Perpustakaan Digital ITB

Ultra high hardness (UHH) armor steel with hardness value more than 570 HB has potential to be used in the fabrication of combat vehicle hull to increase ballistic protection and mobility. During fabrication, UHH steel is difficult to be welded due to susceptibility to hydrogen induced cracking (HIC). Determination of welding parameter is important to produce good and free-defect welded joint. In this study, low hydrogen welding was carried out using Gas Metal Arc Welding (GMAW) with various types of electrode and heat input. Ferritic (ER70S-6) and austenitic (ER307) electrodes were compared focusing on the microstructure and mechanical properties at heat input 1.2 kJ/mm, 1.6 kJ/mm, and 2.0 kJ/mm. Results of the study shown no crack was found in all specimens with different welding parameters. The microstructure on weld metal of ferritic electrode shows acicular, widmanstatten, and polygonal ferrite morphology. Acicular ferrite decreases as the heat input increases. Austenitic electrode shows delta ferrite in austenite matrix with ferrite-austenite (FA) solidification mode and ferrite number 8%. The distance between the delta ferrite is getting further as the heat input increase. The microstructure has correlation with mechanical properties (hardness, tensile strength, and impact). The lowest hardness value was found on weld metal (ferritic 231 HV; austenitic 176 HV), while the highest hardness value was found on rehardened HAZ (ferritic 622 HV; austenitic 597 HV). The hardness value of tempered HAZ (428 HV) is lower than base metal (611 HV) indicating HAZ softening. The welded joint failed on the weld metal with tensile strength 734 MPa (ferritic) and 711 MPa (austenitic) higher than electrode specifications of ferritic electrodes 593 MPa and austenitic 640 MPa. The impact test shows the root layer of ferritic electrode had significant effect on the impact value of the subsize specimen with a value 1.00 J/mm2 (with root) and 0.65 J/mm2 (without root). Crack did not appear in the weld metal area after the firing test.