Gx51 optical microscope

Metallographic samples were ground and polished using Buehler metallography equipment. Light Optical Microscopy (LOM) was undertaken using an Olympus GX51 optical microscope (Olympus Corporation, Tokyo , Japan) and Stream Essentials (ver. 1.9.1.) on etched samples using Kroll’s Reagent (92 vol % H₂O, 6 vol % HNO₃ and 2 vol % HF) for 5 to 10 s to reveal the microstructure. For electron microscopy analysis, a Zeiss MERLIN scanning electron microscope (SEM) (Zeiss, Oberkochen, Germany) in conjunction with a backscatter detector (BSD) was used.
Individual grain width measurements were done with Stream Essentials software tools. Volumetric α/β phase fractions were assumed to coincide with the area phase fractions of a 2D micrograph. Minimum and maximum grain widths were measured as mean values using the line-intercept method automated in Matlab 2017a through the help of Matlab’s Image Processing Toolbox.
X-ray diffraction was done using a Bruker D2 Phaser diffractometer (Cu Kα source) with a standard Bragg Brentano geometry. Accusation dwell time was 0.75 s at steps of 0.01°.

A 400 kW induction heater (Termetal, Graniczna, Poland) was used for the tests, designed for rods of 35–70 mm diameter. In these tests, coatings on steel rods, diameters 35, 45 and 70 mm, were tried. Three protective substances in diverse concentration, diluted with water were tested:

Berulit—concentration; 100%, 66%, 50% and 33%

Bonderite—concentration; 100%, 66% and 50%

Condursal—concentration; 100%

Pieces of metal rods were pre-coated by immersion then dried in a vertical position. After drying out, rods were heated in an induction heater in which they were moving towards the exit, where the temperature measurement was taken. The temperature was 1150–1180 °C which meant that they were fit for forging. Nevertheless, after heating the rods were placed for cooling in the air. Finally, decarburization was analyzed. It showed that among all rod’s diameters, those with the least diameter were decarburized the most, that is, rods of 35 mm diameters. Therefore, based on these rods, the analysis of the coatings and decarburization correlation was carried out. Samples similarly to the bars from previous section were cut perpendicular to the bar axis, the cross section was polished and pre-etched with Nital 3%. The microstructure observations were done on the Olympus GX-51 optical microscope.

To define heating time corresponding to induction heating, a few trials were carried out. First the numerical modelling of heating process was performed to obtain real temperature of rod inside and on the surface during induction heating. Another test show that within 5 min, in a furnace of 1200 °C decarburization similar to an industrial one takes place. Therefore, steel samples C45 were coated by three different products of various concentrations, and after that, they were heated for 5 min in a furnace without a protective atmosphere in a temperature 1200 °C. They were then cooled in the air, and decarburization of the surface layer was examined. In order to evaluate the effect of the coatings on the decarburization of the surface layer the microstructure analysis was performed. Samples were cut perpendicular to the bar axis, the cross section was polished and pre-etched with Nital 3%. The microstructure observations were done on the Olympus GX-51 optical microscope (Olympus, Tokyo, Japan).