PRECIPITATION OF CARBIDES AND NITRIDES OBSERVED BY THERMOELECTRIC POWER MEASUREMENTS IN LOW CARBON ALUMINUM KILLED STEEL.

Changes in thermoelectric power and electrical resistivity during 10 min/20 C isochronal annealing were observed in several quenched or cold rolled specimens of low carbon aluminum-killed steel. Annealing stages corresponding to the precipitation of metastable carbide, cementite, iron-nitride, and aluminum-nitride could be followed by the thermoelectric power measurements with fairly good accuracy. In the quench-aging of the steel, a weak unidentified peak was detected around 380 degree C, which could not be found by resistivity measurements. Based on the abnormally high ratio of thermoelectric power increment to resistivity decrement, it was postulated that the aging on heating from about 20 to 100 C immediately after cold rolling (70% reduction) was associated with dissociation of point defect-nitrogen complexes to form AlN molecules. The 400 to 480 C stage in the isochronal annealing of the cold rolled specimens could be identified as due to pre-precipitation clustering of AlN.