Laser beam welding of ultrahigh strength martensitic steels with active cooling

Abstract

Recent steel developments create new possibilities for steel construction and lightweight design. The reason for this is the ultrahigh strength of particular steel grades. Fully martensitic steels like 22MnB5 or S1100 QL gain ultimate yield strengths of up to 1500 MPa. Therefore, those grades are predestined for crash relevant or highly loaded parts, and they gain increasing importance as construction materials. As promising as the martensitic ultrahigh strength steels are as construction materials, there are some issues when it comes to joining. The heat input into the material harms the costly adjusted material properties. Thus, each welding process reduces the strength in the heat-affected zone considerably. The key to reduce the drop in strength is to conduct the welding heat out of the work piece. The quicker the heat is conducted the lesser it can harm the material properties. Therefore, it is vital to select an active cooling device capable of conducting a large amount of heat out of the work piece in a very short period. Preliminary tests have shown that it is possible to reduce the width of the heat-affected zone to 50%. Therefore, the work pieces were cooled by liquid nitrogen before and straight after the welding process. To improve the cooling conditions, a clamping device including the possibility of active cooling via water or a mixture of liquid and gaseous nitrogen is designed. The clamping jaws are made out of copper for the maximum heat conduction, and it is possible to cool the weld seam directly through a nitrogen gas flow. Combined with a cooling device which mixes fluid and gaseous nitrogen, gas temperatures down to −150 °C are technically feasible. The setup is used to join martensitic steels by laser beam welding. The effects of a temperature range from room temperature down to −50 °C on the mechanical properties are investigated and discussed.