain boundaries causing a higher sensitivity of grain boundaries to general chemical etching solutions, resulting the higher contrasting resolution of grain boundary areas.