Laser welding of ultra low carbon steels

Abstract

Ultra low carbon steels which carbon contents are from 0.001 mass% to 0.072 mass % were laser welded at different travel speeds using a 300W YAG laser and a 2.4kW CO2 laser facility. The microstructures and mechanical properties have been studied. The amounts of polygonal ferrite, bainitic ferrite and low carbon martensite were observed and the percentage of microstructures were measured as a microstructural composition of weld metals by a point counting method. The micro structural compositions of laser welds of ultra low carbon steels were changed by carbon content of steel and travel speed of laser welding. Mechanical test showed that the hardness of welds and the tensile strength of welded joints depended on the microstructural composition.As a result the amounts of bainitic ferrite and low carbon martensite increased with the increases of carbon content of steel and travel speed. The welded joint of 0.014%C steel welded at a travel speed of 16.7mm/s show a tensile strength of 500N/mm2 and the elongation of 44%. The welded joint of 0.051%C steel shows the high strength of 600N/mm2 and the elongation of 36%. The higher quality is due to the microstructures which are mixtures of bainitic ferrite (=BF) and low carbon martensite(=M) with a small amount of polygonal ferrite(=PF). It is suggested that bainitic ferrite has a higher strength and elongation. The percentage of low carbon martensite increases with carbon content of steel and travel speed. The effects of martensite on the hardness and tensile strength of laser welds was significantly high. In addition, CCT diagrams for estimating the hatdness and microstructural composition of laser welds of ultra low carbon steels have been studied in present work.Ultra low carbon steels which carbon contents are from 0.001 mass% to 0.072 mass % were laser welded at different travel speeds using a 300W YAG laser and a 2.4kW CO2 laser facility. The microstructures and mechanical properties have been studied. The amounts of polygonal ferrite, bainitic ferrite and low carbon martensite were observed and the percentage of microstructures were measured as a microstructural composition of weld metals by a point counting method. The micro structural compositions of laser welds of ultra low carbon steels were changed by carbon content of steel and travel speed of laser welding. Mechanical test showed that the hardness of welds and the tensile strength of welded joints depended on the microstructural composition.As a result the amounts of bainitic ferrite and low carbon martensite increased with the increases of carbon content of steel and travel speed. The welded joint of 0.014%C steel welded at a travel speed of 16.7mm/s show a tensile strength of 500N/mm2 and the elongat...