Characterization of heat affected zone for tig torch welded high strength low alloy steel with microalloying elements addition

Abstract

High strength low alloy (HSLA) steels possess an excellent combination of strength and toughness obtained by suitable alloying design and thermo-mechanical controlled processing. However, the strength and toughness combination are deteriorated by the welding parameters and the thermal cycles that the steel experiences during welding. Since welding is an unavoidable stage in HSLA steel manufacturing, it is essential to produce welded sections with as low heat energy as possible, while preserving an appropriate joint geometry and properties. Heat affected zone, particularly adjacent to the weld pool region, has higher hardness and lower fracture toughness compared with the substrate material. The deterioration of heat affected zone (HAZ) mechanical properties are attributed to the formation of martensite-austenite (M-A) constituents and local brittle zones (LBZ). Therefore, the main aim of this research is to improve the HAZ mechanical properties such as tensile strength, hardness and impact toughness of welded high strength low alloy (HSLA) steel using TIG torch melting at different welding process parameters with and without microalloying elements addition (Ti and V). The research investigation was conducted in three-phases. The first phase involves the experimental designs by Taguchi method and producing the welding track under different welding parameters such as welding current, welding voltage, welding speed and gas flow rate with and without microalloying element addition (Ti and V) using powder preplacement and TIG torch welding process. Secondly, optimization the input parameters with the responses to the heat affected zone properties of hardness, tensile strength, and impact toughness. In the last phase, characterization and evaluation of the welded HSLA steel specially HAZ in terms of microstructure, microhardness, tensile strength, and impact toughness. The HAZ microstructural characterization was performed using OM, SEM-EDX, and XRD analyzer. The results showed that the highest tensile strength achieved was 692.85 MPa and 729.80 MPa with Ti and V microalloying element additions, respectively. The impact toughness was 81 J and 76 J for Ti and V addition, and the hardness attained was 202 Hv for both Ti and V microalloying additions. The different ferrite phases formed in the HAZ including acicular ferrite and ferrite with secondary phase aligned along with the bainitic microstructure due to the enhancement of the grain refinement in the HAZ morphology. The best-optimized welding parameters achieved by Taguchi S/N ratio analysis were current, 100 A; voltage, 40 V; speed, 1.5 mm/s; and argon flow rate 20 L/min. The validation of the Taguchi predictive model and optimal parameters for HAZ responses shows that their prediction accuracy error is within the acceptable limit. The improvement of tensile strength value for the HAZ was ≈ 4.20 % for Ti addition and ≈ 5.20 % for V addition, and the average increment of impact toughness value was ≈ 30.36 % for Ti addition and ≈ 37.46 % for V addition. However, the reduction of hardness value for the HAZ was ≈14.5% for Ti addition and ≈19% for V addition compared to the TIG welded sample without the additions of microalloying elements. Due to the positive outcome on the mechanical properties and metallurgical characteristics of the HAZ obtained using the addition of microalloying elements (Ti and V), it can be said that this technique is suitable for improving the welded HAZ mechanical and microstructural performance of HSLA steel.