Tailor Welded Blanks (TWBs) can offer significant benefits in terms of weight reduction, cost and performance. This paper considers the influence of the laser weld on the formability of TWBs in a range of steels with yield strengths from 150 MPa to 585 MPa. The formability of a laser weld in Erichsen tests is related to the weld hardness and the weld width. The hardness can be related to the carbon equivalent of the steel and the weld width depends primarily on heat input. The measurements of cooling rates and laser beam coupling have been made to assess the effect of welding parameters on heat input. Predictions of the maximum heat affected zone (HAZ) hardness are made from the chemical composition, sheet thickness and welding speed. Further work is being carried out to improve predictions of weld metal hardness from computer models.