Matching the analysis and properties of ultra-high-strength steel with weld metal from iron powder low-hydrogen type electrodes found possible.A history of the development at Cambridge of a notch tensile test for the investigation of brittleness in structural mild-steel is described. The test procedure and the methods of assessing the temperature limit at which the material becomes brittle are outlined in Part I.Investigations of material from an oil storage tank and from ships which fractured in service show that this notch tensile test gives good correlation with these casualties whatever the nominal stress in the plates at the time of the fracture. The results of Charpy V-notch and Izod tests are given for comparison, and it is shown that the energy absorption criterion used in these tests may be misleading (Part II).