Study on microstructure and mechanical properties of boron added carbon steel joint by laser-arc hybrid welding

Abstract

Boron added carbon steels show excellent properties such as hardenability. It is known that boron segregates at proeutectic austenite grain boundaries and prevent the austenite transformation to ferrite. As a result, martensitic transformation is promoted during cooling and shows high hardness than general carbon steels. Segregation behavior of boron in steel is affected by process conditions such as austenization temperature and cooling rate. It implies the welding process condition can also extremely affect to segregation behavior of boron. Among the welding processes, an arc welding and a laser welding are common in the field. The arc welding process is easy and convenient when an object is thick, but it has demerits when an object is thin because of its large fusion area. On the other hand, the laser welding process has merits of deep fusion depth and fast welding speed. Because of these merits and demerits of welding processes, industrial field tried to combine the arc and the laser welding processes as hybrid welding. The hybrid welding process has unique heat source model different from existing welding processes, it can show unpredictable results such as phase transformation and precipitation of impurities. We observed how boron and other impurities behave in carbon steels under the hybrid welding process using transmission electron microscopy and scanning electron microscopy. Finally, we analyzed mechanical properties related to the behavior of specific elements.