Tanaporn Rojhirunsakool

This study investigates the optimization of stress-relief heat treatment for overlay weld repair of severely used rail tracks. The research focuses on 900A rail steel samples that have been in service for over 20 years, exhibiting significant wear and plastic deformation. Pre-welding heat treatments at 300 °C, 400 °C, and 500 °C were applied to reduce service-induced residual stresses before flux core arc welding (FCAW) repair using high-Mn filler material. The effects of these treatments on residual stress distribution, microstructure, hardness, and wear resistance were systematically analyzed. Scanning electron microscopy (SEM), X-ray residual stress analysis, microhardness testing, and pin-on-disc wear tests were employed to characterize the samples. Results indicate that stress relief at 300 °C and 400 °C effectively reduces residual stress while maintaining or enhancing hardness and wear resistance. However, treatment at 500 °C led to over-tempering, resulting in decreased hardness and increased wear rates. The 400 °C treatment emerged as the optimal temperature, providing a balanced combination of reduced residual stress, favorable microstructure, and improved mechanical properties across all welded zones.

Pearlitic steel experiences excessive corrosion in a hot and humid atmosphere. The multiphase bainitic/martensitic structure was developed for a better combination of strength and ductility, especially rolling contact fatigue, but little attention to corrosion has been investigated. Corrosion behaviors of multiphase steels obtained from bainitic-austempering (BAT) and bainitic-quenching and -partitioning (BQ&P) processes were investigated via immersion and electrochemical tests in 3.5 wt.% NaCl solution. The corroded surface and rust after immersion and electrochemical tests were analyzed via electron microscopy, Fourier transform infrared spectra, and x-ray diffraction. The multiphase bainite + martensite/retained austenite island showed higher corrosion resistance than that of the pearlitic one. The acicular bainite obtained from the BQ&P process showed slightly higher corrosion resistance than the granular bainite + martensite structure obtained from the BAT process.

This research work aims to study the effects of pre-weld and post-weld heat treatment conditions in the laser welding process on the microstructure of cast nickel based superalloy grade, GTD-111. Pre-weld heat treatment conditions consist of a solution treatment at a temperature of 1473 K for 7.2 ks and three over-aging processes at temperatures of 1393, 1413, 1433 K for 7.2 ks. All as-received and pre-weld heat treated specimens received the same post-weld heat treatment, which consisted of a solution treatment at a temperature of 1473 K for 7.2 ks and precipitation aging at a temperature of 1118 K for 86.4 ks. From the results, it can be seen that the higher temperature of the solution treatment with respect to the pre-weld heat treatment provided coarser gamma prime particles precipitating in a gamma matrix with a more uniform microstructure and very fine gamma prime precipitated particles. Furthermore, when followed by the post-weld heat treatment or standard heat treatment of the alloy, the final microstructures of all as-received and pre-weld heat treated specimens were very similar with regard to uniform gamma prime particles. No significant effect of pre-weld heat treatment conditions on the microstructure was found after post-weld heat treatment.

The effect of rhenium and cobalt additions on the microstructure and gamma prime phase stability of modified nickel-based superalloy, grade MGA1400 by means of a vacuum arc melting method with different contents of rhenium (Re) and cobalt (Co) was investigated. After being subjected to heat treatment, the size of the gamma prime particles was slightly reduced with increasing rhenium content. Then, the alloys were exposed at a temperature of 1173 K for 1800 ks. The results showed that increasing the rhenium addition leads to a slower coarsening rate of the gamma prime particles. Furthermore, the shape of the gamma prime particles changed to a more cubic shape. However, an excessive rhenium content (>1.21 wt.-%) promotes the formation of a topologically close-packed phase after a 360 ks exposure at 1173 K. It can be concluded that an increase in rhenium content can result in a lower coarsening rate of the gamma prime particles. The cobalt addition is beneficial for impeding or slowing the precipitation of topologically close-packed phases at an elevated temperature.