低炭素鋼の高温延性および溶接割れにおよぼす窒化アルミの影響

鋼中の窒化アルミの研究-III

抄録

Effects of aluminum nitride (A1N) precipitates on the high-temperature ductility and on the welding crack at heat-affected zone were investigated. Small tensile test pieces were made of the steels containing different amounts of aluminum and nitrogen (Table 1 and 2), and were drawn at 500-1000°C The specimens of C. T. S. (controlled thermal severity) test Fig.
1) were used for the investigation of the welding crack at the heat-affected zone. (1) The high-temperature ductility of low-carbon steel as cast is decreased extremely by the addition of aluminum (Fig. 2 and 3). The ductility of low-carbon steel at a temperature lower than 800°C is higher than the one of medium-carbon steel.
2) The ductility during rapid-cooled from 1350°C of low-carbon steel as cast or as forged is decreased at a temperature higher than 850°C by the addition of aluminum (Fig. 5). The ductility of it during slow-cooling from 1350°C is not decreased by the addition of aluminum unlike the ductility of medium-carbon steel (Fig. 4). The ductility without preheat-treat ment of low-carbon steel as forged is decreased at a temperature higher than 850°C by the addition of aluminum.
3) Presumably there is a relation between the decrement of the high-temperature ductility of low-carbon steel by the addition of aluminum and the morphology of aluminum nitride in the steel (Photo. 1 and 2).
4) In the heat-affected zone directly under weld-deposit of the steel containing more aluminum and nitrogen, the precipitates of aluminum nitride scatter undissolved by welding heat. The precipitates in the steel containing less aluminum and nitrogen are dissolved in the heat-affected zone by welding heat. The aluminum nitride precipitates which exist dispersedly undissolved by welding heat in the heat-affected zone seem to promote the transformation during cooling after heated by welding heat, and consequently the welding crack in the heat-affected zone (Photo. 3) is decreased by the addition of large quantities of aluminum and nitrogen.
5) Presumably there is a relation between the increment of the welding crack in the heataffected zone by the addition of aluminum up to 0.01-0.02% and the decrement of the hightemperature ductility by the addition of aluminum. It is considered that the hot crack in the heat-affected zone initiates on account of the decrement of the high-temperature ductility by the precipitation of aluminum nitride, and that consequently the cold crack in the heataffected zone is increased with the hot crack as its starting point.
6) The welding cracks in the heat-affected zone observed in the steel as cast are much more than those in the steel as forged, and are not decreased by the addition of great quantities of aluminum unlike with the steel as forged. This phenomenon seems to be based on the fact that the ductility at high temperature is decreased by formation of the line of aluminum nitride precipitates and the network of sulphide inclusions.