Welding method for high crack sensitivity of Q&T steel
Components for combat vehicles need (such as body panzers, main battle tank, armored personnel carrier) to be made of high strength and hardness steel. However, during and after the welding process is complete, this steel often leaves cracks. Quenched and Tempered Steel is made of Hot Rolled Plate Steel (thickness 8 mm), which is heat-treated with quench and temper to increase strength and hardness. The novelty of this research is the welding method to create a welded joint consisting of fine structure, high strength, and high hardness produced. This joint is produced by manual gas metal arc welding. The scheme of investigation: a) The first step. Preparation of welded specimen 120×100×8 mm in size (Fig. 3). The specimen is divided into five parts, each is given code SS (without heat treatment), S750 (heating at 750 °C), S800 (heating at 800 °C), S850 (heating at 850 °C) and S900 (heating at 900 °C). Heating rate used=10 °C/minutes. b) The second step. Heating specimen S750 at 750 °C and holding for 30 minutes, then quenching in the water medium. The same way applied to specimens S800, S850, and S900. c) The third step. The observation of metallography, hardness, and impact energy was done for SS, S750, S800, S850, and S900. d) The fourth step. Removing the first layer of the weld in half-plate thickness using a hand grinding machine of each specimen, and continue to the second layer welding. e) The fifth step. The second layer of the welds is ground in half and proceed to the final welding. f) The sixth step. Discussion of observations. g) The seventh step. Conclusions.
The results of the tests carried out on KSTA 500 Steel include the chemical composition of base metal; microstructure and hardness for standard and water quenched weld joint. Medium carbon steel is equivalent to Quenched and Tempered Steel used in this study and has a high cracking susceptibility.
The microstructure for the standard welded joint is dominated by martensite when quenched and tempered steel made, and martensite produced when water quenched heat treatment is conducted on the welded joint.Water quenched weld joint shows the finer microstructure of the heat-affected zone, but weld metal tends to be coarse and brittle. The highest hardness is achieved after 850 °C water quenching, i. e., base metal=578 VHN, heat affected zone=555 VHN, fusion line=457 VHN, and weld metal=252 VHN.
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