The investigation of mechanical behavior of surface modification treatments, GTA welding process and nitriding of AİSİ 4340

ABSTRACT PhD Thesis THE INVESTIGATION OF MECHANICAL BEHAVIOR OF SURFACE MODIFICATION TREATMENTS, GTA WELDING PROCESS AND NITRIDING OF AISI 4340 Soner BUYTOZ Fırat University Graduate School of Natural and Applied Sciences Department of Metallurgy Education 2004, Page: 174 In this study, the effects of the carbide and other phase in microstructure to the hardness of material and its abrasive wear behavior have been investigated, while modifying the features of surface by three different processes to the high strength-low alloy AISI 4340 steel. For this purpose, the surface modification by hardening/tempering procedure, nitrocarburizing and tungsten inert gas (GTA) process from the traditional surface modification techniques has been performed to AISI 4340 steel specimens. The optical and scanning electron microscope (SEM) and energy dispersive spectrograph (EDS) micro analyses in investigating miscrostructures and X-ray diffraction analyses have been utilized. Then, the hardness of the surface modified has been measured by microhardness devices and the abrasion resistance of surface has been found by the pin-on-disc tests. After abrasion tests, the wear rates of specimens has been obtained extracting the mass losses. The AISI 4340 specimens in the first group have been tempered at 450, 550 and 650 °C temperatures and 60, 120 and 240 minutes, respectively after the austenite at 850 °C temperature and hardening process. The lath type martensite from AISI 4340 material has been obtained by the austenite and hardening processes. The ferrit and dispersed carbides in ferritic matrix have been encountered by tempering the martensite structure at different temperatures and tempering period. The value of abrasive wear was observed at high amount, because the processing period was not enough for 450 °C. The minimum wear rate in abrasive wear tests of the specimens which were tempered by the martensite structure was obtained from the material tempering during 2 hours at 650 °C. The mass loss in this material decreased because it underwent to recristalization and the dislocation areas relatively, decreased. The preliminary heat treatment at 350 °C temperature and the nitrocarburizing process at 580 °C and at 30, 60, and 120 minutes, respectively were performed to AISI 4340 specimens in the secondary group. After this processes, Fe3N and Fe3N/Fe4N phases were discovered in the microstructure. Also, while the period extends, both the layer thickness and microhardness increased. The hardness values XVchanged between 700 and 750 HV depending on the process period. Minimal mass loss was measured from nitrocarburized material during 120 minutes, depending on nitrocarburizing conditions. Doing the modification at different amounts by FeCrC and FeCrC/WC alloy powders and the gas tungsten arc (GTA) methods to AISI 4340 specimens in the third group, the microstructures, hardness and wear characteristics of the surfaces have been experimented. In the FeCrC specimens with surface modifications, both hypoeutectic and hypereutectic microstructures were formed. The hypoeutectic microstructure consisted of austenite phase with surface center cubic (fee) and eutectic M7C3 (M= Cr, Fe) carbides with hep lattice structure. On the other hand, the hypereutectic microstructure consisted of primary M7C3 carbides and eutectic austenite phases. The dentrite structure having Fe, Cr and C with relatively high power density occurred because of insufficiently powder portion and high heat input. In the surface modifications done by WC, M7C3, M23C6 (M= Fe, C, W), W2C and Fe3W3C carbides having Fe, C, W at high amount along with the dentrites formed towards the coating/substrate interface have been encountered. The hardness values of substrate increased because of existing phase and carbides in the microstructure. While the hardness values of materials with surface modification done by FeCrC coatings changed between 712-861 HV, it is switched between 950-1150 HV, adding WC. Moreover, WC coatings reached to 1 200 H V. in the modifications done by FeCrC, the specimen whose maximum mass loss is high and powder portion and heat input is low, was also obtained. In the surface modifications done by adding WC to FeCrC, the mass loss is lower than that of FeCrC coatings. The mass losses in the materials with surface modifications done by WC is lower than those of FeCrC and FeCrC/WC, since the microstructure of WC coatings consisted of hard carbides such as WC and W2C. Keywords: Surface modification, high-carbon ferrochromium, abrasive wear. XVI