During Baosteel high level strategic planning exercises it was identified that large diameter thick walled (~ 30mm) X70 and X80 pipe is a high priority and has very high value market for Baosteel but HAZ toughness is a huge barrier to success. In addition, there hasn’t been much data in the HAZ toughness of X70 and X80 pipe including tests of the fusion line. This project aims at investigating low temperature HAZ toughness in high heat input seam welds of thick walled X70 and X80 pipe using sophisticated thermomechanical simulation, steel chemistry design evaluation and fitness for purpose studies.