Laser beam welding of carbide free bainitic steel

Abstract: Because of its very good mechanical properties and its relatively low costs
carbide free bainitic steel has become an interest for research and
development during the recent years. In this work welding tests on these
steels are discussed for the first time. It is known that welding of steels
with a medium or high carbon content leads to brittleness in the heat
affected zone and the fusion zone because of martensite and crack formation.
The steel used in this work had a carbon content of 0.54 % and a silicon
content of 1.44 % to prevent carbide precipitation. It was austempered at 300
°C which lead to a carbide free bainitic microstructure with an average
hardness of 442HV0.5. A ytterbium fibre laser was used for on plate bead
welding and a furnace or an induction coil was used for heat treatment. The
different experiments were equal in laser parameters but differed in pre- and
post weld heat treatment.

Unsurprisingly, the welds showed martensite and hardness values up to
796HV0.5 when welded without heat treatment. But certain heat treatment
methods could lead to different microstructures and improved hardness values.
The most promising results were achieved by a combination of pre- and
post-weld heat treatment at 300 °C with a maximum hardness of 504HV0.5 and by
a heat treatment method which was based on the quenching and partitioning
concept and had a maximum hardness value of 426HV0.5.