An investigation of microstructural change of low alloy steel AISI 4150 by Seebeck coefficient

T Samran, P Termsuksawad


Low alloyed steels, whose hardness can be modified by heat treatment, have been widely used in various applications. After heat treatment, they are normally destructively characterized and tested by many approaches via optical microscope and hardness testing. It is useful to develop a non-destructive method relating to its properties and microstructures. The proposed non-destructive approach in this study is the Seebeck coefficient measurement. The materials in this study were carbon steels AISI 4150 rod with a diameter of 1.3 cm and length of 3 cm. The specimens were heat-treated at 900ºC for 1 hour and were then cooled to room temperature in furnace and in various mediums: air, oil and water. In addition, one of the samples was cooled in salt bath at 350ºC for 1 hour and then cooled in water. X-ray diffractometry (XRD) and optical microscopy (OM) were used to characterize their crystal structures and microstructures, respectively. The Seebeck coefficient was measured relative to that of copper. The result indicated that the Seebeck coefficients of the treated samples are negative and their magnitudes are inversely related to hardness. The Seebeck coefficient also tends to be related to the crystal structure. Microstructure investigation, in addition, revealed that the magnitude of the Seebeck coefficient increases with a decrease of grain size. In conclusion, the Seebeck coefficient measurement could possibly be applied to study the microstructure of low alloyed steels.

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