A constitutional diagram for supermartensitic weld metals defining compositional ranges producing a martensitic microstructure and including information on solidification mode and retained austenite is introduced. The diagram was used to formulate supermartensitic metal cored wires depositing low C (<0.010 %) "fully" martensitic weld metals. Mechanical properties of SAW, MIG and TIG welds were correlated to microstructure, welding procedure and PWHT for all-weld metals and for simulated production welds in supermartensitic pipe and plate material. Weld metal strength overmatched parent material strength provided excessive residual austenite contents were avoided. With few exceptions, the maximum hardness was below 350 HV10. The highest impact toughness was achieved for low oxygen, "fully" martensitic, Mo-alloyed weld metals achieving up to 150 J at -40° C in TIG welding. Fracture toughness testing showed that a CTOD value of minimum 0.15 mm at -20° C is realistic for MIG welds. A short (5 min/ 620° C) PWHT increased impact and fracture toughness and decreased hardness. Preliminary results from SSC testing indicate that welds produced with supermartensitic consumables have adequate corrosion resistance. It is shown that supermartensitic consumables used with realistic fabrication welding procedures produce high quality welds with mechanical and corrosion properties matching requirements.