BS in Materials Engineering
Materials Engineering Department
Grain growth during austenitization has a negative effect on fatigue strength. Several methods have been investigated in order to determine an accurate method of measuring the austenite grain size of 4340 steel. The McQuaid-Ehn method, an industry standard for evaluating austenite grain size, has been recognized to produce inaccurate results due to the coarsening of grains during extended austenitizing. A method utilized for hypoeutectoid steels, outlined by the ASTM-E112 standard, is investigated to potentially obtain more accurate results by reducing the duration of austenitizing. Studies regarding effective etching procedures for revealing austenite grains without austenitizing are also addressed. Samples are prepared by heat treating to induce temper embrittlement in order to improve etching at these grain boundaries. Although temper embrittlement is undesirable for components in service, the process proved to effectively delineate austenite grain boundaries when etched with a picric acid based reagent. This method may be utilized to acquire an accurate measurement of austenite grain size during processing. The picric acid reagent was unsuccessful when applied to the McQuaid-Ehn and Hypoeutectoid samples. The use of a nital etchant also proved to be ineffective for the McQuaid-Ehn, Hypoeutectoid and temper embrittlement methods utilized by this investigation.