Effect of preheating and preplaced filler wire on microstructure and toughness in laser-arc hybrid welding of thick steel

Abstract

Acicular ferrite (AF) is the most important microstructural constituent to achieve high toughness at low temperatures in weld metal of steels. This is due to the relatively small grain size and large misorientation angles. AF is known to form at non-metallic inclusions (NMIs), but under high cooling rates, as in deep and narrow laser-arc hybrid welding (LAHW), this phenomenon is scarcely studied. In deep and narrow LAHW, insufficient transportation of filler wire to the root results in low amount of NMIs, thus bainite-martensite mainly forms due to fast cooling. In this work, a 45 mm thick high strength low alloy steel was welded by double-sided LAHW using different groove preparations. The effect of different cooling times on the microstructure in the weld metal and the heat-affected zone was studied. A low fraction of AF and high hardness were achieved in the root of weld metal when using standard LAHW. This was related to a rapid cooling time (Δt8/5 < 1.0 s) and a low population of NMIs due to lack of filler wire. Micro X-ray computed tomography revealed centerline solidification cracks and porosity. Two external methods were conducted to assist the formation of AF and to reduce hardness, base plate preheating and preplaced metal-cored filler wire into the weld groove prior to welding. Preheating reduced hardness and increased the AF fraction due to increased cooling time. In addition, preheating showed to mitigate porosity. Preplaced filler wire provided an enhanced population of NMIs in the root thus significantly increased the fraction of AF. It was found that the diameter and chemical composition of NMIs has a strong correlation with the cooling time Δt8/5. High impact toughness (> 35 J) was achieved at −50 °C by combining preheating and preplaced filler wire, and up to 45 % fraction content of AF was reached. However, many NMIs were still inactive due to a small diameter (< 200 nm) and unfavorable chemical composition related to the high cooling rate. The external methods had no influence on the occurrence of weld centerline cracks in the root, which will require further attention to secure mechanical properties and integrity.