Microstructural and Mechanical Properties Examination of High-Power Diode Laser-Treated R260 Grade Rail Steels Under Different Processing Temperatures

In the present study, high-power diode laser surface treatment was implemented to R260 grade steel with three different processing temperatures (1100 degrees C, 1200 degrees C, and 1300 degrees C) at the laser power of 1750W and scanning speed of 6mm/s, in order to identify the effect of various processing temperatures on the mechanical performance. According to the test results, the laser-treated sample at 1300 degrees C showed much better mechanical performance among the other laser-treated samples. It was found that the laser-treated sample at 1300 degrees C had about 3 times more surface hardness, a 43 pct increase in yield strength, and a 53 pct increase in toughness value compared to the untreated sample. Microstructural investigations showed that this surface treatment did not only generate a martensitic structure with a certain depth but also provided the formation of a fine pearlitic structure contributing to an increase in the mechanical properties. As a conclusion, it was found that the processing temperature is one of the critical factors affecting the mechanical properties in the laser hardening process. Moreover, the results demonstrated that this treatment method might be an alternative method to enhance the mechanical properties of existing rail steels online without the need for rail disassembly, reducing operational costs

Thermodynamics and kinetics of the formation of Al2 O3/ MgAl2O4/MgO in Al-Silica metal matrix composite

The formation of Al2O3, MgAl2O4, and MgO has been widely studied in different Al base metal matrix composites, but the studies on thermodynamic aspects of the Al2O3/ MgAl2O4/MgO phase equilibria have been limited to few systems such as Al/Al2O3 and Al/SiC. The present study analyzes the Al2O3/MgAl2O4 and MgAl2O4/MgO equilibria with respect to the temperature and the Mg content in Al/SiO2 system using an extended Miedema model. There is a linear and parabolic variation in Mg with respect to the temperature for MgAl2O4/MgO and Al2O3/MgAl2O4 equilibria, respectively, and the influence of Si and Cu in the two equilibria is not appreciable. The experimental verification has been limited to MgAl2O4/MgO equilibria due to the high Mg content (&ge;0.5 wt pct) required for composite processing. The study has been carried out on two varieties of Al/SiO2 composites, i.e., Al/Silica gel and Al/Micro silica processed by liquid metallurgy route (stir casting route). MgO is found to be more stable compared to MgAl2O4 at Mg levels &ge;5 and 1 wt pct in Al/Silica gel and Al/Micro silica composites, respectively, at 1073 K. MgO is also found to be more stable at lower Mg content (3 wt pct) in Al/Silica gel composite with decreasing particle size of silica gel from 180 micron to submicron and nanolevels. The MgO to MgAl2O4 transformation has taken place through a series of transition phases influenced by the different thermodynamic and kinetic parameters such as holding temperature, Mg concentration in the alloy, holding time, and silica particle size.<br /