In-situ SEM observation of grain growth in the austenitic region of carbon steel using thermal etching.

ABSTRACT

A novel heat stage, recently developed for use within the Scanning Electron Microscope, has facilitated Secondary Electron imaging at temperatures up to 850°C. This paper demonstrates one of the applications of in-situ elevated temperature Scanning Electron Microscope imaging observation and quantification of grain growth within the austenitic region of carbon steels. The resulting Secondary Electron data have used the technique of thermal etching to capture possible 'abnormal grain growth' in the austenitic region. Previous ex-situ and post-heating results from carbon steels indicate normal, non-linear grain growth. Therefore, this new dataset provides greater insight into the heat treatment of steels. From comparison of the in-situ data with the overall grain growth, measured ex-situ, it is further concluded that abnormal grain growth is representative of the growth at temperature. Thus, the heating and cooling parts of the heat treatment are likely to account for the non-linearity previously documented in ex-situ results and, hence, the range of powers recorded when fitting power law models for steel grain growth. The ability of data derived from in-situ thermal etching to represent the microstructure of the entire surface and the bulk material is also considered. LAY DESCRIPTION A novel heating stage has recently been developed for use within the Scanning Electron Microscope (SEM); an instrument that uses electrons to image specimen surfaces at very high magnifications. The development of the heating stage has facilitated imaging at temperatures up to 850°C of the structure and topographic features of metals using two different detectors. This study focusses on observation and quantification of grain growth in steels at temperatures of 800  C. In Materials Science, grains refer to crystals of varying, randomly distributed, small sizes that together make up a solid metal. The temperature of 800  C is used as it is the desired temperature to heat treat steels in order to produce more favourable physical properties. It is also the temperature above which the material undergoes a phase change; phase change is a transition where the atoms rearrange from one order within a grain to another. In the case of steel, at room temperature atoms will be in what is called a ferrite phase (one order) but at 800  C, they will be in a different order within the grains, known as the austenite phase. Hence, the uniqueness of this dataset as the grain growth captured is in the high temperature steel phase of austenite. The steel samples used are made up of 0.4% Carbon, 99% iron and some manganese and other trace elements. The resulting data have, for the first time, shown so called 'abnormal grain growth' which is represented by a linear relationship between grain size and time. Abnormal grain growth is also observed in the images where it can be seen how larger grains grow at a high rate at the expense of smaller ones. Previous data taken after cooling of steels indicate normal non-linear grain growth. Therefore, it is reasonable to suggest, this new dataset provides greater insight into the heat treatment processing of steels, demonstrating that they are potentially more complex than previously thought.