Correlating Macroetch Response to Microstructural Features
A PES internship summer project investigated how chemical segregation in steels can impact the appearance of macroetch test samples. The purpose of this project was to compare corresponding macroetch and microetch features to analyze how the underlying microstructure can affect the stronger macroetch process. Three different alloys were examined as part of the project, namely 4340, 4140, and 440C. Segregation involves the partitioning of elements to localized regions as a molten metal cools and solidifies. It can often coincide with regions where defects or discontinuities are present, including interdendritic regions, grain boundaries, or phase distribution changes. Segregation produces bands or pockets of different chemical composition from the bulk chemistry, resulting in localized mechanical property differences that may impact the material's overall performance, depending on the application. One example is that alloy segregation can ultimately produce a difference in the hardenability of the steel between alloy-rich and alloy-lean areas.
Recent work focused on analyzing the macroetch response of these carious alloys and then microetching and microscopically examining the smaller section that were cut from the original macros. Correlation of the microstructures and macrostructures of these materials could then be studied in detail. Photo documentation was performed and hardness testing was completed in the regions containing evidence of chemical segregation. In most cases, the results demonstrated that the areas in question had much higher hardness values than the bulk microstructure throughout the sample. Microscopically, it was shown that the black macroetch features corresponded to areas of fresh martensite. The segregated areas in the grade 440C material, however, consisted of high concentrations of primary carbides. The presence of segregation was confirmed after phase analysis and hardness tests were completed for each of the three alloys. For the 4340 material, areas of segregation showed Microindentation hardness values of up to 15 HRC points (converted from Vickers) higher than the bulk hardness. For the 4140 material, the hardness value differences in the segregated regions were measured to be up to 2o HRC points higher than the bulk. The segregated areas in the 440C exhibited the largest differences in hardness between regions, with the segregated areas exhibiting hardness values of up to 44 HRC points higher than the general microstructure.
When the samples were macroetched, the areas of segregation showed up as dark spots. This response was obtained with varying etch temperatures and times (within allowable limits per specifications), indicating that the etch response is not artificially manufactured by the preparation/etch process. PES plans to publish a technical paper documenting the results and conclusions of this study.
Entry Originally Posted September 2021